Transducer coupling system

A transmission system is disclosed having an improved means for coupling sound to and from a transducer. The system includes a main sound channel and an associated cavity; damping elements are selectively positioned in or adjacent the cavity to provide a suitable acoustic impedance in the desired operating frequency range to effect a smooth acoustical operation.

BACKGROUND OF THE INVENTION 
The prior art discloses electro-acoustic apparatus wherein the associated 
transducer is located in a housing and the sound output or sound input is 
coupled through suitable acoustic ducts or tubing to the hearing canal or 
source of sound. One common application is as components of headsets worn 
by telephone operators. Another application is as transducers, that is as 
microphones and receivers in hearing aids wherein the transducer is 
located within the hearing aid housing. In the application of the 
transducer as a hearing aid receiver, an acoustic passage from the 
receiver extends through part of the hearing aid and connects to a length 
of flexible tubing which couples into an ear mold that substantially seals 
to a portion of the hearing aid canal. The ear mold creates an effectively 
closed chamber to couple the sound from the receiver to the ear drum. Also 
a duct may be used to couple sound from an appropriate operative location 
to the microphone. 
The sound passage, usually comprising a duct or channel, can be of a single 
length of tubing, but commonly the duct comprises several pieces of tubing 
which pieces may be of different areas and which may be joined to provide 
the necessary length of duct. 
For example, a common length of ducts used in hearing aids to form the 
sound channel from the receiver to the chamber adjacent the ear is of the 
order of two to three inches. Tubing of the foregoing length possesses 
resonant modes within the frequency range of the normal hearing and these 
resonances create varying impedance and transmission parameters than can 
interact with the impedance and transmission parameters of the receiver to 
provide a fluctuating transmission characteristic to the hearing canal. 
Damping elements or materials have previously been added to prior art 
systems to dampen the amplitude of these fluctuations. There are several 
places where the insertion of a damping element into the acoustic path has 
been considered to be functionally acceptable. One such location is in, or 
adjacent, the receiver wherein the damping elements may actually form a 
part of the receiver. Another location applicable in a behind-the-ear 
hearing aid is where the flexible tubing is attached. More specifically, 
in a receiver for such a behind-the-ear hearing aid application, the most 
effective point at which to install the damping element is at the end of 
the duct where it connects to the chamber adjacent the ear drum. At this 
latter point, a damping element of proper impedance can produce a smooth 
transmission characteristic. Likewise, when a damping element is provided 
for a microphone, a damping element is conveniently located adjacent the 
sound input to the duct. However, locating the damping element for the 
receiver system at the end of the channel adjacent the ear drum chamber; 
or, locating the damping element at the sound input of the microphone 
system develops practical problems, particularly for the reasons which 
will now be discussed. 
Damping elements to be useful must consist principally of acoustic 
resistance. Known methods of obtaining resistance in acoustic paths (in 
the frequency range at which the earphone system is to function) comprise 
the provision of small passages or holes to assure that the resistance or 
dissipative component of the total impedance is more effective than the 
inertance component of the impedance. However, when the damping element 
which comprises small passages is used with a receiver in a hearing aid, 
the passages are readily stuffed or clogged by the excretions normally 
present in the ear; or, when a similar damping element is used with a 
microphone such as with a headset, the damping element may become 
contaminated or plugged up by moisture and dust. Accordingly, designers 
have heretofore attempted a compromise between locating the damping 
elements at a point where the element is most effective, but very apt to 
malfunction, or locating the damping element at a point where it is 
partially effective and reasonably free of malfunction. 
Accordingly, it is a principal object of the present invention to provide 
an acoustic transmission system wherein the damping elements may be 
positioned, where there is minimal likelihood of malfunctioning, and still 
be of maximum effectiveness. 
It is another object of the present invention to provide an acoustic 
transmission system having improved means for reducing the amplitude of 
fluctuations of the transmission characteristics within the operating 
frequency range. 
It is another object of the present invention to provide an acoustic 
transmission system which enables the location of the damping elements at 
a point other than at the end of an acoustical duct, but which damping 
elements obtain a high degree of effectiveness. 
The foregoing and other features and objects of the present invention will 
be apparent from the following more particular description of the 
preferred embodiments of the invention as illustrated in the accompanying 
drawings wherein:

DESCRIPTION OF THE INVENTION 
Refer to FIG. 1 showing a headset with acoustical transducers; that is, 
with both a microphone and with a receiver. In FIG. 1, the earphone system 
1 includes a headpiece 2, an assembly housing 3, a microphone 4 (indicated 
by the dotted lines), and a receiver 5 (also indicated by the dotted 
lines). The microphone 4 connects to a duct assembly 6 constructed in 
accordance with the invention, as will be explained. 
