Amplifier Loudness: Watts and Decibels
This was a “Question of the Day” post from Max in Stanhope, PA. He sent me an email asking about how many watts his amp should be. In his email he told me he was looking to get a new tube amp and was concerned about the wattage. He was savvy enough to be concerned about having an amp that was too loud and would not give him the sound he wanted.
Lets start with a brief definition of watts. The watt is a unit of energy and is named after the 18th century engineer James Watt. There is more technical information about power but lets keep it simple because we are talking about a tube guitar amp not the Space Shuttle..! The Watt is actually just a measure of power. The next part of the scenario we have to look at is the decibel. A decibel is actually two words, ‘deci’ (meaning one-tenth) and ‘bel’ (named after Alexander Graham Bell, the inventor of the telephone). A ‘bel’ is a unit of sound and a deci-bel is one-tenth of a bel…..(now you know)! The decibel is just a measure of loudness. So what do each of these have to do with the tube guitar amplifier?
From the above definitions watts and decibels are two different things. You can have a relatively low powered amp that is very loud and would be able to keep up with a drummer on a stage with no problem. Just because you have more watts does not always mean you are going to be louder and the watts increase is not directly proportional to the decibel increase. Take for example two Marshall heads. Let’s say one is rated for 50 watts and the other 100 watts. By looking at the watts you would think that the 100 watt head would be twice as loud because it has twice the power right? Wrong! The loudness difference between the 50 watt amp and the 100 watt amp is only 3 decibels (dB) which is not really a drastic difference. It would take 10 times the power of the 50 watt head to be twice as loud as the 100 watt head which is 500 watts! Ouch…..!
There is also the issue of speaker efficiency to consider as well. The efficiency of a speaker which is also called “sensitivity” is a measure of the speaker’s output (in decibels) with a specific amount of amplifier power (watts). Most often this is calculated by placing a microphone that is connected to a sound level meter at a distance of one meter away from the speaker. One watt of power is then sent to the speaker while the sound level meter measures the loudness in decibels. The output level results in a measure of efficiency. Another way to look at it is how efficiently does the speaker use the power. For example a speaker rated at 90dB will take twice the amplifier power to be the same volume as another speaker rated at 93 dB efficiency just like in our Marshall 50 watt vs 100 watt example. By the same token a speaker rated at 93dB will need ten times more power than a speaker with a 103 dB efficiency rating to be at the same volume. So you can now see how a relatively low powered amp with highly efficient speakers can be louder than a higher powered amp with much less efficient speakers. I’ve actually seen this situation a few times over the years.
That’s enough of the electronics textbook. Lets get back to our man Max. In a case like this with the question “how many watts should my amp be” you have to start by asking yourself…”What will the amp be used for”? Are you just going to play at home? Will you use the amp for some small clubs and rehearsal? Do you plan on playing a lot of outdoor events? The next question you should ask yourself is “Do I want the amp to sound perfectly clean or do I want it to break up a bit”? This opens up a bit of a can of worms because a Master Volume can tame a loud, high powered amp to sound overdriven at lower volumes. But lets say Max is looking at amps without a Master Volume so it will be easier to make a decision.
You do not need a very high powered amp for at home only use. The good ol’ Fender Champ which is rated at about 6 watts is perfect for home use. Just ask Jason Isadore at myfenderchamp.com about them. Some of the newer low powered amps like the Blackheart’s are perfect for home jamming and some recording. Doing some club work will require some more power though. Get on a stage with a drummer and a few other instruments and you will need to step up the wattage into the 20-40 watt range. This will give you enough volume and punch for many situations. Obviously the 20 watt amp will probably break up a little sooner so if you like that sound that’s the way to go. The 40 watts will give you some more clean headroom and a clearer, cleaner tone. Playing outdoors can be challenging because your 20 watt amp that sounded unreal in the small to mid sized club could potentially sound like a popcorn fart outside. I know, I have been there and do not want to go there again. I had that experience once and now when I play outdoors I bring a pair of 45 watt 4×10 amps and I’m usually totally cool with that. You could afford to move up again to the 50-100 watt range if you are playing large venues and outdoors. Generally speaking a lower powered amp will have to be turned up louder to the volume necessary for a live gig but will be pushed into overdrive and not play too clean. Some players really like this and others do not. It is all subjective.
So there you have it Max. Ask yourself the two questions: “What will the amp be used for”? and “Do I want the amp to sound perfectly clean or do I want it to break up a bit”? Answering these two questions and going by the general guidelines I laid out should make it easier to choose the right amp for you. The next step would be to try the amps you are considering and see which one works for you. Thanks for hitting me up and I hope this helps. As always please feel free to ask me more questions if you have them…..I’m always happy to help!!