Axiom Home Page Forums General Discussion Technical Questions Definition of a sub's "headroom" ?

It's a term that's thrown around sub reviews (here and elsewhere) quite often. If I look back at my posts I'd guess I myself may even have used it, but I admit to not having a full understanding of what it really means.

Can one of the resident experts elaborate on what the term means, and how it is measured?

Thanks.

Jason

Headroom refers to the ability of a system to reproduce the dynamics of the source material.

Let's say you have a speaker that is 92dB/meter/Watt efficient. This means that the speaker will produce 92dB of sound pressure level when it is placed 3 feet away from the listener and supplied with a 1 watt input. So this speaker will produce about the same sound pressure level as a jack hammer that is 9 feet from the listener. Prolonged exposure to this level will damage hearing.

You typically wouldn't sit 3 feet away from this speaker and you wouldn't listen with an average level of 92dB. You might sit 9 feet away and listen at 75dB (about the level of a noisy office). In this case, you would need an amplifier with about an eighth of a watt of power.

So you can buy an eighth of a watt amplifier and dance until your heart's content if you listen to the compressed garbage that my son listens to. This is the kind of music that doesn't have any dynamics. It's all been recorded at the same level so that it will sound loud. Listen to Headley and you'll know what I mean.

If you listen to music that's been recorded properly, it has loud and soft passages. If you want to listen to the soft passages at 75dB, then you'll be totally hosed when it comes to the loud passages. Your amp won't have the necessary power to produce the peaks because it lacks...you guessed it...adequate headroom.

So how much headroom do you need? Most agree that 18dB of headroom should serve the vast majority of users. So, if you are still using the same speaker, at 9 feet away and you are happy with the 75dB average level, you need an amp that will take you to 93dB. So you'll need an 11W amp. You didn't read it wrong. That's 11 as in eleven. So you would say that you listen to 1W and have 10W of headroom.

Now if you wanted your average level to be 85dB, then you would need a 110W amp. In this case, you would say that you listen to 2W and have 108W of headroom.

In my case, I listen to a half watt and have 89.5W+ of headroom. At an average listening level of 80dB for music, I have 24dB of headroom.

It's the same idea for a subwoofer. If you set the sub to produce 80dB and that sub requires a half watt for that SPL, a 110dB cannon shot will require 500W of electrical power into that sub for a clean cannon effect. This amount depends on room loading but it's a good approximation. So the sub will have 499.5W of headroom.

Awesome explanation, thanks Mojo.

Well my amp does go to 11 so I'm set...

Mojo, excellent explanation of headroom. Thanks.