as shown in fig1 a closed sound delivery system 10 in accordance with the invention includes a sound generating unit 12 and a sound receiving unit 14 . the sound generating unit 12 includes a sending transducer 16 having a planar , circular diaphragm 18 disposed between electrostatic plates 20 , 22 . a toroidal suppressor element 24 is disposed adjacent the transducer 16 to suppress acoustical energy provided at peripheral regions thereof . a central , cylindrical aperture 26 in suppressor element 24 receives and passes therethrough an acoustical signal from transducer 16 to an air filled , hollow acoustical transmission tube 28 , which may have an inside diameter of 3 . 56 mm and a length of 4 inches . tube 28 has one end 30 adjacent transducer 16 and an opposite end 32 . conductors 34 provide an electrical signal to transducer 16 for conversion to a corresponding acoustical signal . at the end 32 opposite transducer 16 , tube 28 is adapted to engage the temporal bone 36 and substantially close an aperture therethrough leading through an air space 38 in front of the eardrum 40 of an animal such as a cat . an acoustical resistance 42 is disposed at transducer end 30 of tube 28 . the sound receiving unit 14 includes a receiver transducer 50 having a circular , planar diaphragm 52 which is acoustically coupled through a cavity 54 to one end 56 of an acoustical transmission tube 58 , which may be a no . 17 hypodermic needle . an opposite end 60 of tube 58 may extend through the side wall of tube 28 near the end 32 thereof to terminate so as to receive acoustical signals occurring in the air space 38 in front of eardrum 40 . a collar 61 extends about transducer 50 and defines cavity or volume 54 . electrical wires 62 couple transducer 50 to an electrical filter 64 having electrical output wires 66 carrying a wide band electrical signal with a substantially flat frequency response characteristic from approximately 2 hz to 30 khz . electrical filter 64 provides a low frequency attenuation which decreases at 6 db per octave at higher frequencies to compensate for the 6 db per octave high frequency attenuation provided by an acoustical filter resulting from cavity 54 and an acoustical termination resistance 68 disposed at end 56 of tube 58 . an additional acoustical resistance 70 may be disposed at end 60 of tube 58 . fig1 and 2 illustrate a sound system and its electrical network equivalent which is in accordance with a typical embodiment of this invention . transducer 16 of the sound source 12 is a low acoustic output impedance electrostatic transducer which converts the electrical signal supplied via wires 34 into an acoustic output signal . in one embodiment of the invention , the transducer 16 is a srx mk iii push - pull electrostatic earspeaker which is widely used in a stax high fidelity headset . it should be noted , however , that a variety of transducer elements can be employed . transducer 16 is retained within a rigid structure which , for convenience , is not shown , and radiates sound from both sides of diaphragm 18 . sound from the side of diaphragm 18 not facing eardrum 40 can radiate into either an open or a closed space . sound from the side of diaphragm 18 facing the eardrum 40 is collected in volume 26 which is formed primarily by suppressor element 24 . suppressor element 24 , in conjunction with transducer 16 , establishes a smooth , relatively flat and extended pressure response at end 30 of tube 28 by attenuating sound radiating from the periphery of diaphragm 18 facing eardrum 40 . this avoids peaks and dips in the frequency response due to reinforcement and cancellation of sound waves which travel different distances . acoustic resistance 42 is mounted at the end 30 of tube 28 at its junctions with volume 26 . acoustic resistance 42 may be any material exhibiting the desired acoustic resistance . many materials which exhibit an acoustic resistance are known in the art . one such material which has been found to be suitable for the practice of this invention is an electro - formed mesh manufactured by buckbee mears company . sound flowing through resistance 42 propagates down tube 28 to the eardrum 40 where it is sampled by probe tube 58 . probe tube 58 uses air as a transmissive medium and can be either open or can contain resistance 70 across its end 60 nearest the eardrum 40 . sound entering probe tube 58 travels to resistance 68 and into volume 54 which includes receiver transducer 50 as a wall . receiver transducer 50 is a small electroacoustic transducer element which converts the acoustic signal in volume 54 to an electrical signal which can be monitored via wires 62 . for convenience , the supporting structure associated with the probe tube 58 , acoustic resistance