On the loudness of complex stimuli and its relationship to cochlear excitation

Two experiments were conducted to reexamine the relation between loudness and the bandwidth of complex stimuli. In experiment 1, experimental stimuli consisting of a 2000-Hz pure tone and ten computer-generated multitonal complexes ranging in bandwidth from 0.26-3.16 oct, logarithmically centered at 2000 Hz, were matched in loudness to a 2000-Hz comparison pure tone presented at 90, 70, and 30 dB SPL. The SPL of the experimental stimuli required for equal loudness was linearly related to bandwidth (in octaves) for each of the three comparison stimulus levels. In experiment 2, the loudness behavior of narrow-bandwidth stimuli within the previously reported critical band region was examined. The results indicated a linear relation similar to that obtained in experiment 1. These results are consistent with an auditory filter concept in which frequency is continuously encoded along the basilar membrane and in which loudness of complex stimuli is linearly related to area of excitation.     

On the relation between the growth of loudness and the discrimination of intensity for pure tones

The intensity jnd is often assumed to depend on the slope of the loudness function. One way to test this assumption is to measure the jnd for a sound that falls on distinctly different loudness functions. Two such functions were generated by presenting a 1000-Hz tone in narrow-band noise (925-1080 Hz) set at 70 dB SPL and in wideband noise (75-9600 Hz) set at 80 dB SPL. Over a range from near threshold to about 75 dB SPL, the loudness function for the tone is much steeper in the narrow-band noise than in the wideband noise. At 72 dB SPL, where the two loudness curves cross, the tone's jnd was measured in each noise by a block up-down two-interval forced-choice procedure. Despite the differences in slope (and in sensation level), the jnd (delta I/I) is nearly the same in the two noises, 0.22 in narrow-band noise and 0.20 in wideband noise. The mean value of 0.21 is close to the value of 0.25 interpolated from Jesteadt et al. [J. Acoust. Soc. Am. 61, 169-176 (1977)] for a 1000-Hz tone that had the same loudness in quiet as did our 72-dB tone in noise, but lay on a loudness function with a much lower slope. These and other data demonstrate that intensity discrimination for pure tones is unrelated to the slope of the loudness function.     

Loudness reduction and adaptation induced by a contralateral tone

An intermittent tone in one ear may induce a large decline in the loudness of a continuous tone in the contralateral ear [Botte et al., J. Acoust. Soc. Am. 72, 727-739 (1982)]. To uncover the basis for this induced loudness adaptation, the method of successive magnitude estimations was used to measure the loudness of a test tone in one ear during and after a single presentation of a brief inducer tone in the contralateral ear. Duration and frequency of the inducer were varied. The frequency of the test tone was set at 500, 1000, or 3000 Hz. Both inducer and test tones were at 60 dB SPL. When the inducer lasted 5 s or more and was at the same frequency as the test tone, the loudness of the test tone was reduced by 80% to 100% while the inducer was on. As the inducer frequency moved away from the test tone, the loudness reduction declined gradually except for a more marked drop at the point where the frequency separation exceeded the critical bandwidth. Loudness remained depressed after the inducer went off. Additional measurements showed that the amount of loudness reduction corresponded closely to the measured movement of the inducer's sound image away from the center of the listener's head (decentralization).     

