Does vibrating a guitar make any difference?

Guitarists have always believed that two identical guitars, each played exclusively by different players with different styles, will end up sounding quite different from each other. Others make sure to keep their guitars in front of their stereo speakers so that whenever they play their favourite music the guitar will vibrate in sympathy and take on the tone of the music it “hears”.

Is there anything in this?

Recently electromagnetic vibrators have been produced to artificially “play in” new instruments. Even I can tell that a brand-new, just strung up guitar sounds pretty raw compared to its sound a few weeks or months later, so I bought a Tonerite vibrator to experiment with.

The Tonerite fits between the strings down near the bridge and has a choice of settings:

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When the device is turned on, you can feel the whole instrument vibrate from top to bottom. Tonerite recommend you vibrate for about 3 days for a new instrument, and periodically repeat the treatment to “liven up” older instruments.

But does it do any good?

My trusty tap hammer says that it does, in fact, make a difference. Here is the tonal signature of one of my 12-string guitars before and after the vibration treatment at the very beginning of the instrument’s life:

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The blue signature is the instrument’s response before vibrating, and the red afterwards.

It’s pretty clear that the “after” picture is an improvement on the “before”. Entirely new formants (peaks in the response) have formed right across the spectrum from 200 to 1000Hz, and the sensitivity has improved pretty much across the spectrum. My ear agrees that the instrument sounded far richer and livelier after the treatment.

Some caution is needed in looking at the increased response overall, because the traces were produced by two separate and therefore not identical taps, so the “after” trace could have been a heavier tap despite my best intentions.

(This leads to the search for a standard tap device that delivers the same impulse to the guitar every time to make direct comparison more viable – but that’s the subject for another day.)

However, what makes me think that the two are similar enough is that the first peak at about 120Hz is what’s called the coupled Helmholtz response. I wouldn’t expect this to change much after vibration because it is produced by the overall “coupled” response of the top, the air in the soundbox, and the back. This peak is a little higher in the “after” response, which implies that my tap was a bit harder for this one.  But the two peaks are similar enough to show that the overall improvement in response after vibration is real, and not just the effect of bumptious tapping.

When you look at tonal signatures such as this, keep in mind that the response (the height of the trace) is measured in deciBels (dB). This can be tricky, because a difference of +10dB actually means a difference in sound power of 10 times. A difference of about 3dB means a doubling of sound power. This means that the red “after” line shows a hugely increased performance across the spectrum.

Why does it work? My belief is that when a guitar is built many localised stress points are set up in its structure. Both the resin glue and the wood will “creep” under the influence of mechanical vibration, smoothing out those localised stresses and improving its ability to respond evenly across the spectrum.

Because it’s not possible to have two identical guitars, it’s hard to say whether different players would have a different final effect – but it’s a reasonable thing to suggest.

In a separate blog I’ll explain what the peaks on a guitar’s tonal signature actually mean. On another I’ll show how my mighty hammer and I produce the traces, with the help of Audacity and Excel.

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