Handbook for Sound Engineers

1612 Chapter 46

environment. The 0 line represents the ambient (no sig-
nal) state of the medium being modulated. This would
be ambient atmospheric pressure for an acoustical wave,
or zero volts or a dc offset for an electrical waveform
measured at the output of a system component.
Fig. 46-10 shows the same waveform, but this time
played over a loudspeaker into a room and recorded.
The waveform has now been encoded (convolved) with
the response of the loudspeaker and room. It will sound
completely different than the anechoic version.
Fig. 46-11 shows an impulse response and Fig. 46-12
shows the envelope-time curve (ETC) of the loud-
speaker and room. It is essentially the difference
between Fig. 46-9 and Fig. 46-10 that fully character-
izes any effect that the loudspeaker or room has on the
electrical signal fed to the loudspeaker and measured at
that point in space. Most measurement systems attempt
to measure the impulse response, since knowledge of
the impulse response of a system allows its effect on

any signal passing through it to be determined,

assuming the system is linear and time invariant. This

effect is called the transfer function of the system and

includes both magnitude (level) and phase (timing)

information for each frequency in the pass band. Both

the loudspeaker and room can be considered filters that

the energy must pass through en route to the listener.

Treating them as filters allows their responses to be

measured and displayed, and provides an objective

benchmark to evaluate their effect. It also opens loud-

speakers and rooms to evaluation by electrical network

analysis methods, which are generally more widely

Figure 46-7. A room with RT 60 >2 seconds.

Figure 46-8. A room with RT 60 = 2 seconds.

Figure 46-9. Amplitude versus time plot of a male talker
made in an anechoic environment.

"zero" line

Amplitude

Ambient level

Time

Figure 46-10. The voice waveform after encoding with the

room response.

Figure 46-11. The impulse response of the acoustic envi-

ronment.

Figure 46-12. The envelope-time curve (ETC) of the same

environment. It can be derived from the impulse response.

Time

Amplitude