U.S. patent number 3,718,747 [Application Number 05/167,172] was granted by the patent office on 1973-02-27 for electrocoustic pipes for electronic organs.
This patent grant is currently assigned to D. H. Baldwin Company. Invention is credited to Daniel W. Martin, John L. Stein.
United States Patent |
3,718,747 |
Martin , et al. |
February 27, 1973 |
ELECTROCOUSTIC PIPES FOR ELECTRONIC ORGANS
Abstract
An acoustic radiation system for an electronic organ, in which
separate radiators are used for each tone color, the radiators
being physically arranged as in a pipe organ and each including a
cylindrical pipe having an upwardly directed loudspeaker in its
upper end, the loudspeaker being of the minimum diameter capable of
acoustically radiating the band of frequencies over which it may be
driven, and the pipe having an enclosed volume appropriate to form
a Helmholtz resonator tuned near the lowest frequency of the band,
the pipe itself being mechanically rigid in terms of the material
of which it is fabricated and also acoustically damped at higher
frequencies by virtue of acoustical absorbent material which is
distributed internally of the pipe so as to damp out standing
waves. Each pipe may be provided with a mouth, shaped and
positioned to simulate visually the mouth of a true organ pipe, but
of a size acoustically selected to resonate with the pipe enclosure
at the desired low frequency to flatten the frequency response over
the radiated band of frequencies.
Inventors: |
Martin; Daniel W. (Cincinnati,
OH), Stein; John L. (Cincinnati, OH) |
Assignee: |
D. H. Baldwin Company
(Cincinnati, OH)
|
Family
ID: |
22606246 |
Appl.
No.: |
05/167,172 |
Filed: |
July 29, 1971 |
Current U.S.
Class: |
84/600; 84/DIG.1;
84/644; 84/DIG.17; 84/718 |
Current CPC
Class: |
G10K
11/04 (20130101); Y10S 84/17 (20130101); Y10S
84/01 (20130101) |
Current International
Class: |
G10K
11/00 (20060101); G10K 11/04 (20060101); G10h
001/00 () |
Field of
Search: |
;84/1.01 ;179/101
;181/31B |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
G A. Briggs, Loudspeakers, pages 192, 193, 206, 207, 217-219,
October 1959, Wharfedale Wireless Works Limited, IDLE, England.
.
A. B. Cohen, Hi-Fi Loudspeakers and Enclosures, pages 258-261,
298-304, Hayden Book Company, Inc., New York..
|
Primary Examiner: Wilkinson; Richard B.
Assistant Examiner: Weldon; U.
Claims
I claim:
1. In an electroacoustic transducing system for transducing tone
signals generated by an electronic organ within a structure having
a sound reflecting ceiling surface, said electronic organ having
provision for generating in separate leads tone signals
corresponding with different tone colors, the combination of
an elongated hollow internally sound reflecting tube having its
longitudinal axis vertical and its lower end totally closed, said
hollow tube having a depth to width ratio greater than 4 to 1 and
having a longitudinal axis;
a loudspeaker having a sound radiator, said sound radiator having a
sound radiating area substantially equal to the cross sectional
area of said tube and said loudspeaker being mounted at the upper
end of said hollow tube so as to radiate directly upwardly toward
said sound reflecting surface, said sound radiator having a sound
radiating area designed to radiate the lowermost frequencies of a
selected one of said stops essentially omnidirectionally and the
highest notes of said one of said stops only upwardly along said
axis for reflection downwardly by said reflecting ceiling surface,
said hollow tube including acoustic absorbing material in its
bottom for damping acoustic reflections from said lower end, said
hollow tube including at least one internal discrete narrow
acoustic absorbing ring secured to the inner surface of said tube
and located to damp only relatively few selected frequencies of
said tone color which might otherwise occur at levels not
appropriate to said selected tone color, said tube being
acoustically rigid and having a volume selected to resonate as a
Helmholtz resonator near the lowest frequency of said selected one
of said stops.
2. The combination according to claim 1, wherein said hollow member
has a substantially rectangular radiative port near its lower end,
said port being sized to resonate with the volume of the hollow
member at a frequency near the low-frequency limit of the range of
said signals.
3. The combination according to claim 1, wherein said first
mentioned acoustic absorbing material is a substantially
conically-tapered hollow in a mass of sound-absorbent material,
said hollow having its apex at said closed end.
