Bertram Scharf

Focused auditory attention and frequency selectivity

The probe-signal method (Greenberg & Larkin, 1968) was used to determine the percentage of trials in which unpracticed subjects detected (two-interval, forced-choice) a soft, expected sound as compared with an unexpected sound. Pure tones at or near an expected frequency were detected in about 90% of the trials. Tones more than one-half critical band away were detected near chance (50%). Complex sounds (a band of noise or a multitone complex) were detected better if they were inside the same critical band as the expected signal than if they were outside the band. A signal that differed spectrally from the expected sound was not detected even though it had the same low pitch, based on a common fundamental frequency. The results may mean that under some conditions focused attention alters sensitivity in the auditory system.

Dichotic Summation of Loudness

The over‐all loudness of a pair of equally loud tones presented dichotically, a different frequency to each ear, is independent of the frequency separation between the two tones. This rule applies over the whole range of frequency separations tested, from 0 Hz to several thousand hertz; hence, dichotic tone pairs are just as loud as a binaural pair (same frequency to each ear). Both binaural and dichotic pairs, however, are generally less than twice as loud as a component presented by itself. The over‐all loudness is invariant even though at narrow frequency separations the components of a dichotic pair interfere with each other so that each component is softer when heard in the presence of the other than when heard by itself. At wide separations, where two distinct auditory images are reported, the loudness of a component is the same whether presented with or without the contralateral tone.

Model of Loudness Summation Applied to Impaired Ears

The loudness of complex sounds composed of three or four pure tones was measured as a function of the over‐all spacing ΔF between the lowest and highest components. The measured relation between loudness and ΔF was compared to calculations from Zwickers model of loudness summation. In eight ears with a conductive impairment, loudness summated normally and as predicted by the model; loudness remained approximately constant as a function of ΔF near threshold and increased with ΔF beyond the critical band at higher sensation levels. In eight ears with a cochlear impairment, loudness did not change with ΔF at any tested sensation level. This invariance of loudness was not predicted by tire model nor was it found in six normal ears tested in the presence of a 90‐dB uniform masking noise intended to simulate the cochlear impairment. Under masking, loudness summated as predicted. The unexpected results in cochlear pathology were ascribed, tentatively, to a possible widening of the critical band.

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 tones 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.

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.

Critical Bands and the Loudness of Complex Sounds Near Threshold

The loudness of multitone complexes and bands of white noise was measured as a function of band width at levels between 5 and 35 db above threshold. When account is taken of the changes in the loudness that are due to the nonuniformity of the ears sensitivity at low levels, the spreading of energy appears to have the same general effect on the loudness of both line spectra and continuous spectra centered at frequencies as low as 400 cps and as high as 5000 cps. At 5 db SL the spreading of energy does not increase the loudness, and may even decrease it. Above about 10 db SL, the spreading of energy in a complex sound over more than a critical band increases the loudness. As the sensation level is raised up to 35 db, the spreading of energy becomes more and more advantageous to loudness. The critical band defines the limits within which the spreading of energy leaves the loudness of a complex sound unchanged. The widths of the critical bands that were measured in these experiments on loudness summation bel...

Loudness enhancement: Induced loudness reduction in disguise? (L)

Two opposite sequential loudness effects concern the effect of a stronger Tone 1 on the loudness of a subsequent weaker Tone 2, as assessed by loudness matches with Tone 3. Loudness enhancement is reported when Tone 1 precedes Tone 2 by 50 to 100 ms. Loudness recalibration (or induced loudness reduction) is obtained for delays of about 1 s. This letter argues that what appears as an enhancement of Tone 2’s loudness is, in fact, an induced reduction of Tone 3’s loudness, which occurs because Tones 1 and 3 are at the same frequency. Preliminary experiments support this analysis.

Loudness Summation and Spectrum Shape

The loudness of three‐tone complexes centered at 2000 cps was studied as a function of the intensity relations among the three components. Two types of spectra were investigated: (1) peaked spectra, in which the intensity difference between the middle component and the less intense side bands was varied from 0 to 35 rib, and (2) sloped spectra, in which the slope of the connected heights of the three spectral lines of the complex was varied from 0 db per octave to + and −∞. Generally, three‐tone complex whose over‐all spacing was greater than a critical band was loudest, at a given SPL ,when its spectrum was flat, i.e., when the components were equally intense and, more important, equally loud. Complexes whose over‐all spacing was slightly less than a critical band changed little in loudness as a function of spectrum shape. The results are related to the assumption that the loudness of a complex sound is the sum of the loudnesses of the component critical bands.

Loudness of Complex Sounds as a Function of the Number of Components

The loudness of complex sounds was studied as a function of the number of components. Complexes of two, three, four, and eight tones, and a band of white noise were matched in loudness to a 1500‐cy tone. The over‐all spacing, ΔF, between the lowest and highest components of these stimuli was held constant at either 175, 1600, or 3400 cps. All the complexes were centered around 1500 cps when ΔF was either 175 or 1600 cps. At each of the four levels tested (25, 50, 75, and 90 db SPL), loudness remained essentially unchanged when only the number of components within a complex was varied. This invariance of loudness occurs even though inhibition may be greater within the complexes that contain a larger number of components. It is suggested that the increased inhibition may be offset by greater loudness summation when the energy is more evenly distributed within the complexes.

Loudness adaptation and excitation patterns: Effects of frequency and level

Simple loudness adaptation for pure tones was measured at frequencies from 0.125 to 16 kHz and at sensation levels from 5 to 60 dB. Sixteen young listeners with normal hearing participated in four experiments. Most of the loudness measurements were obtained by the method of successive magnitude estimation; some were also obtained by loudness matching. The two indices of loudness adaptation gave similar results. At all frequencies, loudness adaptation increased as sensation level decreased. After 6 min, average loudness declined at most frequencies by about 20% at 40-dB sensation level (SL) and by between 70% and almost 100% at 5-dB SL. Adaptation also increased with increasing frequency, and was especially marked at 16 kHz, where loudness declined more than 60% at a sensation level as high as 40 dB. Most of the adaptation occurred usually within the first 3 min of exposure, but loudness continued to diminish at a slower rate up to around 6 min. The dependence of loudness adaptation on frequency and level can be largely accounted for by the restricted-excitation-pattern hypothesis. Adaptation is assumed to take place when excitation is restricted to a narrow region of the cochlea. This hypothesis is supported by a quantitative analysis based on excitation patterns derived from measurements of masking.