Have you ever seen anything like this?

[Jo Dusepo]

Have you ever seen anything like this? This is an Iranian Tanbur but the same process could be applied to any bowlbacked (or even flatbacked) instrument in the same way. There's an extra half or slightly less of a body over the soundboard. It seems to reflect the sound in an interesting way.

https://www.youtube.com/watch?v=XGDmZz8Pwho

[Marshall Dixon]

Very interesting! and sounds like a fairly dramatic difference in tone.

Yes, I've been doing some experimentation with an additional plate on the inside of the guitar. My idea was to increase the length of travel for the lowest sound wave to affect a lower timbre. Sort of like a transmisssion line loudspeaker where the length of the chamber is equal to the 1/4 wavelength of the fundamental resonant frequency of the speaker.

In my searches on the subject I found the concept put into practice by several makers in the past and one currently. In the 1930's Maccaferri made his "internal resonator" guitar and before him there was Lucien Gelas, who made a guitar with an additional resonator plate on the top, but oriented in the opposite direction as the one on the tanbur in the video. Here's a Gelas instrument from the 1920's:

The Ramirez company still makes their "de camera" model with an (2 cm?) inner ledge around the inside of ther guitar.

I've done this with two guitars and have two more underway. I find the results pleasing. Currently my attempts have focused on the angle of the inner plate, and their length is midway, to coincide (in theory at least) with the node of the sound wave in hopes of producing a gain in combining their energy.

My next experiments will shorten the length of the inner plate to coincide with the node of the 3rd harmonic. Here is a picture of my prototype model from about a year ago.

inner membrane .png (224.74 KiB) Viewed 34288 times

[Eric Crawford]

This is neat. I’ve had similar thoughts of using speaker construction tactics while planning my current acoustic bass guitar build, but didn’t know where to start.
I would like to hear/ see more of this.
Thanks

[Marshall Dixon]

This is neat. I’ve had similar thoughts of using speaker construction tactics while planning my current acoustic bass guitar build, but didn’t know where to start.
I would like to hear/ see more of this.
Thanks

If you're relying on a totally acoustic instrument without amplification then you'll need to trick every bit of energy out of that box.
A bass guitar has a low note around 40 Hz and the quarter wavelength is over 2 meters. Even an upright bass isn't quite long enough.

In his article on the "Basics of Air Resonances" in American Lutherie #1 of 1985, WD Allen states that the resonant frequency is a function of the pipe's length and varying the diameter has no effect on frequency. (Though there are resonances of the cross section and I think they do have some effect.) It doesn't have to be round, and any shape of cross section will have the same frequency for the same length. He also states "that the only requirement is that cross section be constant the full length." Obviously mine in the picture doesn't have a constant depth, but it works.

Rafal Turkowiak is a luthier that has built with "acoustic tubes" inside the neck. Also consider the Weissenborn lap guitar with it's hollow neck. I think the principle Allen refers to is at work in these examples.

I can't remember where I read it, or the exact reason, but there is some advice to not obstruct the depth of the instrument at the soundhole.

One thing has impressed me about this is just how much force the air puts out. The slightest tap with the pad of a fingertip on the back will be felt on the inner plate (or the top). The gap between the back and inner membrane acts like a soundhole and the area of the gap is about equal to the area of a typical soundhole. I'm thinking that adjusting the gap area could be a further way to control the response.

I've been working on this in guitars with standard length but narrow (33 cm)lower bout and shallow (85/90 mm) depth, trying to isolate the effect of length. There is a point at which theory and calculations end and the only way to find out is to try it. This video that Jo posted has been a real inspiration to me. Most people I know don't get too excited about this stuff.

[Eric Crawford]

I assume I’ll need amplification but I would love to be able to play a duet or quiet trio without it.
I am now thinking about your pictured example, but turning it sideways.
What if I used thin sheets from the top to the back, in lieu of parallel bracing to create an “acoustic tube” with an f hole at each end? I believe I could make a reasonably sized instrument with a 2 meter long chamber with the bridge in the center.
This is an interesting rabbit hole, definitely worth some sketches and getting some inexpensive wood to experiment with.
What did you mean by “not obstructing the depth at the sound hole “ ?
How am I supposed to do the things I am supposed to be doing once my brain starts running around in circles with such an interesting concept?

[Marshall Dixon]

What did you mean by “not obstructing the depth at the sound hole “ ?

I just can't remember the specific reason. I recall it has something to due with a particular resonance that is related to the depth there at the soundhole. It made sense to me, but that isn't a guarantee of good practice.

The first time I tried this out in an existing guitar I taped a fan folded cardboard cutout to the lower harmonic bar angled downward toward the tail, leaving a gap. It was a completely sloppy fit but I measured around 6Hz lower tap tone. Not wanting to attach anything to the top I opted for the simplest thing I could think of. (Maccaferri glued his internal resonator to the top in close proximity to the sides and back. At a minimum of 82 vibrations per second it seems like a recipe for disaster, and it was. They came loose.)

My second attempt was a plate that runs along the top of the linings, but as the back is dished the cross section isn't truly constant.
One thing I considered in the next two was that the sound pressure is greatest at the ends and a tapered narrowing of the plate might produce a megaphone effect. In the video Jo posted the top plate of the tanbur has this attribute and also, as I mentioned in my last post, there will be a soundhole effect from the opening at the front. I didn't see a soundhole in the second instrument played, though maybe there is one on the back. So the effect is coming from the air resonance between the two plates, and undoubtedly the overlying plate contributes to the resonances with it's vibrations.

