Some standard content:
1 Scope
National Standard of the People's Republic of China
Measurement procedure for silencers
Acoustics--Measurement procedure for silencers This standard specifies the measurement method and requirements for silencers. It includes laboratory measurement and field measurement methods. The measured quantity is mainly the insertion loss of the silencer, and the following quantities can also be measured: airflow noise sound power level;
silencer pressure loss and resistance coefficient.
This standard is applicable to pipe silencers with mainly resistive properties. 2 Reference standards
GB3102 Acoustic quantities and units
Levels of acoustic quantities and their reference values
GB 3238
GB3240
GB3241
GB 3785
GB3947
Common frequencies in acoustic measurements
Electrical and acoustic properties and test methods of 1/1 and 1/3 octave band filter sound level meters for sound and vibration analysis Acoustic terminology
Measurement specification for hall reverberation time
GBJ 76
GB6881
Acoustic noise sound source power level determination Reverberation chamber precision method and engineering method GB6882
Acoustics
Noise sound source power level determination Anechoic chamber and semi-anechoic chamber precision method 3 Terms
3.1 Insertion loss of silencer GB/T 4760—1995
Replaces GB4760--84
The reduction in the sound power level of the noise radiated from the pipe mouth before and after the installation of the silencer. Symbol: D; Unit: decibel, dB
3.2 Anechoic terminal
Anechoic terminal
The end of the pipe where the silencer measures are taken. It makes the noise reflected from the pipe mouth back to the inside of the pipe negligible. 3.3 Substitute for silencer Empty pipe with the same length and the same channel cross section as the silencer to be tested. 3.4 Measurement for substitute Measurement performed when the silencer is replaced by a substitute pipe. 3.5 Static measurement Measurement performed when the airflow source is turned off and no airflow is applied. 3.6 Measurement under low noise flow Measurement performed when the noise source is turned off and low noise is stable. 3.7 Static pressure
Approved by the State Administration of Technical Supervision on June 15, 1995 256
Implementation on February 1, 1996
GB/T 4760—1995
When the airflow passes through the wall of the pipe with a small hole, the gas pressure measured at the small hole. Symbol: p, unit: Pa. 3.8 Total pressure
When the pipe mouth is closed and facing the direction of the airflow, the gas pressure measured in the pipe. Symbol: pt; unit: Pa. 3.9 Kinetic pressure
The kinetic energy per unit volume in the airflow. Symbol: pv; Unit: Pa. Note: ① At a given measuring point, the dynamic pressure is proportional to the square of the airflow velocity, and the sum of the dynamic pressure and the static pressure is equal to the total pressure. ② Along the cross section of the pipeline, the dynamic pressure corresponding to the average airflow velocity is the average dynamic pressure, and the sum of the average dynamic pressure and the static pressure is the average total pressure. 3.10 Pressure losspressureloss
The reduction in the average total pressure at the inlet and outlet of the muffler when the air falls steadily. The temperature of the muffler is shown in Figure 1.
High service: point: the same as the product of the sea.
The lower limit is shorter than the lowest center
The center line coincides. The transition duct side
, the limit of the column.
For typical design of silencer terminal, please refer to the attached
Center frequency, Hz
Sound pressure reflection coefficient r
4.3 Noise source
GB/T4760-1995
Table 1 Limit of sound pressure reflection coefficient of silencer terminal
125 and above
4.3.1 The noise source is a device that provides stable noise to the main pipeline, usually composed of one or more speakers. The speaker should be installed in a speaker box connected to the main pipeline. The total area of the connection should not be less than 40% of the channel area. Vibration isolation measures should be taken at the connection between the speaker box and the pipeline.
4.3.2 The speaker box should be made of thick material. The speaker should be fixed on the baffle plate inside the box, and a vibration isolation pad should be installed between the speaker and the baffle plate. A proper amount of sound absorbing material should be installed in the speaker box.
