It is important to calibrate transducers used for sound, shock, and vibration to ensure the accuracy of measurements made with them. Proper calibration also ensures that the results measured with the transducers are comparable with the results measured by others. The accuracy of the calibration must also be known. The transducer system and the calibration method used must perform the calibration with a known accuracy. If these conditions are met, then the calibration is termed as traceable. It should be noted, however, that this traceability is not an indication necessarily of highly accurate measurements, but simply that if the uncertainty is known, the measurements can then be compared with valid measurements made by others, which have also been made using proper calibration procedures. Thus measurements made with transducers and calibration procedures are of no use unless the associated uncertainty in measurement is also known. New and upgraded international standards now provide a variety of calibration methods together with their suitability for ensuring traceability.
Transducers should be calibrated on site before and after each measurement or set of measurements and periodically at service centers or national metrology laboratories. Reference [7] describes calibration procedures for microphones and sound intensity systems. Reference [18] discusses the calibration of shock and vibration transducers. References [7, 18] also describe the traceability of sound and vibration measurements, and Ref. [19] describes the traceability of shock and vibration measurements to national and international standards in detail.
Microphones, calibrators, and SLMs all need to be calibrated. The most common requirement is the calibration of microphones for their sound pressure response, but calibrations for sound intensity, sound power, and acoustic impedance measurements are also commonly needed [7]. Nedelitsky in Chapter 108 of Ref. [8] and others have also discussed the problems of calibrating microphones.
When a microphone is introduced into a sound field, it causes undesirable reflections and hence changes the sound field. This effect is more pronounced at high frequency. Ideally we should like to measure the sound pressure in the unperturbed or “free field,” but this is difficult. It is extremely important that manufacturers and users calibrate microphones accurately. Unfortunately, some users do not calibrate microphones sufficiently frequently to ensure the accuracy of sound pressure level readings. As already discussed, microphones may be designed to have three flat frequency responses. TEDS microphones can be used to obtain all of the three flat frequency responses. The two main types of microphones in use are the free‐field type (primarily used for outdoors measurements) and the diffuse‐field type (mainly used for indoors measurements). Pressure‐field microphones are mostly used for coupler measurements, except for the recently developed flat surface microphone.
As regards calibration of microphones, the pressure response may be considered to be the response of the microphone to a uniform pressure applied over its diaphragm. This is the response of an ideal microphone of zero size introduced into a free progressive plane wave field. However, when an actual finite size microphone is introduced into such a field, reflection, and diffraction are caused, which give a different microphone response called the free‐field response [2]. Because the microphone can be oriented at any arbitrary angle in the plane wave field, perhaps the pressure response is more fundamental. There are several ways of measuring the pressure response of a microphone: [20, 21] (i) pistonphone, (ii) driven‐diaphragm‐type calibrator, (iii) electrostatic actuator, (iv) reciprocity method, and (v) substitution method. The only absolute method of calibration is the reciprocity method. This is somewhat complicated and time‐consuming; and the other four methods, although not giving absolute calibration, are more convenient and normally sufficiently accurate. See Ref. [7] and Chapter 108 in Ref. [8].
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