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'Speaker headroom'...and it's (possible) effect on current draw
Posted by joshwill0001 on 09-26-2015 09:26 AM:

G'day guys...sorry to lay down a noob question but it really just stands to reason with me. Yet I cannot find any backup discussions on the topic even after a couple of days on the forums and google! In a nutshell...a (hypothetical)4 x 100W RMS amp is hooked up to 4 speakers. Simple setup. However the installer has allowed 'headroom' for his speakers (say they all are rated at 60W RMS). Now Ohms Law, inefficiency of amps, even losses through cable lengths have all been done to death in regards to current requirements AT 400W RMS...however seeing as this particular installer has allowed headroom and his speakers will never see the full 400W then shouldn't that affect the calculation for current requirement? This is obviously inapplicable in my example as a stock setup would handle that fine either way; however looking at 2-3-35k systems it therefore leads to alternator, battery and wire size ramifications to list only a few. Obviously a difficult one to sit here right now and say that said installer would only use 60W on each channel so work the formulas from that with 4 x 60 = 240 etc... However the 'theory' is correct...isn't it??!! Obviously said installer is myself and when looking at 2-3kW system it becomes relevant if most speakers have a ~70% headroom...at least I cannot see how the formulas behind it could lead anywhere else...? I may be missing or forgetting something incredibly simple so I do apologise. However when the ubiquitous question of 'how many Amps do I need for my 35,000 watt RMS system' is asked the assumption is nearly always taken that the 35k is continuous (relative to my query anyway, obviously there are reasons why the 35k is not the applicable figure and these also have been discussed sufficiently for mine), however the idea of 'tangible headroom' and its affect on current draw I feel hasn't been discussed directly...rubbish; I'm sure it has but I just haven't come across it! Still I want to be certain and hence the question! Apologies for the simplicity of the question but though I have done a pile of installs in my time I have never done one 'specifically' like this (which is my own personal install....finally!). So I want to get it right. FYI I am running an unknown combination of Alpine PDX-M12, PDX-V9, PDX-F6 and PDR-F50 amps in some sort of active arrangement. 90% on a 3 way front with rear fill and a sub(s). One or more amps will drop out due to not having a complete picture yet. Thanks guys for any help! J



Posted by N2Audio on 09-27-2017 03:54 PM:

I realize this is 2 yrs old, but I checked in to see if this place was still even up and this question happened to interest me... It has nothing to do with the power of the speakers. But regarding "dialing back" an amp -- of course that will effect its current demand. And power output of an amp is FAR from continuous. It's only continuous if the signal being played is a sine wave at the edge of clipping. Music is only 25-50% of that. There's also efficiency to consider. A good class D at 80% efficiency requires ~1250w rms to generate that 1000w of output. However, since we play music and not sine waves it would only require ~300wrms at lowish volume, maybe 500-600 at very high volume. That's with gains optimized for output. If you're gain limiting output to protect speakers, then those power numbers will drop fast. This is why it's generally no problem to run a 1000-1500w rms sound system with a stock alternator and battery, unless, of course you're one of those bass heads that's blasting it into clipping 24/7.