Newcomers to the field of acoustics are often overwhelmed by the number of different level metrics used to characterize environmental and community noise. To make matters worse, almost all of these metrics are abbreviated with capital Ls and reported in units of decibels. The purpose of this article is to help distinguish between three of these Ls: L_peak, L_max, and L_min. Of the three, L_peak is the easiest to understand, so we’ll start there.

L_peak

L_peak stands for peak level. The peak level is simply the level of the highest measured pressure. Think of L_peak as the highest point of elevation on a pressure waveform. L_peak values are computed from the raw pressure waveform. Often, L_peak is used to quantify impulse noise (short, fast sound). Some standards like L_peak values to be reported with a C frequency weighting filter applied, and such levels are reported as LC_peak.

L_max

L_max stands for maximum rms level. Rms stands for root mean square. Without going too much into the math of it, the root mean square is the “average” value of the pressure.  The L_max is the level of this rms “average” signal. As a result, L_max values arey always less than L_peak values. A pure tone will give you an L_max value that is 3 dB below the L_peak value. Additionally, there is no way to directly compute L_peak from L_max. If you are unsure whether a pressure waveform is an rms waveform, check if it was recorded with a time weighting (usually fast or slow time weighting.) All time-weighted signals are rms signals. While the L_peak can let you know the maximum pressure measured with a sound level meter, the L_max gives a better picture of what the general maximum level was in a measurement.

L_min

L_min stands for minimum rms level. L_min is the same as L_max except that it reports the lowest rms “average” level of a pressure signal. L_min gives a good picture of the sound level of the background noise during a measurement. You can think of L_min as the baseline for the noise floor—the sound level above which a sound must be to be heard “above the noise”.

Wrapping it Up

As mentioned earlier, these levels are reported in decibels. (If you need a refresher on decibels, check out this article.) Often levels are passed through a frequency-weighting filter, so don’t be surprised to see an A, C, or Z with any of these metrics (e.g. LA_max.) Each of the three metrics talked about here— L_peak, L_max, and L_min—give acoustical engineers and consultants important insight into the nature of measured sound and potential dangers associated with that sound. If any of these metrics gets too high, hearing damage and loss become real threats. If you are worried about sound levels, consider contacting an acoustical consultant to learn more about steps that can be taken to protect your hearing and the hearing of those around you.

Further Reading:

https://en.wikipedia.org/wiki/Sound_level_meter