Minimum Audible Field Research Articles

Psychophysical audiogram of a California sea lion (Zalophus californianus) listening for airborne tonal sounds in an acoustic chamber

Many species-typical audiograms for marine mammals are based on data from only one or a few individuals that are not always tested under ideal conditions. Here, we report auditory thresholds across the frequency range of hearing for a healthy, five-year-old female California sea lion identified as Ronan. Ronan was trained to enter a hemi-anechoic acoustic chamber to perform a go/no-go audiometric experiment. Auditory sensitivity was measured first by an adaptive staircase procedure and then by the method of constant stimuli. Minimum audible field measurements were obtained for 500 ms frequency-modulated tonal upsweeps with 10% bandwidth and 5% rise and fall times. Thresholds were measured at 13 frequencies: in one-octave frequency steps from 0.1 to 25.6 kHz, and additionally at 18.0, 22.0, 36.2, and 40.0 kHz. Sensitivity was greatest between ~0.9 and 23 kHz, with best hearing of 0 dB re 20 μPa at 12.8 kHz. Hearing range, determined at the 60 dB re 20 μPa level, extended from ~0.2 kHz to 38 kHz. Sensitivity was comparable to that of three sea lions tested in similarly controlled conditions, and much better than that of two sea lions tested in less controlled conditions. [Work supported by ONR.]

水中聴覚における外耳道の空気泡の影響について

In order to examine the effects of air bubble in the ear canal for underwater hearing, detailed measurement of minimum audible field (MAF) in audio frequency (50Hz-8kHz) and comparison of loudness were carried out at two conditions as below, 1) the air bubble remains in the ear canal and 2) the ear canal is filled with water. As the result, in the high frequency region over about 300 Hz, it is highly sensitive to hearing in the case that the air bubble remained in the ear canal. In the low frequency region under 300 Hz, it becomes more sensitive in the case that the ear canal was filled with the water by removing the air bubble.The reverse phenomenon of “hearing” at about 300 Hz is explained by difference between frequency characteristics of the occlusion effect (air conduction) and the effect in which the ear canal was filled with the water (bone conduction).

Perceptual speech processing and phonetic feature mapping for robust vowel recognition

We propose perceptual speech processing and phonetic feature mapping, which are inspired by the human auditory perceptual characteristics. The proposed perceptual speech processing is based on three perceptual characteristics and consists of three independent processing steps: masking effect, minimum audible field renormalization, and mel-scale resampling. They remove unperceptible spectral components, and adjust the magnitude and frequency scales of speech spectra, respectively. We apply these three processing steps to the speech spectrum sequentially to generate a new speech signal representation called the perceptual spectrum. For Mandarin vowel recognition, nine representative vowels are selected as references and similarity measures to these reference spectra, called phonetic features, are then generated from the perceptual spectrum. These phonetic features then serve as speech parameters in a continuous HMM-based recognition, stage. With these two techniques, a high recognition accuracy on Mandarin vowel phonemes has been achieved. Further experiments confirm that significant improvement on recognition robustness with respect to speaker variation and noise contamination can also obtained.

Reducing hearing protector test time with a minimum audible pressure to field transfer function

Effective hearing-loss prevention programs should train workers to properly fit hearing protection devices. In order to minimize the training time for protector fitting, unoccluded minimum audible field threshold (MAF) for narrow-band noise could be estimated from the worker’s annual minimum audible pressure (MAP) audiogram. Murphy et al. previously reported a transfer function for 5- and 1-dB audiometry for 75 and 10 subjects, respectively [W. J. Murphy, J. R. Franks, S. L. Hall, and E. F. Krieg, J. Acoust. Soc. Am. 101, 3126 (1997)]. Significant differences at several frequencies between the 1- and 5-dB groups prompted further study of the transfer function. This paper reports MAF and MAP results for 44 subjects tested with both 1- and 5-dB audiometry and 1-dB sound field. Pure-tone thresholds were significantly different, for 1- and 5-dB step sizes. The binaural reference equivalent threshold sound pressure level (RETSPL) sound-field thresholds were compared with the RETSPL thresholds for monaural listening in a sound-field listed in ANSI S3.6-1996, Table 9. The binaural thresholds were found to be significantly higher (p<0.05) than the standard values at all frequencies except 8000 Hz.