4.3.3 When the equivalent diameter of the main duct is greater than 300mm, more than two loudspeakers shall be used. The performance of each loudspeaker shall be the same. The loudspeakers shall be symmetrically installed relative to the pipe axis, and their sounds shall be excited in phase. 4.3.4 The loudspeaker shall be excited by a wide-band noise signal. The range of the center frequency of the noise signal shall refer to the provisions of GB3240. 4.3.5 During the test, the octave sound pressure level radiated by the noise source to the main duct shall not be less than 90dB, the sound pressure level shall remain stable, and the range of change over time shall not be greater than ±0.5dB. 4.4 Low-noise airflow sourceWww.bzxZ.net
4.4.1 The low-noise airflow source is a device that provides low-noise and stable airflow to the main duct. Its main body is usually a fan with sufficient air volume and air pressure. The fan shall be installed in an enclosure with good sound insulation performance, and the fan outlet shall be connected to the pipeline system with a flexible pipe. 4.4.2 The airflow source system shall have control devices such as valves or speed change to adjust the airflow speed in the muffler to be tested. 4.4.3 A silencer should be connected between the fan and the main duct to ensure that the noise entering the main duct from the fan within the test frequency range will not affect the measurement results.
4.4.4 The noise and airflow entering the main duct should be able to be adjusted independently of each other. According to actual needs, the noise propagation direction and the airflow direction can be the same or opposite.
4.5 Receiving room
4.5.1 The receiving room is an enclosed space for measuring the noise radiated by the duct outlet. The receiving room is usually a reverberation room or a semi-anechoic room. 4.5.2 When the receiving room is a reverberation room, the outlet of the main duct should be inserted into the reverberation room by 1 to 1.5m to make the indoor sound field more diffuse, as shown in Figure 2a.
4.5.3 When the receiving room is a semi-anechoic room, the plane of the main duct outlet should be aligned with the rigid wall so that the indoor sound can strictly meet the semi-free field condition, as shown in Figure 2b.
Test silencer
Reverberation chamber outlet
Expansion surface
aReverberation chamber pipe
Rigid surface
Test silencer
Pipe outlet
Inlet
bSilencer chamber device
Figure 2 Receiving chamber
4.6 Measuring instruments
GB/T 4760-1995
4.6.1 The acoustic quantity directly measured during measurement is mainly the octave band or 1/3 octave band sound pressure level, which should be measured using a sound level meter and a matching filter or other measuring instruments with equivalent functions. The sound level meter used should comply with the provisions of GB3785 on type 1 sound level meter, and the matching filter should comply with the provisions of GB3241.
4.6.2 Before and after each test, the sound level meter should be calibrated with a sound level calibrator with an accuracy better than ±0.5dB. The difference between the two calibrations should not be greater than 1 dB.
4.6.3 The aerodynamic quantities directly measured during the measurement are mainly air velocity and static pressure, which should be measured using a Pitot tube and a static pressure tube in conjunction with a micromanometer. If there is a fixed orifice plate flow measurement device in the upstream pipeline, the average air velocity can also be calculated from the air flow rate based on the cross-sectional area of the pipeline.
4.6.4 The aerodynamic performance measurement device should be inspected and calibrated regularly. 5 Determination of muffler insertion loss
5.1 Overview
5.1.1 When determining the insertion loss of a muffler, an empty pipe test should be performed before installing the muffler. The sound pressure level of each octave band or 1/3 octave band of the noise radiated downstream should be measured at a given measuring point, and the sound power level should be calculated from the sound pressure level at each measuring point. Then replace the replacement pipe with a muffler and conduct the test, keeping the noise source conditions the same as those in the empty pipe test, and properly adjust the airflow source system so that the airflow velocity in the main pipe is also the same as that in the empty pipe test. Measure the corresponding sound power level of each frequency band, and calculate the insertion loss of each frequency band from the difference between the two sound power levels. 5.1.2 Low noise airflow test should be conducted for each working condition, that is, corresponding measurement should be conducted under the condition of closing the noise source, and the measured sound pressure level of each frequency band should be taken as the background noise level. The correction value K is calculated according to Table 2 based on the difference between the original measured sound pressure level and the background noise level. Table 2 Background noise correction value K of frequency band sound pressure level Sound pressure level difference, dB
Correction value K, dB
Experimental data of sound pressure level difference <4 dB can be recorded, but for reference only. g～10
10 or more
5.1.3 When multiple measuring points are selected, the frequency band sound pressure level of each measuring point should be averaged according to the energy law, and then the correction value K is calculated according to Table 2 based on the difference between the average sound pressure level and the average background noise level. In the calculation process, one decimal place should be retained, and the final result of the frequency band insertion loss should be an integer value.
