Hi everyone! Has anyone experimented with the resonant frequency of an acoustic bass' body by adjusting the dimensions of the sound hole?
I have a five-string, purely acoustic Thunderchief which has quickly become my favourite instrument. Without electronics, it is not very useful live, but it has been wonderful for practice and teaching, where it saves on setup time. I also really enjoy its acoustic tone and am impressed by its acoustic projection.
I have noticed that the body has a resonant frequency of around 75Hz and the notes around D2 and Eb2 are consequently a little louder and fuller than that rest. I have noticed this particularly when trying to record with the bass or the few times that I've tried to use it live with a condenser mic in acoustic settings ( with IEMs, a Cajon and subsequent lower stage volume). I believe that this is known as Helmholtz resonance. Reducing the area of the sound hole by placing my hand or a flat object over part of it lowers this resonance and also seems to make it less pronounced so that the notes are more even across the instrument's range. This reduces the bass' overall projection and the fullness of the higher notes, but it also makes the lower notes sound a little fuller. As with most things this is a tradeoff. It seems that the Helmholtz resonance of most acoustic six-string guitars is near the fundamental of the lowest notes of the instrument, which I suspect lends itself to a more even note-to-note volume and tone. Would I be correct in thinking that an acoustic instrument's body is effectively a high-Q high-pass filter, or am I over-simplifying things? Of course, it wouldn't just be the Helmholtz resonance that affects the tone and frequency response. Would the vibration modes of the soundboard have a greater affect on the upper frequency response of the instrument, while the lower frequency response is dominated by the air resonance?
When I can afford it (musical equipment is not cheap for South Africans), I would like to install a K&K pickup and would also like to make a wooden sound hole cover to avoid feedback. This made me consider making a sound hole cover with an adjustable port/opening (similar to the top sound hole of this classical guitar:
Projector Model – Classical) so that I could fiddle with the body's resonance. I would use this feature primarily for recording, where evenness in tone is more critical than projection and it would hopefully allow me to rely less on notch filters. This wouldn't do any damage to the bass, would be removable when not needed/wanted and would be a fun project.
Has anyone ever tried anything similar or done any experimentation with adjusting the resonance of an acoustic instrument? Do you notice a similar resonance with your acoustics? I would be interested to hear!