Differences between binaural sound-field thresholds and monaural audiometric thresholds

Pure-tone audiograms under earphones and noise-band open-ear sound-field thresholds were collected from more than 150 subjects during hearing protector testing. The pure-tone audiograms were administered with 5-dB resolution to normal-hearing subjects (≤25 dB HTL) with an audiometer using TDH-50 headphones in standard cushions. Sound-field thresholds were determined by presenting a third-octave band noise in a reverberant room for the same subjects at resolution of 1 dB [Franks et al., Ear Hear. 13, 2–10 (1992)]. The subject’s best pure-tone threshold for either ear at each frequency was converted to dB SPL for comparison to the sound-field thresholds. The transfer function between pure-tone, minimum audible pressure, to diffuse sound-field, minimum audible field, was calculated for each of the test frequencies 125–8000 Hz, including 3150 and 6300 Hz. A second experiment was performed with ten subjects where each received three pure-tone audiograms at 1-dB resolution, as well as three sound-field noise-band audiograms at 1-dB resolution. The ten-subject data set was used to test the robustness of the larger data set. The results may be useful for predicting the open-ear sound-field thresholds from a subject’s pure-tone audiogram and in establishing guidelines about test precision of sound-field audiometry.

Normal hearing threshold levels in the low-frequency range determined by an insert earphone

Hearing threshold levels have been determined in the low-frequency range (20-500 Hz) on a group of 30 young normal-hearing subjects using monaural stimulus presentation through an insert earphone (Etymotic Research ER-3A). A retest was performed on half of the group to provide data on test-retest reliability. The mean hearing threshold levels obtained agree closely with the Minimum Audible Field data of ISO 226, however, with some deviation at the very lowest frequencies below 40 Hz. The test-retest difference results yielded mean values that averaged 1.15 dB with an average standard deviation across test frequencies of 3.9 dB. The results show that low-frequency hearing thresholds for pure tones of frequencies from 40 Hz and up can be determined with acceptable validity and reliability by the use of this type of insert earphone.

Hearing Thresholds and Equal Loudness Contours in Free Field at Frequencies below 1 kHz

Hearing thresholds for pure sinusoidal tones were determined in a free field at frequencies from 25 Hz to 1 kHz. Contours of equal loudness were determined at the same frequencies at loudness levels from 20 phon to 80 phon in steps of 20 phon. 12 subjects participated. The psychometric method used for the threshold determinations was the method of limits. The deviations from minimum audible field values given in ISO/R226 were small (3 dB at most frequencies). For the measurement of equal loudness the reference was a 1 kHz tone, and other tones were compared to that. The psychometric method was the method of limits. The resulting equal loundess contours were positioned at much higher levels than those of ISO/R226. Similar results have recently been reported by others. At low loudness levels the results were in good agreement with the curves obtained by Fletcher and Munson, but at high loudness levels they were also above their curves. Some of the points of equal loudness were also determined by a maximum likelihood estimation method. The results differed slightly from those obtained by the method of limits.

An Investigation into the Microstructure of the Low Frequency Auditory Threshold and of the Loudness Function in the near Threshold Region

The minimum audible field (MAF) is the sound pressure level at the threshold of audibility. The MAF threshold contour clearly demonstrates the variations in sensitivity of the auditory system with frequency. This frequency dependent sensitivity is also apparent at higher intensities, the importance of which is demonstrated by the extensive use of the dB(A) weighting in noise measurements. The sparse data available on low frequency auditory thresholds, and on the subjective effects of low level low frequency noise in the threshold region, indicated that a study of low frequency thresholds and near threshold equal loudness contours would fill a significant gap in the understanding of sound perception in this frequency range. This investigation demonstrates the existence of wide variations in individual sensitivity to low frequency sound. The diversity in auditory response to low frequencies between individuals should therefore be a prime consideration in low frequency noise control.

