U.S. patent number 3,796,839 [Application Number 05/285,012] was granted by the patent office on 1974-03-12 for loud speaker system.
This patent grant is currently assigned to Dukane Corporation. Invention is credited to William R. Torn.
United States Patent |
3,796,839 |
Torn |
March 12, 1974 |
LOUD SPEAKER SYSTEM
Abstract
This invention provides a loudspeaker system having two coaxial
cone diaphragm type nested speakers (woofer and tweeter) for
handling an extended range of audio frequencies. The speaker system
embodying the invention is characterized by a phasing baffle plate
extending part way from a region spaced near the rim of the tweeter
toward the woofer cone diaphragm. By controlling the dimensions,
spacing and material of the phasing baffle plate, the overall
frequency response for the two speakers may be rendered
substantially uniform over the extended range.
Inventors: |
Torn; William R. (St. Charles,
IL) |
Assignee: |
Dukane Corporation (St.
Charles, IL)
|
Family
ID: |
23092364 |
Appl.
No.: |
05/285,012 |
Filed: |
August 30, 1972 |
Current U.S.
Class: |
381/432; 381/182;
381/402; 381/354 |
Current CPC
Class: |
H04R
1/24 (20130101) |
Current International
Class: |
H04R
1/24 (20060101); H04R 1/22 (20060101); H04r
009/06 () |
Field of
Search: |
;179/115.5R,115.5PS,116
;181/31R,31B |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Claffy; Kathleen H.
Assistant Examiner: Kundert; Thomas L.
Attorney, Agent or Firm: Kahn; Robert L.
Claims
1. A self-contained loudspeaker combination comprising a
cone-diaphragm type woofer and a cone-diaphragm type tweeter, each
cone-diaphragm carrying a voice coil operating in an annular air
gap of a magnetic circuit and having a stationary mounting rim for
supporting an annular hinge portion at the large cone end, the
woofer being for low-frequency operation and having its large end
cone diameter of at least about 8 inches, the tweeter being for
higher frequency operation and having its large end cone diameter
substantially less, means for supporting the tweeter in coaxial
nested relation within the volume defined by the woofer cone, and a
pushing self-supporting flat baffle plate of material, whose
thickness is between about one-tenth and about three-fourths inch,
said plate having a curved outer edge and a window therethrough to
provide an inner edge, means for supporting said plate in
symmetrical relation to the coincident diaphragm axes and
perpendicular thereto, said plate being small enough to be
supported within the large diaphragm end portion with its outer
edge stopping short of the large cone rim portion by between about
one-half inch to the order of about 1 inch, depending upon the
woofer speaker size, the plate inner edge being dimensioned and
located with reference to the tweeter diaphragm mounting rim so
that an acoustically suitable peripherally continuous extended air
gap is provided, said plate material having suitable acoustic
properties and the plate and spacing dimensions, shape and axial
location being so selected that said speaker combination, with
phasing baffle plate, when mounted for normal speaker operation
with a conventional baffleboard will have a desired uniform
frequency response over the desired audio-frequency range and be
substantially free of resonance peaks or holes having abnormal
amplitudes in such range, even though the same speaker combination
minus the phasing baffle plate, when tested, will have at least one
resonance or anti-resonance peak of abnormally large amplitude
within the audio-frequency range, said phasing baffle plate
smoothing out the frequency response characteristic and improving
speaker performance, the addition and mounting of said phasing
baffle plate permitting engineering efforts to be directed thereon
to improve frequency response whereby tooling changes for
manufacturing the speaker combination proper may be
2. The construction according to claim 1 wherein the woofer cone
diameter ranges from about 8 inches to about 15 inches, said
phasing baffle plate
3. The construction according to claim 1 wherein the plate is of
particle
4. The construction according to claim 1 wherein the plate is of
styrofoam having a smooth, moulded outer surface.
