Sound Pressure Level

Sound Pressure Level is an expression of the magnitude of sound waves at some particular point in space. It is comparatively easy to measure, and there is a clear, if inexact, correlation between Sound Pressure Level and perceived loudness. Sound Pressure Level is expressed in decibels, where 0 decibels Sound Pressure Level is the "threshold of audibility" or the softest sound our hearing can detect.

In 2006: So far, so good. However, in noise pollution measurements, two corrections relating to the way we hear loudness as well as various frequencies at soft Sound Pressure Levels are added. The first is called "A-weighting," and such levels are expressed as dBA Sound Pressure Level. The primary effect of A-weighting is that it heavily discounts low frequencies, turning down the first octave (31 Hz.) by about 33 dB, the second octave (63 Hz.) by about 21 dB, the third by about 11 dB, and the fourth by about 4 dB. The result of this is that A-weighted measurements of a given level are usually between 5 and 8 dB softer than the C-weighted or broadband measured level. Simply put, if a broadband sound has a measured level of 90 dB SPL, its A-weighted measured level will probably be between 82 and 85 dBA SPL. Below is a brief tour of the various A-weighted levels between what we can't detect because it's too soft and what happens when one of the space shuttles decides its time to head on out while we're standing on the gantry. Meanwhile, the second correction has to do with how much time we take to measure the level. Typical measurements done with a sound pressure level meter are so-called fast levels, where the needle jumps around quite actively for most complex sounds. When we log the highest of these, it is called Lmax. Meanwhile, for most noise pollution measurements, we take a power-based average over time of the the sound for the period we are interested in. This is called Leq and it can't be done with an inexpensive meter. Depending on the nature of the sound, Leq will generally be 10-12 dB softer thn Lmax. We need to pay attention to this difference. An Lmax broadband reading of a given sound may be up to 20 dB louder than the dBA Leq measurement of that same sound. Unfortunately, this distinction is not clearly understood by many, including, sadly, the National Institute of Health, who in my opinion inaccurately describes decibels and dangerous levels, by failing to distinguish between A-weighted Leq measures (which they refer to as simple decibels) and broadband or C-weighted Lmax levels, which are the levels you would be most likely to make with an inexpensive SL meter.

• 0 dBA Sound Pressure Level is the threshold of audibility. It is the softest sound we can detect. It is far softer than our environment, and most of us have never heard such a soft sound.
• 30 dBA Sound Pressure Level is the sound level of a quiet room, with all doors and windows closed, and no ventilation systems on. We characterize it as "extremely quiet." A normal classroom at Berklee College of Music, during the day but when nothing is happening in that room, has a Sound Pressure Level of between 35 and 45 decibels.
• 60 dBA Sound Pressure Level is the sound level of somebody speaking to you in normal voice, as heard from a few feet away.
• 70 dBA Sound Pressure Level is the sound level of easy listening music over loudspeakers where we are using it as a sort of pervasive noise perfume -- a little loud for talking over in intimate circumstances, but not as loud as a typical unamplified live performance. 70 A-weighted decibels Sound Pressure Level is also the sound level in a quiet car at 60 mph with the windows rolled up.
80 dBA Sound Pressure Level is moderately loud sound. Typically, you can listen to music over your hi-fi system at 80 dBA Sound Pressure Level without making your neighbors complain.
• 90 dBA Sound Pressure Level is quite loud. It is the about 8 dB louder than the sound level I use for mixing (which is to say that I set the monitor levels so that 0 VU on the console output meters yields a Sound Pressure Level of 90 C-weighted decibels -- more about this later). Students often tell me that they feel my 90 decibels Sound Pressure Level is quite loud, and they would prefer to mix at a somewhat softer level.
• 100 dBA Sound Pressure Level is extremely loud. It is about as loud as acoustical instruments (except drums and trombones) can play.
• 110 dBA Sound Pressure Level is really loud. It can only be obtained through sound reinforcement. It will be irritating and painful for many of the listeners. It is the sound level of a circular saw cutting through hardwood, or a chain saw at full bore, as heard by the user.
• 120 dBA Sound Pressure Level hurts! It is the sound level of jet engines, et al, from a safe distance away, say 100 feet. It hurts, really hurts!
• 130 dBA Sound Pressure Level is the sound level of contemporary Formula 1 race cars under full throttle from 15 feet away, or the sound level inside a Winston Cup stock race car.
• 140 dBA Sound Pressure Level is the sound level of jet engines or rocket motors or howitzers or other implements of mayhem as heard close up.
• 150 dBA Sound Pressure Level is the sound level of a .357 Magnum revolver heard from a foot away from the muzzle (off to the side, of course -- head on is even louder but wouldn't really matter anymore...). This is a peak level that fortunately lasts for only a few milliseconds. Do not try this at home!

Sound Pressure Level is measured with a special meter-plus-microphone device. Cheap Sound Pressure Level meters are available from Radio Shack for under $50. Usually, they read out in decibels Sound Pressure Level and dBA (switch selectable) and with fast or slow detection times. Fast detection times give a display that correlates quite well with how we hear, and are averages of the most recent 1/10th of a second of sound. The result is that the levels jump around a lot, and it is often difficult to determine what the ongoing average Sound Pressure Level is. The slow detection option averages the levels of sound over the most recent second of time. This yields a considerably steadier meter reading (the meter will act like it is damped by molasses!) while exposed to music, speech or other rapidly changing sounds. Cheap meters usually cannot measure very soft levels (below 40 decibels Sound Pressure Level) nor can they make Leq measurements.