Why Does My Amp Sound Different At High Volume?

Posted on by Nick Beatty

Have you ever sat at home and basked in your “lion riding a motorcycle” guitar tone, only for it to end up sounding all “duck chewing on a balloon” at your next gig or band practice? I know I have. If you’re the kind of bats-for-breakfast, skulls-on-mic-stand, brutal guitar warlord sort for whom LOUD is life, then you’re probably cranked 24/7 and have no idea what I’m talking about. However, for those of us who cut our teeth on modelers with headphones or amps turned down to law-abiding levels, having to adjust to being audible over a drum kit can be a very frustrating experience.

Why does my rig sound so different at high volume?

It can often be the case that whilst you excel at dialing-in great muscular slabs of rock through your 50w Velociraptor half-stack at home, by the time you crank the master volume up to levels where everyone in the room wishes that they weren’t, one of three things happens:

  1. It sounds like first gear in a dragster made entirely out of cats.
  2. It sounds like somebody trying to start a car inside a hippopotamus.
  3. It somehow manages to sound like both of these things, at the same time.

Curse you, treacherous thunderbox! Clearly 50 watts just isn’t going to cut it. Either that or you need to get one of those $400 true bypass reverberocitors at the end of your chain. Yes, that’s almost definitely it. Sell all of this junk and try again.

Velociraptor 50w half-stack

Oh, you’re back? You got the reverberocitor? Was it the reissue or the… oh, an original? Cool, and you picked up a matching 4×12 too I see? Nice.

It still sounds like a cat dragster, doesn’t it?

*sympathetic hand upon your shoulder*

This sounds like a job for …

Fletcher, Munson and the curves that made them famous

Meet Harvey “Excellent” Fletcher and Wilden “Wyld Stallyns” Munson – one a fearless scientist and the other an engineer with a pencil behind his ear probably, who worked for Bell Labs in the early 1930s. Harv and Wildman had been tasked with figuring out the most efficient (least expensive) way of transmitting a telephone call that would be intelligible enough to allow somebody to solicit political donations from you at 7am on a Sunday. In order to do this, they had to study how our ears work and which frequencies are most effective for the carrying of hearable human chingwag.

Today this telephonic twosome are here to explain the way of things with a series of lines, that look suspiciously like they might be from that thing that you flunked in high school. Let us imagine them now, bouncing out onto the stage and high-fiving like Bill & Ted. Most triumphant science-chart, guys!

Fletcher Munson "Equal Loudness" curves

Okay, so modern boffinology has tinkered with their original lines a bit since back in the day, and my drawing skills are not what they could be, but those are still some jolly impressive squiggles, I think you’ll agree? The one at the bottom there looks like it might be great fun to go down on an inner-tube! That being said, there is pertinent, bushy-eyebrowed science going on here, the upshot of which is pretty much:

Stuff sounds different when it’s loud.

Stuff also sounds different when it’s bold and in a bigger font, but that’s a topic for another time.

Wait, what the heck is a phon?

Phon, from the Greek word “phone”, meaning “whoa there brainiac, with the Greek all of a sudden,” is a unit of perceived loudness. At 1db, a sound with a frequency of 1kHz (1,000Hz) will sound 1 phon loud.

“Hz”, as we all know, is lazy physics-jargon for “Hertz” which is the internationally agreed upon unit of measurement for “car rentals per second.” The Hertz was named for Heinrich Rudolf Hertz, who is possibly the most German-sounding man who ever lived.

A sine wave, made of freshly knitted Hertz

As we can see from this compelling wobbleplot, Herr Hertz’s impressive unit can be used to describe the number of cycles per second of a sine wave which, by happy coincidence, is exactly what microscopic old ladies use to knit sound.

So what about these curves?

Fletcher and Munson started out with a 1kHz tone as a reference, followed by a 2kHz tone, played into the headphone-clad baconholes of an entire legion of hapless test subjects. The power of the 2kHz tone had to be lowered in order to make it seem as if it was the same volume as the 1kHz tone. The human ear, it turned out, was a lot more sensitive to the 2kHz sound than its 1kHz colleague.

