Originally posted by BruceWane
What you are looking for is called "efficiency". It is expressed as decibels measured at a distance of 1 meter with 1 watt input power.
Ampeg's current 4x12 (SVT-412HE) is rated at 98Db, which is kinda low for a typical bass cab, but not too bad (compare to an Eden or SWR 4x10 at around 103Db). If your Mesa cab is loaded with EVM's (as a lot of Mesa cabs are), I'd expect the Ampeg to be a lot less loud. EVM 15's are VERY efficient - around 103DB each, so a 2x15 would get you 106DB.
Well, The assumptions above aren't totally correct.
First, it generally takes about 6 dB to notice a significant difference in "typical" usage. You may be able to "detect" a 3 dB difference, but it may not sound like very much. That's assuming there are no big response differences, which would likely be much more obvious to the ear than 3dB efficiency. Generally a 10 dB increase is accepted as "twice as loud", but that can vary.
The more dynamic range in the sound, the more difference in effciency it typically takes to notice a difference. With a sine wave, 1 dB or even less may be audible. (1 dB was originally supposed to be the least change that was perceptible to the ear).
So "a lot less loud" with a 4 dB nominal difference may be stretching a point pretty far.
Next, two identical speakers are not 2X more efficient than 1, nor will 2 speakers necessarily produce twice the power with the same input (if the impedances are equal).
Only if the two draw more power than one will they produce more output (if the two are 4 ohms and the one is 8, for instance).
So two 8 ohm cabinets, one with a single speaker, and one with two, will be the same loudness with the same input if the speakers all have the same efficiency. The power is simply divided between them and each gets less input.
So two 103 dB speakers don't add up to a 106 dB total unless something else changes also.
It is possible for multiple speakers to "beam" the sound more, which will put more sound straight out front, instead of spreading it out. That tends to raise the SPL out in front, and lower it elsewhere. A 4 speaker cab has some advantage there over a one or two speaker cab, as it will probably do the "beaming" to a lower frequency.
If two cabinets are less than about 6 dB different in 1 watt SPL, the response characteristics and any "beaming" effects may make more audible difference than the bare driver efficiency. Your mileage may differ.
The efficiencies of speaker drivers have been a strange spec. Some companies have rated all speakers with 2.83 volts in , which is 1 watt at 8 ohms, but two watts at 4 ohms. So a 4 ohm speaker would get a higher number than an 8 ohm speaker, since they "called it" 1 watt input regardless.
Some made 3 ohm or 6 ohm speakers, and called them 4 or 8 ohm speakers, again getting a bigger efficiency number with the "standard" 2.83 volts input.
Small differences in efficiency may or may not really exist, and may or may not be even audible. I will agree EVM speakers are efficient though.
It's best to try the systems next to one another and see what you think. Efficiency ain't everything.
