Ohms vs. Db

[r100gs]

Can someone in layman terms explain the the relationship between two different speakers, one rated at 8 ohms and one at 4 ohms, but both have a sensitivity rating of 90db. Which is the more efficient speaker? I understand that the 4 ohm speaker will be more difficult to drive, but if your amp is 4 ohm rated you can take advantage of the additional wattage. In my mind I would say the 8 ohm speaker is more efficient, but with the additional wattage the 4 ohm speaker may be also.

[chrispy35]

They both have the same efficiency (dB relative to 1 W input).

The 4 ohm speaker will require more current at a lower voltage to reach 1 W than the 8 ohm speaker will.

e.g.

A 4 ohm load will consume 1 W with 2 volts across it. It will draw ~0.5 amps.

An 8 ohm load will consume 1 W with ~2.8 volts across it. It will draw ~0.35 amps.

Chris P.

[r100gs]

They both have the same efficiency (dB relative to 1 W input).

The 4 ohm speaker will require more current at a lower voltage to reach 1 W than the 8 ohm speaker will.

e.g.

A 4 ohm load will consume 1 W with 2 volts across it. It will draw ~0.5 amps.

An 8 ohm load will consume 1 W with ~2.8 volts across it. It will draw ~0.35 amps.

Chris P.

Thanks for the reply Chris, I'm still don't completly understand it. More layman terms (dumbass) would help be. If you had an amp that could double its power, would the speakers equal?

[chrispy35]

Amps are voltage gain devices (small voltage in multiplied by fixed power amp gain equals large voltage out).

The power supply design of the amp will determine what the maximum voltage is that the amp can put out and what amount of current it can put out at that voltage. When an amp doubles power in going from an 8 ohm to a 4 ohm load, it is the same as saying that the amp is capable of supplying twice as much current at max voltage than is required by an 8 ohm load.

Power is voltage x current.
Current is voltage/impedance.

The end result is that if your amp can supply enough current at max voltage output to drive a 4 ohm load (i.e. double power into 4 ohms), the 4 ohm speaker will go louder than the 8 ohm speaker if the speaker efficiencies are the same.

Hope that helps,

Chris P.

[r100gs]

The end result is that if your amp can supply enough current at max voltage output to drive a 4 ohm load (i.e. double power into 4 ohms), the 4 ohm speaker will go louder than the 8 ohm speaker if the speaker efficiencies are the same.

Hope that helps,

Chris P.

Thanks Chris, That was the answer I was looking for.

[mjb]

Chris, are you saying that 4 ohm (or lower) speakers are therefore 'better' in so far as they can achieve a higher SPL for a given Amp - all other things being equal?

[whoaru99]

The 4 Ohm speaker will play 3dB louder compared to the 8 Ohm speaker - assuming all other things are equal and assuming the amp will "double down".

[chrispy35]

Like whoaru99 said, 3dB louder.

It's very unlikely that all other things will be equal between two different speakers though.

Also, the stated 4 ohm impedance is just a nominal value. Across the audible frequency range the speaker impedance will vary +/- from that value so it may dip to a value that does cause problems for an amp where an 8 ohm nominal speaker will likely not have impedance dips that cause problems for the same amp.

Chris P.

[BTB]

Guys..

Not trying to sound smart or anything... but surely if a 4 or 6 or 8 ohm speaker produces 90dB SPL measured at a distance of 1m driven by 1 watt, then it produces 90dB... period.

"Normal" amp behaviour under dynamic conditions aside... How can the rated nominal impedence of the loudspeaker affect overall SPL in any way? Really, just a question.

And then, chrispy35...

An 8 ohm speaker can cause as many "problems" for the driving amp as any other, many mainstream brands, like B&W for instance are well known for this (e.g a 703, rated at 8ohms nominal will drop as low as 3ohms in the bass and lower midband). What is the basis for your statement?

[whoaru99]

Not trying to sound smart or anything... but surely if a 4 or 6 or 8 ohm speaker produces 90dB SPL measured at a distance of 1m driven by 1 watt, then it produces 90dB... period.

Sure, if you change the voltage output of the amp (change the volume setting) the playback volume changes/is compensated accordingly.

"Normal" amp behaviour under dynamic conditions aside... How can the rated nominal impedence of the loudspeaker affect overall SPL in any way? Really, just a question.

If the 90dB/W/m 8 Ohm speaker is disconnected and a 90dB/W/m 4 Ohm speaker is substitued, the 4 Ohm speaker will play 3dB louder - assuming the amp is capable of providing the same output voltage at 4 Ohms as it did at 8 Ohms.

One amp I have will do 200wpc at 8 Ohms and 400wpc at 4 Ohms. Therefore, if I merely substitute a speaker of equal sensitivity, but cut the impedance from 8 to 4 Ohms, the sound will be 3dB louder from the 4 Ohm speaker at any given setting of the volume control compared to the 8 Ohm speaker. Again, this assumes no change has been made to the volume control position and that the amp can drive the same voltage into the 4 Ohm speaker as it can into the 8 Ohm speaker. Of course, if one adjusts the volume control -3dB then the speakers will again be at equal volume because the output voltage has been changed.

EDIT: One thing to be mindful of is how the speaker sensitivity is rated. Some are dB/W/m some are dB/2.83V/m. If the rating methodology is the second listed, a 4 Ohm speaker will inherently be rated 3dB more sensitive than an 8 Ohm speaker if all other things are equal.

Since these two rating methods are used by different mfg, one needs to be sure that one is comparing apples to apples because 90dB sensitivity by one method is not necessarily 90dB by the other method.

[chrispy35]

An 8 ohm speaker can cause as many "problems" for the driving amp as any other, many mainstream brands, like B&W for instance are well known for this (e.g a 703, rated at 8ohms nominal will drop as low as 3ohms in the bass and lower midband). What is the basis for your statement?

Just hanging around here is my main basis for the statement. I know B&W speakers are well known for their impedance dips. I was just generalizing that for instance, if 8 ohm speakers are known to dip to 3 ohms, it seems plausible to me that 4 ohm speakers may dip lower. I did qualify the statement with 'likely'.

Chris P.

[Glen B]

The PSB Stratus Gold i is a 4-ohm speaker that dips to 2.72 ohms at 97Hz. However, its not that simple. The impedance plot show a significant impedance rise in the midbass at the same time, presenting a severe phase angle and difficult load for the driving amplifier.