Traditional Music Performance

Traditional music performance generally takes place in reverberant halls, usually with a raised stage and sufficient volume and floor area to accommodate an economically viable audience in order to support the performance. There are significant differences between the acoustics of and expectations for venues for “acoustic” performance (i.e. “classical” music, et al) and venues for amplified performance (i.e. “rock,” “pop” music, et al).

During such events, an audience is present, and the music performance is generally highly ritualized, with extremely clear and well established cultural expectations on the part of the audience and the performers. There is a clear psychological interaction between the performers and the audience.

Venue Sizes and Acoustics

A reasonable minimum dimension for any live music venue is equivalent to a single low-frequency wavelength, or approximately 50 feet. Indeed, few venues do not have a longest dimension that is significantly greater than this distance. Venue sizes rise to >>1,000 square meters and many tens of thousands of cubic meters volume (for cathedrals). Not all venues are suitable for all genres of music. The general acoustic behaviors needed for various types of music venues are comparatively well-known and understood.

Sound Pressure Levels

The noise floor for a venue with audience present is seldom less than 40 dBA SPL (approximately equivalent to NC 35). Individual direct sound pressure levels from acoustic instruments range from 3 dB above whatever noise floor is present to about 110 dB SPL (at 10 feet from the source). Electric instruments often obtain levels 10-15 dB above that level (also at 10 feet from the loudspeaker). Sustained acoustic levels in the reverberant field of a concert hall seldom exceed 110 dB SPL, while peak levels may reach 120 dB SPL. Again, electric amplification yields levels 10-15 dB above that. Therefore, we can characterize the dynamic range of performed music as approximately 60 dB (50-110 dB SPL). It is reasonable to speculate that for electrically amplified performance the noise floor goes up as the level goes up and that a similar, or even lesser, dynamic range exists for “electric” genres, even though a greater dynamic range could be available for such genres.

Instrumental and Ensemble interactions with Venue

For acoustic musical performance, considerable attention is usually paid to the acoustics of the venue. Concerns pertaining to the arrival time of early reflections in the hall, lateral reflected energy, on-stage acoustics (for the benefit of the performers), and the duration and spectra of the reverberant decay all are addressed. In venues used for amplified performance, such concerns are a bit simpler, consisting primarily of attempting to reduce the amount of reflected and reverberant information as much as possible, and leaving such elements under the control of the amplified sound reinforcement system.

Loudspeakers Playing Back Music

Loudspeakers playing music mostly exist in a different set of environments (except when they are being used to reinforce acoustic instruments in live performance) than live performance venues. It is useful to consider the physical conditions and constraints of loudspeakers in such environments.

Loudspeakers Play Music In Small Rooms, usually simulating larger concert venues of all types and sizes

Loudspeakers generally play back music in small rooms, i.e. rooms whose largest dimension is significantly less than 50 feet. This means that, in general, the longest wavelengths will be folded back into the acoustic space to create interference patterns before they have fully propagated, for an inherently degraded reproduction.

An important feature of these rooms, vis-à-vis live performance venues, is their variability. While live performance venues may have some commonalities and consideration given to their suitability for music performance, the great majority of small rooms in which loudspeakers are placed have no acoustical attention paid to them at all, other than possible sound isolation standards. Further, the variability in shape, volume, surface treatment, noise floor and other relevant characteristics are significantly greater than for live performance venues. In fact, the only general commonalities that can be described for such small rooms are a volume of less than 300 cubic meters, a ceiling height of less than 3 meters and a predisposition (in residential cases) for rectangular enclosures.

In arrays, loudspeakers are generally called upon to generate reproduced sound environments of spaces with significantly greater volumes than the volume of the playback space, and usually those reproduced environments have much longer reverberant times than the playback spaces (2000-5000 vs. 100-400 milliseconds). The spatial cues for such large virtual spaces are transmitted to the listener by both the direct sound and early reflections in the playback room, and the integrity of those cues is dependent on both high and low frequency spectral content.

Directivity

In general, loudspeakers generate sound omnidirectionally at long wavelengths (>5 feet/<200 Hz.) and increasingly directionally at short wavelengths (<1 foot/>1 kHz.). A traditional design goal has been to maintain flat on-axis amplitude response. Therefore, power response has necessarily rolled off as a function of frequency. This significantly affects the interaction of the traditional loudspeaker with the room in which it is placed. Both early reflections and reverberant energy derived from loudspeaker emission have significantly degraded high frequency content.

