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Relationship between speaker sensitivity and amplifier power

By zeshane
Dec 15, 2008
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  1. Ok I have done a lot of reading on the subject and still few things confuse me. The questions would be in detail as I have to make them specific to my system. I have the following setup:

    A/V Receiver: Yamaha RX-V463 (105 watts per channel)
    Infinity P250 fronts (150 watts, 92db sensitivity)
    Infinity P150 rear surround (100 watts, 88db sensitivity)
    Infinity C25 Center (100 watts, 90db sensitivity)
    Infinity PS* Sub (100 watts)

    I have connected the setup to my PC through SPDIF digital output on my audigy card. I use winamp for music.

    The amp has volume threshold from -80 to +16 and right now I have fixed the volume at -20. I control the volume from my PC. Usually for movies i have to turn up the volume to -15 or sometimes -10 as the SPDIF disables the volume control from the PC.

    My question is, at -20 how much amplifier power am I actually using to drive the speakers? Do I run the risk of clipping them at this level?

    Other than the main volume, each speaker's volume can be adjusted. The threshold is -10 to +10. I have kept the surround speakers at +7 and while watching movies I turn up the center to +6. How does this effect the overall volume?

    One last question, how does speaker sensitivity reflects at different volume levels on the amp? Say at -20db on the amp, what relationship does it have with the 92db speaker sensitivity of my fronts?

    Would appreciate if someone can clarify these queries.
  2. Vehementi

    Vehementi TechSpot Paladin Posts: 3,199

    Well firstly, you're posting this in the wrong forum altogether... try an audio-specific forum. Just because I'm nice, sitting at work in the last few minutes of the day, and sngx was kind enough to post this in the IRC channel, I'll try to answer some of that for you :)

    Your Yamaha does 105wpc at what impedance, and what impedance are each of your speakers?

    Your receiver will not clip your speakers at -20db. A well designed one wouldn't clip even at full volume. But, whether or not your sound card will send a clipped signal, or even your audio programs will is another issue. Try to go no higher than 75% of the max on your volume levels throughout your system - that should keep you pretty safe. Most receivers these days have clipping protection built in as well, and will shut off automatically. Clipping is also dependent on the amount of headroom the built-in amplifier on your receiver has (ie, amount of extra juice it can call upon in a transient, like a lightning strike, explosion, or the cannons in the 1812 Overture).

    How much amplifier power you're using depends entirely upon the signal, as well as the gain settings... it can't be accurately judged without some means of measurement.

    The 92, 88db etc sensitivity numbers are meaningless without some unit of measure. Usually it's "92db 1W/1m" which as you could probably figure out means that if you put 1W of power through the speaker, it will produce 92db with a microphone placed 1 meter away from the speaker. It is also meaningless without the impedance of the speaker. For instance, on the same amplifier, an 8 ohm speaker with an impedance of 92db 1W/1m is twice as efficient as a 4 ohm speaker with the same sensitivity - 92db 1W/1m. This is because the 4 ohm speaker is getting twice the power the 8 ohm speaker is due to the impedance.

    On speaker sensitivity, you're just amplifying those specific speakers over the baseline, so for instance if you had the volume at -20db and you are running your surrounds at +7, you'd just be running your surrounds at -13db. Which wouldn't clip unless your receiver is an absolute piece of garbage :)

    Speaker sensitivity has nothing to do with the different volume levels on your amplifier. At -20db on the amplifier, that is only a designation of amplification, ie, a volume meter. It has no relation to the efficiency (sensitivity) of your speakers.

    Hope this helps.
  3. zeshane

    zeshane TS Rookie Topic Starter

    Thanks for your detailed answer and courtesy to reply even though I know this is not an audio forum. However I would take the liberty and discuss a few more things on the same topic if you dont mind.

    My speakers and the amp are all at 8 Ohms impedance. Is there a formula to calculate the watts being driven by the amp at a specified volume?

    I usually keep the main volume and wave volume on my PC to the highest so that I dont have to increase the volume on my amp too high but like you said a signal could also be clipped. So is it safe to keep the volume too high on my PC?

    Last night I was watching Sin City on DTS and the sub was getting real good exercise. Every gunshot or a punch shook my bed while I was watching the movie. At the same time, I was afraid that I might be driving it too high although the sub volume was set to -2 on a threshold of -10 to +10. The main volume was set at -20 and since the signal was coming through digital SPDIF output, the PC volume controls were not working. When I switched to Dolby Digital 5.1, the sound was numbed alot. I guess the DTS version was deliberately kept at such a high volume. Even the grittiness in the male narration voices were coming from the sub.

    Thank you for bearing with me. I have been an audiphille for a long time but getting a complete 5.1 system has changed things completely for me. So you can guess I'm a newbie at this.

    Hope to hear from you.
  4. Vehementi

    Vehementi TechSpot Paladin Posts: 3,199

    No problem, and this time I just had a Dad's root beer so am in a great mood :)

    Like I said - the number of watts cannot be determined like that. With a music, movie, or some other type of dynamic signal it changes constantly - with the signal. If you're playing a test tone though, that's another story, as it's just a sine wave. If you really want to measure the wattage, hook up a multimeter and use Ohm's Law to figure it out and remember that watts = volts X amps. And keep in mind that 8 ohms is the nominal impedance for your speakers, the DC resistance is altogether different and is typically below the 8 ohm mark, like around 6.5. A speaker's resistance changes with the frequency of the signal it is played, it will typically be lower than the rated impedance in bass frequencies, and steadily rising throughout the audio spectrum depending on the design of the speaker's crossover (if it's a speaker with multiple drivers).

    The 105W output of your receiver is just a nominal rating as well, and should be attached to some kind of THD (total harmonic distortion) rating. On a graph, the THD will steadily increase with the power output, until it reaches the clipping point where it goes exponential. Typically, high end equipment is underrated, and will come with a birthsheet for that specific amplifier. Take a look at this one for a car amp made by Arc Audio:

    [​IMG]

    That amp is rated for 150W but actually puts out close to 180 before it reaches 1% THD (which is undetectable by the human ear at any frequency - and at lower frequencies, such as bass, 10+% THD is already being produced by the speaker, but is undetectable).

    Turn the volume down on your PC, and always try to adjust the volume from the receiver if you can. Keep in mind your amp can theoretically go all the way up to +16 with no problems. You gotta keep that below 100% or you run the risk of sending your amp into clip protect which will shut it down for a few seconds right in the middle of a great action scene :) Like I said around 75% is a good rule to have.

    The particulars of DTS/DD 5.1/whatever the hell else is beyond me, and meant for someone much nerdier :) I go with what sounds best, and in your case, sounds like DTS is.

    Either way, I have no idea how I learned all this stuff, I guess it just took some time of being really into audio :) Best of luck!
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