Detection of Modulated Tones in Modulated Noise by Non-human Primates

In natural environments, many sounds are amplitude-modulated. Amplitude modulation is thought to be a signal that aids auditory object formation. A previous study of the detection of signals in noise found that when tones or noise were amplitude-modulated, the noise was a less effective masker, and detection thresholds for tones in noise were lowered. These results suggest that the detection of modulated signals in modulated noise would be enhanced. This paper describes the results of experiments investigating how detection is modified when both signal and noise were amplitude-modulated. Two monkeys (Macaca mulatta) were trained to detect amplitude-modulated tones in continuous, amplitude-modulated broadband noise. When the phase difference of otherwise similarly amplitude-modulated tones and noise were varied, detection thresholds were highest when the modulations were in phase and lowest when the modulations were anti-phase. When the depth of the modulation of tones or noise was varied, detection thresholds decreased if the modulations were anti-phase. When the modulations were in phase, increasing the depth of tone modulation caused an increase in tone detection thresholds, but increasing depth of noise modulations did not affect tone detection thresholds. Changing the modulation frequency of tone or noise caused changes in threshold that saturated at modulation frequencies higher than 20 Hz; thresholds decreased when the tone and noise modulations were in phase and decreased when they were anti-phase. The relationship between reaction times and tone level were not modified by manipulations to the nature of temporal variations in the signal or noise. The changes in behavioral threshold were consistent with a model where the brain subtracted noise from signal. These results suggest that the parameters of the modulation of signals and maskers heavily influence detection in very predictable ways. These results are consistent with some results in humans and avians and form the baseline for neurophysiological studies of mechanisms of detection in noise.     

Distortion-product otoacoustic emissions study of the noise-induced toughening effect in rats

OBJECTIVE: To compare the toughening effects in rats induced by pure tones and a broadband noise (BBN). MATERIAL AND METHODS: Sprague-Dawley female albino rats (n = 148; 8-10 weeks old) were used. Three experimental groups were established as follows. Toughening only: 38 rats, divided into 3 subgroups, were exposed to different conditioning sounds (2 and 4 kHz and a BBN of 0.25-6 kHz, respectively) at 75-85 dB sound pressure limit (SPL) for 8 h/day for 10 days. Acoustic trauma only: 54 rats, divided into 3 subgroups, were exposed to different conditioning sounds as above for 24 h at 100-110 dB SPL. Toughening plus acoustic trauma: 56 rats, divided into 3 subgroups, were exposed to different conditioning sounds as above, followed 8 h later by traumatic exposure to the conditioning sound at 110 dB SPL for 24 h. 2f1-f2 distortion-product (DP) otoacoustic emission measurements were obtained from the right ear of each animal pre-exposure, immediately post-exposure and after 8 h of the traumatic or conditioning exposure. RESULTS: In our control DPgram response, the maximum amplitude occurred at the highest frequencies (2, 3, 4, 5 and 6 kHz). No statistical differences between the control DPgram and the DP toughening (2 and 4 kHz and BBN)responses were found. Only 2 and 4 kHz frequencies induced a protective effect against traumatic sound exposures to the same frequencies, and this finding was statistically significant. CONCLUSION: The toughening phenomenon induced using 2 and 4 kHz pure tones and BBN in rats does not modify the DPgram response. Nevertheless, only 2 and 4 kHz frequencies induce a protective effect against traumatic sound exposures to the same frequencies.     