5.1.4 When measuring the insertion loss of the muffler, the reverberation chamber method should generally be used first. According to objective needs and possibilities, the semi-anechoic chamber method or the pipe method can also be used.
5.2 Reverberation chamber method
5.2.1 When the reverberation chamber is used as the receiving chamber, the sound power level radiated from the pipe mouth into the receiving chamber should be measured in accordance with the provisions of GB6881. For the insertion loss, only the relative change of the sound power level can be measured. 5.2.2 During the test, the microphone should be fixed on the bracket, and the measuring point should be able to be accurately positioned repeatedly. Generally, the microphone should be connected to the analyzer with an extension cable so that it can be operated outdoors. 5.2.3 Carry out the empty pipe test and the corresponding low-noise airflow test, measure the frequency band sound pressure level and background noise level at each measuring point, and calculate the average frequency band sound pressure level L and the corresponding correction value Ki according to the provisions of 5.1.3 and 5.1.2. 5.2.4 Replace the pipe with a muffler and conduct the corresponding low-noise airflow test. Measure the frequency band sound pressure level and background noise level at each measuring point. Calculate the average frequency band sound pressure level Lp2 and the corresponding correction value K2 according to 5.1.3 and 5.1.2. 5.2.5 The insertion loss of each frequency band is determined by the following formula: D - L - Lp + K, - K
GB/T 4760-1995
5.2.6 When the sound power spectrum of the noise source actually used is known, the A-weighted insertion loss DA (see Appendix A) can be calculated from the measured insertion loss of each frequency band.
5.2.7 For the muffler to be tested, such as disc muffler, muffler elbow, etc., which has a different air inlet direction from the main pipe, it can be installed at the outlet of the main pipe in the receiving room and then measured according to 5.2.1 to 5.2.6. .5.3 Semi-anechoic chamber method
5.3.1 When a semi-anechoic chamber is used as a receiving chamber, the sound power level radiated from the pipe mouth into the receiving chamber shall be measured in accordance with the provisions of GB6882. For insertion loss, only the relative change of the sound power level may be measured. 5.3.2 During the test, the microphone shall be equipped with a nose cone, and the installation conditions shall meet the requirements specified in 5.2.2. 5.3.3 Measure the insertion loss in accordance with the provisions of 5.2.3 to 5.2.7. 5.4 Pipeline method
5.4.1 When the insertion loss is measured by the pipeline method, the measuring point shall be selected in the middle of the straight part of the downstream pipeline of the muffler outlet. The main pipeline outlet shall have a muffler terminal device that meets the requirements of 4.2.6. 5.4.2 When the equivalent diameter of the pipeline is not greater than 100mm, only one measuring point may be selected. The measuring point position should be selected on the pipeline axis, or on the rigid side wall. When the equivalent diameter of the pipeline is greater than 300mm, multiple measuring points should be selected along the cross section of the pipeline. Generally, four measuring points should be selected as shown in Figure 3.
●Measuring point
a Rectangular cross section
Figure 3 Typical positions of four measuring points
5.4.3 During the test, the microphone can be directly fixed at the measuring point, or the microphone or probe can be used for measuring the airflow direction. The maximum cross-sectional area of the measuring device is 5.4.4 When the airflow velocity in the main pipeline is greater than 15m/s, the measuring point should be selected on the rigid side of the microphone or probe, and a fine mesh or micro-perforated plate or other sound-guiding and sound-permeable layer should be installed inside. The fixing device on the pipe wall is shown in Figure 4.
