Which are the input data useful for the implementation of an absorbing lining in a test room?

The input data useful for the implementation of an absorbing lining in a test room are primarily the use of the room (for acoustic measurement rooms: normative requirements are associated with anechoic or semi-anechoic rooms), the envisaged type of metrology and the frequency band of interest, the nature of the objects under test (size spectrum), the physical setting (area of available surfaces, acceptability of overweight for supports for the wall or for the roof and when applicable for the floor). In very many cases, the reaction to fire of considered materials is also taken into account as well as considerations of appearance (architectural aspect, colors, materials ...).

In case of achievement in the automotive industry, or for other industrial applications related to the industries of transportation or of energy (aero-acoustic wind tunnels), specific constraints related to resistance to impacts or to weather damages may necessitate the use of particular absorbing linings (with waterproof, washable cover) or the use of stainless steels.

Other parameters may be involved such as the authorized mass, the required longevity or the available budget as well as (sometimes) thickness of the lining.

Which absorbing linings in a test room?

As for absorbing linings for testing rooms such as anechoic or semi-anechoic rooms (including: aero-acoustic wind tunnels) are concerned, they generally consists of multilayer modules sometimes (quite often: made of mineral wool or of polyester wool or of melamine foam) often covered by a surfacing (most often: a fiber glass or a polyester cover) and (in many cases) and by a perforated protection (mostly: galvanized or painted steel). A powder coating usually provides the best protection against corrosion: classification up to C5 according to ISO 12944 while providing a visula aspect appropriate for the the context.

In some cases, the frame of the plane absorbing lining in a test room can be made of stainless steel (SS 304, SS 316) or aluminum.

As required, the absorbing lining may be, for some applications, protected by a waterproof surfacing (polyurethane film and for extreme cases: painting).

In order to increase the absorption coefficient in a given frequency range, an intermediate plate (usually made of steel) and also an additional spacing absorbent layer may be inserted into the absorbing linings for test rooms, the performance such absorbing panels then being (partially) based on the effect of resonant membrane (also known as resonator).

In some cases, absorbent linings for test rooms are in the form of wedges (i.e.: pyramid-shaped subsets) while in other cases, compact absorbers or asymmetrical structured absorbers are used.

The prediction of the performance of plane absorbing linings for testing rooms can be performed with the simulation software SILDIS.

Verification of acoustic performance of absorbing linings for testing rooms can be made in accordance with ISO 354 Acoustics - Measurement of sound absorption in a reverberation room or in accordance with the standard NF EN ISO 10534-1 Acoustics - Determination of sound absorption coefficient and impedance in impedance tubes - Part 1: Method using standing wave ratio

Why choose an absorbing lining type "Compact Broadband Absorber" in a test room?

Such a choice is appropriate in all cases where it is lokked for an absorbing lining having both a very good absorption factor, with a minimum thickness, with minimum a cost and with minimum installation costs. The lining type "Broadband Compact Absorbers" - in English abbreviation BCA "Compact Broadband Absorbers", in German BKA "Breitband Kompact Absorbern") is plane: it is particularly appropriate for testing rooms such as anechoic or semi-anechoic rooms (including: aero-acoustic wind tunnels), particularly in the automotive industry.

Why choose an absorbing lining type "Asymmetrical Structured Absorber" in a test room?

Such a choice is appropriate in all cases where it is looked for an absorbing lining having both a very good absorption factor, with a minimum thickness, with a minimum cost and with minimum installation costs. The lining type type "Asymmetric Structured Absorbers" - in English abbreviation ASA "Asymmetric Structured Absorbers", in German ASA "asymmetrisch Strukturierten Absorbern"), has a total thickness less than that of wedges: it is particularly appropriate for testing rooms such as anechoic or semi-anechoic rooms where no metallic perforated protection of the absorbing lining is desired.

Which a performance for an acoustic testing room?

The performance of an acoustic test room (anechoic or semi-anechoic rooms, including: aero-acoustic wind tunnels), in terms of cut-off frequency (frequency above which the absorption coefficient at normal incidence of the absorbing element of the partitions is greater than 99% for wall, roof and where applicable - for anechoic rooms- floorl) is characterized by a measurement of a lining sample in a plane waves impedance tube. By extension (in fact: so shortened), the cutoff frequency is often referring to (in practice) the frequency above which the conditions inside the test room are those of a free (acoustic) field (in the case of a semi anechoic room: with a reflective floor), and sometimes: taking into account the tolerances set by the standard NF EN ISO 3745.

As an order of magnitude, a cutoff frequency below 50 Hz can be achieved with an absorbing lining with a careful design and with a careful implementation.

The performance of an acoustic test room (anechoic or semi-anechoic rooms, including: aero-acoustic wind tunnels), in terms of quality of the free field is very frequency dependent and is primarily related to the performance of the absorbing lining (for wall, roof and where appropriate - for anechoic chamber - floor) (characterized by their absorption coefficient at normal incidence).

This performance can be expressed in terms of deviation of measured sound pressure levels with respect to the theoretical levels obtained using the law of the inverse square of the distance (i.e. in terms of deviation from the spatial sound decay in a free field) by octave or 1/3 octave bands at specified locations.

For orders of magnitude deviations of measured sound pressure levels with respect to the theoretical levels obtained using the law of the inverse square of the distance up to 1.5 dB at frequencies below 630 Hz, up to 1.0 dB at frequencies between 800 and 5000 Hz, up to 1.5 dB at frequencies above 6300 Hz can be obtained in the case of testing stations in accordance with standard NF EN ISO 3745.

The prediction of the performance of acoustic test rooms in terms of cutoff frequency and of quality of the acoustic free field can be performed with the simulation software SILDIS.

The performance of an acoustic test room (anechoic or semi-anechoic rooms, including: aero-acoustic wind tunnels), in terms of sound insulation is very frequency dependent and is primarily related to airborne sound attenuation of the envelope (characterized by its sound reduction index) and also the effectiveness of the vibration control device (characterized by its filtration rate).

This performance can be expressed in terms of background nosie, i.e. in terms of A-weighted overall sound pressure levels or in terms of sound pressure levels in octave bands inside the test room.

For orders of magnitude, inside the testing room, A-weighted overall sound pressure levels of 10 dBA or less or sound pressure levels in octave bands of 0 dB or less can be obtained in the case of some performing testing facilties (always with a special construction).

the verification of the performance of acoustic test rooms (and their qualifications) can be conducted according to standard NF EN ISO 3745 Acoustics - Determination of sound power levels of noise sources - Precision methods for anechoic and semi anechoic rooms.

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