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: Lpeak, Lmax, and Lmin. Of the three, Lpeak is the easiest to understand, so we’ll start there.
Lpeak stands for peak level. The peak level is simply the level of the highest measured pressure. Think of Lpeak as the highest point of elevation on a pressure waveform. Lpeak values are computed from the raw pressure waveform. Often, Lpeak is used to quantify impulse noise (short, fast sound). Some standards like Lpeak values to be reported with a C frequency weighting filter applied, and such levels are reported as LCpeak.
Lmax 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 Lmax is the level of this rms “average” signal. As a result, Lmax values arey always less than Lpeak values. A pure tone will give you an Lmax value that is 3 dB below the Lpeak value. Additionally, there is no way to directly compute Lpeak from Lmax. 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 Lpeak can let you know the maximum pressure measured with a sound level meter, the Lmax gives a better picture of what the general maximum level was in a measurement.
Lmin stands for minimum rms level. Lmin is the same as Lmax except that it reports the lowest rms “average” level of a pressure signal. Lmin gives a good picture of the sound level of the background noise during a measurement. You can think of Lmin 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. LAmax.) Each of the three metrics talked about here— Lpeak, Lmax, and Lmin—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.