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Secrets of Amplifier and Speaker Power Requirements Revealed

138K views 179 replies 49 participants last post by  Taylorectx123 
#1 · (Edited)
Secrets of Amplifier and Speaker Power Requirements Revealed

As audio/video hobbyists, most of us grew up thinking that if we have an amplifier with 50 watts of rated output power into 8-ohm speakers, and that combination produces reasonably clean and loud music, then by doubling the amplifier power to 100 watts per channel, the system would then play twice as loud. Many readers likely still believe that. Not so.

Although it's not the easiest thing to comprehend, doubling the amplifier power does not double the loudness. In the above example, the sound from the speakers would not be "twice as loud"; it would only be "a little louder," an increase of 3 decibels. How loud is that? Hearing tests with large groups of people have revealed that a one-decibel (1 dB) change in loudness is approximately the smallest audible step that the average listener can detect, so an increase of 3 dB most listeners term "slightly louder."

So why doesn't that 100-watt amplifier always sound twice as loud? Because the acoustic decibel--the decibel (dB) being the unit of measurement used worldwide to quantify the acoustic loudness of sound--has a peculiar relationship to amplifier power output measured in electrical watts. That relationship is called "logarithmic." If that word gives you an instant headache (nightmares of high-school math), then here's a simpler explanation:

If a sound gets louder by 3 decibels or "slightly louder," it takes twice as much electrical power from your receiver or amp to produce that modest increase. Therefore, a 100-watt amplifier will produce sound only slightly louder than a 50-watt amplifier.
Incidentally, if you'd like a kind of immortality, be terribly clever and work out a system of measurement. It may be named after you. The "decibel," one tenth of a bel and named for BellAlexander Graham Bell, recognizes his contributions to the understanding of sound. Likewise, we have to thank James Watt, Georg Simon Ohm, and Heinrich Hertz for their contributions to the industry. And then there's the Lofft, a measurement of neighbors' tolerance to testing new speaker systems . . .

So far, so good. But what if it's party time, and you're listening to music "very loud," a level defined as about 90 dB Sound Pressure Level (SPL), and your speakers are gobbling up swings of 15 to 20 watts per channel on those musical peaks.

Drink in hand, you advance to the volume control on your receiver thinking, "I'll just crank this up to make the music twice as loud," and you turn up the volume control until there's a 10 dB increase in the sound level. Now your party-time goal of "twice as loud" will make huge electrical demands on your nice little multi-channel receiver or power amp. The receiver must deliver ten times as much power to double the subjective loudness. Between 6 dB and 10 dB is double the volume level, where 6 dB is four times the power and 10 dB is 10 times the power. In the aforementioned example, the amp must produce 150 to 200 watts per channel for those peaks in loudness. Therefore, every 10-dB increase in acoustic loudness--from 80 dB to 90 dB, or 90 dB to 100 dB--requires ten times as much electrical power in watts.

That's all very well if you have a monster amplifier or multi-channel A/V receiver with huge reserves of power output (most of us don't). If not, watch out. Your receiver or amp may "clip" or distort (or both), which will put a clamp on the output of the amp. When you push your amplifier into overload or "clipping," several things may happen. First, the top and bottom of the waveforms (representing the audio signals) are clipped off, generating distortion. Next, the amplifier's protection circuits are activated, removing those portions of the signal that are causing the overload, generating distortion. And finally, the amplifier's power supply may fluctuate according to the demands of the music signals.

Not everyone is affected by this scenario, of course. Some people (increasingly few, it seems) don't listen to loud music. They like background levels, and with average speakers, background levels demand 1 watt or less of amplifier power. Or they may have very efficient speakers (Klipsch, Cerwin-Vega, Tannoy, and the like) that will play extremely loud using modest amplifiers, the trade-off being a very large degradation in tonal accuracy, a definite harshness, and a complete loss of off-axis performance that accompanies horn-loaded designs. But in many situations, speakers will be damaged and distorted sound will offend many ears.

No discussion of decibels, acoustic loudness, and electrical watts is complete without an explanation of loudspeaker "sensitivity." (Another way to define a speaker's sensitivity is to look at how efficiently the speaker converts electrical power, in watts, to acoustic sound output in decibels.) Let it be said in a general way that speakers are not very efficient or sensitive devices. They need a lot of electrical power input to produce relatively little acoustic output. Nevertheless, speakers do vary quite a bit in sensitivity.

To determine a speaker's sensitivity, we feed the speaker with 1 watt of amplifier power, using a test signal of pink noise, and measure in decibels how loud the sound is at a distance of 1 meter (about 3 feet). A lot of domestic hi-fi speakers measure in at about 89 or 90 dB SPL at 1 meter. Larger speakers, with bigger woofers and more drivers, typically produce greater acoustic output; smaller bookshelf models have to work harder, and their output is typically less, often between 86 and 88 dB SPL at 1 meter.

