Читать книгу Engineering Acoustics - Malcolm J. Crocker - Страница 14

Real‐world problems in the control of noise and vibration in aircraft, appliances, buildings, industry, and vehicles require the measurement of particular environmental parameters such as sound pressure, force, acceleration, velocity, displacement, etc. This process is often performed by using acoustical and vibration transducers. Vibration and acoustical sensors are transducers which convert a measured physical property (e.g. the vibration of a body or the propagation of a sound wave) into an electrical signal (voltage or charge). These electrical signals are often conditioned to provide signals suitable for the measurement devices. The signals are then amplified, attenuated, or transformed so that they can subsequently be analyzed and/or processed to provide the data of particular interest in the time domain and frequency domain. The information provided by these analyses is widely used to assess sources of noise and vibration, and design proper engineering control measures. For some cases, such as simple measurements of the A‐weighted sound pressure level, only limited amounts of processing are needed. In other cases with more sophisticated measurements, special analysis and processing is required. Such examples include modal analysis, sound intensity, wavelet analysis, machinery condition monitoring, beamforming, and acoustical holography, with which quite complicated signal analysis and processing may be needed. Some years ago, almost all measurements were made with analog equipment. Many analog instruments are still in use around the world. However, by using analog‐to‐digital conversion, increasing use is now made of digital signal processing to extract the required data. This is done either in dedicated instruments or by transferring measurement results onto computers for later processing by software.

Real‐time analysis in the frequency domain has many applications, including noise and vibration studies where the signal is nonstationary with time. Such applications include machinery vibration analysis, bearing noise, transient analysis, acoustic emission, speech analysis, music, and others. The goal of this chapter is to define the main types of signals used in noise, shock, and vibration control and also to serve as an introduction to signal analysis. The discussion in this chapter is kept mainly descriptive and those readers requiring a further mathematical discussion of signal analysis are referred to more detailed treatments available in several books [1–9].