HP manual Verifit and RM500SL Test Signals and Analysis, Introduction, Test signals

VERIFIT and RM500SL Test Signals and Analysis			Bill Cole BASc, PEng
1 Introduction		Some useful facts about broad-band signals
The Verifit and RM500SL are full-duplex	Ø	Overall SPL is the SPL in a band containing all significant frequency
dual-channel audio measurement systems		components of the signal.
designed for the testing and fitting of all	Ø	Spectrum level is the SPL in a band 1 Hz wide.
types of hearing instruments and many	Ø	Band SPL is the SPL in a restricted frequency range. If the signal is
assistive listening devices. They provide	Ø	uniform in the band, band SPL = Spectrum level + 10*log(bandwidth).
an acoustic test chamber for the testing of		A spectrum is the band SPL, or spectrum level, in a series of
		adjacent bands.
devices coupled to standard 2 cm3
couplers as well as sound-field speaker(s)	Ø 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,
for real-ear measurement (REM) of hearing		the overall SPL, the band SPL and the spectrum level are the same.
aid performance. The Verifit test chamber	Ø	A white noise signal has a spectrum level that is independent of
contains two loudspeakers for the		frequency (i.e. constant SPL per Hertz).
functional testing of directional hearing	Ø	A pink noise signal has a spectrum level that is inversely proportional
instruments. Test signals are delivered to		to frequency (i.e. constant SPL per octave).
the device under test via the test chamber	Ø	A fractional octave band (e.g. 1/3 octave) has a bandwidth that is
loudspeaker(s), the test chamber telephone		proportional to frequency.
magnetic-field simulator (TMFS), the test	Ø A pink noise signal has a flat spectrum when analyzed in fractional-
		octave bands.
chamber magnetic field test loop (Verifit	Ø	A critical band is a band within which the loudness of a continuously-
only) or the sound-field loudspeaker(s).
		distributed signal of constant SPL is independent of bandwidth.
Device output signals are measured in the
2 cm3 coupler via the coupler microphone or in a real ear via the probe microphone. Data from the 2 cm3 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

Four types of test signals are currently available. These are tonal signals, pseudo-random noise, digitized real speech signals and the ICRA distorted speech signal. Test signals are generated in real time by the digital signal processor (DSP) or played from 16 bit binary audio files stored in the on-board flash memory. In the Verifit, these audio files 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.

2.1Narrow-band signals

2.1.1Pure tones

Pure-tones are used in the ANSI hearing aid tests, in Multicurve 2 cm3 frequency response tests (gain or output), in 2 cm3 and real-ear single frequency (manual) tests, in real-ear insertion gain tests and in tone-burst maximum output (MPO) tests. Frequencies used are 1/12 octave except for the MPO test which uses 1/3 octave frequencies. Levels from 40 to 90 dB SPL are available at the reference microphone. Control of the level at each frequency is maintained by measuring the frequency response of the signal path to the reference microphone 256 ms before each test using a 256 ms, 50 – 55 dB SPL, multi-tone complex or a 256 ms chirp, depending on the test to follow. Drive levels at each frequency are then set to achieve the desired band levels at the reference

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