Handbook for Sound Engineers

1618 Chapter 46

46.3.5.1 Dissecting the Impulse Response

The audio practitioner is often faced with the dilemma
of determining whether the reason for bad sound is the
loudspeaker system, the room, or an interaction of the
two. The impulse response can hold that answer to these
and other perplexing questions. The impulse response in
its amplitude versus time display is not particularly use-
ful for other than determining the polarity of a system
component, Fig. 46-15. A better representation comes
from squaring impulse response (making all deflections
positive) and displaying the square root of the result on
a logarithmic vertical scale. This log-squared response
allows the relative levels of energy arrivals to be com-
pared, Fig. 46-16.

46.3.5.2 The Envelope-Time Curve

Another useful way of viewing the impulse response is
in the form of the envelope-time curve, or ETC. The
ETC is also a contribution of Richard Heyser.^2 It takes
the real part of the impulse response and combines it
with a 90 degrees phase shifted version of the same,
Fig. 46-17. One way to get the shifted version is to use
the Hilbert Transform. The complex combination of

these two signals yields a time domain waveform that is

often easier to interpret than the impulse response. The

ETC can be loosely thought of as a smoothing function

for the log-squared response, showing the envelope of

the data. This can be more revealing as to the audibility

of an event. The impulse response, log-squared

response, and energy-time curve are all different ways

to view the time domain data.

46.3.5.3 A Global Look

When starting a measurement session, a practical

approach is to first take a global look and measure the

complete decay of the room. The measurer can then

choose to ignore part of the time record by using a time

window to isolate the desired part during postprocess-

ing. The length of the time window can be increased to

include the effects of more of the energy returned by the

room. The time window can also be used to isolate a

reflection and view its spectral content. Just like your

life span represents a time window in human history, a

time window can be used to isolate parts of the impulse

response.

46.3.5.4 Time Window Length

The time domain response can be divided to identify the

portion that can be attributed to the loudspeaker and that

which can be attributed to the room. It must be empha-

sized that there is a rather gray and frequency-depen-

dent line between the two, but for this discussion we

will assume that we can clearly separate them. The

direct field is the energy that arrives at the listener prior

to any reflections from the room. The division is fairly

distinct if neither the loudspeaker nor microphone is

placed near any reflecting surfaces, which, by the way,

is a good system design practice. At long wavelengths

Figure 46-15. The impulse response, SIA-SMAART.

Figure 46-16. The log-squared response, SIA-SMAART.

Figure 46-17. The envelope-time curve (ETC),

SIA-SMAART.