Page 40 - 2013 Spring

(timbre), humans can track complex reflective patterns in rooms and adjust our processes to compensate for much that they might otherwise disrupt in our percep- tions of where sounds come from, and of the true tim- bral signature of sound sources. In fact, out of the com- plexity of reflected sounds we extract useful informa- tion about the listening space, and apply it to sounds we will hear in the future. We are able, it seems, to separate acoustical aspects of a reproduced musical or theatrical performance from those of the room within which the reproduction takes place. This appears to be achieved at the cognitive level of perception – the result of data acquisition, processing and decision making, involving notions of what is or is not plausible. All of it indicates a longstanding human familiarity with listening in reflective spaces and a natural predisposition to adjust- ing to the changing patterns of reflections we live in and with. The inevitable conclusion is that all aspects of room acoustics are not targets for “treatment”. It would seem to be a case of identifying those aspects that we can, even should, leave alone, and focusing our atten- tion on those aspects that most directly interact with important aspects of sound reproduction—reducing unwanted interference on the one hand or, on the other hand, enhancing desirable aspects of the spatial and timbral panoramas.” (Toole, 2008, p.171).
There is a caution to be noted here. It is that adaptation takes time. When we are moving around we hear things that may gradually disappear when we sit down, or which may not be identified at all if one is seated when the sound begins
A dramatic example of the power of this adaptation is described in Section 11.3.1 (ibid), where three very good loud- speakers were subjectively compared to each other in four dif- ferent rooms. In addition to live (listener in the room) double- blind, randomized, comparisons, binaural recordings were made for subsequent comparisons using insert earphones. It turned out that when the comparisons were organized in the manner of the live tests, one room at a time, the binaural test results were essentially the same as the live results. Statistically, the variable “loudspeaker” was highly significant (p = 0.05) and “room” was not a significant factor. Then those same bin- aural recordings were presented in a different sequence, allow- ing each loudspeaker in each room to be compared to each other. The results were very different: “room” was a highly sig- nificant variable (p = 0.001) and loudspeaker was not a signif- icant factor. The sound of the room had merged with the sound of the loudspeaker and could not be separated because listeners had no opportunity to adapt. In this version of the test, the sounds of the different rooms were more distinctive than the sounds of the different loudspeakers. Among other things this is a caution to observe when performing binaurally recorded subjective comparisons.
Characterizing the sound source: collecting the data
Describing the three-dimensional sound fields emanat- ing from voices and musical instruments could be one of those endless tasks because they exist in infinite variations.
However, describing loudspeakers intended to reproduce voices and musical instruments is entirely feasible, indeed desirable, if one expects to reproduce those sounds without degradation. Ideally, we would look for indications of trans- parency, or “neutrality”. Because we listen in reflective rooms, it is necessary to make many measurements.
Beginning in the early 1980s I collected data on loud- speakers over full horizontal and vertical orbits. It was very revealing of what listeners were responding to when judging sound quality (Toole, 1985, 1986). A smooth and flat on-axis frequency response was a starting point. As loudspeakers improved, it became clear that the loudspeakers awarded the highest subjective ratings also had relatively smooth sound power—i.e. relatively constant directivity vs. frequency. It was also shown that these anechoic data were capable of closely predicting steady-state room curves measured at the listening positions in a small room (Toole, 1986, Figures 18 – 20). This provided the basis for taking the technique to a higher level by combining measurements made at different angles to estimate the sounds arriving at a listener’s ears in more generalized listening rooms (Devantier, 2002).
What we now call the “spinorama” consists of 70 ane- choic measurements made at 2 m at 10° increments on hori- zontal and vertical orbits, frequency resolution 2 Hz (1/20- octave smoothed). These data are then processed to reveal:
• The on-axis curve: important to design engineers and solo listeners.
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Music, Rooms, and Listeners 39