An electro-acoustic transducer for a musical instrument, for producing electrical signals indicative of sound produced by playing of the instrument. The transducer comprises a housing of plastic material containing a capacitor microphone arrangement which comprises a first metal foil electrode within a sheath of metal foil which defines a second electrode. A dielectric separates the two electrodes. The capacitor microphone arrangement is sandwiched between a relatively rigid wall of the housing and a relatively flexible body of plastic material within the housing. The housing is adapted to be mounted in intimate contact with a surface of the instrument which vibrates in accordance with sound produced by playing of the instrument, so that the vibrations are transmitted to the electrodes. A constant electrical potential difference is applied to the electrodes and oscillatory voltages produced by the vibration of the electrodes are amplified by an amplifier to produce an electrical signal indicative of the music produced by the instrument.

FIELD OF THE INVENTION 
This invention relates to an electro-acoustic transducer for providing 
electrical signals indicative of sound produced by a musical instrument. 
BACKGROUND TO THE INVENTION 
When making a magnetic tape recording in a recording studio, of a band or 
orchestra including many different instruments, it is usual to provide 
respective microphones for the instruments or for groups of the 
instruments, and to record the electrical signals from the microphones 
separately on respective tracks of a multi-track magnetic tape. The 
microphones may be of any of the conventional types, capacitor microphones 
for example, each mounted to produce electrical signals in response to air 
vibrations produced by playing of the instrument to which the microphone 
is directed. After recording, the recorded signals from the different 
tracks are mixed and re-recorded onto typically a twin track magnetic tape 
to produce a master tape from which a stereo gramophone record can be 
made. The mixing of the signals permits a desired balance between the 
relative amplitudes of the instruments to be achieved. For the mixing to 
be effective, it is desirable that the signals recorded on the different 
tracks of the multi-track tape are each representative substantially only 
of one of the instruments or a predetermined group of instruments, but in 
practice this is difficult to achieve for certain instruments, 
particularly drums and also stringed instruments such as pianos. In the 
usually cramped conditions of a recording studio, a conventional 
microphone positioned to detect sound produced by a piano usually also 
detects sound produced by adjacent instruments; this degrades the 
recording unless other measures are taken. 
A similar difficulty arises when electrical amplifiers and loudspeakers are 
used to amplify a live performance of a band in a concert hall. Usually 
separate microphones are used for different instruments, and the signals 
therefrom are mixed selectively before being amplified and fed to 
loudspeakers in the hall. However, for certain instruments, the 
microphones tend to detect sound not only from the instrument to which 
they are directed, but also from adjacent instruments. 
This problem is particularly serious for a piano, and a conventional 
microphone directed at a piano will respond not only to the sound from the 
piano but also to sound from adjacent instruments, with the result that it 
is often not possible to give the sound from the piano a desired 
prominence in the sound emanating from the loudspeakers. 
In order to overcome these problems for a piano, a pick-up device has been 
devised which consists of a support member extending transversely of the 
strings of the piano, on which is mounted a series of magnetic pick-ups 
which produce electrical signals in response to oscillations of the 
strings when the piano keys are struck. The device is however relatively 
expensive due to the number of individual magnetic pick-ups required for 
the many strings of a piano. Also, the device requires a complicated 
adjustment prior to use because in order to obtain a uniform amplitude 
response for the different notes of the piano, the gains of the outputs 
from the numerous pick-ups need to be adjusted individually. Additionally, 
the pick-ups all need to be close to the strings in order to obtain a 
satisfactory response, which requires a delicate adjustment. Moreover, if 
the piano is moved, for example by being pushed on or off a stage, or by 
being transported in a truck between concert halls, the setting of the 
pick-up device becomes disturbed, and the time consuming and complicated 
adjustment often needs to be carried out after such moves of the 
instrument. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an electro-acoustic 
transducer which overcomes the aforesaid problems and difficulties of 
prior art microphones and pick-ups. 
It is a more particular object of the present invention to provide a 
simpler and less expensive electro-acoustic transducer which will respond 
substantially only to the musical instrument to which it is attached. 
