Adjustable speaker system and method of adjustment

A speaker assembly and its method of operation are described wherein a plurality of drivers are spatially adjustable relative to each other and to a selected critical listening point in order to establish maximum sound coherency for the system. Means for adjustment and calibration of the relative positions of the drivers are provided to facilitate their location upon a mounting sub-assembly, interchange of components within the total speaker assembly, and synchronization of the drivers for one or more desired listening positions within a listening environment. The speaker system preferably includes a low frequency range sub-assembly having one or more drivers supported in fixed relation within the total speaker system, a housing for the low frequency range sub-assembly including selective band-pass energy filtration means for isolating each driver from its enclosure and an internal structure providing increased structural rigidity and reducing non-linear turbulence within the enclosure.

BACKGROUND OF THE INVENTION 
The present invention relates to improvements in loudspeaker accuracy in 
the field of high fidelity sound reproduction and more particularly to a 
method and apparatus for achieving such improvements. 
The prior art has long been concerned with apparatus and methods of 
operation for speaker systems in order to accurately reproduce various 
sounds from an electronic input. 
It has been thought by many that the ideal speaker would be a point-source 
from which all the frequencies being reproduced would emanate. Limitations 
of technology have prevented the construction of such a driver. In 
attempting to achieve sufficient acoustical output across the entire 
audible frequency band, the prior art has generally resorted to the use of 
a plurality of drivers of differing construction, each type reproducing a 
specific portion of the frequencies within that band. Moreover, it has 
been generally recognized that linear reproduction of the audible 
frequency range is important to achieve realism in the reproduced sound. 
More recently, the importance of polarity and phase alignment of the 
drivers within the system, relative to each other, has been recognized. 
U.S. Pat. No. 3,824,343 issued July 16, 1974 to Dahlquist identifies and 
deals with the problem of "time delay distortion" as a barrier to 
"coherent sound" in multiple driver systems. Dalquist observes that, in a 
multiple driver system where all drivers are mounted in the same 
mechanical plane, the leading edge of a single pulse (an electrical test 
signal which most meaningfully simulates a short duration musical 
transient) applied to the system input will be reproduced acoustically by 
the system as a series of pulses that will appear to the listener as a 
distorted form of the original pulse. This "time delay distortion" has a 
number of causes. Initially, a lower mass driver-diaphragm, such as a 
tweeter dome, can be expected to reach more quickly to the pulse than the 
more massive woofer cone, so its portion of the pulse will arrive at the 
listener's ear before that of the woofer. In addition, the various 
elements in a crossover network may effect a group delay on portions of 
the input signal's spectrum. Thus, while the total energy output of a time 
delay distorted system may be identical with that of the applied signal, 
the waveform generated by the system may be substantially different. 
In order to compensate for the differing "rise time" of each type of 
driver, defined as the inherent time lag between impression of a voltage 
and driver response, Dahlquist contemplated a fixed relative arrangement 
of the drivers so that the pulse wave form front contribution from each 
driver would be theoretically synchronized and would reach the ear of the 
listener at the same time. For this purpose, the higher range speakers 
were located somewhat further away from the listener than the lower range 
speakers. 
However, because significant distances existed between the drivers of the 
above system while the location of the drivers relative to each other 
within the system were fixed, the proper pulse-arrival time alignment of 
the above system has been found in the present invention to be not only 
dependent upon freedom of arrangement for the speaker system within a 
selected listening environment, but more particularly, upon the location 
of the listening point relative to the speaker system. Speaker systems of 
the type contemplated by the present invention may be used in many 
different types of environments including, for example, studios or homes. 
In many such environments, there exist certain constraints which limit 
freedom of location for the speaker system. At the same time, a listener, 
particularly one with a discerning ear, desires to establish the speaker 
system so that maximum sound coherence is achieved at a predetermined 
critical listening point. Location of the critical listening point may or 
may not be determined by choice. 
