Loudspeakers and Rooms for Sound Reproduction—A Scientific Review*

PAPERS Loudspeakers and Rooms for Sound Reproduction—A Scientific Review* FLOYD E. TOOLE ([email protected]) Harman International Industries, Inc., Northridge, CA 91329, USA The physical measures by which acousticians evaluate the performance of rooms have evolved in large performance spaces—concert halls. They rely on assumptions that become progressively less valid as spaces get smaller and more acoustically absorptive. In listening rooms the loudspeakers and the rooms interact differently below and above a transition region around 300 Hz, similar to the Schroeder frequency in large rooms. Above this transition we need to understand our reactions to reflected sounds; below it the modal behavior of the space is the dominant factor. A review of the scientific literature reveals that natural reflections in small rooms are at levels where they are perceptible, and their subjectively judged effects range from neutral to positive. At low frequencies the long-standing problem of room reso- nances can be alleviated substantially through the use of multiple subwoofers, thereby pro- viding similarly good bass to several listeners in a room. A provocative observation has to do with human adaptation to the complexities of reflective rooms, and the extent to which it allows us to localize sounds correctly in direction and distance, and to hear much of the true timbral nature of sound sources. In the case of loudspeakers, an analysis of comprehensive anechoic data is found to be sufficient to provide a good prediction of sound quality, above the low-bass frequencies, as subjectively judged in a normal room. Although the interactions of loudspeakers and listeners in small rooms are becoming clearer, there are still gaps in our understanding. A number of these are identified and are good opportunities for future research. 0 INTRODUCTION spaces ever since Sabine’s pioneering work over a century ago. The design of loudspeakers, listening rooms, and a com- More recent studies have examined which aspects of the bination of these for sound reproduction has evolved with sounds arriving at listeners’ ears contributed to each of the relatively little direction from the acoustic and psycho- identifiable perceptions that, in combination, yielded a sat- acoustic sciences. Only recently has significant effort been isfying concert-hall experience. So even though the sound put into understanding the relationships between what is fields in large, relatively reverberant halls were strongly measured and what is heard in rooms of domestic size. The diffuse, it was recognized that human listeners could re- acoustical measures by which the excellence of rooms is spond with some degree of independence to components judged originated in investigations of performance of that sound field occurring at specific times, or arriving spaces—concert halls and auditoriums. The sound from specific directions. Consequently there now are sev- sources, voices and musical instruments, were considered, eral measures of sound as it decays in specific time inter- as a group, to be essentially omnidirectional, and acousti- vals, and the proportions of sound arriving from different cal measurements in such spaces use omnidirectional directions, all as functions of frequency [1]. sound sources. The premise underlying these measures The essential point about this situation is that the archi- was that the later reflections in the sound field developed tectural space—concert hall, auditorium, or jazz club—is into something that was essentially diffuse and statistically part of the original performance. To such an extent is this random, and that this diffuse—reverberant—sound field so that classical composers routinely adapted their com- extended to the bounds of the space. Reverberation time, a positional styles and instrumentation to fit the acoustical measure of the rate at which this sound field decays, has character of specific performance spaces [2]. been integral to acoustic studies of large architectural In sound reproduction the situation is different. Mea- sured reverberation times in domestic spaces fall in the range of 0.3–0.6 s, in contrast to the objectives for perfor- *Manuscript received 2006 February 17; revised 2006 April 10. mance spaces of about 1.5–5 s, depending on the type of J. Audio Eng. Soc., Vol. 54, No. 6, 2006 June 451 TOOLE PAPERS music. This means that it is unlikely that the decaying we measure differences that we seem not to hear. His sound field in a listening room would significantly impact colleague, James Moir, added in discussion: “Finally, in the apparent decay of a concert-hall recording, or those my view, if a room requires extensive treatment for ste- made in larger recording studios. reophonic listening there is something wrong with the ste- In addition, loudspeakers have well-defined directional reophonic equipment or the recording. The better the ste- characteristics and most are significantly more directional reophonic reproduction system, the less trouble we have than groups of musical instruments. The small rooms in with room acoustics.” which we listen for recreation or monitoring have larger These observations imply that some of the problem lies scattering objects and more absorbing material in them (in in our interpretations of measurements made in small proportion to their height and volume) than performance rooms. The horrendously irregular steady-state “room spaces. The absorbing material is also not distributed uni- curves” that we see simply do not correspond to what we formly, but is concentrated in rugs, carpets, drapes, large hear. Did our problems begin when we started to make pieces of furniture, and so on. This means that the sound measurements? Are we incapable of hearing these things? field cannot be diffuse and, since all rooms are different, it Or is it that we hear them, but they simply become part of is clear that not all consumers are hearing the same repro- the acoustical context within which other acoustical events duced sounds. Yet there appears to be widespread pleasure occur, and we have some ability to separate the two? The and satisfaction with music and movies, even to the point answer turns out to be some of each. of some general agreement on which ones sound good. Once a loudspeaker is in a room, it may be impossible Recording control rooms have drifted in several direc- to identify the contribution that the loudspeaker and the tions driven by fashion, convenience, and opinion, and room each make to the combined measured result. Without they now cover the range from home studios that acous- those separate data, appropriately targeted remedial mea- tically resemble domestic listening spaces, through various sures for problems are not possible. All of this can only be “engineered” spaces (reflections enriched by diffusers or deciphered through an understanding of how the sound depleted by deflection or absorption), to those that are from a loudspeaker radiates into three-dimensional space, nearly anechoic. Because the control room is a factor in is communicated through a room, and then is perceived by determining what is recorded, it is clear that recordings are a listener. The performance of a loudspeaker is much more not created in a standardized fashion. Yet successful rec- complex than anything revealed in an on-axis anechoic ords are made in all of these venues, and one would have measurement. The perceptual processes of two ears and a great difficulty trying to guess the monitoring circum- brain are vastly more complex than anything revealed in a stances by listening to a recording. room curve or a reverberation time. Therefore in spite of gross acoustical variations in the We need to start again. The following is a review of recording and reproduction domains, the music industry acoustic and psychoacoustic research from disparate has prospered for decades. Could it be that good music and fields, put into a framework that is familiar to the audio musicianship can overcome this acoustical anarchy? Or community. The result is intriguing. It turns out that some could it be that humans have a remarkable ability to rec- of our common practices are less than optimum, and some ognize and compensate for these variations? popularly held notions might need to be revised. It will be Whatever the answer, it seems reasonable to strive to clear that not everything is known, and that there are sev- achieve sound reproduction that is essentially independent eral opportunities for significant and useful research. of the acoustical playback space. Timbral accuracy and impressions of direction, distance, and space ought to be 1 A PERSPECTIVE ON SOUND FIELDS delivered by the multichannel reproduction system with IN ROOMS minimal variation attributable to the listening space. The loudspeakers and the listening room should therefore be Reading some of the current literature on the acoustics neutral conveyors of the artistic experience—to both pro- of small rooms, one could easily believe that reflected fessionals and consumers. Neutral, in this context, does sound is a serious problem in need of immediate and ex- not mean acoustically “dead.” We need some room sound, pensive acoustical treatment. Yet attending live perfor- but what kind of room sound? mances in the world’s great concert venues is to experi- It is interesting to note, at this stage, prescient com- ence almost nothing but reflections, and we would not ments by some early workers in the field. Gilford, in 1959 wish to be deprived of any of them. [3], studied

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