We at Front Of House Magazine have not harped about the usage of Sound Pressure Level (SPL) meters for a while. But that doesn’t mean the SPL police have stopped lurking around at your gigs. All they may do is give you a polite reminder to turn it down. But in a worst-case scenario you could be facing litigation and fines by violating the rules against excessive noise. {mosimage}What I want to do here is offer a refresher on SPL meter basics. That way, you won’t get put in a defensive position by a person armed with an SPL meter giving you the heat and being technically dangerous. And by “technically dangerous,” I mean that SPL meters have plenty of settings that may not be appropriate for the measurements being done.
Back to the Chart
I believe we all have seen the classic Fletcher-Munson chart (Figure 1) and have heard about various SPL levels for various reference levels. A classic example is that normal voice conversation between two persons one meter apart is about 70 dB SPL, with each 10dB above or below representing a doubling or halving of perceived loudness. My favorite loudness reference is the 90 to 120dB SPL range that represents a loud concert performance into the audience seating areas.
Another important SPL reference level is the type of measurement that law enforcement persons may make for neighborhood noise ordinances. It is fairly common for a 50dB SPL restriction taken at 100 to 500 feet away from a suspect noisy building as a nighttime noise ordinance. However, this is not the same kind of SPL measurement as a rock concert seating measurement.
Weightings
Most SPL meters come with two or three “weighting” filters that are provided for different kinds of SPL measurements. The most common weightings are the A-weighting and C-weighting filters. The C-weighting filter is pretty much a non-filter over a 20Hz to 7kHz audio passband, optimized for loud sound measurements, 85dB SPL or louder.
The A-weighting filter is used for quiet or neighborhood noise ordinance measurements 55dB SPL or less, and has a pronounced rolloff of sensitivity at lower audio frequencies. This rolloff begins at about 400Hz and is -20dB down by 100Hz. The reason for this rolloff comes from the Fletcher-Munson curves that show humans cannot hear low frequencies very well at low sound pressure levels.
The B-weighting filter, if provided on your SPL meter, handles the 55dB to 85dB in-between region and is used mostly for occupational sound level measurements in offices. This filter is mostly flat as shown in Figure 1, but begins to rolloff in the 100Hz area given the human hearing rolloff in the figure at medium loudness.
What this means is that if the SPL Police come knocking about noise ordinances, make sure they are making A-weighted and not C-weighted measurements. On the flip side, if you wanted to, you could easily “cheat” the venue loudness rules by using A-weighting instead of C-weighting, and let those subwoofers bark a little louder than they should. But eventually you will get caught on this technicality.
Measuring Sources
When measuring SPL, the location of the measurement is as important as the sound pressure level. For example, we rate loudspeakers at a SPL at one-meter standoff distance with one watt audio signal input. Most manufacturers of speakers do not measure SPL at one meter, but at several meters away and compute the effective SPL if located at one meter. This is done because the high and low frequency drivers are typically not co-located in the same position in the speaker cabinet, and the SPL would change depending where the meter was in close proximity to the cabinet components.
Similarly, a measurement of SPL at the Front Of House position may not be the right measurement for “score” of the venue’s loudness. Not only does the FOH position change on various events, but different speaker hangs will change SPL coverage around the venue. Usually checking SPL at near-stage and back of venue to get a range of SPLs is the goal.
When measuring conventional speaker sources with spherical coverage, not line-arrays, the SPL is expected to drop 6dB with each doubling of distance away from the source (1 meter, 2 meters, 4 meters, 8 meters, etc.). That is why distance away from the sources is such an important factor if you are expected to keep the loudness beneath a certain level.