Accordingly, the predetermined relation of components within a speaker 
system of the type referred to above may not be adaptable to a given 
listening environment. The result may be a need for compromise between 
arrangement of the speaker system within the listening environment and the 
quality of sound reproduced by the system. 
There has thus been found to remain a need for a method and apparatus in a 
speaker system for achieving maximum sound coherency for a plurality of 
drivers of a speaker system relative to a critical listening point of a 
selected listening environment while allowing the location of the speaker 
system and the critical listening point to be dictated at least in part by 
considerations imposed, for example, by the environment itself and/or by 
the listener. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a speaker assembly 
which may be adapted to a listening environment in order to produce highly 
coherent sound at a selected critical listening point in the environment. 
It is also an object of the invention to provide a method for adjusting 
such a speaker assembly in order to achieve this same purpose. 
Generally, the present invention contemplates a speaker assembly including 
a plurality of sub-assemblies, each including single or multiple drivers, 
the various sub-assemblies covering the same or different frequency 
ranges, and being adapted to produce highly coherent sound at a critical 
listening point in a selected listening environment. Within the speaker 
assembly of the invention, selected drivers within the system are 
adjustable relative to the critical listening point. The adjusting means 
include calibration means for determining the relative positions of the 
drivers, for example on a mounting sub-assembly, when adjusted for 
pulse-arrival time alignment at a critical listening point. 
Similarly, the invention contemplates a method for mounting or adjusting a 
plurality of drivers in a speaker assembly, the speaker assembly being 
arranged in a predetermined locale of a listening environment which also 
has a selected critical listening point. According to the method of the 
present invention, the drivers are independently supported with at least a 
portion of the drivers being adjustable relative to the critical point. A 
microphone and/or listener is then placed at the critical listening point 
as a monitor in order to adjust the respective drivers for maximum sound 
coherency of the system at that point. 
The method and apparatus of the present invention also permit a number of 
additional features. For example, drivers adapted for interchangeability 
within the system will have calibrated positions which permit their 
interchange while maintaining substantial sound coherency at the critical 
listening point according to a calibration "correction factor". 
In other words, the calibration correction factor may be engineered to 
compensate for the time delay differences between the interchanged drivers 
and allow the system to correctly maintain sound coherency at the critical 
listening point. 
Furthermore, the invention contemplates construction of a housing, 
particularly for one or more low frequency range drivers, the housing 
including sound absorptive means for isolating the driver or drivers from 
the main part of the housing and an internal structure with sound wells 
behind the driver(s) in order to substantially increase the rigidity of 
the housing and to reduce turbulence, or non-linear movement of air, 
within the housing. 
Additional objects and advantages of the invention are made apparent in the 
following description, having reference to the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A speaker system constructed in accordance with the present invention is 
generally indicated at 11 in FIG. 1. Preferably, the invention 
contemplates a speaker assembly including a relatively large number of 
drivers, or multiple driver sub-components, covering different frequency 
ranges in order to provide various desired characteristics such as 
accurate tonal balance, transient response, and power handling. However, 
for purposes of simplicity, it is believed that the present invention may 
be clearly understood from the simplified representation for the speaker 
assembly 11 which is illustrated in FIG. 1 as including a single low 
frequency range driver, or woofer, unit 13, two midrange units indicated 
at 15 and 19, and a single tweeter unit 17. 
The drivers are illustrated as being supported by two mounting 
sub-assemblies generally indicated at 12 and 21. 
The woofer unit 13 is preferably mounted upon, and in fixed relation to the 
support base 12. The upper mounting subassembly 21 rests upon an adjustable 
support 23, and extends upward from the woofer unit 13 to provide support 
for the other driver units 15, 19, and 17. 
Since the construction of the drivers is generally conventional, with the 
exception of the woofer housing 14, it is not believed necessary to show 
components of the driver units 15, 19, and 17 in greater detail. 
The speaker assembly 11 of the present invention is particularly adapted 
for adjusting the various drivers generally fore and aft relative to a 
critical listening point in a listening environment in order to establish 
maximum sound coherency for the system at that point. 
