directions of propagation of the sound waves are randomly distributed. In a practical sense, the sound pressure levels at all points in the room are nearly the same except near the room boundaries and a sound wave reaching a given point in the room is equally likely to arrive from all directions. direct sound: sound which reaches a given location in a direct line from the source without any reflections. equivalent continuous sound level (L eq ): the level of steady sound which, in a stated time period and at a stated location, has the same sound energy as the time varying sound. If frequency weighting is applied, the equivalent continuous sound level may be designated LA eq to indicate A-weighting or LC eq to indicate C-weighting, etc. See also frequency weighting. fast exponential time weighting: a lowpass filter for the purpose of averaging a signal having a time constant of 0.125 seconds applied to the square of the sound pressure as specified in ANSI S1.4. field sound transmission class (FSTC): a single number rating similar to sound transmission class (STC), except that the transmission loss values used to derive this class are measured in the field. FSTC ratings are typically lower than STC ratings which are measured under laboratory conditions. flanking path: A wall or floor/ceiling construction that permits sound to be transmitted along its surface; or any opening, which permits the direct transmission of sound through the air. freefield: a sound field in which the boundaries have negligible effect over the frequency range of interest. frequency: the number of times that a waveform repeats itself in a given period of time, usually one second, i.e. the number of cycles per second). Unit: Hz. frequency weighting: a prescribed frequency dependent attenuation or amplification applied to measured sound data usually intended to better approximate the sensation of loudness in a human listener. For example, A, B, and C weighting approximate the frequency dependent shape of the equal loudness contours for soft, moderate, and loud sounds. Hertz (Hz): unit of frequency, cycles per second. impact insulation class (IIC): a single number metric used to compare the effectiveness of floor-ceiling assemblies in providing reduction of impact-generated sounds such as footsteps. This rating is derived from values of normalized impact sound pressure levels in accordance with ASTM E492. impulsive sound: sound that is characterized by brief excursions of sound pressure , typically less than one second,whose peak pressure significantly exceeds the background sound pressure. insertion loss: the reduction in sound level at the location of the receiver when a noise reduction measure such as a barrier, attenuator, muffler, etc. is inserted into the transmission path between the source and receiver. Unit: dB. level: the logarithm of the ratio of a given quantity to the reference quantity of the same kind. Levels represent physical quantities such as sound pressure on a logarithmic scale and are
43 of 58 Spendiarian & Willis Acoustics & Noise Control LLC
12/15/2019
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