Normal equal-loudness contours for pure tones and normal threshold of hearing under free-field listening conditions

The orientation sound emitted by the Panamanian mustached bat, Pteronotus parnellii rubiginosus, consists of four harmonics. The third harmonic is 6-12 dB weaker than the predominant second harmonic and consists of a long constant-frequency component (CF3) at about 92 kHz and a short frequency-modulated component (FM3) sweeping from about 92 to 74 kHz. Our primary aim is to examine how CF3 and FM3 are represented in a region of the primary auditory cortex anterior to the Doppler-shifted constant-frequency (DSCF) area. Extracellular recordings of neuronal responses from the unanesthetized animal were obtained during free-field stimulation of the ears with pure tones. FM sounds, and signals simulating their orientation sounds and echoes. Response properties of neurons and tonotopic and amplitopic representations were examined in the primary and the anteroventral nonprimary auditory cortex. In the anterior primary auditory cortex, neurons responded strongly to single pure tones but showed no facilitative responses to paired stimuli. Neurons with best frequencies from 110 to 90 kHz were tonotopically organized rostrocaudally, with higher frequencies located more rostrally. Neurons tuned to 92-94 kHz were overpresented, whereas neurons tuned to sound between 64 and 91 kHz were rarely found. Consequently a striking discontinuity in frequency representation from 91 to 64 kHz was found across the anterior DSCF border. Most neurons exhibited monotonic impulse-count functions and responded maximally to sound pressure level (SPL). There were also neurons that responded best to weak sounds but unlike the DSCF area, amplitopic representation was not found. Thus, the DSCF area is quite unique not only in its extensive representation of frequencies in the second harmonic CF component but also in its amplitopic representation. The anteroventral nonprimary auditory cortex consisted of neurons broadly tuned to pure tones between 88 and 99 kHz. Neither tonotopic nor amplitopic representation was observed. Caudal to this area and near the anteroventral border of the DSCF area, a small cluster of FM-FM neurons sensitive to particular echo delays was identified. The responses of these neurons fluctuated significantly during repetitive stimulation.

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New hearing threshold measurements for pure tones under free-field listening conditions

The Journal of the Acoustical Society of America ◽

10.1121/1.400927 ◽

1991 ◽

Vol 89 (5) ◽

pp. 2400-2403 ◽

Cited By ~ 8

Author(s):

Klaus Betke

Keyword(s):

Free Field ◽

Hearing Threshold ◽

Pure Tones

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Effect of Loudspeaker Position on Differences between Earphone and Free-Field Thresholds (MAP and MAF)

Journal of Speech and Hearing Research ◽

10.1044/jshr.1704.549 ◽

1974 ◽

Vol 17 (4) ◽

pp. 549-568 ◽

Cited By ~ 17

Author(s):

Richard W. Stream ◽

Donald D. Dirks

Keyword(s):

Free Field ◽

Secondary Analysis ◽

Field Measurements ◽

Ear Canal ◽

Pure Tones ◽

Calibration Techniques

Free-field and earphone measurements were obtained from eight practiced listeners under monaural and binaural conditions to assess the hypothesis that a major source of the disparity between minimum audible pressure (MAP) and minimum audible field (MAF) speech thresholds was the position of the loudspeaker relative to the listener’s head. Free-field measurements were made at seven different loudspeaker positions (0, 30, 60, 90, 270, 300, and 330 degrees). Stimuli were spondaic words and pure tones at five octave intervals from 250 to 4000 Hz. The smallest monaural MAP/MAF difference for spondees occurred at 0° azimuth (2.7 dB) and the largest appeared at the 60° near-ear position (7.1 dB). Similar results emerged for spondaic words under binaural conditions, although the magnitude of the changes due to variations in loudspeaker position was reduced considerably from comparable monaural conditions. These results indicated that the disparities in MAP/MAF differences of previous investigations were due principally to the location of the loudspeaker. The differences between MAP and MAF thresholds were compared to other published results on ear-canal pressure measured in free field and under earphone. Secondary analysis of the data suggests that the MAP/MAF differences observed in this study may be related partially to the differences in calibration techniques used to specify the level of the signal in free field and under earphone.

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