Description
This invention relates to a loudspeaker system of tht type having
nested coaxial cone diaphragm type speakers for extended frequency
response. The larger speaker (woofer) is relied upon for handling
the lower frequency range, which may extend from the lowest
frequency to be handled, generally something less than about 50
Hz., to somewhere in the region of about 1,200 Hz. Nesting within
and coaxial with the conical region defined by the woofer diaphragm
is a smaller speaker (tweeter) which is suitably supported on the
woofer. The tweeter is relied upon to handle frequencies from the
order of about 800 Hz. to the maximum value to be handled, such
maximum value being about 8,000 Hz. to 16,000 or more, depending
upon the top frequency to be handled by th speaker system.
The tweeter cone diaphragm may have a diameter in the range of
about 6 inches to about 3 inches and the woofer cone may have a
diameter in the range of about 18 inches down to about 8 inches.
The tweeter has its pot or permanent magnet field structure usually
carried by the permanent magnet structure of the woofer so that the
voice coils and diaphragms of the two speakers may vibrate
independently. Voice currents over the entire frequency range to be
handled are fed to both speakers in which case the mechanical
properties of the woofer and tweeter may be relied upon for
separating the low and higher frequency ranges. Or electrical
networks may be provided for frequency separation of voice currents
to the woofer and tweeter, there being a cross-over range of
frequencies from about 800 Hz. to about 3,000 Hz.
A speaker system embodying the present invention is used as
conventional woofer and tweeter speaker combinations, with
conventional baffle plate for the woofer, all housed in a
conventional cabinet. A cabinet for housing a tweeter and woofer
combination, whether embodying the present invention or not, may
have an open back or may utilize a substantially sealed cabinet
having acoustic absorbing material within the cabinet for loading.
In such a speaker system, it must be borne in mind that the
frequency cross-over network, if provided, or the mechanical
response of the woofer and tweeter is such that from about 800 Hz.
and higher, the response of the woofer becomes weaker with
frequency rise while the tweeter response increases. This gradual
shifting of the speaker response is thus generally operative over a
substantial frequency range from the order of about 800 Hz. to the
order of about 3,000 or more Hz.
PRIOR ART AND ITS DRAWBACKS
In designing a high quality audio frequency amplifier and speaker
system, the frequency response characteristics of an amplifier as
one means and that of a speaker system as a transducer are
separately designed. This is due to the general practice of
engineering and production of audio frequency amplifiers as one
field of commercial activity and speakers or speaker systems as
another field of commercial activity. Even if one source provides
amplifiers and speakers, the above practice of separately
engineering the two is still generally true. The technical
qualifications for the two fields are quite different.
In theory, an audio frequency amplifier having a uniform frequency
characteristic when feeding a high quality speaker system also
having a uniform frequency response characteristic should provide
an overall uniform frequency response over the frequency range
being handled. In practice, this ideal is rarely if ever
realized.
As a practical matter, the frequency response characteristic of a
speaker or speaker system usually represents an idealized
characteristic derived from smoothing out the actual frequency
response curve. This may be accomplished by rapid frequency changes
over the spectrum. Generally, it is not possible to produce a
speaker system and its component parts free from some sharp
resonance peaks or anti-resonance holes (dips) in the desired
working range. The difficutly is due primarily to the multiplicity
of separate physical parts in a speaker system, each having
individual resonances an being physically or acoustically coupled
to each other. Consequently over the entire frequency spectrum to
be handled by a speaker system, it becomes well nigh impossible to
avoid some resonant peaks or anti-resonant holes within the normal
listening range. The factors which may be involved in having
undesired peaks and holes in the speaker response may relate to
such physical elements as the speaker baskets, the cone diaphragm
and suspension for either or both speakers, the nature of parts and
airchambers providing physical or acoustic coupling between the two
speakers, the voice coils and mountings.
When a speaker system exhibits resonance peaks or holes of
substantial amplitude, a painstaking analysis of possible sources
of trouble as well as some physical changes in the speaker system
may be required to improve the frequency response characteristics.
Such a program for critical analysis and possible physical changes
requires not only a substantial amount of time and expenses for
engineering but may in addition require changes in tooling for the
manufacture of such speakers. The testing requirements involve
actual production samples with production tolerances. Thus unlike
most production procedures, where engineering or production is
condemned because of costs in tooling or production changes,
speakers require production units for testing.
