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Head More Watts Than Cab, 'Watts' The Science?


Billy Apple
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I'm selling a Matamp 212 cab in the Market Place. The cab is 4 ohm and rated at 400w. I've had a PM from a guy with a Markbass LM3, which is rated at 500w at 4 ohm. He seemed to think that the head would break the cab. I know this not to be true, but I don't know the science to explain why.

Can anyone help?

Thanks! :)

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Because the 400 watt rating is based on the speakers' continuous average thermal power handling. Under normal circumstances a 500 watt amp couldn't put out 400 watts average power using a bass guitar (or any musical) signal, so unless you use test tones rather than play music you won't be able to destroy those speakers by overheating.

In the interests of a complete answer, too much very low bass WOULD let you damage them with less than 500 watts, but this is true of nearly every cab even some with much higher thermal power handling, and it's really not something to worry about with your Matamp.

Edited by LawrenceH
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Your amp's rating is the maximum power it will give without distorting. Try turning it up high and play a really loud note and listen. If you can watch the speaker cone. The first snap on the string will make the cone jump forwards and the sound will be really loud. This dies away quite quickly but the sound continues quieter and quieter for a long time depending upon your actual bass. The amp and speaker are only dealing with full power for a few thousandths of a second, the first hit of the pick or your finger. The average power to the speakers is a lot less than the maximum power the amp can produce.

Two things will break your speaker.

Depending upon the design of the actual speaker units the first impact sounds may push the speaker cone too far and it will hammer against the back of the magnet making a loud 'farting' sound or worse still it will go out of the front of the magnet. Good speakers will be designed to prevent this but you can break speakers this way. This isn't in the speakers power rating because it depends upon the detailed design of the cab it is in.

The second way is caused by all the electrical power from your amp heating up the tiny coil of thin wire at the heart of your speaker. The wattage rating of your speaker is actually the amount of heat it can dissipate before it gets hot enough to start damaging things. Speaker components can get as hot as an old fashioned light bulb. Because the important thing is the average power and your sound rapidly decays as you have heard and because of the gaps in the music you can generally reckon a 500W amp flat out but without distortion is probably averaging less than 50W. Usually it will be even lower than this.

A 500W amp averaging 50W isn't going to blow a 100W speaker by overheating never mind a 400W speaker unless you are doing something else unusual. It might 'pop' the 100W speaker but it isn't going to threaten the 400W speaker in all probability. In any case using this system turned up full will drown out the drums and damage the hearing of anyone standing too close.

So long as you don't turn up loud enough to make 'farting' noises or any other unusual sounds and you don't use stupid amounts of distortion/compression or bass boost this combination should go on working for years.

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[quote name='Mr. Foxen' timestamp='1353351721' post='1874141']
Watts don't break cabs. Stupid breaks cabs. Ask him if he's stupid, if he says 'no', you can assure him in that case the cab won't break. That's science.
[/quote]

agree, and put another way, when it sounds like you are destroying your speakers, you are. When it 'farts' turn it down. Action: Point to volume control in exaggerated way.

I find the Crown XTi 6000 has plenty of 'headroom' into the 4" speaker 2.5W bass cab. Just unable to get the volume knob above 0.5 :) Have not destroyed the speaker yet.

Edited by 3below
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[quote name='3below' timestamp='1353791567' post='1878578']
agree, and put another way, when it sounds like you are destroying your speakers, you are.
[/quote]
Except if you're using fuzzy distortion. Fairer to say inexperience, rather than stupidity.

More generally, a factor that people preaching about excursion versus thermal limits ignore, is that the magnet is itself a major thermal design element. Push the voicecoil out of the gap and you rapidly reduce thermal capabilities unless the design allows for this.
The thermal rating tests are bandwidth-limited, but a 'modern' xmax definition can allow the voicecoil to leave the gap by as much as 1/3rd total gap height, greatly reducing the thermal capability. Factor in a player's (perhaps subconscious) compensation for gradually increasing speaker power compression and you can potentially hit a new, compromised thermal limit [i]before you hit xmax, ie no easily audible warning.[/i]
Add to this that newer generation premium drivers can show a less rapid transition from clean to gak [i]above[/i] xmax, all the while reducing thermal capability further and further, and it can be very difficult to judge where the limits are unless you are pretty experienced at blowing that particular speaker model!
I have heard rubbing voicecoils that are probably damaged by overexcursion, but the few times I've looked at the voicecoil of a fully 'blown' speaker I've seen signs of heat damage.

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