The relative role of beats and combination tones in determining the shapes of masking patterns: II. Hearing-impaired listeners

Masking patterns were measured for hearing-impaired subjects with varying degrees of hearing loss. In one set of conditions, three subjects were tested using narrowband noise ('noise') and sinusoidal ('tone') maskers and narrowband noise signals. The maskers had centre frequencies of 0.25, 0.5, 1.0 and 4.0 kHz and levels of 60, 80 and 100 dB SPL. Masking patterns for both the noise and tone maskers showed irregularities ('dips'), especially for signal frequencies up to 500 Hz above the masker frequency. The irregularities occurred for all masker levels and for all subjects for at least one masker frequency and they occurred for a relatively constant range of masker-signal frequency separations, suggesting that they were the result of beat detection. In another set of conditions, masking patterns were measured using two subjects, for a 2.0-kHz tone masker with a level of 100 dB SPL and tone and noise signals. For the tone masker alone (baseline condition), the masking patterns again exhibited prominent dips above, and sometimes below, the masker frequency. The addition of a lowpass noise to the masker, intended to mask combination tones, had little effect for one subject. For the other subject, who had near-normal absolute thresholds at low frequencies, the noise elevated thresholds for masker-signal frequency separations between 500 and 1500 Hz. For this subject, an extra tone with a frequency equal to the masker-signal frequency separation, added in place of the lowpass noise, had a very similar effect to that produced by the lowpass noise, suggesting that he was detecting a simple difference tone in the baseline condition. The addition of a pair of high-frequency tones (MDI tones - intended to reduce the detectability of beats) to the masker elevated thresholds for signal frequencies from 1500 to 2500 Hz for one subject and from 1500 to 3500 Hz for another subject. The addition of lowpass noise and MDI tones to the masker produced masking patterns very similar to those observed when the MDI tones alone were added to the masker. Overall, the results suggest that the irregularities in the masking patterns were caused mainly by the detection of beats and not by the detection of combination tones.     

Distortion-product Otoacoustic Emissions Study of the Noise-induced Toughening Effect in Rats

Objective—To compare the toughening effects in rats induced by pure tones and a broadband noise (BBN). Material and Methods—Sprague–Dawley female albino rats (n=148; 8–10 weeks old) were used. Three experimental groups were established as follows. Toughening only: 38 rats, divided into 3 subgroups, were exposed to different conditioning sounds (2 and 4 kHz and a BBN of 0.25–6 kHz, respectively) at 75–85 dB sound pressure limit (SPL) for 8 h/day for 10 days. Acoustic trauma only: 54 rats, divided into 3 subgroups, were exposed to different conditioning sounds as above for 24 h at 100–110 dB SPL. Toughening plus acoustic trauma: 56 rats, divided into 3 subgroups, were exposed to different conditioning sounds as above, followed 8 h later by traumatic exposure to the conditioning sound at 110 dB SPL for 24 h. 2f1–f2 distortion-product (DP) otoacoustic emission measurements were obtained from the right ear of each animal pre-exposure, immediately post-exposure and after 8 h of the traumatic or conditioning exposure. Results—In our control DPgram response, the maximum amplitude occurred at the highest frequencies (2, 3, 4, 5 and 6 kHz). No statistical differences between the control DPgram and the DP toughening (2 and 4 kHz and BBN) responses were found. Only 2 and 4 kHz frequencies induced a protective effect against traumatic sound exposures to the same frequencies, and this finding was statistically significant. Conclusion—The toughening phenomenon induced using 2 and 4 kHz pure tones and BBN in rats does not modify the DPgram response. Nevertheless, only 2 and 4 kHz frequencies induce a protective effect against traumatic sound exposures to the same frequencies.     

Masked threshold changes associated with angular separation of noise and signal sources

Threshold changes associated with separating a signal source and a masking white noise source from 0 degree to 90 degrees were determined for 0.5, 1 and 8 kHz pure tones and click trains. No changes occurred for the 0.5 and 1 kHz pure tones. Masked thresholds of 8 kHz pure tones and click trains decreased linearly by 9 and 13 dB respectively as angular separation was moved from 0 degree to 90 degrees. Changes in click train stimuli masked thresholds did not change significantly when the ear directed toward the masking source was occluded (11 dB drop at 90 degrees). The absence of changes at low frequencies and the similarity in magnitude of the changes in signals containing high frequency components with the responses to the monaural click trains, suggests that the threshold changes can be attributed to a head shadow effect. The casting of a sound shadow effectively lowers the noise level on the shielded side. These findings question the importance of cross-correlation techniques when detecting signals in noise.     

