How to Measure Amplifier Output Wattage

A Simple Explanation of Watts, Audio Power, and Pissing Off The Neighbors

We’ve all purchased power amps for PA systems, studio monitors, or our cars. We pay top dollar to get as much electrical power (watts) as possible because we want our music louder. Unfortunately, most people who are buying power amps don’t understand how watts work or even what the word “watt” means.

One Watt Can Get Pretty Loud!

To fully understand watts, in audio terms, you need to understand two things.

1. Audio loudness is logarithmic. This means that you have to double the audio power of a speaker system to get a 3dB increase. Do you want more on top of that? You’ll need to double your power again. Want another 3dB? Double it again! Of course, you can also double the quantity of working speakers as well, but that’s another article altogether.
2. One watt is pretty loud already. Take a look at the “sensitivity” spec on a speaker. This determines how efficient the speaker is. In other words, it tells how loudly the speaker will play when given just 1 watt. Yes, a speaker will play just fine with one watt. In these tests, a test microphone is usually placed 1 meter from the speaker. On average, a speaker will hit around 90dB (give or take 5dB which just happens to be a TON!!!!) with just one watt of power. So how loud is 90dB? According to my research, your average push lawn mower is 90dB. Oddly enough, I’ve always worn the best earplugs while mowing for years. So what does that tell you?

Headroom Is Everything When Measuring Amplifier Output

In an ideal world, we’d never run out of gas, milk, or power in our power amplifiers. Unfortunately, unlimited gas, milk, or power does not come cheap.

Depending on our wallets, we have to balance between $$ and headroom (or supply). While we could go out and buy 50 gallons of milk right now, but more than likely 48 and a half of those will be going to waste.

A lot of times audio power is the same way. Basically, I’m saying that if you have unlimited funds for a PA system, go out and buy as many 50,000 watt amplifiers as you need. This will ensure that you never run out of power. Of course, this is 100% total overkill!

You see, if you are playing your music at 96db (Not as loud as a band, but definitely loud enough to piss off the neighbors) with just one speaker with a sensitivity of 90dB @ 1 watt, you need to double your wattage to get up to 93dB. So we now have 2 watts. We need to double the wattage again to get to 96dB. So it looks like we need 4 watts to hit 96dB with our hypothetical speaker.

Back to the headroom concept. If a person knew they would never play louder than 96dB, it would be an easy decision. They could stick with the 4-watt system and piss the neighbors off all day long.

However, especially with PA systems or studio monitors, there is always that chance that we’ll need a few extra dB of power. Maybe there is just one huge bass hit just one time. If we don’t have enough power, we’ll distort for sure, but we may have way bigger problems (read on).

Because people always want to have plenty of power and don’t want to chance running out, they buy 2,000-watt power amps. I can tell you right now that 2,000-watt power amps probably won’t sound noticeably louder than 1,000-watt power amps.

Why? One, because of the way they are used. You will get an extra 3dB of headroom from doubling your power to 2,000 watts, but that’s only a 3db increase. Read on. You’ll see that a compressor or limiter can do a lot more good.

If you are using a 2,000-watt amplifier, let’s see how loud we can expect to go with our one speaker with a 90dB sensitivity.

• 1 watt = 90dB
• 2 watt = 93dB
• 4 watt = 96dB
• 8 watt = 99dB
• 16 watt = 102dB
• 32 watt = 105dB
• 64 watt = 108dB
• 128 watt = 111dB
• 256 watts = 114dB
• 512 watts = 117dB
• 1024 watts = 120dB
• 2048 watts = 123dB

Well, it looks like our 2,000-watt power amplifier will handle a tad under 123dB. Well, guess what! My research tells me that 120dB (which is an RMS type of sound) is the volume of a “Painfully loud jet plane” …..AND WE WENT AHEAD ADDED 3 MORE dB!!

If we just one hit of a snare drum hitting 123dB and then the rest of the band was still sitting at 117dB or so, the extra headroom would be almost meaningless. Of course, jumping from 1,000 watts to 2,000 watts certain will cost you some $$.

A Compressor Or Limiter Can Be More Powerful Than 1,000 Extra Watts

We are talking about how a 2,000-watt amplifier can handle a spike of 123dB. Well, a spike is like a transient like a kick drum or snare drum hit.