The end of the duct assembly 6 is positionable to receive sound from the 
user's mouth. The receiver 5 connects to second duct assembly 7 similar to 
assembly 6; and, the end of the assembly 7 couples to an ear insert 8 and 
thence to the acoustic chamber formed in the user's ear. Electrical leads 
indicated generally at 9 connect the power; and, the signal input/output 
to the earphone system. 
The present invention is useful with transducers generally; that is, with 
both microphones and receivers; and, the following explanation while 
referring, for convenience in explanation, to a receiver and to hearing 
aids is thus also applicable to microphones, and generally to earphone 
systems. 
Refer now to some practical concepts concerning the location of damping 
elements in the acoustical transmission systems in accordance with the 
invention. 
Consider a point for insertion of a damping element along the sound path, 
near the receiver so that a considerable part of the sound path remains 
between this point and the ear chamber or cavity. The length of duct or 
channel (comprising a tube or tubing) which is connected to the ear 
chamber is long enough to provide protection for the damping element and 
is located at a convenient mechanical site. The cavity formed in the ear 
can be approximated by an acoustical compliance having a value between 0.3 
.times. 10-6 and 1.5 .times. 10-6 cgs units. Since most of the disturbing 
impedance interactions observed in practice are found to occur at 1,000 Hz 
and above (see FIG. 2), the tube or tubing is terminated with a negative 
acoustical reactance of less than 530 ohms. The diameters (bores) of the 
tubes or ducts that have been found to be acceptable usually have 
characteristic impedances in excess of 900 ohms, usually about 1500 ohms. 
The characteristic impedances of a tube is approximated by an acoustical 
resistance value, Zo, where: 
##EQU1## 
At the reference point of the connection of the receiver to the tube, the 
impedance looking from the receiver into the tube and toward the ear is 
that of an acoustical transmission line that is terminated in a low 
impedance. The input impedance of such line, at the frequencies at which 
the line is an odd number of quarter wavelengths long, will be high; at 
least many times the characteristic impedance of the tube. Satisfactory 
damping impedances normally have the values of the order of the 
characteristic impedance of the tube, and if inserted at this point, will 
have very little effect at these frequencies. 
In other words, a damping element positioned at one location may produce a 
desirable effect at one frequency, but at another frequency it may be 
ineffective. There may be locations to insert the damping elements, 
usually found emperically, that for a given receiver and tubing produce 
more useful results than other locations. 
One such location and value of damping element may not work equally 
satisfactorily for a variety of tubing lengths and a problem arises since 
the length of the tubing usually varies because it is adjusted to fit the 
particular individual who is to wear the hearing aid. 
This invention discloses an acoustic transmission system which provides an 
output which is substantially smooth and free of high peaks in the 
operating frequency range. A basic concept of the invention is the 
provision of an auxiliary cavity or sound branch functioning as part of an 
acoustic transmission system as shown in the sketches of FIGS. 9a, 9b, and 
9c. 
The acoustic transmission system of FIGS. 9a, 9b and 9c comprises a sound 
channel or main sound passage acoustically coupled to the associated 
transducer. As mentioned above, the invention is applicable to both 
receivers and microphones, hence for purposes of explanation assume the 
transducer of FIGS. 9a, 9b and 9c is a receiver. Thus consider that the 
sound channel conveys sound from the receiver to the ear of the user. The 
auxiliary cavity or sound branch is provided in accordance with the 
invention, and a damping element is positioned as shown in FIGS. 9a, 9b, 
or 9c. In the absence of the auxiliary cavity, at certain frequencies, the 
acoustic impedance of the sound channel becomes very high and therefore, 
there is very little volume velocity at the location of the damping 
element in the sound channel; and, the insertion of a damping element has 
very little effect. However, the insertion of a damping element such as 
shown at the junction of the sound channel and the auxiliary cavity of 
FIG. 9a does have a damping action at these frequencies. The auxiliary 
cavity retains or constrains the sound within the acoustic transmission 
system. 
The configuration shown in FIG. 9a will produce beneficial results at 
locations and frequencies where a damping element positioned in the sound 
channel is ineffective. And, this new construction of FIG. 9a may then be 
combined with the damping element means positioned in the sound channel as 
in FIG. 9b and 9c to obtain even more desirable results. 
FIG. 2a shows, in curve a, the response of a receiver provided with a 
damping element such as indicated in FIG. 9a; and in curve b the response 
of the same receiver with no such damping element. 
FIG. 3 shows in curve a, the response of a microphone provided with damping 
elements such as indicated in FIG. 9a; and in curve b the response of the 
same microphone with no such damping element. 
