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Verifit User’s Guide Version 2.8
Page 49
9.3
Directional function test
FastFacts 9.3: Directional function test
Directional characteristics such as polar plots and Directivity Index (DI) are
typically measured in a large anechoic chamber using sound from one direction
and at one frequency at a time. This means they apply only for a hearing aid set
for linear amplification. These are not conditions representative of real use.
The unique Verifit directional test presents over 1000 tones simultaneously at
different frequencies from both test chamber speakers. The tones are individually
controlled in real time to produce a precise spectrum from each speaker at the HIT
reference microphone.
The coupler SPL is then analysed into two real-time response curves, labeled L
(left) and R (right) to indicate which speaker generated the curve. Since the curves
are generated simultaneously, their difference is not influenced by electronic
processing such as compression or noise reduction but reflects only the effect
produced by the directional microphones in the instrument. The real-time nature of
the test allows the operation of adaptive directional systems to be readily
visualized.
Because the Verifit test chamber is small and anechoic only at higher frequencies,
results are not expected to agree with data taken in large anechoic chambers. It is
a functional test only, providing assurance that the directional microphone array
has been correctly assembled and programmed and has not deteriorated due to
element drift or the clogging of ports. See also
12: Real-Ear Directional Test
.
