Speaker SPL calculator
How loud will it be at the back row, and how many watts does it take? Enter the speaker's sensitivity, your amp power and the distance.
How it works
Sensitivity is the level 1 m away with 1 W in. Every doubling of power adds 3 dB, and every doubling of distance takes away 6 dB.
A speaker with 97 dB sensitivity on 250 W makes about 121 dB at 1 m. At 60 ft (18.3 m) that falls to about 95.7 dB.
Turning that around: to gain 6 dB at the back of the room, you need four times the power, or you need to halve the distance with a delay speaker.
Field notes
- Indoors, real rooms are louder than this. The formula is for open air. In a room, reflections add a reverberant field that stops the level falling as fast past a certain distance. Treat the result as a worst case for direct sound, which is also what decides clarity.
- Line arrays fall off slower up close, closer to 3 dB per doubling of distance in their near field. This calculator is for point-source speakers.
- Check how sensitivity is rated. Some specs use 2.83 V / 1 m instead of 1 W / 1 m. That's the same thing at 8 Ω, but on a 4 Ω speaker 2.83 V is 2 W, so subtract 3 dB to get the 1 W figure.
- Watts are continuous, peaks are not. Program peaks run well above the average level. Leave headroom between the SPL you want and what the amp and speaker can do flat out.
- Mind the speaker's limit. Past the speaker's rated power, extra amp watts don't add level; they add damage. Check the cabinet's maximum SPL spec too.
FAQ
How much louder are two speakers than one?
For two identical speakers playing the same signal to a listener roughly the same distance from both, it's +3 dB in power terms, and up to +6 dB at low frequencies where they add coherently.
Why does doubling the amp only add 3 dB?
Decibels are logarithmic. Double the power is 10 × log10(2) ≈ 3 dB, which most people hear as a small step. Roughly 10 dB sounds "twice as loud", and that takes ten times the power.