Placing the speaker in a room helps (the walls, ceiling, and floor reflect and reinforce the speaker's sound), adding about 4 dB to its output. For example, a speaker like Axiom's M80ti has a measured sensitivity in an anechoic chamber of 91 dB SPL at 1 watt at 1 meter. But putting the M80ti in a room raises its sensitivity rating to 95 dB SPL at 1 watt, 1 meter. A 95-dB sound level happens to be "very loud," as most of us would subjectively describe it. And it is--from 3 feet (1 meter) in front of the speaker. But let's move our listening seat back twice as far, to 6 feet. Guess what happens? We instinctively know that sound gets weaker as the distance from the source is increased, but by how much? A formula called the "inverse square law" tells us that when the distance from the source is doubled, the sound pressure weakens by 6 dB. Among sound engineers, there's a common saying: "6 dB per distance double." So at a 6-ft. distance, the M80ti is now producing 89 dB. Now let's double that distance again to 12 feet, a fairly common listening distance. The speaker now produces 83 dB, which isn't all that loud at all. And if you sat 24 feet away, a not uncommon distance in big rooms, the speaker would produce 77 dB SPL.

But what about stereo, I hear you shout. Here's another oddity of loudness and the decibel. When one speaker is producing a level of 90 dB, adding a second speaker playing at the same level only increases the overall loudness by 3 dB! (The loudness does not double!). So the two speakers in stereo produce a loudness level of 93 dB.

So adding a second M80ti will raise the loudness at 12 feet from 83 dB to 86 dB. And don't forget we're still using 1 watt of amplifier power output into Axiom's most sensitive speaker. But how loud are real-life instruments, orchestras and rock bands? Now, while 86 dB SPL is "fairly loud," it's not nearly as loud as what you might hear from a good seat at an actual rock concert or from an orchestra or pianist in a concert hall. A solo grand piano can reach peak levels of 109 dB SPL, a full orchestra and chorus in a concert hall will measure 106 dB, and a rock group, 120 dB SPL. Now let's try and get our peak speaker sound levels to 96 dB, "twice as loud" as our 86-dB listening level. That isn't that difficult because right now we're only using 1 watt per channel to drive the M80ti's to 86 dB. So we'll need ten times as much power, or 10 watts, to reach 96 dB. Big deal. We've got lots more.

But things begin to change, and rather dramatically. Let's push the M80ti's to what we might experience from a solo grand piano, 109 dB. We're at 96 dB with 10 watts per channel. Let's go to 106 dB. So that requires 10 x 10, or 100 watts. Close, but not quite there yet. Just 3 dB more. Remember, we have to double the power for a 3-dB increase in sound level. So 100 watts becomes 200 watts. Yikes! Our receiver has only 110 watts maximum output! We've run out of amplifier power! And what about the rock concert? Let's lower our expectations and aim for 119 dB. Going from 109 dB SPL, which needs 200 watts per channel, to 119 dB SPL (get out your ear plugs) is another 10-dB jump and--you do the math--that requires 10 x 200, or 2,000 watts per channel!

From all this you can see the huge power requirements inherent in reproducing real-life acoustic sound levels in average or big rooms. The M80ti's are tested to levels of 1,200 watts of input power so they come very close. But the truth is that if we are seeking real-life acoustic sound levels in our listening rooms, there's a very persuasive argument for very large, powerful amplifiers. And if your speakers are less sensitive (and many are), then the power demands rise even more dramatically. Sizeable rooms and greater listening distances will also increase power demands tremendously.

And what many of us don't realize until we hear it, is that clean undistorted loud sound often does not sound that "loud." The key here is that in most or our home listening, there are small amounts of distortion caused by a lack of dynamic headroom (but more on that next month). It's the distortion that makes it sound "loud" in a domestic setting. To remove those distortions and increase dynamic headroom relates to even more power. We've become accustomed to accepting some distortion with our reproduced music, because all amplifier's distortion ratings gradually increase as they approach their output limits or slightly clip the audio signals. When that happens, we turn down the volume, because distortion starts to intrude on our listening pleasure, and it sounds "too loud."

The lesson in all this is that you can never have too much power, and that big amplifiers rarely damage speakers. Little amplifiers driven into clipping burn out speakers. In the scheme of high fidelity, that last barrier to realism is having enough power and being able to approximate real-life loudness levels. The lesson in all this is that you can never have too much power, and that big amplifiers rarely damage speakers. Little amplifiers driven into clipping burn out speakers. In the scheme of high fidelity, that last barrier to realism is having enough power and being able to approximate real-life loudness levels.
Read and learn ^^^^ from Alan Lofft
 
#52 ·
While you were in Meridian did you do any partying with Max? He's a good time. I spent a bunch of time down there too. Did a LOT of peavey service and was one of the few that did mass repairs on the DPC stuff. Can still repair those damn things in my sleep.