Level discrimination of frozen and random noise

This paper examines how the difference limen for level, delta L, is affected by stimulus bandwidth and variability. The delta L's were measured in three normal listeners using an adaptive two-interval, forced-choice procedure. The 30-ms stimuli were a 3-kHz tone and nine noise bands with half-power bandwidths ranging from 50 Hz-12 kHz. Except for the 12-kHz bandwidth, which was a low-pass noise, the noise bands were centered at 3 kHz. The delta L's were measured for both frozen and random noises presented at 30, 60, or 90 dB SPL overall. For frozen noises, the same sample of noise was presented throughout a block of 50 trials; for the random noises, different samples of noise were used in each interval of the trials. Results show that the delta L's are higher for random than for frozen noises at narrow bandwidths, but not at wide bandwidths. The delta L's for frozen narrow-band noises decrease with increasing level and are similar to those for the pure tone, whereas the delta L's for wideband noises are only slightly smaller at 90 than at 30 dB SPL. An unexpected finding is that the delta L's are larger at 60 than at 30 dB SPL for both frozen and random noises with bandwidths greater than one critical band. The effect of bandwidth varies with level: The delta L's decrease with increasing bandwidth at low levels, but are nearly independent of bandwidth at 90 dB SPL. The interaction of bandwidth and level is consistent with the multiband excitation-pattern model, but the nonmonotonic behavior of delta L as a function of level suggests modifications to the model.     

Why can a decrease in dB(A) produce an increase in loudness?

Loudness measured by the method of absolute magnitude estimation is compared to loudness calculated in accordance with ISO 532 B (International Organization for Standardization, Geneva, 1966). The measured and calculated loudness functions exhibit a similar pattern of loudness growth. Both measured and calculated loudness of a complex sound composed of a 1000-Hz tone and broadband noise is a nonmonotonic function of the overall SPL of the complex. The nonmonotonic loudness-growth pattern holds over a 30-dB range from 73.5 to 103.5. To facilitate understanding of the results, a single cycle of data is analyzed in detail. The analysis shows that loudness patterns produced in the auditory system by the tone-noise complex can account for the observed effects. Moreover, they show that the A-weighting and the loudness of the complex are negatively related. This inverse relation means that the A-weighted SPL is an inappropriate and misleading indicator of the loudness of sound combinations with heterogeneous spectral envelopes. Consequently, its suitability for noise control is diminished. A loudness meter that combines the spectral shapes of different sounds to produce an overall perceived magnitude offers greater promise.     

The influence of middle-ear muscle contraction on auditory threshold for selected pure tones

The influence of middle-ear muscle (MEM) contraction on auditory threshold has been measured for pure tones of 0.25, 0.5, and 1.5 kHz. The reflex-activating signal was a 3-kHz pure tone. Signal paradigms were chosen to reduce or eliminate the effects of binaural loudness summation, contralateral direct masking, and contralateral remote and backward masking effects, and to maximize the influence of MEM contraction. Results indicate that under no condition was behavioral threshold affected by the MEM contraction induced using a pure-tone stimulus of 3 kHz, 105 dB SPL.     

Cortical responses to the 2f1-f2 combination tone measured indirectly using magnetoencephalography

The simultaneous presentation of two tones with frequencies f(1) and f(2) causes the perception of several combination tones in addition to the original tones. The most prominent of these are at frequencies f(2)-f(1) and 2f(1)-f(2). This study measured human physiological responses to the 2f(1)-f(2) combination tone at 500 Hz caused by tones of 750 and 1000 Hz with intensities of 65 and 55 dB SPL, respectively. Responses were measured from the cochlea using the distortion product otoacoustic emission (DPOAE), and from the auditory cortex using the 40-Hz steady-state magnetoencephalographic (MEG) response. The perceptual response was assessed by having the participant adjust a probe tone to cause maximal beating ("best-beats") with the perceived combination tone. The cortical response to the combination tone was evaluated in two ways: first by presenting a probe tone with a frequency of 460 Hz at the perceptual best-beats level, resulting in a 40-Hz response because of interaction with the combination tone at 500 Hz, and second by simultaneously presenting two f(1) and f(2) pairs that caused combination tones that would themselves beat at 40 Hz. The 2f(1)-f(2) DPOAE in the external auditory canal had a level of 2.6 (s.d. 12.1) dB SPL. The 40-Hz MEG response in the contralateral cortex had a magnitude of 0.39 (s.d. 0.1) nA m. The perceived level of the combination tone was 44.8 (s.d. 11.3) dB SPL. There were no significant correlations between these measurements. These results indicate that physiological responses to the 2f(1)-f(2) combination tone occur in the human auditory system all the way from the cochlea to the primary auditory cortex. The perceived magnitude of the combination tone is not determined by the measured physiological response at either the cochlea or the cortex.     

