Investigation of sound transmission properties of a medium resulting from sound wave compensation caused by other sound sources
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Secondary source
Sound transmission class
Point source
The sound power level of a sound source is usually determined by sound intensity or sound-pressure measurements either in a free-field environment or in enclosures. Although these measurements provide good comparable results for the middle- and high-frequency range, there are large discrepancies at low frequencies. Using the sound intensity method as reference, it can be shown that these discrepancies are systematic. In the case of measuring sound pressure in a free field, they are due to the near-field error depending on the distance of the measurement surface to the source. Measuring sound pressure in enclosures—especially in rooms with hard walls—interference effects cause a change in the sound power output of the source, depending on the distance of the source to the room boundaries. The effects described above have been investigated theoretically and the results have been confirmed by accurate measurements.
Sound intensity probe
Intensity
Sound speed gradient
Free field
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In this paper,an elliptical micro-loudspeaker system(sound source) is designed and manufactured.The equivalent diameter of sound source is about 0.047 m,which guarantees the spatial high-resolution of sound source when the sound source distance is not less than 1.0 m.The frequency response of sound source within the range of 400 Hz to 20 kHz is good.Under the rated power,the radiation sound pressure level(SPL) of sound source reaches to 75 dB at 1.0 m.The directivity of sound source is approximately consistent with that of local circle pulsating sound source on a rigid sphere,and the non-directivity can be realized in the ±30° region near the positive direction of radiation field.As a result,the sound source with the parameters mentioned above is suitable for(and has been used to) the measure-ments of spatial high-resolution head-related transfer functions.
Directivity
Directional sound
Head-related transfer function
Sound speed gradient
Sound intensity probe
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Sound intensity probe
Intensity
Sound speed gradient
Particle velocity
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The sound intensity scaling method can be used to calculate the source sound power from the array beamforming results. The distance between the sound source and the array and the environmental noise during the test process will affect the accuracy of the sound source intensity measurement. A sound power level solution method for sound intensity scaling suitable for complex environments is verified in this paper. The depth of the sound source is determined by dual arrays. The sound wave propagation and superposition characteristics are used to establish an optimized objective function, to optimize the strength of the target sound source and remove the noise components. Finally, the sound power level of the sound sources after noise reductions are calculated by the sound intensity scaling method. In the semi-anechoic room, white noise signal and impact signal are used as interference noise in the experiments. The results show that this method can effectively reduce the noise of the signal in the noisy environment and improve the calculation accuracy of the sound power level of the sound source. The error of the sound power level calculation results under the influence of white noise and impact noise has been greatly reduced after optimization. The calculation error of sound power level decreased from 3.2 dB–12.2 dB to 0.2 dB–5dB.
Sound intensity probe
Directional sound
Ambient noise level
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Sound intensity probe
Anechoic chamber
Directional sound
Position (finance)
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The outputs of sound sources are being reported in a variety of ways, so that it is difficult to grasp quickly their relative significance. For some kinds of noise, computed sound-power levels are frequently given, whereas only sound-pressure levels are ordinarily measured. The whole picture can be clarified by routine use of source level. The (spherical) source level is the sound-pressure level at a reference distance of 1 m (unless otherwise specified) from an equivalent point source. Source level is equal to sound-power level plus an adjustment for the medium and units of measurement. In a free spherical sound field, sound pressure p varies inversely as distance r; thus, source level is 20 log (p×r/p0×r0), where the subscript 0 identifies reference quantities. Hemispherical source level is the sound-pressure level at reference distance from an equivalent hemispherical source. Axial source level is the sound-pressure level produced at reference distance on the axis of maximum response. Duct source level is the sound-pressure level of a free plane wave in a duct of reference cross section that would transmit sound power equal to that emitted by the actual source into the duct.
