May 14, 2011

Estimating maximum SPL for your mains

There are two figures of interest. One is the theoretical maximum that is easy to determine with a WinISD simulation. Of greater interest is the real world useful output, which will normally be substantially lower.

Let's consider a typical hifi floorstander. In a typical room with a listening distance of three metres with about 100 watts of power we can expect around 90 dB of clean output. We might hit 95 dB if we are lucky. How does that compare to a maximum theoretical figure?

Let's assume we have a pair of 6.5" midbass drivers and a vented alignment. Running full range we can expect around 107 dB @ 1m. Input power 40w and at this point, excursion is the limit. Now if we run it actively, set to small with a high pass at 80 Hz, we can apply more power and get 111 dB of excursion and power limited output. Allowing for distance we subtract about 5 dB and we get either 102 or 106 dB depending on whether we are running full range or with a high pass and more power.

So here we have a discrepancy of 7 - 16 dB between the theoretical maximum and the level that subjectively sounds clean. With this in mind, you might like to consider a realistic estimate to be the output at around 10% power level.

1. Estimate the output based on the maximum power of your amplifier

2. Reduce the output if needed to ensure excursion limits are not exceeded

3. Allow for any high pass filtering

4. Allow for attenuation due to distance - around 3 dB per doubling of distance (this figure is 6 dB in a free field but a useful figure in a real room is closer to half that)

5. Reduce the figure by 10 dB

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