National Academies Press: OpenBook

Acoustic Beamforming: Mapping Sources of Truck Noise (2009)

Chapter: Appendix C - Glossary of Special Terms

« Previous: Appendix B - Vertical Distributions of Noise Sources for Heavy Trucks
Page 78
Suggested Citation:"Appendix C - Glossary of Special Terms." National Academies of Sciences, Engineering, and Medicine. 2009. Acoustic Beamforming: Mapping Sources of Truck Noise. Washington, DC: The National Academies Press. doi: 10.17226/14311.
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Page 78
Page 79
Suggested Citation:"Appendix C - Glossary of Special Terms." National Academies of Sciences, Engineering, and Medicine. 2009. Acoustic Beamforming: Mapping Sources of Truck Noise. Washington, DC: The National Academies Press. doi: 10.17226/14311.
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Page 79

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78 1 Hz spectrum level: The level of a signal spectrum in a frequency band 1 Hz wide, often called spectral density. Aperture: An opening in a microphone array through which sound waves travel. Autospectrum: The averaged magnitude of multiple instantaneous spectra with the coefficients of the components expressed as the square of the magnitudes. A-weighted sound level: The sound level obtained by use of A-weighting. A-weighting accounts for the frequency sensitivity of the human ear by decreasing the sound levels at very low and very high fre- quencies (below approximately 500 Hz and above approximately 10,000 Hz) to approximate the human ear’s sensitivities to those frequencies [units: dBA]. Bandwidth: A frequency range of a segment of the frequency spectrum, which may be specified by its upper and lower cutoff frequencies. Beam width (spot width): The width of the main lobe in the directivity pattern of a microphone array. Beamforming: A signal processing technique used in sensor arrays for directional signal transmission or reception. Cross spectrum: Cross-spectrum analysis is an extension of single- spectrum Fourier analysis (see autospectrum, fast Fourier transform, and spectrum) to the simultaneous analysis of two signals. The pur- pose of cross-spectrum analysis is to uncover the correlations between two signals at different frequencies. Decibel (dB): The unit used to identify 10 times the common logarithm of two like quantities proportional to power, such as sound power or sound pressure squared, commonly used to define the level produced by a sound source. For the A-weighted sound levels, the unit symbol dBA is used. Directivity: A measure of the directional characteristic of a microphone or a microphone array and its sensitivity to sound waves that arrive from various directions of propagation of the incident sound. Doppler shift: The apparent change in frequency of sound waves, vary- ing with the relative velocity of the source and the receiver: if the source and receiver are drawing closer together, the frequency is increased, and vice versa. Equivalent sound level (Leq): The level of a constant sound that, in the given situation and time period, has the same average sound energy as does a time-varying sound. Specifically, equivalent sound level is the energy-averaged sound pressure level of the individual A-weighted sound pressure levels occurring during the specified time interval [units: dBA]. Fast Fourier transform (FFT): An efficient algorithm to compute the discrete Fourier transform of a signal, which decomposes a sequence of values into components of different frequencies (spectrum). This operation is useful in many fields but computing it directly from the definition is often too slow to be practical. An FFT is a way to com- pute the same result more quickly. Free field: A sound field in a homogeneous isotropic medium whose boundaries exert a negligible influence on the sound waves. In prac- tice, it is a field in which the effects of the boundaries are negligible over the frequency range of interest. Frequency: The number of oscillations per second completed by a vibrat- ing object [units: Hz]. Frequency resolution: Minimum difference in frequency between two components of the sound spectrum which still allows resolving two distinct peaks in the spectrum. Grating lobe: A secondary projection in the directivity pattern of a microphone array caused by the interference of individual micro- phone directivities. Hertz (Hz): The unit used to designate frequency. Specifically, the num- ber of cycles per second. Inverse square law: See spherical spreading (divergence). Main lobe: A major projection or protuberance of rounded or globular form, specifically in the directivity pattern of a microphone array. A P P E N D I X C Glossary of Special Terms

