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Earphone enclosures are described which embody an improved acoustical network for enhanced noise reduction and efficient performance for an earphone. The network uses two chambers, one adjacent to the ear and the other contained in a reentrant expanded portion of the enclosure. The two chambers are connected by parallel passages, one defining predominantly a resistance and the other an inertance. It is shown that this form of network is more effective than an arrangement of inertance and resistance in series. Cushions filled with nonfluid gel provide high wall impedance for improved noise attenuation and conformance with circumaural contours of the head. Values of noise attenuation are given, measured by artificial head and psychoacoustic methods. Performance of a typical earphone in the enclosure is shown.

Through cooperation among the Bell Telephone Laboratories, the National Bureau of Standards, and the Electro‐Acoustic Laboratory at Harvard University, the technique of obtaining absolute pressure calibrations of condenser microphones by the reciprocity method has been perfected and standardized to a gratifying degree of accuracy. Numerical data are given for a Western Electric Type 640‐AA instrument which was calibrated at the three laboratories in succession, showing the accuracy to be expected at the present state of the art.

The object of this research was the development of a shield whose noise‐cancellation properties would effectively complement those of a pressure‐gradient microphone, thus providing substantial cancellation at high frequencies where the pressure‐gradient microphone alone has little or no cancellation. In contrast to the normal vented‐cavity type, the new shield is tubular in shape with the microphone and mouthpiece at one (closed) end, and the other end open for breath expulsion. The intended microphone has a cosine directional pattern and is positioned so that its null plane coincides with the tube axis. Therefore, unwanted noise arrives at both sides of the exposed diaphragm in phase, producing no output. Further enhancement of noise cancellation is provided by lining the tube with acoustical absorption along the inner wall. The mouthpiece is located on the side of the tube, directly above the microphone. Thus, speech is effectively received as a result of proximity to the microphone and because of the l...

A study was made to determine the level, spectrum, and intelligibility of speech available for contact transducers, as a function of head location. A specially designed head‐contact microphone was developed for use in these tests. The output of the head‐contact microphone and that of a “flat” acoustical microphone were simultaneously recorded for 10 talkers reciting phonetically‐balanced test sentences. The recorded data were then played back, and trained listeners subjectively evaluated the intelligibility of the contact speech versus the acoustically received speech. Several head areas showed highly intelligible speech with good consonantal pickup, while other regions were notably poor even when equalization techniques were employed. Spectral analysis showed that regions that had adequate high‐frequency energy generally corresponded to those that had exhibited good intelligibility in the listening tests. The head was grouped into four regions—forehead, ear, cheek, and top and back of head. The over‐all intelligibility of each area was evaluated. The forehead region gave the most desirable output for contact pickup. Over‐all output level available at each area was measured and a statistical analysis showed only modest level differences for most regions of the head. [Work supported by the U. S. Army Electronics Command.]

A miniature pressure‐gradient microphone exhibits improved noise discrimination and extended crossover frequency. Ability of a cosine pressure‐gradient microphone to discriminate against the pickup of unwanted noise is known to be limited by the acoustical pathlength between the two sound entrance points in front and back of the diaphragm. Noise discrimination is further limited by unwanted phase shifts occurring in the acoustical treatment surrounding the diaphragm. A miniature, efficient magnetic transducer actuated by a slender drive pin connected to a pressure‐gradient diaphragm permits a reduction of diaphragm size with corresponding decrease of front‐to‐back distance. Added symmetrical acoustical networks avoid differential phase shifts. Further enhancement of noise discrimination stems from a mechanically balanced system that diminishes sensitivity to mechanical vibration. Prototypes exhibit noise discrimination of 40 dB at 200 cps and a crossover frequency above 5 kc/sec. [This work was supported ...

