noise dose and leq

1. Noise Dose and Leq University of Salford Acoustics, Audio & Video Group

2. Intro to Acoustics: Leq 2 Obviously, we need to evaluate the acoustic energy of the time-varying signal.Obviously, we need to evaluate the acoustic energy of the time-varying signal.

3. Intro to Acoustics: Leq 3 Equivalent continuous sound level (graph) Imagine we are exposed to a time-varying SPL. The equiv continuous level is a constant SPL which contains the same energy. It�s a kind of average.Imagine we are exposed to a time-varying SPL. The equiv continuous level is a constant SPL which contains the same energy. It�s a kind of average.

4. Intro to Acoustics: Leq 4 Equivalent continuous sound level (words)

5. Intro to Acoustics: Leq 5 Equivalent continuous sound level (equation) This means that for small time intervals dt, we form p2, which is proportional to intensity. We then multiply this p2 by the short time interval dt for which it exists, to get something proportional to energy. We sum up all these small contributions to the total energy, over a specified time T. Then we divide this energy sum by T to obtain the average energy over this period. Finally we take logs to make it into a decibel. These days, this integration process can be performed by a modern integrating sound level meter. Such a meter samples the pressure at very short time intervals dt. If we do not have one of these to hand (and they can be expensive) we may be able to calculate Leq. This can be done by dividing the record of the fluctuating signal into time periods over which the level does not vary much...This means that for small time intervals dt, we form p2, which is proportional to intensity. We then multiply this p2 by the short time interval dt for which it exists, to get something proportional to energy. We sum up all these small contributions to the total energy, over a specified time T. Then we divide this energy sum by T to obtain the average energy over this period. Finally we take logs to make it into a decibel. These days, this integration process can be performed by a modern integrating sound level meter. Such a meter samples the pressure at very short time intervals dt. If we do not have one of these to hand (and they can be expensive) we may be able to calculate Leq. This can be done by dividing the record of the fluctuating signal into time periods over which the level does not vary much...

6. Intro to Acoustics: Leq 6 Calculation of Leq This procedure can sometimes be applied to industrial noise where a work pattern involves several different time periods at different, but constant, sound levels. This procedure can sometimes be applied to industrial noise where a work pattern involves several different time periods at different, but constant, sound levels.

7. Intro to Acoustics: Leq 7 Leq example An industrial process produces the following constant SPLs over an 8-hour shift: SPL (dB(A)) time (hours) 80 4 75 2 85 2 What is the 8-hour Leq? Ans: 10 lg ( (108x4 + 107.5x2 + 108.5x2) / 8) = 81.4 dB Now try T6 Q5.Ans: 10 lg ( (108x4 + 107.5x2 + 108.5x2) / 8) = 81.4 dB Now try T6 Q5.

8. Intro to Acoustics: Leq 8 Measurement Need an integrating sound level meter Can be A-weighted - LAeq Common in environmental and industrial noise Time period depends on application 12 hr night-time noise survey 24 x Leq,1 hr for traffic noise survey

9. Intro to Acoustics: Leq 9 Predicting human response Human response varies; several factors Can use Leq to define critical dose (at which x% will get y damage after z years Even harder - use Leq to predict sub-damage response - e.g. annoyance Most noise codes are moving to LAeq

10. Intro to Acoustics: Leq 10 Conclusions NIHL related to sound energy Describe time-varying levels with the equivalent continuous level Leq can be measured or (sometimes) calculated LAeq is single most common metric used to predict human response to noise