4. The combination according to claim 2, wherein said port extends
at least at its upper side into a lip opening in said tube sized
for tuning said tube, said lip opening being covered with an
acoustically transparent covering material which visually obscures
said lip opening.
5. The combination according to claim 2, wherein said system
includes: a supporting sleeve member for said tube at least
partially surrounding said tube only at its lower end, there being
a V-shaped opening in said sleeve member under said port, said
V-shaped opening having its apex extending downwardly and having
its base at least as wide as said port, the configuration of said
tube, said V-shaped opening and said port being such as to simulate
a pipe of a pipe organ.
6. The combination according to claim 2, wherein said hollow member
contains in the lower end thereof a conically tapered hollow in a
mass of sound-absorbent material, the apex of said hollow being the
lowermost point of said mass.
7. The combination according to claim 6, wherein said tube contains
at least one narrow ring of sound-absorbent material located
between the upper end of said hollow member and said port, said
ring being positioned to damp only relatively few selected
frequencies of said tone color which might otherwise occur at
levels not appropriate to the selected stop.
8. A tone cabinet for an electronic organ, comprising in
combination:
a main box-like enclosure,
at least one row of electroacoustic pipes extending upward from the
top of said enclosure,
at least a first relatively-large loudspeaker facing outward from
one side of the main enclosure,
at least a second relatively-large loudspeaker facing outward from
the other side of said main enclosure and
a multi-channel electronic amplifier for separately supplying
amplified organ tone signals via separate channels of each
amplifier to each of said loudspeakers, electroacoustic pipes
provided on a per stop basis, wherein each of said electroacoustic
pipes comprises in combination:
an elongated hollow member having a sound reflective interior and
having a length-to-width ratio of the order of 7 to 1,
a further loudspeaker mounted at the top of each of said hollow
members with its front radiating surface directed upwardly, said
loudspeakers having each a radiating area which is minimum for
effective radiation of the frequencies of the stop to be radiated
by that loudspeaker and said pipes having the cross sectional area
of the loudspeaker supported thereby,
means for damping reflection of acoustic waves from the bottoms of
each of said hollow members, and further means for damping out
partials which absent said further means would radiate at
inappropriate amplitudes, and means for connecting said further
loudspeakers to separate channels of said multichannel electronic
amplifier.
9. The combination according to claim 8, wherein said main
enclosure contains:
a third relatively-large loudspeaker facing outward from said one
side of said main enclosure,
a fourth relatively-large loudspeaker facing outward from said
other side of said main enclosure and
a plurality of smaller enclosures, one for each of said first and
second loudspeakers and one for said third and fourth
loudspeakers.
10. In an acoustic radiation system for an electronic organ
comprising an array of acoustic radiators arranged to visually
simulate the pipes of a pipe organ, said acoustic radiators being
located in a room having an acoustically reflective ceiling located
above said radiators and a plurality of seats facing said
radiators, the combination wherein each of said acoustic radiators
includes a vertical hollow pipe having its lower end closed and its
upper end terminated by a loudspeaker, said loudspeaker being
arranged to radiate vertically upwardly toward said ceiling, said
loudspeaker and said pipe having substantially the same transverse
areas, a separate source of multi-partial tone signals
corresponding with a stop of said organ and comprising plural
frequencies of the musical scale having partials appropriate to
said stop coupled to drive each of said loudspeakers, respectively,
each of said loudspeakers having a radiative area selected to
effectively radiate the lowermost frequencies of said tone signals
applied to that loudspeaker, each of said hollow pipes being
arranged to provide an enclosure for its associated loudspeaker
which by virtue of its internal volume and construction is resonant
near said lowermost frequencies of said tone signals and contains
substantially no internal acoustic standing wave patterns at said
lowermost frequencies, whereby different partials of said tone
signals produce diverse acoustic radiation patterns, the higher
partials radiating directionally only toward said ceiling and the
lower partials radiating toward said ceiling and also in azimuth,
said seats being located at levels near the levels of the lower
ends of said pipes and well below the levels of said
loudspeakers.
11. In an electronic organ system, said electronic organ system
including an electronic organ providing tone signals pertaining to
separate stops of said organ on separate leads, the combination
comprising a plurality of vertical hollow tubes, one for each of
said stops, a loudspeaker providing a vertically upward radiation
pattern secured only at the upper end of each of said tubes, means
totally closing the lower end of each of said tubes, acoustic
damping means located in the bottom of each of said tubes for
damping reflections of acoustic waves upwardly from said bottom of
each of said tubes, and means for connecting each of said
loudspeakers to a different one of said leads to be driven only by
tone signals pertaining to a separate stop of said organ.
