General - Understanding Sound Measurement and the Law


Sound is difficult to measure, as the act of putting something in the sound field often impacts that very field. Some of the simple concepts of sound measurement and how they relate to the noise at work regulations are mentioned here.


In Peak we only focus on Noise Dose. We use this measurement as it provides a easily understandable expression of the relationship between the amount of time spent in noise and the intensity of that noise.


In this section we hope to give some basic definition to the following concepts that lead us to calculating Noise Dose:






What is Sound Pressure?

Sound Pressure is measured in Pascals (Pa), and describes the pressure amplitude from the mean value -  Sound pressure. Acoustic pressures are normally small relative to the mean value, e.g. 1 Pa, is a fairly loud sound in air (94 dB SPL), but atmospheric pressure is 101,325 Pa. Much like an electrical signal, the average sound energy can be found by squaring the signal, averaging the results and square-rooting, known as the RMS. - Root mean square



What is Sound Pressure Level (SPL)?


Sound Pressure Level (SPL) is the most used quantity in sound measurement, so much so that is often called sound level, or just level. Sound Pressure Level describes the power in a sound signal relative to the threshold of human hearing (the quietest sound that humans can hear). SPL is expressed in decibels as:

(1)    SPL = 10 x log10 (p / p)where p is the average measured pressure and p is the reference pressure 0.0002 Pa.




A Sound Pressure Level of 0 dB is the threshold of human hearing (the quietest sound people can hear) and upwards from that sounds are compared to the sound energy at that level, say 10 dB is 10x the energy and 20 dB is 100x the energy.


SPLs can be calculated by measuring over any length of time and with any frequency weightings applied. Common SPL measurements include:

L = Equivalent continuous A-weighted sound level measured over 10 seconds. ← As measured in the FocusLite

L = Equivalent unweighted continuous sound pressure level measured over n hours.


Sound Pressure Levels are usually measured on site by acoustic consultants who are hired to asses noise using a Sound Level Meter. Typically the measured SPL will change based on how close you are to the object that is making sound and other factors like the environment around the object, e.g. a room or open field.



What is Sound Power Level (SWL)?


Sound Power Level (SWL) defines the total noise energy that is output from a device. This value is typically used in defining the sound output of power tools.


By making assumptions about distance, spacial environment and directivity of the sound source you can estimate the Sound Pressure Level from the Sound Power Level, but these assumptions must be valid.


In a typical example, being 10m from a sound source and 1m from a sound source will result in a difference of 20 dB (x100 the energy) to the Sound Pressure Level at the receiver position, but the Sound Power Level of the device will remain the same.



What is Peak Sound Pressure Level (LPeak / LCPeak)?


A regular Sound Pressure Level measurement is the average energy of the sound. As most sounds fluctuate in amplitude the average level will be lower than the absolute peak level.


In some instances, e.g. gun shots, a very loud peak sound level can cause hearing loss instantaneously. Below is an example sound where the average level is stated as the RMS (Root Mean Square), and the peak level is also indicated.



In the Noise at Work regulations, the L measurement is C-weighted, before the peak level is calculated.



What is Daily Personal Exposure Level (LEP,d or LEX,8hr) / What is Noise Dose (100%)?


Noise exposure is accumulated by spending time in noise. By increasing the time spent in noise, or increasing the noise level you are in, you will increase the overall noise exposure.


Noise exposure can be represented in two ways, as a Noise Dose (as a % of the allowable dose) or as a Personal Exposure Level (as an SPL in dB normalised to an 8 hour working day). Dose is easier to interpret but they both mean the same thing.


Let's take an example day where a worker spends two hours in 90 dB(A) noise and then 4 hours in 80 dB(A) noise.:


The noise exposure for the day is the total noise energy that has been accrued. In this example:


90 dB(A) noise for 2 hours = 79 % Noise Dose,

80 dB(A) noise for 4 hours = 16% Noise Dose.


This means in total, this person received 79 + 16 = 95% noise dose for the day. This noise dose can be converted into an LEP,d/LEX,8hr to be able to compare with the noise at work regulations regulations. Again, hiding some maths, 95% Noise dose is the same as an LEP,d = 84.8 dB(A).

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