How does the number of watts on an amplifier work?

Question

So I am going out to buy a custom surround system, probably a top-of-the -line one.

I read on the description of an amplifier something like "65 watts x 6 speakers + 300 watt subwoofer"

However, I can rarely a satisfactory speaker exactly 65 watts RMS or a subwoofer at 300 RMS.

My question is, can I hook up a speaker that runs at less watts RMS than what the amplifier outputs or should I find the speaker at the exact number of watts the amplifier outputs?

Can I do the opposite? i.e. hook up a 85 watt RMS speaker to the amplifier?

Answer 1

No need to get exact wattage for speakers; however, if they are way off, they can be either underused or in danger to blow up.

For example, buying a 1000 W speaker if the max/speaker is 65 W is a waste of money. A 20W speaker can't also take full advantage of your amp.  

Answer 2

Wattage ratings are tricky. There's no real set standard. A thing to look at in the receiver is the watts per channel and how they've measured it. You'll often see it followed by a THD%. Total Harmonic Distortion is how much the amplifier will distort the electical signal when it's amplified. Typically the lower the THD the better. Some manufacturers are notorious for rating their HTIBs as 140W/channel @ 10%THD. High end receivers will rate theirs at 0.1% or lower. The 140W @ 10%THD might actually rate only 8W @ 0.05% THD.

Another thing is that a 140W/Channel receiver might not actually put out 140W/Channel when they are all being powered. Sometimes the manufacturer will rate the wattage on only one or two channels driven. Such a system, using my 140W example, may only pump out 110W/Channel all channels driven. The spec should list how the wattage measurement was derived.

Speaker ratings can be somewhat misleading also. Another stat to look at is sensitivity. That will be listed in dB and is what the speaker level should be at 1 watt at one meter away. Therefore a 250W 87dB sensitivity speaker will output ~110dB at 250 watts. A 125W 96dB sensitive speaker will output ~116dB at 125 watts. You need to double the wattage to gain 3dB which is double the sound.

The wattage on speakers can be rated as peak or sustained power handling also. Some speakers might be rated at 150 sustained watts and be able to handle brief peaks of 400 watts.

As for amp power compared to speaker power handling, it is generally safer to match a slightly higher rated speaker to a slightly lower rated amp. That way if someone cranks it while you're away they're not going to damage your speakers. If you over power your speakers too much it will make them work harder than they physically can which may cause the motor in the woofer/tweeter to get too hot and burn/melt or tear the woofers or pop the tweeters.

You don't want to under power the speakers though as that can be worse than over powering them. If you had a weak amp (say 40Watts) powering low sensitivity (say 86dB) speakers you would have to turn the receiver up a lot higher which could cause clipping. Clipping is when the top of the amplified wave is cut off. This is bad distortion and can damage some speakers as the wattage of the clipped signal might become higher than what the speaker can handle. It doesn't sound very good either.

Answer 3

You dont worry about matching these numbers.

SPEAKERS: The wattage is the power at which the speakers will be destroyed - not what you try to match.

RECEIVERS: In truth - your receiver outputs about 3-15 watts per channel over the length of a movie.  But many shows have large swings in volume. To increase the volume by 1 db - you have to double the power.  This is why we have receivers that pump out 80-120 watts per channel.

And you can ... use different options to make a receiver seem more powerful than it is.

Short answer: Stick with Dennon, Yamaha RX-V, Pioneer Elite or Sony ES models of receivers. These companies make honest units with honest/conservative power ratings.

POWER:

Ok - when a receiver says it outputs 80 watts - READ THE FINE PRINT. Good manufacturers will state that this power comes out with:

- All channels driven

- With an 8 ohm load (non-inductive)

- RMS power is measured, not just peak

- All frequencies from 20 to 20 thousand

- Total harmonic distortion below 0.07%