Detection of pure-tone amplitude modulation as a function of sensation level from 8 to 14 kHz

Amplitude modulation detection thresholds were obtained for pure-tone stimuli of 8, 10, 12 and 14 kHz at 5 dB intensity increments from 10 to 65 dB sensation level. Performance at 8 and 10 kHz was a non-monotonic function of sensation level for all four subjects with the largest difference limen measured near 30 dB sensation level and optimal performance at the highest sensation level (60 dB). Weber fractions at 12 and 14 kHz appear dependent on each subject's high frequency hearing profile; i.e., the difference limens remain high and either increase or remain essentially constant at high sensation levels only when the frequencies tested are near a particular subject's upper limit of hearing.     

Some static characteristics of the stapedial muscle reflex

The stapedial muscle reflex was investigated in a group of normal and sensorineural ears. Pure tones of 250, 500, 1 000, 2 000, and 4 000 Hz, 600 Hz bandwidth narrow-band noise centered at 500, 1 000, 2 000, and 4 000 Hz and modified wide-band noise were utilized as reflex producing stimuli. All stimuli were presented through either of 2 prototype reflex-indicator systems generating probe tones of 625 and 800 Hz developed in the Audiology Department of Sahlgren's Hospital, or a Madsen ZO-70 electro-acoustic impedance meter generating a probe tone of 220 Hz. Reflex thresholds for pure tones and noise stimuli were determined with the prototype system delivering a probe tone of 800 Hz for normal and sensorineural ears. the average reflex threshold for pure tone stimuli approximated 85 dB hearing level for normal ears and 86 dB for the ears showing a sensorineural hearing loss. No differences were found for an ascending or descending approah to threshold or by measuring reflex response for changes in amplitude or amplitude phase. The hearing level limits for normal reflex response found in this study for pure tones between 250 and 4 000 Hz ranged from a lower limit of 75 dB to an upper limit of 95 dB. Comparison of reflex thresholds for 500 and 4 000 Hz for the 3 reflex-indicator systems resulted in most sensitive thresholds for the 220 Hz probe tone system (82.5 dB) and least sensitive thresholds for the 625 Hz probe tone system (88.0 dB). Narrowband and white noise stimuli produced reflex thresholds approximately 15 dB more sensitive than for pure tones. As was true for pure tones, aspproaching threshold from above or below, or measuring reflex response as change in amplitude or amplitude phase showed no difference. Altered middle ear pressure by Valsalva and Toynbee maneuvers in normal ears elevated reflex thresholds as much as 20 dB for pressure changes exceeding ±50 mm water pressure. Studies of reflex growth for pure tone stimuli increasing or decreasing in intensity shows a steeper response pattern for normal ears than those with sensorineural hearing loss and more shallow response for both grops at 4 000 Hz than for lower frequencies. An abnormally flat reflex growth rate may be of diagnostic significance.     

Temporal modulation transfer functions in the European starling (Sturnus vulgaris): I. Psychophysical modulation detection thresholds

Temporal modulation transfer functions (TMTF) were obtained from four European starlings (Sturnus vulgaris) using a psychophysical Go/NoGo procedure combined with the method of constant stimuli. The TMTF for a continuous, broad-band noise of 55 dB SPL had a low-pass characteristic with a cut-off frequency of 123 Hz. For an 800 ms gated stimulus of the same sound-pressure level, the TMTF had the shape of a band-pass filter with the most sensitive modulation frequency at around 20 Hz. At 75 dB the band-pass shape of the TMTF was preserved, whereas at 35 dB SPL the TMTF had a low-pass characteristic. The cut-off frequency of the TMTF for continuous noise depends on which part of the spectrum carries the information on the envelope fluctuations. If only sound energy below 1 or 1.5 kHz is modulated, then the cut-off frequencies are 40 and 38 Hz, respectively. If only sound above 3 kHz carries the information on the modulation, then the cut-off frequency is 125 Hz and the shape of the TMTF is similar to that found for broadband noise. The results are discussed with respect to the coding of sinusoidal amplitude modulations by the auditory system and to different measures of time, frequency and intensity resolution in the starling.     

