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Sensitivity and Resistance

post #1 of 14
Thread Starter 
Hi all,

I've been an appreciator of good audio for years and have very much enjoyed a casual pursuit of what sounds nice to my own imperfect ears. But recently I've been delving into the more technical side of audio simply because it's intriguing and there are a couple things that I wish were a little easier to find good explanations for. This was mostly sparked because I was looking into a pair of Pioneer SP-BS22-LR speakers in order to create a cheap, fun stereo to put in my family room. Some of the reviews for these speakers were quick to point out that they have a low sensitivity of 85db and fairly low impedance of 6 ohms, which they said means that you should feed them more than minimal power. So my big question is this: how do you "calculate" how much power an amp or receiver will give to the speakers? Like, for example, if you were to use a Denon AVR-E200 (just the first receiver I clicked on in their site...), how would you know, given the sensitivity and resistance of the Pioneers, how much power the receiver is ACTUALLY providing? And is there a measure of sensitivity for a reciever as well, or just speakers? I thought maybe that S/N was the equivalent at one point, but now I'm not so sure. I'm just trying to find a good example I can use to complete the "big picture" here, so hopefully I'm not making this too confusing...

Thanks for the help!!
post #2 of 14
Quote:
Originally Posted by ikylerobert View Post

how would you know, given the sensitivity and resistance of the Pioneers, how much power the receiver is ACTUALLY providing?
You can't. First off, you can't measure watts going into the speaker, only volts. Even knowing that you can't easily determine power via Ohm's Law, as the impedance of a speaker is not a constant value, it's different at every frequency. Besides, what does it matter? So long as the receiver is rated to deliver more power than the speaker is rated to handle you'll be able to drive the speaker to full output. Probably more, as speaker ratings are thermal, where the voice coils will burn out, and in real world use they usually reach their mechanical maximum output capability well below the thermal rating.
BTW, 85dB isn't particularly low sensitivity. Beware of claims for much more than 87dB for single woofer speakers, or more than 90dB for dual woofer speakers, they're usually bogus.
post #3 of 14
Thread Starter 
So then you wouldn't worry about hooking up a pair of speakers rated to handle 80 watts @ 6ohms to a receiver that's rated at 150 watts @ 6ohms, for example? And what you're saying is that there really isn't too much of a danger of harming the amp or speakers doing this? I know people all over tell you "you don't wanna burn out your amp" or "you don't want to underpower your speakers". So I'm a little curious
post #4 of 14
The Denon e200 is rated at 75W/ch so you won't be overpowering anything.
Quote:
Power and Processing:

5-channel amplifier
75 watts per channel into 8 ohms (20-20,000 Hz) at 0.08% THD, with 2 channels driven
Dolby® and DTS® surround sound decoding

Whatever 150W rating you saw isn't close to being accurate.
http://www.crutchfield.com/p_033AVRE200/Denon-AVR-E200.html?tp=179&awkw=52150898545&awat=pla&awnw=g&awcr=22742609185&awdv=c#overview-tab
post #5 of 14
Thread Starter 
I was talking in generalities, sorry about that. Should have been a little clearer. I guess what I'm not finding an answer to yet is "when should I start being worried about harming equipment?" I'm starting to feel like there isn't a hard and fast rule for this now...
post #6 of 14
Thread Starter 
And also, is it very noticeable if you give speakers too little power?
post #7 of 14
Quote:
Originally Posted by ikylerobert View Post

Hi all,

I've been an appreciator of good audio for years and have very much enjoyed a casual pursuit of what sounds nice to my own imperfect ears. But recently I've been delving into the more technical side of audio simply because it's intriguing and there are a couple things that I wish were a little easier to find good explanations for. This was mostly sparked because I was looking into a pair of Pioneer SP-BS22-LR speakers in order to create a cheap, fun stereo to put in my family room. Some of the reviews for these speakers were quick to point out that they have a low sensitivity of 85db and fairly low impedance of 6 ohms, which they said means that you should feed them more than minimal power. So my big question is this: how do you "calculate" how much power an amp or receiver will give to the speakers? Like, for example, if you were to use a Denon AVR-E200 (just the first receiver I clicked on in their site...), how would you know, given the sensitivity and resistance of the Pioneers, how much power the receiver is ACTUALLY providing? And is there a measure of sensitivity for a reciever as well, or just speakers? I thought maybe that S/N was the equivalent at one point, but now I'm not so sure. I'm just trying to find a good example I can use to complete the "big picture" here, so hopefully I'm not making this too confusing...!

