U.S. patent number RE31,679 [Application Number 06/344,353] was granted by the patent office on 1984-09-18 for loudspeaker enclosure with acoustical compliance resonating with port mass at frequency below system resonance.
This patent grant is currently assigned to Bose Corporation. Invention is credited to Thomas A. Froeschle, William P. Schreiber.
United States Patent |
RE31,679 |
Froeschle , et al. |
September 18, 1984 |
Loudspeaker enclosure with acoustical compliance resonating with
port mass at frequency below system resonance
Abstract
A multiple driver loudspeaker system comprises two angularly
spaced rear rectangular baffles each nearly filled with four
closely spaced full-range small loudspeakers with a port tube
passing through the center of each rear baffle and the junction
therebetween. A front baffle carries a small centrally located
loudspeaker. The front loudspeaker is backed by a cavity that is
vented through the port tube at the intersection between the rear
baffles. Each of the remaining loudspeakers is backed by a cavity
with the four cavities associated with each rear baffle being
vented through the associated port tube through channels located at
the front of each cavity. Two bullets are cantilevered from the
front baffle rearward and essentially concentric within the
respective centrally located ports. The volume of each of the nine
cavities is substantially the same. The nine loudspeakers are
connected in phase and in series and energized through an active
equalizer having a sharp low-frequency cutoff that prevents the
loudspeakers from being energized with appreciable energy much
below the port tube and cavity resonance of each ported enclosure,
typically 40 Hz while coacting with the loudspeakers to provide
substantially uniform acoustic power radiation over substantially
the full audio frequency range. In an alternative embodiment of the
invention there is a common cavity behind eight drivers, and a pair
of port tubes include damping material inside to lower the Q of the
resonant elements comprising the cavity and port tubes. .Iadd.
Inventors: |
Froeschle; Thomas A.
(Southborough, MA), Schreiber; William P. (Ashland, MA) |
Assignee: |
Bose Corporation (Framingham,
MA)
|
Family
ID: |
26993880 |
Appl.
No.: |
06/344,353 |
Filed: |
February 1, 1982 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
82299 |
Oct 5, 1979 |
|
|
|
Reissue of: |
719943 |
Sep 2, 1976 |
04146745 |
Mar 27, 1979 |
|
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Current U.S.
Class: |
381/89; 181/145;
181/147; 181/156; 381/103; 381/111; 381/349; 381/351; 381/98 |
Current CPC
Class: |
H04R
1/2819 (20130101); H04R 1/227 (20130101); H04R
3/12 (20130101); H04R 1/2834 (20130101); H04R
1/2826 (20130101) |
Current International
Class: |
H04R
1/28 (20060101); H04R 001/28 () |
Field of
Search: |
;179/1E
;181/144-148,151,156 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Stellar; George G.
Attorney, Agent or Firm: Hieken; Charles
Parent Case Text
REFERENCE TO PRIOR COPENDING APPLICATION
This is a continuation of reissue application Ser. No. 82,299 filed
Oct. 5, 1979, now abandoned. .Iaddend.
Claims
What is claimed is:
1. A loudspeaker system comprising,
means defining an enclosure for accommodating a plurality of like
high-compliance loudspeaker drivers characterized by potential
instability associated with variations of the characteristics
between drivers when operating in the same frequency range and
sharing a common cavity with one or more port tubes or drone
cones,
cavity defining means formed with a corresponding plurality of
driver openings each for accommodating a respective loudspeaker
driver and characterized by acoustic compliance,
said loudspeaker drivers each seated in a respective one of said
driver openings and connected in phase,
said cavity defining means having at least one mass opening for
accommodating means for providing acoustic mass that resonates with
said acoustic compliance at a predetermined mass-compliance
resonant frequency in the low range of audio frequencies,
said means for providing acoustic mass seated in a respective mass
opening,
said cavity defining means being constructed and arranged to
provide an air channel common to all said loudspeaker drivers,
and means for preventing the cones of said loudspeaker drivers from
exhibiting out-of-phase movement when said loudspeaker drivers are
connected in phase and energized with an electrical signal having
spectral components in the low range of audio frequencies embracing
and near said mass-compliance resonant frequency.
2. A loudspeaker system in accordance with claim 1 wherein the
embracing frequency range is from just below said predetermined
frequency to the fundamental resonance of said loudspeaker system
related to the volume of said cavity defining means and the
electromechanical parameters of said loudspeaker drivers.
3. A loudspeaker system in accordance with claim 1 wherein said
cavity defining means defines a cavity for each driver adjacent to
a respective driver opening and said means for providing acoustic
mass comprises common port tube means for venting said cavities
outside said enclosure and further comprising,
aperture defining means defining a coupling aperture between each
cavity and said common port tube means of cross sectional area
small enough to provide substantial acoustic isolation between
adjacent cavities at low bass frequencies and large enough to
transmit air therethrough at said low bass frequencies for coupling
each cavity to said common port tube means.
4. A loudspeaker system in accordance with claim 3 wherein the
volume of each of said cavities is substantially the same.
