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145 â¢ Ambient noise. The prevailing general noise existing at a location or in a space, which usually consists of a composite of sounds from many sources near and far. â¢ Background noise. The general composite nonrecognizable noise from all distant sources, not including nearby sources or the source of interest. In a building or airport, background noise consists of a large number of distant noise sources. â¢ Decibel (dB). Ten times the logarithm (base 10) of a ratio. The decibel is a measure on a loga- rithmic scale of the magnitude of a particular quantity such as sound pressure, sound power, or sound intensity with respect to a standardized quantity. â¢ Decibel, A-weighted (dBA). The sound pressure level in decibels as measured on a sound level meter, using the internationally standardized A-weighting filter or as computed from sound spectral data to which A-weighting adjustments have been made. A-weighting de-emphasizes the low and very high frequency components of the sound in a manner similar to the response of the average human ear. A-weighted sound levels correlate well with subjective reactions of people to noise and are universally used for community noise evaluations. â¢ Digital signal processor (DSP). See Appendix F. â¢ Equalization. See Appendix F. â¢ Gain before feedback (GBF). See Appendix F. â¢ Flutter echo. This phenomenon is a distinct sound reflection pattern that may occur in the presence of large flat or parallel surfaces. For instance, a hand clap will echo repeatedly. â¢ Frequency. The number of oscillations per second of a periodic noise (or vibration) expressed in Hertz (abbreviated Hz). Frequency in Hertz is the same as cycles per second. â¢ Human factors. The study of how humans behave physically and psychologically in relation to particular environments, products, or services. Also known as ergonomics. â¢ Equivalent level (Leq). The level of a steady noise which would have the same energy as the fluctuating noise level integrated over the time period of interest. Leq is widely used as a single number descriptor of environmental noise. This energy average is not the same as the aver- age sound pressure levels over the period of interest, but must be computed by a procedure involving summation or mathematical integration. â¢ Noise reduction coefficient (NRC). The NRC is a simple representation of the amount of sound energy absorbed on striking a particular surface. An NRC of 0 indicates perfect reflec- tion; an NRC of 1 indicates perfect absorption. â¢ Octave band and 1/3 octave band. One octave is an interval between two sound frequencies that have a ratio of two. For example, the frequency range of 200 Hz to 400 Hz is one octave, as is the frequency range of 2000 Hz to 4000 Hz. An octave band is a frequency range that is one octave wide. A standard series of octaves is used in acoustics, and they are specified by their center frequencies. In acoustics, to increase resolution, the frequency content of a sound or vibration is often analyzed in terms of 1/3 octave bands, where each octave is divided into three 1â3 octave bands. A p p e n d i x d Glossary
146 improving intelligibility of Airport Terminal public Address Systems â¢ Outdoor-indoor transmission class (OITC). OITC is a standard used for indicating the rate of transmission of sound between outdoor and indoor spaces in a structure. It is dif- ferent from the STC because it uses a source noise spectrum that considers frequencies down to 80 Hz and is weighted more to lower frequencies. A single number classification, specified by the American Society for Testing and Materials (ASTM E 1332, issued 1994), that establishes the A-weighted sound level reduction provided by building facade compo- nents (i.e., walls, doors, windows, and combinations thereof), based on a reference sound spectrum that is an average of typical air, road, and rail transportation sources. The OITC is the preferred rating when exterior facade components are exposed to a noise environment dominated by transportation sources. â¢ PA. This is a system with an electronic sound amplification and distribution system which contains a microphone, amplifier, and loudspeakers, used to allow a person to address a large public area. â¢ Rapid Speech Transmission Index (RASTI). The rapid speech transmission index was devel- oped as instrumentation evolved to measure STI more efficiently. â¢ Reverberation time (RT, RT60). The characteristic rate at which sound decays in a room. It is a function of volume and effective acoustical absorption. Various formulas can be used to calculate reverberation time, all of these formulas address nominal geometry and acoustical absorption conditions. â¢ Signal-to-noise ratio (SNR). The signal-to-noise ratio (SNR) is a measure of how clearly a signal can be heard above noise, and it is a critical factor for speech intelligibility. SNR is defined as the ratio of the information (or signal) over the interference (noise). Given that sound and noise (unwanted sound) are commonly measured as sound pressure levels using decibels (dB), the ratio of the sound pressures can be equally expressed as the difference in decibels. Industry practice thus uses SNR to quantify the difference between the PA system sound level and the background noise level (e.g., HVAC noise). On a more basic level, SNR can be viewed as the effect of any unwanted sound that degrades intelligibility, such as sound lingering from announcements due to excessive reverberation. â¢ Sound absorption coefficient (`). The absorption coefficient of a material is the ratio of the sound absorbed by the material to that absorbed by an equivalent area of open window. The absorption coefficient of a perfectly absorbing surface would be 1.0 while that for concrete or marble slate is approximately 0.01 (a perfect reflector would have an absorption of 0.00). â¢ Sound pressure level (SPL). The sound pressure level of sound in decibels is 20 times the loga- rithm to the base of 10 of the ratio of the RMS value of the sound pressure to the RMS value of a reference sound pressure. The standard reference sound pressure is 20 micro Pascals as indicated in ANSI S1.8-1969, âPreferred Reference Quantities for Acoustical Levels.â â¢ Speech Transmission Index (STI). The most widely accepted quantitative measure of intel- ligibility is the Speech Transmission Index (STI), which is defined in IEC 60268-16:2011. STI values range from 0 to 1, with numbers close to 1 achieving high levels of intelligibility. This quantitative measurement method relies on comparing a known signal broadcast through the loudspeaker with the sound measured at the receiver (e.g., height of the human ear); the test signal covers the frequency range of human speech with a specific sequence of periodic (repeating) signals. â¢ Sound Transmission Class (STC). STC is a single number rating, specified by the American Society for Testing and Materials, which can be used to measure the sound insulation proper- ties for comparing the sound transmission capability, in decibels, of interior building parti- tions for noise sources such as speech, radio, and television. STC is used extensively for rating sound insulation characteristics of building materials and products.
Glossary 147 â¢ Speech Transmission Index for PA (STIPA). When RASTI was applied to PA systems, short- comings in the RASTI method were revealed, leading to the development of STIPA. A known signal is broadcast over the PA system, and an STI value can be determined based on what is measured. Jan Verhave and Herman Steeneken developed the STI-for-PA method based on extensive research to make it practical to measure the intelligibility of PA systems. â¢ Total Harmonic Distortion (THD). See Appendix F. â¢ Voice Address (VA). More general purpose than a PA system, a VA system might be used for emergency or internal useânot for general purpose public messages and announcements.