Plate Reverb Size and Thickness.

[cdhoffman]

I am in the infancy stages of planning a small (6" X 18") plate reverb.

I'm reading that typical plate sizes are around 4' x 6 ' and steal gauge is 24 to 28Ga. Will I achieve a similar result by scaling down the thickness in proportion to the change in size? If so, should I scale by the change in length (2/3) or width (1/3).

  -- Or --

The idea of scaling is unnecessary; 24 to 28Ga will do, even given the plate's small dimensions. 

Perhaps naively I am expecting to get a plate reverb sound, albeit short decay and it will in some way resemble a small room. I'm now looking for a contact driver that won't be too heavy so as to prevent the steel from ringing in an effective way and the right circuit to drive the speaker.

This is my first time casting the net into this forum, I appreciate any and all help that someone can provide.

Thanks

 

[Matt Nolan]

It won't work.

Resonant frequencies will be higher and decay time will be much shorter, complexity will be much less, there will be strong resonant modes that stand out above the rest.

And, your maths is off. 6" is (1/8) of 4' and 18" is (1/4) of 6'. Modal frequencies of a metal plate are approximately proportional to thickness and to the inverse square of length. So 1/8 the length would require 1/64 of the thickness to stay the same (apart from decay time). This is impractically thin.

 

[cdhoffman]

Thanks for the reply.

Modal frequencies of a metal plate are approximately proportional to thickness and to the inverse square of length. So 1/8 the length would require 1/64 of the thickness to stay the same (apart from decay time). This is impractically thin.

Clearly I need to do more research. Can you tell me where you found that modal frequencies are proportional to inverse square of the length? I'd like to read more about this.

 

[emrr]

This why the small ones were gold foil, opinions seem to vary greatly on their sonic qualities.  I've never worked anywhere that had one. 

My Ecoplate III (the small one, 3'x4'-ish) does a good small/medium space, can't at all do what my Ecoplate I (the big one 4'x7'-ish) does, and vice versa. 

 

[Matt Nolan]

Thanks for the reply.

Modal frequencies of a metal plate are approximately proportional to thickness and to the inverse square of length. So 1/8 the length would require 1/64 of the thickness to stay the same (apart from decay time). This is impractically thin.

Clearly I need to do more research. Can you tell me where you found that modal frequencies are proportional to inverse square of the length? I'd like to read more about this.

Well, I know this because I'm often tuning pieces of metal for orchestral percussion sections and the likes, but you can find useful information in books such as "Music, Physics and Engineering" by Harry F Olson.

 

[Rob Flinn]

This why the small ones were gold foil, opinions seem to vary greatly on their sonic qualities.  I've never worked anywhere that had one. 

My Ecoplate III (the small one, 3'x4'-ish) does a good small/medium space, can't at all do what my Ecoplate I (the big one 4'x7'-ish) does, and vice versa.

The Gold foils sound a bit like being in an Oil Tanker compared to a 140.    I like the Ecoplate 1, I used to have one.  They have nice bright smooth sound.  Must get my 140 going !

 

[pvision]

Could you modulate the audio onto a high-frequency carrier, like FM radio, send it to a small plate and demodulate it afterwards?

You'd need pickups that worked to very high frequencies (mod freq + 20 KHz approx). Would the resonances be out-of-band, I wonder?

I love the idea of taking a simple idea and making it complicated!

Nick Froome

 

[cdhoffman]

Thanks for your replies everyone.

Could you modulate the audio onto a high-frequency carrier, like FM radio, send it to a small plate and demodulate it afterwards?

That's a terrific idea! I would certainly not have thought of that on my own. Assuming that it works in theory, my questions then are:

  1) What's the best way to go about selecting a high-frequency to use as the carrier signal?

  2) Is it possible to detune the oscillator of a radio transmitter to cause it to carry within a lower range, or would it be wiser to design a oscillator and modulator (not sure of its proper name) circuit?