5. A loudspeaker system in accordance with claim 1 and further
comprising,
active equalizing means including means for coupling signal
spectral components of substantially the full range of audio
frequencies to the in-phase-connected drivers, means for
establishing a substantially uniform radiated power response of
said system as a function of frequency at least in the range of
frequencies between said predetermined mass-compliance resonant
frequency and the fundamental resonance of said system, and means
for sharply attenuating signal spectral components below a
predetermined frequency at or slightly below said predetermined
mass-compliance resonant frequency.
6. A loudspeaker system in accordance with claim 5 wherein said
means for establishing includes means for establishing a
substantially uniform radiated power response as a function of
frequency of said system for substantially the full range of audio
frequencies.
7. A loudspeaker system in accordance with claim 5 wherein the
attenuation imparted to signal spectral components by said means
for sharply attenuating below said predetermined frequency is at
least substantially 18 db per octave.
8. A loudspeaker system comprising,
means defining an enclosure for accommodating a plurality of like
loudspeaker drivers,
cavity defining means formed with a corresponding plurality of
driver openings each for accommodating a respective loudspeaker
driver and characterized by acoustic compliance,
said cavity defining means having at least one mass opening for
accommodating means for providing acoustic mass that resonates with
said acoustic compliance at a predetermined mass-compliance
resonant frequency in the low range of audio frequencies.
means for preventing the cones of said loudspeaker drivers from
exhibiting out-of-phase movement when said loudspeaker drivers are
connected in phase and energized with an electrical signal having
spectral components in the low range of audio frequencies embracing
and near said mass-compliance resonant frequency,
wherein said cavity defining means defines a cavity for each driver
adjacent to a respective driver opening and said means for
providing acoustic mass comprises common port tube means for
venting said cavities outside said enclosure and further
comprising
aperture defining means defining a coupling aperture between each
cavity and said common port tube means of cross sectional area
small enough to provide substantial acoustic isolation between
adjacent cavities at low bass frequencies and large enough to
transmit air therethrough at said low bass frequencies for coupling
each cavity to said common port tube means,
and wherein said driver openings are on the rear of said enclosure
and further comprising,
at least another port tube means,
said cavity defining means defining at least another cavity
adjacent to a driver opening at the front of said enclosure and
connected to said another port tube means.
9. A loudspeaker system in accordance with claim 8 wherein all said
port tube means vent to the rear.
10. A loudspeaker system in accordance with claim 8 and further
comprising,
a plurality of like loudspeaker drivers connected in phase each
seated in a respective one of said driver openings.
11. A loudspeaker system in accordance with claim 8 wherein the
volume of each of said cavities is substantially the same.
12. A loudspeaker system comprising,
means defining an enclosure for accommodating a plurality of like
loudspeaker drivers,
cavity defining means formed with a corresponding plurality of
driver openings each for accommodating a respective loudspeaker
driver and characterized by acoustic compliance,
said cavity defining means having at least one mass opening for
accommodating means for providing acoustic mass that resonates with
said acoustic compliance at a predetermined mass-compliance
resonant frequency in the low range of audio frequencies,
means for preventing the cones of said loudspeaker drivers from
exhibiting out-of-phase movement when said loudspeaker drivers are
connected in phase and energized with an electrical signal having
spectral components in the low range of audio frequencies embracing
and near said mass-compliance resonant frequency,
said means for providing acoustic mass seated in a respective mass
opening and comprising port tube means for venting said cavity
means outside
said port tube means including means defining a tapered annular
hollow region for establishing substantially laminar airflow
therein.
13. A loudspeaker system in accordance with claim 12 wherein said
means defining an annular region comprises a bullet cantilevered
from a member forward of said port tube means.
14. A loudspeaker system in accordance with claim 13 wherein said
bullet extends through the entire length of said port tube
means.
15. A loudspeaker system comprising,
means defining an enclosure for accommodating a plurality of like
loudspeaker drivers,
cavity defining means formed with a corresponding plurality of
driver openings each for accommodating a respective loudspeaker
driver and characterized by acoustic compliance,
said cavity defining means having at least one mass opening for
accommodating means for providing acoustic mass that resonates with
said acoustic compliance at a predetermined mass-compliance
resonant frequency in the low range of audio frequencies,
means for preventing the cones of said loudspeaker drivers from
exhibiting out-of-phase movement when said loudspeaker drivers are
connected in phase and energized with an electrical signal having
spectral components in the low range of audio frequencies embracing
and near said mass-compliance resonant frequency,
said cavity defining means defining a cavity for each driver
adjacent to a respective driver opening and said means for
providing acoustic mass comprising common port tube means for
venting said cavities outside said enclosure,
aperture defining means defining a coupling aperture between each
cavity and said common port tube means of cross sectional area
small enough to provide substantial acoustic isolation between
adjacent cavities at low bass frequencies and large enough to
transmit air therethrough at said low bass frequencies for coupling
each cavity to said common port tube means,
wherein said common port tube means includes means defining a
tapered annular hollow region for establishing substantially
laminar airflow therein.
16. A loudspeaker system in accordance with claim 15 wherein said
means defining an annular region comprises a bullet cantilevered
from a member forward of said common port tube means.
17. A loudspeaker system in accordance with claim 16 wherein said
bullet extends through the entire length of said common port tube
means.
18. A loudspeaker system in accordance with claim 17 wherein said
port tube means is formed with a folded over lip facing the forward
member adjacent each aperture for establishing smooth air flow at
the entrance of said port tube means.