In the careful section method
Weigh the measuring device
Root measures. Typical: Transformer
Flow guide
a Microphone
GB/T 4760—1995
Flow guide sound layer
Vibration isolation wall
Transmitter
To microphone
b Probe
Figure 4 Typical device with measuring point on rigid side wall 5.4.5 Determine the insertion loss in accordance with 5.2.3 to 5.2.7. 6 Determination of sound power level of airflow noise
6.1 Overview
Flexible pipe
6.1.1 The quantity to be measured in this chapter is the sound power level of the noise generated by the airflow in the muffler radiated to the downstream pipe. During the test, the noise source should be turned off and the measurement should be carried out under the condition of low noise airflow. 6.1.2 When measuring the sound pressure level, for each given air flow velocity, a corresponding empty pipe test should be performed, and the measured sound pressure level of each frequency band should be used as the background noise level.
6.1.3 When multiple measuring points are selected, the frequency band sound pressure level of each measuring point should be averaged according to the energy law, and then the correction value K should be calculated according to Table 2 based on the difference between the average sound pressure level and the average background noise level. The background noise correction of the sound power level of each frequency band airflow noise is taken as the K value. During the calculation process, one decimal place should be retained, and the final result of the frequency band sound power level should be an integer value. 6.1.4 The A-weighted sound power level E can be calculated from the measured sound power level of each frequency band (see Appendix A (Supplement)]. 6.1.5 When measuring the sound power level of airflow noise, the reverberation chamber method should generally be used first. According to objective needs and possibilities, the semi-anechoic chamber method or the duct method can also be used.
6.2 Reverberation chamber method
6.2.1 When a reverberation chamber is used as a receiving chamber, the sound power level radiated from the pipe mouth into the receiving chamber shall be carried out in accordance with the provisions of GB6881. For the sound power level radiated from the airflow noise in the muffler to the downstream pipe, the correction value Xr of the reflection at the end of the pipe shall be added. The value of X can be calculated based on the frequency parameters of the pipe mouth [see Appendix B (Supplement)].
6.2.2 Install the microphone in accordance with the provisions of 5.2.2. 6.2.3 When testing after installing the muffler, the frequency band sound pressure level at each measuring point shall be measured first, and then the average sound pressure level Lp shall be calculated according to the energy law.
6.2.4 When replacing the muffler with a replacement pipe for an empty pipe test, the airflow source system shall be appropriately adjusted to keep the airflow velocity in the main pipe the same as when the muffler is installed for the test. The average background noise level is obtained from the frequency band sound pressure level at each measuring point, and the background noise correction value K is calculated. 6.2.5 Before the test, the reverberation time t of the reverberation chamber in the test frequency band shall be measured in advance according to the provisions of GBJ76, and the volume V and total surface area S of the reverberation chamber shall be recorded. The frequency band sound power level Lw of airflow noise is calculated by the following formula. Lw= L, - 10log + 10og+10log( 1+SA
13.5-K+X,
Wherein: Lw
GB/T 4760--1995
frequency band sound power level, dB (reference value is 1pW); frequency band average sound pressure level, dB (reference value is 20 μPa); reverberation time of reverberation chamber, s,
volume of reverberation chamber, m;
Vo---- 1 m;
In - wavelength corresponding to the center frequency of the frequency band, m; K - background noise correction value, dB;
X - pipe end reflection correction value, dB.
6.3 Semi-anechoic chamber method
6.3.1 When a semi-anechoic chamber is used as a receiving chamber, the sound power level radiated from the pipe mouth into the receiving chamber shall be carried out in accordance with the provisions of GB6882. For the sound power level radiated from the airflow noise in the muffler to the downstream pipe, the pipe end correction value X'r shall be added. The X' value can be calculated based on the pipe mouth frequency parameters (see Appendix B).