Point source
Sound speed gradient
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Classical acoustical equations relate the sound power of a source to sound-pressure level at a given distance from that source. Sound intensity of a source can be measured, sound power determined by integration over the radiation area, and the sound-pressure levels at a distance from the source can be calculated. An assumption is made when the sound power to sound-pressure calculation is made, that at low frequencies (20–100 Hz) the waves have propagated and the resultant sound-pressure level is from the source. This work tests the reliability of that assumption. A speaker was suspended inside three different size boxes, and powered by a noise generator bandpassed at discrete frequencies. Sound intensity measurements were recorded near field at the same discrete frequency. Free-field sound pressure measurements were then recorded at distances from the boxes. This paper reports the results of the measurements.
Sound intensity probe
Sound speed gradient
Free field
Intensity
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Similar to air reverberation chambers, non-anechoic water tanks are important acoustic measurement devices that can be used to measure the sound power radiated from complex underwater sound sources using diffusion field theory. However, the problem of the poor applicability of low-frequency measurements in these tanks has not yet been solved. Therefore, we propose a low-frequency acoustic measurement method based on sound-field correction (SFC) in an enclosed space that effectively solves the problem of measuring the sound power from complex sound sources below the Schroeder cutoff frequency in a non-anechoic tank. Using normal mode theory, the transfer relationship between the mean-square sound pressure in an underwater enclosed space and the free-field sound power of the sound source is established, and this is regarded as a correction term for the sound field between this enclosed space and the free field. This correction term can be obtained based on previous measurements of a known sound source. This term can then be used to correct the mean-square sound pressure excited by any sound source to be tested in this enclosed space and equivalently obtain its free-field sound power. Experiments were carried out in a non-anechoic water tank (9.0 m × 3.1 m × 1.7 m) to confirm the validity of the SFC method. Through measurements with a spherical sound source (whose free-field radiation characteristics are known), the correction term of the sound field between this water tank and the free field was obtained. On this basis, the sound power radiated from a cylindrical shell model under the action of mechanical excitation was measured. The measurement results were found to have a maximum deviation of 2.9 dB from the free-field results. These results show that the SFC method has good applicability in the frequency band above the first-order resonant frequency in a non-anechoic tank. This greatly expands the potential low-frequency applications of non-anechoic tanks.
Anechoic chamber
Sound speed gradient
Directional sound
Free field
Sound intensity probe
Reverberation room
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Method for the measurement of the underwater transient sound characteristics in a reverberation tank
A method to evaluate the sound power of transient sound sources in reverberation tanks is proposed and tested. The method is based on the steady state sound field characteristics in closed space, which are analyzed by normal-wave theory. To eliminate the interference caused by the boundary of the sound field, the volume integral of the spatial mean-square pressure in local space is measured and the sound power density of the non-anechoic is then obtained. Using the relationship between the sound field in the enclosed space and that of the free field, the sound power of the sound source can be obtained using the measured spatial mean-square pressure. The sound power of an impulsive sound generated by a spherical sound source is measured in a reverberation tank and the results are compared with the sound power measured in anechoic pool to verify the accuracy of the proposed method. The proposed method can be used to measure the radiated sound power of different types of transient sound sources in reverberation tanks.
Anechoic chamber
Reverberation room
Sound speed gradient
Transient (computer programming)
Sound intensity probe
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Because of its plane-wave assumption at measurement points, the pressure method is in essence an approximation in determining the sound power of a source. When using the sound intensity method, the measurement errors are theoretically due only to the finite number and locations of the measurement points. In the present paper, the degrees of measurement errors of the two methods are evaluated using a sound source model that comprises multiple point-like spherical sources placed above an infinitely large rigid floor. Using such a sound source model, it is possible to evaluate the measurement accuracies of the sound pressure and the sound intensity methods because the radiated sound power is exactly calculated for arbitrary distributions of point-like sources. By randomly changing the distribution and phases of the point-like sources, it is possible to evaluate the statistical aspects of errors such as mean, standard deviation, maximum and minimum errors. One application of this source model is a comparison of the sets of measurement points proposed by standards. A new set of measurement points is also proposed.
Sound intensity probe
Intensity
Point source
Sound speed gradient
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