Subjective loudness level measurements and equal loudness contours in a bottlenose dolphin (Tursiops truncatus)

Loudness level measurements in human listeners are straightforward; however, it is difficult to convey the concepts of loudness matching or loudness comparison to (non-human) animals. For this reason, prior studies have relied upon objective measurements, such as response latency, to estimate equal loudness contours in animals. In this study, a bottlenose dolphin was trained to perform a loudness comparison test, where the listener indicates which of two sequential tones is louder. To enable reward of the dolphin, most trials featured tones with identical or similar frequencies, but relatively large sound pressure level differences, so that the loudness relationship was known. A relatively small percentage of trials were "probe" trials, with tone pairs whose loudness relationship was not known. Responses to the probe trials were used to construct psychometric functions describing the loudness relationship between a tone at a particular frequency and sound pressure level and that of a reference tone at 10 kHz with a sound pressure level of 90, 105, or 115 dB re 1 μPa. The loudness relationships were then used to construct equal loudness contours and auditory weighting functions that can be used to predict the frequency-dependent effects of noise on odontocetes.     

Perceptual interactions in the loudness of combined auditory and vibrotactile stimuli

The loudness of auditory (A), tactile (T), and auditory-tactile (A+T) stimuli was measured at supra-threshold levels. Auditory stimuli were pure tones presented binaurally through headphones; tactile stimuli were sinusoids delivered through a single-channel vibrator to the left middle fingertip. All stimuli were presented together with a broadband auditory noise. The A and T stimuli were presented at levels that were matched in loudness to that of the 200-Hz auditory tone at 25 dB sensation level. The 200-Hz auditory tone was then matched in loudness to various combinations of auditory and tactile stimuli (A+T), and purely auditory stimuli (A+A). The results indicate that the matched intensity of the 200-Hz auditory tone is less when the A+T and A+A stimuli are close together in frequency than when they are separated by an octave or more. This suggests that A+T integration may operate in a manner similar to that found in auditory critical band studies, further supporting a strong frequency relationship between the auditory and somatosensory systems.     

Critical modulation frequency based on detection of AM versus FM tones

The ratios between the modulation index (eta) for just noticeable FM of a sinusoidally modulated pure tone and the degree of modulation (m) for just noticeable AM at the same carrier and the same modulation frequency were measured at carrier frequencies of 0.125, 0.25, 0.5, 1, 2, 4, and 8 kHz. Signal levels were 20 dB SL and 50 dB SPL or 80 dB SPL. At low modulation frequencies, for example, 8 Hz, AM and FM elicit very different auditory sensations (i.e., a fluctuation in loudness or pitch, respectively). In this case, eta and m show different values for just noticeable modulation. Since both stimuli have almost equal amplitude spectra if eta equals m (m less than 0.3), the difference in detection thresholds reflects differences in the phase relation between carrier and sidebands in AM and FM. With increasing modulation frequency, the eta-m ratio decreases and reaches unity at a modulation frequency called the "critical modulation frequency" (CMF). At modulation frequencies above the CMF, the same modulation thresholds are obtained for AM and FM. Therefore, it can be concluded that the difference in phase between the two types of stimuli is not perceived in this range. At center frequencies below 1 kHz, where phase errors caused by headphones and ear canal presumably are small, the CMF is useful in estimating critical bandwidth.     