Labyrinthine enclosures have been used in speaker boxes for nearly 100 years. Look up Stromberg-Carlson and Victor Orthographic from the past. For plenty of food for thought do a search for "acoustic labyrinth loudspeaker."

Just forget about the other things you're supposed to be doing... it works for me.

[Eric Crawford]

Thanks.
Acoustic labyrinth loudspeaker is a deceptively simple concept. Looking at my notes taken during a meeting this morning I find 4 different possible baffle patterns in the margins. 2 putting the bridge at an end, and 2 with the bridge in the center of the labyrinth.
I think my next step is to make a simple box body and a neck with a couple of strings and start the trial and error process.
If I can find a pattern that chambers the body into an effective acoustic labyrinth increasing volume at lower frequencies without causing other problems, I’ll try and incorporate it into a trial instrument.
Thanks for the ideas!

[Marshall Dixon]

Increased volume is a slippery concept. Our ears hear better in the mid frequency range, so a boost in bass could be perceived as increased volume. Think of it as you would an equalizer in that it levels out the response, although it does have an effect on the higher registers.

My last two were similar sized one without the partition for comparison. My brother in law tried them out and one comment was “sounds like a little reverb going on.” I imagine it could work into an annoyance if taken too far.

One of my mainstay references is Erik Jansson’s Acoustics for Violin and Guitar Makers. I printed it out and put it in a 3 ring binder. It’s available online and it’s free: https://www.speech.kth.se/music/acviguit4/

Another is The Big Red Book of American Lutherie Volume I from the Guild of American Luthiers; well worth the price. Beside the article by WD Allen (which I merely touched on) it has many articles of interest.

[Eric Crawford]

Thank you.
I’ve been reading Jansson, and whatever I can find on this concept.
That isn’t a lot, and most of it seems to have been put on the web or referenced by you.
I believe my first prototype build for a semi acoustic bass (as much acoustic as possible!) will be something like a large dreadnaugt size, a single sound hole, with double sides a few cm apart that form a transmission line from the bridge around the body to the sound hole. I’ve made a couple of sketches and it looks like if this is done carefully a chamber of 2 meters in length can be made and parallel sides can be avoided.
Do you think there is a “sweet spot” in the relationship between the size of what I’m thinking of as the “bridge chamber” which in my model would be the driver, and the volume of the channel?
I don’t want to constrain the motor part where the vibrations start but also can’t help but think the transmission line requires a certain minimum volume to be effective.

[Alan Carruth]

Fred Dickens did an experiment where he varied the depth of a guitar to see what happened to the 'main air' resonance. He started with sides 6" deep (in BRW!) and measured the air mode pitch. Then he cut the back off, reduced the side height by 1", and put it all back together, re-doing the measurement. He kept going until he'd gotten down to 2" depth or less. In the end the 'main air' resonant pitch was (drum roll), 7% higher than it was with 6" deep sides; a little more than a semitone.

If the box had been a real Helmholtz resonator the pitch would have risen by quite a lot, but the 'main air' resonance is not simply a Helmholtz mode. It's actually the lower part of a 'bass reflex couple' between the Helmholtz (A-0) mode and the loudspeaker-like 'main top' resonance. Rossing found the 'real' isolated A-0 of a classical guitar to be around 120-125 Hz, and the isolated 'top' resonance at around 160-180. Top motion changes the air pressure in the the box, and air moving in and out of the hole pushes on the top, so the two couple. This pushes the resulting pitches away from each other, usually to something like 100 and 20 Hz. The change in pitch difference depends on how strongly the two modes couple, and the amount each one changes depends on the relative masses, I believe.

Making the box shallower does two things:
i) it raises the 'real' Helmholtz pitch, and
ii) increases the strength of the coupling between the 'air' and 'top' modes. The rise in the 'real' A-0 pitch is easy to see. The increased coupling happens because a given movement of the top plate produces a larger change in pressure in the box, and vice versa, of course. Normally you'd expect the 'air' pitch to rise as the box gets shallower, but the increased coupling strength tends to push the pitch back down, and the two pretty well cancel each other out here. When I talked to Fred about the experiment (which I don't think he ever published) he didn't say anything about the timbre of the shallow box, or any change in the 'main top' mode pitch, and I didn't think to ask.

I did try cutting down the sides on a round hole archtop classical once. Iirc, I took them dowm from ~3-1/2" to a bit less than 3", but I'm not totally sure. The 'main air' pitch rose by about 2.5 Hz (84 to 86.5), while the 'main top' pitch rose 9Hz (170-179). The stronger couple produced a larger gap, and the top changed more than the air.

Although I have not cut down other instruments I do have records of ones with deeper than normal sides. The main thing I see in the spectrum is that the 'main air' peak tends to be lower in height; again, due to less pressure change for a given volume change from top displacement. The 'air' peak may or may not be wider, but it looks it. A wider peak (lower Q value) would indicate more loss, and it's hard to see why a deeper box would do that. Still, the timbre does tend to be less 'focused' on the deeper body. What's interesting is that the bass tends to sound 'fuller' even though there is actually less output in the A-0 range. What they sound like and what they do can be different things.

[Teresa Wiggins]

It reminds me of a Strumbly.