6.3.2 Install the microphone in accordance with 5.3.2. 6.3.3 Measure the average sound pressure level L of the frequency band and the background noise correction value K in accordance with 6.2.3 and 6.2.4. 6.3.4 Measure the distance r from the center of the main pipe mouth of the measuring point, and the sound power level Lw of the airflow noise is calculated by the following formula. Lw = L, + 20 logr + 8- K + X'r Where: Lw-
frequency band sound power level, dB (reference value is 1pW); L,
frequency band average sound pressure level, dB (reference value is 20μPa); distance from measuring point to pipe mouth center, m;
K background noise correction value, dB;
X'——pipe end reflection correction value, dB. 6.4 Pipeline method
6.4.1 Select measuring points and measuring devices according to 5.4.1 to 5.4.4. 6.4.2 Measure the frequency band average sound pressure level L and background noise correction value K according to 6.2.3 and 6.2.4. 6.4.3 Measure the cross-sectional area S of the downstream pipe of the muffler.The sound power level Iw of the airflow noise is calculated by the following formula: Lw = L, + 10 logS, - K
Wherein: Lw——
band sound power level, dB (reference value is 1pW); L,—band average sound pressure level, dB (reference value is 20 μPa); S,——m2; K background noise correction value, dB;
——pipe end reflection correction value, dB.
7 Determination of pressure loss and resistance coefficient
7.1 Overview
+**(3)
7.1.1 During the test, a measurement section should be selected in the middle of the straight part of the pipe at the inlet and outlet of the muffler. At a given airflow velocity, measure the average dynamic pressure and static pressure on the two sections respectively, and calculate the average total pressure. The pressure loss is obtained from the reduction of the average total pressure at both ends of the muffler, and the resistance coefficient is obtained from the ratio of the pressure loss to the average dynamic pressure. When the cross-sectional area of the pipes at both ends of the muffler is the same, the pressure loss is equal to the difference in static pressure at both ends. When the muffler is installed at the outlet of the main pipe in the receiving room, its pressure loss can be measured by measuring the total pressure at the inlet end of the muffler and the static pressure in the receiving room respectively, and the difference is the pressure loss of the muffler. 7.1.2 Generally, it is advisable to use a Pitot tube to measure the dynamic pressure and velocity of the airflow, a static pressure tube to measure the static pressure of the airflow, and a calibrated micromanometer to measure the pressure difference.
7.1.3 When the airflow and noise are measured simultaneously, the measuring points should be selected on different sections to avoid mutual interference. 7.2 Determination of airflow velocity
7.2.1 When measuring the average velocity of the airflow in the pipeline, the measuring section should be selected in the straight part of the pipeline to arrange the measuring points. The number of measuring points should be no less than 8, and the measuring points should be evenly distributed on the entire cross section according to the area. 7.2.2 Place the Pitot tube so that the end is at the measuring point position and in the direction of the airflow. With the help of a micromanometer, read the dynamic pressure readings at each measuring point. The airflow velocity at each measuring point is calculated by the following formula: t—20)/2p/1/2
293.
Where: v—-airflow velocity, m/s;
airflow temperature, ℃,
pgas density, kg/m,
p, —airflow flow pressure, that is, the difference between total pressure and static pressure, Pa. The average airflow velocity element along the cross section is equal to the arithmetic mean of the airflow velocities at each measuring point. 7.2.3 During actual measurement, the average airflow velocity can be calculated by the following formula: am
Where: —average airflow velocity, m/s;
Um—airflow velocity at the center of the cross section, m/ss-correction factor.
. (6)
α is related to the shape of the pipe section and the flow velocity, but the range of variation is not large. It can be calculated according to the actual measurement results obtained according to the provisions of 7.2.1 to 7.2.2 as follows:
7.2.4 If there is no air leakage in the pipe system and there is a flow measurement device at the air flow source, the average air flow velocity at a given cross section can also be calculated by the following formula:
Where: Yuan---—average air flow velocity, m/s; U—air flow volume flow, m\/s;
S—cross-sectional area, m\.
7.3 Determination of static pressure and total pressure
(8)
7.3.1 When measuring the static pressure p in the pipe, only one measuring point can be selected. The static pressure tube can be separated from the Pitot tube or another small hole can be opened in the pipe wall to connect it.
