Huron & SNAP Documentation

Signal Generation via MatLab Scripts
Caution
THE ENGINEERING TOOLS
rolloff with increasing frequency. These test signals are safer
for electroacoustic systems and are easier on the ear than white
noise.
The third variety of test signals is a type of swept sine wave.
These signals (sNN_MMMk.sim, where NN is the number of
octaves of the sweep, and MMM is the length of the signal in
kilosamples) contain a sinusoid who's instantaneous frequency
sweeps up at a constant number of octaves per second until it
reaches the Nyquist frequency (in this case, half the sampling
frequency). This imparts the signals with two important
properties:
1. Pink spectrum. These signals will not damage tweeters in
loudspeakers if used at moderate volumes.
2. Harmonic distortion rejection. Because the time duration
between the instantaneous excitation frequency and its
harmonics is constant in the test signal, any harmonic
distortion in the system under test will manifest itself as
narrow peaks very late in the impulse response. This
typically corresponds with a region containing no relevant
information, and so it can be truncated (removed entirely)
from the measurement. To take advantage of this property
select a test signal of much greater duration than the
response of the system under test.
These properties mean that the swept sine test signals are the
best choice for electroacoustic measurements, which can have
large harmonic distortion components.
When using these test signals, it is necessary to choose not only
the appropriate length, but the number of octaves to be covered.
This will depend on the frequency range of interest, and the
physical capabilities of the test source. To find the lowest
instantaneous frequency in a given test signal, divide the sample
rate by:
2 (NN+1)
The Matlab function swptsine.m in the huronmat
subdirectory will allow the creation of your own variations of
these test signals. Type “help swptsine” in Matlab for
more details.
CAUTION: Using a sweep of too many octaves could cause
damage to loudspeakers. These signals contain low frequencies
at very high levels, and can cause excessive cone displacement
in speaker drivers. If in doubt start with a signal of fewer
octaves and visually inspect the cone displacement. The low
frequency limit of your tests will depend on the physical
capabilities of your speaker. Using lower volume levels can
alleviate this problem.
HURON TECHNICAL MANUAL PAGE 202