The acoustic transmission system of the invention comprises a damping 
element with a cavity formed behind the damping element. One useful 
embodiment of the inventive concept can be appreciated by reference to the 
sketch of FIG. 10. In FIG. 10, a receiver connects through a common or 
receiver duct R to a main duct M and also to auxiliary duct X. In the 
sketch of FIG. 10, the length of the auxiliary tubing is substantially 
equal in length to the length of the main tubing. The auxiliary tubing is 
blocked or plugged at the end remote from the receiver and fitted at the 
end closer to the receiver with a damping element D. A similar damping 
element D1 is placed in the tubing leading to the ear canal. The main duct 
M and auxiliary duct X provide alternative paths for the sound as it comes 
from the receiver. When the damping elements D and D1 are the 
characteristic impedance of the ducts, the acoustic impedance, when 
looking from the receiver toward the connection of the main and auxiliary 
tubes, is the characteristic impedance of the tubes except at very low 
frequencies. Thus, the impedance at this point is no longer substantially 
fluctuating, but is relatively constant; and, the transfer impedance from 
this junction to the ear canal is approximately equal to the 
characteristic impedance. The volume velocity (V) flowing into the ear 
canal chamber is approximately the sound pressure (P) at the junction 
divided by the characteristic impedance (Zo). 
Another way of considering the foregoing is that the acoustical admittance 
of the main duct is complementary to the acoustical admittance of the 
auxiliary duct. As the frequency varies, the acoustical admittance of the 
main duct will change, and the acoustical admittance of the auxiliary duct 
will also change in an equal amount, but in an opposite or complementary 
manner. The result is that the combined admittance of the main and 
auxiliary ducts remain essentially constant and the output response is a 
relatively smooth curve as shown in FIG. 2. 
Since the joining of the two tubes produces a combined impedance equal to 
the characteristic impedance of the tubes, it also forms a suitable 
termination for an additional length of the receiver duct R that connects 
the junction of the main and auxiliary tubes to the receiver. The 
impedance presented to the receiver is also the characteristic impedance 
of the tubes and the value of the transfer impedance remains equal to the 
characteristic impedance. 
With this form of the inventive construction, the same transfer 
characteristic is provided utilizing substantial lengths of tubing, as is 
obtained by mounting the receiver at the ear canal with a damping element 
installed at the output of the receiver. 
An important advantage of the invention is that the transfer characteristic 
remains substantially unaltered regardless of the length of the main 
acoustical transmission duct, so long as proper damping elements are used, 
and so long as an appropriate auxiliary duct is provided. 
FIG. 4 illustrates a behind-the-ear hearing aid 11 embodying the principles 
of the present invention. The hearing aid 11 is illustrated as comprising 
a housing 13 which may be formed of a molded plastic. As is well known, 
the housing 13 includes a receiver 15, associated circuitry and power 
supply (not shown) for electronically processing the incident sound waves. 
Acoustic transmission channels couple the sound to a chamber formed 
adjacent the ear drum as will be explained. 
In the construction shown in FIG. 4, the behind-the-ear hearing aid 11 
includes a conventional hook portion 17 which is formed to go over the top 
of the ear and to be supported thereby. The end of the hook portion 17 
couples through a suitable flexible tubing 19, which is inserted on the 
end of portion 17, to an ear mold 21 fitted in the ear of the user as is 
well known in the art. 
As described hereinabove with respect to the sketch of FIG. 10, the 
inventive construction of FIG. 4 provides an improved acoustical coupling 
duct system 27 including a three outlet junction or connector 25 for 
coupling sound from the receiver 15 to the ear cavity. More specifically, 
a duct or tubing 23 couples sound from the receiver to the vertical arm of 
an inverted T-shaped junction 25 (see also FIG. 4a). In the drawing of 
FIG. 4a, acoustical damping elements 28 and 29 are positioned respectively 
in the horizontal arms 26 and 36. Horizontal arm 26 couples through a 
tubing or duct 24 positioned in hook portion 17 to the flexible tubing 19. 
The other arm 36 of the T-shaped junction 25 connects to an auxiliary 
acoustical assembly comprising a tubing 31 and a tubing extension 33 
terminated by a plug 35 and mounted in folded-back relation with respect 
to tubing 31. 
The operation of the structure of FIG. 4 is as described with respect to 
the sketch of FIG. 10. As explained above, the impedance of the main 
acoustical duct 27 when looking from the junction into the duct is that of 
an acoustical transmission line that is terminated by a very low 
impedance. The auxiliary duct 31 and the extension 33 measured from the T 
junction 25 to the remote end or plug 35 is substantially equal to the 
length of the main duct 27 measured from the T junction 25 through tubing 
24 and 19 to the open end 30 of the tubing 19 which couples to the ear 
cavity. Damping elements 28 and 29 have the same characteristic impedance 
as the associated tubing. 
