Niek J. Versfeld

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.

Extended speech intelligibility index for the prediction of the speech reception threshold in fluctuating noise

The extension to the speech intelligibility index (SII; ANSI S3.5-1997 (1997)) proposed by Rhebergen and Versfeld [Rhebergen, K.S., and Versfeld, N.J. (2005). J. Acoust. Soc. Am. 117(4), 2181-2192] is able to predict for normal-hearing listeners the speech intelligibility in both stationary and fluctuating noise maskers with reasonable accuracy. The extended SII model was validated with speech reception threshold (SRT) data from the literature. However, further validation is required and the present paper describes SRT experiments with nonstationary noise conditions that are critical to the extended model. From these data, it can be concluded that the extended SII model is able to predict the SRTs for the majority of conditions, but that predictions are better when the extended SII model includes a function to account for forward masking.

The relationship between the intelligibility of time-compressed speech and speech in noise in young and elderly listeners

A conventional measure to determine the ability to understand speech in noisy backgrounds is the so-called speech reception threshold (SRT) for sentences. It yields the signal-to-noise ratio (in dB) for which half of the sentences are correctly perceived. The SRT defines to what degree speech must be audible to a listener in order to become just intelligible. There are indications that elderly listeners have greater difficulty in understanding speech in adverse listening conditions than young listeners. This may be partly due to the differences in hearing sensitivity (presbycusis), hence audibility, but other factors, such as temporal acuity, may also play a significant role. A potential measure for the temporal acuity may be the threshold to which speech can be accelerated, or compressed in time. A new test is introduced where the speech rate is varied adaptively. In analogy to the SRT, the time-compression threshold (or TCT) then is defined as the speech rate (expressed in syllables per second) for which half of the sentences are correctly perceived. In experiment I, the TCT test is introduced and normative data are provided. In experiment II, four groups of subjects (young and elderly normal-hearing and hearing-impaired subjects) participated, and the SRTs in stationary and fluctuating speech-shaped noise were determined, as well as the TCT. The results show that the SRT in fluctuating noise and the TCT are highly correlated. All tests indicate that, even after correction for the hearing loss, elderly normal-hearing subjects perform worse than young normal-hearing subjects. The results indicate that the use of the TCT test or the SRT test in fluctuating noise is preferred over the SRT test in stationary noise.

The optimum decision rules in the same-different paradigm

In this paper we derive the optimum (likelihood-ratio) decision statistic for asame-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 asame-different task.

The dynamic range of speech, compression, and its effect on the speech reception threshold in stationary and interrupted noise

Changes in the speech reception threshold (SRT) after amplitude compression of speech or speech in noise may be due to changes in the dynamic range of the speech signal. However, current models set up to predict the speech intelligibility consider the dynamic range of speech to be fixed regardless of the type of compression. The present paper describes two experiments with normal-hearing subjects to examine the effect of the dynamic range on the SRT in stationary and interrupted noise after wide dynamic range compression. The dynamic range has been varied by compression or expansion of only the speech signal, leaving the masking noise unaltered, or by compression or expansion of the mixed speech-in-noise signal. The results show that compression affects the SRT, both in a positive or a negative direction, not only due to dynamic range but also due to distortion of the speech signal.

The optimum decision rules for the oddity task

This paper presents the optimum decision rule for an m-interval oddity task in whichm-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 anm-interval oddity task corresponds to the special case of highly correlated observations. However, as is also true for thesame-different paradigm, there exists a different optimum decision rule when the observations are independent. The relation between the probability of a correct response andd’ 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.

Prediction of the intelligibility for speech in real-life background noises for subjects with normal hearing.

Objectives: The speech reception threshold (SRT) traditionally is measured in stationary noise that has the long-term average speech spectrum of the target speech. However, in real life the instantaneous spectrum of the background noise is likely to be different from the stationary long-term average speech spectrum noise. To gain more insight into the effect of real-life background noises on speech intelligibility, the SRT of listeners with normal hearing was measured in a set of noises that varied in both the spectral and the temporal domain. This article investigates the ability of the extended speech intelligibility index (ESII), proposed by Rhebergen et al. to account for SRTs in these real-life background noises. Design: SRTs in noise were measured in 12 subjects with normal hearing. Interfering noises consisted of a variety of real-life noises, selected from a database, and chosen on the basis of their spectro-temporal differences. Measured SRTs were converted to ESII values and compared. Ideally, at threshold, ESII values should be the same, because the ESII represents the amount of speech information available to the listener. Results: SRTs ranged from −6 dB SNR (in stationary noise) to −21 dB SNR (in machine gun noise). Conversion to ESII values resulted in an average value of 0.34, with a standard deviation of 0.06. SRT predictions with the ESII model were better than those obtained with the conventional SII (ANSI S3.5-1997) model. In case of interfering speech, the ESII model predictions were poorer, because additional, nonenergetic (informational) masking is thought to occur. Conclusions: For the present set of masking noises, being representative for a variety of real-life noises, the ESII model of Rhebergen et al. is able to predict the SRTs of subjects with normal hearing with reasonable accuracy. It may be concluded that the ESII model can provide valuable predictions for the speech intelligibility in some everyday situations.

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.

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.

Learning effect observed for the speech reception threshold in interrupted noise with normal hearing listeners.

Traditionally, the speech reception threshold (SRT) for sentences in noise is measured in stationary speech-shaped noise; however, non-stationary masking noises are gradually becoming more common. A previous study by Rhebergen et al. () suggested that a learning effect might be present for the SRT in interrupted noise but not in stationary noise. The current study tested if a stable SRT for female or male speech is reached by determining SRTs in listeners with normal hearing in either stationary or 8-Hz interrupted noise after five replications. Contrary to repeated SRT measurements in stationary noise, a significant improvement was observed for SRTs in interrupted noise with replication. For both speech materials, after five replications, the SRT in interrupted noise improved about 3 to 4 dB in comparison to the first SRT. Stable thresholds seem to be reached after about two replications. This experiment shows that there is a substantial learning effect present in SRT measurements with interrupted noise but not in stationary noise. With non-stationary noise, it is recommended to include a repeated measures design.