Method for the selection of sentence materials for efficient measurement of the speech reception threshold.

A method is described to select sentence materials for efficient measurement of the speech reception threshold (SRT). The first part of the paper addresses the creation of the sentence materials, the recording procedure, and a listening experiment to evaluate the new speech materials. The result is a set of 1272 sentences, where every sentence has been uttered by two male and two female speakers. In the second part of the paper, a method is described to select subsets with properties that are desired for an efficient measurement of the SRT. For two speakers, this method has been applied to obtain two subsets for measurement of the SRT in stationary noise with the long-term average spectrum of speech. Lastly, a listening experiment has been conducted where the two subsets (each comprising 39 lists of 13 sentences each) are directly compared to the existing sets of Plomp and Mimpen [Audiology 18, 43-52 (1979)] and Smoorenburg [J. Acoust. Soc. Am. 91, 421-437 (1992)]. One of the outcomes is that the newly developed sets can be considered as equivalent to these existing sets.

Preference judgments of artificial processed and hearing-aid transduced speech.

In order to assess the relative importance of various signal processing algorithms and distortions on hearing-aid preference, male and female speech was manipulated in a number of ways and subsequently presented to normal-hearing and hearing-impaired subjects (the latter having a mild sensorineural high-frequency hearing loss). Signal manipulations were artificial (e.g., filtering, compression, peak clipping, or adding noise) or were actual dummy-head recordings of five different hearing aids. Listeners judged the sounds in a pairwise-comparison format. Their task was to indicate the "hearing aid" they would prefer assuming they had to wear it all day. The data were analyzed with multidimensional scaling techniques; Principal Components Analysis revealed that the first two dimensions on which preference judgments were based, can be interpreted as (1) intelligibility or clarity, and (2) distinction between signal distortion and added background distortion. Furthermore, the results showed that normal-hearing subjects generally preferred the original signal, whereas hearing-impaired subjects were inclined to choose the signals with a high-frequency emphasis. Severe band-pass filtering or low-frequency emphasis were disliked, as was to be expected. Surprisingly, however, a soft background noise (S/N ratio of 25 dB) was often among the least preferred of all signals. The differences in preference between the five hearing aids were small, but consistent. For hearing-impaired subjects, hearing-aid ordering could be accounted for by the amount of low-frequency cutoff; for normal-hearing subjects both high- and low-frequency cutoff played a role. Results of a retest experiment with normal-hearing subjects, about one year later, showed that subjects' criteria remain remarkably stable.

Annoyance caused by sounds of wheeled and tracked vehicles.

In a laboratory experiment, the relationship between the annoyance caused by sounds of tracked and wheeled vehicles was investigated. Sounds of various vehicles, ranging from a main battle tank (MBT) to a passenger car, were recorded at several distances. Subjects were presented with these sounds and were asked to judge the annoyance if they were exposed to them at home on a regular basis. Overall, sounds emanated from military tracked vehicles were judged to be less annoying than civil passenger cars, provided that the indoor A-weighted sound-exposure levels (ASELs) were the same. It should be noted that receiver distance and operating conditions were not specifically matched for military and civil vehicles but were chosen to represent typical ranges of values found in practice. For steady-speed conditions, the sounds were judged to be equally annoying if the (indoor) ASELs of the tracked armored personnel carrier (APC) and the MBT exceeded that of a passenger car by 5 and 10 dB, respectively. In specific driving conditions such as alternate accelerations and decelerations, however, these differences were smaller. For ASELs below 55 dB, passenger cars were found to be more annoying than heavier wheeled vehicles, such as a bus or a truck with a trailer. The data indicate that the difference between the high-frequency part and the low-frequency part of the spectrum might play a role in the annoyance. With respect to noise-zoning procedures it is highly interesting that the outdoor ASEL could predict the annoyance, as rated indoors, better than could any indoor noise measure. The 5- and 10-dB bonuses to the indoor level of the APC and the MBT, respectively, were reduced to 1 and 3 dB if the dose was measured outdoors.

Discrimination of changes in the spectral shape of noise bands.

