Noise Descriptors for Environmental Noise

Noise from different sources can be measured or described in different ways. There are alternative ways to measure or describe even the same type of noise source! Scientists have conducted surveys and laboratory studies aiming to develop descriptors to best correlate community response to various environmental noise sources. This is why different countries have adopted different noise descriptors for assessment of different community noise sources.

The n-Percent Exceeded Level, L_n

In most cases, the sounds and noises we hear are not steady. Apart from variation in tones, the magnitude or the sound pressure level of a sound or noise changes with time.

The n-percent exceeded level, L_n, is the sound pressure level exceeded for n percent of the time. In other words, for n percent of the time, the fluctuating sound pressure levels are higher than the L_n level. L_n can be obtained by analysing a given noise by statistical means.

If the noise is being measured on the A-weighting scale, L_n is often written as L_An.

Let us explain this noise descriptor with some illustrations.

The change of magnitude of a sound or noise over time can be represented by this graph. Please click on the demo button to see the details.

L₁₀, L₅₀ and L₉₀

The commonly used value of n for the n-percent exceeded level, L_n, are 10, 50, and 90.

L₁₀ is the level exceeded for 10% of the time. For 10% of the time, the sound or noise has a sound pressure level above L₁₀. For the rest of the time, the sound or noise has a sound pressure level at or below L₁₀. These higher sound pressure levels are probably due to sporadic or intermittent events.

L₅₀ is the level exceeded for 50% of the time. It is statistically the mid-point of the noise readings. It represents the median of the fluctuating noise levels.

L₉₀ is the level exceeded for 90% of the time. For 90% of the time, the noise level is above this level. It is generally considered to be representing the background or ambient level of a noise environment.

For a varying sound, L₁₀ is greater than L₅₀ which in turn is greater than L₉₀. The following graph illustrates L₁₀, L₅₀ and L₉₀.

Chart illustrates relation between L10, L50 and L90

Use of L₁₀ to Describe Traffic Noise

The early definitive work on traffic noise was done in England around the 1960's. Extensive research carried out in England showed that the descriptor L₁₀(18 hour), the noise level exceeded for 10% of the time between 6 a.m. and 12 midnight, correlated well with the community response. L₁₀(18 hour) of 68 dB(A) at 1m outside facade of dwellings was adopted in the UK as the eligibility criterion for acoustic insulation.

Traffic noise planning standard in Hong Kong is L₁₀(1 hour) of 70 dB(A) measured over the peak hour of traffic. The standard was formulated after review of similar standards in other developed countries. Field measurement in Hong Kong also revealed that L₁₀(1 hour) of 70 dB(A) over the peak hour is roughly the same as L₁₀(18 hour) of 68 dB(A), for the kind of normal traffic flow situations encountered in Hong Kong.

The traffic noise planning standard, which is L₁₀(1 hour) of 70 dB(A), means that when this limit is just met, traffic noise will exceed 70 dB(A) for 10% of an hour. For the remaining time, the traffic noise will be less than 70 dB(A).

The following diagram explains the concept more clearly. Please click on the demo button to see the details.

Equivalent Continuous Sound Pressure Level, L_eq

This is a widely used noise descriptor that is commonly adopted in many developed countries. It is the constant noise level which, under a given situation and time period, contains the same acoustic energy as the actual time-varying noise level. As L_eq measures the energy content of a noise over a period of time, noise with different characteristics, such as fluctuating (e.g. from traffic) or impulsive noise (e.g. from hammering) as described in the next section, can give the same L_eq Level.

When noise or sound is measured in dB(A), it is customary to denote the equivalent continuous sound pressure level as L_Aeq.

Let us explain L_eq or L_Aeq by using an illustration. Please click on the demo button to see the details.

Equivalent Continuous Sound Pressure Level, L_eq for Continuous, Fluctuating and Impulsive Noises

Environmental noise is often non-steady. But there are some exceptions. Noise from air-conditioners is relatively steady.

A fluctuating noise has its magnitude varying quite considerably over time. Examples are road traffic noise, rock music and noise from helicopter flyover.

For an impulsive noise, the level rises sharply and then falls rapidly. Examples include hammering, shooting and firecracker noise.

Let us look at the equivalent continuous sound pressure levels of different types of noises as below. Please note that although the types of noise can be different, traffic noise and hammering may produce the same level of L_Aeq if the sound energies produced are the same at the receiver. Please click on the demo button to see the details.