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

OCR for page 78
79 Microphone array: A collection or arrangement of multiple microphones in the A-weighted sound level measured during a transient noise in a certain order for conducting special measurements of sound. event. SEL represents the sound level of the constant sound that would, in one second, generate the same acoustic energy as did the Non-specular reflection: The change of direction which the ray of actual time-varying noise event [units: dBA]. sound experiences when it strikes upon a surface and is thrown back into the same medium making the angle of reflection NOT equal to Sound intensity: The rate of flow of sound energy through a unit area the angle of incidence, unlike in mirror reflection. normal to the specified direction at the point considered. Octave: The interval between two sound frequencies having a ratio of 2. Sound (noise) source: The object which generates the sound. Octave band: A frequency range which is one octave wide. Standard Sound power: The rate per unit time at which sound energy is radiated octave bands are designed by their center frequency. from a source. Octave band center frequency: The geometric mean of the upper and Sound power level (PWL): A measure in decibels of the rate at which lower frequencies of the octave band. Standard octave band center sound energy radiates from a sound source. Specifically, it is the total frequencies in the audible range are 31.5, 63, 125, 250, 500, 1000, energy per second produced by a sound source, and expressed in deci- 2000, 4000, 8000, and 16,000 Hz. 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 1012 watts [units: dB]. One-third octave: The interval between two sound frequencies having a ratio of the cube root of 2. Sound pressure level (SPL): A measure in decibels of the magnitude of the sound. Specifically, the sound pressure level of a sound, in deci- One-third octave band: A frequency range which is one-third octave bels, is 10 times the logarithm to the base 10 of the ratio of the wide. Standard one-third octave bands are designed by their center squared pressure of this sound to the squared reference pressure. The frequency. reference pressure is usually taken to be 20 micropascals [units: dB]. One-third octave band center frequency: The geometric mean of the Spatial resolution: Determination or breaking up of the image of a upper and lower frequencies of the one-third octave band. Stan- sound source into separate, constituent elements or parts in the space dard one-third octave band center frequencies (in Hz) in the audi- domain. ble range are: Spectrum: The frequency content of the noise produced by the source. 25.0 50.0 100 200 400 800 1,600 3,150 6,300 12,500 The noise is broken into multiple periodic (frequency) components, 31.5 63.0 125 250 500 1,000 2,000 4,000 8,000 16,000 each with an amplitude and phase. 40.0 80.0 160 315 630 1,250 2,500 5,000 10,000 20,000 Spherical spreading (divergence): The spreading of sound waves from Overall sound pressure level (OASPL): The logarithmic level of the a point source in a free field, resulting in a diminution in sound pres- total energy contained in the noise spectrum, determined as the sum sure level with increasing distance from the source. For waves having (based on energy) of the sound pressure levels of all frequency com- wave fronts that are concentric spheres (spherical waves), the sound ponents [units: dB]. In this report this operation is applied to the pressure level decreases 6 dB for each doubling of distance from the A-weighted sound levels (in dBA). source (inverse square law). Pink noise: Noise with a continuous frequency spectrum and with Spiral angle: The angle between a twisted spoke of a spiral microphone equal power per constant percentage bandwidth. For example, equal array and the array radial axis [see Figure 1(a)]. power in any one-third octave band. Wavelength: The physical distance between identical points on succes- Resonance: The tendency of a system to oscillate at its maximum am- sive waves. plitude, associated with specific frequencies known as the system's resonance frequencies (or resonant frequencies). Weighting: An additive (or subtractive) factor by which the sound pres- sure level at certain frequencies in an acoustic measurement is in- Sound exposure level (SEL): A time-integrated metric (i.e., continu- creased (or reduced) in order for that measurement to be more repre- ously summed over a time period) which quantifies the total energy sentative of certain simulated conditions. See A-weighted sound level.