Discrimination experiments were performed for a change in the spectral shape of noise bands. The subject's task was to discriminate noise bands with a positive spectral slope from those with a negative spectral slope. Thresholds were measured at several bandwidths and center frequencies, as well as for several noise samples. Experiments were performed while roving the overall intensity. At a fixed center frequency of 1 kHz, sensitivity was best for bandwidths of 3-6 semitones (ST). At larger bandwidths, thresholds increased only slowly. At a fixed bandwidth of 1 ST, threshold hardly changed as a function of the center frequency. At a fixed bandwidth of 58 Hz, threshold was lowest near 500-1000 Hz. Model calculations show that the EWAIF model [Feth, Percept. Psychophys. 15, 375-378 (1974)] can account for the present results if the signal's bandwidth does not exceed 1 ST. The IWAIF model [Anantharaman et al., J. Acoust. Soc. Am. 94, 723-729 (1993)] can account for the present results only if the signal's bandwidth is smaller than 1 ST but larger than about 25 Hz. Results obtained with broadband signals could be described only qualitatively with the multichannel model [Durlach et al., J. Acoust. Soc. Am. 80, 63-72 (1986)]. Then, the model needs the assumption that either the output of the different frequency bands cannot be optimally combined, or that only two bands are used in the discrimination process. The present results are compared with those obtained with two-tone complexes measured under identical conditions [Versfeld and Houtsma, J. Acoust. Soc. Am. 98, 807-816 (1995)].

The optimum decision rules in the same-different paradigm.

In this paper we derive the optimum (likelihood-ratio) decision statistic for a same-different paradigm. The likelihood ratio is dependent on the degree of correlation between the two observations on each trial. For the two extreme cases in which the observations are either independent or highly correlated, the optimum decision rule is identical to each of two previously suggested decision rules. For these two cases, the receiver-operating characteristic (ROC) curves are calculated. Finally, an experimental procedure is suggested for assessing the decision rule actually used by the observer in a same-different task.

The optimum decision rules for the oddity task.

This paper presents the optimum decision rule for an m-interval oddity task in which m-1 intervals contain the same signal and one is different or odd. The optimum decision rule depends on the degree of correlation among observations. The present approach unifies the different strategies that occur with "roved" or "fixed" experiments (Macmillan & Creelman, 1991, p. 147). It is shown that the commonly used decision rule for an m-interval oddity task corresponds to the special case of highly correlated observations. However, as is also true for the same-different paradigm, there exists a different optimum decision rule when the observations are independent. The relation between the probability of a correct response and d' is derived for the three-interval oddity task. Tables are presented of this relation for the three-, four-, and five-interval oddity task. Finally, an experimental method is proposed that allows one to determine the decision rule used by the observer in an oddity experiment.

Discrimination of changes in the spectral shape of two-tone complexes.

Discrimination experiments were performed for changes in the amplitude of two-tone complexes. Thresholds were measured, among other things, as a function of bandwidth and center frequency. Usually, the overall intensity was roved in each and every presentation to prevent subjects from using loudness cues. The results show that changes in the spectral shape of broadband two-tone complexes can be perceived with and without a roving intensity level. For narrow-band signals, attempts have been made to explain the results in terms of the EWAIF model [Feth, Percept. Psychophys. 15, 375-378 (1974)]. This model could account for the data for signal bandwidths of 1 semitone or less. Modified EWAIF models did not yield better predictions. Thresholds for a change in the spectral shape were measured as a function of sensation level. The "near-miss" to Weber's law was not observed. This result is consistent with excitation pattern models.

Perception of spectral changes in multi-tone complexes.

Amplitude changes of the spectral components of a complex tone, relative to each other, are usually well perceived, even if the over-all intensity is kept fixed. Three experiments are reported: Experiment 1 dealt with the detectability of amplitude changes in two-tone complexes of fixed frequencies. Experiment 2 examined detection of slope changes in ramp-shaped spectral envelopes of two- and three-tone complexes as a function of spectral spacing. As a control experiment for some conditions a roving intensity level was used. Experiment 3 investigated the detectability of changes in the spectral slope of multi-tone complexes as a function of the number of components. The results of the experiments show that detection of spectral changes in a sound is strongly dependent on the frequency spacing of the components. It is concluded that the auditory system is capable of comparing the relative energy distributions over different critical bands. Within a critical band there exists an optimum frequency separation with respect to the detection of relative amplitude change.

Pitch motion with random chord sequences.

Perception of global pitch motion was studied through psychoacoustic experiments with random chord sequences. Chords contained either six or eight (fixed) tone elements, being sinusoidal, sawtooth-like, or Shepard tones, which were either on or off according to a probability controlled by the experimenter. Sequences of 2, 4, 5, or 8 chords were used. Identification by subjects of the perceived direction of overall pitch motion (up or down) was found to be well accounted for by a model in which the ultimate pitch motion percept is given by a sum of contributions from selected element transitions--that is, transitions between adjoining tone elements in successive time frames only. In its simplest form, this dipole contribution model has only one free parameter, the perceptual noise for an element transition, which was estimated for various acoustic tone representations and chord arrangements. Results of two experiments, which were carried out independently in two different laboratories, are reported.