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Efficiency

 

Bronze Member
Username: Sjordan872

Post Number: 15
Registered: 01-2004
I posted the following on the Speaker page but got no responses. Any thoughts from this side?

Can someone please explain this to me in English? If a speaker is rated at 86db compared to another one at 92db which one puts more strain on the amp? Does more strain mean using more electricity hence costing me more to run the darn thing? Same thing with ohms. Does it take more power (hence more dinero) to run a 4ohm speaker comapered to an 8ohm speaker? Sorry to sound ignorant. Thanks guys.
 

Silver Member
Username: John_a

Post Number: 172
Registered: 12-2003
Sound pressure level or "loudness" is measured in dB (decibells).

Speaker efficiency is quoted as so many dB, and it means that the speaker gives that many decibels, measured 1 metre away, for a standard input into the speaker, usually a 2.83 V standard test signal.

To get the same volume of sound your amp needs to supply more power to an 86 dB speaker than to a 92 dB speaker. Power is measured in Watts.

Resistance aka impedance is how much resistance the speaker offers to the electrical current the amp supplies. A 4 Ohm speaker offers less resistance than an 8 Ohm speaker. Therefore the amp can give more Watts to the 4 Ohm speaker. There is a trade-off, though, because driving an easier load (a smaller impendance in Ohms) lets the amp supply more current (measured in Amperes or Amps). Some amps easily go into overload protection with 4 Ohm speakers, because they are delivering more current that they are designed for.

Resistance in Ohms equals potential in Volts divided by current in Amps.

Power in Watts equals potential in Volts mutiplied by current in Amps.

You pay for electricity usually in units of kW-hours. I don't know how much a kW-hour costs where you are, but your electricity bill should tell you. A kW-hour is a unit of energy that can supply a power of 1000 Watts (1 kW) for one hour. 1000 Watts is about one electric radiator, half an electric kettle, ten domestic light bulbs, or, say, four complete HT systems running flat out. Most likely when watching movies you are using not much more than a couple of light bulbs (say 200 W).

Any change in speaker efficiency is not going to make much difference to your power bill. Imagine you saved 10 percent (20 W-hours) by changing speakers. While watching a movie for two hours you will get about the same cost benefit from that speaker swap as using a vacuum cleaner for 1.2 less minutes. Leave one light on overnight and you have blown the saving you would have made after watching 40 movies!
 

Unregistered guest
I'll add that at 86 dB vs 92 dB (at the same power and distance, often 1W at 1m, and both in either live room or anechoic chamber) is a 6 dB difference. To increase sound pressure by 3 dB requires doubling the power. So in this case it is doubled twice, or a factor of 4.

You need 120W to get the same loudness from the 86dB speakers as you get using 40W from the 92dB speakers.

Note that this also means there's not a whole lot of difference between, for example, 80W and 100W.
There is the same audible difference between 1W and 10W as there is between 10W and 100W.

I typically listen to a movie between 1W and 10W on Klipsch LaScale speakers (rated at 106 db at 1m and 1m).

Hope this helps.
 

Silver Member
Username: John_a

Post Number: 174
Registered: 12-2003
Thanks, Peter.

There are two other things I would like to add, maybe, in case Blazer or anyone else goes away with the idea that a more efficient speaker is always a better speaker.

1. Yes, twice the power adds 3 dB. But to get twice the perceived volume of sound, you need to add 10 dB. So, at the same power, the 92 dB speaker sounds less than twice as loud as the 86 dB speaker. Having speaker A with +6 dB efficiency over speaker B is not such an advantage as inspecting power requirements suggests.

2. There is often a trade-off in speaker design between efficiency and a flat frequency response. 106 dB is certainly high efficiency, and your amp may run such a speaker more within its limits, but it does not automatically mean a better sound. Getting efficiency without paying the penalty of colouration may be one of the few areas where speaker design has improved.

There was some stuff on power requirements of speakers from Jan 30 on Receivers: Amp/Receier Help.

Hope this helps.
 

Unregistered guest
Thanks John,

You're quite correct that efficiency doesn't corrolate with sound quality. My first pair of speakers had 94 db/1W/1m and sounded very coloured. The Klipsch sound much better (also cost CDN$3000 a pair) but are still very bright. What I liked about the Klipsch when I got them was the solid bass that wasn't wishy-washy. A drum sounds like a drum. I bought them over 20 years ago as a teenager when I had a sideline job as a "mobile disco". The Klipsch impressed the heck out of people used to see small Bose 301 at parties.

It's true that 10dB makes for a doubled impression, but it's often difficult to get really realistic levels (read "loud") from a 86 dB speaker. But I think we're on the same page. While 106 dB is very rare, you'd need 100W to get the same sound level I get from 1W.
 

Silver Member
Username: John_a

Post Number: 175
Registered: 12-2003
Peter,

Yes, we agree. My very inefficient KEFs (83 dB) I bought in 1980 and I wouldn't change them for the second-room stereo job they now still do. Going back to Blazer's question, the last thing you worry about is the cost of the electricity...

Hope we did this in English as Blazer requested.

Best wishes.
 

Unregistered guest
Yes, the last thing you need to worry about is the cost of electricity indeed!

In fact, if you live in a cold climate where you heat your home, turning on appliances doesn't cost you anything if you already heat your home using electric heat. The electric baseboards will simply turn on less often while your appliances heat the home.
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