What is an Anechoic Chamber?

The anechoic chamber is not only a special laboratory for acoustic testing, but also an important part of the test system. In fact, it is also one of the acoustic testing equipment. Its acoustic performance indicators directly affect the accuracy of the test. The anechoic room is divided into a full anechoic room and a semi-anechoic room. The six layers of the room are called full anechoic chambers, and are generally referred to as anechoic chambers. Among the six sides of the room, only five or four sides are covered with sound absorption layers, which are called semi-anechoic rooms. The main function of the anechoic chamber is to provide a free or semi-free sound field space for acoustic testing. The sound absorption processing is the key to ensure good free sound field performance after the anechoic chamber is completed, and most of them adopt sound-absorbing cleats with strong sound-absorbing ability or flat-plate thin-plate resonance sound-absorbing structure.

Refers to a room without reflections. In the anechoic chamber

The anechoic chamber is not only a special laboratory for acoustic testing, but also an important part of the test system. In fact, it is also one of the acoustic testing equipment. Its acoustic performance index directly affects the accuracy of the test. The main purpose of the anechoic chamber is to test anti-noise transmission, receiver sensitivity, frequency response, and directivity. The frequency range of this transmitter and receiver must ensure clear language communication, generally around 200-4000Hz.

According to the purpose of the anechoic chamber and the original room conditions, the acoustic design indicators are as follows:

(1) The designed anechoic chamber is a full anechoic chamber, and a working ground network is required.

(2) Acoustic insulation design to achieve the noise floor index.

(3) The sound absorption design realizes the free sound field index, and the measurement error is required to be within ± 1dB, or there are special requirements such as measuring the direction of the sound source.

(4) The noise reduction design achieves aerodynamic performance.

(5) Vibration isolation design realizes isolation of low-frequency vibration generated around the anechoic chamber.

(I) Sound and vibration isolation

Through on-site data collection and field inspection, determine the low-frequency noise source and environmental noise near the location of the anechoic chamber to be built, and determine the design plan based on the analysis of the collected results. In order to improve the sound insulation and vibration isolation effects, generally separate "House in room" type sound insulation structure.

(2) Floating the ground

In order to isolate the solid sound caused by the impact, the floating floor of the elastic cushion is used for vibration isolation. The method is to lay a layer of glass wool insulation board with a thickness of 15cm (10cm after compaction) as a vibration-isolating elastic cushion on the original floor, and then make a layer of reinforced concrete floor with a thickness of 20cm on top of it, and the outer wall A gap of 5cm is left to prevent rigid connection with the external wall and isolate the introduction of solid sound in the building and outdoors.

(Three) the partition

A layer of 24 cm thick brick wall is laid on the floating ground as the inner wall, and a gap of 20 cm is left between the wall and the outer wall. The mortar of the brick wall requires full mortar to prevent the gap from leaking sound.

(Four) sound insulation ceiling

Considering the construction and reducing the weight of the sound insulation flat roof, the double-layer steel mesh cement plaster is used, and the sound insulation flat roof with a 10cm air layer in the middle is characterized by high sound insulation and light weight. In order to achieve the maximum effective height of the anechoic chamber, the girders supporting the floor slabs protrude downward.

(V) Sound insulation door

The anechoic chamber door has sound insulation and sound absorption functions. It is composed of a sound insulation door and a sound absorbing sharp split door. It is located on the separation wall between the instrument room and two single-open steel composite structure sound insulation doors. And the sound absorption of the inner wall pulls the door. Its characteristic is to greatly reduce the space occupied by the general push-pull sound-absorbing tip split door, and the switch is also very convenient. Because the design of the anechoic chamber uses short sound-absorbing spikes, a set of sound-absorbing spikes against the wall surface are installed inward, leaving a neutral position to solve the door pull position.

