Free Sound Field Research Articles

Measuring the Free Field Correction for a New Condenser Microphone

In the development of the new Bruel & Kjaer Measurement Condenser Microphone, it was found necessary to thoroughly investigate the diffraction characteristics and to use several calibration techniques to establish the difference between the microphone's free field and pressure response. The techniques and results of these investigations are described as the following: (A) Measuring the pressure distribution around a model of the microphone placed in a free sound field and the sensitivity distribution over the microphone diaphram. In conjunction with measurements made with a specially designed concentric electrostatic actuator, the effective sound pressure increase on the diaphram is calculated. (B) Measuring the microphone sensitivity as a function of frequency according to the free-field reciprocity method, and comparing the results with the pressure response of the same microphones measured by means of the electrostatic actuator method. The free-field correction obtained by the two different methods (A) and (B) showed excellent agreement.

Experiments on the Bone-Conduction Threshold in a Free Sound Field

Earlier investigators of the free-field bone-conduction threshold occluded the auditory meatus with plugs and muffs and reasoned by various arguments that the real-ear attenuation obtained was less than the attenuation provided for air-conducted sound. The conclusion was that the sound perceived reaches the auditory system via bone conduction. The present experiments are a check on these conclusions for the frequencies from 1000 to 8000 cps. With the auditory canal occluded by plugs and muffs the whole skull, or sections of it, was covered with a 2-in. layer of absorbent cotton. The muffs were not covered. A higher free-field threshold with the cotton indicated that without cotton the sound was reaching the ear via bone. [The sound pressure level (SPL) at the surface of the muffs was not altered by the surrounding cotton.] Cotton over the entire head raised the bone-conduction threshold by 10 to 18 db from 2000 to 8000 cps (8 subjects). Bone-conduction thresholds determined previously were confirmed from 1000 to 8000 cps. The front and back of the head were approximately of equal sensitivity with respect to bone conduction. Covering only the side of the head facing the sound source raised the threshold more than covering the “shadow” side alone. The results suggest a possible method for improving the effectiveness of special noise-attenuating devices.

Experiments on the Bone-Conduction Threshold in a Free Sound Field

Earlier investigators of the free-field bone-conduction threshold occluded the auditory meatus with plugs and muffs and reasoned by various arguments that the real-ear attenuation obtained was less than the attenuation provided for air-conducted sound. The conclusion was that the sound perceived reaches the auditory system via bone conduction, i.e., not through the auditory meatus. The present experiments are a check on these conclusions for the higher frequencies. With the auditory canal occluded, the whole skull or sections of it, excluding the muffs, was covered with a 2-in. layer of absorbent cotton. A higher free-field threshold with the cotton indicated that without cotton the sound was reaching the ear via bone. (The SPL over the muffs was not altered by the cotton.) Cotton over the face and back of head raised the bone-conduction threshold by 10 to 18 db (2000 to 8000 cps). Bone conduction thresholds measured previously were confirmed from 1000 to 8000 cps. Front and back of the head were approximately of equal sensitivity with respect to bone conduction. Covering only the side of the head facing the sound source raised the threshold more than covering the “shadow” side alone. The results suggest a possible method for improving the effectiveness of special noise attenuating devices.

In Search of the Bone-Conduction Threshold in a Free Sound Field

Several investigators have attempted indirect determination of the bone-conduction threshold in a free sound field. It has recently become possible to attenuate sound transmitted through the auditory meatus to a level below that of a sound reaching the ear through other pathways. By varying the properties of sound-attenuating devices, and with the help of resonance phenomena, it could be shown that the remaining hearing was actually controlled by body conduction, if not more specifically by bone conduction. According to these measurements, the “minimum audible field” for body conduction (MAFB) exceeds 80 db re 0.0002 dyne/cm2 at low frequencies and decreases gradually to a minimum of about 45 to 50 db between 1500 and 2000 cps. For higher frequencies it increases somewhat again. The difference between body conduction and air conduction seems to be maximal in the vicinity of 500 cps—approximately 65 db. At frequencies around 100 cps the difference is of the order of 50 db. This difference holds also for the frequency range between 2000 and 6000 cps. Between 1500 and 2000 cps the difference is only about 45 db.

In Search of the Bone-Conduction Threshold in a Free Sound Field

Several investigators have attempted indirect determination of the bone-conduction threshold in a free sound field. It has recently become possible to decrease the intensity of sound transmitted through the auditory meatus to a level below that of a sound reaching the ear through other pathways. By varying the properties of sound-attenuating devices, and with the help of resonance phenomena, it could be shown that the remaining hearing was actually controlled by body conduction, if not more specifically by bone conduction. According to these measurements, the “minimum audible field” for body conduction (MAFB) exceeds 80 db re 0.0002 dyne/cm2 at low frequencies and decreases gradually to a minimum of about 45 to 50 db between 1500 and 2000 cps. For higher frequencies it increases somewhat again. The difference between body conduction and air conduction seems to be maximal in the vicinity of 500 cps, amounting to approximately 65 db. At frequencies around 100 cps the difference is of the order of 50 db. This difference holds also for the frequency range between 2000 and 6000 cps. Between 1500 and 2000 cps the difference goes down to about 45 db.