12. The combination according to claim 11, wherein each of said
tubes has an enclosed volume appropriate to form a Helmholtz
resonator tuned near the lowest frequency of the tone signals its
associated loudspeaker is called on to radiate.
13. The combination according to claim 12, wherein each of said
tubes is sufficiently mechanically rigid that it does not
acoustically radiate.
14. The combination according to claim 13, wherein each of said
tubes includes a mouth shaped and positioned to simulate visually
the mouth of a true pipe organ but of a size selected to resonate
with the enclosure of said tube near said lowest frequency.
15. The combination according to claim 11, wherein said tube has a
radiative port near its lower end, said port being sized to
resonate with the volume of said tube at a frequency near the
low-frequency limit of the range of said signals.
16. The combination according to claim 11, wherein said first
mentioned acoustic absorbing material is a conically tapered hollow
in a mass of sound-absorbent material.
17. The combination according to claim 11, wherein said port
extends at least at its upper side into a lip opening in said tube
sized for tuning said tube, said lip opening being covered with an
acoustically transparent covering material which visually obscures
said lip opening.
18. The combination according to claim 11, wherein said system
includes: a supporting sleeve member for said tube at least
partially surrounding said tube only at its lower end, there being
a V-shaped opening in said sleeve member under said port, said
V-shaped opening having its apex extending downwardly and having
its base at least as wide as said port.
19. The combination according to claim 11, wherein said hollow
member contains in the lower end thereof a conically tapered hollow
in a mass of sound-absorbent material, the apex of said hollow
being the lowermost part of said mass.
20. The combination according to claim 19, wherein said tube
contains at least one narrow ring of sound-absorbent material
located between the upper end of said hollow member and said port,
said ring being positioned to damp selected frequencies of said
tone color which might otherwise occur at levels not appropriate to
the selected stop.
21. In an electronic organ system for operation in a room having a
reflecting ceiling including an electronic organ having separate
output signal leads for separate stops of said organ, separate
acoustic radiating means for each of said stops, each of said
radiating means including a hollow pipe and an upwardly directed
radiator covering the top of said hollow pipe, means closing the
lower end of said pipe, a discrete mass of acoustic damping
material located at said lower end of said pipe for damping
acoustic waves, said pipe being acoustically rigid and having an
enclosed volume appropriate to form a Helmholtz resonator tuned
near the lowest frequency of said stop, said loudspeaker having the
minimum radiative area capable of effectively radiating the band of
frequencies covered by said stop, said pipe having an axis directed
toward said reflecting ceiling, the radiating area of said
loudspeaker being such that the higher frequencies of said stop are
reflected from said ceiling in greater proportion than are the
lower frequencies and said lowest frequency is radiated
approximately omnidirectionally.
22. In an electronic organ, lead means for separately conveying
tone signals corresponding with separate stops of said organ, a
separate Helmholtz resonant hollow tube for each of said stops
visually simulating a pipe organ and having an upwardly directed
loudspeaker at its upper end and a mass of acoustic wave absorbing
material internally in the lower end, and means connecting said
leads individually to said loudspeakers, each said tube providing
an enclosure for one of said loudspeakers, and means for damping
out standing waves internally of said tube, the wall of said tube
being acoustically rigid.
23. The combination according to claim 22, wherein each of said
tubes includes an acoustically radiative mouth visually simulating
the mouth of a pipe of a pipe organ.
24. A radiative system for an electronic organ having separate
output leads on a per stop basis, comprising a plurality of
vertical acoustically rigid pipes each including internal means for
damping out standing waves, and each including a loudspeaker
located in its upper opening, said pipes providing enclosures for
said loudspeakers, and means separately connecting said leads to
said loudspeakers.
25. The combination according to claim 24, wherein each of said
loudspeakers has a radiative area selected to radiate approximately
omnidirectionally the lowest frequencies of the stop radiated by it
and only vertically upwardly the highest frequencies of said stop.
Description
BACKGROUND OF THE INVENTION
Acoustical measurements and recordings of the tones of organ pipes,
supplemented by acoustic theory, have led to the following
observations on the acoustical nature of organ pipes as sound
sources:
1. Most of the sound from flue pipes comes from the open end and
the mouth. (For closed pipes, radiation occurs from the mouth only,
but these are relatively few in number and generally rather
undeveloped harmonically in their tones.) For open pipes more tone
radiates from the open end than from the mouth because the open end
is much larger in area than is the mouth.