In this connection, reference is made to FIGS. 2-4 which illustrate various 
selected listening points and environments. The locale for the speaker 
assembly is generally indicated at 32 in each of FIGS. 2-4. A first 
critical listening point is indicated in each of the three figures at 30 
while a second selected critical listening point is indicated at 34 in 
FIG. 4. 
The manner of adjustment for the drivers in the speaker assembly to achieve 
maximum sound coherency is described in greater detail below in connection 
with FIGS. 2-4. It is initially noted, however, that while at least some 
of the drivers in the speaker assembly of FIG. 1 are adjustable fore and 
aft relative to the critical listening point, the present invention 
contemplates adjustment of the drivers along any axis relative either to 
each other or to a critical listening point in order to achieve a desired 
time or phase alignment. It is also noted that while in the speaker 
assembly of FIG. 1, and as described in the preferred embodiment of the 
invention, the adjustment apparatus for the drivers is calibrated in order 
to provide benefits such as those described above, the present invention 
contemplates any adjustment means for the drivers whether or not such 
means are calibrated. Referring to FIG. 1, it is noted that where rods are 
used to support and adjust a driver, such as the tweeter 17, one rod may be 
calibrated, as rod 22, while the second, as rod 24, need not be calibrated. 
Referring to FIG. 5, it may be seen that the driver housing is rigidly 
affixed to an elongated rod 22 via a mounting such as 47. The rod is 
slidably mounted through a hole in a mounting block 44, which is in turn 
rigidly mounted as part of the mounting sub-assembly 21. The calibration 
markings are generally indicated at 46, and may be indexed, for example, 
to the face of the mounting block 44. Means for locking the rod in 
position are provided by a set screw, indicated at 45. 
Referring to FIG. 6, the position of the woofer 13 relative to the other 
drivers in the system is adjusted by sliding the mounting sub-assembly 21 
along a calibrated track 23 which is in turn rigidly affixed to the woofer 
housing 14. The calibration markings are generally indicated at 49, and may 
be indexed, for example, to the front edge of the mounting sub-assembly 21. 
Locking bolts, indicated at 48, are provided to lock the assembly 21 in 
place. 
Referring now to FIGS. 2-4, and particularly to FIG. 2, which is 
representative of the prior art, the low frequency range unit 13 and the 
high frequency range unit 17 are arranged in a co-planar relationship. An 
electrical sound signal in the form of a square wave as generally 
indicated at 27 is applied to the drivers 13 and 17 from a suitable source 
26 through a crossover network 25. Without further consideration of factors 
present in the drivers and in the crossover network, it may be assumed for 
purposes of example that reproduced sound components 28 and 29 from the 
respective drivers 13 and 17 are slightly out of phase as they reach the 
listener, as represented by the delay component generally indicated at 33. 
Thus, the listener at critical listening point 30 would hear the reproduced 
sound components in a distorted blur, without the time domain coherence 
contemplated by the present invention. 
Referring now to FIG. 3, a similar situation to that of FIG. 2 is 
represented. However, in this case the drivers 13 and 17 are positioned 
with a tweeter 17 somewhat behind the woofer 13, as is generally 
contemplated in the above cited Dahlquist patent. It may be seen that the 
delay between the sound components 28 and 29 as heard by listener at 
critical listening point 30 has been eliminated, rendering a coherent 
sound. It should be noted that the speaker system 32 of FIG. 3 is capable 
of producing coherent sound for only one critical listening point, and 
that said point will be at a fixed distance from the speaker system. 
Referring now to FIG. 4, the same speaker system from FIG. 3 is indicated 
at 32. In this case it may be seen that a second listener at critical 
listening point 34 would hear a delay, or incoherence, between reproduced 
sound components 28 and 29. Further adjustment of the speaker system would 
be necessary in order to provide sound coherency at the critical listening 
point 34. This illustrates the need for apparatus and means to provide 
maximum sound coherence at a variety of given critical listening points. 
