FONIX® 8000 - Frye Electronics

General Operation 37
2.3.1.4 Pure-tone Filter
Pure-tone signals are usually flat-weighted. That is, in a pure-tone sweep, each
tone usually has the same amplitude as every other tone in the sweep. In contrast,
the wideband Composite and Digital Speech signals are usually speechweighted.
That is, the lower frequencies of the wideband signals have higher
amplitudes than the higher frequencies, simulating the long-term average of
speech.
2.3.2 Understanding Composite Signals
There are two types of composite signals: Composite and Digital Speech. The
Composite signal is a continuous broadband signal containing 79 different frequencies
presented simultaneously. This signal is usually “speech weighted,”
which means that the lower frequencies have a higher emphasis than the higher
frequencies.
The Composite signal is both a faster and a more realistic signal than a puretone
sweep because there is no waiting for a progression of tones to complete,
and, like speech, a broad spectrum of frequencies is used simultaneously. The
Composite signal updates several times a second.
Digital Speech is an interrupted version of the Composite signal used for testing
high-end digital hearing aids. Many high-end digital aids (though not all)
use a technology called “speech enhancement” or “noise reduction.” These aids
respond to any continuous signal as if it were noise, and lower the gain at the
offending frequencies. Unfortunately, these aids regard the Composite signal or
pure-tone sweeps as noise, making them difficult to test using traditional methods.
Digital Speech was developed as a way to test these high-end hearing aids.
Instead of presenting a continuous signal, it presents an interrupted signal that
the aid regards as speech instead of noise.
The Composite and Digital Speech signals can be used with several different
types of speech weightings: ANSI, ICRA and FLAT. Flat is generally only used
for specific research purposes—it contains no speech weighting and is difficult
for most hearing aid circuits to process. See Section 2.3 for a discussion of the
ANSI and ICRA speech weightings.
A significant advantage of using a composite signal is that each frequency present
in the signal can be individually controlled in amplitude and phase. As
the analyzer goes through the leveling process, each component is adjusted
to produce a signal that is optimally accurate at the reference point where the
leveling microphone is located. The FONIX 8000 Test System is able to equalize
the amplitudes to within 0.25 dB for coupler measurements made with the
Composite signal. The spectrum limits are broadened to within 2 dB for real-ear
measurements.