Underwater hearing sensitivity of two ringed seals (Pusa hispida).

Minimum audible field, underwater audiograms from 1 to 90 kHz were obtained for two ringed seals (Pusa hispida). The audiograms exhibited a uniform sensitivity, to within ± 7 dB, in the frequency range 1 to 45 kHz. Above 45 kHz the threshold increased at a rate of 60 dB per octave. The lowest threshold was −32 dB relative to 1 μbar (68 dB re 1 μPa) at 16 kHz. The audiograms were similar to those of other phocid seals so far studied.

Brownian motion in the cochlear partition.

Estimates have been made of the Brownian-pressure fluctuations in the air. These are 18 dB below threshold (minimum audible field) for 3000 Hz. It is thus necessary to look into the ear itself to see whether thermal fluctuations limit the sensitivity of the ear. It is assumed that the first stage of amplification of acoustical energy takes place at the hair cell, and that it is the relative shear displacement between the top of the hair cell and the stereocilia that is the relevant parameter. Two estimates of Brownian fluctuation are made. The first estimate assumes a connection rigid enough to provide the greatest flow of acoustical energy to a hair cell—i.e., proper impedance match—and loose enough so that each hair cell has independent thermal displacements. The second assumes a rigid connection between the stereocilia and the tectorial membrane and thus the source of noise is the fluctuation in amplitude of the basilar membrane displacement. The impedances used in making these estimates were based on the in vivo measured impedances at the eardrum and the anatomy of the middle ear. The first estimate gives a singal-to-noise (S/N) ratio of 22 dB at threshold for 3000 Hz. The second estimate gives an (S/N) ratio of −33 dB. This calculation shows that it is necessary to assume that the stereocilia are attached to the tectorial membrane and that the hair-cell bodies are imbedded in the organ of Corti with some degree of rigidity. There is some direct experimental evidence for the assumption of rigid attachment, and there is much indirect evidence from ultrastructural studies. A critical evaluation is made of the assumptions and of the data used in the calculations.

Earphone versus sound-field threshold sound-pressure levels for spondee words.

This experiment sought to establish the difference between earphone and sound-field measures of spondee threshold SPL (sound-pressure level). Recorded spondee words were delivered to both a conventional and an insert-type earphone as well as to a single loudspeaker that generated the signal in the sound field. Monaural spondee threshold SPL's were established for both normal hearing and hypoacousic subjects under the three listening conditions. The intensity level of the spondee words under the three conditions was expressed as the SPL of an “equivalent” speech spectrum noise. For both groups of subjects, the mean sound-field threshold SPL was about 7.5 dB lower than that measured under the conventional earphone and approximately 12.5 dB lower than that established under the insert-type earphone. These results tend to confirm the reality of the “missing 6 dB.” Further, they indicate that the difference between minimum audible pressure (MAP) and minimum audible field (MAF) is a product of both diffraction and “closed-ear” effects. The latter effect appears to vary as a function of the volume of air enclosed by the pressure transducer.

Minimum Audible Field of Cat

Minimum Audible Field of Cat

A Wide-Band Carrier System for Condenser Microphone

A carrier microphone system has been developed for measuring very low sound pressure levels over a bandwidth from static pressure to 100 000 cps. The system is suitable for use with conventional condenser microphones and other types of capacitive transducers. This paper discusses (1) the problems of carrier systems vs direct bias systems in achieving low noise in the low- and high-frequency region, (2) a method of using tuned circuits at the carrier frequency without restricting the input bandwidth to the Q of the individual tuned circuits, and (3) the use of electrostatic methods of actuation to obtain equivalent pressure calibration in a carrier system. An equivalent acoustic noise floor less than Fletcher's minimum audible field curve over the entire audio bandwidth has been achieved. Pressure calibration of condenser microphones to 100 kc has been accomplished with the system described. (This work was supported in part by a U. S. Air Force contract.)

