WTF is the Difference Between a 50 Watt and a 100 Watt Amp?

Besides the obvious difference of 50 watts, the answer I am thinking of is 3db.  Taking for granted that all else is equal, a 100 watt amp’s perceivable volume will be 3db louder than a 50 watt.  Why does this matter and why did I care to learn this?  A while back I thought I was in the market for a 100 watt Marshall, it was the foundation for my favorite guitarists’ tones.  I quickly realized that in order to get their tone with a 100 watt Marshall I would quickly become very unpopular with the neighbors.  The part that really confused me was that when I tried the 50 watt version of the same amp I could barely tell the difference in volume.  Up to then I thought 100 watts was twice as loud as 50 watts, turns out I was wrong.

What is the relationship between watts and loudness?  Here is a breakdown of what I found practical from a guitarist perspective:

• The intensity of the sound coming from a guitar amp, we often refer to as loudness or volume, is technically called sound pressure level (SPL).
• SPL is measured in decibels (db).
• Every time the SPL increases by 10db, it’s perceived volume is doubled. 120db sounds twice as loud as 110db and 130db sounds twice as loud as 120db.
• Every time the SPL decreases by 10db, it’s perceived volume is halved.  110db sounds half as loud as 120bd.
• The smallest change in db the average person can detect is 3db.
• Doubling an amp’s watts will increase its SPL by 3db, halving its watts will decrease its SPL by 3db.

I could now see that the difference in loudness going from a 100 watt amp to a 50 watt amp is a reduction of 3 db, or just audibly quieter.  For an amp to actually sound half as loud as a 100 watt it would have to be 10 watts.  Conversely, if you want an amp to be twice as loud as a 50 watt it would need to be 500 watts.

One of the variables that will also affect the SPL of an amp is the sensitivity of the speaker it is driving.  If you have ever looked at a guitar speaker’s specs you might have noticed the sensitivity rating, its usually in the range of 95db to 100db.  Look at the specs for a Celestion Vintage 30, you will find it under General Specifications.  This speaker has a sensitivity of 100db, this means that if you send a 1 watt signal to the speaker you will get a measurement of 100db at a distance of one meter from the center of the speaker cone.  A speaker with a sensitivity of 95db would only produce a signal of 95db.  If I have a 100 watt amp pushing a speaker with a sensitivity of 100db and swap the speaker for one that has a sensitivity of 97db, I will have decreased the amps SPL by 3db which is the same as if I were to have halved the amps watts.  This is a good bit of info if you want to drive your amp harder without increasing the volume, you can swap your current speaker for one with a lower sensitivity rating.

Now I will attempt to summarize with a bit of math, break out your scientific calculator if you want to follow along.  If you hate math here is a link to a SPL Calculator I found online.  This calculator has some added parameters such as the proximity of the speaker to a wall and your listening position.  To get the bare SPL input 3.28 feet (1 meter) for the distance and “Away From Walls” for speaker placement.

The formula for figuring out an amps SPL is:

SPL=10*log(amps watts)+speaker sensitivity

example 1: 

100 watt amp and a speaker with a sensitivity of 100db

10*log(100)+100=120db

10 watt amp and a speaker with a sensitivity of 100db

10*log(10)+100=110db

You can see  that there is a 10db reduction going from 100 watts to 10 watts resulting in a halving of the perceived volume.

example 2: 

100 watt amp and a speaker with a sensitivity of 97db.

10*log(100)+97=117db

50 watt amp and a speaker with a sensitivity of 100db

10*log(50)+100=116.9db or round it to 117db

This shows how changing the speaker sensitivity can alter the amps SPL.  Both amps have the same SPL despite having a 50 watt difference in output power.

You may be wondering about multiple speakers, such as a 4-12 cabinet.  Every time you double the number of  speakers you add another 3db of output. Here is an example with a speaker with a 100db sensitivity rating:

1 speaker = 100db

2 speakers = 103db

4 speakers = 106db

It’s not every time you add a speaker, it’s every time you double the speakers.  That’s why it only goes up 3db from two to four speakers, you are doubling the number of speakers.

So a 100 watt amp driving a 4-12 cabinet would look like this:

10*log(100)+106=126db

There are many other variables involved that can affect an amps loudness.  The amp’s output transformer, EQ design,  speaker cabinet design, and room acoustics can play a role in how loud an amp sounds.  This information is provided to give a better understanding of watts, how they work, and to show you how to calculate a ballpark SPL of your current or future amps.  Hope you enjoyed it.

 

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