First off you are asking the wrong question if you want an affirmative answer.

Sensitivity and resistance are the wrong parameters to look at if you want to know about real power and speakers.

The closest thing to the right answer is that you set up a 2 channel audio interface that measures voltage and current. Current is easier than you may think - just stick a 0.1 ohm resistor in series with the speaker and measure voltage across it. Then you run some software that multiplies the two signals at points in time throughout the measurement interval. Add some calibration and voila, watts. I seem to recall that a commercial software package called Spectra Lab will do this.

Problem is your measurement will yield zero unless you apply a signal, which begs the question of what signal to apply. Give you a hint - you will get a different number for every different signal. Ouch!

I think the question you actually want to know the answer to is how do I know that my amp will drive this speaker loud enough and clean enough. The answer is that depends on you and your listening room. That is one parameter that I will happily admit is subjective, which is what is the right loudness for you?

Rule of thumb is that you can probably hit reference levels if you have speakers with average efficiency, sit less than 12 feet from the speakers and have a 100 wpc stereo amp. With 85 dB/W speakers you might be 2-3 dB shy of reference levels which is not the end of most people's life. Reference levels are loud for a lot of people, even too loud.
post #8 of 14
Thread Starter 
Quote:
Originally Posted by arnyk View Post

First off you are asking the wrong question if you want an affirmative answer.

Sensitivity and resistance are the wrong parameters to look at if you want to know about real power and speakers.

The closest thing to the right answer is that you set up a 2 channel audio interface that measures voltage and current. Current is easier than you may think - just stick a 0.1 ohm resistor in series with the speaker and measure voltage across it. Then you run some software that multiplies the two signals at points in time throughout the measurement interval. Add some calibration and voila, watts. I seem to recall that a commercial software package called Spectra Lab will do this.

Problem is your measurement will yield zero unless you apply a signal, which begs the question of what signal to apply. Give you a hint - you will get a different number for every different signal. Ouch!

I think the question you actually want to know the answer to is how do I know that my amp will drive this speaker loud enough and clean enough. The answer is that depends on you and your listening room. That is one parameter that I will happily admit is subjective, which is what is the right loudness for you?

Rule of thumb is that you can probably hit reference levels if you have speakers with average efficiency, sit less than 12 feet from the speakers and have a 100 wpc stereo amp. With 85 dB/W speakers you might be 2-3 dB shy of reference levels which is not the end of most people's life. Reference levels are loud for a lot of people, even too loud.

Okay, so my original question now appears to be a nearly irrelevant one. So then maybe a simpler question is, again, do I risk damaging equipment by going over the theoretical power ratings for the speakers? Thank you for the helpful answer, by the way!
post #9 of 14
Quote:
Originally Posted by ikylerobert View Post

do I risk damaging equipment by going over the theoretical power ratings for the speakers?
Yes, but they'll sound really bad with high level distortion long before you reach that point. It's when you ignore the warning sound of distortion that you damage speakers.
Quote:
And also, is it very noticeable if you give speakers too little power?
There's no such thing as too little power. They simply won't go as loud. However, if you have a small amp and try to get too much out of it that will create just as much, if not more, harmonic distortion as driving the speakers too hard. That harmonic distortion can harm tweeters. Insuring that you won't have this issue is why you should size the amp power rating at or more than the speaker power rating.
post #10 of 14
Thread Starter 
Quote:
Originally Posted by Bill Fitzmaurice View Post

Yes, but they'll sound really bad with high level distortion long before you reach that point. It's when you ignore the warning sound of distortion that you damage speakers.
There's no such thing as too little power. They simply won't go as loud. However, if you have a small amp and try to get too much out of it that will create just as much, if not more, harmonic distortion as driving the speakers too hard. That harmonic distortion can harm tweeters. Insuring that you won't have this issue is why you should size the amp power rating at or more than the speaker power rating.

