LEVELS and DECIBELS

• Decibel (dB)

A logarithmic ratio of two quantities - one-tenth of a Bel. The Bel is given as: log₁₀(quantity₁/quantity₂) thus the decibel is given by:

• Sound Power Level (SWL, also L_W, or sometimes PWL)

Sound power level - the logarithmic measure of sound power in comparison to a specified reference level of 10^-12W

• Sound Intensity (I)

A measure of energy flowing through an area in a given time which is equivalent to power/area (watts/m²). Sound intensity is a vector quantity as the energy is passing in a particular direction.

• Sound Intensity Level (SIL or L_i)

The logarithmic measure of the sound intensity relative to a reference intensity of 10^-12W/m². If I is the measured pressure then:

The SIL equation produces a sound intensity level value in decibels (dB)

• Sound Pressure (p)

The pressure deviation from the local ambient pressure caused by a sound wave. pressure is a measure of force (F) per unit area (S). So p=F/S. The SI unit for sound pressure is the Pascal (Pa), where 1Pa = 1N/m²

• Sound Pressure Level (SPL or L_p)

The logarithmic measure of the rms sound pressure relative to a reference pressure of 20μPa. If p is the measured pressure then:

The SPL equation produces a sound pressure level value in decibels (dB)

• L_eq

The equivalent continuous sound pressure level. The level of a steady, continuous noise which has the same total energy as the real fluctuating noise measured over the same time period.

• L_Aeq

An A-weighted L_eq - one of the best descriptors of human perception of sound pressure level.

• L_max

Maximum sound pressure level measured over a measurement period

• L_min

Minimum sound pressure level measured over a measurement period

• L₁, L₁₀, L₅₀, L₉₀ etc

Statistical parameters that describe the sound pressure level exceeded for a certain percentage of the measurement period. e.g. L₁₀ is the level exceeded for 10% of the time, L₉₀ is the level exceeded for 90% of the time. These can be modified by the appropriate weighting e.g. L_A50 is the A-weighted level that is exceeded 50% of the time during the measurement period.

• Reference Quantities

These values are taken from BS EN 21683:1994 - Acoustics—Preferred reference quantities for acoustic levels. Table 1 shows the various acoustic levels expressed in decibels:

Table 2 shows the preferred reference quantities expressed in SI units: