Time Weighting: SPL Fast, Slow, Leq, and Peak Explained

Time Weighting: SPL Fast, Slow, Leq, and Peak Explained

Often discussed in SPL, different frequency weighting curves can help inform more accurate measurements based on context. In addition to frequency weighting, it is important to consider how signals are looked at in the time. 

Typically, noise exposure limits are specified as single numbers. This is useful when measuring steady-state noise sources such as industrial machinery, but live music and other similarly dynamic noise sources require different parameters to measure accurately. Since real-world signals, such as that of a musical program, change in level from moment to moment, our measurements can produce different results depending on the period of time over which they take place. For instance, a transient event such as a drum hit  will have a high peak value, but a lower value over time due to the silence in between hits. Longer measurement periods allow the sound engineer at a concert to mix a dynamic show.

Sound level measurements in Smaart fall into three different categories of time-domain weighting: Exponential Average, Equivalent Continuous Sound Level (Leq), and Peak (instantaneous level).

Exponential Average

Exponential averaging is the most common category of time-domain weighting, often seen on basic sound level meters. It comes in two varieties: SPL Fast, which uses a time constant of 125 milliseconds, and SPL Slow, which uses a time constant of 1 second. Exponential averaging is, in a sense, like applying a low-pass filter to the signal, but in the time domain. Doing so “dampens” the reading against quick jumps in level, but keeps it trending towards the current level.

Although SPL Fast and Slow are the most commonly available (and used), they don’t offer enough context to allow us to make meaningful statements about level trends over time.

Equivalent Continuous Level

A more useful metric for sound level monitoring applications is Equivalent Continuous Sound Level, or Leq. Leq is a time-averaged measurement of total sound energy over a specified time period, presented as a single dB value. Since live events tend to have high dynamic range and crest factor, Leq is typically the most useful metric for evaluating level trends between sets, individual songs, or even sections within a song. For this reason, Leq measurements spanning 10 or 15 minutes are the most implemented when setting sound level limits at live events.

In Leq readings, the number in the suffix indicates the measurement period in minutes alongside the corresponding frequency weighting letter. For instance, an SPL measurement labeled as LAeq 15 uses an A-weighted frequency curve, averaged over 15 minutes. In Smaart Suite and Smaart SPL, you can configure an unlimited number of Leq measurements with averaging times from 1 second to 24 hours.

Peak/Instantaneous

The “Peak” measurement always shows the maximum instantaneous level reached within a measurement interval. It is useful for evaluating compliance with regulations seeking to protect employees against high level impulses such as those created by industrial machinery, explosives, and firearms.