enough watts to drive these speakers? Help?

Greetings all,

I've got a friend with 89db efficent speakers playing in a 12'x15'x8' bedroom. He's looking at getting a 8 or 10 watt, low powered SET amp and is concerned if his speakers will play "loud enough".
He usually listens to classical and jazz at "moderate" levels at the loudest.

I compute that he should be getting 95db using only 4watts and the room should give a little more, perhaps 2db.

Are my calculations correct?

Anyone else that has a similar setup and can comment if it's loud and dynamic enough?

Thanks and happy listening!
Your calculations are a little optimistic based on my experience and will vary according to speaker design. Having said that, i don't see a problem with 8 - 10 watts of power feeding "normal" impedance speakers in this size room. That is, so long as one doesn't expect "thunder" and / or the amp uses tubes in the output stage. Tubes overload much more gracefully ( as a general rule ) than SS designs, producing more listenable results even when the amp is temporarily over-driven. Something along the lines of a 300B ( or something similar in output ) should work pretty well. Just be careful as some small amps are WAY over-rated AND don't like low impedances at all. Sean
I use a 90db efficient speaker.I was happy with the InSound/Omega Mikro 10 w/ch prototype V amp,but settled on a set of Viva Verona XLs {26w/ch,pure class-A}.
Your first watt will produce an 89db SPL,at 10 watts you should yeild a 10 db increase in SPL.Doubling the power will add 3db.
I would be more concerned with the impedance presented by the speaker and complexity of the crossover.
Your calculations assumed that your friend would be listening at 1m (3.3ft) from the speaker in his bedroom. Would that really be the case?
If not, one loses 6dB when one moves back from 1m to 2m. Then one loses 3dB when one moves back from 2m to 3m. If your friend is 3m (~10ft) from speakers, the 89dB efficient speaker yields 80dB SPL at his listening position using 1W. To get 90dB SPL, he'll need 10X the power i.e. 10W. At this pt. the amp runs out of gas. There'll be a 3dB increase in SPL owing to 2 speakers i.e. stereo music. Room can re-inforce to some degree but it is hard to tell.
Ill designed xovers in the speakers can eat up some watts.
If he's willing to not crank it up, 8-10W/ch should be fine if everything else has a reasonable design (a bit nebulous but I hope that you can understand what I mean).
Bombaywalla, wouldn't moving back from 1 meter to 2 meters result in a 3 dB loss, not a 6 dB loss?
Bombay pointed out an important factor i.e. seated listening distance vs apparent spl. Having said that, one can't calculate the reduction in spl based on distance without knowing what type radiation pattern the speaker makes use of. Some speakers "project" sound into the distance in a more linear manner than others.

As a side note, the first receiver that i ever had was rated at 12 wpc driving speakers that were around 88 dB's or so. While it wasn't the cleanest ( not by a LONG shot ), this little "beast" could play pretty loud. This was with highly compressed hard rock i.e. "demanding" music that put a steady strain on the amp / power supply. Given that tubes tend to clip in a much more "usable" manner ( SS sounds like HELL ), and the dynamic nature of most classical and jazz music, even with "conventional" front firing boxes, i think that he'll be okay. That is, so long as "moderate" is all that is required. One would also have to assume that this is a secondary system since it is going in the bedroom. Sean
My listening room at home is just about the same size as your friends. My current system there is Soliloquy 5.3 speakers which are 90db floorstanders, paired off with George Wright Mono 10's which are 10 watt monoblocks (push-pull 2A3 based amps). Plenty of power for the size of the room, but as has been said, don't expect chest-pounding bass. My system can play at very 'satisfying' volumes and remains engaging at low volume as well. YMMV depending on which amp and which speakers you end up pairing off. Being a 2A3 based system it is all about midrange and leaves a bit to be desired in the low end. I've tried in the same room pairing off 8 watt 300B SET amps with 89db Silverline SR-17 monitors. Though they did sound wonderful at very moderate volumes, this combination did not work at all well when the volume went over about 88db (I'd guess). Numbers don't always tell the whole story. But I think the general speculations and expectations given his taste for jazz is a good bet for a nice solution if done with care and consideration for synergy.

Good points Marco. My comments referencing the 300B pertained to some "manufacturers" claiming that their single 2A3 per channel ( or similar tube ) will do 8 - 10 wpc. Since you're amp is not a SET and each tube is only doing half the work, it is possible for two 2A3's to make that kind of power.

The reason that i mention this is that i remember a speaker manufacturer measuing a certain SET amp that was advertised at 8 wpc. The person that had bought their speakers was using this amp and complained that the speakers weren't as sensitive as the manufacturer stated. That is, 8 watts should have driven the speakers, but it wasn't. The speaker manufacturer wanted to see what the deal was, so they checked the amp. When it came down to it, the amp was having a hard time doing 3-4 wpc, which just wasn't enough. As such, one should look at the tube compliment of the output stage along with the rated power output. If you've got one tiny tube per channel, don't count on ore than a very few watts at most. Bigger tubes typically have bigger plates and can pass more power, offering higher output levels. One can also achieve this by paralleling multiple tubes or running multiple tubes in push-pull fashion ( like Marco's amps ). Buyer beware... Sean

Just saw your comments.
6dB loss in SPL per doubling of distance is correct! We are not talking about a power measurement here; rather, a voltage measurement when we read off SPL on a meter. Hence, 20*log() rather than 10*log(). Another thing is that since it is a ratio (SPL at 1m vs. SPL at 2m) when we do a divide operation, the impedance cancels out.
Hope that this clarifies.
Bombaywalla, I understand where you got the 6 dB figure (voltage), which can throw confusion into the mix, depending on whether we are talking about voltage, or power readings. But I thought that actual acoustic sound waves (spl) were a measure of POWER, and not voltage. The spl meter converts this to a reading which represents dB's of power not voltage. But I will do further research, since you now piqued my curiosity!
Think of it as surface area. As the distance doubles the surface area quadruples...equal to -6dB/unit surface area.
I didn't read anyone's reminder that sensitivity values are usually expressed at 1k, which can be far less meaningful unless one knows the low frequency rolloff (and impedence). Since it's possible for a "high" efficiency speaker to have much lower response, let's say, at 50Hz than a lower efficiency one with flatter bass, one can indeed be fooled into predicting that a low power amp might have an easier time with the former...especially if a low impedence couples with a large phase angle diff in the bass. So it's not so simple...especially with a tube amp.