A, B, C and Z weighting

A sound level meter measures sound pressure, but the number on its display is more useful if it reflects how the ear responds. Weighting filters do this by emphasising frequencies the ear hears well and reducing the others before the level is computed.

The standard weightings

The definitions are in IEC 61672-1. Level values are written dB(A), dB(C), or with the letters as subscripts, for example L_Aeq for the equivalent continuous A-weighted level.

What the curves look like

A-weighting removes a great deal of the low frequencies, so a measurement in dB(A) hardly shows rumble or bass-heavy music. C-weighting passes most of it. Use the A and C weighting tool for any frequency.

A rough bass indicator

Measure the same sound in dB(C) and dB(A). If dB(C) is more than about 10 dB higher, a large share of the energy is at low frequencies. This is useful for judging loud music at events, where regulations may refer to both.

Time weighting

The meter also averages the squared pressure over time:

• Fast (125 ms) follows changes quickly.
• Slow (1 s) gives a steady reading for fluctuating sources.
• Impulse and Peak have special rules for short events.

A weighting letter and a time weighting together make up a reading such as L_AF (A-weighted, fast) or L_CPeak.

Limits

• The weightings are based on pure tones at fixed levels. Real sound is more complex, and real loudness depends on level, as in the equal-loudness contours.
• A-weighting understates the harm of low-frequency sound and underrepresents the loudness of bass at high levels.
• It is a measurement convention, not a model of the ear.

Class 1 and class 2

The standard defines two accuracy classes. Class 1 meters have tighter tolerances and are used for official measurements. Class 2 is adequate for general work. Phone apps are neither unless calibrated.

Draft: written by an AI assistant, awaiting technical review.