Here is what I used to use for stereo amps. It is a generalization. It also assumes some things. One is you have to know the sensitivity of your speaker. A speaker that is 85 dB sensitivity (average home speaker) means 1W will give you 85 dB with your head 1 meter in front of it. most guitar and bass speakers have a higher sensitivity than this, and thus will play louder with less watts. (however sensitivity isn’t usually given by bass cab manufacturer like it is in home audio speakers). holding your head 1 meter in front of a speaker is not realistic in the real world, and sound diminishes quickly with distance. so the farther away/bigger the room the more watts you need, which is why the rolling stones require a billion zillion watts in yankee stadium.
anyhoo, here’s the chart. explanations will follow.
Watts to dB
1W = assume 85dB/w/m = 85dB
(so 1W gives you 85dB at 1 meter away from speaker)
2w = +3 dB = 88dB
4w = +6 = 91dB
(notice we have to double the watts each time to get an additional +3dB)
8w = +9 = 94dB
16w = +12 = 97dB (2x loud as 1w)
32w = +15
64w = +18
the sweet spot IMO:
**128w = +21 = 106 dB (2x loud as 16W)
**256w = +24 = 109 dB
**512w = +27 = 112 dB
1024w = +30 = 115 dB (2x loud as 128W)
2048w = +33
4096w = +36
8192w = +39 = 124 dB (2x loud as 1024W)
some sample spl readings at performances:
New age: 60-70 dB
Folk: 75-90 dB
Jazz: 80-95 dB
Pop: 90-95 dB
Classical: 100 dB
Rock: 95-110 dB
Heavy metal: 110 dB
It’s kind of accepted that the smallest increase in dB that can be easily heard is +3 dB (i.e. increasing 5 dB to 8 dB sounds “a little louder”). In order to get this you need to DOUBLE the amp’s watts. A +10 dB increase is perceived as twice as loud. So as you can see as amps get bigger you get insane diminished returns. Which is why I think buying huge amps is a waste of money. Some people will argue that a gigantic amp gives you added “headroom” (peak transients/loud short bursts) but the usefulness of this seems to be controversial amongst engineers (and basses don’t have nearly as much peaks as guitars). To me, for most people the sweet spot zone for amps is marked above, which is why most bass amps are between 100-500w.
tldr: get a 500W amp. do you need a super monster 1000W amp? it’s only a little louder than a 500W amp. also notice you can use this chart in interesting ways. want a practice amp? 15W is almost as loud as 50W and might be a lot cheaper. (all things being equal of course. a low quality speaker may sound like crap for reasons that have nothing to do with it’s power). play jazz? pick a smaller lighter cheaper amp that puts out around 100W.
Crown amplifiers has a great article on this.