ASP Testing
with the FONIX 6500 I/O Option
by Larry Revit, hearing scientist
Frye Electronics continues its commitment to provide the tests required
to analyze ASP (Automatic Signal Processing) hearing aids. ASP circuits
are designed to decrease low-frequency gain as the level of low-frequency
noise in the environment increases. One of the best ways to check this function
of an ASP circuit is to run an input/output (I/O) curve using a low-frequency
signal. An I/O curve will show you the kneepoint of ASP action (the SPL
required to trigger the circuit) and the amount of gain reduction provided
as the level of low-frequency noise increases.
However, the I/O test conditions required for ASP circuits are different from those for regular AGC circuits. AGC circuits have fast attack times, because they are designed to protect against any unwanted loud sounds, sudden or otherwise. ASP circuits have slow attack times. They are designed to be triggered by long-term, as opposed to transient, low-frequency energy. For example, you would want an ASP circuit to be triggered by the "drone" of a motor, but not by the "thud" of a door closing. This avoids the annoying "pumping" sounds you would get with a fast attack time. Because of the slow attack time of ASP, in order to get a proper measure of the action of an ASP circuit, longer stimulus times must be used. That is, there must be a delay between the onset of the test signal and the acquisition of the measurement of the response: You have to give the ASP circuit a chance to react before measuring it.
With software Version 2.38, the FONIX 6500 I/O Option allows you to select the delay time between stimulus onset and acquisition of measurement necessary for testing ASP. Delay is selectable via the I/O menu. The default setting is one-half secong, which is long enough for many circuits. But some circuits take longer to react. With the 6500 I/O Option, the delay may be varied between 0.1 and 3.0 seconds. (The shorter times are for regular, fast-acting, AGC circuits.)