trying to understand power...

[Ryanschmidt224]

how do you determine how many watts are being pushed to the speakers on a tower?
If i am running a 50W RMS speaker and have a 75 watt/channel amp, how do i know at what point i'm pushing 50W through the speaker. I try and avoid asking broad questions like this on a forum but i couldn't find any good information on google, I may just not know what to search for... Links would be appreciated. I'm very new to building stereos, so i am trying to learn as i go.
Thank You,
Ryan Schmidt

[sofaspud]

As far as I know the only way to tell is to measure it. The 50W RMS isn't a special number, where 49W is fine and 51W will cause the speaker cone to catch fire and implode. If you don't crank the amp to maximum or send a distorted signal to the speakers, I wouldn't worry about it much. Then again, I wouldn't try to test it by running it hot for extra long periods either.

[Michel]

Ryan,

I have been asked this question countless times by people trying to make sure they would not burn their speakers with an amplifier having a power rating higher than the speakers. They usually hated my answer because at first glance it makes no sense.
here it is:

The higher the power of the amplifier (to a point), the less chance you have to burn your speakers.

I told you it would make no sense but here is how to make sense of it:

What usually burn speakers is clipping distorsion and this usually occurs when you are using and amplifier that is not powerful enough for what you want it to do.
When an under powered amplifier is pushed too hard it will generate square waves which are extremely harmful to your speakers. The first one to go will be the tweeter as it is more fragile than the woofer and if you push your amp harder it can even generate DC currents which can destroy your woofer and some times even parts in the cross-over.

Trying to match your speakers and amplifiers by power ratings is not the way to go. Choose your speakers by how they sound and make sure you use an amp that generates enough power for the listening levels you want to attain. Even if your amp has a rating of 75 W per channels and you speakers are rated for 50 you should not run into problems.

[Ryanschmidt224]

Thank you michel,
I have read into that and after a bit it made sense, what i dont understand is what is the purpose of a speaker having a 50rms if i can push 100W through it?
Will 50W give me optimal sound quality?

[Michel]

Music is made of a complex set of different frequencies mixed together which are never stable in time and vary constantly. When they put a rating on a speaker or an amplifier it usually is done with pure and constant tones which do not really happen in real life situations. I must admit it may happen more with todays electronic music but I am sure you get my point.
I said usually because as time goes by, some manufacturers have twisted ways to measure or probably invented those ratings to please or attract unaware consumers that they have become in many cases, useless.
Even when they say the whole truth and you have a 100 W amplifier and a 50 W speaker, the possibilities you have to blow a speaker is extremely remote. There is no definite answer as to when it is too much, you have to use your common sense and if your speaker has too much on its plate it will distort and you will be able to hear it and react accordingly by lowering the volume control to a more sustainable level. Even then, I would not be surprised that the amplifier was distorting, not the speaker.

[Ryanschmidt224]

Thank you,
That helps out alot.
Ryan

[bunkie]

The first thing you need to understand is that amplifiers don't "push" power, speakers draw it from the amp. A speaker is an electrical load. Think of it this way: if you want to push a box along the floor a given distance in a given time (the definition of power, by the way), the heavier the box, the more force it will take to do the job.

There are a number of factors here. The largest one is how loudly are you asking your speakers to play? That's equivalent to how quickly you want the box to get from point A to point B. It's represented by the voltage of the output signal. Louder equals higher voltage. The second factor is the impedance of your speakers. This is the load. Ohm's law tells us that voltage, impedance and the amount of current the amp can supply are all related. Power is determined by two different equations. The first relates power to current and voltage: Power = current time voltage (P=I*V). The second related Power to current and impedance: Power = current squared * impedance (P= I**2 *V). As the output voltage rises into a given impedance, the current must rise as well. This the the limiting factor of all amplifiers. How much current can they supply? If the demand (defined by the output voltage and the impedance of the speaker) exceeds the available current, you get clipping. Clipping is very bad for speakers especially tweeters as it concentrates more output current in the higher frequencies. This is why underpowered amps kill tweeters. It doesn't take very much current to kill a tweeter. A more powerful amplifier can respond to the higher demand without clipping.

Of course, with more power come more responsibility. It is easier to damage woofers with bigger amps. But, again, it's a function of demand (volume). The real danger is transient peaks. Back in the turntable days, a really big amp turned up very loud combined with an errant drop of the needle on the disk could easily break a woofer by sending lots of current through the speaker.

