Question about headroom and multiple cabs

[ras1983]

I always see many TB'ers run two cabs that together have a higher power rating than their head. how does this work if head X puts out 400W @4ohms and Joe Blogg wants to run two 8-ohm cabs rated at 600W each (for a total of 1200W).

being wired in parallel, will each cab recieve 400W or will the 400W be split across the two for 200W each?

 

[Richard Lindsey]

I always see many TB'ers run two cabs that together have a higher power rating than their head. how does this work if head X puts out 400W @4ohms and Joe Blogg wants to run two 8-ohm cabs rated at 600W each (for a total of 1200W).

being wired in parallel, will each cab recieve 400W or will the 400W be split across the two for 200W each?

200 W each. The power handling of the cab has nothing to do with the power the amp puts out. Suppose you have a pint of beer, and three empty jugs, one holding a pint, the second a quart, and the third a gallon. Whichever empty jug you pour the pint of beer into, there's still only going to be a pint of liquid in it. The bigger jugs will just have more unused capacity.

 

[Mcrelly]

I always see many TB'ers run two cabs that together have a higher power rating than their head. how does this work if head X puts out 400W @4ohms and Joe Blogg wants to run two 8-ohm cabs rated at 600W each (for a total of 1200W).

some run two cabs rated lower than their amp. I've got two 300w speakers and a 1200w amp. I USE TO run two 250w cabs with 350w into 4ohms. I like to have more reserved amp power than reserved cabinet handling.

NEITHER situation is "safe" for the speakers, but in my second scenario I was more likely to NOT have enough volume. both situation were two 12" speakers, but underpowering speakers can yield sluggish and undynamic sound in high volume situations.

like a high power sports car at highway speeds you may not need the extra power all the time, but when you do ZZZOOOOMMMMM you have what you need. most speakers can handle almost twice what they are rated for if it is only for a second or two.

 

[ras1983]

some run two cabs rated lower than their amp. I've got two 300w speakers and a 1200w amp. I USE TO run two 250w cabs with 350w into 4ohms. I like to have more reserved amp power than reserved cabinet handling.

NEITHER situation is "safe" for the speakers, but in my second scenario I was more likely to NOT have enough volume. both situation were two 12" speakers, but underpowering speakers can yield sluggish and undynamic sound in high volume situations.

like a high power sports car at highway speeds you may not need the extra power all the time, but when you do ZZZOOOOMMMMM you have what you need. most speakers can handle almost twice what they are rated for if it is only for a second or two.

That's the answer i was looking for. it just seemed odd that people would run cabs and power them well below their rating.

 

[billfitzmaurice]

just seemed odd that people would run cabs and power them well below their rating.

There's nothing odd about it at all. The power rating of a speaker tells you how much juice you can safely put into it before you blow it up; it tells you nothing about how loud it will go, nor for that matter how much power you can put into it and still have it sound good. Few speakers will work at more than half their power rating before they distort heavily. So if, for instance, you have a speaker rated at 500 watts and you're using a 200 watt amp and it's as loud as you want it to be then you do have a large enough amp.

On the other hand having two or even four times the amp power available than the speakers are rated for is not a bad thing, as that ensures that your amp will never be taxed, which also leads to distortion. It's just like the big V8 as opposed to a high-revving four. They'll both get you to 75 MPH, but the V8 will never have to exceed 4,000 RPM to do it.

 

[Richard Lindsey]

That's the answer i was looking for. it just seemed odd that people would run cabs and power them well below their rating.

As Bill said, it's not odd at all. There's no particular reason you have to run speakers at or near their rated capacity. There *is* a good argument for having a bigger amp than you actually need. When you run an amp that puts out more power than your cabs are rated for, whatever benefit you get from that comes entirely from having a bigger amp than you need. Effectively none of the benefit comes from having cabs with lower power ratings; that really has no bearing on it. Thus, if you need a 400 W amp to do your gig, and you buy a 1200 W amp, your benefit (i.e., headroom) is effectively the same whether your cabs handle 500 W or 1200 (assuming equivalent sensitivity and frequency response). In addition, if you have a 600 W amp, switching from a 750 W cab to a 300 W cab of the same sensitivity and frequency response does not give you one iota more headroom, because you haven't increased the power-available-to-power-needed ratio, which is what determines headroom.