It was not uncommon to use a "hole" for the vent. Hell, I designed an enclosure recently that never transpired that had just the same thing. Looked damn good on paper too.

Not many pro subs go much below 40, the Q-Wave stuff does now.... Not much below. Hell, the SP series subs did not get much below 50!

I worked a bit in the field development of the BWX series. Broke 4 at a time without the coil looking anything but brand new. Then they got the suspension issues worked out :D
 
#53 ·
OK - A little more OT!


"Max" rings a bell - it was in the mid 90's. Our regional Peavey Rep came down there with me - can't remember his name ATM. I remember Marty McCann (spelling) giving most of the lectures. Spent a lot of time in the fish bowl and auditorium jamming, too :)

Agreed on the SP118 subs - useless below 50Hz :) Their BW suspensions always seemed to be a limiting factor to me, too. The coils can take the power for sure, and they make a killer "Woofer" - but Peavey makes their compromises - they rob Peter (low bass) to pay Paul (efficiency and low cost). I much prefer my JBL 2241G's where Peter and Paul both end up rich (by robbing my wallet!) :p Peavey's BWX and Low Riders seem to be a step in the right direction. It's cool that you were involved with their development IMNSHO...

Back to the "Power" talk :smash:

:cool:
 
#54 ·
#55 ·
Odd - I'm not sure if I agree with all of R.C.'s findings with regard to clipping and how the energy is dispersed through the octaves. He appears to be talking about clipping a single low-frequency sinewave, and is not taking into account clipping complex full-range waveforms. He seems to suggest that clipping a full-range system WILL NOT blow tweets - but we know that is not the case.

JBL certainly states otherwise:

http://www.jblpro.com/pub/technote/lowpower.pdf

I dig and respect R.C. and all, but I'll believe JBL/Harmon (and my own Live Sound experience) well before some car audio buff :)

And even though the clipped harmonics are distributed throughout the frequency range, when you clip a sub amp that's actively crossed, ALL of this energy still goes to the Woofer's coil (the woofer has no passive x-over to filter this energy out). Plus the much higher "duty-cycle" of the clipped waveform further increases RMS Power...

:cool:
 
#57 ·
That makes more sense IMO. So it's mainly that the full-range amp can keep increasing the HF content's power (since it is already relatively small compared to the bass) when the user is slamming the low-frequency spectrum into clipping. As long as the woofer can handle the total RMS heat of the clipped amp (generally a max of 2x clean rated RMS power), the woofer will keep on chugging just fine (but sound like ass). That combined with the small increase in harmonics due to clipping the woofer, and your tweets are toast just as JBL mentioned above...

:cool:
 
#60 ·
That makes more sense IMO. So it's mainly that the full-range amp can keep increasing the HF content's power (since it is already relatively small compared to the bass) when the user is slamming the low-frequency spectrum into clipping. As long as the woofer can handle the total RMS heat of the clipped amp (generally a max of 2x clean rated RMS power), the woofer will keep on chugging just fine (but sound like ass). That combined with the small increase in harmonics due to clipping the woofer, and your tweets are toast just as JBL mentioned above...

:cool:
I don't see how that's possible if your tweeters have proper filters in place. Again, if you are fully saturating the channels that feed your tweeters, creating an excessive clipped state, then this only seems to point to inadequate application.

If you're not clippin' ur just trippin'. lol
 
#61 ·
Found this thread looking up "headroom". The concept just doesn't make sense to me. Aren't your speakers going to distort at the same wattage whether its from a 100 wpc amp or a 200 wpc amp? For example, if the speakers are rated at 60 wpc and they distort at 80 wpc, wouldn't a 200 wpc amp be pointless? I'm probably oversimplifying it or I just have it all wrong, but can someone please explain? I'd really like to know if I should buy that 200 wpc amp :)
 
#89 ·
Found this thread looking up "headroom". The concept just doesn't make sense to me. Aren't your speakers going to distort at the same wattage whether its from a 100 wpc amp or a 200 wpc amp? For example, if the speakers are rated at 60 wpc and they distort at 80 wpc, wouldn't a 200 wpc amp be pointless? I'm probably oversimplifying it or I just have it all wrong, but can someone please explain? I'd really like to know if I should buy that 200 wpc amp :)
This would have to assume that the speakers resistance stays constant, which we know is not the case. Put100 watts of power to a sub playing sine waves at the subs resonant frequency and then do the same with sine waves playing 2 octaves north of that and see which sounds like it distorts more.