[Lynn Olson]

"Speaker Power" is usually a number picked out of thin air, with a fair amount of input from the marketing staff. A much more meaningful number is efficiency (stated in XXdB/meter/watt, or occasionally XXdB/meter/2.83Vrms).

Unfortunately the efficiency number goes over the head of most buyers, and the knowledgeable ones might decide against a speaker if the stated efficiency is too low. Even though it's an informative spec (typically, the most informative one published), it's kind of a lose-lose situation for the manufacturer, so it's often omitted, or if published, exaggerated by 2~3 dB so the speaker seems more efficient than it really is. (Tube-amp owners are the ones that care about efficiency, since tube amps usually have less power than even modestly rated home-theater receivers.)

So most manufacturers pick a likely number that more or less lines up with the most popular amplifiers - so, a very typical "power rating" for a speaker aimed at consumers (not prosound) is "recommended power, 75 to 200 watts". Not by accident, this covers most home-theater receivers and audiophile transistor amps. The buyer is happy, and the manufacturer is happy too.

The reason the power rating has little meaning is pretty simple: music has a very wide dynamic range. Even heavily compressed MP3's have a peak-to-average ratio of 6 dB (that's a 1:4 power ratio), and uncompressed music is around 14 dB (that's a 1:25 power ratio). So the brief peaks - those things that clip the amplifier and make it sound gritty and harsh - are much, much louder than the average level of the music.

If you turn the amp up to "eleven", party-style, the peaks will all be clipped, and maybe some of the average as well. If the listener is drunk enough, they won't notice the clipping. They might even like it. A few seconds of heavy clipping will take out most tweeters, unless there is some kind of auto-protect circuit in the crossover. Keep it up long enough, and the other drivers will fail too, starting with the mids, and ending with the woofers. The resistors in the crossover at that point will be a charred ruin, and the circuit board won't look too good either. Maybe the cabinet would be worth saving, but the rest would be gone.

Notice that it wasn't power that killed the speaker, but gross abuse - a power amplifier run into heavy clipping for an extended length of time. In practice, when a manufacturer gets a speaker return with a burned voice coil, or worse, burned crossover components, the warranty claim is disallowed, and it's up to the consumer to buy a set of new drivers and/or crossover components.

Since a low-power amplifier is likely to clip earlier than a high-power monster, the speaker might actually do better under stress with the bigger amplifier, although in terms of dB, an amplifier with twice the power is merely 3 dB louder, barely noticeable when the owner is in a party mood. When the owner is bound and determined to play it as loud as possible, they almost always turns up the amp until it is heavily clipping, so the speaker gets whacked anyway. As mentioned above, it isn't really the power, but the clipping that does the damage, but this is something that manufacturers have told the public for thirty years and the message still hasn't sunk in. Most people the world over associate the "cranked" sound with heavy clipping, so that's what they do.

P.S. If buyers really cared about how loud their speakers played, they'd seek efficient speakers, since every 1 dB increase in efficiency means 23% less amplifier power is required for the same sound level. Unfortunately, more efficiency also means the speaker gets bigger - sometimes a lot bigger - and also more expensive. Watts are cheap these days, so consumers buy big amps instead. I've been to plenty of hifi shows where the amps were bigger than the speakers.

[ScottG]

Originally Posted by Lynn Olson

"Speaker Power" is usually a number picked out of thin air, with a fair amount of input from the marketing staff..

LOL - so much it hurts, and all because it's SO TRUE.

[Lynn Olson]

What the market really needs, even though audiophiles would all sneer in four-part harmony, is a "crank" control, marked zero through eleven. This would mimic the sound of a guitar amp as it is progressively pushed further into overload. A guitar-amp simulator has a much friendlier harmonic structure than the square waves transistor amps put out when they reach the limit, it would protect the transistor amp from ever reaching the brick-wall of hard clipping, it would protect the tweeter as well, and, last but not least, it would sound way louder! Most consumers would love it! Just what they always wanted - a party knob!

But audiophiles and the dim-witted reviewers would be horrified. Thanks to them, we ain't got no tone controls, even though we need them just as much as we did back in the Fifties and Sixties. (What? Recordings are better now? Surely you jest!) Since the "cranked" sound is so popular, why not give people what they want, without the burnt offering of all those drivers, crossovers, and power transistors?

This might sound tongue-in-cheek, but I'm serious. Why shouldn't XYZ transistor amp with all those Class AB watts also have the provision to go insanely loud without taking out the speaker? With digital trickery almost free these days, why not?