2. An array of pipes in a pipe organ frequently includes many pipes
which are situated behind a "forest" of other pipes, so that the
weaker sounds radiated from the mouths of the rear pipes are
further weakened when reflected by the pipes farther forward. Only
at the lower frequencies (where the sound easily passes around the
other pipes without reflection) do the sounds from the mouths
emerge from the "forest" at full power.
3. The sound from the open end of the organ pipe is directional
along the axis of the cylindrical pipe at very high frequencies,
and less directional as frequency decreases. Experimental data
confirm that the same physical laws apply to the directivity of
this sound and to the directivity of other sound sources of similar
diameter (such as loudspeaker cones).
4. Because of the length of the larger pipes and the typical
elevated location of the arrays of smaller pipes, the sound sources
in a pipe organ are usually high above the floor, especially
relative to the listening audience.
5. Because of the high location of the sources and the upward
directivity of the high harmonics, the sound coming directly to the
listeners from the pipe contains relatively less high-frequency
power than the sound which is reflected from the ceiling and walls
of the room toward the listener. This is particularly true for
installations of exposed pipes within reverberant rooms, as
distinguished from tone chambers.
Realistic organ tones can be produced from electrical tone waves,
if the following conditions are fulfilled:
1. The electrical input wave-form to the loudspeaker resembles
closely the corresponding pipe tone wave-form.
2. The loudspeaker unit faithfully converts the input to sound
output without audible distortion.
3. The loudspeaker and the loudspeaker enclosure combine to radiate
tone in a pipe-like manner.
4. The loudspeaker enclosures are located, oriented and grouped to
resemble a spatial array of organ pipes.
5. The array is installed within an acoustical environment suitable
for a pipe organ.
Fulfillment of the first condition depends upon pipe tone research
and the development of suitable electrical circuitry. The second
involves careful selection of a loudspeaker unit. Condition 5 is a
matter of installation room acoustics. This application relates
primarily to the fulfillment of conditions 3 and 4, assuming that
conditions 1, 2 and 5 are met.
Efforts toward this end, i.e. to transform facsimile electrical
signals into fully authentic pipe tones, have now led to the
invention of loudspeaker enclosure structures which, for both
acoustical and visual reasons, resemble real pipes.
It is not new broadly to direct a loudspeaker axis upward, in
producing electronic organ tones. Early tone cabinets radiated the
medium and high-frequency sounds from the top upward, and the
low-frequency sounds from ports in the side of the cabinet.
However, in order to match the acoustical radiation pattern of a
true pipe by means of a loudspeaker and enclosure, it is necessary
to decrease the diameter of the space behind the loudspeaker
diaphragm approximately to organ pipe size. Reduction is limited to
the diameter of the smallest loudspeaker unit which can adequately
produce the tones to be called forth. For high-pitched pipes or
stops, the loudspeaker diameter can be small, and for low-pitched
pipes or stops, the diameter must be larger.
For efficient tone production at low and medium frequencies, with
flat frequency response, one must enclose the back of the
loudspeaker in sufficient volume to prevent the back enclosure from
unduly stiffening the diaphragm acoustically. When the back
enclosure has the diameter of the loudspeaker rim, it is thus
necessary for the length of the pipe to be large compared to the
diameter. This acoustical requirement leads into the range of
dimensional proportions for tone radiators according to the present
invention, which are also typical of organ pipes (e.g., five or 10
to one).
The use of long enclosures has the additional acoustical advantage
of raising physically the easily localized sources of high-pitched
tones and harmonics high above the listeners, as they usually are
in a pipe organ installation. A practical advantage follows that a
small floor area is needed for an electroacoustic pipe or for an
array of these. But acoustically the lower frequencies, which
radiate in all directions and directly to the listeners in a hall,
follow different paths than do the higher frequencies, which
radiate chiefly upwardly and must be reflected and dispersed
towards the listeners. In order to supplement tone radiation at
lower frequencies, where an organ pipe mouth is most efficient, we
have provided a bass reflex port in each electroacoustic pipe, the
port being both a radiator and a resonating device, dimensioned to
resonate acoustically with the enclosure at or slightly below the
lowest frequencies required of the pipe. The port is located where
the mouth of a conventional pipe would be, so that the combined
tone radiation from the top of the pipe and the side of the
enclosure via the port will be pipe-like.