(Six) sound-absorbing spikes

The design of the sound absorption spike is the decisive factor for ensuring the sound field characteristics of the anechoic chamber and the lower limit frequency of the test. In order to maximize the effective space, the length of the spike is determined by the cutoff frequency and 1/4 wavelength theory. The specific calculation method is L = 1/4 * (340 / Fc). Fc is the cutoff frequency; 340 is the propagation speed of sound waves in the air, the unit is m / s; L is the theoretical length of the sound absorption and muffler to reach the cutoff frequency. 4mm cold-drawn steel wire is used as the skeleton, and it is filled with environmentally friendly formaldehyde-free sound-absorbing and sound-absorbing materials. It is cut with custom molds and filled after cutting to ensure that each sharp split has a consistent and beautiful appearance. The glass wool does not leak, and the inner layer is cut with a new high-weave white glass cloth. The seams are bonded with Velcro. The outer layer is made of fire-resistant gray-white flame-retardant hole cloth. The overall cut ensures uniform specifications. Everywhere is at the bottom of the wedge, sealed by hand.

(7) Ground network structure

For testing convenience, the anechoic chamber is equipped with a working ground network. According to the height of the anechoic chamber, the ground network is set at 64cm from the ground. The working ground network should have sufficient strength and stiffness on the one hand to ensure safety; on the other hand, the sound reflection of the ground network is not allowed to affect the sound field characteristics. To this end, 4 high-strength steel wires are selected, and the two ends are respectively connected to flower basket screws and pull hooks fixed on the wall ring beam. The flower basket screws are used to tighten the steel wires to keep the ground net straight and the wire spacing is 10 cm.

The ground network is designed with a 1m × 1m manhole at the corner of the wall, so as to install the spikes on the ground under the network, and if necessary, enter the space below the ground network for maintenance.

Due to actual needs, since the 1970s a

Whether the performance of the anechoic chamber meets the requirements for use is generally verified by the method of checking the free field, that is, the sound pressure generated by the point sound source should be inversely proportional to the distance to the sound source. The deviation between the measured sound field and the ideal free field is Main indicators used to measure the performance of the anechoic chamber. In general acoustic testing, this deviation is required to be no greater than ± 1dB; for microphone calibration, this deviation is required to be no greater than ± 0.1dB near the calibration distance.

In addition to meeting the requirements of the free field, the anechoic chamber also requires a lower noise floor. Therefore, it is necessary to take some measures between the anechoic chamber and the foundation.

The calibration of the anechoic chamber was originally only specified in Appendix A of the national standard GB6882 or ISO3745 "Acoustics-Determination of Sound Power Level of Noise Source-Anechoic Chamber and Semi-anechoic Chamber Precision Method". JJF1147-2006 "Specifications for Calibration of Acoustic Characteristics of Anechoic Chambers and Semi-anechoic Chambers" was issued in 2006, which specifies the determination and evaluation of acoustic characteristics of anechoic and semi-anechoic chambers in detail.

There are two main technical indicators of the anechoic chamber: the frequency range and spatial range of the free sound field ^[1]

When designing the anechoic chamber:

(1) The test items of pure tone signals and the test items of wideband noise signals have different requirements for interface sound absorption coefficients.

(2) What follows is the design of sound-absorbing structures.

For sound-absorbing structures that require a sound absorption coefficient 0.99, a sharp wedge shape is generally used. Because of the sound absorption mechanism of porous materials, there are a large number of air gaps inside the material, forming thin tubes or even capillaries. When sound waves are transmitted, the vibrations of the sound waves in the thin tubes are converted into thermal energy due to internal friction. The sound absorption ability is related to the porosity of the material (for example, the porosity of glass wool is about 96%), the flow resistance and the fiber structure of the material. Simultaneously. The frequency characteristics of sound absorption are related to the thickness of the material, that is, the lower limit frequency of the maximum value of sound absorption is about 1/4 wavelength corresponding to its thickness. To make low-frequency sound absorption good, it is necessary to increase the thickness of the porous sound-absorbing material. However, due to the material's flow resistance, it is not possible to increase the thickness arbitrarily to extend low-frequency absorption. Various porous materials have their effective thickness.

