A,B, and C Contours

Standard filter contours are used to make the instrument more nearly approximate the normal human ear. The different contours were intended to match the ear at different sound intensities.

This is the Weighting control on a research-quality General Radio sound level meter. The weighting marked 20kHz means that it has a certified flat response to 20kHz. Most low-cost sound level meters cannot merit such certification, and typically have just two weightings, A and C. The C weighting is reasonably close to a flat decibel response.

A Contour Filter

The A-contour filters out significantly more bass than the others, and is designed to approximate the ear at around the 40 phon level. It is very useful for eliminating inaudible low frequencies.

Sound Measurement in dBA

When making practical assessments of the sound level of a concert or as a part of a general survey of ambient sound levels, the type of measurement which is usually made is that of the level in dBA. This measurement is made with a sound level meter with an A contour filter which provides the best instrument match of the ear's equal loudness curves for soft sounds in the neighborhood of 40 dB. When this filter, (which is one of the standard contour filters of most sound level meters) is used, the levels should be recorded as dBA rather than dB. The unit dB implies that all frequencies in the audible frequency range are treated equally - something very different from what the ear does. A good practice in sound level surveys is to measure all levels in both dBA and in the flat-response mode where the intensity in decibels (dB) is measured directly. Measurements made in dBA approximate the loudness level in phons and are practical to make with a standard meter.

B Contour Filter

The intermediate B-contour aproximates the ear for medium loud sounds. It is rarely used.

C Contour Filter

The C-contour does not filter out as much of the lows and highs as the other contours. It approximates the ear at very high sound levels and has been used for traffic noise surveys in noisy areas.