Developing a family of frequency response curves for AGC types of hearing instruments using swept pure tones at varying input levels often produces erroneous results. This problem is caused by exceeding the threshold for activating the AGC circuit at some frequencies but not at other frequencies during the pure-tone sweep, thereby producing a different frequency response from that which would be obtained with a complex input signal such as speech-shaped noise. This measurement artifact may be minimized by ensuring that the threshold for activating the AGC circuit is either always exceeded or never exceeded during the development of a frequency response curve. Three input signals are compared for developing a family of frequency responses for an AGC hearing aid: (1) swept pure tone, (2) swept pure tone with bias tone added, and (3) shaped broad-band noise. The shaped broad-band noise appears to be the input signal of choice.
Standardized methods for the primary free-field calibration of laboratory standard microphones deal with Type L (ANSI S1.10-1967, R1977) “one-inch” diameter microphones. However, the use of “1/2-inch” diameter microphones for measurement of the sound pressure level in acoustic fields is increasing. Consequently, the NBS has developed a fixed-cost measurement service for the free-field calibration of these microphones by the reciprocity method over the range 2.5 kHz to 20 kHz. For this service, the apparatus and procedures, including essential properties of the anechoic chamber in which the calibrations are performed, are described. Opportunities for improvements are noted. The frequency-dependent positions of the apparent acoustic centers of the microphones were obtained. The overall uncertainty estimate for free-field calibration, expressed as the sum of the magnitude of credible bounds on the systematic component (s) and the random component (2 σ , where σ is the standard deviation) is 0.16 dB or better (s=0.06 dB, 2 σ =0.10 dB) at frequencies 1.25 kHz ⩽ f ⩽ 5kHz, and 0.07 dB or better (s=0.02 dB, 2 σ =0.05 dB) for 5 kHz ⩽ f ⩽ 20 kHz. Comparison for given microphones of the measured difference between free-field and pressure response levels with the difference calculated by diffraction theory (derived by Matsui) indicates agreement of 0.16 dB or better in the low-frequency range (1.25 kHz to about 4 kHz) where free-field reciprocity measurements encounter the greatest experimental difficulties. This agreement is consistent with the estimated uncertainties of free-field and pressure calibration by the reciprocity method.
The frequency responses of hearing aids measured in a free field differ from those measured on the head of a person or a manikin due to the scattering of the sound by the head and the torso. In order to compare and interpret the response of hearing aids located on the head at various frequencies it is necessary to know precisely the spatial pressure distribution. The amplitude and phase of the acoustic pressure were measured alongside a manikin's head in increments ranging from 2 to 5 mm with frontal sound incidence . The acoustic driver was located in front of the manikin at distances of 1.0 and 3.5 m from the ear-canal axis. The test frequencies were the octave band center frequencies from 0.5 to 4.0 kHz and the third-octave band center frequencies t•rom 4.0 to 8.0 kHz. The sound pressure level varies smoothly, as a function of position, alongside the head for frequencies equal to or less than 2.0 kHz. At frequencies equal to or greater than 4.0 kHz the pressure level changes rapidly with position. Particulary severe pressure minima were found to exist around the pinna at 6.3 and 8.0 kHz. The smoothing effect of test signals using pink noise of 6% and 29% bandwidth on the acoustic pressure variation alongside the head and behind the pinna is also shown.
A previously unrecognized anomaly of pitch perception was discovered by H. F. Stimson while testing his hearing. Signals above 3500 Hz did not produce the same pitch in each ear; the right ear's sensation was that of a sound at a lower frequency, for which he also had normal pitch perception. Between 3700 and 5200 Hz, the pitch perceived in the right ear was independent of the frequency of the stimulus. As intensity was raised, additional pitches appeared; and for an intense sound, a chord was heard whose constituents were not harmonically related. Stimson can match these anomalous pitches with sinusoidal stimuli of the appropriate lower frequency within about 2%; part of this variation is a change of pitch with intensity. Tones in his normal pitch range having a pronounced overtone structure are perceived as multiple. Overtones falling in the anomalous range give rise to the anomalous pitch. This would indicate a Fourier type of analysis before pitch recognition. Beats are not excited by interference between the pitches evoked by signals in the anomalous range and those heard normally. Loudness studies show signal powers to be additive. Tone-masking and loudness summation experiments yield data suggesting that the pitch recognition mechanism lies in that part of the auditory system in which loudness is perceived—i.e., beyond the cochlea.
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