Sound intensity probes

Direct measurement of the velocity or the amplitude of displacement of an imaginary particle submitted to an ultrasonic field is not easy. Filipczynski [132] suggested the use of a capacitance probe method in which the vibrations in the medium are picked up by a diaphragm. The displacement of the diaphragm is measured with an electrostatic microphone, and this is then related to the particle displacement. Sound intensity is given by the relation shown in Eq. (28) where r = particle displacement. The method can be used up to a frequency of 300 kHz. 

The response of hydrophones or microphones can be calibrated in terms of either sound level (dB) or acoustic pressure either of which are related to ultrasound intensity. The Fourier transform of these give a frequency-dependent signal, which is also related to sound intensity. It is possible therefore to obtain a plot of sound level (and thus theoretically ultrasound intensity) against frequency. However, as already mentioned, ultrasound power measurements using acoustic probes are not straightforward and require preliminary calibration of the probes with another... 

Sound intensity meters. A sound intensity meter comprises a probe and an analyzer. The analyzer may be of the analog, digital, or FFT (fast Foiuier transform) type. The analog t5q)e has many practical disadvantages that make it suitable only for surveys and not precision work. 

Sound intensity probes. There are several probe designs that employ either a number of pressure microphones in various configtu ations or a combination of a pressure microphone and a particle velocity detector. The first type of probe uses nominally identical pressure transducers that are placed close together. Various arrangements have been used with the microphones either side by side, fiice to face, or back to back. Eadi configuration has its own advantages and disadvantages. 

The equipment under test is divided into a convenient number of subareas that are selected to enable a well-controlled probe sweep over the subarea. Guidance is given on the sweep rate and the line density. Measurement accuracy is graded according to the global pressure-intensity index, LK This is the numerical difference between the sound intensity level and the sound pressure level. If this... 

Sound source location. Since a sound intensity probe has strong directional characteristics there is a plane at 90° to the axis of the probe in which the probe is very insensitive. A sound source just forward of this plane will indicate positive intensity, whereas if it is just behind this plane the intensity will be negative (Fig. A-12). 

This property of the probe can be used to identify noise sources in many practical situations. The normal procedure is to perform an initial survey of the noise source to determine its total sound power. The probe is pointed toward the source stem to identify areas of high sound intensity. Then the probe is reoriented to lie parallel to the measurement surface and the scan is repeated. As the probe moves across a dominant source the intensity vector will flip to the opposite direction. 

If the measurements in the right hand room are carried out using sound intensity, then it is necessary to reduce the amount of reverberation in this room. Since the probe can measure the sound intensity coming through the panel then flanking transmission is no longer a limitation. 

Sound source location. Diuing two noise surveys the sound power levels from two different designs of lube oil console were meastued using the sotuid intensity meter. The overall sound power levels were 93 dB(A) and 109 dB(A). The two consoles had electrically driven pumps and both emitted strong tonal noise. In the first case the tonal noise was centered on 8 kHz using the sound intensity probe it was possible to home in on the piunp suction pipe as a major noise source. This was confirmed by vibration velocity measiuements. 

TLS have been probed effectively in ultrasonic experiments. An interesting strategy is used in these measurements. This is the so-called hole burning technique. When the intensity of the acoustic pulse is sufficiently high, the two states in TLS become equally populated and therefore the absorption from the acoustic field becomes saturated. As a result, the subsequent pulse suffers a reduced attenuation. Thus the high intensity sound pulse has burnt a hole in the occupation number at an energy, E =hco with a width Aa>. This is illustrated in Figure 9.06 for the... 

A caveat must be sounded as to the use of intensity (or lifetime) data accrued through the use of triplet probes in sensing polymeric transitions in this manner. The probe/label data are complementary but vicarious in the nature of the information relayed to the observer. 

Methods for measuring ultrasonic power have been reviewed [42] but, in short, there does not seem to be a simple method for the quantitative measurement of local ultrasonic intensity when cavitation is present. Pugin has developed a number of methods for the characterisation of sound fields in a variety of reactors [43]. These were used to develop profiles of the acoustic intensity for both cleaning probes and probe systems with a view to examining the reproducibility of reactions. 

The Scandinavian proposed standard (DSF 88/146) was developed for the determination of the sound power of a sound source under its normal operating conditions and in situ. The method uses the scanning technique whereby the intensity probe is moved slowly over a defined surface while the signal analyzer time-averages the measured quantity during the scanning period. 

FIG. A-12 Sound source location using the intensity probe. (Source Altair Filters International Limited.)... 

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