Therefore, in order to extend the high sound absorption characteristics to low frequencies, the porous material is made into a wedge shape. From the cross-section of the spike structure. It is a gradual transition from an air medium to a porous material. The acoustic impedance has a gradual process, so that the sound energy can be transmitted to the deep part of the spike structure and converted into heat energy to be consumed.

Of course, it is necessary to design a sound absorption coefficient above 0.99. In addition to the parameters of the material itself, it is also related to the shape of the wedge (the angle of the wedge and the ratio of the wedge to the tip). The total length of the spikes determines the lowest frequency of the maximum sound absorption coefficient (generally, the lowest frequency with a sound absorption coefficient greater than 0.99 is the cut-off frequency of the spikes). The total length of the spikes corresponds to a frequency of 1/4 wavelength. If we use the resonance sound absorbing structure of the base of the wedge and the depth of the cavity after the wedge. The cut-off frequency can also extend slightly to low frequencies.

In the case of testing broadband noise signals, especially the measurement of the sound power level of the noise source in a semi-anechoic room, in many cases it is not necessary to adopt the design of a cusp sound absorption structure. For example, when designing the sound power measurement of a large motor for a certain enterprise and designing a semi-anechoic chamber, a multi-resonance sound absorbing structure with a three-layer curtain is used to test different materials of fireproof cloth in a low-frequency standing wave tube to change the rigid wall. The result is that the sound absorption coefficient above 100Hz is greater than 0.86, and the design task of the semi-anechoic room is completed very economically.

(3) Considerations about the size and shape of the anechoic chamber.

Generally speaking, the architectural shape of the anechoic chamber hardly needs the shape of a spherical, columnar or arc surface. Because if the sound absorption coefficient of the sound absorption structure is completely greater than 0.99, the shape of the shell has little effect; but in the case where the sound absorption coefficient is much lower than 0.99, at least below the cutoff frequency of the sound absorption structure, the sound absorption The coefficient drops sharply, the large concave surface will produce focused acoustic defects, and it is impossible to obtain an approximate free sound field.

For the test of the machine's radiated noise power, the general measurement points should be arranged in the space around the equipment, so most of them are designed as square or rectangular semi-anechoic rooms. Both the length, width, and height can be estimated, that is, the dimensions of the side length and height are determined according to the measurement distance, measurement position, and allowable deviation from the free sound field required by the relevant test standards. Of course, there will be room for due consideration, and future considerations Possible device size.

For parameter measurement of electroacoustic devices, if the sound source (speaker) is placed in the center of the anechoic chamber. If the microphone is placed in the axial or diagonal direction of the plane (generally the test distance is 1m, for large-sized speakers and speaker systems such as line arrays, a larger test distance is required), the size of the anechoic chamber is larger. The general consideration is to set the center of the sound source and microphone test line at the center of the anechoic chamber, and the test line is along the diagonal of the plane, and the shape of the anechoic chamber is rectangular. This arrangement maximizes the space savings in the anechoic chamber. After the completion of the free sound field identification, the sound removal source is placed in the center of the anechoic room for measurement. In this case, the range of the free sound field within a certain deviation (± ldB, ± 2dB, etc.) is obtained. In addition, the test sound source is placed in the future. Place the speaker under test. Detect how far (in the plane diagonal direction) the test distance is from the ideal free sound field.

Full-anechoic room Semi-anechoic room

Simple anechoic room

Sound absorption material type

Glass wool cleats Metal cleats Metal sound-absorbing board (for flat anechoic room)

Sound insulation board type

Metal sound insulation board Metal sound insulation board

Isolator type

Spring vibration isolator, rubber vibration isolator, glass wool vibration isolator

Ground type of full anechoic room

Steel wire soft ground net, steel hard ground net

Cut-off frequency can be as low as 63Hz
Noise floor is not higher than 20dBA
Meet the requirements of ISO3745 GB 6882 and various industry standards

Automotive, electromechanical or electro-acoustic industries

Anechoic room, semi-anechoic room for acoustic detection of each product, anechoic room, anechoic box for detection of mobile phones or other communication products