Louder sounds can produce less forward masking: effects of component phase in complex tones

The influence of the degree of envelope modulation and periodicity on the loudness and effectiveness of sounds as forward maskers was investigated. In the first experiment, listeners matched the loudness of complex tones and noise. The tones had a fundamental frequency (F0) of 62.5 or 250 Hz and were filtered into a frequency range from the 10th harmonic to 5000 Hz. The Gaussian noise was filtered in the same way. The components of the complex tones were added either in cosine phase (CPH), giving a large crest factor, or in random phase (RPH), giving a smaller crest factor. For each F0, subjects matched the loudness between all possible stimulus pairs. Six different levels of the fixed stimulus were used, ranging from about 30 dB SPL to about 80 dB SPL in 10-dB steps. Results showed that, at a given overall level, the CPH and the RPH tones were louder than the noise, and that the CPH tone was louder than the RPH tone. The difference in loudness was larger at medium than at low levels and was only slightly reduced by the addition of a noise intended to mask combination tones. The differences in loudness were slightly smaller for the higher than for the lower F0. In the second experiment, the stimuli with the lower F0s were used as forward maskers of a 20-ms sinusoid, presented at various frequencies within the spectral range of the maskers. Results showed that the CPH tone was the least effective forward masker, even though it was the loudest. The differences in effectiveness as forward maskers depended on masker level and signal frequency; in order to produce equal masking, the level of the CPH tone had to be up to 35 dB above that of the RPH tone and the noise. The implications of these results for models of loudness are discussed and a model is presented based on neural activity patterns in the auditory nerve; this predicts the general pattern of loudness matches. It is suggested that the effects observed in the experiments may have been influenced by two factors: cochlear compression and suppression.     

Dependence of loudness growth on skirts of excitation patterns

Over a range of 50 dB, the loudness of a 100-Hz tone was measured in the presence of a broadband noise with a low-frequency cutoff at 200 Hz. The noise was varied in intensity along along with the tone so that the signal-to-noise ratio remained constant at either 0 or--10 dB. Listeners judged the loudness of the tone by loudness matching, magnitude estimation, and magnitude production. The noise markedly decreased the tone's rate of loudness growth but not the range over which loudness grows. The overall decrease in steepness of the 100-Hz loudness function was greater than that previously reported at higher frequencies. It is hypothesized that the decrease was greater because the spread of excitation at 100 Hz was more effectively contained than at higher frequencies. Support for this hypothesis is given by measures of intensity discrimination at 100 Hz.     

Loudness recalibration as a function of level

Recent research on loudness has focused on contextual effects on loudness, both assimilation and recalibration. The current experiments examined loudness recalibration [Marks, J. Exp. Psychol. 20, 382-396 (1994)]. In the first experiment, an adaptive tracking procedure was used to measure loudness recalibration as a function of standard- and recalibration-tone level. The standard-tone frequencies were 500 and 2500 Hz and the levels were 80-, 70-, 60-, and 40-dB SPL, and threshold. Seventeen dB of loudness recalibration was obtained (combined over both frequencies) in the 60-dB SPL condition. This amount of loudness recalibration, while substantial, is still less than that obtained by Marks (22 dB), using the method of paired comparisons. The second experiment sought to duplicate Marks' earlier experiment [Marks, J. Exp. Psychol. 20, 382-396 (1994), experiment 2]. The results of this experiment (21 dB) were almost identical to those obtained by Marks. The results of experiment 1 indicate that loudness recalibration is maximum when the recalibration tone is moderately louder than the subsequent standard tones. Relatively little loudness recalibration is exhibited when the standard-tone level equals the recalibration-tone level. In addition, there is no loudness recalibration at threshold. The tracking procedure also identified that the onset of loudness recalibration is very rapid. The difference between the maximum loudness recalibration obtained at each frequency (11 dB at 500 Hz, 6 dB at 2500 Hz) suggests that loudness recalibration is dependent upon the frequency of the standard tone.     