Reference is now made to FIGS. 7-9 in order to disclose preferred 
construction details for the woofer housing 14. Generally, the woofer(s) 
13 are mounted on a front panel of a woofer housing as illustrated for 
example at 40 in FIGS. 7 and 8. 
Referring particularly to FIG. 7, the woofer unit fragmentarily represented 
at 13 is secured to the front panel 40 by one or more mounting screws such 
as indicated at 36. Sound absorbing material is arranged between the front 
panel 40 and the woofer unit 13 as well as between the front panel 40 and 
the remainder of the woofer housing 14 in order to isolate the woofer 
housing 14 from mechanical vibrations induced by the frame of the woofer 
unit 13, thus significantly reducing unwanted acoustical radiation from 
the housing 14 itself. The front panel 40 may, for example, be formed from 
a rigid material such as plywood. The sound absorbing means contemplated by 
the present invention comprises a sandwich panel arranged on the forward 
surface of the front panel 40 adjacent to the housing 14 as indicated at 
41. Both of the sandwich panels 39 and 41 are of known construction which 
is not shown in greater detail herein. However, it is noted that each of 
the panels is of a laminate construction employing special sound absorbing 
plastics, and may include layers of metal plate. 
The effect of this use of the sound absorbing layers as illustrated in FIG. 
7, is to prevent the mechanical transmission of vibrations from the woofer 
frame 13 to the front panel 40, as well as from the front panel 40 to the 
woofer housing 14. Thus, there may be a significant reduction of 
structural vibration within the housing and a significant reduction of 
their accompanying re-radiation to the air, while at the same time 
maintaining the woofer unit(s) in rigid relation to the other drivers 
within the system in order to maintain maximum sound coherency. 
It should be noted at this point that, in the prior art, woofer units have 
some times been isolated from their housings by, for example, soft rubber 
bushings which, although perhaps effectively achieving mechanical 
isolation, may not be able to maintain the woofer in rigid relation to the 
other drivers of the system and thus not be able to maintain maximum sound 
coherency as contemplated by the present invention. 
In any event, the front panel 40 is secured to the woofer housing 14 by one 
or more screws 38 passing through the front panel and the sandwich plate 
41. The screws 36 are similarly passed through the woofer frame 13, the 
sandwich plate 39, the front panel 40, and are engaged by the nuts such as 
that indicated at 37. It may be seen that the nuts 37 are also isolated 
from both the woofer frame 13 and the sandwich plate 41 in order to 
further prevent sound transmission from the woofer 13 to the housing 14. 
Referring also to FIGS. 8 and 9, the woofer housing 14 is preferably 
constructed to achieve enhanced rigidity of the housing as well as damping 
of sound waves and reduction of non-linear air flow or turbulance within 
the housing. Since the low frequency range units of many speaker systems 
may generate considerably more mechanical energy and movement of air than 
other drivers in the system, the proper damping, or control, of these 
components is important if accurate, coherent sound is to be reproduced by 
the system. 
For this purpose, a plurality of angled dividers 51 is arranged in spaced 
apart relation within the housing 14 to form sound wells extending 
rearwardly from the woofer driver(s) 13 in parallel relation with its 
axis. The dividers 51 are mechanically interconnected with each other and 
the inside surfaces of the housing 14, for example, by glue and 
"tongue-in-groove" construction, so that the sides of the housing are 
rigidly braced and the sound wells are separated from each other. The 
sound wells 42 all open forwardly toward the woofer(s) 13 and are 
individually filled with sound damping material such as, for example, 
fiber batting sold for example under the trademark Dacron. Thus, as sound 
waves propagate rearwardly from the driver(s), they pass into the separate 
sound wells 42 which prevent lateral turbulence effects from developing 
within the woofer housing 14. A brace element 43 also extends from the 
rear surface 50 to a central position on the front panel 40 in order to 
further stabilize the front panel and rigidify the entire housing 14. 
Referring momentarily to FIG. 8, it may be seen that the rear surface 50 
for the sound wells 42 could also form the back of the housing 14. 
