Acoustics & Noise Control: Putting it all together
When a sound is generated by a source within a room, there are two ways that the sound reaches the receiver, and the sound that is heard by the receiver is a combination of the two. The sound reaches the receiver by both a direct path and a path caused by reflections on one or more surfaces of the room. The various sound paths within a room can be shown as a ray trace as shown in Figure 1. The time it takes for the sound to reach the receiver depends on the length of the path the sound takes. The longer the path of the sound, the longer the time it takes to reach the receiver. Thus, it is apparent that the sound taking the direct path reaches the receiver before any of those which are reflected. In the same way, sounds that are reflected off several surfaces take longer in reaching the receiver than those which only go through one reflection.
Figure 1 Sound paths in a room.
The direction at which the sound reaches a receiver also affects the perception of a sound. Lateral reflections or reflections that reach the receiver from the side help in giving the sound a spatial characteristic. Whereas rear reflections, or reflections that reach the receiver from behind, tend to confuse the listener especially if there is a long delay. In larger rooms, the number of reflecting surfaces and their longer propagation times, greatly affect the perception of a sound within the room.
When a sound pulse is generated in a quiet room, the perceived duration of the pulse may be longer than the pulse's actual duration. This is due to the delayed arrival of the reflected sounds. The time difference between the end of the actual pulse and the end of the perceived pulse is called the decay time.
Reverberation is defined as the persistence of sound due to repeated boundary reflections after the source of sound has stopped. This phenomenon should not be mistaken for an echo, in which distinct repetitions of a sound can be heard. A room's reverberation time, also called T60, is the time in seconds that is required for the sound level to decay by 60 dB after a sound source is abruptly turned off. The principal acoustical design factor for a room is its reverberation time. The reverberation time of a room depends on the volume of the room and the absorptive characteristics of the surfaces in the room. Excessive reverberation gives the perception of "boominess", or makes the room sound "live." Although this is favorable for musical performances, it reduces the intelligibility of speech by causing phrases and syllables to overlap and thus confuse the listener. A lack of reverberation time results in the room sounding "flat" or "dead."
The optimum reverberation time for a particular usage of a given space is determined by consulting a chart such as that of Figure 2. The chart contains plots of reverberation time at 500 Hz as a function of the volume of a given space for various classes of usage.
Figure 2 Reverberation Criteria. (Source: Engineering Acoustics & Noise Control)
The reverberation time of a room is frequency dependent, and is usually measured for each octave band. The reason for this is that a one-octave change in frequency anywhere within the audible range is perceived to have the same significance. For example, if the frequency of a sound was to change from 1000 to 2000 Hz, the change would be easily detected by the human ear, whereas a frequency change from 1000 to 1010 Hz would probably be imperceptible. The ten standard octave bands have frequencies centered at 31.5, 63, 125, 250, and 500 Hz, and 1, 2, 4, 8, and 16 kHz. Each octave band center is approximately double the preceding one as is its bandwidth. Because human response to various frequencies is non-uniform and non-linear, certain frequencies must be allowed to persist for a longer period of time than others. Research has shown that low frequencies require a longer reverberation time than high frequency sounds for a given space to be rated "acceptable" by the listener. Therefore, the determination of the proper reverberation time for a given space not only depends on Figure 2 but must also be adjusted by using Figure 3.
Figure 3 Variation in reverberation time with frequency. (Source: Engineering Acoustics & Noise Control)
The figure above indicates that the optimum reverberation time for all frequencies above 500 Hz is constant as determined from Figure 2. The optimum reverberation time for lower frequencies is found by multiplying the T60 at 500 Hz by a correction factor determined from Figure 3.
Since the volume of a room is usually fixed, its reverberation time is controlled by the type of surfaces used in the venue. When a sound strikes a surface, a portion of its energy is reflected off the surface, some is absorbed by the surface and dissipated as heat, and the rest is transmitted through the surface. The amount of sound energy absorbed is usually governed by the density of a material. A reflective material is usually called a "hard" surface and conversely an absorptive material is called a "soft" surface. The mounting of a surface also affects its absorptive qualities. If a surface is not firmly mounted, and is able to vibrate at the frequency of the impinging sound, the material will tend to vibrate at this frequency using the energy of the impinging sound. This causes the absorption of the sound energy in a way similar to that of suspension systems. This effect may be a disadvantage or used as an advantage such as in the use of suspended acoustical tile, depending on the required acoustical characteristics for the room.
The absorptive characteristics of a surface are determined empirically and are quantified as the material's absorption coefficient (a). The absorption coefficient of a material is the percentage of incident sound that is not reflected. Thus a perfectly absorptive surface would have a coefficient of 1. Like many acoustical properties, absorption coefficient is a function of frequency. Because of this, the absorption coefficients of materials are usually given for each octave band. Generally, absorption coefficients are proportional to frequency, and therefore materials are less absorptive at lower frequencies.
Therefore, by changing the surfaces in a given volume, or room, we are able to change its acoustical qualities. A "hard room" or a room with concrete walls and a hardwood floor would have a relatively long reverberation time such as a gymnasium. The addition of carpeting, drapes, acoustical paneling, an acoustic ceiling, soft furniture, or other items with high absorption coefficients changes the room characteristics to an acoustically "medium" or "soft room."