Table of Contents
Advertisement
6.4 Speech Level Meter
In the context of speech intelligibility measurements, special care is needed when deter-
mining the sound pressure level of running speech. As human talkers use their articulators
to form speech utterances, they alternately produce high-intensity bursts of acoustic energy
and naturally occurring pauses. Across the length of a spoken sentence, periods of silence
roughly account for half of the total time (length) of the sentence. Those pauses do not
necessarily contribute to the intelligibility of the sentence, but they do have an influence on
the overall (equivalent continuous) sound pressure level of the sentence. There is a firmly
established relation between speech-to-noise ratio (SNR) and speech intelligibility - but
how do we determine the speech level to calculate (SNR)?
It has been shown that removing the pauses from speech increases the correlation between
SNR and subjective speech intelligibility. In the 1990s, a method was developed within Eu-
ropean ESPRIT project 2589 (SAM) to assess speech levels while removing the influence
of natural pauses. This method is known as Speech Level Meter (SLM) and is included in
IEC-60268-16 as part of a normative annex (Annex J). This Speech Level Meter module is
a direct implementation of this method.
The procedure consists of the compilation of a level histogram, which normally reveals
a bimodal distribution: a low-level mode which represents the background noise, and a
higher-level mode which represents speech. The RMS of speech and background noise can
be calculated separately by summing the two modes in the histogram separately. The crite-
rion for separating the two modes is a threshold that is set 14 dB below the SPL presumed
to correspond to speech.
Estimated speech level (left) and histogram on which the estimate is based (right)
To use the Speech Level Meter, simply carry out a measurement while sampling running
speech. The SPL of speech and background noise are calculated, as well as the total LAeq.
VOX-01 Operating Manual
49
Advertisement
Table of Contents
