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sequence of sensory inputs have arisen from the same source. Such betting principles could take advantage of properties of sounds that had a reasonably high probability of indicating that the sounds had a common origin.” (ibid, p.24).
Of comparable interest is the fact that listeners formed remarkably similar and consistent sound quality ratings when listening to widely different kinds of music, most of which was created in recording studios. Listeners in the tests never heard it in the control room and thereby had no true reference of excellence—there was only one’s internal gener- ic sense of what it might have sounded like. But, without a certain mental image of perfection, one may be able to rec- ognize imperfections—aberrations, colorations or distor- tions that are not part of any natural sounds. Evidence that this might be so could be seen in the descriptive reports pre- pared by the listeners. Detailed essays incorporating some- times colorful language described unpleasant attributes asso- ciated with low scores, while high scores were justified with few words of flattery. Could the “best” loudspeaker simply be the one perceived to be “least bad”?
Summarizing what these listeners seem to have done, they first separated the timbral contributions of the loud- speakers from those of the rooms in which they were evalu- ated. This was done while listening to program material for which they could not have had a “live” reference experience. And, finally, they were able to identify, and rate, loudspeak- ers according to the degree of imperfection. All highly rated loudspeakers conformed to very simple “motherhood” objec- tives: smooth flat axial frequency response, relatively con- stant directivity, and low distortion. This is remarkable, yet my colleagues and I have conducted hundreds of such tests over about 35 years, and there have been no surprises.
Closing the loop
Until recently the observed relationships between meas- ured data and listening test results have been entirely subjec- tive. That we had no numerical correlations didn’t mean that we couldn’t see what good loudspeakers “looked like” in spin- oramas. Nevertheless, ultimately, the objective was to devise an algorithm for processing measured anechoic data that yielded predictions of subjective ratings in rooms. In 2004, Olive (Olive 2004a, 2004b) assembled data on 70 loudspeak- ers that had been used in competitive analysis of products at Harman International. They ran the gamut from large expen- sive floor standing units to small bookshelf units. For each of them he had the results of double-blind listening tests in a room, and anechoic spinorama data. Based on years of obser- vation, and psychoacoustic research data, he created metrics and exercised them in a multiple regression model. The result was a correlation coefficient of 0.86 between a rating predict- ed from anechoic data, and the results of listening tests con- ducted in a normal listening room. Clearly this is not guess- work. This is benchmark research, but it is not a complete answer. In these tests the listening room was a constant fac- tor, meaning that all room mode and adjacent boundary issues were fixed. And, this was a domestic/control-room- size room. Comparably competent listening tests and corre- lations have yet to be done in large venues.
About 30% of the factor weightings leading to sound qual- ity ratings related to bass performance. Therefore, in calibrat- ing systems in different venues, in addition to spinorama data, we will need some in-room measurements, and possibly some room-specific adjustments at low to mid frequencies.
The roles of room acoustics, acousticians and psychoacousticians
In auditoriums for sound production, the room is part of the performance, and therefore it matters greatly. The science applicable to this is very well documented, and research is ongoing. However, the small rooms in which we are enter- tained at home, and control rooms in which music record- ings and many components of film soundtracks are created, are very different matters.
The room is the dominant factor at low frequencies— standing waves, and the manner in which sources and lis- teners interact with them are the central issues. Room dimensions, acoustical absorption and its placement, loca- tions of sound sources and listeners, are prime determinants of the spectral and temporal quality of bass that is heard. Massive amounts of low frequency damping helps, but is costly and/or bulky—not compatible with common notions of interior décor. Because all modes are not equally ener- gized by woofers and not equally heard by listeners, the tra- ditional “ideal room” investigations do not yield generaliz- able solutions. The supposedly advantageous dimensional ratios apply only to predetermined arrangements of sound sources and listeners within the room boundaries. With multiple sound sources operating independently (i.e. con- nected to separate channels) the acoustical coupling to the room modes is simply not predictable. However, if the mul- tiple sources of low frequency energy are driven by the same signal (bass management in surround processors), it is pos- sible to employ strategies of constructive and destructive interference among the low-frequency room modes to con- trol the modes in a perfectly rectangular space that are and are not energized. This allows the placement of multiple lis- teners in regions where the bass may be more uniform and more similar.
Taking this to a higher level, one that includes rooms of arbitrary shape and allowing for more flexible arrangements of listeners and subwoofers, it is possible to process the sig- nals supplied to each subwoofer, manipulating the room modes so that the result is a more uniform bass performance at several listening locations, and a superior bass perform- ance in all locations. Interestingly this can be very successful with no low-frequency absorption other than that naturally occurring in the room boundaries. Adding absorption sim- ply makes it easier. All of this is discussed in detail in Toole, 2008, Chapter 13, and references therein.
Above what I call the transition frequency (called the Schroeder crossover frequency in large auditoriums), around 200-300 Hz in domestic-size rooms, the direct sound and first reflections dominate what is measured and heard, meaning that loudspeaker directivity is a major factor, as well as the frequency-dependent absorption at the reflection points. Evidence suggests that listeners prefer loudspeakers
42 Acoustics Today, April 2013