1.Introduction
To reasonably reduce investment costs and improve the utilization rate of the testing laboratory, a testing room capable of testing various types of products is needed. Therefore, the original semi-anechoic chamber, which could only test a single model of Ba4, has been redesigned and transformed into a multifunctional acoustic testing room. This new facility can detect noise from various air conditioning units, refrigeration compressors, air conditioning fans, and their components. By connecting ductwork to the outside, it can also conduct duct machine noise testing and, with a water system, can perform noise testing for water source heat pumps. Additionally, with a refrigerant system in place, it can conduct noise testing for compressors.
2. Principles of the Acoustic Room Method
The acoustic room method involves placing the sound source inside an acoustic chamber or semi-anechoic chamber for measurement. The inner walls of the chamber are lined with sound-absorbing materials that can absorb over 98% of incoming sound energy. The sound inside the chamber is primarily direct sound, with very little reflected sound. If the floor of the acoustic chamber is not covered with sound-absorbing surfaces but instead has a solid reflective surface, it is referred to as a semi-anechoic chamber. During measurement, it is assumed that there is an enclosing surface surrounding the sound source. The sound source is completely enclosed within it, and the enclosing surface is divided into n surface elements, each with an area of Δs. The sound pressure level ( L_P ) on each surface element is measured, and the sound power level of the sound source is calculated using the following formula:
[ L = L_t + 10 \log S_0 ]
Where ( S_0 ) is the total area of the enclosing surface.
When the area of each surface element is equal, the average sound level is:
[ \tilde{L_t} = 10 \log \left[ \frac{1}{S} \sum 10^{\frac{L_{P,i}}{10}} \right] ]
When the area of each surface element is not equal, the average sound pressure level is:
[ L_t = 10 \log \left[ \frac{S}{\sum S_i 10^{\frac{L_{P,i}}{10}}} \right] ]
3. Design of the Acoustic Testing Room
The design of the acoustic testing room is based on GB6882-86 “Determination of Noise Source Sound Power Level – Precision Method for Acoustic Chambers and Semi-Anechoic Chambers” and standards such as ISO3745-2003E and ISO7779-1999E. It incorporates the requirements for sound field characteristics of the acoustic chamber and the specific requirements of national standards for refrigeration and air conditioning equipment, adding multiple testing functions based on the original design.
(1) According to GB/T19232-2003 “Fan Coil Units,” for units with outlet static pressure, noise measurement must be conducted at the return air inlet duct.
(2) GB/T18836-2002 “Ducted Air Conditioning (Heat Pump) Units” stipulates that for ceiling-mounted indoor units, a 2m long damped duct must be connected, and rated external static pressure should be applied at the supply and return air outlets to adjust static pressure. To avoid the influence of supply air, the outlet must be connected to the testing room.
(3) For GB/T18836-2002 “Water Source Heat Pump Units,” noise tests must be conducted under conditions close to nominal cooling.
(4) For GB/T10079-2001 “Piston Single-Stage Refrigeration Compressors” and GB/T19410-2003 “Screw Refrigerant Compressors,” the noise indicators of the units must be tested under specified conditions, requiring a fluorine system design in the acoustic testing room.
Based on the above standards and the requirements for the types of tested products, the design of the acoustic testing room has been enhanced to include a water system, a refrigerant circulation system, and a duct system, allowing for the testing of various refrigeration and air conditioning equipment and their components.
The design is primarily a semi-anechoic chamber, with the floor being a terrazzo surface while the other five sides are lined with sound-absorbing wedges. If sound-absorbing wedges are also placed on the floor, it can be considered a full anechoic chamber. The net dimensions of the acoustic chamber are 6m × 5.2m × 4.6m, with a cutoff frequency of 63Hz. The hemispherical free field has a radius greater than 2m, and the measurement error is less than 1dB.
The acoustic room is isolated from the outside, using a double brick wall sound insulation structure, and is equipped with two soundproof doors, with the inner door being a rotating soundproof door. The background noise level in the semi-anechoic chamber is 14.5 dB(A). The inner room uses vehicle engine suspension for vibration isolation, with a total of 22 pairs of springs. Each spring is sealed in an iron box with grease, with a free height of 288.5mm and a compressed height of 230mm, allowing a maximum load of 7t per spring. The natural frequency of the acoustic chamber is 2.14Hz. This design not only isolates external noise but also eliminates the impact of external vibrations on the interior of the acoustic chamber.
