C “ Reflected sounds can be responsible for acoustical defects such as echoes,sound foci,dead spots and room flutters. “

ARCHITECTURALACOUSTICS

- rinsha.m.gafoor

• A sound reflected off a surface that arrives at the listener after the direct sound.

• Echoes are reflections that can be heard distinctly and separately from the early reflected and reverberant sound.

o Sound that reaches a listener in a room by a path involving reflections from its boundaries always travels a greater distance than does the sound that comes by the direct path.

o If the difference in these two path lengths is as great as 65ft,which corresponds to a time difference of about 0.06 second, the delay in the arrival of the reflected sound is sufficient to enable a listener to hear it as a separate sound ; ie, the delayed reflection produces an echo.

o Even when the difference in the two path lengths is somewhat less than 65ft , but greater than about 50ft,the delayed reflection may have damaging action.

echo

o It tends to blur or even mask the direct sound.

o Delayed reflections are most detrimental when they are concentrated or focused on a highly reflective concave surface.

o In contrast, they are least damaging when diffused by convex surfaces.

o Ironically, echoes are most commonly detected in the front rows of an auditorium and onstage. This results from the front row being farthest from the rear wall, thus generating the largest path length difference between the direct sound and the sound radiating directly from the rear wall or the combination of the ceiling and the rear wall.

Defect

Cause Solutions

Echoes

• Unsuitable shape

• Remote reflecting surfaces

• Avoid unsuitable shape

• Make offending surfaces highly absorbent

• Providing rough and porous interior surfaces.

o Undesirable acoustical phenomena associated with the shape of a room is flutter echo.

o It occurs between a pair of parallel (opposite) walls in a room.

o Most noticeable in rectangular room when one pair of opposite walls is smooth and highly reflective and the other two opposite walls are treated with absorptive materials.

o A flutter echo can be diagnosed : hand clap

a sharp hand clap produces a multiple echo as the impulse is reflected back and forth between the pair of reflective walls.

If the distance between these walls is

>50 ft - flutter is slow, single hand clap is heard as series of “puts” (put,put,put….)gradually dying away to inaudibility.

Walls nearer-the successively reflected impulses recur more frequently and the series is heard as a prominent flutter or even as a “dry rattle”.

Parallel walls are 8-10 ft apart – sound of a single pulse like a hand clap or a snap of fingers is heard as “buzz” which dies away quite rapidly.

room flutter

o Room flutter frequently occurs in uncarpeted rooms where the ceiling and floor are highly reflective and the walls are broken with windows , doors, hangings, pictures etc.

o Troublesome in broadcasting or sound-recording studios and is annoying in all speech , music , living , or work rooms.

o Flutter echoes can be eliminated by

avoiding the use of parallel pairs of walls , or by “breaking up” the uniformity of such walls with doors, windows , book shelves , hangings , paintings ,splays or patches of absorptive materials.

A very small departure from parallelism between the pair of flutter producing walls , such as a change in the direction or slope of one wall by as little as one part in twenty will suffice.

Flutter echoes can be acoustically treated with careful placement of sound absorption materials such as foam or wall panels on the walls or ceiling tiles, baffles or banners in the ceiling. The idea here is to absorb the sound wave at one or both surfaces and keep that sound wave from reflecting of the surface backtowards the noise source.

Flutter echoes can also be acoustically treated with the use of sound diffusers. Sound diffusers are multi-faceted, slotted or curved materials that are reflective in nature and are designed to scatter or redirect sound waves.

The sound diffusers can break up flutter echo within a room by taking the sound waves and sending them in different directions and eliminating the repetitive reflections caused by reflective, parallel surfaces.

ACOUSTICAL DESIGNING IN ARCHITECTURE BY JOHN WILEY & SONS ,INC. , LONDON

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