As explained above, the acoustic impedance when looking at the joint pair 
is the characteristic impedance of the ducts; the impedance is relatively 
constant and the transfer impedance from the junction to the ear cavity is 
approximately equal to the characteristic impedance. 
Since the joining of the two ducts 24 and 19, and of tubing 31 and 
extension 33 produces a combined impedance equal to the characteristic 
impedance of the ducts, this also forms a suitable termination for the 
additional length of the duct 23 connecting the junction to the receiver 
15. The impedance presented to the receiver 15 is also the characteristic 
impedance of the ducts and the transfer impedance remains equal to the 
characteristic impedance. 
Note that the cross sections of the ducts may have differing diameters. For 
example, the auxiliary duct 31 may be of a different diameter than the 
main duct 27 comprising tubing 19 and 24. The varying diameters of the 
various segments of the ducts and the length of the various segments may 
be emperically selected to provide a desired deviation from the 
transmission characteristics obtainable when the ducts are of the same 
diameters. 
In FIG. 5, the present invention is utilized with a microphone in an 
eyeglass type of hearing aid 40. The temple 51 of the eyeglass is 
constructed by molding two mating halves 52 and 53 of the temple and then 
joining or glueing the two halves together to form the eyeglass temple 51. 
Sound channels or ducts 27A and 19A are formed as halves of cylinders in 
the two halves 52 and 53, and when the two halves are joined, a passageway 
is formed for sound. 
The main sound duct 27A includes an opening 55 at the forward end of the 
temple 51 to admit sound and convey the sound through duct 27A, and a 
three-inlet junction or connector 23A to the microphone 4A. 
In accordance with the invention, a damping element 28 is positioned 
adjacent the junction of duct 27A and Y-shaped connector 23A. One end of 
auxiliary duct 19A connects to the other inlet of the junction 23, and the 
remote end of duct 19A is sealed and auxiliary damping element 29 is 
positioned adjacent the junction of duct 19A and connector 23A. The 
operation of the structure of FIG. 5 is the same as that described above, 
and the response is as substantially shown in curve a of FIG. 3. 
In the embodiment of FIG. 6, the invention is utilized with a receiver of 
an eyeglass type of hearing aid 41. In this embodiment, the main 
acoustical tubing or duct 27 couples through a suitable fitting 42 to the 
main duct 19; and, the auxiliary tubing 31 extends along the length of the 
eyeglass temple 43. In this construction, the connector 45 is also a 
three-outlet junction, in essentially a Y-shape, which couples the main 
duct 27 and the auxiliary duct 31 through the common duct 23 to the 
receiver 15. 
The operation of the structure of FIG. 6 is as described above. 
Another embodiment of the present invention is shown in FIG. 7 wherein a 
channel 47 is formed in a removable hook portion 17A of the hearing aid 
housing 13. The channel 47 is connected through a flexible tubing 23A to 
the receiver 15. The channel 47 includes a branch 49 formed near the end 
of the hook portion 17A which branch couples to an auxiliary tubing 32 
terminated by a plug 35. A damping element 29 is placed in the branch 49 
at a point near the junction of branch 49 and channel 47. A second and 
similar damping element 28 is placed in the channel 47. The end of hook 
portion 17A couples to flexible tubing 19, which in turn, is mounted in 
the ear mold 21. 
Still another embodiment of the invention is shown in FIG. 8 wherein the 
main duct 27 and the auxiliary duct 31 are positioned to extend alongside 
one another within the housing 13 and hook portion 17B. The adaptation of 
this arrangement of parallel ducts to the microphone application as shown 
in FIG. 1 is straight forward. 
Note also that this embodiment, as in the embodiment of FIG. 7, the hook 
portion 17B may be removable from the main housing 13. The main duct 27 
and auxiliary duct 31 extend throughout the length of the hook portion 17B 
and throughout the length of the flexible tubing 19A. The length of the 
main duct 27, which has an open end, is the same as the length of the 
auxiliary duct 31 which is plugged by plug 35. Damping elements 28 and 29 
are positioned respectively at the junction of main duct 27 and common 
duct 23, and auxiliary duct 31 and duct 23. The operation of the structure 
of FIG. 8 is similar to that described above for FIG. 4. 
While the invention has been particularly shown and described with 
reference to preferred embodiments thereof, it will be understood by those 
skilled in the art that various changes in form and details may be made 
therein without departing from the spirit and scope of the invention.