Loudspeakers 641this technique have several common elements. The
technique involves sampling the signal at a point in the
chain prior to the input of the loudspeaker (input), as
well as the signal from a test microphone (output). The
output may be sampled at a later time in order to
account for the time required for sound to propagate
from the loudspeaker to the test microphone. As with
MLS testing, cross-correlation between input and output
signals will yield the impulse response of the loud-
speaker. It is also possible to perform an FFT on both
input and output signals and obtain the transfer func-
tion of the loudspeaker by complex division.
The dual-channel approach has the advantage of
allowing a wide range of signals, including music, to be
employed as excitation. The commercially available
implementations of this technique incorporate several
refinements of the basic procedure described above, and
these systems offer the possibility of measuring the
response of a sound system while it is in operation.
SNR is a possible issue with this form of measure-
ment, so averaging is generally performed to improve
the accuracy of the results. Additionally, the spectrum
of the input signal may not contain sufficient energy at
all frequencies to sufficiently excite the system under
test. For this reason, a coherence function is used to
indicate those frequency ranges where the signal energy
is insufficient to yield good results.
17.11.5 Swept Sine Measurements
Although the chart recorder is a swept sine measure-
ment, it fails to take advantage of all the possibilities
offered by the use of a sweep (also known as a chirp) as
a test signal. Dick Heyser developed and patented a
technique known as time delay spectrometry, or TDS. In
TDS, the analyzer’s receiving circuitry employs a band-
pass filter, the center frequency of which is swept in
synchronicity with the frequency of the signal applied to
the loudspeaker. A delay may be applied to the sweep of
the bandpass filter to account for the amount of time
required for sound to propagate from the device under
test to the microphone, hence the name of the technique.
The bandpass filter will reject frequencies that are
displaced by some amount from its center frequency. If
the receive delay for the filter is chosen appropriately,
the analyzer will admit the direct signal from the loud-
speaker, while simultaneously rejecting signals that
have been reflected from environmental surfaces,
thereby traveling a longer path and arriving later than
the direct signal. The effect of this ability to reject
unwanted reflections is the creation of a time window,
even though the data is taken in the frequency domain.
Additionally, the bandpass filter attenuates broadband
noise by a much greater amount than it does the direct
signal from the loudspeaker.
Due to the inherently high SNR of TDS, averaging
of multiple tests is usually unnecessary. The number of
samples analyzed is also not a function of the time
window, as it is with an FFT-based analyzer. Further-
more, the bandpass filter removes distortion products,
so TDS is intrinsically more capable of separating the
linear transfer function from distortion products. It is
also possible to use the technique to track specific
harmonics while rejecting the fundamental.
Loudspeaker test instrumentation is more powerful
and less expensive now than at any other time in the
history of loudspeakers. While the current situation
makes it possible to gather ever more detailed informa-
tion about the behavior of loudspeakers, it is important
to keep in mind the basics of instrumentation and spec-
trum analysis. This awareness will assist in identifying
loudspeaker data that is suspect or incomplete.Reference- R. H. Small, “Constant-Voltage Crossover Network Design,” Journal of the Audio Engineering Society, Vol. 19,
no. 1, January 1971.
BibliographyA. A. Janszen, “An Electrostatic Loudspeaker Development,” J. Audio Eng. Soc., Vol. 3, no. 2, April 1955.
A. H. Benade, Fundamentals of Musical Acoustics Oxford, University Press, 1976.
A. N. Theile, “Loudspeakers in Vented Boxes: Part I & II,” J. Audio Eng. Soc., Vol. 19, no. 5, and 6, May and June
1971.
A. Wood, Acoustics, New York: Dover Publications, Inc., 1966.