Why do higher frequencies get louder?

[triden]

Say I am playing my guitar and I am playing on a fret high up in the neck as to get a lower frequency sound...but with a single pluck I slide my finger down the fretboard on the same string, and as the frequency increases, so does the volume. It gets much louder. It as almost as though the string increases in amplitude for some reason as I slide my finget to get a high frequency. Does anybody know why?

[Scottmoose]

Our hearing isn't flat. As the frequency of the note you play increases, you're most likely moving further up into our most sensitive hearing region: see the attached Fletcher Munson & equal loudness curves.

[tinitus]

I suppose that its mostly your speaker/xo that does it, or relation between speaker and amp, not uncommon even with hifi gear, unfortunately

Also guitar speakers mostly have much higher sensitivity somewhere above 1khz

[triden]

It is an acoustic guitar, there is no amp.

Thanks for the image scottmoose, very interesting.

[t_blizzard]

Modal analysis on acoustic guitars has shown that string amplitude is a function of the distance between where you fret and the bridge break angle.

However, the amplitude changes are small and most likely you are hearing the non-linearities from your magnetic pickups in your guitar.

Most electric and acoustic magnetic pickups are not flat across the audio band. (and far from it)

Some designs rise in sensitivity due to coil resonance and some roll of due to high leakage inductance within the pick-up design.

just food for thought............

[triden]

Interesting, however, I do not have magnetic pickups in my guitar. It is purely acoustic - just a soundbox. When I slide my finger down to get a higher frequency, the volume difference is very noticeable. My guitar instructor confirmed the same thing, but he doesn't know why.

If you say that the amplitude is a function of the distance, then I could see that being the main reason. I would just like to know how that function was derived...or what is the physical explanation behind it.

[t_blizzard]

Laser Inferometry shows many modal patterns that exist on an acoustic guitar tops.

High frequencies are spread all over different areas.

Low frequencies shift around as a function of frequency.

String resonance combined with an effiecient guitar top could explain what you are hearing.

Don't rule out room resonance as well.

Hope this helps.

[Cal Weldon]

I'm wondering if you guys are trying too hard to answer. I think Scott's table explains it best.

[t_blizzard]

I am very familiar with Fletcher-Munson curves.

The Fletcher–Munson curves are one of many sets of equal-loudness contours for the human ear, determined experimentally by Harvey Fletcher and W A Munson, and reported in a paper entitled "Loudness, its definition, measurement and calculation" in J.Acoust. Soc Am.5, 82-108 (1933).

The graph that is posted represents the sound pressure required to maintain a perceived "equal loudness" across the audio band.

Our hearing sensitivity to sound pressure waves is actually the inverse of that graph.

The human ear is most sensitive to frequencies in the mid band between 800Hz and 4kHz.

The string frequencies on the upper scale of an acoustic guitar will approach 6khz - 8khz where human hearing is actually less sensitive.

[AdamThorne]

I think it's probably the human auditory sensitivity thing. But since I like to make stuff up I'll throw out some other ideas...

The closer to the center of the string you pick the more fundamental you get. Further from the center = more uhh... overtones? Harmonics? (test this yourself by picking down by the tremolo - as far from center as possible - to good effect when busting out a little "Wish You Were Here") So you move your fret hand but your pick hand stays in place. Play low notes and you pick far from the center of the string. Move up the fretboard and you are picking closer to the center. So more energy goes into the harmonic, and it sounds louder.

And also, it's known that hearing damage occurs from the top of your hearing range down. Well, loss goes like that, but so does non-loss hearing damage. High notes are where your hearing is already damaged, so your threshold of pain is lower, and the sound is more 'piercing'.

You obviously need a BSC on your guitar. The baffle rolloff is caused by the width of the body. You could try a really wide guitar, or one with a round body. The guitarist for U2 has done some work simulating this effect. "The Edge" is his name...

The energy density on a plucked string is greater for a shorter string. The energy transfer to air function is independent of string length. So notes picked from a shorter string transfer more energy to the air and are louder.

There, some of that sounded pretty good. All made up, of course, I wouldn't believe a word of it if I were you. But as long as one lets his conversational compatriots know when he's BSing, it can be a an entertaining and harmless pasttime.

If one *doesn't* let others know, then it's just entertaining.