19. A loudspeaker system in accordance with claim 18 wherein there
are four of said cavities surrounding said common port tube means
and each of said coupling apertures is located between said lip and
said forward member and embraces an angle of substantially
90.degree. about the axis of said common port tube means,
20. A loudspeaker system in accordance with claim 19 and further
comprising a second of said common port tube means formed with a
folded over lip facing said forward member with an additional four
of said cavities surrounding said second common port tube means
with a coupling aperture between each of the latter cavities and
said second common port tube located between the latter lip and
said forward member and embracing an angle of substantially
90.degree. about the axis of said second common port tube
means.
21. A loudspeaker system in accordance with claim 20 wherein the
eight cavities extend between said front member and respective rear
angled baffles forming an obtuse angle with each other.
22. A loudspeaker system in accordance with claim 21 and further
comprising at least a third port tube means,
said cavity defining means defining a ninth cavity adjacent to a
driver opening in said front member,
said third port tube means being for venting said ninth cavity to
the outside of said enclosure and extending from said ninth cavity
through an opening embracing the junction between said rear angled
panels.
23. A loudspeaker system in accordance with claim 22 and further
comprising loudspeaker drivers connected in phase seated in each of
said driver openings.
24. A loudspeaker system in accordance with claim 23 and further
comprising,
active equalizing means including means for coupling signal
spectral components of substantially the full range of audio
frequencies to the in-phase-connected drivers, means for
establishing a substantially uniform radiated power response of
said system as a function of frequency at least in the range of
frequencies between said predetermined mass-compliance resonant
frequency and the fundamental resonance of said system, and means
for sharply attenuating signal spectral components below a
predetermined frequency at or slightly below said predetermined
mass-compliance resonant frequency.
25. A loudspeaker system in accordance with claim 24 wherein said
means for establishing includes means for establishing a
substantially uniform radiated power response as a function of
frequency of said system for substantially the full range of audio
frequencies.
26. A loudspeaker system in accordance with claim 24 wherein the
attenuation imparted to signal spectral components by said means
for sharply attenuating below said predetermined frequency is at
least substantially 18 db per octave. .Iadd. 27. A loudspeaker
system comprising,
means defining an enclosure for accommodating at least one
loudspeaker driver,
said enclosure including cavity defining means formed with at least
one driver opening for accommodating said loudspeaker driver and
characterized by acoustic compliance,
said loudspeaker driver being seated in said driver opening,
said cavity defining means also having at least one mass opening
for accommodating means for providing acoustic mass that resonates
with said acoustic compliance at a predetermined mass-compliance
resonant frequency in the low range of audio frequencies,
said means for providing acoustic mass seated in said mass
opening,
said loudspeaker system having a fundamental resonance determined
by the volume of said enclosure and the electromechanical
characteristics of said driver at a frequency significantly higher
than said predetermined mass-compliance frequency,
and active equalizing means including means for establishing a
substantially uniform radiated power response of said system as a
function of frequency at least in the range of frequencies between
said predetermined mass-compliance resonant frequency and said
loudspeaker system fundamental resonance, and means for sharply
attenuating signal spectral components below a predetermined
frequency at or slightly below said predetermined mass-compliance
resonant frequency. .Iaddend..Iadd.
A loudspeaker system in accordance with claim 27 wherein said
loudspeaker system fundamental resonance is at least twice said
predetermined mass-compliance resonant frequency. .Iaddend..Iadd.
29. A loudspeaker system in accordance with claim 5 wherein the
fundamental resonance of said system is at least twice said
predetermined mass-compliance resonant frequency. .Iaddend..Iadd.
30. A loudspeaker system comprising,
means defining an enclosure for accommodating at least one
loudspeaker driver,
said enclosure including cavity defining means formed with at least
one driver opening for accommodating said loudspeaker driver and
characterized by acoustic compliance,
said loudspeaker driver being seated in said driver opening,
said cavity defining means also having at least one opening for
accommodating means for providing acoustic mass that resonates with
said acoustic compliance at a predetermined mass-compliance
resonant frequency in the low range of audio frequencies,
said means for providing acoustic mass seated in said mass opening
and comprising port tube means for venting said cavity means
outside said enclosure,
said port tube means including means defining a tapered annular
hollow region for establishing substantially laminar airflow
therein. .Iaddend..Iadd. 31. A loudspeaker system in accordance
with claim 30 wherein said port tube means has an inside end inside
said enclosure and an outside end adjacent to the region outside
said enclosure and said means defining an annular region comprises
a bullet cantilevered from a member that is nearer said inside end
than said outside end of said port tube means. .Iaddend..Iadd. 32.
A loudspeaker system in accordance with claim 31 wherein said
bullet extends through the entire length of said port tube means.
.Iaddend..Iadd. 33. A loudspeaker system in accordance with claim
27 wherein said loudspeaker driver is a full-range driver.
.Iaddend.
Description
BACKGROUND OF THE INVENTION
The present invention relates in general to loudspeaker systems and
more particularly concerns a novel loudspeaker system characterized
by unusually realistic reproduction of sound that is compact and
relatively easy and inexpensive to manufacture in large quantities
while maintaing good quality control and producing relatively high
sound levels in response to relaively low input electrical power
levels. The present invention achieves the performance level of the
internationally famous BOSE 901 DIRECT/REFLECTING loudspeaker
system described in Bose U.S. Pat. No. 3,582,553 and embodies the
principles of that patented invention and the invention described
in Bose U.S. Pat. No. 3,038,964.