It is another object of the invention to provide an electro-acoustic 
transducer which can be easily and simply installed on an instrument 
without the need for the complex adjustment procedure of the known pick-up 
device aforesaid. 
In accordance with the present invention, I provide an electro-acoustic 
transducer which in use is attached to a musical instrument on a surface 
thereof which vibrates in accordance with sound produced by playing of the 
instrument, such that the vibration of the surface causes the transducer 
to produce electrical signals indicative of the sound produced by playing 
of the instrument. The transducer of the invention consists of a housing 
of plastic material in which is housed a capacitor microphone arrangement 
comprising two overlying electrodes between which is a layer of 
dielectric. In use, a substantially constant electrical potential 
difference is applied to the electrodes. The housing is arranged to be 
mounted in contact with the instrument surface such that the vibrations 
produced on playing of the instrument are transmitted to the electrodes 
such that they are displaced in accordance with the vibrations, thereby 
producing an oscillatory voltage between the electrodes indicative of the 
sound produced by playing of the instrument. The voltage developed across 
the electrodes is fed to a sensing circuit which includes an amplifier 
that amplifies the oscillatory voltage separately from the constant 
potential difference applied to the electrodes and hence the output of the 
amplifier is an electrical signal indicative of the sound produced by the 
instrument. 
The transducer according to the present invention has the advantage that, 
because it is in use attached to a surface of the instrument and is 
responsive to the vibration of the surface, it responds substantially only 
to the instrument to which it is attached, and its output is not degraded 
by sound from adjacent instruments. Moreover, the transducer of the 
invention responds to the entire fundamental frequency range of most 
musical instruments, so that the complicated balancing adjustment of the 
prior art magnetic pick-up device is not required. Also, the installation 
of the transducer of the present invention is a simple matter; it can be 
stuck to the instrument with self-adhesive tape or can be screwed in 
position if a more permanent affixation is required. Accordingly, the 
transducer does not require complex readjustment each time the instrument 
is moved. 
In a preferred embodiment of the invention, the capacitor microphone 
arrangement is sandwiched between a relatively flexible body of plastic 
material and a relatively rigid support member of plastic material for 
attachment to the instrument surface. It has been found that this 
preferred arrangement provides for improved transmission of the surface 
vibrations of the instrument to the electrodes. 
A particularly preferred form of the capacitor microphone arrangement 
consists of a first electrode in the form of an elongate foil of 
electrically conductive material, a second electrode comprising an 
elongate flat sheath of electrically conductive foil within which the 
first electrode is received and a dielectric between the electrodes 
electrically insulating them from one another. 
The housing preferably comprises an elongate box of plastic material having 
a lid, the capacitor microphone arrangement being sandwiched between a 
wall of the box or the lid, and a body of plastics material which is more 
flexible than the material of the box or lid. 
Further features, objects and advantages of the present invention will 
become apparent from the following description of two embodiments thereof 
given by way of illustrative example with reference to the accompanying 
drawings.

DESCRIPTION OF PREFERRED EMBODIMENTS 
The transducer shown in FIGS. 1 to 3 comprises an elongate box 1 open on 
one side and having a rectangular base 2, in which is received a capacitor 
microphone arrangement 3 connected to an output jack socket 4, the box 1 
being filled with a cold curing polyurethane compound 5 and being closed 
by a lid 6. The capacitor microphone arrangement 3 is thus encapsulated 
within a housing of plastic material. In use, the base 2 of the transducer 
is mounted in intimate contact with a surface of a musical instrument, 
which surface resonates in accordance with the sound produced by playing 
of the instrument. A sensing circuit such as shown in FIG. 4, is connected 
to the capacitor microphone arrangement 3 through the jack socket 4, to 
provide electrical signals indicative of the sound produced by the musical 
instrument. 
The structure of the transducer will now be described in detail with 
particular reference to FIG. 2 in which the various layers thereof are 
shown on an enlarged scale for purposes of clarity. 