The depth of the tweeter within the woofer cone may be varied and
effect some changes (good or bad) in the frequency response
characteristics of this speaker combination. For the most part
however, such a procedure does not alter the basic frequency
characteristics. The generally rough frequency response
characteristic of such a speaker combination usually remains.
THE INVENTION GENERALLY AND ITS ADVANTAGES
The invention generally contemplates the addition of an annular
member of suitable material attached to one of the speakers,
preferably the tweeter, at the anchorage of the large tweeter cone
diaphragm end and within the space encompassed by the woofer cone
diaphragm. The annular member is preferably flat, extending beyond
the rim of the tweeter diaphragm in a plane generally normal to the
coincident cone axes. The nature of the material may vary widely
and should be self supporting or have reenforcements to provide a
self-supporting structure and such material should have good
acoustic properties. Such materials as natural or synthetic woods,
wall-board, plastic sheet, particle board material may be used. Any
material that is useful as a sounding board may be used. The
thickness is one of the dimensional parameters which must be
determined by testing. Generally, material from about one-eighth
inch to three-fourths inches may be used. While metal may be used,
it should be much heavier than conventional sheet metal. As a rule,
metal should be about one-sixteenth inch or thicker. Lighter
materials like styrofoam, having a smooth outer molded skin, are
satisfactory in which case a thickness of about three-eighths inch
to about three-fourths inch may be good, depending upon desired
frequency characteristics.
Considerable latitude in the shape of inner and outer plate edges,
plate thickness, spacing between inner plate edge and tweeter cone
outer edge, spacing between plate outer edge and woofer cone wall
is permissible, all such variables affecting the frequency response
of the speaker system. The space between the outer edge of the
phasing baffle plate and adjacent woofer cone wall should be at
least about an inch for a woofer, 12 inch cone and more for a 15
inch cone. Too narrow a space between the woofer cone wall and
plate outer edge tends to throttle low frequency response.
The invention directs engineering efforts to improve speaker
response from a production speaker system (that has been reasonably
well designed) to a phasing baffle plate by controlling the above
identified parameters. The relative spacing between woofer and
tweeter within the woofer cone and relative spacing of phasing
baffle plate along the cone axis are also parameters which can be
operated on with engineering and testing. Once a phasing baffle
plate for a particular speaker combination has been satisfactorily
determined, then quantity production of the entire speaker system
can proceed. The selection of relative woofer and tweeter sizes
will be governed for the most part by conventional engineering
technique. There are so many variables in adapting a phasing baffle
plate to a speaker combination that an improved frequency response
characteristic can be obtained by concentrating engineering changes
on the phasing baffle plate and, for the most part, leaving the
woofer and tweeter speakers as is.
Commercially, the invention makes possible the quantity production
of speaker combinations having exceptionally desirable frequency
response characteristics at a cost substantially less than
conventional high quality speaker systems.
In a simple embodiment of the invention, a phasing baffle plate is
flat with a suitably shaped window therethrough providing an inner
edge which will normally be partly or wholly spaced from the
mounting rim of the tweeter to provide an air path. The extent of
the air path is an important parameter and is experimentally
determined. The plate has an outer edge portion extending toward
the woofer diaphragm, the plate being disposed in acoustically
satisfactory relation with respect to the concentric rims of the
two speakers. It is possible to have the outer edge of the plate
non-circular or non-symmetrical with respect to the axis of the
tweeter. The latter modification may provide different effects on
the frequency response characteristic along the axis of the speaker
combination or off the axis.
Similarly, while the phasing baffle plate is preferably flat, a
dished plate shape may be used. In such case, having the phasing
baffle plate concave when viewed from the voice coil end of the
cone may be more desirable.
A combination of low and high frequency range speakers embodying
the invention can be marketed like present day speaker
combinations, where the woofer and tweeter are rigidly secured to
each other, the speakers being adapted to have the woofer mounting
rim bolted to a conventional baffle board forming part of the
housing or cabinet in which the speaker combination operates. It is
understood that the woofer and tweeter combination must be operated
in an appropriate mounting for developing its frequency response
characteristics. Usually the manufacturer of the speaker
combination prescribes or recommends a desired mounting for the
speaker in a housing or cabinet.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be disclosed in connection with drawings
wherein:
FIG. 1 is a plan view of a speaker combination provided with a
phasing baffle plate embodying the present invention, the broken
lines suggesting a mounting for the speaker combination.
