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Old 31st May 2013, 07:54 PM   #1
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Default Why 4 Ohm Speakers?

Hi,
I have been auditioning a cheap, but very nice sounding Chinese made chip amp board with a variety of different speakers.
With a pair of 15 inch 15 Ohm vintage "Monkey Coffins" I get get some serious volume and the little heatsink only gets a couple of degrees over ambient.
Same volume with 8 Ohm speakers the heatsink is warm and with 4 Ohm speakers the heatsink gets bloody hot.
The only reason I can imagine for the development of 4 Ohm speakers is that the extra current is required for cheaper magnets with weaker magnetic fields?
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Old 31st May 2013, 08:03 PM   #2
adason is offline adason  United States
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you are comparing speakers with not only different (decreasing) resistance, but with different, most likely decreasing efficiency too

to answer your question, why 4 ohm? they produce low efficiency speaker at 4 ohms, so it actually makes some sound, if they would make it 16 ohms, it would be quiet like headphones...

power is cheap...and you chip amp is weak

Last edited by adason; 31st May 2013 at 08:09 PM.
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Old 31st May 2013, 08:54 PM   #3
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What is the reason for producing low efficiency speakers if it's not the cost?
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Old 31st May 2013, 09:19 PM   #4
chrisb is online now chrisb  Canada
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The DC resistance or AC complex impedance of a loudspeaker's voice coil does not necessarily have any relation to its sensitivity / efficiency.

Indeed, if higher impedance drivers, such as in your 15ohm vintage monkey coffins, have both more voice coil in the gap and higher magnetic flux density at that point, they may well be substantially more efficient than modern / lower impedance drivers. Keep in mind that your drivers may harken back to an era when amplifier power was much more expensive per watt than these days.

Why low impedance these days? At least one reason would relate to what Adason noted - power is cheap these days, and within limits of their SOA most SS amps can deliver substantially more power into lower impedance loads (albeit with more distortion)

It's only one of numerous factors that are part of the calculus of compromise with which a driver / system engineer wrestles.
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Old 31st May 2013, 09:29 PM   #5
wg_ski is offline wg_ski  United States
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Quote:
Originally Posted by radiosmuck View Post
What is the reason for producing low efficiency speakers if it's not the cost?
Because efficient speakers are the size of monkey coffins and people want the size of Bose cubes.
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Old 31st May 2013, 09:37 PM   #6
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4 ohm is huge in car audio. If you tell a kid his 100 watt amp will give him 200 watts if he uses 4Ω speakers, wuduyathink he's gonna pic?

I see little to no reason to have 4 ohm drivers in either HiFi or PA.
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Old 31st May 2013, 10:05 PM   #7
Zero D is offline Zero D  United Kingdom
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@ radiosmuck

4 Ohm Amplifiers, for the same power output as an 8 Ohm Amp, don't need as high a voltage power supply to achieve the same wattage. So "can" be more effecient, depending on the speakers used !

That's why vehicle Amplifiers were designed for 4 Ohms, initially. In recent times some are now capable of 2 - 1 Ohms, an naturally increased power outputs.

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@ Cal Weldon

Hi, the reason 4 Ohms is often used in PA systems, is to save $ on extra Amps, plus the weight advantages that go with it.
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Old 31st May 2013, 11:17 PM   #8
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Quote:
Originally Posted by Zero D View Post
@ radiosmuck

4 Ohm Amplifiers, for the same power output as an 8 Ohm Amp, don't need as high a voltage power supply to achieve the same wattage. So "can" be more effecient, depending on the speakers used !

That's why vehicle Amplifiers were designed for 4 Ohms, initially. In recent times some are now capable of 2 - 1 Ohms, an naturally increased power outputs.

*
@ Cal Weldon

Hi, the reason 4 Ohms is often used in PA systems, is to save $ on extra Amps, plus the weight advantages that go with it.
Vehicle amplifiers and portable PA equipment need to run on low voltage batteries and high current, thus 4 Ohm speakers are the choice if you want to make a lot of noise and get noticed.
I seem to remember now that the recommended impedance for the TDA7247 is 8 Ohm.
I would still like to know why 4 Ohm speakers are in home hi-fi?
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Old 31st May 2013, 11:39 PM   #9
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I don't have technical background many of you have, but perhaps the reason is as simple as the frequency response from a given driver. Recently I have purchased large quantities of ScanSpeak drivers in both 4 and 8 ohm configurations, because while they are physically the same, the different impedance produces different frequency responses.
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Old 1st June 2013, 12:01 AM   #10
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Safety can be a factor, when driving higher ohm high power speakers the voltage in the speaker cables can reach dangerous levels, by using a load with less ohms you keep the voltage lower for a given power output
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