However, the surface 50 is preferably spaced apart from the back of the 
woofer housing in order to form a space that may be used for further 
damping of sound waves or perhaps, for example, for storage or other 
purposes. 
DESCRIPTION OF THE METHOD OF OPERATION 
While it is believed that the method of operation contemplated by the 
present invention, for the adjustment of drivers relative to a critical 
listening point in order to establish maximum sound coherency at that 
point, is apparent from the above description, that method is described 
below in order to facilitate a more complete understanding of the 
invention. 
In general, it may be desirable for reasons such as apparent image size to 
mount the drivers in a relatively spaced-apart configuration. It then 
becomes particularly desirable to establish a proper relation between the 
drivers and the critical listening point in order to achieve coherent 
sound reproduction at that point. Thus, while the present invention 
contemplates in general any adjustment of drivers along any axis within a 
speaker system, the invention particularly contemplates adjustment fore 
and aft of selected drivers within the speaker system illustrated in FIG. 
1, assuming that the spaced-apart relation of the drivers has been 
predetermined in order to achieve various desired sonic characteristics. 
It should be noted that the method described below would serve equally 
well for adjustment of drivers along any axis in order to obtain maximum 
coherency of sound at a given desired point. 
The pulse or phase response characteristics for any of a variety of drivers 
may be calculated or experimentally determined in a known manner. However, 
the present invention contemplates a method for adjusting the positions of 
drivers within a system relative to a critical listening point in order to 
compensate for the rise time characteristics of the drivers themselves, 
and also for effects that may be caused by associated equipment in the 
entire sound reproduction system. 
With the speaker system 11 established at a predetermined locale within a 
selected listening environment, one or more critical listening points are 
also determined within the environment. In order to accurately monitor the 
emanations of the various drivers, a monitor such as a microphone or the 
like is mounted at one of the critical listening points, and coupled to a 
time domain measurement device such as an oscilloscope (not shown). 
The woofer unit 13, being fixed in relation to the critical listening 
point, is used to establish a reference time delay component between the 
speaker system and the critical listening point. A sharp pulse is applied 
to the woofer unit via all, or a significant portion of, the sound 
reproducing system. The time elapsed from the generation of the electrical 
pulse and the arrival of its leading edge is then noted. The same pulse is 
then applied in turn to each of the remaining driver assemblies 15, 17, 
and 19 in turn. The position of each driver is then adjusted so that the 
leading edge of its reproduced pulse arrives at the critical listening 
point at the same time as the previously determined reference pulse. Thus, 
all the drivers may be adjusted so that their reproduced sound components 
will come into synchronization for a listener at that critical listening 
point. 
The position of each driver assembly may now be noted using the calibration 
systems illustrated in FIGS. 5 and 6. The relation between the drivers may 
now be recorded in connection with the first critical listening point. 
This process of adjustment, calibration, and recording may be repeated for 
each of any additional critical listening points. 
It is apparent that all or part of the speaker system 11 could be 
disassembled and reassembled with the proper relation between the drivers 
being reproduced without repeating the monitoring steps described above. 
At the same time, the speaker system could easily and repeatedly be 
adjusted to produce maximum sound coherency at any of the previously 
determined critical listening points, by simply referring to the 
calibrations recorded for those points as described above. 
It is also apparent that interchangeable driver components for a speaker 
system could be designed with precalibrated positions, in order to 
facilitate their interchange into a previously adjusted speaker system 
while substantially maintaining maximum sound coherence as described 
above. 
Various other modification and changes will be readily apparent within the 
scope of the present invention in accordance with the method and apparatus 
described above. Many variations upon the mounting sub-assembly are 
possible beyond that illustrated in FIG. 1. For example, components could 
be ceiling mounted, or mounted with a motor drive to facilitate automatic 
adjustment of their relative positions. While the preferred embodiment of 
the present invention includes calibration means for reasons described 
above, any means for adjustment of drivers, with or without calibrations, 
according to the method described above would fall within the scope of the 
invention. Accordingly, the scope of the present invention is defined only 
by the following claims.