FIG. 1A is a plan view of a modified embodiment of the
invention.
FIG. 1B is an enlarged detail on line 1B--1B of FIG. 1A.
FIG. 2 is a transverse sectional view along line 2--2 of FIG. 1,
the section line going around the center bolt for ease of
illustration.
FIG. 3 is a frequency response curve of a conentional speaker
combination having a 12 inch woofer and a 5 inch tweeter without
the new phasing baffle plate, the X axis showing, logarithmically,
frequencies ranging from 20 Hz. up to about 20,000 Hz., the Y axis
showing decibel (dB) values from about 75 to over 100.
FIG. 4 shows a corresponding frequency response characteristic for
the same speaker combination of FIG. 3, the combination in this
instance being provided with a phasing baffle plate of 45 pound
density particle board having a thickness of substantially
three-eights of an inch with concentric circular inner and outer
edges 8-1/4 inches and 4-1/4 inches diameters respectively, washers
51 for spacing the phasing baffle plate being one-sixteenth inch
thick, the frequency scale and dB scale being the same as FIG.
3.
FIG. 5 shows a frequency response curve of the same speaker
combination as set forth in connection with FIG. 3 wherein voice
currents were fed only to the tweeter, the speaker combination not
having the phasing baffle plate.
FIG. 6 shows the frequency response curve of the tweeter in the
same combination of speakers as before, with the phasing baffle
plate, the woofer being inoperative.
FIG. 7 is a frequency response curve of the same speaker
combination provided with the phasing baffle plate identified in
connection with FIG. 3 with the woofer only operating and the
tweeter or high-frequency speaker being inoperative.
FIG. 8 is a simple circuit diagram illustrating a network for
separating and feeding high-frequency voice currents to the
tweeter.
DESCRIPTION OF A SIMPLE WOOFER AND TWEETER SPEAKER COMBINATION
MINUS INVENTIVE FEATURE
Permanent magnet 10, generally of toroidal shape, has pole faces 11
and 12. Pole faces 11 and 12 have cemented or otherwise joined
thereto bottom and top pole plates 13 and 14 respectively. The pole
plates are of soft iron or soft steel, with the outer portion of
permanent magnet 10 projecting beyond plates 13 and 14. The inner
edges of permanent magnet 10 are laterally and outwardly offset
from inner edge of face 15 of top plate 14. Bottom plate 13 is a
solid disc having a countersunk central aperture 17 through which
extends bolt 18. Cylindrical pole piece 20 is disposed about the
body of bolt 18. In order to minimize magnetic reluctance, the
opposing surfaces between the magnet and the pole plates as well as
pole piece 20 are ground to a smooth finish and the opposing
surfaces are cemented together with a thin layer of cement.
Resting against the top face of top pole plate 14 is annular flat
portion 21 of a steel (usually soft) basket 22 which extends
upwardly (as seen in FIG. 2) from flat portion 21 and outwardly to
provide mounting flange or rim 24. Basket 22 as a rule has windows
punched out therefrom to reduce weight and improve the speaker
acoustic properties.
The outer cylindrical surface of pole piece 20 cooperates with
inner cylindrical pole face 15 of top plate 14 to provide an
annular air gap within which voice coil 26 operates. As is well
known, woofer voice coil 26 consists of a number of turns of wire
wound over a fabric or synthetic bobbin of suitable stiffness.
During woofer operation, the voice coil moves axially in response
to voice currents, the amplitude of excursion being a function of
the amplitude of voice currents in the voice coil. The voice coil
operates in a narrow air gap about the outer surface of pole piece
20, the gap defining close spacing between opposed pole
surfaces.
