1.4 Sound Level Descriptors/Metrics for Impulsive Sound Events

No consensus exists regarding the criteria for the measurement and evaluation of impulse sounds. Impulse sound refers to a sound signal of short duration (generally less than one second), particularly of high intensity, abrupt onset and rapid decay, often with a rapidly changing spectral composition.

Some railway activities produce impulsive sounds. These include: the coupling of cars when assembling a train, take-up of car slack when trains start to move or immediately prior to stopping, shunting of railway cars in rail yards and on rail sidings and repair work in railway repair shops. Figure 3 above illustrates a typical time history of impulsive noise.

To evaluate possible annoyance to the public, the following measurable impulse sound level parameters may be important: peak overpressure, rise time, time-duration, impulse noise spectrum and impulse-noise energy.

Various jurisdictions rely on different standards for measurement and assessment of impulsive noise including the use of maximum unweighted peak dBZpk (linear), dBCpk, maximum unweighted impulse response dBZi (linear), the maximum A-weighted impulse response dBAi (events may be used singularly or averaged on the basis of logarithmic averaging)^{Note 5}. Some jurisdictions simply use the dBA metric with the meter response set to the “fast” response; i.e. dBZf. The use of any one of these metrics may be considered acceptable provided that the results are also reported in dBAi.

Additional information on impulse sound, its measurements and adjustments, can be found in CAN/CSA-ISO 1996 (CSA 2005), ANSI standard S12.7 (R2006) (ANSI 2006).