Minimum Audible Field of the Cat

In the course of our studies of persistent and temporary threshold shifts in the cat, audiograms have been obtained on a total of 39 monaural cats. These cats were trained and the thresholds were obtained by methods similar to those used in previous investigations. There is good agreement between our results and those from various laboratories, e.g., Neff and Hind [Acoust. Soc. Am. 27, 1955]. Intra-cat and inter-cat variabilities in thresholds will be presented for three modifications of the psychophysical method of limits. Details of the methods and their practicability will be discussed. [This research was supported by the U. S. Air Force under Contract No. AF 33(616)-3844, monitored by Aero Medical Laboratory, Directorate of Research, Wright Air Development Center, Wright-Patterson Air Force Base, Ohio.]

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.

Free-Field Thresholds vs. Pressure Thresholds at Low Frequencies

In an earlier paper data were reported indicating there was no difference between minimum audible field and minimum audible pressure at low frequencies. No satisfactory explanation was available at that time as to why these results differed from previously published data except the statement that body noises, breathing, etc., had been minimized. Further work has now produced a satisfactory explanation. Furthermore, if a well-sealed volume of 6 to 7 cm3 is maintained invariant over the eardrum and the mechanical isolation of the signal source progresses from good to bad, the difference between MAF and MAP varies from 0 to about 16 db. Data have been obtained for deviation of MAP from MAF as the volume and mechanical isolation are kept constant, the seal being broken by measurable amounts. Finally, to study the effect for the smallest possible volume, the plugged ear was fed acoustically from a high impedance source that was well-isolated mechanically. With this arrangement the results indicate that for the 2-cm3 volume the difference between MAP and MAF can be made less than 4 db by carefully controlling the subject. This control involves the position of the teeth, tongue, arms, etc.

Free-Field Thresholds Versus Pressure Thresholds at Low Frequencies

In 1933 Sivian and White closed their paper in the Journal of the Acoustical Society with the statement that at low frequencies the difference between the thresholds for minimum audible field and minimum audible pressure could not be explained. To investigate this problem further monaural field thresholds and monaural pressure thresholds have been obtained, the latter for cavities varying from 6 cc to over 800 cc. A probe tube, attached to a W.E. 640-AA condenser microphone, was inserted near the ear drum and the sound pressure determined with the probe remained invariant under transformations of volume. Similarly the same probe was used to determine the pressure for free field. When sufficient care is taken to eliminate muscle noises, breathing noises, etc., and to insure monaural listening, there appears to be no significant difference between minimum audible field and minimum audible pressure at low frequencies. This result is based on measurements made with a limited number of subjects; however, to secure more representative data measurements are being continued with a larger number of subjects.

ON MINIMUM AUDIBLE SOUND FIELDS

The minimum audible field (M.A.F.) has been determined from data taken on 14 ears over the frequency range from 100 to 15,000 c.p.s. The observer is placed in a sound field which is substantially that of a plane progressive wave, facing the source and listening monaurally. The M.A.F. is expressed as the intensity of the free field, measured prior to the insertion of the observer. Similar data are presented for binaural hearing, over the range from 60 to 15,000 c.p s., obtained with 13 observers. At 1000 c.p.s. the average M.A.F. observed is 1.9 × 10−16 watts per cm2, corresponding to a pressure 71 db below 1 bar. Included are data showing how the M.A.F. varies with the observer's azimuth relative to the wave front. Another type of threshold data refers to minimum audible pressures (M.A.P.) as measured at the observer's ear drum. The differences obviously to be expected between M.A.F. and M.A.P. values are due to wave motion in the ear canal and to diffraction caused by the head. The M.A.F. data are discussed in relation to the M.A.P. determinations from several sources. Some possible causes of difference between the two, which are due to experimental procedure and may add to the causes already mentioned, are pointed out.