The bobbin of the voice coil is securely attached, as by cementing,
to annularly corrugated spider 28 whose outer edge 29 is securely
attached to adjacent basket portion 21. The inner edge of spider 28
is also attached to small end 30 of woofer cone diaphragm 31. Cone
diaphragm 31 has its large cone edge 32 attached to or extending
into annularly corrugated cone diaphragm hinge portion 33, the
outer edge of which is securely clamped at rim portion 24 of the
basket. Bolts 35 extend through openings in rim portion 24 of the
basket and cooperate with cardboard ring 36 for tightly clamping
the various parts together. Bolts 35 are used to secure the rim
portion of the speaker basket to a baffle board of housing for
mounting a speaker in a suitable manner.
To protect the woofer voice coil and the air gap in which it
operates from dirt and dust, cover 37 of foam rubber or plastic is
provided. Foam dust cover 37 is cemented to the cone diaphragm 31
near the small end thereof. Disposed about bolt 18 above the top of
pole piece 20 is rigid mouting spacer 39. This spacer is preferably
of non-magnetic metal and extends through the central portion of
dust cap 37. Boll 18 has its threaded end portion 40 screwed into
the bottom of a tweeter cone-type speaker unit to support the same
in coaxial relationship with the woofer unit which has just been
described. In general, the tweeter speaker unit is a small version
of the structure so far described and requires no detailed
description. In this particular tweeter unit, basket 42 does not
have windows as is true of the woofer basket. However some tweeters
have apertures through the basket to change acoustic
characteristics. The tweeter basket has mounting rim 43. The voice
coil in each of the two speakers is connected by wires to an outer
terminal board, such wires being led along a path to prevent
excessive vibration and crystallization of the wire metal.
Referring to FIG. 8, a cross-over network for a speaker combination
is illustrated. Audio frequency voice currents are fed to wires 61
from a suitable source, usually an amplifier. Wires 61 are
connected to voice coil 26 of a cone diaphragm type of loudspeaker
62, as for example the low-freqency speaker illustrated in FIG. 2.
Branching from wires 61 are wires 64 and 65. Wire 64 is connected
through series connected capacitors 66 to 67 to one terminal of the
voice coil of high-frequency or tweeter speaker 69. Capacitors 66
and 67 have suitable values for passing audio frequency currents.
Connected between the common junction of capacitors 66 and 67 is
audio frequency inductor 70, the bottom terminal of which is
connected to line 65. The voice coil for high-frequency tweeter 69
is connected between the free terminal of capacitor 67 and supply
wire 65.
The combination of capacitors 66 and 67 and inductor 70 cooperates
to pass voice currents having frequencies above about 600 Hz.
Generally a sharp cutoff for high frequencies is not desirable.
However networks simplicity or complexity may be used. No great
precision in establishing a frequency cross-over region is
necessary since the large and small speakers themselves function to
effect frequency separation.
The speaker arrangement so far described is well known and widely
used. In the case of a 12 inch speaker, the large cone diameter
(usually measured at the outer rim edge of the basket) may be any
one of 8 inches, 10 inches, 12 inches or 15 inches (these four
sizes being quite common) and have tweeters which may have large
end cone diameters of about 3 inches or less for an 8 inch woofer,
about 3 or 5 inches for 10 and 12 inch woofer or 4 or 6 inches for
the tweeter for use with 15 inch woofers.
As has been previously indicated, for testing a speaker combination
unit, as illustrated in FIG. 2, must be a production unit and must
be suitably mounted in a baffle board for testing acoustic
response. As shown by the curve in FIG. 3, a coaxial speaker
combination utilizing a 12 inch woofer and a 5 inch tweeter tested
out with holes at about 1,100 Hz. and 2,000 Hz., a high resonance
peak at about 4,500 Hz., a bad drop at about 7,000 Hz. followed by
another bad drop at about 8,000 Hz. and bad drops at about 11,000
and about 13,000 Hz.
To correct these bad drops and objectionable peak at around 4,500
Hz., various expendients had been relied upon. For example the
tweeter axial position can be adjusted without too much trouble to
move the tweeter diaphragm axially along the space within the large
diameter cone. Such a change or adjustment however has limited
effects and will not smooth out the usually rough frequency
response curve. It had been the practice for an experienced speaker
engineer to visually analyze the response curve (as FIG. 3 for
example) and quess where the trouble is. By playing with the sizes
and nature of various parts making up the speaker and the gauge of
metal subject to possible vibration, some changes in the
characteristic curve may be effected. However such changes are
usually in the realm of production changes, are expensive, time
consuming and may involve changes in tooling.