The relationship of pulsed, continuous, and warble extended-high frequency thresholds

The relationship among pulsed, continuous and warble tone 10, 12, 14, and 16 kHz thresholds of six normal hearing young adults was studied. It was found that pulsed and continuous tones elicited essentially equivalent thresholds at all frequencies tested. Warble tone thresholds were markedly better than unmodulated thresholds at 14 and 16 kHz. This threshold discrepancy is believed to be attributable to hearing the lowest frequency components of the warble tone when a sloping audiometric configuration exists. The audiologist is cautioned against using warble tones in extended-high frequency testing.     

The relative role of beats and combination tones in determining the shapes of masking patterns at 2 kHz: I. Normal-hearing listeners

Masking patterns for a 2-kHz sinusoidal masker at 45, 65 or 85 dB SPL were measured for three normal-hearing subjects, using a 3-AFC method with feedback (condition 1). The patterns showed distinct irregularities, particularly at the highest masker level. In condition 2, a lowpass noise was added to mask combination tones. The noise increased thresholds mainly for the 85 dB masker, for signal frequencies of 2.3-3.0 kHz. In condition 3, a pair of high-frequency tones ('modulation detection interference (MDI) tones') was used to introduce beats at the same rate as produced by the interaction of the masker and signal. Thresholds were higher than for condition 1, particularly for signal frequencies adjacent to the masker frequency. In condition 4, the lowpass noise was presented simultaneously with the MDI tones. Thresholds were well predicted as a combination of the effects of the lowpass noise and the MDI tones. In condition 5, a pair of low-frequency MDI tones was added to the masker. The thresholds had the same overall pattern as in condition 4. We conclude that the shapes of masking patterns measured using a 2-kHz masker are influenced by the detection of beats for masker-signal frequency separations up to at least 300 Hz and by the detection of combination tones for separations between 300 and 1000 Hz.     

Impact of infrasound on the human cochlea

Low-frequency tones were reported to modulate the amplitude of distortion product otoacoustic emissions (DPOAEs) indicating periodic changes of the operating point of the cochlear amplifier. The present study investigates potential differences between infrasound and low-frequency sounds in their ability to modulate human DPOAEs. DPOAEs were recorded in 12 normally hearing subjects in the presence of a biasing tone with f(B)=6Hz and a level L(B)=130dB SPL. Primary frequencies were fixed at f(1)=1.6 and f(2)=2.0kHz with fixed levels L(1)=51 and L(2)=30dB SPL. A new measure, the modulation index (MI), was devised to characterise the degree of DPOAE modulation. In subsequent measurements with biasing tones of f(B) = 12, 24 and 50Hz, L(B) was adjusted to maintain the MI as obtained individually at 6Hz. Modulation patterns lagged with increasing f(B). The necessary L(B) decreased by 12dB/octave with increasing f(B) and ran almost parallel to the published infrasound detection threshold. No signs of an abrupt change in transmission into the cochlea were found between infra- and low-frequency sounds. The results show clearly that infrasound enters the inner ear, and can alter cochlear processing.     

Equivalence and test–retest reproducibility of conventional and extended-high-frequency audiometric thresholds obtained using pure-tone and narrow-band-noise stimuli

Objectives: Extended high-frequency (EHF) audiometry is useful for evaluating ototoxic exposures and may relate to speech recognition, localisation and hearing aid benefit. There is a need to determine whether common clinical practice for EHF audiometry using tone and noise stimuli is reliable. We evaluated equivalence and compared test-retest (TRT) reproducibility for audiometric thresholds obtained using pure tones and narrowband noise (NBN) from 0.25 to 16?kHz. Design: Thresholds and test-retest reproducibility for stimuli in the conventional (0.25–6?kHz) and EHF (8–16?kHz) frequency ranges were compared in a repeated-measures design. Study sample: A total of 70 ears of adults with normal hearing. Results: Thresholds obtained using NBN were significantly lower than thresholds obtained using pure tones from 0.5 to 16?kHz, but not 0.25?kHz. Good TRT reproducibility (within 2?dB) was observed for both stimuli at all frequencies. Responses at the lower limit of the presentation range for NBN centred at 14 and 16?kHz suggest unreliability for NBN as a threshold stimulus at these frequencies. Conclusion: Thresholds in the conventional and EHF ranges showed good test-retest reproducibility, but differed between stimulus types. Care should be taken when comparing pure-tone thresholds with NBN thresholds especially at these frequencies.     

