Reverberation room measurement to ISO 354: procedure, specimen, cost

ISO 354 describes the measurement of sound absorption in reverberation rooms. The reverberation room creates a diffuse sound field in which sound reaches the specimen from all directions. That corresponds to the situation in real rooms. The result is the sound absorption coefficient αs per one-third-octave band, i.e. the share of the incident sound energy that the product does not reflect back.

What is measured is the product, not just the material: build-up, thickness, perforation, fleece and mounting type all enter the result. That is why the reverberation room measurement is the evidence that acousticians and specifications demand for acoustic panels, ceiling tiles, ceiling rafts, screens or curtains. The impedance tube measurement to ISO 10534-2, by contrast, characterises small material samples at normal sound incidence.

The standard requires a room volume of at least 150 m³; 200 m³ is recommended for new rooms. Above 500 m³, air absorption limits the accuracy at high frequencies. Typical test rooms lie between 200 and 400 m³. The walls are acoustically hard, no pair of room dimensions stands in a simple whole-number ratio, and diffusers on the ceiling and walls provide the necessary diffusivity of the sound field.

Temperature and humidity are recorded because the air itself absorbs. The empty-room measurement and the measurement with the specimen must take place under comparable conditions, otherwise air absorption distorts the difference.

Planar absorbers are tested as a single contiguous rectangular area. ISO 354 requires 10 to 12 m²; for rooms above 200 m³ the upper limit may grow by the factor (V/200)^(2/3). The aspect ratio lies between 0.7 and 1, and no edge of the specimen is closer than 1 m to a room edge. In practice around 11 m² is usually sufficient. The edges are covered with an acoustically hard frame so that the cut faces do not absorb as well.

The mounting type co-determines the result and is therefore stated in the test report. ISO 354 defines standardised mounting types: type A lies directly on the surface, type B is bonded directly to a solid substrate, type E has a cavity behind it, given in millimetres (E-200 stands for a 200 mm gap, typical of suspended ceilings), type G applies to curtains at a distance from the wall. Each mounting type is a separate measurement.

Single objects such as ceiling rafts, baffles, screens or furniture are not measured as an area but as objects. As a rule, three identical samples are enough. The result is the equivalent absorption area per object; the rating is governed by ISO 20189. For all specimens: send the production version, describe the build-up in writing, label the samples.

First the reverberation time of the empty reverberation room is measured, then the reverberation time with the specimen installed. Both per one-third-octave band from 100 to 5000 Hz, i.e. in 18 bands. Excitation is by interrupted noise or via the integrated impulse response; the evaluation usually uses T20 from the decay range between 5 and 25 dB below the initial level.

For a reliable average the standard requires at least 12 decay curves per band from several loudspeaker and microphone positions, for example two loudspeaker and six microphone positions. Microphones are at least 1.5 m apart, 2 m from the loudspeaker and 1 m from room surfaces and the specimen.

The equivalent absorption area of a room follows from Sabine: A = 55.3 · V / (c · T), with V the room volume, c the speed of sound and T the reverberation time. In addition, air absorption is subtracted with the term 4 · V · m. The specimen yields the difference A_T = A_2 (with specimen) minus A_1 (empty). The sound absorption coefficient is then αs = A_T / S, with S the specimen area.

For single objects, A_T is divided by the number of objects and stated as equivalent absorption area per object in square metres. The calculator below shows the relationship for any reverberation room: enter volume, specimen area and both reverberation times, read off αs.

ISO 11654 turns the 18 one-third-octave values into a single number. First the one-third-octave values are averaged to practical sound absorption coefficients αp per octave band from 250 to 4000 Hz and rounded to 0.05; values above 1.00 are set to 1.00. Then a standardised reference curve is shifted in steps of 0.05 until the sum of the unfavourable deviations is at most 0.10. The value of the shifted curve at 500 Hz is the weighted sound absorption coefficient αw.

The absorption class follows from αw: A from 0.90, B at 0.80 to 0.85, C at 0.60 to 0.75, D at 0.30 to 0.55, E at 0.15 to 0.25. If the measured curve exceeds the reference curve in one range by at least 0.25, a shape indicator L, M or H is appended. αw and class always apply only to the measured build-up including the mounting type.

A test report to ISO 354 describes the test object (build-up, dimensions, mass per unit area or density, mounting type), the test room (volume, surface area), climate, measurement method and number of measurement positions. Its core is the table with αs per one-third-octave band, usually with the underlying reverberation times and as a curve. To ISO 11654, αp per octave, αw, absorption class and shape indicator are added.

For the specification, what counts is αw with mounting type plus the band values. A report without the mounting type is only half the information: a ceiling tile measured with E-200 can perform much worse bonded directly (type A).

Test laboratories in Germany do not publish prices for ISO 354 measurements; quotes are available on request. Via Acoustic Index, a reverberation room measurement costs EUR 590 per measurement, including mounting in the reverberation room and a test report with αs, αp, αw and absorption class. Several measurements cost less per measurement. Specimens are accepted from all over Europe; an e-mail is enough.

What determines the price is the number of build-ups: every mounting type and every variant is a separate measurement. Anyone who wants to verify several thicknesses or cavities plans them as a measurement series and benefits from the lower price per measurement.

The impedance tube to ISO 10534-2 measures small round samples at normal sound incidence. It is fast and inexpensive, ideal for material development and for material data. But it does not provide product evidence: mounting type, perforation, frame and edge effects do not occur, and the values are not comparable with αs from the reverberation room.

For specifications, design software and comparison with competitors, the reverberation room measurement is what counts. On Acoustic Index, data sources are therefore labelled: reverberation room measurement of the product, impedance tube, material measurement or manufacturer’s datasheet.

Reverberation rooms differ in volume, diffusivity and the edge length of the specimen. Round-robin tests show deviations between laboratories of a few hundredths up to around 0.1 in αs, especially below 250 Hz. Comparisons between products are therefore most reliable when mounting type and specimen size are the same; ideally the measurements come from the same laboratory.

Values above 1.0 do occur. They arise from diffraction at the specimen edges, which enlarges the effective area, and are a measurement effect of the method, not a product feature. ISO 354 leaves them as they are, ISO 11654 caps αp at 1.00. Claims such as αw 1.2 are not credible.