element 68 , and receiver transducer 50 is not shown . in one embodiment , receiver transducer 50 is a bruel and kjaer 4134 precision condenser microphone which is widely used for acoustic measurements . it should be noted however that a variety of receiver elements can be employed . furthermore , it should be noted that acoustic resistances 68 and 70 may be any material exhibiting the desired resistance when mounted at the ends 56 and 60 of probe tube 58 . fig2 shows an electrical network 80 which is an electrical analog of the sound delivery and monitoring system described in fig1 . as shown in fig2 the portion of the circuit to the left of node c represents the sound delivery portion 12 of the system 10 , whereas that to the right of node c represents the sound monitor part 14 of the system . element 82 between node c and ground represents the impedance at the eardrum 40 . the electrical equivalent of the impedance at the eardrum 40 of a cat is shon by way of simplified example as a series combination of a capacitor and a resistor . in the embodiment shown , motor transducer 16 and suppressor element 24 produce a smooth relatively flat and extended pressure response in volume 26 and are represented as an analog voltage source 84 in the circuit . volume 26 has as its electrical analog capacitor 86 . it is important to realize that volume 26 does not significantly alter the impedance of transducer 16 , but is instead merely a byproduct of the geometry of suppressor element 24 which , in conjunction with transducer 16 serves to produce a substantially frequency independent pressure at the volume side of resistance 42 . resistor 88 connected between nodes a and b is the electrical analog of acoustic resistance 42 which serves to increase the impedance seen by delivery tube 28 as represented by electrical transmission line 90 . acoustic resistance 42 is constructed such that the electrical resistance of resistor 88 is approximately equal to the characteristic impedance 23 of the delivery tube 28 and accordingly minimizes wave reflections from the transducer end of the tube 28 . the characteristic impedance 23 of tube 28 is selected so as to minimize wave reflections from the eardrum end 32 of the tube 28 . probe tube 58 has as its electrical analog electrical trnsmission line 94 having characteristic impedance 25 . resistances 96 and 98 are the electrical analogs of acoustic resistances 70 and 68 respectively . it is important to recognize that resistance 70 is not essential for proper operation of the probe tube monitor 14 . those skilled in the art will recognize that resistance 70 essentially serves to increase the input resistance of the monitor system 14 at the expense of sensitivity . on the other hand , resistance 68 serves to minimize wave reflections from the receiver end 56 of the probe tube 58 and theoretically should have a value equal to the characteristic impedance of the tube 58 . those skilled in the art will also recognize that volume 54 having an electrical analog of capacitance 100 must be sufficiently large that the magnitude of its reactance is small compared to resistance 98 at the lowest distributed resonance of tube 58 . the reactance of volume 54 and transducer 50 in conjunction with acoustic resistance 68 corresponding to electrical analogs 100 , 102 and 98 respectively produce a 6 db per octave roll off at frequencies above a lumped element resonance point produced by the reactance of volume 54 and receiver element 50 in conjunction with the reactance of the air mass in tube 58 . it is desirable to have volume 54 as small as possible in order to extend the flat portion of the low frequency response as high as possible while still maintaining the volume sufficiently large that its reactance is small compared to the terminating resistance 98 at the frequency of the first distributed transmission line resonance of tube 58 . the size of volume 54 should also be influenced by mechanical constraints such as required tube length and maximum diameter . since the input impedance of the probe system is in parallel with that of the eardrum , it is advantageous that the magnitude of the former be much greater than that of the latter so that loading effects by the monitor need not be a major consideration in the design of the delivery system 10 . capacitor 102 represents to a first order of approximation , the acoustic behavior of the transducer 50 in one embodiment of the invention . fig3 shows the frequency response of a probe tube monitor constructed in accordance with the typical embodiment of this invention that is illustrated in fig1 . the data shown are the electrical output of the receiver element for constant sound pressure at the entrance of the probe tube . whereas curve a shows the unequalized response , curve b shows the response after external electrical equalization of 6 db / octave is provided above approximately 2 . 