An auditory negative after-image as a human model of tinnitus

The Zwicker tone (ZT) is an auditory after-image, i.e. a tonal sensation that occurs following the presentation of notched noise. In the present study, the hypothesis that neural lateral inhibition is involved in the generation of this auditory illusion was investigated in humans through differences in perceptual detection thresholds measured following broadband noise, notched noise, and low-pass noise stimulation. The detection thresholds were measured using probe tones at several frequencies, within as well as outside the suppressed frequency range of the notched noise, and below as well as above the corner frequency of the low-pass noise. Thresholds measured after broadband noise using a sequence of four 130-ms probe tones (with a 130-ms inter-burst interval) proved to be significantly smaller that those measured using the same probe tones after notched noise at frequencies falling within the notch, but larger for frequencies on the outer edges of the noise. Thresholds measured following low-pass noise using the same sequence of probe tones were found to be smaller at frequencies slightly above the corner, but larger at lower, neighboring frequencies. This pattern of results is consistent with the hypothesis that the changes in auditory sensitivity induced by stimuli containing sharp spectral contrasts reflect lateral inhibition processes in the auditory system. The potential implications of these findings for the understanding of the mechanisms underlying the generation of auditory illusions like the ZT or tinnitus are discussed.     

Intervention Planning for Children with Communication Disorders

Modulation thresholds for sinusoidally amplitude-modulated broadband noise were obtained from normal-hearing and sensorineural hearing-impaired listeners as a function of modulation frequency. The resulting temporal modulation transfer functions (TMTFs) indicated that the impaired listeners were generally less sensitive than the normals to amplitude modulation and, unlike previously published data from normal-hearing listeners, TMTFs in the impaired listeners were level dependent: sensitivity to modulation, particularly for modulation frequencies greater than 100 Hz, decreased with decreases in level. TMTFs were also obtained with band-limited noise from the normal-hearing listeners: the noise was low-pass filtered at 1.6 kHz after modulation and was generally presented with a 1.6-kHz high-pass masker. The TMTFs in the low-pass condition were similar to the TMTFs obtained with broadband noise from the impaired listeners, suggesting that the impaired temporal processing in the hearing-impaired listeners is a result of a narrower effective, ‘internal’ bandwidth. Increment thresholds for continuous broadband and low-pass noise were obtained in conditions similar to those in which TMTFs were obtained. In general, a similar power-law relationship between modulation threshold and increment threshold was found to exist for both the normal-hearing and the hearing-impaired listeners.     

Temporal modulation transfer functions in normal-hearing and hearing-impaired listeners

Modulation thresholds for sinusoidally amplitude-modulated broadband noise were obtained from normal-hearing and sensorineural hearing-impaired listeners as a function of modulation frequency. The resulting temporal modulation transfer functions (TMTFs) indicated that the impaired listeners were generally less sensitive than the normals to amplitude modulation and, unlike previously published data from normal-hearing listeners, TMTFs in the impaired listeners were level dependent: sensitivity to modulation, particularly for modulation frequencies greater than 100 Hz, decreased with decreases in level. TMTFs were also obtained with band-limited noise from the normal-hearing listeners: the noise was low-pass filtered at 1.6 kHz after modulation and was generally presented with a 1.6-kHz high-pass masker. The TMTFs in the low-pass condition were similar to the TMTFs obtained with broadband noise from the impaired listeners, suggesting that the impaired temporal processing in the hearing-impaired listeners is a result of a narrower effective, 'internal' bandwidth. Increment thresholds for continuous broadband and low-pass noise were obtained in conditions similar to those in which TMTFs were obtained. In general, a similar power-law relationship between modulation threshold and increment threshold was found to exist for both the normal-hearing and the hearing-impaired listeners.     