GB/T 4760—1995
7.3.2 The average dynamic pressure p of the airflow along a given cross section can be approximately calculated by the following formula: , =αpvm
Where: p-average dynamic pressure of the airflow, Pa;
pvm——dynamic pressure at the center of the cross section;
α——correction factor, determined by formula (7). 7.3.3 The average total pressure P of the airflow along a given cross section can be calculated by the following formula: p=p+ ps
Where: pt-
average total pressure of the airflow, Pa;
p. Average dynamic pressure of the airflow, Pa
static pressure of the airflow, Pa.
7.4 Determination of pressure loss
7.4.1 The pressure loss of the muffler is calculated by the following formula: Ap = pt — p2
Wherein: △p — pressure loss, Pa;
pm — average total pressure at the inlet end, Pa;
pt2 — average total pressure at the outlet end, Pa.
7.4.2 When the cross-sectional area of the pipes at both ends of the muffler is the same, the pressure loss can be calculated by the following formula: Ap p — ps2
Wherein: △p — pressure loss, Pa;
static pressure at the inlet end, Pa;
static pressure at the outlet end, Pa.
7.5 Determination of resistance coefficient
The resistance coefficient of the muffler is calculated by the following formula: g = Ap/ pu
Where: ——
Drag coefficient;
Ap——Pressure loss, Pa,
p——Average dynamic pressure of airflow in the channel, Pa. 8 Field measurement method of muffler
8.1 Overview
·(10)
·(11)
·(12)
（13)
8.1.1 The acoustic quantity measured on site is mainly the insertion loss of the muffler under actual use conditions. When measuring before and after installing the muffler, care should be taken to keep the experimental conditions and sound field distribution basically unchanged. The insertion loss is determined by the difference between the sound pressure 264
level obtained by two measurements before and after the given measuring point.
GB/T 4760—1995
8.1.2 At a given measuring point, the A sound level before and after the installation of the silencer and the octave band sound pressure level with a center frequency of 63 to 8000 Hz should generally be measured respectively. The A-weighted insertion loss and the insertion loss of each octave band are obtained from the difference between the two measurements. 8.1.3 If necessary, the pressure loss of the silencer can be measured on site in accordance with the provisions of Article 7. 8.2 Measuring instruments
8.2.1 On-site sound pressure level measurement should generally use a sound level meter that complies with the provisions of GB3785 for type 2 or above. Other measuring instruments with equivalent accuracy can also be used. The octave filter used to measure the octave band sound pressure level should comply with the relevant provisions of GB3241. 8.2.2 Before and after each measurement, the sound level meter should be calibrated with a sound level calibrator with an accuracy better than ±0.5dB. The difference between the two calibrations should not be greater than 1dB. Sound level calibrators and sound level meters or other measuring instruments should be calibrated in accordance with relevant regulations to ensure the accuracy of the test instruments.
8.3 Measurement of insertion loss
8.3.1 Sound field conditions
During the measurement process, the sound field distribution before and after the installation of the muffler should be basically the same. In general, the sound field distribution can be considered to remain approximately unchanged when the following conditions are met:
The size and shape of the pipe mouth are basically the same before and after the installation of the muffler; a.
b. The relative position between the pipe mouth and the main reflection surface within 3.5m around it remains unchanged before and after the installation of the muffler.