Vocal responses to unanticipated perturbations in voice loudness feedback: an automatic mechanism for stabilizing voice amplitude

The present study tested whether subjects respond to unanticipated short perturbations in voice loudness feedback with compensatory responses in voice amplitude. The role of stimulus magnitude (+/- 1,3 vs 6 dB SPL), stimulus direction (up vs down), and the ongoing voice amplitude level (normal vs soft) were compared across compensations. Subjects responded to perturbations in voice loudness feedback with a compensatory change in voice amplitude 76% of the time. Mean latency of amplitude compensation was 157 ms. Mean response magnitudes were smallest for 1-dB stimulus perturbations (0.75 dB) and greatest for 6-dB conditions (0.98 dB). However, expressed as gain, responses for 1-dB perturbations were largest and almost approached 1.0. Response magnitudes were larger for the soft voice amplitude condition compared to the normal voice amplitude condition. A mathematical model of the audio-vocal system captured the main features of the compensations. Previous research has demonstrated that subjects can respond to an unanticipated perturbation in voice pitch feedback with an automatic compensatory response in voice fundamental frequency. Data from the present study suggest that voice loudness feedback can be used in a similar manner to monitor and stabilize voice amplitude around a desired loudness level.     

Spectral loudness summation as a function of duration

Loudness was measured as a function of signal bandwidth for 10-, 100-, and 1000-ms-long signals. The test and reference signals were bandpass-filtered noise spectrally centered at 2 kHz. The bandwidth of the test signal was varied from 200 to 6400 Hz. The reference signal had a bandwidth of 3200 Hz. The reference levels were 45, 55, and 65 dB SPL. The level to produce equal loudness was measured with an adaptive, two-interval, two-alternative forced-choice procedure. A loudness matching procedure was used, where the tracks for all signal pairs to be compared were interleaved. Mean results for nine normal-hearing subjects showed that the magnitude of spectral loudness summation depends on signal duration. For all reference levels, a 6- to 8-dB larger level difference between equally loud signals with the smallest (delta f = 200 Hz) and largest (delta f = 6400 Hz) bandwidth is found for 10-ms-long signals than for the 1000-ms-long signals. The duration effect slightly decreases with increasing reference loudness. As a consequence, loudness models should include a duration-dependent compression stage. Alternatively, if a fixed loudness ratio between signals of different duration is assumed, this loudness ratio should depend on the signal spectrum.     

Interactions between test- and inducer-tone durations in induced loudness reduction

A tone usually declines in loudness when preceded by a more intense inducer tone. This phenomenon is called "loudness recalibration" or "induced loudness reduction" (ILR). The present study investigates how ILR depends on level, loudness, and duration. A 2AFC procedure was used to obtain loudness matches between 2500-Hz comparison tones and 500-Hz test tones at 60 and 70 dB SPL, presented with and without preceding 500-Hz inducer tones. For 200-ms test and comparison tones, the amount of ILR did not depend on inducer level (set at 80 dB SPL and above), but ILR was greater with 200- than with 5-ms inducers, even when both were equally loud. For 5-ms tones, ILR was as great with 5- as with 200-ms inducers and about as great as when test and inducer tones both lasted 200 ms. These results suggest that (1) neither the loudness nor the SPL of the inducer alone governs ILR, and (2) inducer duration must equal or exceed test-tone duration to yield maximal amounts of ILR. Further analysis indicates that the efferent system may be partly responsible for ILR of 200-ms test tones, but is unlikely to account for ILR of 5-ms tones.     

