Audioscan Verifit User Manual download pdf (Page 144)

1 Introduction

The Verit and RM500SL are full-duplex dual-chan-

nel audio measurement systems designed for the

testing and tting of all types of hearing instruments

and many assistive listening devices. They provide

an acoustic test chamber for the testing of devices

coupled to standard 2 cm

couplers as well as sound-

eld speaker(s) for on-ear measurement of hearing

aid performance. The Verit test chamber contains

two loudspeakers for the functional testing of direc-

tional hearing instruments. Test signals are deliv-

ered to the device under test via the test chamber

loudspeaker(s), the test chamber telephone mag-

netic-eld simulator (TMFS), the test chamber mag-

netic eld test loop (Verit only) or the sound-eld

loudspeaker(s). Device output signals are measured

in the 2 cm

coupler via the coupler microphone or in

a real ear via the probe microphone. Data from the 2

coupler may be viewed as coupler SPL, coupler

gain or as estimated SPL in the ear canal (simulated

real-ear measurements, S-REM). Data from the probe

microphone may be viewed as ear canal SPL, as ear

canal SPL re normal hearing threshold (i.e. dB HL), as

insertion gain or as ear canal SPL in the context of an

auditory area (Speechmap®).

2 The test signals

Sinusoidal, pseudo-random noise and digitized real

speech signals are provided. Two signals derived

from real speech, the International Speech Test Signal

(ISTS) and the single-talker International Collegium of

Rehabilitative Audiology (ICRA) signal, are also pro-

vided. Test signals are generated in real time by the

digital signal processor (DSP) or played from 16 bit

binary audio les stored in the on-board ash memo-

ry. In the Verit, these audio les are up-loaded from

the internal compact disc (CD) drive at power up. In

order to provide a repeatable acoustic signal to the

device under test, a reference microphone is used in

conjunction with a digital control loop to maintain

the desired band level at each frequency. Live audio

(speech, music etc.) may also be used as a test signal

but it is not controlled for spectrum or level.

Some useful facts about broad-band signals

Overall SPL is the SPL in a band containing all signi-

cant frequency components of the signal.

Spectrum level is the SPL in a band 1 Hz wide.

Band SPL is the SPL in a restricted frequency range.

If the signal is uniform in the band, band SPL = Spec-

trum level + 10*log(bandwidth).

A spectrum is the band SPL, or spectrum level, in a

series of adjacent bands.

For a broad-band signal, the overall SPL is greater

than the band SPL and the band SPL is greater than

the spectrum level. For a pure tone, the overall SPL,

the band SPL and the spectrum level are the same.

A white noise signal has a spectrum level that is inde-

pendent of frequency (i.e. constant SPL per Hertz).

A pink noise signal has a spectrum level that is in-

versely proportional to frequency (i.e. constant SPL

per octave).

A fractional octave band (e.g. 1/3 octave) has a band-

width that is proportional to frequency.

A pink noise signal has a at spectrum when ana-

lyzed in fractional-octave bands.

A critical band is a band within which the loudness

of a continuously-distributed signal of constant SPL

is independant of bandwidth.

Bill Cole BASc, PEng

USA 800-265-2093 519-268-3313 FAX 519-268-3256 www.audioscan.com

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Audioscan Verifit User Manual Page 144

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