Even then, the fundamentally rough frequency response of the two
way speaker combination still remains. Additionally, makers of
equipment having speakers as parts thereof are usually unable to
persuade the speaker manufacturer to improve the frequency response
characteristic unless leverage in the form of large orders or
general customer demand is present. There are good guality speakers
available but the prices frequently cannot be justified by the
economics of the situation.
Modification of the Speaker Combination in Accordance with the
Invention
In order to incorporate the invention in a conventional speaker
combination construction as described previously, apertured phasing
baffle plate 50 is provided. This plate is bolted or otherwise
secured to rim 43 of tweeter basket 42.
In accordance with the invention, an air path is provided between
the tweeter rim and inner edge 54 of phasing baffle plate 50. The
size of the air path is one of the parameters which must be
experimentally determined by a skilled speaker engineer to provide
a desired frequency response characteristic. A simple means for
creating such an air path is illustrated in FIGS. 1 and 2. Spacer
washers 51 of suitable material as iron, aluminum, etc. are
disposed about bolts 52 between tweeter rim 43 and the inner edge
portion of plate 50 to secure plate 50 in desired spaced relation
thereto. In the speaker system tested, for which frequency response
characteristics are given, washers 51 were one-sixteeth inch thick,
for three-sixteenth inch bolts.
No attempt has been made to show parts or air spaces in proper
proportion in the drawings. The air spaces created by washers 51
may range from that created by a thin washer at each bolt 52 to
thick washers, depending upon the circumferential length of all the
air spaces due to washers 51. Large thickness of washers 51 at each
bolt 52 may be avoided by removing plate material at the inner edge
or adopting a different plate support means.
Referring to FIG. 1A, plate 50' has inner edge 54' generated by a
four sided window or aperture, the sides being curved outwardly.
The aperture is larger than the tweeter rim. Fingers 55 of suitable
material (the material may be the same as plate material) are
securely attached, as by cementing, to one face of plate 50 and
extend radially inward to the cone axis overlie tweeter rim 43.
Bolts or screws 52' (with or without spacer washers) extend through
tweeter rim 43 and anchor plate 50' to the tweeter rim. By
controlling the shape and dimensions of the window or aperture of
phasing baffle plate 50', any desired size for the air path around
inner edge 54' of plate 50' may be provided.
As an example, each finger 55 may be of particle board,
three-eighths inch thick, where it is secured to the bottom face of
plate 50', and one-sixteenth inch thick where it extends over
tweeter rim 43 and about seven-sixteenths inches wide. The length
may be about 1-3/4inches. The outer circular edge of plate 50' may
have a diameter of about 8-3/4inches and thickness of about
one-half inch. The radius of the circle upon which the four arcuate
sides of the window lay can be substantially 4-1/2inches. The
window is symmetrical with respect to the circular outer edge,
arcuate window sides being similar. The entire plate is useful with
a 5 inch tweeter having a 12 inch woofer. In this instance, plate
50' is bevelled at the outer edge. This is not essential. By
reversing fingers 55 prior to cementing or by reversing the entire
plate, after the fingers are secured, to change plate position,
some changes in the response curve may be obtained.
In case the tweeter mounting rim is unsuitable for attaching a
phasing baffle plate, it is possible to attach mounting fingers to
the plate, such fingers extending outwardly toward the woofer
mounting rim and be secured thereto.
It is generally desirable to have the phasing
It is generally desirable to have the phasing baffle plate
positioned so that it lies within th woofer cone. If it projects
forwardly beyond the woofer, it may be difficult to dispose a
fabric covering for decoration. On the other hand, having the
tweeter too deep in the cone is not desirable for reasons of sound
dispersion. In general, conventional testing can be used to
determine the most desirable location.
