speaker impedence and sensitivity

ronaldJ · 02-07-2006, 09:35 PM

I'm auditioning speakers and need a little help in understanding the relationship between accuracy, impedence, and sensntivity. 1st speaker (paradigm studio 100 ) is rated at 90 db into 8 ohms. 2nd (revel F-32 ) is rated at 86 db into 6.5 ohms. Does impedence influence sensitivity or does a lower impedence speaker just need more power to push. Does lower sensitivity mean less accuracy, or just less efficiency?

DavidSGT · 02-07-2006, 11:09 PM

Hiya RonaldJ,

FWIW, I would think that if it sounds good to your ears after say half hour of listening at a decent volume(read loud), then all the specs in the world won't matter, at least to me

Regards.
David SGT

Chu Gai · 02-08-2006, 09:11 AM

You'll find examples of high and low sensitivities in whatever nominal rating you can think of, so no, there is no inherent realtionship between the two. As the impedance of the speaker drops, more current will be required. My recommendation would be to get both speakers into your home and listen to them for a period of time, then choose the one that you like best. Even with speakers of relatively equivalent FR made by competent designers there will be some difference in sound so accuracy can be somewhat subjective here. There are numerous reasons for this but one which should not be underestimated is the effect of the room upon the speaker's sound.

FeisalK · 02-08-2006, 10:58 AM

I'm thinking the only reason they publish these figures is so you can gauge how much power (amplification) you need to drive them to your desired listening levels.

John Garcia · 02-08-2006, 11:21 AM

Impededance DOES influence sensitivity. A 4 ohm load will give you a 3dB gain in sensitivity because a 4 Ohm speaker will draw twice as much current as an 8 Ohm speaker (per Ohm's Law) with the same sensitivity.

Impedance varies with frequency also, so nominal impedance is more of a guideline because it is the impedance with no signal applied. The lower the nominal impedance, the more power you are likely to need to power the speaker because it will dip down to even lower impedance much more quickly than a speaker with a higher nominal impedance. Having said that, the Stuidos are listed as "8 Ohm compatible" not nominal, and they have been measured to drop fairly low in impedance.

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Jacob C · 02-08-2006, 05:54 PM

Sensitivity is measured at 1 watt so the fact that a 4 ohm speaker draws more current, and therefore more watts, doesn't matter. That said, a high quality amp could drive a less sensitive low impedance speaker to the same volume and a higher impedance higher sensitivity speaker. In practice the effect depends on the amp and you don't generally see a full doubling of power into 4 ohms compared to 8. So while the impedance can affect the apparent sensitivity it is impossible to determine how much without also knowing the behavior of the amplifier.

John Garcia · 02-08-2006, 06:44 PM

Quote:

Sensitivity is measured at 1 watt so the fact that a 4 ohm speaker draws more current, and therefore more watts, doesn't matter.

It does matter. The same amp is now working nearly twice as hard with the 4 Ohm speaker to deliver the same SPL, that means you need 4 times the headroom for peaks as you would with a similarly spec'ed 8 Ohm speaker. So instead of using 25% of amp "x's" capability, you are now using 50% of it's capbability, leaving you with much less headroom. I agree that it is dependent on the amplifier's capability; as long as the amp can handle dynamic peaks for a given speaker at a desired SPL, it will be OK. If you drop a 4 Ohm load on an amp that is 4 Ohm stable but can't deliver enough power to handle peaks, then it can become an issue.

1W@1m is a little different for a 4 Ohm speaker also, as it is drawing 2W to achive that SPL.

Jongyoon Lee · 02-08-2006, 09:38 PM

Quote:

I'm thinking the only reason they publish these figures is so you can gauge how much power (amplification) you need to drive them to your desired listening levels.

Jacob C · 02-09-2006, 06:35 AM

I was assuming that the voltage would be the same. It seems (to me anyway) to be a fairly reasonable assumption. (though maybe not strickly true) Power(watts)is a function of voltage and current(amps). The correlation is positive so if the current goes up the power must as well unless the voltage drops enough to offset the increase in current.

Heres an example that may clear things up.

Amplifier: 100w@8ohms and 180w@4ohms.
Speakers: Speaker A is 8ohm with a sensitivity of 92.
Speaker B is 4ohm with a sensitivity of 91.
Speaker C is 4ohm with a sensitivity of 89.

At a given setting speaker B will be the loudest, then speaker A then C. Why? The extra current/power will make up for the lower sensitivity. In the case of Speaker C the extra power will not be enough to overcome the difference in sensitivity. So while speaker B may play louder on a given system speaker A is still more efficient as with equal amounts of power (watts, and therefore current assuming voltage is the same) it will play louder. The effect impedance has on how loud the speaker plays could be thought of as a factor in apparent sensitivity. 1w@1m is a specific set of conditions. Sensitivity is measured this way to make it easy to compare speakers and match them with an appropriate amount of power. It could just as easily be done 10w@3m as long as everyone did it the same way.

Another thing to note is the increased distortion levels when driving lower impedance loads. On good amps its not a big deal since the distortion is inaudible. It is, none the less, higher.