Both that system and a preferred embodiment of this system include
eight speakers on a pair of rear panels or baffles that each form
an angle of about 30.degree. with the wall upon which the rear
speakers direct their sound and one speaker on the front panel or
baffle that faces the normal listening area. This arrangement
radiates the desired ratio of about 8:1 reflected sound to direct
sound while projecting the image of a musical performance located
on a stage that is about a foot behind the wall when the enclosure
is about a foot in front of the wall so that it is possible to hear
the full stereo spread from a wide range of listening positions
including directly in front of one enclosure. That system and a
preferred embodiment of the present invention also both include an
active equalizer for establishing essentially uniform acoustic
power radiation as a function of frequency over substantially the
entire audio frequency range.
The BOSE 901 loudspeaker system has received an unprecedented
series of rave reviews in the United States and many other
countries. While that system performs well when driven with power
amplifiers of moderate capacity, higher power amplifiers are
required to produce high acoustic levels at the lower audio
frequencies.
It is known in the prior art to use ported enclosures to obtain
higher acoustic power levels at lower frequencies with a given
electrical input power. And a simple port works satisfactorily in a
conventional woofer-tweeter loudspeaker system and is used, for
example, in the BOSE Model 301 DIRECT/REFLECTING loudspeaker
system. However, it was discovered that simply porting the
loudspeaker cabinet in the system described in U.S. Pat. No.
3,582,553 did not provide .[.satisfatory.]. .Iadd.satisfactory
.Iaddend.performance. It was discovered that in the vicinity of
port resonance all the small loudspeakers did not operate in phase
with the result that the excursions of the drivers working together
increased to compensate for the excursions of the out-of-phase
drivers, causing the drivers to enter the nonlinear region of
operation at relatively modest .[.and.]. sound levels.
Accordingly, it is an important object of this invention to provide
an improved loudspeaker system.
It is another object of the invention to achieve the preceding
object while retaining all the performance advantages of the BOSE
901 loudspeaker system described in U.S. Pat. No. 3,582,553.
It .Iadd.is .Iaddend.a further object of the invention to achieve
one or more of the preceding objects while increasing the ratio of
acoustic power radiated to input electrical power in the bass
frequency range.
It is a further object of the invention to achieve one or more of
the preceding objects .[.witha.]. .Iadd.with a .Iaddend.compact
ported cabinet.
It is still a further object of the invention to achieve one or
more of the preceding objects with a structure that is relatively
inexpensive and easy to manufacture through mass production
techniques while maintaining high quality control.
It is a further object of the invention to achieve one or more of
the preceding objects with multiple in-phase-connected full-range
loudspeakers in a ported enclosure while solving the problem noted
above.
SUMMARY OF THE INVENTION
According to the invention, there are a plurality of loudspeaker
driver means connected in phase, port tube or drone cone means for
coacting with cavity means to establish a cavity-port-tube or drone
cone resonant frequency in the low range of audio frequencies, and
means for preventing the driver means cones from moving
out-of-phase in the low range of audio frequencies where the
port-tube or drone cone means is effective in reducing driver cone
excursions.
According to a more specific form of the invention, there are a
plurality of loudspeaker driver means connected in phase, means
defining a cavity associated with each loudspeaker driver means,
port tube means for venting the cavity to the outside, and aperture
defining means defining apertures between each cavity and the port
tube means for coupling the cavities to the port tube means.
Preferably the loudspeaker driver means are like in-phase-connected
full-range drivers typically connected in series closing the
associated cavities at an end thereof opposite the end near which
the coupling aperture is located with the port tube means venting
to the outside through a port opening in a baffle carrying the
associated drivers clustered thereabout. Preferably, there are two
rear angled baffles each carrying a cluster of four drivers
surrounding a respective port opening. Preferably, there is a front
baffle carrying a single driver connected in phase with the other
drivers closing one end of a cavity and vented through a tube at
the other end passing through the junction between the two rear
baffles. Preferably, the volume of each cavity is substantially
equal to that of all the others. Preferably, there is a bullet
means concentric within each port tube means and
.[.cabntilevered.]. .Iadd.cantilevered .Iaddend.from the front
baffle coacting with the surrounding port tube means to comprise
means for establishing laminar air flow within the tube means.
Preferably, there is active electrical equalizing means coacting
with the loudspeaker drivers and assembly for establishing
substantially uniform acoustic power radiation as a function of
frequency over substantially the full audio frequency range and
characteristized by a sharp cutoff below a frequency corresponding
substantially to the port tube-cavity resonant frequency typically
at substantially 40 Hz. Preferably, the means defining the
cavities, the tube means and the bullets comprise molded plastic
components.
According to one aspect of the invention, the cavity defining means
comprise means for preventing the driver means cones from moving
out-of-phase. According to another aspect of the invention,
Q-reducing means for reducing the Q of the resonant system
comprising cavity defining means and port tube means comprises
means for preventing out-of-phase cone movement.