The Pressure Distribution in the Auditory Canal in a Progressive Sound Field

The variation of the sound pressure along the auditory canal was determined experimentally on a number of subjects, male and female, placed in a progressive sound field. This was accomplished by insertion of a small flexible probe microphone at various positions along the auditory canal. The subjects were placed in front of a loudspeaker in a room free from acoustic wall reflections. The free sound field at the subjects' location was essentially that of a plane progressive wave. The measurements were carried out over the significant range of audiofrequencies for various orientations in azimuth of the subjects with respect to the sound source. The sound pressure at the eardrum is found to be greater than the free-field pressure. The average ratio of these two quantities is a function of frequency, and reaches values of about 20 db in the vicinity of 3000 c.p.s. The human ear is thus an effective acoustic “amplifier.” The increase in sound pressure at the eardrum over the free-field pressure is caused by a combination effect of diffraction by the head and pinna and resonance in the auditory canal. The measurements of the sound pressures at several other positions along the auditory canal serve to separate these two phenomena to a certain extent and to furnish additional information about the pressure distribution. Most of the data were obtained with a group of male subjects, but measurements on a few women did not show any marked discrepancies.

A Precision Method for the Determination of the Velocity of Sound in Air

A method has been developed and refined in the laboratory at U.C.L.A. for the measurement of the velocity of free progressive sound waves in air. The method involves wave-length measurements in a free sound field of known frequency. A directional source radiating “spherical” waves is placed at one end of the free field room. Measurements of positions of equal phase along the axis of the source are made with a movable microphone. The phase measurements are made on the screen of an oscilloscope. The amplified voltage developed by the microphone is applied to the vertical plates of the oscilloscope and a voltage derived from the source oscillator is applied to the horizontal plates. Closure of the ellipse on the oscilloscope screen is used to determine position of equal phase. The microphone is moved away from the source on a steel track, the position of the microphone being remotely controlled by Selsin motors. The positions of equal phase are recorded by photographing a reference scale against a steel tape stretched parallel to the axis of the source. The camera and reference scale move with the microphone. Wave-length measurements are made over an interval of 10 meters. The source frequency is determined by beating one of its harmonics with the carrier frequency of a commercial broadcasting station. Harmonics of the order of the 100th are generated in a suitable diode circuit. Temperature measurements are made by means of thermocouples distributed along the line of measurement. Humidity is determined by a gravimetric method. A sample of air of known volume is passed through a drying tube and the change in weight of the tube determined. Since it is unnecessary for the operator to enter the room during or immediately previous to making the measurements temperature gradients and turbulence are at a minimum.

SENSITIVITY OF THE EAR

THE difficulty in securing accurate values of the minimum stimulus that the ear can detect is great. In the first place there are differences in the hearing mechanisms of various individuals tested. Except for those with some definite hearing defect, these differences largely depend on age ; as one grows older the response of the ear becomes less, chiefly at the higher frequencies. Measurements of minimum hearing stimulus thus vary with the age of those being tested. Another difficulty is in measuring the minimum stimulus. According to an article by W. A. Munson (Bell Lab. Rec., 21, No. 10 ; June 1943) two references have been widely used. One is the rms. pressure of the sound wave at a point in a free sound field where the listener's ear will later be placed. This is the simpler of the two, and is particularly applicable to studies of the usual mode of hearing, that is, without the use of telephone receivers or similar devices. The other is the pressure at the ear-drum. This reference avoids uncertainty as to how the pressure, measured in the air previously to the listening test, would be modified at the ear-drum by the head and ear of the listener, and is particularly useful in studying the mechanism of hearing. It involves great technical difficulties, however, since the space in front of the ear-drum is small and almost inaccessible.

Effect of Cavity Resonance on the Frequency Response Characteristic of the Condenser Microphone

The effect of the cavity before the membrane in the usual condenser microphone is to cause an increase of pressure at the membrane over that which exists in the undisturbed (free) sound-field, particularly at high frequencies. An approximate theory of the effect is given and a specimen case is calculated. In this case the pressure ratio reaches a maximum of 2.5 at 3000 cycles. Experimental values obtained with the Rayleigh disk are compared with the computations and are in general agreement. A modified design for a condenser microphone without a diaphragm cavity, and adapted for spherical mounting, is described. The temperature coefficient of sensitivity due to the unequal thermal expansions of a duralumin membrane in a steel mounting was found to be 0.6 per cent per degree C. The importance of taking the cavity and reflection effects into consideration in the construction of curves showing the overall fidelity of broadcast transmitters is stressed on account of its intimate relation to the design of radio receivers for uniform frequency response.