Attenuation of ear enclosures suffers limitations owing to resonance between earcup mass and compliance of cushions and the circumaural flesh. Worthwhile improvements in attenuation at this frequency and below are obtained by selectively coupled volumes (Helmholtz/Absorbers) tuned to this resonance. If the unwanted resonance falls below the communication spectrum of interest, then the added volumes produce no lowering of acoustical impedance load to the headphone transducers. A jel‐filled cushion is described. Since the jel does not escape through small holes or tears, it may be contained in an envelope of thin, limp plastic material. A special cross‐sectional configuration permits the cushion to conform to the circumaural region with little or no stretching of the envelope material, thus retaining the soft jel properties. [Work supported by the U. S. Army Electronics Command.]

Microphones, to be used at altitude or in the increased pressures of underwater diving, pose the need for calibration at abnormal ambient pressures. Free‐field measurements are usually not feasible, since variable ambient pressure anechoic chambers are not normally available. Pressure calibration in a coupler can be performed conveniently in a small inexpensive variable‐pressure tank. This paper describes such a system, utilizing a commercially available coupler with minor modifications. Various corrections, occasioned by the abnormal static pressures, are discussed.

Work was undertaken to develop a microphone that would provide highly intelligible speech reception by direct contact with the head. Since investigation (Paper 7D4) showed that the forehead provided the bighest quality speech, efforts were concentrated in this region. In order to avoid a loss of high‐frequency response owing to flesh compliance, the total mass of the transducer had to be limited to below 1 g for a unit of small size. This was most readily accomplished with a miniature, inertial‐type transducer, which, additionally, provides the proper response‐frequency characteristics. Articulation tests, with the microphone located on the forehead, indicated an average PB word score of 90% in quiet surroundings. Good rejection of ambient acoustical noise was observed. The ultimate noise rejection is limited by noise‐induced vibrations transmitted through the head. The microphone totally frees the mouth area of obstacles located in front of the lips, and its miniature size suggests several applications. ...

With conventional earphone listening, accurate simulation of the direction of sound arrival is impossible because the sound field is not fixed in space but rotates with the head. A system for overcoming this limitation has been developed. Signals are transmitted by electromagnetic or underwater radiation to two directional receptors attached to the listener's head. The receptors are connected to the earphones through a Ster-Bin network. This network divides each of the signals from the receptors into two parts. One part is fed into its corresponding earphone; the other is delayed with respect to the first by the time interval required for sound to travel normally between the two ears, and is fed into the opposite earphone. In addition, both parts are modified at high frequencies to simulate the diffraction effects attributable to the head. The varying relative signal strengths from the directional receptors, as a function of the direction of signal arrival, modified by the Ster-Bin network, provide the listener with orientation capability. Further apparatus improvement and applications to aerospace and underwater swimmers' communications devices are envisioned.

The deep‐sea hard‐hat diver must be supplied with a copious air flow through his helmet at rates far in excess of his need for life oxygen. If reckoned in terms of free air movement, this flow increases with depth of dive and is required to reduce carbon dioxide content resulting from exhaled diver's breath. At a typical depth, the flow rate may be 28 ft3/min or about 800 liters/min. All this air passing through a number of control valves, entrance ports, and discharge valves causes much turbulence, with attendant noise known to reach well into the damage risk levels. This paper explores some special problems encountered in the measurement of diver helmet noise and describes a series of noise measurements performed in a typical helmet under simulated diving conditions.

A pressure‐gradient hydrophone exhibiting uniform response and precise polar pattern has been developed based on the moving‐coil principle. The coil, under mass control, closely matches the impedance of the medium contributing to high sensitivity. Equivalent‐circuit analysis demonstrates how to select the electrical‐termination resistance for any desired degree of damping and for proper phase‐shift control. Theoretical performance is compared with measurements using a CBS Laboratories gradient hydrophone calibrator, and in free field. [Work supported by U. S. Naval Air Development Center, Johnsville, Pennsylvania, under subcontract from Sanders Associates, Nashua, New Hampshire.]