Numerous other features, objects and advantages of the invention
will become apparent from the following specification when read in
connection with the accompanying drawing in which:
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a perspective view of a loudspeaker assembly accoding to
the invention showing the three rear-venting port tubes;
FIG. 2 is a front view of a loudspeaker assembly according to the
invention with the front grill cloth removed to show the mounting
of the front driver and the base of the bullets that are
cantilevered from the front baffle into the port tubes of the rear
drivers;
FIG. 3 is a perspective view of an enclosure as seen from the right
rear without the top and sides;
FIG. 4 is a top view of the assembly with portions cut away and
some portions in section to illustrate the internal structure;
FIG. 5 is a rear view of a loudspeaker assembly according to the
invention with the grill cloth removed;
FIG. 6 is a combined block-schematic circuit diagram of one channel
of a system according to the invention with a preferred form of
active equalizer;
FIG. 7 is a graphical representation of typical responses of the
equalizer of FIG. 6 plotted to a common frequency scale;
FIG. 8 is a fragmentary view of a baffle portion illustrating
structural details of means for accommodating twist-and-lock driver
assemblies;
FIG. 9 is a diagramatic representation of an alternate embodiment
of the invention having means for lowering the Q of a resonant
system comprising acoustic compliance provided by a cavity and
acoustic mass provided by port tubes;
FIG. 10 is a top plan view of an assembled cabinet with drivers
installed and the top removed.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
With reference now to the drawing and more particularly FIG. 1
thereof, there is shown a perspective view of a loudspeaker
assembly according to the invention as seen from the left rear. The
assembly includes top, bottom, left and right panels 11, 12, 13 and
14, respectively. Left and right grills 15 and 16, respectively,
cover respective angled rear baffles 52 and 33 that each carry four
closely-spaced full-range drivers connected in series (FIG. 5)
through which left and right port tubes 17 and 18, respectively
protrude for venting four cavities behind each respective baffle as
described below. Left and right port tubes 17 and 18 surround
respective concentric tapered left and right bullets 21 and 22 that
comprise means for establishing laminar air flow in the annular
region of slightly tapered radial width between the bullet and the
surrounding tube.
A central port tube 23 protrudes through a collar 28 defining a
port opening in a junction between the rear baffles for venting the
cavity behind the driver on the front baffle (FIG. 2).
Referring to FIG. 2, there is shown a front view of a loudspeaker
assembly according to the invention with the front grill cloth
removed to illustrate certain structural features. The same
reference symbols identify corresponding elements throughout the
drawing. The front baffle 24 is preferably made of high impact
plastic, such as styrene, and formed with a central opening 25 for
accommodating front loudspeaker driver 26' and left and right
openings 26 and 27, respectively, surrounded by annular rims (not
visible in FIG. 2) to which the bases 31 and 32 of left and right
bullets 21 and 22 are secured to cantilever bullets 21 and 22 from
front baffle 24. Recess 19 is for accommodating the fastener that
secures a logo to the front of the assembly.
Referring to FIG. 3, there is shown a perspective view of an
assembly according to the invention without the top and side panels
as seen from the right rear illustrating structure defining the
respective cavities for the rear drivers. Right rear baffle 33 is
formed with four openings 34, 35, 36 and 37, for accommodating
respective drivers, each at one end of a respective cavity closed
at the other end by front baffle 24. A horizontal partition 41 and
portions of port tube 18 separate the cavity behind opening 36 from
the cavity behind opening 37, and apertures 42 and 43,
respectively, couple these cavities into the entrance of port tube
18 adjacent front baffle 24 over lip portions 44' and 45',
respectively, that help establish a smooth flow of air from each
cavity into the associated port tube. The area of each of
aperatures 42 and 43 is large enough to avoid audible noises when
reproducing passages in the low .[.bas.]. .Iadd.bass
.Iaddend.region while being small enough to prevent acoustical
coupling between drivers. A suitable cross sectional area for each
aperture is 2.25 square inches. The aperature areas were
established by pushing the tubes as close to the front baffle as
practical without producing undesired audible noises when the
drivers were energized with a low frequency signal. Studs 38 are
for receiving staples to secure the grill cloth assembly.
Vertical partition 44 and inwardly sloping partition 45 isolate the
cavity before opening 36 from the cavity before opening 35.
Similarly vertical partition 46 and downwardly and inwardly sloping
partition 47 isolate the cavity before opening 37 from the cavity
before opening 34. It is desired that the cavity behind each driver
be of substantially the same volume. By making panels 45 and 47
slope inwardly, cavity volume is added to the cavities adjacent the
sides to compensate for the shorter span between front panel 24 and
rear panel 33 for these cavities as compared with the cavities
nearer the center.
Referring to FIG. 4, there is shown a top view of the loudspeaker
assembly according to the invention partially in section and with
portions at different depths cut away to illustrate features of the
invention. The cavity behind front driver 26 is defined by the
generally cylindrical member 51 connected at the rear to separate
port tube 23 that vents through collar 28 at the junction between
the .[.twp.]. .Iadd.two .Iaddend.angled rear panels 33 and 52. A
vertical partition 53 extends above and below cylindrical member 51
and port tube 23 separates the cavities associated with the inner
pairs of rear drivers.
