On the importance of early reflections for speech in rooms.

TLDR: The speech intelligibility test results confirm the importance of early reflections for achieving good conditions for speech in rooms and show that for common conditions where the direct sound is reduced, it is only possible to understand speech because of the presence ofEarly reflections.

Abstract: This paper presents the results of new studies based on speech intelligibility tests in simulated sound fields and analyses of impulse response measurements in rooms used for speech communication. The speech intelligibility test results confirm the importance of early reflections for achieving good conditions for speech in rooms. The addition of early reflections increased the effective signal-to-noise ratio and related speech intelligibility scores for both impaired and nonimpaired listeners. The new results also show that for common conditions where the direct sound is reduced, it is only possible to understand speech because of the presence of early reflections. Analyses of measured impulse responses in rooms intended for speech show that early reflections can increase the effective signal-to-noise ratio by up to 9 dB. A room acoustics computer model is used to demonstrate that the relative importance of early reflections can be influenced by the room acoustics design.

Figures

Figure 8. Measured values of the early reflection benefit in four rooms intended for unamplified speech communication.

Figure 3. Mean intelligibility scores ± 1 standard deviation for each sound field condition for all listeners for ‘direct only’ and ‘direct+early’ reflections cases.

Figure 12. Calculated 1 kHz ERB values versus source-receiver distance for the ‘live’, ‘dead’ and ‘optimized’ room design cases. (Table V includes descriptions of the designs).

Table II. Summary of acoustical measures for the Direct only and Direct+early reflections sound field cases. (Noise level 47.6 dBA).

Table I. Orientation of loudspeakers relative to the listener. Horizontally straight ahead of the listener at ear level is 0 degrees in both planes.

Figure 4. Mean intelligibility scores for each sound field condition for ‘direct only’ and ‘direct+early’ reflections cases plotted for each hearing loss group.

Full text

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Journal of the Acoustical Society of America, 113, June 6, pp. 3233-3244, 2003-
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On the importance of early reflections for speech in rooms
Bradley, J. S.; Sato, H.; Picard, M.
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On the importance of early reflections for speech in
rooms
Bradley, J.S.; Sato, H.; Picard, M.
NRCC-45431
A version of this document is published in / Une version de ce document se trouve dans :
Journal of Acoustical Society of America, v. 113, no. 6, June 2003, pp. 3233-3244
http://irc.nrc-cnrc.gc.ca/ircpubs

On the Importance of Early Reflections for Speech in Rooms
J.S. Bradley and H. Sato
National Research Council, Ottawa, Canada, K1A 0R6
M. Picard
Ecole d’orthophonie et d’audiologie, Université de Montréal, Montréal, Canada, H3C 3J7
Shortened title: “Early reflections for speech”
PACS numbers: 43.55.Hy, 43.71.Gv
Received: ___________________________________________________________
Early reflections for speech - 1

ABSTRACT
This paper presents the results of new studies based on speech intelligibility tests in
simulated sound fields and analyses of impulse response measurements in rooms used for
speech communication. The speech intelligibility test results confirm the importance of
early reflections for achieving good conditions for speech in rooms. The addition of early
reflections increased the effective signal-to-noise ratio and related speech intelligibility
scores for both impaired and non-impaired listeners. The new results also show that for
common conditions where the direct sound is reduced, it is only possible to understand
speech because of the presence of early reflections. Analyses of measured impulse
responses in rooms intended for speech show that early reflections can increase the
effective signal-to-noise ratio by up to 9 dB. A room acoustics computer model is used to
demonstrate that the relative importance of early reflections can be influenced by the
room acoustics design.
Early reflections for speech - 2

I INTRODUCTION
The beginnings of our understanding of how we perceive sound reflections that arrive
within a short time after the direct sound can be traced to the work of Joseph Henry in the
1850s [1]. Work by Haas [2] and Wallach et al. [3] showed explicitly how early
reflections are integrated with the direct sound to make the direct sound seem to be
effectively louder. Lochner and Burger [4] carried out extensive experiments to
determine exactly how delayed reflections affected articulation test results as a function
of the amplitude and delay time of an early-arriving reflection. Although these early
results demonstrated an extensive understanding of the benefits of early reflections to
speech intelligibility in rooms, they sometimes focussed on the negative effects of early
reflections. For example, Haas considered the point at which early reflections became
disturbing. Although the early work of Lochner and Burger and others provides a solid
basis for the importance of early reflections, they provide little information on the
expected improvements to speech intelligibility scores in actual rooms due to early
reflection energy.
Lochner and Burger’s results suggest that speech energy in early arriving reflections is
more or less equivalent to similar amounts of direct speech energy in terms of increasing
speech intelligibility scores. They developed the concept of useful-to-detrimental sound
ratios in which ‘useful’ is defined as the sum of the direct and early-reflected speech
energy and ‘detrimental’ is the sum of the later-arriving speech reflections and the
ambient noise. This concept has been shown to be well correlated with speech
intelligibility test scores in a wide range of rooms [5-7] and to be strongly correlated [8]
with the newer speech transmission index (STI) concept.
Early reflections for speech - 3