Acoustics and vibrations

Acoustics

Nowadays we are living in an increasingly noisy environment. We are surrounded by busy roads, railways, and in some places even airports. Some working environments are also more or less noisy, especially in heavy industry. Moreover, we are occasionally exposed to disturbing noise at our homes, both because of the neighbours, as well as because of various sports and cultural events in the neighbourhoods or because of the proximity of heavy industry and transport infrastructure. Daily exposure to excessive noise damages our hearing, so the number of hearing impairments is increasing.

Disturbing noise needs to be measured correctly, so several different instruments for measuring acoustic quantities have been developed. The basic instrument is a sound level meter, which generally measures the sound pressure level in a wide or narrow frequency range. The key element of a sound pressure meter is a measuring microphone, which converts an acoustic signal into an electrical signal with a certain microphone sensitivity. The measurement of acoustic parameters is generally highly dependent on ambient conditions (i.e., air temperature, pressure, and relative humidity), so sound level meters would generally display a different reading depending on the day-to-day ambient conditions compared to a reading measured under reference laboratory conditions. Therefore, the sound level meter must be calibrated under given ambient conditions prior to the use with an appropriate acoustic calibrator that generates a constant acoustic pressure over a much wider range of ambient conditions. Sound level meters measure only sound pressure at a certain moment or. within a short interval, however they do not give cumulative noise, or. sound doses received by a person over a longer period (i.e., several minutes, hours). This acoustic quantity is measured by an acoustic dosimeter.

Vibrations

We talk about acoustics when we are dealing with fluctuations in air pressure, i.e., oscillations of air particles. On the other hand, we are talking about vibrations when we are dealing with the oscillation of solid surfaces. Each rotating or a moving object evokes certain vibrations. Vibrations could be periodic which most often occur because of surface oscillations or as rotation of various motors. The vibrations are measured with vibration meters, which are usually based on accelerometers. Reliable and accurate measurement of periodic vibrations is important since a change in vibration may indicate damage to the machine or engine, so the operator can stop operation even before costly and dangerous mechanical damage could occur. Periodic vibrations are usually sinusoidally shaped, are long-lasting, and generally have a relatively low amplitude of accelerations (i.e., vibrations have small velocities and small displacements). In addition to periodic vibrations, we also know vibration transients (shock vibrations), which occur when there are sudden changes in acceleration, velocity, or displacement (e.g., a car crash, an object falling to the ground, etc.). Proper measurement of such transients is necessary in the modern world mainly due to the increasing number of product testing in accordance with various standards. In the case of vibration transients, the duration of the transient and the amplitude of the acceleration or deceleration.