3 khz by filter 64 . by contrast , curve c shows the response of a typical conventional prior art probe tube monitor . it should be noted that the equalized response is substantially flat to 30 khz , although not specifically shown because of limitations of the automatic equipment used to plot the data . fig3 also shows the frequency response of the sound delivery portion 12 of the system 10 as measured using monitor 14 just described . shown is the output of the receiving unit 14 equalized by filter 64 for constant voltage applied to the transducer 16 of the delivery portion 12 of the system 10 . curve d shows the response when the sound delivery tube 28 is terminated acoustically in its characteristic impedance . curve e shows a typical response when the delivery tube 28 is terminated by the eardrum of a live cat . the low frequency plateau in the curve e is an inherent property of the embodiment described and can be eliminated if desired by either simple low frequency electrical equalization of the driving voltage or by implementing the acoustic equivalent of an inductance and resistance in series to ground anywhere between nodes b and f . it should be noted that the response extends smoothly to 30 khz , although not directly shown . it should also be noted that to obtain the actual responses it is necessary to correct those shown in curves d and e by the monitor calibration of curve b . under most conditions however , this correction is negligible and need be applied only in the most exacting applications . in one example suppressor element 24 has an outside diameter of 1 . 72 inch , an inside diameter of 0 . 5 inch and a longitudinally extending thickness of 0 . 091 inch . the volume of cavity 26 occupies 0 . 3 cc corresponding to a capacitance 86 of 0 . 2 μf . transmission tube 28 has an inside diameter of 3 . 56 mm corresponding to a characteristic impedance of 410 ohms and acoustical resistance 42 corresponds to a resistor 88 of 405 ohms . below approximately 1 khz the impedance at the eardrum has a large reactive component which causes wave reflections at the driving frequency . in order to avoid resonances which destroy the flat frequency response at these lower frequencies tube 28 should be shorter than one - fourth wave length at a frequency of 1 khz and below . if tube 28 is longer than 4 inches the frequency of the distributed transmission line reflective resonances becomes undesirably low . collar 61 has an inside diameter of 0 . 485 inch and cavity 54 provides a volume of 0 . 02 cc . the transducer 50 provides a further equivalent volume of about 0 . 01 cc for a total equivalent volume of about 0 . 30 cc . the corresponding capacitors 100 and 102 are about 0 . 014 μf and 0 . 007 μf . transmission tube 58 has an outside diameter of 1 . 5 mm and an inside diameter of 1 mm , providing an acoustical characteristic impedance of 5 . 3 k ( cgs ). acoustical resistance 68 corresponding to resistor 98 has a value of 5 . 1 k . acoustical resistance 70 corresponding to resistance 96 if 0 for maximum high frequency sensitivity , but may be non - zero if a higher input impedance is required for a specific application . while a particular arrangement of a closed sound delivery system in accordance with the invention has been shown and described for the purpose of enabling a person of ordinary skill in the art to make and use the invention , it should be appreciated that the invention is not limited thereto . accordingly , any modifications , variations or equivalent arrangements within the scope of the attached claims should be considered to be within the scope of the invention .
a closed sound delivery system includes a sound generating unit coupled to provide sound to an ear in response to electrical input signals and an electrically equalized sound receiving unit coupled to accurately receive acoustical signals provided by the sound generating unit in the vicinity of an ear . the delivery system provides a pressure at the eardrum with a frequency response which remains flat up to about 30 khz . the sound generating unit includes a low acoustical output impedance transducer and a suppressor element which attenuates acoustical energy emanating from all but a small central region of the transducer to minimize frequency response variations resulting from constructive and destructive interference of acoustical energy originating at different physical positions . the receiving unit has a 6 db per octave high frequency roll off due to the characteristics of its terminating elements while an electrical high pass filter compensates for this roll off .