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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!!

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