Acoustic bass guitar experimentation

[Beer of the Bass]

Like quite a few people on here, I bought one of the Harley Benton acoustic basses from Thomann which were being sold as unplayable "deko" models due to some minor flaws. I've been enjoying playing mine and it's a fun instrument to noodle on in the front room. I'd found that while the acoustic volume was not terrible, the tone is a little thin, without much bass underpinning the low notes.
I'd seen the Planet Waves O-Port mentioned as something which could help the low end of ABGs - it's a funnel shaped plastic ring which fits inside the soundhole. I did a little reading about the concept and found out that the idea had been used by classical guitar luthiers as well as on the Selmer Macaferri guitars - it was originally called a Tornavoz. It is essentially a tube in the soundhole which lowers the resonant frequency of the box, similar to adjusting the port tuning in a speaker cab.
I came across this link about trying a makeshift Tornavoz on a guitar orchestra Contrabass guitar, and decided to give it a go. [url="http://www.hago.org.uk/faqs/contrabass/theory.php"]http://www.hago.org....bass/theory.php[/url]
I used a sheet of craft cardboard, formed a cylinder to fit the soundhole and taped it in place. Surprisingly, it does seem to have the desired effect. The volume of the instrument is not much affected but the low notes now have a bit more depth to them. I tried a couple of different depths of tube, a longer 70mm tube helped the low notes but thinned things out a little higher up. A shorter 55mm tube was a better compromise. I may try and find a sturdier, better looking way to do it, but it would appear that the idea is one that works for these basses. The effect is fairly subtle but does make a satisfying difference in my opinion. I thought I would mention it here in case anyone else wanted to try it...

[Geek99]

Thanks for the tip

[alyctes]

Interesting... How does that work acoustically? I don't get it.

[Beer of the Bass]

[quote name='alyctes' timestamp='1424392038' post='2695977']
Interesting... How does that work acoustically? I don't get it.
[/quote]

I'm not great at explaining the physics, but it helps to look up Helmholtz Resonance. A cavity with an opening or port in it, like an acoustic guitar body, has a resonant frequency which is affected by the volume of the cavity and the area and length of the port. This resonance provides some reinforcement to the the bass, not just at this frequency but in a band around it - if you've ever tried using one of those feedback-buster soundhole plugs you can hear that the instrument sounds thin and quiet without this effect.
You can hear this resonance by damping the strings and humming low notes into the soundhole until you find the one where you can hear the body boom. On this bass, this was around the lowest G I can hum, which is the pitch of the open G string on a bass (around 100Hz). Lengthening the opening lowers this frequency. I'm not certain where it is with the tube in place, as G is near the bottom of my vocal range and I can't hum as low as the new resonance! The bass reinforcement which comes from the soundhole now happens slightly lower, making the instrument a little deeper sounding.
It might not do something useful on every instrument, but it's an interesting thing to try.

Edited February 20, 2015 by Beer of the Bass

[AndyTravis]

I had one of the oport things stashed away in my old work when rep said we should try it...

Left the job and didn't bring it home, gutted.

It did work.

Edited February 20, 2015 by AndyTravis