Personal observations suggest a trend in the spectral composition of typical contemporary live sound mixes.
Most mixes appear to show an gentle continuous declining slope from low to high frequencies in order of 3 to 6 dB per octave (left hand chart in figure 1).
That’s 27 (9 x 3) to 54 (9 x 6) dB of tonal change respectively, in the 9 octaves between 32 Hz and 16 kHz.
A glance at the A-weighted values in the right hand chart of figure 1 shows that, depending on the slope, these tonal trends will peak out first in the octaves between 250 Hz and 1 kHz. These are typically the first frequencies that reach the SPL limit, if applicable. Not the low end.
The left hand chart reveals that at 40 Hz, 134 dB of unweighted pressure for the steepest slope (most sub heavy mix) will do, in contrast to the overestimated value of 140 dB for 103 dB(A) full range. That’s 50% less subwoofers!
The tables in figure 3 inform us that we therefore could cover twice the distance (16 meters 52 ft) with the same amount of subwoofers required for the 103 dB(A) full range SPL at merely 8 meters (52 feet)!
The gentlest slope (least sub heavy mix) could cover even 64 meters (210 feet) using the same amount of subwoofers to achieve 123 dB of unweighted pressure for 40 Hz at 64 meters (210 feet).
So depending on the application, typically between 10 and 32 subwoofers in half-space, with music exhibiting a dynamic range of 12 dB or less, goes a long way. If you would like to build in some headroom, scale the number of subwoofers accordingly. 140% for 3 dB and 200% for 6 dB. Unfortunately, in practice, sound systems tend to go as loud as we can afford…
Flown subwoofer configurations should be upscaled by as much as 6 dB, to account for the lack of advantageous coupling with the floor in favor for more even coverage.
Coverage control in general, by means of cardioid configurations and/or arrays comes at a price which is typically a reduction in SPL and should be compensated accordingly. The End-Fire array is an exception.
So ultimately, the number of subwoofers required depends on:
—The unweighted spectrum of the program material and how this translates to the weighted sound level meter
—The dynamic range of the program material
—The distance that needs to be crossed
—Half-space (ground stacked) or full-space (flown)
—Coverage control
—Headroom
But rarely is the number of subwoofers, required to achieve values of 103 dB(A) at 40 Hz (140 dB SPL at 1 m [3 feet.]), called for.
Watch the video by Meyer Sound below to see how program material affects maximum SPL.
Real World SPL from Meyer Sound on Vimeo.
Read and comment on the original article here.
Based in The Netherlands, Merlijn van Veen is a consultant specializing in sound system design and optimization, and he’s also a noted audio educator.