What if we could compress or limit the entire mix to smash down those spikes and then bring up everything else? With hard limiting, we can ensure that we never have out of control spikes jumping out of nowhere.

By doing this, we don’t need nearly as much power because we’ve reduced our need for headroom. However, our system will sound much louder because all the RMS stuff (constant signal like a guitar or bass) can be cranked louder.

(It’s important to note that one or two loud spikes that are limited probably won’t even be noticed, but will allow you to be much louder with less power!)

This is the same basic concept used to master modern CDs. All CDs have a limit of 0. (They use negative numbers with digital for some reason). So if your cd sounds too quiet, it’s probably because you’ve got a few peaks that are forcing all the other signals down. A limiter or compressor is generally used to correct this (although most of the magic is in the mix.

50 Watt vs 100 Watt Guitar Amplifiers Is A Crock

I’ve heard people say that they really want to cut the volume of their guitar amplifier, so they take out two tubes or they flip a switch which takes out two tubes via a switch.

Well, guess what. These people are full of crap. If you can’t take a 50 Watt amplifier with a 4×12 cabinet and blow everyone’s ears all to hell, then something is wrong with your amp.

50 Watt guitar amplifiers are incredibly loud. Theoretically, a 100-watt tube guitar amp should only be about 3dB louder. 3DB is certainly louder, but it’s not like going from a whisper to a scream…not even close!!! It’s more like going from “really painfully loud” to “painfully loud”.

Actually, the real reason that people prefer 50 watts or 100-watt amplifiers has a lot more to do with tone. 50-watt guitar amplifiers typically sound quite a bit different than 100-watt guitar amplifiers.

100-watt guitar amps tend to be thicker, but possibly duller sounding. 50-watt amps tend to be brighter and possibly a tad thinner. Enough about guitar tone, that’s a whole other article.

What If I Don’t Have Enough Power?

Most people don’t understand that one of the leading causes of blown speakers is not using enough power. When you crank up your music, you should imagine a wave going up and down inside the amplifier.

Unfortunately, your wave can only get so big. There is a maximum limit. (Electronics nerds call this “compliance”). Anyway, this the max an amplifier will spit out without “clipping”.

Imagine a nice pretty sine wave. It’s pretty as long as it’s not smashing the ceiling (or max limit) of the amplifier. If you turn the music up even louder, the wave will get bigger inside the amplifier. Then, the wave will smash the ceiling. Unfortunately, the ceiling in an amplifier doesn’t give.

It’a brick wall. The result is the top and bottom of waves get smashed in and turn into a horizontal line. The resulting signal looks like you simply clipped the bottom and top of the wave off.

This is what a distorted signal looks like. A 100% distorted signal is called a square wave. Instead of being a pretty, flowing sine wave, it looks like jagged teeth. Of course, you don’t care about that. What you care about is that clipping in a PA system is extremely audible distortion! It sounds wretched!! Even worse, you will blow your speakers!

How Clipping Blows Your Speakers

The interesting thing about clipping is that you can take a 10-watt amp and blow just about any speaker by playing a clipped signal long enough.

Imagine our pretty little sine wave again. It flows up and down in a smooth way. This wave actually represents how the speaker is moving in and out.

Well, if we clip this wave hard enough to turn it into a jagged, square wave, the speaker must follow this path. This means the speaker jerks in quicker than usual and must hold it. The speaker jumps out quickly and has to hold it.

This “holding” is kind of like lifting weights where you hold the weight in a certain place just to feel it burn. Of course, in weight lifting, this burn is a good thing.

It’s working your body harder. However, in our audio situation, that burn is also working the speaker harder….a lot harder!!! It doesn’t take long of a speaker jumping out….holding….jerking in…holding for the speaker to overheat and blow. Speakers just weren’t designed to function in this way.

So make sure that you have enough power (or compress your signal more) to ensure that you never exceed the compliance (max ceiling or headroom) of your amplifiers.

Conclusion

The worst thing you can do is clip your amplifiers. The biggest boys can just buy mega-powered speakers that will never run out of power. That, however, is not very $$$ efficient.

My recommendation is to use a compressor and/or limiter to smash them out of control transient peaks. You simply don’t need as much headroom then. The harder you compress your signal, the more RSS volume you can safely squeeze out of your amplifier.