Audioscan Verifit - Manual (Page 73)

Verifit ® User's Guide Version 3.12  May 2015
14 On-Ear Instrument Measures
A number of the tests covered in Test box measures can also be performed with the hearing instrument coupled
to the ear of a client rather than to a 2cc coupler. The on-ear results may be expected to differ from the test box
results because of the different acoustic environment and the different acoustic load.
On-ear directional test overview
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 Verifit
directional test measures directional performance in real-time while using speech. It is compatible with all forms
of hearing aid processing, including frequency lowering, feedback compensation, and other non-linear
algorithms.
The Verifit directional test presents speech at selectable levels from the front speaker and noise, at a selectable
signal-to-noise ratio, from the rear speaker. At periodic intervals, the level of the speech and noise is
momentarily reduced and an additional burst is played from either the front or rear speaker to measure the
response from that direction. This burst is extremely fast and will not influence the behavior of the hearing aid.
The coupler SPL is analyzed into two curves labeled F (front) and B (back) to indicate which speaker generated
the curve. The real-time nature of the test allows the operation of adaptive directional systems to be readily
visualized.
Optionally, speech can be disabled which will cause the directional test to alternate noise bursts from the front
and back speakers. This test may not produce a directional response with all hearing aids.
Once directional performance is observed, the test may be terminated.
Because this measurement is carried out in a real room, results are not expected to agree with data taken in large
anechoic chambers. Due to strong reflected signals, measurements made in small reverberant rooms will show
small separation between front and back response curves, just as the benefits of directional technology will
become less apparent in similar situations. When performed in a sound booth or large acoustically 'dead' room,
this test can be used to demonstrate and verify directional benefit and to provide 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.
On-ear directional testing
Best results are obtained using external front and rear sound-field speakers positioned well away from reflective
surfaces. See External sound-field speaker setup. Refer to On-Ear Measures - Setup to prepare the system for use
and for proper positioning of the client and the probe tube.
1. Insert the earmold or custom hearing instrument into the ear, being careful not to advance the probe tube
further into the ear canal. A lubricant applied in the vicinity of the probe tube will ease insertion and prevent
slit leaks.
2. Press <Tests> then highlight and <PICK> [On-ear measures], then highlight and <PICK> [Directional].
3. Highlight and <PICK> Test 1, 2, 3 or 4. Two real-time curves appear on the screen - the heavier one is the
response to the front speaker signal and the lighter one the response to the back speaker signal. The two
curves will be nearly identical for a non-directional instrument in a sound booth or acoustically 'dead' room.
Setting the instrument for directional operation should produce a separation of the curves.
4. On the Test control poster, highlight and <PICK> the Level window, then highlight and <PICK> a level
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