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Old 25th April 2013, 01:00 PM   #1
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Default Why 2 ohm for OEM/Factory car audio??

I have noticed many factory car audio speakers are 2-ohms?? I'm curious why they would use 2-ohms speakers?? I understand the math of it, more power from lower voltage, but at the expense of more current for a given power rating.

One OEM automotive sound engineer I spoke with talked about using 12-ohm speakers for higher efficiency. less current draw and hence load on the alternator,( which got them better MPG numbers,) but would require a higher voltage swing for a given power rating.

2-ohm speakers have never made much sense to me. especially in a car where you are plagued with such a weak electrical system to begin with. so I'm curious why OEM's do so? what am I missing??

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Old 25th April 2013, 01:14 PM   #2
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When you only have 12 VDC to work with you either have to boost the supply or go to very low impedance drivers. The less you boost the supply, the better the efficiency- high input-to-output ratios become problematic, though anything can be done if you really want to. A bridge amp with 12 VDC supplies can apply maybe 23 Vpp or 8.1 Vrms. That's only 8.2 watts into 8 ohms but 32.8 watts into 2 ohms. If I'm too asleep to do the math right hopefully somebody will correct it.
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Old 25th April 2013, 02:24 PM   #3
godfrey is offline godfrey  South Africa
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The other thing to bear in mind is that the kind of knuckle-draggers who think multi-kilowatt car systems are a good idea probably don't know anything about electrical safety. Operating at lower voltage / higher current reduces the risk of the idiots accidentally electrocuting themselves.

Apparently it's frowned upon in the industry for a manufacturer to be seen to be encouraging natural selection.
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Old 25th April 2013, 02:47 PM   #4
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I would guess it's a simple as them trying to squeeze every last drop of audio power out of 12V without investing in costly step-up power supplies. Then they get to use it as a "mutli-hundred watt audio system" marketing angle.
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Old 27th April 2013, 08:50 PM   #5
TMM is offline TMM  Australia
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Originally Posted by Zero Cool View Post
One OEM automotive sound engineer I spoke with talked about using 12-ohm speakers for higher efficiency. less current draw and hence load on the alternator,( which got them better MPG numbers,) but would require a higher voltage swing for a given power rating.
You can't get something for nothing otherwise we'd all be using very high impedance speakers for their 'efficiency'. 1watt at a low current and a high voltage is no different to 1watt at a high current and low voltage in terms of efficiency - you are still going to get a 1watt draw @ 12v (=0.083Amps) at the alternator/battery.

Perhaps he might have meant that they used 12ohm speakers so they could parallel them and drive multiple speakers from a single amplifier channel - that would give significant gains in efficiency because you don't have the idle current draw of a larger number of amplifiers.

Higher impedance systems do have less losses in the speaker cables because the impedance of the cable becomes insignificant compared to the speakers - we are however talking fractions of a watt, not enough to affect the MPG of a car. Higher impedance also allows for thinner (and cheaper) cable.
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Old 28th April 2013, 03:48 AM   #6
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It is simply - as mentioned - the issue of having a boatload of current available, and not much voltage. That's where it got started.

Today the car amps all use DC-DC converters, so in truth there isn't much benefit, except again, you can keep the voltage down and swing more current, which does make it easier to make high power class AB amps, or even class H. But now with Class D amps, the thing is changed a bit... but the "standard" of getting down to two ohms was already established.

Also, there are those that will parallel up speakers no matter what... so the need to not let the magic smoke out in the amp design is/was a factor.

At least that's my take on it...
http://www.bearlabs.com -- Btw, I don't actually know anything, FYI -- every once in a while I say something that makes sense... ]
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