Steady-state evoked responses to sinusoidally amplitude-modulated sounds recorded in man

Steady-state potentials evoked in response to binaural, sinusoidally amplitude-modulated (AM) pure tones and broadband noise signals were recorded differentially from position F4 and the ipsilateral mastoid on the human scalp. The responses elicited by the AM stimuli were approximately periodic waveforms whose energy was predominantly at the modulation frequency of the stimulus. The magnitude of responses was between 0.1 and 4 microV for modulation frequencies between 2 and 400 Hz imposed on a 1-kHz carrier signal. The magnitude of the responses increased linearly with log modulation depth for low (4 Hz) and high (80 Hz) modulation rates. The response magnitude also increased linearly with the mean intensity of the sound for intensities up to 60 dB above the subject's pure tone threshold; at higher levels the response saturated. The relationship between response magnitude and modulation frequency (the modulation transfer function) was a lowpass function for both pure tone and broadband noise carrier signals. The modulation transfer functions were similar to those obtained from human psychophysical measurements where spectral cues are either unavailable or not used by the subject. The responses also contained a significant component at the second harmonic of the modulation frequency. The magnitude of this component was greatest at modulation rates between 5 and 20 Hz. The responses elicited by ipsilateral and contralateral monaural stimulation were approximately equal in magnitude, and binaural stimulation produced a potential 30% greater than the individual monaural responses. It is suggested that the evoked response represents the entrained neural activity to temporal amplitude fluctuations, and reflects the psychophysically measured performance of the auditory system for the detection and analysis of amplitude modulation.     

Efficient stimuli for evoking auditory steady-state responses

OBJECTIVE: To compare the magnitudes of the steady-state responses evoked by several types of stimuli, and the times required to recognize these responses as significant. DESIGN: In the first two experiments, we examined auditory steady-state responses to pure tones, broadband noise and band-limited noise. The stimuli were amplitude modulated in the 75 to 100 Hz range with sinusoidal or exponential envelopes. A third experiment investigated the effects of exponential envelopes on the responses to broadband noise. The final experiment examined auditory steady-state responses evoked by rapidly presented transient stimuli, such as clicks, brief tones and brief noise-bursts. All stimuli were presented dichotically at intensities 30 to 50 dB above behavioral thresholds. The subjects were adults, who drowsed or slept during the recording sessions. RESULTS: The responses to the noise were larger than the responses to the tones. At an intensity of 32 dB nHL, the average amount of time needed to obtain significant responses for the amplitude-modulated noise was 43 sec and the maximum time was 2 minutes. The average time for pure tone stimuli was approximately 2 minutes but 25% of the responses remained undetected after 5 minutes. Combining the responses to all the frequency-specific stimuli showed results similar to using noise stimuli. Using exponential envelopes did not increase response amplitudes for noise stimuli. At 45 dB nHL, the steady-state responses to clicks and other transient stimuli were larger than responses to the broadband noise. The average time to detect steady-state responses to transient stimuli was approximately 20 sec, which was a little faster than for amplitude modulated noise. CONCLUSIONS: Auditory steady-state potentials evoked by amplitude modulated noise or transient stimuli might be useful in providing rapid and objective tests of hearing during screening procedures. Another approach might be to record responses to multiple frequency-specific stimuli and to evaluate the combined responses for a rapid indication that some hearing is present.     

