1. Reflection
In completely free space, sounds travel outwards from their source with diminishing intensity until all the energy has been dissipated in the ever-widening wavefront or lost as heat in the air itself. By contrast, sound waves in a concert hall are repeatedly turned back on themselves and bounced in criss-cross patterns throughout the enclosed space. The audience therefore hears not only the direct sound but also a mixture of later—and weaker—sounds. These multiple reflections are delayed in accordance with the extra distance travelled; and they diminish in intensity through normal dissipation and absorption at each boundary reflection.A near-perfect reflector such as a polished wood floor will reflect almost 100 per cent of the incident energy, but soft furnishings, porous fibre-tiles, or pliant panels will absorb part of that energy. In practice, the different kinds of absorber are frequency-selective, and good acoustic design depends on careful disposition of various absorbers to control reflections evenly at bass, middle, and treble frequencies.
2. Directional effects
Sound sources are described as directional or non-directional depending on whether they are physically large or small compared with the wavelength of the musical notes being radiated (see acoustics, 10). For similar reasons, reflectors of different sizes and shapes may modify the distribution, i.e. diffusion, of sounds throughout a room not only by frequency-selective reflection but also by re-radiating some bands of frequencies uniformly and others in a directional manner. If curved surfaces are essential, a convex shape is generally preferred because it tends to scatter sounds and help produce uniform listening conditions. Concave surfaces focus sounds back along their axis and give rise to local echoes or dead spots.A domed ceiling is a classic example of the concave shape to be avoided. The Royal Albert Hall in London is perhaps specially unfortunate in having both an oval plan and a high domed ceiling focusing sounds on to parts of the audience area. The effects of marked echoes were complained of for many years until arrays of ‘flying saucers’ were suspended beneath the huge dome. The underside of the saucers is convex, to scatter the upward-travelling sound waves, and their tops carry absorbent material to capture any sounds that missed the saucers on the way up and rebounded from the ceiling. In the same way, recesses or coffering should be of generous proportions so that their scattering effect will be felt through most of the frequency spectrum.
3. Reverberation
Although members of an audience receive the direct sound followed by a wedge or ‘tail’ of countless reflected waves, they are not normally conscious of these as separate entities or echoes. The hearing mechanism (see ear and hearing) works in such a way that sound repetitions arriving within about 1/20; of a second of each other are run together and heard as one. Note, however, that ‘flutter echoes’ can arise between parallel walls.The prolongation effect is known as ‘reverberation’. A smooth decay is to be preferred, secured by careful acoustic design to produce evenly diffused sounds. The time taken for sounds to fall to inaudibility is called the ‘reverberation time’ (strictly the time to fall to a millionth of its original value, or to −60 dB). Reverberation time increases in direct proportion to the volume (size) of the enclosure—the greater distances stretching the decay period—but is reduced by the introduction of absorbent materials. An audience also mops up sound energy quite effectively, so rehearsals in an empty hall sound much more reverberant than the actual concert. To reduce this difference, modern concert-hall seating can be designed so that each seat absorbs about the same amount of sound whether occupied or not.
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4. Designing for good acoustics
For speech, the principal criteria for design are adequate loudness and a high degree of intelligibility. This suggests a short reverberation time; yet too dry an acoustic will lack the reflected energy needed to carry adequate sound levels to listeners at greater distances from the platform. Attention to room shape and seating layout is necessary, and a sloping or raked floor will help to give listeners in the back row a clear view of the speakers and a better chance of hearing properly.For music, there are additional acoustic requirements, making acoustic design as much art as science. From an examination of existing halls generally rated as having ‘good acoustics’, Leo L. Beranek, for example, listed 18 criteria of quality in his book Music, Acoustics and Architecture. Historically, increasingly large halls have been built, with correspondingly greater reverberation times, as the size of orchestras has grown. Thus Baroque and chamber music are suited to a reverberation time of less than 1.5 seconds, Classical music about 1.7, and Romantic music about 2.2 seconds. A longer decay at low frequencies makes for fullness of tone or warmth, whereas good definition or clarity demands a rise at high frequencies.
Modern concert halls often incorporate some means of varying the reverberation characteristics to suit different musical or non-musical events. A good example is Symphony Hall in Birmingham (opened in 1991 ) where a movable circular canopy over the platform area directs sound towards specific regions of the auditorium, and reverberation chambers round the periphery can be opened to increase reverberation time.
Performing musicians naturally demand a sense of ease and power in producing adequate tone without fatigue. This is helped by strategic placing near the players of reflecting surfaces which also enable them to hear each other clearly. There seems no doubt that composers of all periods consciously or unconsciously wrote in such a way as to suit the environment in which their music would be performed.
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