Verification of sound intensity measuring instrument in traveling wave field
 

 
  Sound intensity is a vector indicating the strength and direction of the sound energy flow. The size of sound intensity is the sound energy of a certain point in the sound field per unit time passing through a unit area perpendicular to the direction of particle velocity. The sound intensity measuring instrument is an instrument that measures the component of the sound intensity in the air medium in the direction of the probe axis. Through multi-point sound intensity measurement, the intensity distribution of sound energy flow on a plane near the sound source can be obtained to study the radiation characteristics of the sound source. Through the sound intensity measurement, the sound power level of the sound source can also be determined.
 
  Sound intensity measuring instrument includes two parts: sound intensity probe and sound intensity processor. The sound intensity probe uses a pair of sound pressure microphones separated by a certain distance. The sound intensity measuring instrument should give the octave or 1/3 octave spectrum of sound intensity and sound pressure, and provide A weighting function. The sound intensity meter has two accuracy levels, level 1 and level 2. The requirements for level 1 are stricter than level 2. When the sound intensity meter is in use, a sound intensity calibrator is usually used for self-calibration. The calibrator simulates the sound pressure and phase difference on the two microphones of the sound intensity probe for a given sound intensity, and it is usually used in conjunction with a certain sound intensity probe. Since this is a simulation method, the degree of sound intensity simulated by the calibrator needs to be verified separately.
 
   A direct verification method is used here. A sound field with a known sound intensity is generated in the space. Use the verified sound intensity meter to measure and compare the measurement results to determine the measurement deviation of the verified sound intensity meter.
 
   This method has been submitted to the National Quality Supervision and Inspection* as a draft of the verification regulations for sound intensity measuring instruments.
 
   1. Contents and principles of verification
 
  Sound intensity measuring instrument needs to be verified mainly including the deviation of the sound intensity level and the measured value of the sound pressure level in each 1/3 octave band, and the sound pressure-residual sound intensity index. When the axis of the two microphones of the sound intensity probe is perpendicular to the direction of sound intensity, the sound intensity component should be zero. Because there is no sound energy in the direction perpendicular to the sound intensity. However, because the phase characteristics of the two microphones and amplification channels of the sound intensity probe are not completely consistent, the sound intensity meter will measure a certain sound intensity value. This sound intensity reading is called the residual sound intensity. The residual sound intensity is proportional to the sound pressure level. The sound pressure-residual sound intensity index is defined as the difference between the measured sound pressure level and the residual sound intensity level when the two microphones of the sound intensity probe are subjected to the same pink noise. The index is generally given in the form of a 1/3 octave spectrum. The larger the sound pressure-residual sound intensity index, the stronger the suppression ability of the sound intensity meter to the sound waves incident perpendicular to the axis of the probe, and the better the directivity.
 
  2. Verification of sound intensity level and sound pressure level deviation of sound intensity meter
 
The test device for measuring deviation of sound intensity level and sound pressure level of    sound intensity meter is shown in Figure 1. In an anechoic room, the far field of the speaker box can be approximated as a series of plane traveling waves. The axis of the two microphones of the sound intensity probe is consistent with the propagation direction of the sound wave. The direction of the probe is reversed by 180°, and the reverse sound intensity component can be measured. The standard microphone is placed next to the sound intensity probe. The measuring amplifier is a high-precision amplifier. Comparing the sound pressure level measured by the standard microphone and the measuring amplifier and the sound pressure level measured by the sound intensity meter, the deviation of the sound pressure level measurement of the sound intensity meter can be obtained. The deviation of the sound intensity level measured by the sound intensity level measured by the sound intensity meter and the sound intensity level calculated by the formula (1) can be obtained.
 
  3. Verification of sound pressure-residual sound intensity index level of sound intensity meter
 
   Input the output signal of the pink noise signal generator directly to the power amplifier. Remove the isolation column of the sound intensity probe and close the two microphones to each other. Place the sound intensity probe near the central axis of the sound source and about 2m away from the sound center of the sound source. Make the axis of the sound intensity probe perpendicular to the direction of the incident sound wave, so that the two microphones receive the same sound pressure signal.
 
  Measure the sound pressure level and sound intensity level with a certified sound intensity measuring instrument. The center frequencies are from 50 Hz to 6.3 kHz. The sound pressure level measured at this time minus the sound intensity level is the sound pressure-residual sound intensity index.