NVH Laboratory (semi anechoic chamber of the whole vehicle) is usually used to measure the indoor and outdoor noise and vibration of 2WD or 4WD vehicles. The test time is usually when the vehicle starts, slides and has a constant speed. The measured contents include air intake and exhaust, engine room, steering wheel and seat guide rail vibration.
Vehicle anechoic chamber is a very important place in acoustic experiment and noise test. Its function is to provide a low noise test environment in a half free field space. Main functions of complete vehicle silencing room:
● provide acoustic semi free field environment
● low noise measurement environment
Semi free field space
The main function of the vehicle anechoic chamber is to provide a half free field space for acoustic testing. Semi free field means that when sound wave propagates in infinite space, there is no reflector (surface) except the ground.
The half free field space in nature means that there is a total reflection surface in the free space, and the sound wave can only propagate freely in the 180 ° hemispherical direction. Common semi free space, such as large open-air parking lot on cement ground, outdoor basketball court, parking apron, etc.
The specific physical definition of sound wave propagation in semi free field space is:
● the sound pressure of point sound source attenuates with distance, which is the inverse square law of sound energy;
● sound pressure level is equal to sound intensity level at normal temperature. This is the theoretical basis for measuring sound power in anechoic chamber.
background noise
Another function of the vehicle anechoic chamber is to provide a low background noise environment to meet the requirements of the test environment. According to ISO3745-2003 Acoustics-Determination of sound power levels of noise sources using sound pressure. Precision methods for anechoic and hemi-anechoic rooms、. ANSI S12.55-2006 acoustics - Determination of sound power levels of noise sources using sound pressure - Precision methods for anechoic and hemi anechoic rooms and the national standard gb6882 precision method for determination of sound power levels of acoustic noise sources anechoic chamber and semi anechoic chamber. Within the test frequency range, the sound pressure level of background noise is at least 6dB lower than that of the measured sound source, preferably 12dB lower.
cut-off frequency
NVH anechoic chamber is designed to simulate semi free field space indoors, so the sound absorption coefficient of each indoor surface is required to be more than 99%. It is impossible to design a 100% sound absorber that can achieve half frequency band (20hz-20000hz). Because common materials are easy to absorb high-frequency sound waves, while low-frequency absorption is related to the thickness of the material (the length of the wedge). The cut-off frequency means that above this frequency, the sound absorption system of the wall can ensure 99% of the sound absorption coefficient. In practical engineering, the semi free sound field space refers to the sound wave above the cut-off frequency. The lower the cut-off frequency, the greater the length requirement of the wedge. In general, the length of the wedge is applicable to the 1/4 wavelength theory.
Above the cut-off frequency, the complete vehicle anechoic chamber meets the free field conditions. Below the cut-off frequency, the complete vehicle anechoic chamber cannot meet the free field conditions, and the free field correction shall be carried out according to GB 6882 during measurement.
Identification method of NVH anechoic chamber
In practical engineering, the free space of sound field is the key basis to identify the quality of anechoic chamber. ISO 3745、. ANSI s12.55 and the national standard GB 6882 provide an identification method for identifying the free space of the silencing room of the complete vehicle.
In addition, the background noise is another important index of the vehicle anechoic room. The background noise obtained in the measurement is the background noise of the instrument plus the background noise of the anechoic chamber.
2、 Acoustic performance parameters of semi anechoic Laboratory for automobile
1. size and main technical indexes of semi silencing Laboratory for complete vehicle
※ design standard for complete vehicle silencing room ISO 3745 ANSI s12.55 and GB 6882
※ cut off frequency 125Hz (can be extended to 50Hz as required)
※ background noise 16dB (a) (11db (a) with special design)
※ external dimension
※ internal dimensions shall be designed according to requirements and standards
※ available space
2. acoustic performance parameters
2.1 sound absorption performance
The sound absorption coefficient of the wedge provided for installation shall be 0.99 at the cut-off frequency (take the wedge with the cut-off frequency of 125Hz as an example)
Frequency (Hz) sound absorption coefficient
125 0.99
250 0.99
500 0.99
1000 0.99
2000 0.99
4000 0.99
8000 0.99
2.2 silencing performance
NVH acoustic laboratory can provide an approximate semi free field environment with a frequency range from 125Hz to 8000Hz, and its inverse square law performance deviation meets the requirements of the following international standards ISO 3745, ANSI s12.55 and GB 6882:
The measurement requirement is to use a single frequency signal and select the center frequency of 1/3 octave of the frequency range
2.3 sound isolation performance
The sound insulation characteristics of NVH semi anechoic room are determined according to the design requirements and the surrounding environment. The patented technology of "turbocharged microcirculation frequency conversion silencing ventilation system" (Patent No.: 200520108948.5) of Guangzhou danpin Artificial Environment Technology Co., Ltd. can meet the following sound insulation characteristics:
Octave band (Hz) 125 250 500 1000 2000 4000 8000
Transmission loss (DB) 283139485863
3、 NVH lab components
3.1 professional sound-absorbing wedge material (Patent No.: 200720155131.2)
Super green environmental sound-absorbing cotton is sampled, and the surface is treated with glue curing. Dust proof treatment technology is adopted between the glass wool and the finish to make the wedge surface easy to clean and ensure long-term stable acoustic performance parameters.
3.2 wedge door (Patent No.: 200920279477.2)
The wedge door adopts a metal perforated sound-absorbing wedge structure, so that the wedge door and the internal wedge form a complete sound-absorbing surface. The wedge door adopts a special hinge, so that the door can be opened and closed easily without deformation for a long time. When the wedge door is opened, it can be designed to be staggered with the wedge teeth of the inner wall, saving space.
3.3 sound insulation door (Patent No.: 200920279477.2)
The door is a very important factor in the design of anechoic room (anechoic room of automobile). A good sound insulation door should have good sound insulation performance and optimized edge effect. The 1.5mm thick cold-rolled steel plate is used, with formed sound-absorbing cotton added inside. The hinge is an imported product, and the whole door floats when opening the door. When returning the door, the high-density magnetic edge banding is semi closed to ensure that its acoustic treatment can match the acoustic treatment of other surrounding parts to eliminate edge leakage as much as possible. The strength meets the operating requirements and will not deform after long-term use. At the same time, the outer surface is sprayed with plastic to make the color consistent with the wedge.
3.5 floating ground
When the external noise is high or vibration measurement is required, the floating structure is usually used to isolate the external influence. The damping materials that play a key role in the floating structure have the following damping methods:
◆ spring structure: the vibration isolation frequency can reach (1Hz. This structure is used when low noise requirements are very high. However, the disadvantages are: high cost, difficult maintenance, and rust prevention of the spring.
◆ rubber damping pad structure: the vibration isolation frequency can reach (7Hz. This structure is used when low noise requirements are high.
◆ glass wool felt damping structure: the vibration isolation frequency can be less than 15Hz, and the cost of this structure is low.
In the actual project, what kind of vibration reduction is adopted depends on:
● required background noise
● surrounding environment, noise source and vibration source
The original ground or terrazzo with sound absorption coefficient less than 0.06 or geostrophic shall be used as the ground.
3.6 lighting
Provide incandescent lamps that can meet the requirements of indoor lighting. The background noise and sound reflection of incandescent lamps shall be minimum. At the same time, the consistency and aesthetics between the lamps and the overall indoor design shall be considered.
3.7 circuit system
Except for lighting, all circuit conduits, wires and controls shall be installed behind the wedge to avoid sound reflection.
3.8 wiring pipeline
The routing pipeline shall meet the requirements of the required line passing without any impact on the test.
