Seriously BIG car audio visual systems!

[damienengland]

Dave, awesome post. Like others, i took all day to get through this but you're bang on. I haven't studies sound engineering but have spent many years installing audio, including car stereo (Morley Hi-Fi and Micro Electronics), home theatre (electrical contractor with a West Coast Hi-Fi contract) and finally commercial sound systems in bars and clubs etc.

The keys to power are exactly as you said and i have always been told to work our the needs of a venue then double it so the amp racks are essentially idling when in use. A couple of notes i would add include checking out the given Total Harmonic Distortion (THD) specification vs RMS power and also remember the power supply is critical to the peak performance requirements we are seeking.

Have a look at the specs on this Alpine Mono Block amp as an example-

Specifications
CEA-2006 Power Rating (40hm @ 14.4V = 1%THD+N), S/N 80dBA (Ref. 1W into 4Ohm): 1000W x 1
RMS Power (at 14.4V THD+N,20Hz-200Hz)
Per channel into 2 ohms : 1000W x 1 (<=1%THD+N)
Per channel into 4 ohms : 1000W x 1 (<=1%THD+N)
Dimensions
Heat Sink (WxHxD) in: 10-1/8" x 7-9/16" x 2-7/16"
Heat Sink (WxHxD) mm: 257mm x 192mm x 62mm
Bass Engine®
Crossover Frequency : 50Hz-200Hz, -24dB oct (LP)
General
Input Sensitivity (For Rated Power): 0.1 V to 8.0V
Input Impedance: 10k Ohm
Power Requirements: 14.4V DC (11V to 16V)
Speaker Impedance: 4 or 2 ohm
Frequency Response: 20Hz-200Hz (20Hz/-3dB)
Signal to Noise (S/N): 110dBA (Reference to Rated Power)
- See more at: http://www.alpine.com.au/showItem.ph....HZlb9G1U.dpuf

A couple of the keys here are that the RMS rating is given for a consistent 14.4VDC supply with a 1% THD (plus noise) and the frequency response is only quoted at 200hZ maximum so i take it there is some fudge factor in this amps performance. Even so, i bet this amp does very well connected to a sub woofer!

[SLR_dave]

A couple of the keys here are that the RMS rating is given for a consistent 14.4VDC supply with a 1% THD (plus noise) and the frequency response is only quoted at 200hZ maximum so i take it there is some fudge factor in this amps performance. Even so, i bet this amp does very well connected to a sub woofer!

Great example. For a premier manufacturer producing some of the best car audio this doesn’t make heaps of sense, when really it could be made very clear. And all manufacturers are doing it of course, the market requires it.

The specs you have quoted from this manufacturer are for one of their top notch, high performance single channel subwoofer amplifiers (some other manufacturers are calling these “monoblock” amplifiers).

Its actually designed to power a subwoofer only. (This is why the very limited 20Hz-200Hz) frequency range. Its mono, and will not be able to reproduce frequencies higher than 200hz with much volume or accuracy at all. This is because it is not intended too, its job is all in the sub frequency range. As you say, its most likely it would perform really decently in those ranges. (although for $800 just to power a subwoofer it would want to)!!

I noticed manufacturers using 14.4V as a standard quoted supply voltage a few years back, that seems fair enough to me actually, (they have to use something, and most cars will have around 14.5 avail when running) but yes I agree it may also allow them to tweek the final output specification upward depending on how they’re measuring it, but, theres other stuff in there too that is fairly meaningless.

For instance, it seems to be claiming 1000W RMS (including noise as you point out) going into 2 ohms, and also 1000W RMS (including noise) going into 4 ohms, (ok, assuming that its feasible that the electronics can do this at either 2 or 4 ohms... ), where is the explanation of exactly what it means. What are they actually saying? It’s a fairly important spec for sales.

Remembering that its only producing a very small amount of the audible frequency range (20-200Hz) and that it seems to have a “crossover” (probably a High Pass Filter – adjustable you’d hope at that price) that can operate at around 50Hz – again it doesn’t say if its fixed, or goes up, or down from 50Hz , which will further limit this very narrow frequency range, so, what frequencies will it actually be able to produce to provide its optimum “1000W RMS”?

…..1000 Watts of what exactly?.

If you think about it, its producing such a limited frequency range, that its performance is going to be closer to a version of Peak Music Power (perhaps continuous, and before distortion), but I don’t know if that (if thats what it is) should really be called 1000W RMS - but theres no way of knowing reading the specs, what is actually being described.

Now because its only a mono (one channel) amp, and the range is so low, and giving the manufacturer the benefit of the doubt lets assume it is able to produce, from an amplifier that size, some form of continuous bass frequency operating at 1000W. Lets imagine the current drain.

Power (Watts) = Volts (V) x Amps (A).
W/V = A
1000 / 14.4 = 69.4


Close enough to 70 Amps!
So , for the amplifier to produce the claimed 1000W of power, it would need to be able to draw 70 Amps or thereabouts continuous current draw from the battery or the charging system. .

The draw on the charging system would be unsustainable very quickly. And on top of that, you still need another amplifier around the same size to run the front channels. (so say 120 Amp current draw for both).

Now I’m not saying that this doesn’t happen, it can happen quite easily, a big amplifier and Sub set up installed improperly is easily likely to threaten a cars charging system, and, can (for instance) cause fires, stop the headlights, or windscreen wipers from working properly, etc it happens! Installation is a serious issue with big amps in cars, extra batteries, beefed up alternators, even an extra alternator in some cases may be needed or justified.

It is quite possible to achieve 1000W RMS in a car, (if one really wants it) but, it makes me think twice about whether a big brand would sell an amplifier that big on the net, it needs professional installation so in the back of my head I wonder, perhaps another discrepancy there.

So yep a good example. (And all the manufacturers are doing it). All that said however, I reckon, reading carefully, (guessing between the lines basically) specs like these can actually give you a vague idea of the capability of the unit in question because, you can judge how far the manufacturer is prepared to be caught out lying blatantly – some lower quality brands may not care, the better ones will care more, whilst these specification descriptions may not be at all clear, for the higher end manufacturers the gear still has to sound like it meets what those specifications may seem to claim, so with care you can still make a fairly informed choice.

This doesn't mean that the specifications given above can't be believed, its just that its not easily clear to see exactly what they are specifying.

They are after all a top brand, if you have their components and they are set up correctly its pretty hard to go wrong. This is perhaps why its a good example, they make great gear, you'd have to imagine its an excellent piece of equipment, but even their specs aren't clear about the RMS rating.

So coming to terms with the issue of volume and “Watts”/ "Watts RMS" not necessarily being what they say they are (and most often, many many miles apart) is really important in selecting car (or any) audio system. A more important specification for all amps, head units, and speakers is the Frequency Response figure, (distinct from frequency range) from this you can get an idea of overall sound quality produced by a component (as opposed to volume) but this is turning into another essay atm so maybe more a little later.

[damienengland]

Power (Watts) = Volts (V) x Amps (A).
W/V = A
1000 / 14.4 = 69.4


Close enough to 70 Amps!

Dave, another great thesis and its awesome to see Ohms law getting a workout on MySandman. With reagrds to the current draw, i have seen this in action. I had a Sony X-Plod (1000W RMS branded) amp running two Alpine 12 inch subs some years ago and suffered several failed alternators in my VT 5lt common door. I connected an amp meter to the alternator wire and measured a max draw of 120A with everything on such as headlights, A/C, sound system up full etc etc. Its an area not many people think about.