The volume of generally cylindrical member 51 is substantially
equal to the volume of each of the other eight cavities in the
enclosure and coacts with port tube 23 to establish a cavity-port
tube resonance of substantially 40 Hz. Each of the other cavities
coacts with the associated port tube to establish a cavity-port
tube or mass-compliance resonance of substantially 40 Hz.
While the invention may be practiced by fabricating the various
partitions and other members as separate pieces, in a preferred
form of the invention, the front baffle, the partition 53, the port
tube 23 and the generally cylindrical member 51 is a unitary
structure formed by injection molding, each rear baffle, the
associated port tube and associated partitions is a unitary piece
formed by injection molding, and the bullets are unitary pieces
formed by injection molding. The preferred material is plastic. A
feature of the invention is that only three molds are required, one
for the front baffle assembly, a second for the rear baffles and a
third for the bullets because bullets 21 and 22 are identical and
rear baffles 33 and 52 are identical, collar 28 being formed of two
semicircular portions that mate together. The result of this
arrangement is high reproducibility at relatively low cost while
establishing good acoustic properties.
A feature of the invention resides in having all the cavities
vented toward the rear. It has been discovered that venting to the
rear where the loudspeaker assembly is closer to the wall results
in improved bass response as compared with the conventional
approach of venting to the front. There is no problem with the wall
obstructing the flow of air from the vents because the preferred
position of the loudspeaker assembly is about a foot from an
adjacent wall.
Referring to FIG. 5, there is shown a rear view of the loudspeaker
assembly with the rear grill cloth removed.
Referring to FIG. 6, there is shown a combined block-schematic
circuit diagram of an exemplary embodiment of one channel of an
active equalizer connected to a receiver for energizing nine
drivers in series according to the invention. For stereo there are
two of these channels. Representative parameter values are set
forth.
Referring to FIG. 7, there is shown a graphical representation as a
function of frequency of the response of the active equalizer shown
in FIG. 6 for the extreme settings of the mid-bass and treble
controls, the middle curve being the normal setting and the effect
of moving the below 40 switch to the decreased position. The
circuitry includes a number of features. There is a 3-pole sharp
cutoff network that effectively sharply cuts off the response below
32 Hz, a frequency slightly below the cavity-port tube resonance of
about 40 Hz. Another feature is the compensation for driver rim
resonance in the region between 1 and 2 kHz. Still another feature
is the provision of the mid-bass control which affects the response
between 100 and 300 Hz to accommodate for various listening
environments and the treble control which affects the response only
above 2 kHz.
Referring to FIG. 8, there is shown a portion of a baffle
illustrating structural details preferably molded therein for
accommodating twist-and-lock drivers. A baffle includes for each
driver an opening 62 inside a depressed annular surface for
accommodating a mating rear annular surface on a driver when the
driver is mounted in opening 62. The diameter of opening 62 is just
large enough to accommodate the portion of the driver basket
rearward of the mating rear annular surface. Three equiangularly
spaced recesses for accommodating mating tabs of a driver are
defined by structures 64 open at the counterclockwise edges for
receiving the driver tabs and are formed with notches 65 for mating
engagement with corresponding protrusions on the driver to lock the
driver in place when twisted fully clockwise. The span of the slit
in a direction perpendicular to the baffle is preferably slightly
less than the driver tab thickness so that rotating a driver
clockwise until a driver protrusion mates with a notch 65 results
in each tab being firmly engaged while the outside surface of a lip
on the driver basket parallel to the driver axis snugly engages the
wall portions 66 extending perpendicularly from the baffle to
establish a substantially fluid-tight seal with a driver without
gaskets, other soft material such as Mortite or screws to
significantly reduce assembly costs while improving
reliability.
Referring to FIG. 9, there is shown a diagramatic representation of
an alternate embodiment of the invention in which a pair of drivers
71 and 72 .[.pratially.]. .Iadd.partially .Iaddend.enclose a common
cavity 73 vented through a port tube 74 having flocked material 75
or other suitable acoustic damping material for reducing the Q of
the resonant system comprising cavity 73 and port tube 74. This
aspect of the invention may also be embodied with a loudspeaker
system of the type disclosed in U.S. Pat. No. 3,582,553 with all
the drivers partially enclosing a common cavity and preferably
having a port tube venting the common cavity through each rear
panel, each port tube having flocked material or other suitable
acoustic damping material inside the tube. In this embodiment the
bullets may be omitted when properly designed in accordance with
principles discussed below. Other means for reducing the Q may be
practiced, for example, placing damping material inside or across
one or both openings of the port tube, lining or filling the cavity
with damping material, or other suitable means.
Referring to FIG. 10, there is shown a top view of the cabinet
shown in FIG. 4 with top panel 11 removed and rear upper drivers
35' and 36' visible seated in openings 35 and 36, respectively, and
rear upper drivers 39 and 40 visible. Front driver 26' is shown in
broken outline. The nine drivers are connected in phase in series
as shown in FIG. 6.
Having described the physical arrangement of the invention and some
important features, it is appropriate to consider certain
principles of operation. One aspect of the invention is concerned
with reducing cone excursion at a given sound level. An important
function of the present invention is to improve the linear motion
of the cones and significantly increase the dynamic range over
which the loudspeaker system accurately reproduces the bass notes
of musical instruments. To this end there are .[.14.].
