The requirements for noise testing can be somewhat unclear given the general lack of familiarity with acoustic phenomena. This article describes the three basic categories of noise testing. It serves to present the fundamentals of each test type to enable a better understanding of the applicable test specifications.
AIR-BORNE NOISE TESTS
Air -borne noise tests measure the noise levels generated by the test item. It is performed in an anechoic chamber that serves to eliminate interference from intruding sources. It also eliminates reverberations (echoes) within the chamber. The chamber is constructed of modular steel panels with interior surfaces of anechoic wedges on all interior surfaces.
STRUCTURE-BORNE NOISE TESTS
Structure-borne noise is self-induced vibration that is transmitted through equipment and ultimately into foundations and surfaces, where the vibrational energy is radiated as airborne sound. Another definition is sound that travels over at least part of its path by means of a solid structure. Testing is done by measuring the test items vibrational response.
ACOUSTIC NOISE TESTS
Simulates the environment of an external acoustic field and observes its effects on the test item. Of importance for equipment operating near aircraft or large industrial equipment operating near aircraft or large industrial equipment, engines, etc. Problems caused by acoustic noise include chafing and failures of wires, cracking of printed circuit boards, vibration of optical elements, etc. The testing is performed in the reverberation chamber, which is similar to the anechoic chamber except that the wedges are removed.
A n anechoic chamber is a room with special walls that absorb as much sound as possible. "Anechoic" means "without echoes." Sometimes the entire room even rests on shock absorbers, negating any vibration from the rest of the building or the outside.
The material covering the walls of an anechoic chamber uses wedge-shaped panels to dissipate as much audio energy as possible before reflecting it away. Their special shape reflects energy into the apex of the wedge, dissipating it as vibrations in the material rather than the air. Anechoic chambers are frequently used for testing microphones, measuring the precise acoustic properties of various instruments, determining exactly how much energy is transferred in electro-acoustic devices, and performing delicate psychoacoustic experiments.
A sound field is a region where there is sound. It is classified according to the manner and the environment in which the sound waves travel. Some examples will now be described and the relationship between pressure and intensity discussed. This relationship is precisely known only in the first two special cases described below.
The Free Field
This term describes sound propagation in idealized free space where there are no reflections. These conditions hold in the open air (sufficiently far enough away from the ground) or in an anechoic room where all the sound striking the walls is absorbed. Free field propagation is characterized by a 6 dB drop in sound pressure level and intensity level (in the direction of sound propagation) each time the distance from the source is doubled. This is simply a statement of the inverse square law. The relationship between sound pressure and sound intensity (magnitude only) is also known. It gives one way of finding sound power which is described in the International Standard ISO 3745.
The Diffuse Field
In a diffuse field, sound is reflected so many times that it travels in all directions with equal magnitude and probability. This field is approximated in a reverberant room. Although the net intensity is zero, there is a theoretical relationship which relates the pressure in the room to the onesided Intensity, Ix. This is the intensity in one direction, ignoring the equal and opposite component. One-sided intensity cannot be measured by a sound intensity analyzer but it is nevertheless a useful quantity: By measuring pressure we can use the relationship between pressure and one-sided intensity to find the sound power. This is described in ISO 3741.
Measuring Sound Intensity
Sound pressure is not equally sensed by the human ear at different frequencies; the human ear is more sensitive to sound in the frequency range of 1 kHz to 4 kHz than to sound at very low or high frequencies. Higher sound pressures are therefore acceptable at lower and higher frequencies than in the mid range. Knowledge about the human ear is important in acoustic design and sound measurement. To compensate, sound meters are normally fitted with filters adapting the measured sound response to the human sense of sound. Common filters are dB(A), dB(B), and dB(C)
dB(A) - The decibel A filter is widely used. dB(A) roughly corresponds to the inverse of the 40 dB (at 1 kHz) equal-loudness curve for the human ear. Using the dB(A) filter, the sound level meter is less sensitive to very high and very low frequencies. Measurement made with this scale are expressed as dB(A).
dB(B) and dB(C) - The decibel C filter is practically linear over several octaves and is suitable for subjective measurements at very high sound pressure levels. The decibel B filter is between C and A, however, the B and C filters are seldom used.
The dB A, B, and C criteria could be observed on the graph below. It can be seen how the dB(A) filter is used to adjust sound at frequencies in the high and low ranges of human hearing.