The geometry of room surfaces greatly influences the behavior of reflections. Parallel reflective surfaces are common in many rooms, and can be acoustically problematic if sound reflects back and forth between the surfaces. These repetitive reflections, called flutter echoes, can be very audible as a series of impulses. Flutter echo can also be perceived as a pitch or timbre coloration, which degrades sound quality and speech intelligibility.
A room can be tested for flutter echoes by simply clapping hands together and listening for a ringing or fluttering high-frequency sound. (For example, flutter echo can be heard in most stairwells because of the many parallel walls.) The experiment should be repeated at different places in the room where more focused echoes may occur. More rigorously, the reverberation characteristic of the room can be plotted; an impulse is sounded and the energy decay is plotted over time until it disappears relative to the background noise level. An echo will appear as a spike in the sound decay slope; flutter echoes will appear as a series of spikes regularly spaced over time. A plot of a typical flutter echo is shown in Fig. 2-1; the periodic spikes are clearly visible.
Absorption is usually the easiest solution for echoes. Once the walls involved in the echo are identified, absorption can be placed on those surfaces. For example, in the case of a flutter echo, absorbing panels can be placed on one or both parallel walls. As another example, a room with a wood parquet floor and a plaster ceiling could have significant flutter echo; the floor and/or the ceiling must be treated (for example, with carpet and absorptive tiles, respectively) to eliminate the flutter echo. It is important to remember that echoes are created by specific surfaces; if absorption is placed on a surface that is not creating the echo, the echo will be unaffected. For example, in the example above, placing absorbers on the walls would not affect the floor/ceiling flutter echo.
In some cases, instead of using absorption, diffusers can be used to break up echoes. For example, diffusers would be a good choice if it is important to maintain sound energy levels in a room; adding absorption would decrease energy levels. In new construction, walls can be splayed to prevent flutter echoes; a 10:1 splay (1 ft for 10 ft of wall length) is satisfactory. Care must be taken to ensure that another (third) wall does not complete the flutter echo loop.
Reflective concave surfaces will focus sound, creating an area of higher sound level at the expense of lower level elsewhere. This is contrary to the usual need for uniform distribution throughout a room. A domed ceiling is an example of a concave surface, and a common trouble spot for acousticians. Large convex reflective surfaces, unlike concave surfaces, can provide welcome diffusion. Sound striking the convex surface reflects in many directions, distributing a broad bandwidth of sound throughout a room.
shows some of the reflections in a rectangular room traveling from a loudspeaker to a listener. For clarity, only reflections from one loudspeaker are shown. The reflections are individually identified by letters (A–G). Through simple computations based on perfect reflections and inverse square propagation, the magnitude and delay of each reflection are estimated. In particular, the reflection level and delay can be calculated from:
Reflection level = 20log [(Direct path)/(Reflected path)]
Reflection delay = [(Reflected path) − (Direct path)]/1130
All of these reflections are adjustable in regard to amplitude, although the delay values are fixed by the room geometry and dimensions. A given reflection can be reduced in amplitude by applying an area of absorbing or diffusing material at the point of reflection. For example, squares of absorbent on the floor, the side and front walls, and the ceiling can greatly reduce or essentially eliminate the reflections. Absorbers of different absorbing efficiency will affect the reflection amplitude. An acoustician would adjust the amplitude of the reflections to achieve the sense of spaciousness and the stereo image qualities desired. Also, the frequency response of the reflection will be varied depending on the type of absorber.
With respect to reflections, we then have a choice. Some reflections can be adjusted to provide the desired front image and degree of spaciousness, or they can be eliminated (such as the early reflections at the listening position of some control room designs).