Speaker Efficiency Question?

see Rabelais' responses

http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1108496641&read&3&4&

>>I was told that if a speaker is of the 4 ohm variety that you have to reduce the efficiency rating by 5 dbs.
Well, it depends........
On what?? It depends on how the manuf. spec'd the efficiency. Did he say 93dB/1W or did he say 93dB/2.83V?? If he said, 93dB/1W then it doesn't matter whether the speaker is 8Ohms or 4Ohms 'cuz 1W is 1W & in an 8Ohms speaker it'll create 2.83Vrms (as Eldartford wrote) & in a 4Ohm speaker it'll create 2Vrms. In this case, take the efficiency value as correct.
If he wrote 93dB/2.83V then you'll have to reduce the efficiency by 3dB ('cuz the speaker manuf. "cheated" by inputting more power - 2W as Eldartford wrote - into the speaker. Naturally, if he doubled the power, he's going to get a better efficiency!)

If the efficiency is 93dB/1W then you might be compromised by your Thor amp when running it off the 4 Ohms tap (if it has one) since the voltage swing is reduced => music amplitude into the speaker is less.
If the speaker efficiency is 93dB/2.83V then you'll have to subtract 3dB & you'll about 2dB better than your Quads. 2dB is a reasonable (58%) improvement in that you need this much less power from the Thor. Might just do the trick for you depending upon what you listen to & how loud.
FWIW. Hope that this helps.

A 3 dB increase in sound-pressure level requires a doubling of the amplifiers output power. A four ohm speaker requires more power to drive than an eight ohm speaker.

A loudspeaker with a sensitivity of 88dB at 1W would produce 91dB with 2W, 94dB with 4W, and 97dB with 8W, for a speaker with an 88dB sensitivity to produce a 109dB peak would require a 128W amp. An speaker rated at 91dB would only need 64W to reproduce the same peak.

That means an 8 ohm speaker will be easier for you amp to drive. The ability for your amp to achieve those peaks, which occur in virtual all music is greater when you use the more efficient speaker, so 8 ohm loads are easier to drive.

So your last sentence is correct, it will take twice as much power to drive the 4 ohm load in such a way as to reproduce all the action of the music.

This is based on the information that Robert Harley wrote in his book "The Complete Guide to High-End Audio."

Hi KJL,

I beleive you would want to use the term "sensitivity" rather than "efficiency".

Best,

Barry

Buy a 16 ohm Tonian Lab TLM1 at 98db sensitivity and don't worry about any of this... just enjoy the music :-)