Calculating the Hearing Threshold from the Stapedius Reflex Threshold for Different Sound Stimuli

The acoustic stapedius reflex depends not only on stimulus intensity, but also on stimulus frequency, as far as reflex threshold, amplitude of response and reflex decay are concerned. The stapedius reflex threshold (SRT) for wide-band noise has been proved lower than that for pure tones. Our own investigations dealt with the relation between the SRTs for pure tones, white noise, and a 24-tone mixture (one single tone at every critical band width) in normal-hearing persons and patients suffering from sensorineural hearing loss

In normal hearing, the SRT for pure tones was measures at 70-85 dB (73-105 dB SPL) above the normal-hearing threshold in a free sound-field, the SRT for white noise (average) at 46.5 dB (68.5 dB SPL) and the SRT for the 24-tone mixture at 47.0 dB (67.2 dB SPL). In 125 patients (223 ears), the difference level between the mean SRT for tones of 0.5-4 kHz and the SRT for white noise (dl 2) was correlated with the difference level between the mean SRT for tones of 0.5-4 kHz and the mean hearing threshold for tones of 0.5-4 kHz (dl 1). The functions were found to be roughly linear: dl 2 = dl 1/2.5 and dl 1 = 2.5 dl 2. Validity: 73% ～ 10 dB; 17% ～ 15 dB, and 10% ～ 20 dB. Since the dB-value of the mean SRT for pure tones of 0.5-4 kHz can be read from the audiogram, it is possible to calculate the mean hearing threshold for 0.5-4 kHz from dl 2 with equal validity: mean hearing threshold 0.5-4 kHz = SRT tones -2.5 dl 2

In cases of falling threshold curves the results become less exact and in addition to the SRT for white noise and pure tones, the SRT for two tone-mixtures was determined, namely for one low-pass noise consisting of 12 sine waves of 100-1 600 Hz, and for one high-pass noise consisting of 12 sine waves of 1.8-13.5 kHz. In normal hearing, the dl 2 for the low-pass noise is ～ 15 dB and for the high-pass noise ～ 20 dB (3:4). In falling audiograms, the dl 2 for the high-pass noise was found to be equal or lower than that for the low-pass noise. Investigations in sleeping children indicated that these relations were practically unchanged

A correlation of the free-field findings with the loudness calculated by Zwicker's procedure showed that the triggering of the stapedius reflex by wide-band sounds is due to the centrally summated loudness and not the SPL     

Laser Doppler measurements of cochlear blood flow during loud sound exposure in the guinea pig

This investigation examined the effects of loud sound of different frequencies and intensities on cochlear blood flow as measured by the laser Doppler flowmeter. Cochlear blood flow was measured in anesthetized guinea pigs during a 1 h exposure to either a 2, 4, or 12 kHz pure tone or high-pass noise (10-40 kHz) at 90, 103, or 110 dB SPL. Cochlear function was assessed using the compound action potential audiogram before and after exposure. There was no change in blood flow in the second turn with a 2, 4, or 12 kHz tone but there was a significant (P less than 0.05) decline in flow in the first cochlear turn at the end of either the 12 kHz tone or high-pass noise exposure at 103 and 110 dB SPL. There were elevations in the thresholds of the cochlear compound action potential after all but the 90 dB exposures to 12 kHz or high-pass noise. No such changes were observed in blood flow or electrophysiology in control animals. These findings demonstrate that there is a small but significant decline in cochlear blood flow with high intensity sound exposure. However, the relationship between this change in blood flow and the development of cochlear damage is unclear.     

The influence of moderate-intensity noise on the compound action potential evoked by tone bursts in the guinea pig, Cavia porcellus

Noise-induced changes in the compound action potential (CAP) evoked by tone bursts in the frequency range 0.5-24 kHz were studied in 15 pigmented guinea pigs by means of chronically implanted electrodes positioned near the round window. The animals were exposed for 120 h to continuous pink noise at the intensities 80, 90 and 100 dB SPL. During the exposure period, all the animals exhibited an exponential rise in CAP threshold, leveling out after 24-72 h (asymptotic threshold shift, ATS). The largest threshold shifts were recorded during exposure to 100 dB SPL, for frequencies in the range 8-12 kHz. In the recovery phase, after the end of noise exposure, the threshold to tones at all frequencies tested fell exponentially, reaching the original level in about 72 h in all cases.