A local concert hall with a capacity of about 1000 has complex baffles and excellent acoustics, except for a few seats to one side near the rear. From there, a percussion stroke tails off into a little bleat. Why does this happen and can it be fixed?
鈥� In any room, a listener hears sound directly from the source, followed by many reflections from the walls, floor and ceiling. The unique pattern of reflections at each location determines the sound field there.
In a concert hall with good acoustics, the later a reflection arrives, the lower its intensity. The little bleat that your correspondent hears at the rear of the concert hall indicates that a series of late reflections arrives with more intensity than the ones preceding them.
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To solve this problem, you must determine which room surfaces participate in these strong late reflections. An acoustical consultant would be able to identify the surfaces of interest. The complex baffles may or may not be involved; the problem may instead originate with the walls. When the surfaces are located, a sound diffuser should be applied to them. This is a special surface treatment designed to spread reflected sound energy in all directions, reducing the reflection intensity in any single direction.
Of course, a sound absorber will also reduce the reflection intensity, but would permanently remove valuable sound energy from the room.
Jonathan Rathsam, Department of Electrical and Computer Engineering, Ben-Gurion University of the Negev, Beer-Sheva, Israel
鈥� The questioner may be experiencing what refer to as a flutter echo. This occurs usually 鈥� but not exclusively 鈥� between two parallel walls and is the result of 鈥渢rapped鈥� sound bouncing back and forth between the two surfaces with the listener in the middle. Our suggestion is based on the questioner describing a 鈥渓ittle bleat鈥�. 鈥淏leat鈥� suggests a regular, repeating pattern of sound and 鈥渓ittle鈥� suggests it is significantly weaker than the direct impulse sound of the percussion.
We hear the reflection of a sound as a separate sound (an echo) only if there is sufficient delay between the original and its reflection for the two to be processed as distinct sounds by the brain. If not, the two will be integrated and we will hear only a single sound. For this to be the case, the time delay must be larger than the integration time of the ear (about 50 milliseconds) so we surmise that the seating position puts the questioner at least 8.5 metres from one of the reflecting surfaces involved. This ensures that the round trip to and from the reflector takes more than 50 milliseconds.
The solution could take one of several forms. The first would be to change the angle of the problem surface to break the reflection pattern. The second would be to place sound-absorbing material on the wall to suppress reflections, or the surface could be reshaped so it diffuses and scatters the sound instead of contributing to a 鈥渂leat鈥�.
Incidentally, coming from New Zealand, we are in a pre-eminent position for interpreting the sound made by our most numerous inhabitant 鈥� the sheep.
鈥淐oming from New Zealand, we are in a pre-eminent position for interpreting the sounds made by sheep鈥�
鈥淎coustics 424鈥� class, School of Architecture and Planning, University of Auckland, New Zealand