8.3.2 Measurement point position
8.3.2.1 The position of the measurement point relative to the pipe mouth should remain unchanged before and after the installation of the muffler. The location of the measuring point is shown in Figure 5: 0
a Before installing the silencer
b After installing the silencer
Figure 5 Location of measuring points
In the figure, A is the measuring point, B is a point on the edge of the pipe mouth, and for rectangular pipes, the midpoint of the longer side should generally be taken; r is the distance between A and B, and the connecting line AB must be on the same plane as the central axis OO of the pipe mouth; α is the angle between AB and OO'. 8.3.2.2 For air intake or exhaust silencers with an equivalent diameter of the pipe mouth not exceeding 1m, generally one measuring point can be taken; for silencers with an equivalent diameter of the pipe mouth exceeding 1m, 3 or more measuring points should be selected around the central axis of the pipe mouth. 8.3.2.3 For various types of air intake or exhaust silencers, the distance r between the measuring points should generally be 1m. When the equivalent diameter of the pipe mouth is less than 0.13m, the r value can also be taken as 0.5m; the angle α should generally be taken as 45°. 8.3.2.4 For gas release and exhaust silencers, the distance r between the measuring points can be taken as 1m or an integer greater than 1㎡ according to the actual situation; the angle α should be taken as 45°, and when the on-site conditions are difficult, it can also be taken as 90° or greater than 90°. 8.3.2.5 The distance between the measuring point and the nearby reflecting surface should not be less than 1m. 8.3.3 Measurement and correction of background noise
GB/T 4760—1995
8.3.3.1 The background noise should be measured before and after the silencer is installed. In general, the A sound level or octave band sound pressure level measured under the condition of shutting down the unit under test can be used as the corresponding background noise level. 8.3.3.2 When the unit under test radiates strong noise through channels other than the nozzle, the background noise level is generally estimated according to the following method:
Select an auxiliary measuring point far away from the nozzle but close to the unit under test. The distance from the auxiliary measuring point to the unit under test is roughly the same as that from the given measuring point. The distance from the auxiliary measuring point to the nozzle should generally be greater than 2m. b. Test the given working conditions under the condition of installing a silencer, and the sound pressure level measured at the auxiliary measuring point is used as the corresponding background noise level.
8.3.3.3 Correct the measured sound pressure level for background noise according to Table 3, and subtract the correction value K from the measured sound pressure level data. Table 3
Difference between the measured sound pressure level and the background noise level dB
Correction value K, dB
Experimental data with a sound pressure level difference less than 3dB can be recorded, but for reference only. 8.3.4 Situations that do not meet the requirements
10 or more
When the conditions on site cannot meet the requirements of 8.3.1 to 8.3.3, the insertion loss cannot be measured. At this time, the difference in sound pressure levels before and after the muffler can be measured in accordance with 8.3.2, and the environmental conditions of the measurement site can be recorded. However, the measurement results do not represent the insertion loss of the muffler and are for reference only.
8.4 Reading method
8.4.1 When measuring with a sound level meter, the "slow" gear should be used. When the swing amplitude of the sound level meter pointer is less than 3dB, the average value between the maximum sound pressure level and the minimum sound pressure level can be read. When the swing amplitude is greater than 3dB, the average value within the 10s observation time should be taken. 8.4.2 For each working condition, two readings should be taken. If the difference between the two readings is greater than 3dB, two more readings should be added. The final result is the average of each reading and an integer value. 8.4.3 For the test of gas release and exhaust silencer, the maximum value under stable emission conditions should be read. 8.4.4 When obvious discrete frequency noise or pulse noise appears, it should be noted in the test report. 8.5 Test conditions
8.5.1 Muffler installation conditions
The muffler installation conditions during the test should be the same as the installation conditions during actual use. 8.5.2 Test conditions
The unit under test and its auxiliary equipment equipped with silencer should be in normal working condition or rated condition during the test. It can be tested under full load or no load conditions, or under maximum noise conditions. The working conditions before and after the installation of silencer must remain the same. 8.5.3 Meteorological conditions
The test process should be carried out under the condition of wind speed less than 6m/s, and a wind hood should be used when the wind speed is greater than 2m/s. 8.6 Test report
The test report should list the following items:
Test location;
b. Test date and time;
Test personnel;
Test equipment and measurement point layout (with attached picture);
Instruments used;
Muffler to be tested
Test conditions;
GB/T4760
—1995
Should indicate the model, serial number and geometric dimensions; Noise test data
Insertion loss;
Should include the sound pressure level measurement values before and after the device is silenced and the background noise level; Meteorological data should include wind speed, temperature and air pressure, etc.; Pressure loss data;
Others;
If necessary, photos of the muffler before and after installation should be attached. 267
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