How Loud is Your Amp?

You see it all the time when this topic comes up: The chart showing the ratios of speaker surface area. It shows that a 15-inch speaker is 1.5 times larger than a 12-inch speaker, which is 1.5 times larger than a 10-inch speaker, which is 1.5 times larger than an 8-inch speaker. So, will you get 1.5 times more loudness out of your amp with the bigger speaker? In a word, NO.

Total speaker surface area has almost nothing to do with perceived loudness. That chart is meaningless, but it certainly helps perpetuate a common amp myth.

Let’s take for example a typical 4x10 Bassman-style amp. It has 312 square inches of speaker real estate, compared to 113 inches for an amp with a single 12-inch speaker. So, is the 4x10 Bassman nearly 3 times louder than the 1x12? No, not even close.

Remember, it takes ten violins to sound twice as loud as one violin. That is an immutable law of psycho-acoustics and the nature of the human ear.

With the 4x10 Bassman you have 4 speakers sharing the output of a 45-watt amp. Each speaker is “seeing” 11.25 watts. Adding a second sound source (another 10-inch speaker) that carries the same signal does not double the perceived loudness, but makes the sound seem only slightly greater. The Bassman’s four 10-inch speakers each driven by 11.25 watts will have the same total perceived loudness of a single 10-inch speaker driven by the full 45 watts.

Now we will hear owners and makers of 4x10 amps cry, “But the four tens move more air!” Perhaps, but “moving more air” has the same effect on perceived loudness as speaker area. By itself, it has almost no effect.

The proponents of “bigger is always louder” are confusing sound intensity with sound loudness. Sound intensity can be measured by instruments as a linear curve, while sound loudness is perceived by the human ear as a logarithmic curve. (Keep in mind the ten violins.) When you stand in front of your big amp and feel all that air making your pants cuffs flutter, it might impress you but it has almost nothing to do with the audience’s perceived loudness of your amp.

If speaker surface area does not affect perceived loudness, what does? Simply put, the two most important factors are 1) the power you apply to the speaker and 2) the efficiency of the speaker. When the bigger-is-better crowd insists they need a big amp for bigger rooms, they are right only to the extent the bigger amp is more powerful. But a lesser-powered amp with a more efficient single 12-inch speaker can have the same perceived loudness as a typical Bassman-style amp. Better tone, too.

Additionally, the smaller amp is likely to weigh less and cost less. It is axiomatic with vintage-style tube amps that a small amp turned up sounds better than a big amp turned down. And since your amp can be mic’ed through the PA in nearly any gig, the reasons for using a big multi-speaker amp are pretty weak.