Microphone array: A collection or arrangement of multiple microphones in a certain order for conducting special measurements of sound. Non-specular reflection: The change of direction which the ray of sound experiences when it strikes upon a surface and is thrown back into the same medium making the angle of reflection NOT equal to the angle of incidence, unlike in mirror reflection. Octave: The interval between two sound frequencies having a ratio of 2. Octave band: A frequency range which is one octave wide. Standard octave bands are designed by their center frequency. Octave band center frequency: The geometric mean of the upper and lower frequencies of the octave band. Standard octave band center frequencies in the audible range are 31.5, 63, 125, 250, 500, 1000, 2000, 4000, 8000, and 16,000 Hz. One-third octave: The interval between two sound frequencies having a ratio of the cube root of 2. One-third octave band: A frequency range which is one-third octave wide. Standard one-third octave bands are designed by their center frequency. One-third octave band center frequency: The geometric mean of the upper and lower frequencies of the one-third octave band. Stan- dard one-third octave band center frequencies (in Hz) in the audi- ble range are: 25.0 50.0 100 200 400 800 1,600 3,150 6,300 12,500 31.5 63.0 125 250 500 1,000 2,000 4,000 8,000 16,000 40.0 80.0 160 315 630 1,250 2,500 5,000 10,000 20,000 Overall sound pressure level (OASPL): The logarithmic level of the total energy contained in the noise spectrum, determined as the sum (based on energy) of the sound pressure levels of all frequency com- ponents [units: dB]. In this report this operation is applied to the A-weighted sound levels (in dBA). Pink noise: Noise with a continuous frequency spectrum and with equal power per constant percentage bandwidth. For example, equal power in any one-third octave band. Resonance: The tendency of a system to oscillate at its maximum am- plitude, associated with specific frequencies known as the system’s resonance frequencies (or resonant frequencies). Sound exposure level (SEL): A time-integrated metric (i.e., continu- ously summed over a time period) which quantifies the total energy in the A-weighted sound level measured during a transient noise event. SEL represents the sound level of the constant sound that would, in one second, generate the same acoustic energy as did the actual time-varying noise event [units: dBA]. Sound intensity: The rate of flow of sound energy through a unit area normal to the specified direction at the point considered. Sound (noise) source: The object which generates the sound. Sound power: The rate per unit time at which sound energy is radiated from a source. Sound power level (PWL): A measure in decibels of the rate at which sound energy radiates from a sound source. Specifically, it is the total energy per second produced by a sound source, and expressed in deci- bels, equal to 10 times the logarithm to the base 10 of the ratio of the power of a sound to the reference power of 10–12 watts [units: dB]. Sound pressure level (SPL): A measure in decibels of the magnitude of the sound. Specifically, the sound pressure level of a sound, in deci- bels, is 10 times the logarithm to the base 10 of the ratio of the squared pressure of this sound to the squared reference pressure. The reference pressure is usually taken to be 20 micropascals [units: dB]. Spatial resolution: Determination or breaking up of the image of a sound source into separate, constituent elements or parts in the space domain. Spectrum: The frequency content of the noise produced by the source. The noise is broken into multiple periodic (frequency) components, each with an amplitude and phase. Spherical spreading (divergence): The spreading of sound waves from a point source in a free field, resulting in a diminution in sound pres- sure level with increasing distance from the source. For waves having wave fronts that are concentric spheres (spherical waves), the sound pressure level decreases 6 dB for each doubling of distance from the source (inverse square law). Spiral angle: The angle between a twisted spoke of a spiral microphone array and the array radial axis [see Figure 1(a)]. Wavelength: The physical distance between identical points on succes- sive waves. Weighting: An additive (or subtractive) factor by which the sound pres- sure level at certain frequencies in an acoustic measurement is in- creased (or reduced) in order for that measurement to be more repre- sentative of certain simulated conditions. See A-weighted sound level. 79

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 635: Acoustic Beamforming: Mapping Sources of Truck Noise explores the acoustic beamforming technique in an attempt to pinpoint and measure noise levels from heavy truck traffic. The beamforming technique uses an elliptical array of more than 70 microphones and data acquisition software to measure noise levels from a variety of noise sources on large trucks—including the engine, tires, mufflers, and exhaust pipes.

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