When the ceiling height permits the electroacoustic pipe to be
long, one can take full acoustical advantage of the additional back
enclosure space for the low-pitched organ stops. This is done by
enlarging the port area beyond conventional pipe-mouth proportions.
Perforating the upper lip area just above the pipe mouth improves
the efficiency for low tones, while enhancing visual resemblance to
a pneumatic organ pipe.
Without any internal acoustical treatment a pipe enclosure would
cause the loudspeaker to respond quite differently for different
notes of the musical scale, because of resonances within the pipe.
The air-column resonances which would produce this non-uniform
response are effectively removed in the present invention by two
types of internal acoustical treatment as follows:
a. An internal conically-tapered sound absorber at the foot of each
pipe, for controlling column resonance at the lower resonance
frequencies.
b. Spaced acoustically absorbent rings within the length of each
pipe, for controlling acoustical resonance at the higher
frequencies.
Custom installation of electroacoustic pipes is feasible because
(1) several standard diameters can be adapted to various stop (or
pitch) requirements, and (2) the lengths can be varied (within
reasonable design limits) to achieve a desired visual result to
complement the architectural design.
For this purpose it is convenient to provide a base board on which
pipe supports (shoes) are preloaded at the factory. The split
shoes, which can be spread apart for insertion of the pipes, may be
secured to a base board by brackets, for example. The split in the
shoe is purposely widened at the top to simulate the appearance of
the conical foot of a pneumatic organ pipe.
The air blown pipes of pipe organs operate on a per note basis. In
the present invention an electroacoustic pipe radiates various
notes of the same organ stop, leading to economies, but also to
wide divergence from other known pipe systems, both pneumatic and
electronic.
SUMMARY OF THE INVENTION
The tones of electronic organ stops are radiated from one or more
rows of vertical pipes each having a loudspeaker facing upward from
its upper end, there being a mouth in each pipe near the bottom end
and sound-absorbing material within the pipe, shaped conically-open
at the bottom and in the form of discrete rings located between the
mouth and the top. In a compact system for a home or small chapel
installation a tone cabinet may support one or more rows of
electroacoustic pipes, with one or more relatively large
loudspeakers facing outward from each edge of the cabinet. Each
pipe radiates various tones developed for one stop of the
electronic organ, or a very few related stops.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a view in perspective of an installation leading to the
invention;
FIG. 2 is a view in elevation of an electroacoustic pipe, having a
portion at the top broken away to show a loudspeaker;
FIG. 3 is a sectional view taken on the line 3--3 of FIG. 2;
FIG. 4 illustrates a broken front view, of a pipe of the type
illustrated in FIG. 1, but a variation therefrom in the bass reflex
port area;
FIG. 5 is a view in elevation of a row of pipes such as might
comprise a typical grouping for the entire Great division of an
organ and including several pedal pipes; and
FIG. 6 is a view in rear elevation of a cabinet version of the
invention, the back panel having been removed to show the contents
of the cabinet.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In FIGS. 1 and 2, a vertical pipe 1, preferably of solid plastic,
thick-walled impregnated paper tubing or formed Fiberglass, has a
loudspeaker 3 mounted at the top thereof. A pair of audio signal
leads 5 may pass down through the inside of the pipe 1 and leave
via a small hole (not shown) near the bottom rear. The bottom of
the pipe 1 may be closed by a plug 6 of suitable solid material.
The pipe 1 may be secured in a vertical position on a floor or
solid panel 7 by a shoe or sleeve 9 shaped, as shown, to expose a
lower portion of the pipe, in correspondence with the tapered shape
of a conventional organ pipe. The sleeve 9 may be fastened in place
by suitable angles 11. According to the teachings of this
invention, a bass reflex port 13 may be located near the lower end
of the pipe 1, as illustrated in FIG. 2. Also, in FIG. 4, an upper
lip area 15 may be open in the pipe 1 and covered with an
acoustically transparent fiber or wire-mesh screen, colored as the
pipe 1 to simulate a closed area. Thereby, the appearance of a
conventional pipe is retained, while enlarging the port 13 to
obtain the additional port area needed to resonate with the
enclosed volume of the pipe at a desired lowest frequency for the
stop being radiated by the pipe. In FIG. 3, in accordance with a
further aspect of the invention, a conically-tapered mass 17 of
sound-absorbent material, such as glass fiber wool, may be located
at the bottom of the pipe to damp the lowest column resonances of
the enclosure. Also, spaced sound-absorbent rings 19 of a similar
material may be distributed along the inside of pipe 1 between the
port 13 and the loudspeaker 3. The rings are so spaced as to damp
higher-frequency acoustical resonances which would otherwise occur
at undesired levels, while mass 17 serves to reduce reflections
from the bottom of the pipe.
