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1. Junior Member
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Sep 2007
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1

Bad information given relating to speaker ohms

Greetings,

I signed up to receive a couple of newsletters and one came today from "This Old House".

In the discussion of home theatre speakers, the writer claims that it takes more power to drive low resistance speakers than it does a higher resistance. This is completely backwards and wrong.

Ohms are a measure of resistance. Higher ohm rating means a circuit has more resistance (not less). For instance, 8 ohm speakers take more power to drive than say, a 4 ohm speaker. The writer claims just the opposite.

2. Banned Rank 2
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Re: Bad information given relating to speaker ohms

Well, I did not see the article but I deal with Ohm's law every day. So lets take an arbitrary voltage of say 70 and see what Ohm's law says.

Volts / Resistance = Amps (Power)

70 / 4 = 17.5 Amps

70 / 8 = 8.75 Amps

Do you have a link to the article?

3. Senior Member Rank 2
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Re: Bad information given relating to speaker ohms

Let me see if I can oversimplify this discussion . Imagine two wires . One is the " hot wire " and the other is grounded . Now place a resistor of a HIGH ohmic value between them . Not much current flow due to the HIGH resistance , right ? Now LOWER the resistance down to virtually nothing . What happens ? Maximum current flow , up to the limits of the source . ( or until the fuse blows )

Which scenario required the strongest source , the high resistance , or the low resistance ?

NOW, to further complicate the discussion , what is the output impedance of the amplifier ?
Probably 8 ohms . There is a reason that amps have an impedance rating . It's got to do with "matching " . An amp with an impedance of 8 ohms , connected to a speaker rated at 8 ohms will be matched and operate at it's designed efficiency . Connect a 4 ohmn speaker to it and it will have to work harder .
Last edited by djohns; 09-07-2007 at 08:01 PM.

4. Re: Bad information given relating to speaker ohms

This one kinda looks like fun ...if you don't mind another 2 cents worth.

In the discussion of home theatre speakers, the writer claims that it takes more power to drive low resistance speakers than it does a higher resistance. This is completely backwards and wrong.
First thing is... a loud speaker system is rated by it's ( supposed ) nominal impedance. The ohms value is assigned as a unit of measure to this impedance.

Usually in audio the power formula is W=VxV/R (Watt=Volt x Volt / Resistance value).

For example :

a source voltage of 40 volts from the amplifier and if the loudspeaker has 8 ohms of impedance, then (W = V x V / R) W = 1600 ÷ 8 = 200 watts of power.

The same 40 volts were delivered by the amplifier to a 4 ohm loudspeaker, then we would have W = 1600 ÷ 4 = 400 watts of power.

Basically where some of the confusion is that in order to achieve the same SPL ( sound pressure level ) from an 8 ohm speaker the power would have to be doubled than that of a 4 ohm speaker.

But , you don't get something for nothing , in this case the strain on the amplifier.

As mentioned earlier in the thread by kentvw and djohns ( who know what they're talking about ) the working power that happens by the formula P/R=I shows it takes more work power ( increase of current ) to drive a 4 ohm load speaker.

OR

P = I x V, so I = P / V

I = 400 watts ÷ 40 volts = 10 amps
I = 200 watts ÷ 40 volts = 5 amps

Ohms are a measure of resistance. Higher ohm rating means a circuit has more resistance (not less). For instance, 8 ohm speakers take more power to drive than say, a 4 ohm speaker. The writer claims just the opposite.
You are thinking of a pure resistance and can be applied to DC fundamentals.

Loud speaker systems have different characteristics with things like coils , capacitors which create different variables known as reactance that's where impedance comes in to play and the fact that the audio source is AC with changing frequencies.

Now loud speakers will also have changing impedances as well for example : an 8 ohm ( nominal ) will have an impedance as low as 3.9 ohm at low frequencies and as high as 20 ohms at high frequencies.

Anyway enough with the techno speak I think I've used up my 2 cents.
Last edited by canuk; 04-21-2008 at 11:56 AM.

5. Senior Member Rank 2
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459

Re: Bad information given relating to speaker ohms

Yeah . What he said .

6. Re: Bad information given relating to speaker ohms

In other words the TOH newsletter article is correct.
Jack

7. Re: Bad information given relating to speaker ohms

Yeah ... what he said.

8. Re: Bad information given relating to speaker ohms

Oh yeah one more thing...

There are some audio equipment manufacturers that rate there equipment as high current output rather than stating it in Watts. The rated Watt output is somewhat ambiguous and has been a bone of contention for years. The rating has been known to border on misleading since you don't know if it's a RMS , continuous or peak and at what percentage of distortion at that rating. I've seen cheapo amplified sub woofers claiming to be 100 watt when in reality they are barely 25 Watts continuous with possibly 100 Watts peak with 10% distortion.

The power ratings of speakers can also be ambiguous as well. For example : if a speaker is rated for 100 Watts is that continuous or peak power handling ? There is a considerable difference between the two since peak power will only be for a brief moment whereas continuous lasts longer. It's the way they can handle the power ( heat ) without frying. So depending on the rated output a speaker of 100 Watts peak may only be able to handle 40 Watts continuous with peaks of 100 Watts.

I think my tab is up to 5 cents now.

9. Senior Member Rank 2
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459

Re: Bad information given relating to speaker ohms

Go on and run a tab , bro ' . I gottcha covered .

10. Re: Bad information given relating to speaker ohms

Thanks djohns.... check ( we say cheque )is in the mail.

I thought those brain cells were dead but once I accessed them it was hard to shut them down.

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