Weighting Curves: An Introduction

Weighting Curves: An Introduction

In the world of SPL measurements, you may encounter a series the letters A, C, and/or Z. What do these mean?

Depending on context, you may need to apply different frequency response filters to your data to make it useful. These filters are called Weighting Curves and should always be mentioned when discussing SPL measurement values, as different curves will produce different results.

A-Weighting

The A-Weighting curve consists of a significant low-frequency roll-off, exhibiting an attenuation of about -20dB at 100Hz, in combination with a more subtle high-frequency roll-off. A-Weighting, while originally intended to correlate with human perception of sound at low levels, more closely correlates with the energy that lies within the human vocal range. It is commonly used for real-time level monitoring by concert mix engineers. As such, most sound level limit policies at live events specify an A-Weighted limit as averaged over a period of time.

In addition, many environmental and nuisance noise regulations in the US specify A-Weighting. This is a holdover from the now-defunct Noise Control Act of 1972. A-Weighting, however, is a poor choice for nuisance noise applications such as live event monitoring due to its low-frequency roll-off. Low frequencies are more likely to generate noise complaints, as they are most likely to propagate far distances outdoors and transmit most efficiently through structures.

A-Weighting is also used for most health and safety regulations concerning sound exposure despite bearing little resemblance to the tonal response of human hearing at high levels. The current audiological consensus is that A-Weighting sound level measurements correlate well with statistical data on noise-induced hearing loss, and reasonably approximate the diffuse-to-DRP transfer function of the human hearing mechanism. For this reason, the A-Weighting curve is considered effective at characterizing sound exposure risk. Despite this consensus, additional research has shown that LF energy can present a hearing health risk at high levels. For this reason, many sound level limits specify a secondary C-weighted limit.

C-Weighting

The C-Weighting curve is effectively broadband, exhibiting only a slight roll-off at each end of the frequency spectrum. Since C-Weighting includes subwoofer frequencies, it is commonly used to characterize broadband content such as live music. The slight roll-offs are sometimes considered useful for restricting the measurement to the range of energy commonly emitted by a sound system, and de-emphasizing low frequency structure-borne vibrations, rumble, and so forth. It is often stated that C-Weighting resembles the tonal response of human hearing at concert levels. However, when compared against the 100-phon Equal Loudness Contour, we see that this is not the case.

The core difference between these two common weighting curves is such: C-Weighting is for when we wish to include most of the low frequency energy in our measurement, and A-Weighting is for when we don’t.

Z-Weighting

The Z-Weighting curve is completely flat or unweighted. It is most often used when needing to accurately characterize energy at the extremes of the audible range (below 30 Hz and above 10 kHz). It is of limited use in the context of live sound but can be useful for loudspeaker testing applications and acoustical evaluations.