Listeners’ speech rate preferences in stationary and modulated maskers
Olympia SIMANTIRAKI(1) , Martin COOKE(2)(1) University of the Basque Country, Spain, olympia.simantiraki@ehu.eus(2) Ikerbasque (Basque Science Foundation), Spain, m.cooke@ikerbasque.org

Abstract
Durational changes to speech can improve intelligibility in noise, but what speech rate would a listener choose
in such conditions if allowed to do so? We introduce a technique which enables listeners to adjust arbitrary
speech properties in real time while performing a speech-in-noise task, and apply it to the question of preferredspeech rate. Native listeners adjusted speech rate while identifying word sequences in quiet, stationary noise(3 SNRs) and modulated noise (5 modulation rates). Listeners preferred faster rates in quiet, increasingly
slower rates in stationary noise as SNR decreased, and increasingly slower rates in modulated noise as envelope
modulation rate increased. These findings complement those obtained using traditional intelligibility measures.
We speculate that for tasks where intelligibility is near to ceiling, any consistent listener preferences might beused as an indicator of optimal listening effort and applied in the design of speech enrichment algorithms. The
listening preference technique can be adapted readily to many scenarios such as different masking conditions(e.g. competing talkers) and applied to other speech features such as F0 and spectral tilt.
Keywords: Speech rate, Speech-in-noise, Speech recognition task

1

INTRODUCTION

Listening to speech under imperfect conditions can reduce intelligibility (1) and lead to greater listening effort(2). Intelligibility enhancement in challenging conditions has been widely investigated, with speech modification
algorithms able to achieve significant improvements (3). Recently, the goal of reducing listening effort hasgrown in prominence. Several studies have focused on listening effort by drawing comparisons amongst different
listener groups (4, 5), maskers (6, 7), speech types (8), synthetic speech algorithms (9) and linguistic aspects ofthe speech signal (10). A number of objective and subjective measures have been used for quantifying listening
effort (2, 11, 12). However, how specific factors such as speech rate impact on listening effort has received littleattention (13). An alternative approach to extracting supra-intelligibility effects of speech is to allow listeners tocontrol some aspect of speech during an intelligibility-based task. A similar approach was used in (14) to study
listener speech rate preferences as a function of age and passage comprehension difficulty in quiet. The current
study extends the listener preference approach to elicit preferred speech rates in a range of adverse listening
conditions.

2METHODS

Listeners were asked to change speech rate in real-time using up/down keys while listening to word sequences.
The task was explained as akin to choosing an appropriate volume for a television: too quiet makes comprehension difficult, while too loud a setting leads to discomfort. Listeners were instructed to choose a speech ratethat allowed them to recognise as many words as possible. Each trial consisted of an adjustment phase followed
by a test phase in which their intelligibility was evaluated via a word identification task. The experiment wasdivided into 9 blocks by condition: quiet and 8 additive noise masking conditions: speech-shape noise (SSN) atSNRs of 0, +6 and +12 dB, and speech modulated noise (SMN) for 5 envelope modulation rates, mixed withspeech at +6 dB SNR. Each block contained 22 trials. Eighteen native Spanish listeners were recruited and foreach one, the experiment lasted around two hours with a short break between each block.

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Intelligibility (%)
Speech rate

fasterStabilisation timeslower

Mod. Rate
SNR (dB)
Masker

0+6SSN+12∞
quiet

2.5slower+61.7slower+6unmod.
+6SMN

1.4faster+6

2.5faster+6

Figure 1. Upper plot: Mean of the percentage of correct words recalled. Middle plot: Mean of the preferredspeech rate. Speech rate values correspond to the times slower/faster with regard to the original(unmod.). Lower
plot: Mean time in seconds that listeners needed for stabilising their preference. Error bars represent ±SE.

3SUMMARY OF THE FINDINGS

Listeners preferred slower speech rates for more challenging conditions (Fig. 1 upper & middle). For the SSNmasker, the preferred rate became slower as SNR decreased (p < .05). This finding is in line with preferred
rates in studies on speech complexity (14) and degraded (reverberant) listening conditions (15). For the SMNmasker, listeners needed more time to express their listening preference compared to the quiet condition (p <.01). It may be that the modulated nature of the masker interferes with that of the target speech, causing
attentional distraction or making it hard to predict when to listen. In all conditions the preferred speech rate
was faster compared to the speech rate of the original recording, in agreement with (14). We speculate thatstabilisation time may be an indicator of listening effort since different times are required to find the value thatmaximizes intelligibility in different conditions. Indeed intelligibility scores are not correlated with stabilization
time (r = −0.14, p = .09).

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ACKNOWLEDGEMENTS
Olympia Simantiraki was funded by the European Commission under the Marie Curie European Training Network ENRICH (675324).

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