.Iadd.fourteen .Iaddend.principal regions inside the enclosure; the
nine cavities behind the drivers, the three port tubes 17, 18 and
23 venting at the rear and the two mixing regions at the front of
port tubes 17 and 18. At times the air in port tubes 17 and 18
moves faster than 60 miles an hour and would produce undesired
audible noise in the absence of specific features of the invention
that aerodynamically establish laminar flow in the mixing regions
and inside tubes 17 and 18. To this end the front ends of tubes 17
and 18 present a .[.cruved.]. .Iadd.curved .Iaddend.surface
established by the folded-over front lip portion, and bullets 21
and 22 are formed as shown to have outer surfaces approaching the
inside surfaces of tubes 17 and 18 at the rear ends, the front
portion curved outwardly as shown to coact with the curved lip at
tubes 17 and 18 to provide a smooth transition region into the port
tubes and a gradually tapered tip at the rear outside tubes 17 and
18. Gradually reducing the cross sectional area of the port tubes
helps keep the air flow laminar and causes the port tubes to
function as a low pass filter which helps confine high frequency
noise inside the enclosure. Thereafter, the air stream diverges. It
is preferable to cantilever the bullets 21 and 22 as shown without
introducing supports in the region between bullet and tube because
the supports might tend to distort the laminar flow and thereby
introduce undesired audible effects.
The air confined in each port tube 18 may be regarded as an
acoustic mass in series with the air in each cavity which may be
regarded as an acoustic compliance in parallel with three other
acoustic compliances each resonated by four times the effective
acoustic mass of the associated port tube to establish a
cavity-port tube resonance at substantially 40 Hz, the same
frequency at which the cavity defined by cylindrical member 51 and
port tube 23 are resonant. At relatively few Hertz below the
fundamental resonance, typically below 32 Hz, the active equalizer
sharply curtails the electrical power to the drivers because
applying increasing levels of electrical power to the drivers at
these frequencies would produce additional deflection of the cone
that might well extend into the nonlinear region without providing
appreciable audible acoustic power. Accordingly, a feature of the
invention is to arrange the active equalizer so that there is a
sharp decrease in response as a function of frequency below about
32 Hz, typically 18 db per octave. It may be advantageous to cut
off sharply below the fundamental cavity-port tube resonance of 40
Hz to maximize dynamic range without significant loss of reproduced
spectral components present in most music. The choice of 32 Hz
still provides adequate dynamic range while facilitating
reproduction of very low bass components present in some music.
It is also desirable to sharply reduce the response of the
equalizer above the highest audible frequency of 15 kHz to prevent
spurious inaudible signals from overloading the amplifiers or
speakers and thereby effectively increase the dynamic range of the
reproducing system for audible frequencies.
The active equalizer according to the invention also is arranged to
help obtain optimum performance in the presence of varying room
acoustics and speaker placement in different listening rooms. The
treble frequency contour control adjusts the high frequencies to
compensate for materials that might affect the high frequency
absorption of the room or for curtains or other lightly absorbing
materials that might be located on the wall behind the speakers
where complete reflection at these frequencies is preferred. The
mid-bass control adjusts for those frequencies most affected by
placing the .[.apeaker.]. .Iadd.speaker .Iaddend.in different
locations in the room and for different amounts of mid-bass
absorbing materials in the room.
As indicated above, in a multiple-driver loudspeaker system where
the drivers operate in the same frequency range and share a common
cavity having one or more port tubes or drone cones, there is a
potential instability associated with variations in the
characteristics between drivers. The problem may occur essentially
in the frequency range between resonance of the cavity acoustic
compliance with the port tube or drone cone acoustic mass,
typically about 40 Hz, and the fundamental resonance of the
loudspeaker system, determined by enclosure volume and driver
electromechanical characteristics, typically about 150 Hz. Consider
the two-driver case represented in FIG. 9 with drivers 71 and 72
connected in phase and electrical forces F.sub.x and F.sub.y
applied to drivers 71 and 72, respectively, driving the cones
inward. If driver 72 is stronger so that the force F.sub.y is
greater than the force F.sub.x, the pressure inside cavity 73 may
cause the cone of driver 71 to move outward in phase opposition to
the movement of the cone of driver 72. In extreme cases the voice
coil on driver 71 may be driven completely outside the air gap, and
this excessive motion will cause undesirable distortion, reduction
in maximum bass output and potential early failure of the driver.
Although this problem may also be encountered in unported systems,
the problem is more severe in ported or drone cone systems in which
the drivers operate over the low audio frequency range where the
port or drone cone is effective in increasing the pressure on the
cones. One means for preventing the in-phase-connected drivers from
having one or more cones move in phase opposition to the other or
others is to divide the enclosure into separate cavities to reduce
coupling as described above. Another means comprises using drivers
with stiff spiders, a less preferred approach because drivers with
stiff spiders are hard to control and lower the bass efficiency of
the system.