The acoustic chamber is equipped with 1,189 Owens Corning sound-absorbing wedges, covered with fireproof fabric, with a density of 30kg/m³ and stable sound-absorbing performance. The dimensions of the sound-absorbing wedges are as follows: the length is 1,000mm, the base height is 200mm, the base length is 400mm, and the base width is 400mm. The top of the sound-absorbing wedge is cut to form a flat head shape. Tests show that the flat head shape and pointed head shape of the sound-absorbing wedge have similar absorption coefficients; cutting off the pointed head has little effect on sound absorption performance but can expand the effective volume of the acoustic chamber. The wedge is fixed with a steel rebar framework, and the distance between the wedge and the wall is 150mm, creating a cavity that effectively increases the length of the wedge and adjusts the resonant frequency, enhancing low-frequency sound absorption characteristics.
The top of the acoustic chamber is designed with a 2T manual hoist to lift duct machines. At the same time, air ducts can be connected to the inlet and outlet on both sides of the duct machine, supported by brackets. When the duct machine is not being tested, the ducts and brackets can be removed, as both can produce reflections. The acoustic chamber also has a water system for testing the noise of water source heat pumps, and a refrigerant system for testing compressor noise.
4. Performance of the Semi-Anechoic Chamber
4.1 Background Noise Measurement of the Semi-Anechoic Chamber
The background noise of the acoustic chamber is 14.3 dB. Since there are no strong noise sources around the chamber, only the background noise under normal conditions was measured, with the results under fan operation shown in Table 1.
| Frequency Band | Normal Condition | Fan On |
|---|---|---|
| 31.5 | 14.3 | 15.6 |
| 63 | 17.0 | 33.5 |
| 125 | 15.0 | 21.5 |
| 250 | 11.0 | 19.0 |
| 500 | 9.0 | 14.5 |
| 1000 | 7.5 | 10.5 |
| 2000 | 6.0 | 7.0 |
| 4000 | 6.5 | 7.0 |
| 8000 | 8.0 | 8.0 |
4.2 Free Field Radius of the Semi-Anechoic Chamber
The measured values of the free field radius of the semi-anechoic chamber are shown in Table 2. Considering the symmetry of the chamber, the sound field measurements were taken in three directions: OA, OB, and OC. From point O, lines were drawn towards points A, B, and C, with heights of 2.8m above the ground. The speaker was placed at point O, and microphones were fixed on wires along OA, OB, and OC. Starting from the center, measurements were taken every 5cm within 1.5m, and every 10cm thereafter until reaching the top of the wedge.
The sound pressure level attenuation curves along the directions OA, OB, and OC are shown in figures.
| Direction | Frequency (Hz) | OA | OB | OC |
|---|---|---|---|---|
| 63 | 3.3 | 3.9 | 3.6 | |
| 100 | 3.3 | 3.9 | 3.6 | |
| 125 | 3.3 | 3.9 | 3.6 | |
| 250 | 3.3 | 3.9 | 3.6 | |
| 500 | 3.3 | 3.9 | 3.6 | |
| 1000 | 3.3 | 3.2 | 3.3 | |
| 2000 | 3.3 | 3.9 | 3.6 | |
| 4000 | 3.3 | 3.9 | 3.6 |
From the test data, it can be seen that at frequencies from 63Hz to 8000Hz, the free field radius in the directions OA, OB, and OC is greater than 3m, meeting the requirements of ISO3745 for precision testing standards.
4.3 Capability for Noise Testing of General Machinery Products
This testing room is primarily used for noise detection and analysis of small general machinery products, in conjunction with the National Compressor Product Quality Supervision and Inspection Center and the National Refrigeration Equipment Product Quality Supervision and Inspection Center, among other departmental quality testing centers. The testing room has a volume of 145m³. According to ISO3745, the volume of the tested sound source should be less than 0.5% of the volume of the acoustic chamber. It can conduct noise detection work for refrigeration compressors with a cooling capacity of less than 50,000 kcal/hour, as well as pumps, small fans, and centrifugal machines with a volume of less than 0.7m³. The testing accuracy can reach precision level.
The modified acoustic chamber has been tested, and its sound field performance indicators meet the requirements for semi-anechoic chambers specified in ISO3745 standards. It is suitable for noise detection of general machinery products with a volume of less than 0.7m³, achieving precision-level requirements. Additionally, this acoustic chamber is a multifunctional laboratory capable of testing the noise of duct machines, water source heat pumps, and compressors, thereby reducing testing costs.