It has been found that the annular space between the outer edge of
plate 50 (or 50' ) and mounting rim 24 of a 12 inch woofer should
be at least 1 inch and preferably greater so that the low frequency
response of the entire speaker combination will not be adversely
effected. For an 8 inch woofer, the annular space may go to
one-half inch while a 15 inch woofer may require more than 1 inch
space. It has already been pointed out that plate 50 (or 50' ) may
be made of various materials having the properties set forth and
that for the most part, depending upon the material, a thickness of
the order of about one-tenth inch is a minimum. As the test curve
indicate, a plate of 45 pound density particle board having a
thickness of three-eights of an inch was satisfactory. The
thickness of the plate is generally not critical.
Referring now to FIG. 4 this curve shows the frequency response
characteristic of the speaker combination used in connection with
the frequency response characteristic illustrated in FIG. 3 except
that, a three-eights inch thick phasing baffle plate of above
identified particle board having cconcentric inner and outer
circular edges whose diameters respectively were 8-1/4and
4-1/4inches was used, the plate being attached to the tweeter rim
by screws as shown in FIGS. 1 and 2 of the drawings, spacing
washers 51 being one-sixteenth thick. The form of the curve shown
in FIG. 4 is generally similar to that of FIG. 3 up to about 1,000
Hz. The drop or hole in FIG. 3 at about 1,100 Hz. has disappeared.
The curve of FIG. 4 has been smoothed out somewhat between 1,000
Hz. and about 2,000 Hz. The hole or drop at about 2,000 Hz. in FIG.
3 has disappeared and the high amplitudes of speaker response in
the range of from about 2,000 to about 6,000 Hz. has been
considerably reduced in FIG. 4. The sharp drops or holes in FIG. 3
for 7,000 Hz. and for about 11,000 Hz. have disappeared in FIG. 4,
a sharp hole or drop in FIG. 4 now appearing at about 13,000 or
14,000 Hz. in FIG. 4.
At this high frequency of well over 10,000 Hz. such a drop or hole
may not be objectionable. This drop in FIG. 4 at about 12,000 Hz.
can be moved toward the higher frequency end of the curve if
desired by making some changes in the phasing baffle plate, such as
for example slightly reducing the diameter of the outer edge. It is
also possible to increase the amplitude of speaker response in the
range of between about 5,000 and about 9,000 or 10,000 by suitable
operation on the phasing baffle plate. The frequency response curve
illustrated in FIG. 4 does show great improvement over the response
curve illustrated in FIG. 3. Insofar as quality of speaker response
is concerned, the curve illustrated in FIG. 4, as distinguished
from the curve illustrated in FIG. 3, reflects a smoother and
substantially better sounding speaker system due to the presence of
the phasing baffle plate. It should be understood that the 45 lb.
density particle board is one example of a readily available
particle board. Denser or less dense materials may be used.
Referring now to the frequency response curve illustrated in FIG.
5, this shows the speaker combination used in connection with
obtaining the response curve illustrated in FIG. 3, the speaker
being a combination of 12 inch and 5 inch speakers with no phasing
baffle plate and wherein the woofer is not connected to receive
voice currents. It will be noted from FIG. 5, tha there is little
or no response below about 500 Hz. At about 1,200 Hz. there is a
deep hole while at about something over 2,000 Hz. there is a
substantial reduction in amplitude. Beyond that frequency, the
response curve illustrated in FIG. 5 is generally similar to the
corresponding frequency range for the curve illustrated in FIG.
3.
The frequency response curve illustrated in FIG. 6 shows the
operation of the same speaker combination with the tweeter only
being operated, the woofer not receiving any voice currents and the
tweeter now having the phasing baffle plate previously described.
It will be noted that the response curve from about 800 Hz. to
about 12,000 Hz. has been smoothed out substantially.
FIG. 7 shows the frequency response curve of the speaker
combination previously identified wherein the phasing baffle plate
is provided and the woofer only operating, the tweeter being
inoperative. The frequency response up to about 800 Hz. is
generally similar to the frequency response curve of FIG. 4 wherein
both speakers are operating with FIG. 7, the response
characteristic falls off quite rapidly. It is evident throughout
the characteristic curve illustrated in FIG. 4 that the tweeter
response, when operating in conjunction with the woofer, in the
combination having the phasing baffle plate, shows that the
contribution of the tweeter in this combination begins to be
significant from about 600 Hz. and up and that the tweeter response
or contribution to the speaker response begins to be the major
factor in frequencies beyond about 2,000 Hz.