The loudness of sounds that increase and decrease continuously in level

A sound at a low level is heard as much softer after having decreased continuously from higher levels than if presented after a period of silence at that same low level. Canévet [Acustica 61, 256-264 (1986)] demonstrated this phenomenon for a tone that (1) decreased from 65 to 20 dB in 180 s; he also presented a tone that (2) increased from 20 dB, or (3) was presented as pairs of bursts at various levels in random order. Below about 40 dB, loudness changed most rapidly in the decreasing condition so that, at 20 dB, the tone was judged ten times softer than in conditions (2) and (3). In the present experiments, magnitude estimation was used to examine the possible role of judgmental biases and adaptation in this rapid loudness decline, which we call decruitment. Results show that decruitment did not come about because subjects made many successive loudness judgments; loudness declined as much when a tone was judged only twice, at the beginning and end of its 180-s decrease. In contrast, interrupting the decreasing tone so that it was heard only at 70 dB and 160 s later at 30 dB greatly diminished the decruitment. Similarly, pairs of 500-ms tone bursts presented at successively lower levels instead of continously decreasing did not show decruitment, suggesting that sequential biases are irrelevant. The likely cause of decruitment is sensory adaptation.     

Differential responses to acoustic damage and furosemide in auditory brainstem and otoacoustic emission measures

Characteristics of distortion product otoacoustic emissions (DPOAEs) and auditory brainstem responses (ABRs) were measured in Mongolian gerbil before and after the introduction of two different auditory dysfunctions: (1) acoustic damage with a high-intensity tone, or (2) furosemide intoxication. The goal was to find emission parameters and measures that best differentiated between the two dysfunctions, e.g., at a given ABR threshold elevation. Emission input-output or "growth" functions were used (frequencies f1 and f2, f2/f1 = 1.21) with equal levels, L1 = L2, and unequal levels, with L1 = L2 + 20 dB. The best parametric choice was found to be unequal stimulus levels, and the best measure was found to be the change in the emission threshold level, delta x. The emission threshold was defined as the stimulus level required to reach a criterion emission amplitude, in this case -10 dB SPL. (The next best measure was the change in emission amplitude at high stimulus levels, specifically that measured at L1 x L2 = 90 x 70 dB SPL.) For an ABR threshold shift of 20 dB or more, there was essentially no overlap in the emission threshold measures for the two conditions, sound damage or furosemide. The dividing line between the two distributions increased slowly with the change in ABR threshold, delta ABR, and was given by delta x(t) = 0.6 delta ABR + 8 dB. For a given delta ABR, if the shift in emission threshold was more than the calculated dividing line value, delta x(t), the auditory dysfunction was due to acoustic damage, if less, it was due to furosemide.     

Equal autophonic level curves under different room acoustics conditions

The indirect auditory feedback from one's own voice arises from sound reflections at the room boundaries or from sound reinforcement systems. The relative variations of indirect auditory feedback are quantified through room acoustic parameters such as the room gain and the voice support, rather than the reverberation time. Fourteen subjects matched the loudness level of their own voice (the autophonic level) to that of a constant and external reference sound, under different synthesized room acoustics conditions. The matching voice levels are used to build a set of equal autophonic level curves. These curves give an indication of the amount of variation in voice level induced by the acoustic environment as a consequence of the sidetone compensation or Lombard effect. In the range of typical rooms for speech, the variations in overall voice level that result in a constant autophonic level are on the order of 2 dB, and more than 3 dB in the 4 kHz octave band. By comparison of these curves with previous studies, it is shown that talkers use acoustic cues other than loudness to adjust their voices when speaking in different rooms.     

Forward-masked monaural and interaural intensity discrimination

Intensity-discrimination thresholds were measured for a 25-ms, 6-kHz pure tone for pedestal levels from 40 to 90 dB sound pressure level (SPL) with and without a forward masker (100-ms narrowband Gaussian noise, N(0)=70 dB). When the masker was present, the masker and probe were separated by 100 ms of silence. Unmasked and masked thresholds were measured in a two-interval monaural procedure and, separately, in a single-interval interaural procedure in which the pedestal and incremented pedestals were presented simultaneously to opposite ears. While the monaural thresholds were elevated markedly by the forward masker for mid-level pedestals, interaural thresholds were nearly unaffected by the masker across pedestal levels. The results argue against the notion that the monaural elevation in forward-masked thresholds is due to degraded encoding of intensity information at early stages of auditory processing.