I realised after finishing part 1, that I’d missed my target by so much I could have shot myself in the foot. Nothing new there then.
What I was really after was some kind of sane explanation regarding how PEAK rating (or MUSIC rating; same difference) is the only one that makes sense in the context of speaker ratings. Instead of this, disappointingly, I was wont to ramble on about spectacles with springs in, and I think camels came into it somewhere. So I am definitely going to be disciplined and not have anything to do with hedgehogs, emu’s or camels. Now where did I put my specs with the springs in……
The problem with rating a speaker cabinet in RMS values, is that RMS, strictly speaking, deals with only one frequency. So an amp, preamp, output level of some ficticious device (we’ll call a toadophone) will be rated in RMS at one particular frequency. Often this frequency is either 1 kHz or 400 Hz. A second or two’s thought will be sufficient to prove that an RMS rating on a speaker cabinet tells you nothing at all that’s useful. This is a happy situation, because a second or two’s thought is all I’m motivated to string together on Tuesdays. Even though it is Wednesday.
What’s wrong with RMS then? Nothing, except that your bass speaker has to take maybe 4 octaves whereas your tweeter has only got to survive 1 octave. If this sounds unfamiliar, then I’m afraid you have to go back to part 1 and suffer camels and spring-loaded specs. Or was it spring-loaded camels? Well, anyway, if we think in terms of PEAK or MUSIC power this problem can be sorted out.
Let’s say we’ve got an amp that kicks out say 50 watts RMS. MUSIC or PEAK rating (for the same thing) works out to be double that (give or take). We saw that in part 1. So your speaker cabinet has to be able to handle 100 watts MUSIC. This means that all the speakers in your cabinet have to at least equal the 100 watts MUSIC output of your amplifier. If they were all exactly the same speakers, each one would take a third of the total 100 watts. So, 33 watts each (ish). A bass player’s 4 x 12 cabinet works exactly like this. 25 watts each for a hundred watt cab.
Taking a look at the picture we used in the last post (sounds like I should be playing a trumpet. No, don’t go there) the three speakers are obviously not the same. This is where the power per octave comes into it. As we said, there’s a thing called a CROSS-OVER in the cabinet, and its job is to sort out which frequencies go to where. These are guesses, but not too far off. The bass speaker gets 4 octaves, the mid-range gets 3 octaves, and the tweeter gets 1 octave.
If we now say that the cabinet in total has to handle 8 octaves then each octave has an eighth of the total, so about 12 watts (just over).
From that, we can see that the bass speaker has to handle 4 of those total octaves, so 4x 12; is around 50 watts.
The Mid range, 3 octaves. So 3×12 or 36 watts. And the tweeter one octave, so 12 watts.
If we want to go to the trouble of dividing those figures by two, we get RMS ratings for each speaker of ; 25 watts bass;
18 watts mid; and 6watts tweeter. Or thereabouts.
Here it is that we get to the point. If we rate the cabinet in RMS we have to state the frequency of the rating. So the cabinet is 50watts at 400 Hz, or 18 watts at 1kHz, or 6 watts at 10 kHz.
This is obviously completely meaningless, and worse than that, misleading. The power rating in PEAK or MUSIC power, however, makes perfect sense. It’s a 100 watts; and that 100 watts is sorted out within the cabinet by the cross-over.
I would like to have put a few more pictures in but I couldn’t think what for. Maybe a picture of my dog then? But I don’t have a dog. “Oww! I thought you said your dog don’t bite!” “That’s not my dog.”