Internationally comparable screening tests for listening in noise in several European languages: The German digit triplet test as an optimization prototype

Abstract

Objective: A review is given of internationally comparable speech-in-noise tests for hearing screening purposes that were part of the European HearCom project. This report describes the development, optimization, and evaluation of such tests for headphone and telephone presentation, using the example of the German digit triplet test. In order to achieve the highest possible comparability, language- and speaker-dependent factors in speech intelligibility should be compensated for. Materials and methods: The tests comprise spoken numbers in background noise and estimate the speech reception threshold (SRT), i.e. the signal-to-noise ratio (SNR) yielding 50% speech intelligibility. Results: The respective reference speech intelligibility functions for headphone and telephone presentation of the German version for 15 and 10 normal-hearing listeners are described by a SRT of −9.3 ± 0.2 and −6.5 ± 0.4 dB SNR, and slopes of 19.6 and 17.9%/dB, respectively. Reference speech intelligibility functions of all digit triplet tests optimized within the HearCom project allow for investigation of the comparability due to language specificities. Conclusions: The optimization criteria established here should be used for similar screening tests in other languages.

Key Words::

• Hearing screening
• speech-in-noise test
• HearCom
• digit triplet test
• number recognition test

Acknowledgements

This work was funded by the EU FP6–004171 HearCom. The following partner sites contributed to this test arsenal: Belgium: University of Leuven, Lab. Exp. ORL; Germany: Hörzentrum Oldenburg and Center of Competence HörTech; Greece: Institute for Language and Speech Processing; The Netherlands: Academic Medical Centre Amsterdam and VU University Medical Centre Amsterdam; Poland: A. Mickiewicz University, Institute of Acoustics; Sweden: Linköping University, Dept. of Audiology; United Kingdom: University of Southampton, Institute for Sound and Vibration Research. The contribution of the scientists involved in these institutions and the coordination work by the HearCom management team is gratefully acknowledged. Further support was provided to the authors by the EFRE-project HURDIG, the Ministry of Science and Culture for the Federal State of Lower Saxony, and the German Research Foundation (DFG). The authors thank Jutta Birkigt (speaker of the German digit triplet test), Timo Bräcker, and Frauke Eenboom for their contribution. The hints and suggestions of two anonymous reviewers are also gratefully acknowledged.

Notes

1. The subjects were not informed about the reduced number of alternatives. Such a restriction of the valid digits for a response would make the listeners” task more demanding and might therefore interfere with the individual test performance. Moreover, the effect of changing the number of alternatives from 10 to 9 would only shift the random hit rate from 10% to 11.1% and the 50%-point on the discrimination function by approximately 0.55% which would be roughly equivalent to a threshold shift of 0.03 dB, which is far below any significant change. Even if three digits are actually not used (as, e.g. in Swedish), the corresponding shift would only amount to 2.1% or 0.09 dB, respectively.

2. The underlying assumption is that the SNR (and not the presentation level of the noise) is the primary control parameter for speech intelligibility as long as the noise level is between reasonable limits (such as, e.g. the perceived categorical loudness of the noise ranging between ‘soft’ and ‘loud’). This assumption is based on a vast body of literature (see Plomp, 1978, and Wagener & Brand, 2005 for a review).

3. The method employed here attempts to compensate for any listener-specific bias by using a pseudo-random assignment of test lists and SNR values to listeners. An alternative, more accurate procedure (suggested by one anonymous reviewer) would be to fit a separate listener-specific discrimination function and then take the mean of the individual SRTs and the geometric mean of the individual slopes to define the overall discrimination function. However, this method is not applicable in this case because the number of measurements for each listener and each test list was too small and the respective SNR values employed were not suitable for estimating the midpoint and slope of the individual list-specific discrimination function in a reliable way.

Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.