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07/05/06 © Etymonic Design Incorporated, 41 Byron Ave., Dorchester, ON, Canada N0L 1G0 Page 3
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digitized speech passages and the single-talker ICRA distorted speech are provided. Each has a duration of 10 –
15 seconds and may be presented as a single passage or in a continuous loop. In order to provide a repeatable
speech signal to the device under test, the signal path must be equalized prior to the presentation of the speech
signal. This is accomplished by presenting a 896 ms pink noise burst at the selected speech level, 256 ms prior
to the start of each speech passage and adjusting a digital filter to provide a flat response at the reference
microphone.
The two “Speech-std” signals are by the same male talker, filtered to provide the long-term average speech
spectrum (LTASS) recommended by Cox & Moore (1988) for average vocal effort. This is the LTASS assumed in
the Desired Sensation Level (DSL) method of hearing aid fitting. The “female” and “child” signals have been
chosen to provide a range of spectra and are presented “as recorded” without any processing. Overall SPLs of
50, 55, 60, 65, 70 and 75 dB (at the reference microphone) are available. Soft and Average levels (50 - 70 dB
SPL) have the same spectrum. A Loud vocal effort filter is applied to the 75 dB level signals. This is shown in
Figure 2.
The ICRA distorted speech signal is a recording of an English-speaking talker that has been digitally modified to
make the speech largely unintelligible. The resultant signal has many of the properties of real speech but has a
harsh sound and lacks harmonic structure. The latter may be significant for hearing aids which use this property
of speech to control noise reduction schemes. The LTASS is similar to the “Speech-std” signals up to 5 kHz.
The test systems provide a simulated real-ear mode (S-REM) in which measurements made in the 2 cc coupler in
the test chamber are converted to estimated real ear SPL. In this mode, the effects of hearing aid microphone
location need to be included in the test signals so spectra are further altered to include the microphone location
effects shown in Figure 3.
LOUD VOCAL EFFORT EFFECT re nominal band SPL
-5.00
0.00
5.00
10.00
15.00
20.00
200 250 315 400 500 630 800 1000 1250 1600 2000 2500 3150 4000 5000 6300 8000
BAND CENTER FREQUENCY (Hz)
BAND SPL (dB re NOMINAL)
Figure 2: Loud vocal effort effect in dB re nominal band levels. Nominal band levels are band levels
for an overall SPL of 65 dB. This shaping is applied to the 75 dB SPL speech and ICRA signals.
In addition to these well-controlled and repeatable signals, live speech may be used as a test signal. It will, of
course, be unequalized and at an uncontrolled level.