This was extremely helpful, thank you very much! That's kind of a load off of my mind. So given this information on power handling, is resistance more or less of a factor to worry about? Like, if a receiver is only rated as low as 8 ohms but you have speakers that are rated at 6?
post #11 of 14
Quote:
Originally Posted by ikylerobert View Post

This was extremely helpful, thank you very much! That's kind of a load off of my mind. So given this information on power handling, is resistance more or less of a factor to worry about? Like, if a receiver is only rated as low as 8 ohms but you have speakers that are rated at 6?
It's not resistance, it's impedance, and the speaker impedance must not be lower than what the receiver is rated for. Most high end receivers are rated for 4 ohms, but many low and middle range receivers are only rated for 6 ohms, so you could not use a 4 ohm speaker with those.
post #12 of 14
Quote:
Originally Posted by ikylerobert View Post

Quote:
Originally Posted by arnyk View Post

First off you are asking the wrong question if you want an affirmative answer.

Sensitivity and resistance are the wrong parameters to look at if you want to know about real power and speakers.

The closest thing to the right answer is that you set up a 2 channel audio interface that measures voltage and current. Current is easier than you may think - just stick a 0.1 ohm resistor in series with the speaker and measure voltage across it. Then you run some software that multiplies the two signals at points in time throughout the measurement interval. Add some calibration and voila, watts. I seem to recall that a commercial software package called Spectra Lab will do this.

Problem is your measurement will yield zero unless you apply a signal, which begs the question of what signal to apply. Give you a hint - you will get a different number for every different signal. Ouch!

I think the question you actually want to know the answer to is how do I know that my amp will drive this speaker loud enough and clean enough. The answer is that depends on you and your listening room. That is one parameter that I will happily admit is subjective, which is what is the right loudness for you?

Rule of thumb is that you can probably hit reference levels if you have speakers with average efficiency, sit less than 12 feet from the speakers and have a 100 wpc stereo amp. With 85 dB/W speakers you might be 2-3 dB shy of reference levels which is not the end of most people's life. Reference levels are loud for a lot of people, even too loud.

Okay, so my original question now appears to be a nearly irrelevant one. So then maybe a simpler question is, again, do I risk damaging equipment by going over the theoretical power ratings for the speakers? Thank you for the helpful answer, by the way!

The frank answer is that you risk damage to your speakers every time you play them. Its a matter of more risk or less risk...

IME if you hook a 10 watt amp to speaker that is rated at 200 watts, you almost certainly won't be able to hurt it.

IME if you hook a 100 watt amp to speaker that is rated at 200 watts, good chance you won't be able to hurt it.

IME if you hook a 1,000 watt amp to speaker that is rated at 200 watts, the wrong part of the wrong recording, a slip of the wrist, and the speaker is toast. If you are very careful, the speaker has a happy life forever.

It is almost like... ...common sense! ;-)
post #13 of 14
Quote:
Originally Posted by ikylerobert View Post

Like, if a receiver is only rated as low as 8 ohms but you have speakers that are rated at 6?

There is one thing about load ratings that needs to be borne in mind is the fact that sine waves are commonly used for testing, while music is generally used for listening. Sine waves are very different from music in a critical area and that is crest factor. Crest factor is the ratio between the peak and average voltage of a signal. Sine waves have a crest factor of 3 dB. Music has a crest factor of from 6 dB to 20 dB. Signals with a higher crest factor make fewer demands on amplifiers, especially the transistor heat sinks, power transformers, and to a lesser degree (IOW you need similar parts no matter what the crest factor is) filter capacitors.

Therefore an amplifier that would overheat on the test bench while driving a test load resistor of say 4 ohms and while playing sine waves, will be capable of playing without overheating if we change from sine waves to music. I've actually done this experiment, and theory equals practice.

The crest factor of music means that an AVR that can operate 2 channels at a certain power level and laod impedance with sine waves, may be able to operate 7-8 channels with music.
post #14 of 14
Thread Starter 
Thanks all for your help!! I have a little better of an idea of how all this stuff works now
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