Page 39 - 2013 Spring

Fig. 2. A concept presented by Benade, (1984) extended by Toole.
er, is a substitute for two ears and a brain.
• To begin with, even though 1/3-octave filters approxi-
mate the critical bands/equivalent rectangular band- width (ERBs) over some of the frequency range, tim- bral cues in the form of beats and roughness originate within each of those bands—we need higher resolu- tion if we are to have an adequate predictor of per- ceived timbre from sound reproducing devices (Toole, 2008 pp. 450-451).
• The common ± 3 dB tolerance is extremely generous, especially because there is no bandwidth associated with it. Humans respond to localized spectral variations at much lower amplitudes (Toole and Olive, 1988).
• The measurements include the room, and the associat- ed non-minimum-phase reflections. Humans treat these very differently than measuring devices, because they arrive at different times and amplitudes, and from directions different from that of the direct sound. What may be perceived as innocent, indeed pleasurable, spa- ciousness to a human may be interpreted as a bump or dip in a measured curve that suggests a need for equal- ization. Evidence of non-minimum-phase phenomena should not be equalized, or what is thought of as a remedial measure has the potential to create audible problems. I suspect that this misuse of equalization is responsible for much of the criticism of it.
• The “room curve” may fluctuate because of amplitude response flaws in the loudspeaker or because of fre- quency-dependent directivity. Equalization can com- pensate for the former, but not the latter. Neither can it compensate for all fluctuations caused by frequency selective absorption at room boundaries. Significant understanding of underlying causes is required before deciding on remedial actions.
• Finally, there is indecision about the target curve to which a sound system is equalized. There is broad agreement that a flat steady-state “room curve” sounds too bright. So, depending on the venue and the pro- gram, different installers/consultants/industries
employ different forms of high-frequency rolloff and/or downward spectral tilts. This is usually done with no knowledge of, or requirements for, the loud- speakers or the rooms, and yet what is measured embraces both. Such practices cannot be generalized.
This incomplete list of issues refers to common practice within the audio industry. But, more seriously, some or all of them are embodied in international standards purporting to set objectives for sound quality within the broadcast and film industries.
Can we do better? Almost certainly. I can think of no better way to introduce the viewpoint than with Fig. 2, begin- ning on the left with the basic observation from Arthur Benade, including my embellishments to bring it into the present context.
This is a significant change in perspective, yet it aligns with everyday experience. We can track a voice as a conver- sation moves from one room, down a corridor to another room. There are huge, complex, changes to the sounds arriv- ing at our ears, and yet subconsciously we know that the sound of that voice remains essentially constant. It is per- ceived as a voice in changing acoustical contexts. Some of us have experienced moving around within a space, listening to a repeated passage of music, noting that what we hear is more stable than the varying details in “room curves” measured where we are located. When we stop moving, and adapt to the acoustical circumstances, rooms tend to become con- texts. To a physicist, a room adds an impossibly complicated distortion of the transfer function between a sound source and a listener. To a listener, a good room embellishes the music. Understanding how reflections are perceived is important (Olive and Toole, 1989).
Adaptation—adjusting to life in an ever-changing (acoustical) world
Chapter 9 in my book discusses adaptation, beginning with:
“In the contexts of precedence effect (angular localiza- tion), distance perception and spectral compensation
38 Acoustics Today, April 2013