They repeated the comparison across a vast array of frequencies, in the most ambitiously irritating experiment of its day, transcripts of which still survive:

Science transcript

This continues for something like 8,000 pages, just for that one test subject.

After tirelessly adjusting levels up or down to where each frequency was perceived to be the same volume, it transpired that the ear was most sensitive in the 1 – 5 kHz range, peaking at about 3.5kHz. Somewhat crucially for our interests – as a stock 50w Velociraptor is significantly louder than the average sales call – they carried out this experiment using 13 different reference levels, 10dB apart.

I’m still not clear about how any of this is relevant?

As someone with roughly the same IQ as a pool table, I must confess that I initially found this whole phon and decibels business to be somewhat baffling. Let’s take another look at our science chart:

Science chart showing Fletcher Munson curves for 60, 80 and 100 phon.

Each colored line upon the chart is a tone that is perceived to be exactly the same volume, from one end to the other.

For example, that yellow line at the bottom there is 60 phon “loud”, which equates to 60dB at 1kHz. If we follow the Hz axis back to 100Hz, you can see that our line has curved up close to 80dB, an increase of almost 20 actual decibels. However, the perceived volume is exactly the same. Your ear is less sensitive to this frequency, so it takes more power to make the sound seem as loud as it was at 1kHz.

So what?” you mutter, rather petulantly for someone your age, “if lower frequencies aren’t as loud, then I’ll just have my bass turned up a bit? That hardly seems worthy of a science chart and a couple of geezers having their own squiggles.

Hold your horses there, impertinent horseholder. Let’s crank our Velociraptor up so that it’s 100dB at 1kHz (100 phon) – the red line up at the top of our chart. In order for a 100Hz tone to sound as loud as a 1kHz tone at 100 phon you only need a boost of about 5 or 6 decibels. It doesn’t take a rocket scientist such as I to know that “5 or 6” is quite a bit less than “almost 20”. Given that decibels are logarithmic – which is beyond the scope of this article because I have no idea what it means – that’s three-and-a-bit times the power versus, I dunno, a squillion* times the power, respectively.

*somewhere around 80 – 100, I think.

Those of you who actually do know what a logarithm is may have already concluded that our 100 phon Velociraptor has now very likely become what professional audio engineers probably almost never refer to as “troublesomely rumblesome”. In short, if you dial in your sound at home and then turn up to gig levels, there’s a good chance that you’re going to find yourself with a lot more low-end than you had intended. At high volume, the bass doesn’t need boosting as much as it did to compete with the mids at a lower “phon”.

In conclusion

So yes, this is a huge amount of words for what is essentially: “stuff sounds different when it’s loud”. I think we often intuitively know this to be the case, and most people probably figure it out through trial and error, but my experience was that understanding why was a huge help in figuring out how to adjust my EQ for different volume levels. At the very least it was reassuring to understand that there was nothing wrong with my rig, or my ears.

As an aside, did you ever have a “Loudness” button on your portable cassette player? I did, probably because I am older than dirt’s dad. As a kid, I cluelessly assumed that you pushed this button when you wanted to brutal some Van Halens into your skull like a pair of bulldozers charging into either side of a satsuma, so I had that bad boy engaged basically all of the time. Ironically, the actual purpose of the “Loudness” button is to boost bass and treble when listening at low levels, in direct accordance with Fletcher and Munson’s magnificent squigglebusiness. For years I thought that Van Halen was some sort of avant-garde, bass-guitar-and-hi-hat duo.

In any case, if you get to a gig, crank up your amp and it sounds nothing like it did at home, try rolling the bass off. Maybe reduce the treble or presence a touch if things sound too shrill.

In closing, I have one important caveat: this article has focussed soley upon why your rig might sound different when it gets louder, as this can often be the opening obstacle for the newly noisy. However, just because your guitar now sounds like a lion riding a motorcycle again, that doesn’t mean you automatically have a killer live guitar tone. This is especially true when your bandmates ruin everything with their selfish playing of other instruments. Holding your own within the maelstrom of a live band mix is an entirely different undertaking, and one that perhaps we shall investigate another day.

Written on March 15, 2014, by Nick Beatty

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