The box 1 is made of 0.002-0.003 inch thick high impact molded polystyrene 
and has attached thereto along the length of the underside of its base 2, 
a strip of double sided adhesive fabric tape 7 for attaching the 
transducer to a musical instrument. The capacitor microphone arrangement 3 
includes a strip of double sided adhesive polyester tape 8, by which it is 
attached to the upper side of the base 2 along its length. The capacitor 
microphone arrangement 3 comprises a first central electrode 9 consisting 
of an elongate strip of aluminum foil received within a second electrode 
10 which comprises an elongate sheath of flexible aluminum foil. The first 
electrode 9 is totally encapsulated with a dielectric 11 which may or may 
not be polarized; in this example, the dielectric 11 comprises a 
self-adhesive polyester tape attached to the electrode 9. A small air gap 
exists between the dielectric 11 and the second electrode. 
The box 1 is filled with the cold curing polyurethane compound 5 above the 
capacitor microphone arrangement 3, the compound typically having a depth 
of 3/8". It will thus be appreciated that the capacitor microphone 
arrangement 3 is received between and in intimate contact with the 
relatively rigid supporting base of the box 1 and the relatively flexible 
plastic material 5. A strip of doubled sided self adhesive polyester tape 
12 is attached on one side to the top of the compound 5, and on the other 
side to the lid 6. The lid 6 consists of polythene sheet and is glued at 
its periphery to a peripheral recess 1a formed in the box 1. 
Referring now to FIG. 1, the jack socket 4 includes two output terminals 
(not shown) which are connected to the first and second electrodes 9, 10 
(FIG. 2) by means of a length of coaxial cable 13 the shielded conductor 
of which is soldered to the first electrode and the shielding conductor of 
which is soldered to the second electrode. A jack plug (not shown) fits 
into the socket 4 and a length of coaxial cable connects the electrodes 9, 
10 to inputs 14, 15, respectively, of the sensing circuit shown in FIG. 4. 
The circuit is arranged to apply a constant electrical potential 
difference between the electrodes 9, 10 of FIG. 2, and to sense and 
amplify oscillatory voltages produced between the electrodes by virtue of 
the microphone arrangement 3 responding to sound produced by the musical 
instrument. 
The sensing circuit is housed in a metal box (not shown) which receives dry 
cell batteries B to drive the circuit. The batteries B establish an 18 
volt rail voltage relative to ground between rails Q1, Q2. The circuit can 
also be driven from the external power source which is usually available 
in recording studios and public address systems that employ a mixer, 
namely the conventional 48 volt d.c. supply known in the art as the 
"phantom" power supply. The phantom supply, when available is fed to the 
circuit by a cannon plug which connects with terminals 16, 17, 18. 
An output from the circuit is taken by means of a jack plug socket 19, or, 
if the phantom power supply is not being used, by means of a cannon plug 
connected to the terminals 16, 17, 18. The cannon plug output is balanced 
relative to ground. 
A switch S1 selects power either from the phantom power supply through 
dropping resistors R1, R2, or from the batteries B, and a d.c. bias 
voltage is applied through a resistor R3 to the terminals 14, 15 so as to 
bias the electrodes 9, 10 of the capacitor microphone arrangement. As will 
be explained hereinafter, in use of the transducer, oscillatory voltages 
will be induced between the terminals 14, 15. The oscillatory voltages 
pass through a capacitor C1 which blocks the d.c. bias, to an input of an 
operational amplifier IC. The amplified output of the amplifier IC passes 
through a capacitor C2, and hence through a resistor R4 to the output jack 
19, and through a transformer T1 to the cannon plug outlet. The 
transformer T1 makes the cannon plug output balanced relative to ground. 
The amplifier IC is provided with a feedback loop including a resistor R5 
and a capacitor C3, which provides for a uniform frequency response. 
The circuit also includes a battery test circuit comprising a push button 
PB, a LED and an associated resistor R6. If the batteries B are in good 
order, the LED will illuminate when the button PB is depressed. The test 
circuit can also be used, when the phantom power is connected, to check 
that the phantom power is reaching the circuit. 
In use of the transducer, it is attached by means of the self adhesive tape 
2, in intimate contact with a surface of a musical instrument which 
vibrates in accordance with the sound produced by playing the instrument. 
