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Riverbank Acoustical Laboratories - Reverb Chambers

Independent Acoustic Test Data



How are reverb chambers used to test absorption?

The measurement of the intrinsic absorptive properties of a material or item is not possible in a normal room, as the reverb field is not diffuse, and the reverb time is too short. The reverb chamber at RAL is a specially designed test facility designed to overcome these limitations and is comprised of a large volume room [about 10,000 cubic feet] with highly reflective surfaces. There are also several diffusion panels in the test chamber including a set of large rotating vanes hung from the ceiling. These vanes are slowly rotated when the chamber is in use taking measurements. The result is a room in which a highly diffuse reverberant field is created [sounds comes from all directions - more or less evenly distributed], and in which, once a sound occurs, it will reverberate [echo] for a long time. Reverb chambers such as the RAL "Room 0" are capable of taking accurate absorption measurements because they have long reverb times and highly diffuse reverb fields.

When conducting standard absorption tests, and seeking to provide for the repetition of such tests, one must define a position for the measurement microphone. In standard tests RAL uses a defined microphone position and sample area. The microphone is mounted on a tilted boom that is slowly rotated during measurements. The boom length can be altered, Mic Boomand the entire rotating system can be tilted, and these factors when maximized assure that the microphone varies its position to the largest degree possible during the measurements. The longer the boom radius is, and the greater the tilt, then the greater the variance in mic position during rotation and the more accurate the measurement. These factors are limited by the fact that the microphone cannot be allowed during its revolutions to come too close to any room surface [including the diffusion panels and vanes].

The final key variable in absorption testing is sample placement. There are defined locations on the test floor typically used for all absorption testing and chosen based on sample size. One of these defined areas is marked permanently by a white square painted on the RAL test chamber floor and which you can see in the picture below. These defined areas have been established from the strictures of testing standards which require a minimum amount of material to used in each test, and from the experience of the laboratory staff in arriving at placements which achieve consistent measurement results.

Once the parameters of microphone and sample placement are fixed, then the chamber is sealed, the lab technicians repair to a control room and introduce a white noise signal into the room and measure the decay of the sound impulse. EngineeringThis process is repeated many times - RAL does the procedure 80 times and then correlates the results to produce a measurement. These results are then compared to similar measurements taken of the chamber while empty. By comparing the rate of the decay of the diffuse reverberant sound field in the empty chamber with the rate of decay that occurs with the specimens in place, the lab is able to measure the absorptive properties of the sample.

The measurements taken in this manner are reported per one-third octave frequency band. The accuracy of the measurements vary per band depending on several factors including the modal structure of the test chamber, the acoustic reflectiveness of the various room surfaces and the reverb time [how long the echo lasts] as to that band of frequencies. In calculating the measurement for each section of the frequency band, the lab can also calculate the accuracy of the measurement - expressed as "% Uncertainty".

Absorption measurement standards such as ASTM C423 do not address taking measurement at very low frequencies. As a result taking absorption measurements below 100 Hz is an "out of spec" proposition. Therefore, in order to rationally review and understand low frequency absorption measurements you must take careful note of the sample placement and give consideration to the % Uncertainty of the measurements of each frequency band. Incomplete absorption measurement reports which do not provide this information should not be taken seriously in the lower frequency levels, as sample placement can have a large effect on measurement results, and it is in the very lowest frequency bands that the inaccuracies of measurement maximize. The RAL reports Ready Acoustics has published on corner mount absorption test includes appendixes which provide all this information. Note that only those frequency bands which have some reasonable accuracy are reported by Ready Acoustics in its product descriptions, and the calculated % Uncertainty is provided in the lab reports we publish, so you can know all the facts related to the measurement.

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