To meet the need for an objective self-test for hearing screening. a new Dutch speech-in-noise test was developed. Digit triplets were used as speech material. The test was made fully automatic, was controlled by a computer, and can be done by telephone. It measures the speech reception threshold (triplet SRT(n)) using an adaptive procedure, in about 3 min. Our experiments showed no significant influence of telephone type or listening environment. Measurement errors were within 1 dB. which makes the test accurate. In additional experiments with hearing-impaired subjects (76 ears of 38 listeners), the new test was compared to the existing sentence SRT(n) test of Plomp and Mimpen, which is considered to be the standard. The correlation between both SRT(n)s was 0.866. As expected, correlations between the triplet SRT(n) test by telephone and average pure-tone thresholds are somewhat lower: 0.732 for PTA(0,5,1,2), and 0.770 for PTA(0,5,2,4). When proper SRT(n) values were chosen for distinguishing between normal-hearing and hearing-impaired subjects, the triplet SRT(n) test was found to have a sensitivity of 0.91 and a specificity of 0.93.

The concept of the Modulation Transfer Function (MTF) can successfully be applied to evaluate the quality of speech transmission from a talker to a listener in an auditorium. Typically, depending on the auditorium acoustics, the intensity modulations contained in the original sound are to some extent reduced when measured at a listener’s location, especially for higher modulation frequencies. The implementation of such an acoustical MTF analysis with a sinusoidally modulated test signal is described in detail. The performance of a sound transmission system as revealed by the MTF can be expressed in one single index (the Speech Transmission Index, STI), which relates well to the performance as determined by intelligibility tests with talkers and listeners. A review is given of a series of studies on various aspects of the chain of relations between auditorium acoustics, MTF, STI, and speech intelligibility, illustrating the use of this approach for estimating speech intelligibility, either from MTF calculations at the design stage of an auditorium or from MTF measurements in actual situations.

A physical method for measuring the quality of speech-transmission channels has been developed. Essentially, the method represents an extension of the Articulation Index (AI) concept, which was developed mainly to account for distortions in the frequency domain (noise, bandpass-limiting). The underlying concept of the present approach, based on the Modulation Transfer Function (MTF) of a transmission channel, has been adapted to account for nonlinear distortions (peak clipping) as well as for distortions in the time domain (reverberation, echoes, AGC). The resulting index, the Speech-Transmission Index (STI), has been correlated with subjective intelligibility scores obtained on 167 different transmission channels with a wide variety of disturbances. The relative predictive power of the STI, expressed in PB-word score, appeared to be 5%. This accuracy is comparable with results obtained from subjective measurements when about four talkers and four listeners are used. Expressed in terms of signal-to-noise ratio, the accuracy is about 1 dB. Pilot studies have been carried out to evaluate the use of the STI for testing digital-speech transmission channels.

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.

The Speech Reception Threshold for sentences in stationary noise and in several amplitude-modulated noises was measured for 8 normal-hearing listeners, 29 sensorineural hearing-impaired listeners, and 16 normal-hearing listeners with simulated hearing loss. This approach makes it possible to determine whether the reduced benefit from masker modulations, as often observed for hearing-impaired listeners, is due to a loss of signal audibility, or due to suprathreshold deficits, such as reduced spectral and temporal resolution, which were measured in four separate psychophysical tasks. Results show that the reduced masking release can only partly be accounted for by reduced audibility, and that, when considering suprathreshold deficits, the normal effects associated with a raised presentation level should be taken into account. In this perspective, reduced spectral resolution does not appear to qualify as an actual suprathreshold deficit, while reduced temporal resolution does. Temporal resolution and age are shown to be the main factors governing masking release for speech in modulated noise, accounting for more than half of the intersubject variance. Their influence appears to be related to the processing of mainly the higher stimulus frequencies. Results based on calculations of the Speech Intelligibility Index in modulated noise confirm these conclusions.