Sound Pressure Levels, Sensitivity and Power

Loudspeakers are typically rated for sensitivity by measuring their output at 1 meter on axis with a signal input of either 1 Watt (power) or 2.83 Volts (the energy needed to generate 1 Watt with an 8 ohm load), in a free field. Typical sensitivities for domestic loudspeakers range from approximately 80 to 92 dB SPL. A typical stereophonic array of domestic loudspeakers will yield an output 4-5 dB above that when each speaker is driven by 1 Watt, and program is correlated at low frequencies (which is typical). Maximum output is generally limited by amplifier power, and by the limitations imposed by the crest factor of the program (maximum sustained average levels are usually 10 db below peak level, except for “hypercompressed” recordings [2]). Assuming 100 Watts of power per channel and a sensitivity of 88 dB SPL (both typical), a single loudspeaker will generate a peak output of 108 dB SPL at 1 meter and a sustained level of 98 dB SPL. Two loudspeakers will yield approximately 102 dB SPL sustained level. At 3 meters (a typical listening distance) the direct output from the two loudspeakers will attenuate by approximately 9 dB, to 93 dB SPL. Depending on the Critical Distance in the room, the direct–plusreverberant level will be approximately 3 dB above that, or 96 dB SPL. This means that in a typical reasonably high-quality domestic listening situation the maximum sustained output will probably be no greater than 96 dB SPL at the listening position, with peaks reaching 106 dB SPL. This is approximately 14 dB below the maximum outputs of live performances of acoustic ensembles and 24-30 dB below the performance levels of “electric” ensembles.

Linearity and Distortion

Compared to electronic circuitry, loudspeakers are comparatively non-linear in their behavior. Transducers exhibit non-linear behaviors as a function of their motional limits, spectral contents at or beyond the limits of the pass-band, thermal changes in the transducer, and asymmetries and irregularities in the motion of the transducer surface. A “good” loudspeaker exhibits harmonic and intermodulation distortion artifacts of somewhat less than 1% of total amplitude through much of its dynamic range and distortion onset levels are usually determined by a 3% distortion threshold (in contrast, a power amplifier with distortion artifacts greater than .01% would be considered poor). As the physical limits of the various transducers are approached, non-linearity increases significantly for the various reasons noted above.

Accuracy

The term “accuracy,” as applied to loudspeakers, is generally taken to mean “spectral resemblance” to various source instruments. This usage is technically a misnomer (as noted above), and systematic studies and evaluations of accuracy, through rigorous blind comparisons of a loudspeaker with a variety of source instruments and ensembles, are simply not undertaken, due to their difficulty, expense and perceived irrelevance.

Loudspeakers can readily be distinguished from source instruments in most cases, and in direct comparisons are found to sound distinctly different from other instruments.[3,4] This is probably due to (a) differences in directivity between the instruments and (b) differences in the nature, size and location(s) of the instruments’ vibrating surfaces.

There are two further issues with accuracy, as it pertains to loudness. First, single loudspeakers (and small arrays) cannot reasonably approach the sound pressure levels obtained by large or amplified ensembles. Such a failing introduces large errors in magnitude, of course, but also spectral errors dues to the changes in frequency response of the human auditory system as a function of loudness. Second, “accurate” sound pressure levels may not be appropriate (or even legal) for much playback. Therefore, it is axiomatic that music playback via loudspeaker may be attenuated by as much as 60 dB (relative to a live performance) for a variety of playback environments. Such attenuation and its sensory and spectral effects are generally ignored in considerations of accuracy. Those effects are nonetheless quite real, and have a significant impact on the “quality of reproduction” of music.

Stereophony and Surround Sound

Stereophony and surround sound involve the use of an array of loudspeakers operating in phase-locked synchrony. Such synchrony is unique to loudspeaker arrays among arrays of acoustic sound sources, and it enables loudspeakers to generate complex sound artifact volleys which in turn permit humans to perceive two particular sensations unique to loudspeakers: phantom images and phantom environments/envelopment. The effect of these images and environments is profound on human listeners, and constitutes one of the most powerful qualities of loudspeaker music.