Another advantageous approach involves controlling the Q of the
resonant system comprising the acoustic compliance of the cavity
and the acoustic mass of the port tubes or drone cones. The
differences between drivers (F.sub.y -F.sub.x)/F.sub.x which can be
tolerated depend directly upon spider stiffness, cavity volume and
inversely upon the number of drivers in excess of one, atmospheric
pressure, the square of the area of the cones and Q+1, where Q is
related to the sharpness of the port tube-cavity resonance. If the
enclosure compliance has a pair of complex zeros associated with
the port tube, which can be determined by solving an equation of
the form
Q may be defined as shown in the equation. The Q is strictly a
function of the enclosure and port tube or drone cone parameters,
and does not depend significantly upon driver characteristics. The
Q may be lowered by restricting the flow of air in the port tube,
for example, by inserting a piece of open-cell foam in the port
tube or fuzzing the inside of the port tube 72 with a flocking
material 72. Alternatively, the inside of the cavity may be
arranged to dissipate energy or combinations of increased
dissipation in the cavity and port tube or drone cone. This damping
increases the stability of the system and increases the tolerable
driver variations without having undesired out-of-phase driver cone
movement.
A function of port tube 75 is to lower the excursion of the drivers
to reduce distortion in a frequency range around the cavity-port
tube resonance. The Q may be lowered from values typically as high
as 5 to 20 down to one or two to increase stability while retaining
the advantage of the port tube in reducing distortion.
While the preferred form of the invention uses port tubes to
provide the effective acoustic mass for resonating with the
acoustic compliance of the cavity, it is within the principles of
the invention to use a drone cone speaker as a substitute for one
or more port tubes for the various embodiments of the invention. In
the embodiment of FIG. 9 the damping means may be applied on the
drone cone and might comprise foam material at the periphery or
roll of the cone or other suitable material having a damping
effect.
The equation relating the tolerable force differences is given
by:
where:
V.sub.o is the enclosure volume,
K.sub.s is the driver spider stiffness,
N is the number of drivers,
Q is as defined above,
P.sub.o is atmospheric pressure, and
A is the area of the driver cones.
It is preferred that the means for damping be inserted in the port
tube or drone cone where velocity is relatively high and relatively
easy to resist for producing the desired Q-reducing dissipation.
However, damping may also be introduced in the cavity, preferably
by means responsive to pressure because pressure is relatively
high, such as movable sides supported in dash pots or by other
suitable dissipative means.
An exemplary embodiment of this form of the invention involved
modifying the commercially available BOSE 800 professional
loudspeaker system having eight in-phase-connected drivers on the
two angled panels and none on the opposite flat panel in a cabinet
13 inches high by 20 inches wide by 12 inches deep with an internal
volume of substantially 1800 in..sup.3 and a port tube venting
through the center of each angled panel of diameter 23/4 inches and
length 9 inches each having an inch length of open cell urethane
foam spanning the tube opening of density of 10 pores per inch. Its
appearance is substantially as seen in FIG. 5 without the bullets
and the central port. The cavity acoustic compliance-port tube
acoustic mass resonance was substantially 50 Hz and the fundamental
resonance of the loudspeaker system substantially 120 Hz.
.[.It.]. .Iadd.If .Iaddend.desired a single shorter port tube may
be used and the port tube or tubes may vent through the side panels
of the enclosure or the flat panel opposite the angled panels.
Venting through the angled panels is preferred because the side and
flat panels may be the sides of a weather-resistant carrying case
with the angled panels covered by a cover making a substantially
fluid-tight seal with the rest of the case when transporting the
system.
In an exemplary embodiment of the invention shown in FIGS. 1-5 the
plastic ports are preferably made of impact polystyrene such as
Monsanto 4200, the width of the assembly is substantially 21 inches
wide, 123/8 inches high and substantially 13 inches deep. The
volume of each cavity is substantially 177 cubic inches. Port tubes
17 and 18 are substantially 9.5 inches long, have an inside
diameter of 1.62 inches and outside diameter of 1.82 inches at the
rear end, an inside diameter of substantially 2.42 inches at the
front end with the outside diameter of the folded over lips being
substantially 3.62 inches and the folded over portion being
substantially 1.00 inches. Center port tube 23 typically has an
inside diameter of 0.65 inch and outside diameter of 0.85 inch and
is substantially 9 inches long, substantially half of that length
extending inside cylindrical member 51 whose inside diameter is
substantially 6 inches and length to the portion that tapers
inwardly at an angle of substantially 30.degree. being 6 3/16
inches. Front baffle 24 is preferably curved along a radius of 35.5
inches.
The loudspeaker drivers are 41/2 inches and may be of the
high-compliance type used in the BOSE 901 loudspeaker each having a
voice coil impedance of substantially 8 ohms connected in
series-parallel with three drivers in each bank to provide a
nominal impedance of substantially 8 ohms; however, the drivers are
preferably high-compliance drivers having a nominal voice coil
impedance of 0.9 ohm established by a single-layer edge-wound
rectangular aluminum wire voice coil connected in series and used
in the BOSE 901 series III loudspeaker commercially available at
the time this patent is granted, which driver is described in
pending application Ser. No. 718,112, now U.S. Pat. No. 4,061,890,
granted Dec. 6, 1977.
It is evident that those skilled in the art may now make numerous
uses and modifications of and departures from the specific
embodiments described herein without departing from the inventive
concepts. Consequently, the invention is to be construed as
embracing each and every novel feature and novel combination of
features present in or possessed by the apparatus and techniques
herein disclosed and limited soley by the spirit and scope of the
appended claims.
* * * * *