The vibrating surface causes oscillation of the electrodes 9, 10 and 
accordingly effects the spacing therebetween, thereby producing an 
oscillatory voltage between the inputs 14, 15 which is amplified by the 
amplifier IC. It has been found that the amplified signals produced at the 
outputs 17, 18, after further amplification and presentation to an 
appropriate loudspeaker, provide sound which accurately represents the 
sound of the instrument to which the transducer is attached. 
It is believed that one of the reasons that the transducer provides such a 
good response both in terms of frequency spectrum and amplitude, is that 
the capacitor microphone arrangement 3 is sandwiched between a relatively 
rigid member (the base of the box 1) in contact with the vibrating surface 
of the musical instrument, and a relatively soft, flexible member (the 
plastic material 5). It is believed that the relatively rigid base of the 
box provides for an efficient transference of the vibrations of the 
instrument to the capacitor microphone arrangement 3, and that the 
relatively soft plastic material 5 allows the vibrations to produce a 
substantial relative movement of the electrodes 9, 10 over a wide 
frequency range and in particular over the entire fundamental frequency 
range of a piano. Moreover, the described transducer has the significant 
advantage that it substantially only detects the sound produced by the 
instrument to which it is attached and does not pick up sound from 
adjacent instruments. The described example of the transducer can be used 
with many instruments, particularly stringed instruments and also drums. 
However, the described transducer can be used with special advantage on a 
piano, and a preferred mounting arrangement of two such transducers is 
shown in FIG. 5, for obtaining a stereo sound picture for the piano, the 
transducers being mounted on the soundboard which underlies the piano. It 
has been found that, when the described example of the transducer is used 
on a piano, the transducer and the sensing circuit provide signals in 
response to the entire fundamental frequency range of the piano. 
Another embodiment of transducer of the invention will now be described 
with reference to FIGS. 6 and 7. This embodiment includes the capacitor 
microphone arrangement 3 previously described, housed within a box 20 but 
attached to the lid 21 of the box, rather than to its base as in the 
previously described embodiment. The lid 21 of the box 20 is adapted to be 
attached in intimate contact with a vibrating surface of a musical 
instrument. 
Within the box 20 is an elongate rectangular block of perspex 22, formed at 
one end with a recess 23 and a channel 24 which receive the cable 13 and 
its soldered connections to the capacitor microphone arrangement 3. The 
surface of the block 22 which faces the lid 21 is covered with a cellular 
adhesive strip 26, such as one of the strips sold under the trade names 
"Perfix" and "Cyamber" by Cyamber Tapes Ltd. of Shoreditch, London, E.1., 
England. The capacitor microphone arrangement 3 adheres to the strip 26, 
and the strip has a cut out portion 27 which allows the cable 13 and its 
soldered connections to sit in the recess and channel 23, 24. 
The capacitor microphone arrangement 3 includes a double sided self 
adhesive strip 28 which holds the arrangement 3 in intimate contact with 
the lid 21. The cable 13 passes out of the box 20 through a hole 29. The 
box 20 is made by vacuum forming from 2 mm thick styrene, such as to have 
a peripheral flange 30 which is welded or glued to the lid 21. The lid 
itself is made of 1 mm thick polystyrene sheet and has dimensions 
81/2".times.11/2". As can be seen from FIG. 6, a small air gap exists 
between the box 20 and the perspex block 22. 
The exterior of the lid 21 is provided with a length of double sided self 
adhesive tape 31 which is used to attach the lid to a musical instrument. 
For a more permanent attachment, the transducer is provided with holes 32 
in the peripheral flange 30 to allow the transducer to be held by screws 
on the instrument. 
From the foregoing, it will be seen that the capacitor microphone 
arrangement 3 is sandwiched between the relatively rigid polythene 
material of the lid 21 which contacts the instrument, and the relatively 
flexible cellular material of the strip 26, and thus the two described 
embodiments both provide similar mountings for the capacitor microphone 
arrangement 3 on the instrument, even though the mounting thereof in the 
box differs somewhat for the two embodiments.