Adapting The Phasing Baffle Plate To a Speaker Combination For
Obtaining The Benefits of the Invention
To adapt a phasing baffle plate to a coaxial speaker combination
and obtain the benefits of the invention, it is necessary first to
obtain with a conventional speaker combination having large and
small production speakers disposed in fixed relation to each other
as illustrated in FIG. 2 of the drawings. Thereafter a phasing
baffle plate must be adapted to the speaker. It is understood that
the speaker combination minus phasing baffle plate will be tested
to show frequency response characteristics over the entire
frequency range to be covered by the speaker combination. The test
should be both along and off the cone axis. Then a suitable
material for the phasing baffle plate, such as particle board, must
be selected. Assuming that concentric outer and inner circular
edges are to be used, select the plate material having the proper
thickness, (for 45 lb. density particle board, a thickness of
three-eighths inch will serve as a starter) cut the window through
the plate. This may be a circular opening or window whose diameter
is large enough to permit mounting the phasing baffle plate upon
the tweeter rim as shown in FIGS. 1 and 2. Then arbitrarily trim
plate material so that the outer edge will have a diameter 2 inches
greater than that of the window. The entire speaker combination
together with the experimental phasing baffle plate must be
suitably mounted in a large baffle board either in a housing or
cabinet or if the baffle board is large enough, the entire
construction can be tested in open air. A conventional frequency
response test is now run and the response curve is plotted. This
response curve should be compared to the response curve of the same
speaker combination with similar baffle board but without the
phasing baffle plate. Different thicknesses of washers 51 should be
tried. Thereafter other baffle plates having increasing diameters
in steps of one-fourth inch for the outer edge should be
substituted in the speaker combination. Beyond a certain point in
the various phasing baffle plate sizes and spacings there will
usually be an optimum response curve, the curve beginning to
deteriorate beyond a certain outer edge diameter of the phasing
baffle plate and/or spacer washer thickness. At or close to the
best dimension for the phasing baffle plate, some small variations
in diameter of both inner and outer edges of phasing baffle plate
may be tried to determine the most desirable frequency
characteristic curve. Due to the number of variables, it will
generally be desirable to vary only one or two parameters at a time
while holding others constant. It is possible to obtain
satisfactory frequency response characteristics for a speaker
combination for a number of parameter values. Some parameters have
greater response effects than others. In general, the air space at
tweeter rim 43 and space between the plate edge and woofer cone are
important. Trying different tweeter locations along the cone axis
may also be investigated. Once a desirable phasing baffle plate is
obtained, no changes in the speakers should be made to avoid making
changes in the plate.
Once the parameters of the phasing baffle plate are determined
experimentally, production of phasing baffle plates determined by
the experiments can proceed. It will be found that a substantial
free annular region between the outer edge of the phasing baffle
plate and the nearest portion of the woofer cone will be indicated,
such annular space as a rule having at least a width radially of
more than one-half inch between the edge of the phasing baffle
plate and adjacent woofer cone. Inasmuch as phasing baffle plates
are easy to make and the plate material and their dimensons are
easily changed, testing of various phasing baffle plates may be
accomplished economically and quickly. Frequency response tests for
speakers are well known.
In testing a speaker combination embodying the present invention,
variations in the shape of the outer edge of the phasing and baffle
plate, such as indentations, will produce different effects on the
frequency response characteristic. In making such changes in the
phasing baffle plate it may be desirable to test the speaker
response not only along the cone axis of the speakers but laterally
away from the axis from a region about 4 or more feet along the
axis from the speaker.
The modified phasing baffle plate shown in FIG. 1A may be used with
equal convenience. Desirable plate shapes, dimensions, and air
spaces may be determined by tests. A skilled speaker engineer will
be able to determine in which direction to change plate dimensions,
plate density, air spacing, etc. to improve speaker response.
* * * * *