Experimental data collected over the years, for defining limits of vibration exposure to human beings, have resulted in a set of vibration criteria specified in ISO Standard 2631. In this article, instrumentation requirements for evaluation of the responses of humans to vibration according to these criteria are described, as well as some of the pitfalls to be avoided during these measurements. Exposure limits for vibration transmitted to the hands and arms of operators of vibrating tools have been suggested in Draft Standard ISO/DIS 5349. A special hand adaptor developed for the measurement of hand-arm vibration transmitted from the handle of such tools is described in the article, and measurement results obtained with it on a chip hammer are illustrated.

The calibration of measurement microphones below 100 Hz is not very well covered by the present IEC standards. The uncertainty increases rapidly and for very low frequencies it goes toward infinity. This paper approaches this issue and presents a unique way to verifying and calibrating the low-frequency response of measurement microphones. Using a small isolated calibration volume and applying a constant force to a large piston inside this volume, you obtain a direct proportional relation between force and sound pressure, allowing calibration of measurement microphones down to 0.01 Hz. LOW FREQUENCY MEASUREMENTSModern technology has caused a more frequent occurrence of low frequency sound phenomena. Thunder, water-falls and wind are sources in nature, but human activity has added many more sources causing distress and annoyance among people sensitive to low frequency sound. Heavy machinery and traffic often produce sound of a continuous cyclic character while aircraft take-off, sonic boom, pile driving, and blasting produce single events of an often startling nature.The very long wave length associated with low frequency waves e.g. 170 m (500 feet) at 2 Hz, makes it very difficult to absorb the energy of the travelling wave. Very low frequencies may travel thousands of kilometers. Free field conditions do not exist and in dosed volumes isothermal to adiabatic conditions play an important role.The measurement of low frequency sound pressure levels calls for special instrumentation not described in the present standards for sound level meters. Sound level meters are only specified for use in the frequency range above 20 Hz. At 20 Hz the tolerance of the weighting is ±2.5 dB and proper standardized calibration procedures including the microphone are non-existing. The consequence is the jump in standardized hearing threshold curves based on discrepancies in measurements reported over many years. Fig.l In order to measure perceived sound pressure levels at the threshold of human hearing, one should cover the frequency range down to 2 Hz and a dynamic range from 120 dB at 2 Hz and up to the maximum level one is interested in. As seen from fig. 2, the equal loudness level contours cover a range of more than 140 dB for full coverage of measurements involving humans.The instrumentation used to measure the sound pressure in the infrasound range 2 -20 Hz must be able to measure correctly in this frequency range. A normal type 1 sound level meter is not well suited. The tolerance on the frequency weighting alone below 20 Hz is ±3 dB and at 16 Hz it is +5 -∞. This does not include the microphone in most calibration situations. The database on which the ISO 226-2003 is based may thus be rather vague at 20 Hz, which may explain the lack of continuity in the equal loudness contour curves shown in fig. 1. The data trace for the infrasound equal loudness contour level may be considered more realistic, as the experimenters may have been more aware of the instrumentation requirements.

No abstract

In the development of the new Bruel & Kjaer Measurement Condenser Microphone, it was found necessary to thoroughly investigate the diffraction characteristics and to use several calibration techniques to establish the difference between the microphone's free field and pressure response. The techniques and results of these investigations are described as the following: (A) Measuring the pressure distribution around a model of the microphone placed in a free sound field and the sensitivity distribution over the microphone diaphram. In conjunction with measurements made with a specially designed concentric electrostatic actuator, the effective sound pressure increase on the diaphram is calculated. (B) Measuring the microphone sensitivity as a function of frequency according to the free-field reciprocity method, and comparing the results with the pressure response of the same microphones measured by means of the electrostatic actuator method. The free-field correction obtained by the two different methods (A) and (B) showed excellent agreement.

Sound power determination using direct measurement of sound intensity is presently under study by ANSI and other standards organizations. When compared with the conventional methods of sound power determination that are based on sound pressure measurements (e.g., ANSI S1.31–S1.36), the intensity approach offers significant advantages. Precision sound power determinations can be made without a special test chamber, and the result can be as much as several orders of magnitude less sensitive to ambient noise. Also, measurements can be made over any convenient control surface in the nearfield or farfield. After a brief introduction, the authors will demonstrate these advantages by performing a series of sound power determinations on a known source using commercially available equipment.