In accordance with further teachings of this invention, an array 20
of pipes, such as illustrated in FIG. 5, may be employed for one or
more divisions of an electronic organ. Lower-pitched stops will use
long, wide pipes, while higher-pitched stops will use shorter,
narrower pipes. The possibilities for spatial distribution and
architectural design are practically unlimited. FIG. 5 illustrates
particularly an array which is effective to radiate the sounds of
the Great and Pedal divisions of an electronic organ, in emulation
of the sounds of a pipe organ.
Although the arrangement of the pipe assembly of FIGS. 1-3 shows a
sleeve 9, it may be desirable to support pipes directly to the
under panel or floor, an area such as 21 (FIG. 5) being finished in
a dark color, the remainder of the pipe, as at 23, being finished
in a lighter color, such as gray, to simulate the appearance of a
conventional organ pipe.
In accordance with a further aspect of the invention, FIG. 6 is a
rear elevation of an electronic organ tone cabinet assembly such as
might be employed with a smaller organ in a home or other
small-room environment. One or more rows of electroacoustic pipes,
as at 25, are shown along the top 26 of a cabinet 27. These pipes
may be used only for higher-pitched and/or higher-harmonic voices,
which are more easily localized by listeners than are lower
frequencies. Enclosed relatively-large loudspeakers 29 may be
employed for the lower-pitched and lesser-harmonic voices of the
Swell division, while enclosed speakers 31 may serve for the
lesser-harmonic and/or lower-pitched voices of a Great division.
Large speakers 33 separately enclosed in the same cabinet may serve
for the Pedal voices. A cabinet such as 27 would preferably be
installed behind the console of an electronic organ and probably in
front of a wall. Appropriate multi-channel power amplifiers 35 may
be located centrally in the cabinet to supply audio signals of
different electronic organ stops to the corresponding
electroacoustic pipes and the cabinet loudspeakers.
Although round pipes have been illustrated herein, it will be
obvious to one skilled in the art that other hollow configurations,
such as square cross-section, can also serve.
With regard to the quantitative aspects of the invention, optimum
dimensions, including port areas and widths may be determined
empirically for larger-diameter pipes, smaller-diameter pipes then
being scaled down to similar proportions. For example, for a 5 foot
long, 12 inch diameter pipe employing a 12 inch loudspeaker, a 3
inch .times. 10 inch port gave optimum "bass reflex" action as well
as visual similarity to a conventional organ pipe. However, when
applying the lowest-range notes to such a structure, additional
volume was needed behind the loudspeaker to reduce the stiffening
effect of the enclosure, which is a function of volume. In
lengthening the pipe from the 5 foot length -- say to about 8 feet
to 10 feet -- a substantially-larger port area was found optimum by
comparing the frequency-response at the port with that on the axis
of the diaphragm. Adding the upper-lip area, such as at 15 in FIG.
4, not only provided sufficient port area for proper resonance
frequency and greater efficiency but, when covered by a perforated
member, the appearance of the port resembled the mouth and lips of
certain conventional organ pipes. Additional port area is
available, if needed, by using a lower-lip 16 (FIG. 4) and covering
it also with fine-mesh screen, finished to be of the same color as
the pipe 1. The optimum port size depends upon the volume of the
pipe and the lowest frequency played on the associated organ
stop.
With regard to the sound-absorbing material, long strips of
tapered-width glass fiber wool of, say 1 inch thickness may be
rolled; or a series of rings of graduated inner diameter may be
stacked, in order to produce the effectively-conical shape
specified for the mass 17. The two hollow rings 19 preferably have
an inside diameter about half that of the pipe and may be about 4
inches wide. For example, they may be built up from 1 inch thick
hollow discs. A spacing in the pipe of about one-third and
two-thirds of the distance from the loudspeaker 3 to the port 13 is
effective, although for a short length only one ring at about
midway in the pipe was found effective. The optimum location for
the rings depends upon the antinodes of these pipe resonances which
require damping.