With that said, with amplifiers you should always buy the amp that gives you the most power for the dollar. Obviously quality build and things like that come into play, but in general you should always get the most bang for the buck when it comes to amps. Headroom is always a huge plus, not the fact that one doesn't want their amp to clip at all.
 
#68 ·
My thinking is inline with yours. A larger amp leaves room to grow and it isn't going to damage anything is you use restraint. I'd rather have clean sound than have an amp that is straining and starts to sound harsh when it is reaching its limit. Even though the average power being used isn't much, the peaks in music can be many times greater. As an example, the LED meters on my home reciever go to 200 watts. If I have it cranked up pretty good, the average power being used may only be a few watts, but the 200 watt lights flicker from time to time.
 
#63 ·
I'd like to bring focus back to the article vs. the subtleties we're slowly introducing....

you can never have too much power... ...Little amplifiers driven into clipping burn out speakers.
Obviously there's a lot of variables to mention, debate, or argue...but I don't see anything wrong with the conclusion?

Drive a small amp into clipping hoping for something amazing; well, you'll get something amazing :) I don't think it means just a little flicker either...I think it's talking about grandma's boy messing with his 16th birthday present.
 
#64 ·
I thought the variables was plain to see, just that some went on a tangent which diverted the flow of things. Basically, you're better off with a bigger amp under control than a smaller amp driven beyond it's limits given that it's clipped power is still enough to overdrive the speaker. Correct?
 
#69 ·
Everyone wants amps that are a bazillion watts, have small footprints, cost only a few bucks, and don't draw any current from the electrical system. But that doesn't really help anyone make decisions about what to buy. The power discussion is VERY relevant. Because the reality is that most people *DO* clip some of their channels some of the time, so it's important to know what it means and whether it's important to spend extra and dedicate more space to other/bigger amps.
 
#70 ·
That's what I'm saying. If your speakers clip at 80 wpc then won't a 100 wpc amp suffice? Wouldn't a 200 wpc amp be wasted? Obviously there's the 'ability to change gear factor' to consider. But, what if you aren't going to change anything? Doesn't that make the "headroom" point mute?
 
#72 ·
its the "green is greener on the other side" syndrome. you have a nicely set up system but you just know that is you replace the tweeters with X brand it will sound 100x better.
 
#73 ·
Or you could be like me and want a simple upgrade that has more flexibility and more power over dinosaur amps that are bulky and limited on power. But I suppose that doesn't put that in the constantly trading category...

However, I'm sure bad decisions as well as forum boners account for most. Heck it took me I don't know how many years to finally switch to an all class D setup while I've sat back and watch a lot of these guys go through them like soiled underwear.
 
#78 ·
Not enough to be anal about if it is used... :D

But seriously, I'd rather experiment with different drivers periodically than to keep switching amps so getting one with enough power to be flexible only makes sense. Now if you keep switching amps to suit every driver you play with then that would be more wasteful than having extra power you don't use IMO.
 
#87 ·
Why would one need to keep switching amps to suit different speakers?! How much extra power do you think that 200 watt amp is going to provide you over the 100 watt amp you currently own? Seems like we have gotten way off target with the main point in dispute:

"clipped" sound waves DO NOT cause damage to speakers themselves. Excessively "clipped" sound waves, which contain significantly more power output than their "non-clipped" or "lightly clipped" sound waves, is what can lead to thermal or mechanical failure in a speaker. This is obviously dependent upon the power of the amp in question, since fully saturating and "clipping" a 1 watt amp will do nothing to any speaker, tweeter or otherwise, that is playing that clipped signal.
 
#79 ·
#92 ·
Is the "lite clipping" you are referring to audible as distortion or in any other way?

I agree with your statement about wasting money on a bigger amp as a 75 wpc amp driving my components plays louder than I ever listen to it. Truthfully, I think a 50wpc amp is plenty of power but we all want the biggest and best money can buy.

I'm just trying to justify not buying that Zapco z150.6 I really want, if you guys haven't noticed:p
 
#84 ·
I don't believe in the concept of setting your amplifier to not burn up drivers. If you have a problem burning up drivers, you're using the wrong drivers for the job. A properly designed system should be designed to withstand the output level requirements that you need.

If your problem is blowing speakers, you have three options.
1) turn the volume down
2) use drivers that can handle more power
3) restrict the bandwidth that the driver is asked to play (e.g. ask your larger drivers to pick up the slack by shifting your crossover points).

All I'm saying is that yes, clipping will deliver more power to the speakers than if you turned the volume knob down to the point where it didn't clip... DUH. :) Overdriving your amp will produce higher power than not overdriving the same amp. But everybody knows that.

The true comparison is whether your drivers would have lived if you had a bigger amp that didn't clip. The answer to that question is a resounding no (again, with the exception of passive tweeters and other delicate high freq drivers...).
 
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