Resonator Cone Physics

[naymi]

Hello all,

I would be intrested in understanding the physics that make a resonator cone work. I haven't been able to find an explanation on this anywhere. The most I've read is that the cone makes the air flow in a determined way which causes the amplification of the sound, but that doesn't really explain much.

I'm not looking to get to the mathematical formulas, but I'd be very interested in a conceptual explanation of the physical phenomenon.

Besides the mere curiosity, I think this is also useful to understand how different setups, manufacturing methods, etc., can affect the sound of a resonator guitar.

Thanks a lot in advance.

[mitchfit]

..."I'd be very interested in a conceptual explanation of the physical phenomenon."...

works the same as a speaker in an amplifier, but instead of moving the speaker with electromagnetism, it is moved by a mechanical connection with the vibrating strings.

mitchfit

[Deleted]

I don't think the air flows at all. Rather, the sound waves travel through the air.
I would guess that the cone vibrates much more than a flattop soundboard, thus being louder than a regular guitar. Bigger cone areas would generally produce more 'volume' although they might not be as loud, depending on what guitar it is.
TT

[naymi]

Hi Mitchfit, your explanation makes sense, but it surprises me, considering the cone is made out of metal. I have never held one in my hand, but I would guess it must be quite rigid, whereas in a speaker I assume the membrane is required to have certain flexibility in order for it to vibrate and generate the (sound) waves in the air.

Deuce, I don't think it's a matter of the cone vibrating more than a flattop soundboard. I think the phenomenons are completely different. In a regular guitar -or so I have always thought- the sound simply "echoes" inside the hollow box of the guitar. In a resonator, even though the cone takes up some of that empty space, the sound comes out even louder. I think it's simply different physics.

Regards.

[bod]

Hi Naymi

Phonographs might seem a bit off topic, but this (link) online 'Introduction to Vibro-Acoustics' from The Victor-Victrola Page could give you a jumping off point in related technologies - and if it serves your interest there are some more detailed technical articles on the same site... 

[pete1951]

Cones are very thin. The rim has a sort of dip spun into it, and that's the `fexi` bit. With a steady down pressure they are very strong, but they need to have a coverplate over them to stop damage from fingers etc. If you look at a speaker cone you will find similar features (though often fibre and plastic rather than metal around the edge)
PT

[davey]

Very good question there. I think it has something in common with a wind-up gramophone where a fixed needle transmits vibrations through a tube which increases in size. Also like a trumpet, I guess.

Having said that, I have no idea how either of those work either. I know a Physics Lecturer, I'll ask him !

I think this question also has a connection to the "gasket under the cone" discussion as a gasket will tend to isolate the action of the cone from the rest of the body.

[davey]

Hey Bod, thanks for that link. Very informative and it seems to explain what's going on. I wonder if anyone ever tried amplifying a guitar in exactly the same way as a Phonograph ? Might be fun to make one.

[tark]

All stringed acoustic instruments work in the same way. The vibrations from the strings are transmitted via a bridge into some kind of soundboard which moves air. The strings have a relatively small surface area so they cannot move much air. The soundboard usually has a much bigger surface area than the strings so it moves more air. It is a question of going from a small surface that vibrates a lot (relatively speaking) to a much larger surface area that vibrates less. This is a process of mechanical impedance transformation. Loudspeaker cones, resonator cones and acoustic guitar soundboards need to be as stiff as possible BUT they are all mounted on flexible surrounds.

The purpose of the body of a guitar (or any other acoustic instrument) is to act as a baffle that forces the air pressure waves from the soundboard (or cone) to radiate out away from the instrument rather than just flowing around the edge to the back of the soundboard / cone. Because of the soundhole/s in the body the body also acts as a Helmholtz resonator which acts to flatten out the main low frequency resonance of the soundboard / cone and extends the low end response of the instrument

The horn of a wind up gramophone or of a wind instrument like the trumpet is also a mechanical impedance transformer. A relatively large movement of a small amount of air at the throat of the horn is converted to smaller movement of a large amount of air at the bell.

[Deleted]

The soundboard of a regular acoustic needs to be less stiff than a resonator body. Its not attached to any flexible surrounds - it needs to vibrate.
Resonator bodies should be as stiff as possible - there are not supposed to vibrate really.

[mitchfit]

..."but it surprises me, considering the cone is made out of metal"..

www.sweetwater.com/store/detail/410XL?adpos=1o1&creative=54988012321&device=c&matchtype=&network=g&gclid=CJ7k7tzTicECFQET7AodK24AuA

same principal, different construction material.

mitchfit

[alexandre]

Sept 30, 2014 19:18:34 GMT mitchfit said:

same principal, different construction material.

Yep, to illustrate the resonator cone physics, here's a prototype of a "paper resonator" made with a real speaker by a DIY enthusiast member of the french forum "slide et résonateurs"...
(here's the link of the original thread: link)

... and it works pretty well !!

In this first video, the setup is not perfect and our friend is telling at the end that he needs a little work to eliminate some buzzing.

[naymi]

Hi Bod, thanks for that link. It doesn't exactly address the question raised here, but it's interesting reading anyway, somewhat related.
I've been doing some additional reading, and my conclusion is that, roughly speaking, it is as Mitchfit stated in the first place. Same principle as in a speaker.
And the more technical explanation of the full process would be as posted by Tark.
One small clarification, however, as Davey compared it to the process in a phonograph or trumpet. I may be wrong, but I think it's actually different. It is a matter of impedance, as Tark said, but in the case of a trumpet or phonograph horn, the moving air particles forming the sound wave collide into a progressively increasing amount of new particles as they advance through the horn shape, preventing them in this way from "bouncing back". (This is well explained in one of the articles of the page that Bod provided). But in the resonator cone, the sound waves don't actually travel THROUGH the cone (in fact, I believe the cone is covered in the smaller end). Rahter, they are CREATED by the movement of the cone. (This is just so you know what I mean, being precise we'd have to say that they are actually created by the vibration of the strings, and then transferred onto the cone).

All this said, there are a couple of questions that come to my mind.
First of all, Deuce, are you sure that, appart from the cone, nothing is supposed to vibrate in a resonator? Including the coverplate? And what about the rest of the top of the guitar body surrounding the coverplate in wooden resonators? Is it made from thick wood to keep it from vibrating, or does it act just like the soundboard of a regular guitar, only with a smaller surface, "complementing" the vibration of the cone?

And one other question, which might go a little bit off topic, but that is probably related to the phenomena discussed here. Why are there only (or at least predominantly) single cone and tricone resonators? Why not two or four cones?

Regards.

[Deleted]

All the bits of a resonator guitar vibrate a bit, but although some of their jobs are not specifically to vibrate (e.g. coverplate, body), the fact they do a bit affects the tone / volume / adds 'colour' to the sound.
BUT, the only things on a resonator guitar that are designed to significantly vibrate are the strings, saddles, bridge and cone.
I think that in the interests of science, you should go now and buy a resonator guitar, and experiment with it.
TT

[resonatorman]

Oct 2, 2014 14:46:31 GMT naymi said:

Why are there only (or at least predominantly) single cone and tricone resonators? Why not two or four cones?

IIRC the Dopyeras experimented with 1 through 4 cones and found the 3-cone solution most pleasant - so the tricones were the first resonators. The single cone came later in an effort to reduce costs to offer cheaper models.