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#78134 - 04/05/08 02:30 PM 4-ohm vs. 8-ohm
Daryl Offline
Gunslinger

Registered: 04/21/06
Posts: 96
An amp will put out more wattage with a 4-ohm speaker then an 8-ohm speaker. Does this mean that with everything else being equal will a 4-ohm speaker be louder then an 8-ohm speaker?
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#78135 - 04/05/08 03:06 PM Re: 4-ohm vs. 8-ohm
Altec Offline
Desperado

Registered: 01/06/08
Posts: 334
No. Impedance has nothing to do with loudness. If a particular 4 ohm speaker is significantly less efficient than a given 8 ohm speaker, it could play less loudly, even if it is supplied with more wattage input.

Efficiency is the rating which determines how loud a speaker will play for a given wattage input. In addition, there is a very definite limit to how loud a particular speaker will play before destruction, and this is determined by characteristics of it's design, usually involving more cost for more robust parts.

Efficiency is rated as a certain number of decibels sound pressure when a speaker is supplied with one watt of power, at a distance of 1 meter. A typical rating might be 89db for 1 watt. A difference of 3dB equals an efficiency difference of a factor of 2. A speaker with a rating of 89db will play twice as loud (3db louder) as a speaker rated at 86db when supplied with 1 watt, regardless of it's impedance.

It's worth noting that the ear does not hear 3db as subjectively "twice as loud" - as a matter of fact, 3db is just audible as a loudness increase (or decrease). To make a subjective difference of being "twice as loud", the increase must be around 10db.

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#78136 - 04/06/08 11:40 AM Re: 4-ohm vs. 8-ohm
nfaguys Offline
Desperado

Registered: 04/09/05
Posts: 500
Loc: Maine
Here's a simple question whose answer I should but do not know.

I bought some KEF tweeters just as backup since they are hard to get. Several ae 8 ohms. So......

If speaker systems with x-overs in cabinet are rated at 4 ohms, and you have a replacement driver at 8 ohms....any problem in using it, for example as a replacement?

TIA
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#78137 - 04/06/08 02:14 PM Re: 4-ohm vs. 8-ohm
mahansm Offline
Gunslinger

Registered: 06/01/07
Posts: 91
Loc: Panama City, Florida
Possibly. The efficiencies may be different between the 4 and 8 ohm parts.
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#78138 - 04/06/08 03:00 PM Re: 4-ohm vs. 8-ohm
Altec Offline
Desperado

Registered: 01/06/08
Posts: 334
Quote:
Originally posted by nfaguys:
Here's a simple question whose answer I should but do not know.

I bought some KEF tweeters just as backup since they are hard to get. Several ae 8 ohms. So......

If speaker systems with x-overs in cabinet are rated at 4 ohms, and you have a replacement driver at 8 ohms....any problem in using it, for example as a replacement?

TIA
The parts values in the crossover were arrived at with calculations using the 4 ohms impedance. If you use an 8 ohm driver in place of an original 4 ohm driver, the crossover frequency will change for that driver, and any voicing applied to that driver will be changed.

I would certainly not install a driver with a different impedance than the one being replaced.

However, I would check that the driver you're replacing is really 4 ohms. Just because the total system is 4 ohms doesn't mean the drivers are automatically 4 ohms.

If you have an ohm meter, check the DC resistance of the voice coils - an 8 ohm driver will measure about 6 ohms, and a 4 ohm driver will measure about 3 ohms.

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#78139 - 04/07/08 12:17 AM Re: 4-ohm vs. 8-ohm
Daryl Offline
Gunslinger

Registered: 04/21/06
Posts: 96
I understand efficiency and that a more efficient speaker is going to be louder at a given wattage than a less efficient one. I'm asking about the system of an amp and speaker. Some subwoofers have dual voice coils and if they are both 4 ohms you can wire it as 2 or 8 ohms. Will the system, as a whole, be louder if wired at 2 ohms than at 8 ohms?
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Sony KDS-55A2000
Onkyo DX-C390
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Philips Pronto TS-1000
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#78140 - 04/07/08 01:17 AM Re: 4-ohm vs. 8-ohm
Altec Offline
Desperado

Registered: 01/06/08
Posts: 334
Dual coil subwoofers are usually made for car audio, where 2 ohms is sometimes used in order to get more power out of an otherwise conventional amplifier (most car power amplifiers now use voltage inverters to get around this).

The amplifier should deliver more power at lower impedances. I would not under any circumstances attach a 2 ohm load to any home theater amplifier.

Whether the speaker will be louder - why don't you hook it up and see?

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#78141 - 04/07/08 03:01 AM Re: 4-ohm vs. 8-ohm
mahansm Offline
Gunslinger

Registered: 06/01/07
Posts: 91
Loc: Panama City, Florida
Regarding the speaker with dual 4 ohm windings. The speaker will pull more power from the amp with the windings in parallel (2 ohms) than it will with them in series (8 ohms) and will play louder. You must be sure that the amplifier is designed to function at a 2 ohm load. There may be problems with sourcing that much current, thermal overload, or stability.
_________________________
Outlaw 990/7500/LFM1+/2xLFM1 EX
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Kef XQ2c center
Kef 3005 surrounds
Samsung HL-67a750
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Toshiba HD-A2
Denon CD
Harmony 880
Comcast HD DVR with external drive
Nintendo Wii

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