Although a cone loudspeaker has been indicated herein, it will be
obvious that any loudspeaker having both front and back radiation
will be suitable. For very high-pitched organ stops (e.g.,
mixtures) it may suffice to use only front radiation from an
unloaded horn-driver unit mounted in the top of the pipe.
One typical installation is illustrated in FIG. 5 of the
accompanying drawings which is drawn to scale. The Bourdon 8 foot
stop would utilize a 12 inch cone, the Great Mixture IV a 4 inch
cone, for example. The sizes of the cones are not critical, but the
usual 12 inch, 8 inch, 6 inch and 4 inch cones which are
commercially available, are employed, the selection principle being
that the cone is as small in diameter as is feasible to radiate
effectively at the lowest frequency which is significant in the
stop involved. The length of the pipe is usually then about seven
to 10 times its diameter, to assure an adequate pipe volume to
avoid stiffening the cone unduly within the ban of frequencies to
be radiated. The Helmholtz resonance of the pipe enclosure is
controlled in part by the area of openings 15 (or 13), some part of
the area of which may be covered with sound transmissive material
and appropriately masked, so the visual appearance of a pneumatic
organ pipe is retained. These same openings act as tone radiators,
as is the case in conventional organ pipes. The material of the
pipe can be heavy paper board, or plastic, but should be of such
character that the pipes do not substantially vibrate mechanically
along their bodies, and so that radiation occurs chiefly from the
pipe mouth and directly from the upper surface of the loudspeaker.
Generally, the latter radiation is by far the greater in
intensity.
The utilization of a small loudspeaker, located at the upper
opening of an elongated electroacoustic pipe, produces two
important acoustical effects. One is that low-frequency tones
radiate efficiently in all directions (see pattern LF of FIG. 2),
and directly reach the listeners located well below the level of
the loudspeaker without the necessity for reflections. High
frequencies on the other hand, radiate along the axis of the cone
in a highly directional pattern, labeled HF in FIG. 2, and
accordingly reach the listener chiefly after reflection and
dispersion from the ceiling or upper enclosing wall, of the hall or
room. Accordingly, the high frequencies or high partials travel
slightly farther than the low frequencies, and also undergo
multiple reflections before reaching the listeners. Conventional
organ pipes provide much the same effect, even though these radiate
on a per note basis while the electroacoustic pipes of the present
invention radiate various notes of the same stop. If one note is
played, in the system of the present invention, and assuming that
that the note has many partials or harmonics, the lower partials
will radiate directly to the listeners but the very high partials
chiefly by reflection. Intermediate partials radiate partly
directly and partly via dispersing or multiplying reflective paths.
This kind of radiation pattern for a conventional organ pipe
contributes to the tonal character of the stop, i.e., there are
relative delays, directivities, and relative intensities, or
partials, which are peculiar to the timbre effect of each stop. In
the electroacoustic pipe very similar characteristics are achieved,
and therefore a close simulation of pipe organ tones, although the
electroacoustic pipes must each handle a range of notes.
Pipes are not entirely new for electronic organs. For example,
refer to the U. S. Pat. No. 3,327,044 to Markowitz. However, this
earlier system employed loudspeakers on a per note basis. Moreover,
these loudspeakers are located at or near the bottoms of the pipes,
so that internal pipe resonances will profoundly affect the
spectrum (harmonic patterns) of the tone radiated from the other
end of the pipe. The goals and results and philosophy achieved by
the Markowitz system, which depend upon internal acoustical
resonance of the pipe, diverge radically from those of the present
system in which internal resonances of the pipe are purposely
minimized in effect.
While stress has been placed on the per stop, or per tone color,
character of the present system, a single pipe, therein, can
radiate a plurality of tone colors, if the acoustical character of
the pipe relates suitably to the frequency ranges of the stops. For
small installations, this is essential. For large installations,
architectural and aesthetic considerations may dictate a very large
number of pipes. For smaller installations, low frequencies may be
radiated by means of an ordinary loudspeaker cabinet, and only high
frequency notes by means of pipes, to achieve the effect of a
dispersed sound source, together with the effect of elevated
sources, of sources deriving in part from a vertically directed
radiator and in part from a mouth, and to achieve the aesthetic
visual influence of perceiving organ pipes visually, and providing
spatially dispersed multiple source radiators.
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