You'd need pickups that worked to very high frequencies (mod freq + 20 KHz approx). Would the resonances be out-of-band, I wonder?

I love the idea of taking a simple idea and making it complicated!

Me too, and I'd love to try this. I think it might be worth the build just to find out the answer. I'm trying to come up with the formula to calculate resonances in the plate and whether or not they'd be out-of-band. Maybe at this smaller scale there's a better material alternative to steel.

Thanks Nick and everyone else for your help.

 

[JohnRoberts]

The root problem is the speed of sound in the metal plate... A smaller plate will exhibit the same propagation speed per unit distance, but have much shorter paths, resulting in much faster decay and shorter initial slaps.

JR

 

[joaquins]

Could you modulate the audio onto a high-frequency carrier, like FM radio, send it to a small plate and demodulate it afterwards?

You'd need pickups that worked to very high frequencies (mod freq + 20 KHz approx). Would the resonances be out-of-band, I wonder?

I love the idea of taking a simple idea and making it complicated!

Nick Froome

Similar to what JR said, the problem is more in the time domain than in the frequency domain. Let's say you could shift the frequency up and down again, if you are processing in real time the reverberation time will be the time the plate keeps ringing, and that will be pretty much the same for higher or lower frequencies, or more like lower at HF. For processing offline could be implemented, let's say you get your track recorded sampled at 48kHz and then you get that signal and put the output at 384kHz so it's 8 times faster and the frequency is 8 times higher. Then you record that back into 384kHz and reproduce it down to 48kHz, that way you have a plate that's 8ish times smaller the one you are listening. Instead of doing that I'd go for digital processing anyway.

Trying to make a plate reverb pocket size, is as trying to make a room verb pocket size. It takes some space. The process described earlier is used in a similar way to reproduce ambience to replace simulations when designing big complex rooms, frequency is shifted and size is reduced to be able to use a model of the room.

The more compact alternative to a physical reverb is the spring reverb, I haven't heard one sounding really good in anything but a guitar, lacks complexity. But maybe building a net of springs with multiple transducers instead of having few springs directly in parallel may allow a bit more complexity. Also combining different springs types and tensions may help. I'm thinking 4 springs in diamond shape supported by other 4 springs, that makes a cell with 2 path, which allows, in a 3D expansion, add as many pairs of springs as wanted (within size limitations) and then the pattern may be repeated. Good thing here is that resonant frequency of springs can be really low and long times in a small size, less than a foot may do the trick. Each node (where 3 or more springs meets) could be dampened or driven individually allowing for tuning of time and path,  with different sources and receivers positions possible. That may worth experimentation, I don't know if that would fit the size you are aiming for but I guess than in a box of 2'x1'x1' there's more than enough space to fit few diamond cells and have a lot of fun. I don't know which kind of transducers are the best for this application, for driving something similar as the plate but with a wire connected to a node. For the receiver maybe a rod to a piezo. All mounted on a modular mecano-like structure to allow moving the holding points and experiment around. May worth playing with...

JS

 

[Matt Nolan]

Could you modulate the audio onto a high-frequency carrier, like FM radio, send it to a small plate and demodulate it afterwards?

You'd need pickups that worked to very high frequencies (mod freq + 20 KHz approx). Would the resonances be out-of-band, I wonder?

I love the idea of taking a simple idea and making it complicated!

Nick Froome

This does nothing to help the problem. If anything it makes it worse as there is more damping in metals at higher frequencies.

What you could do is make the reverb non real time. If you played the audio through at, say, 4x the original sample rate (i.e. sped up, not re-sampled) and record the output like this too, then slow the recorded output back down to normal sample rate, then you might get something longer, more complex and less peaky. This too would suffer a bit from the damping at higher frequencies, but then you don't tend to want a bright reverb in general.

 

[Rob Flinn]

The more compact alternative to a physical reverb is the spring reverb, I haven't heard one sounding really good in anything but a guitar, lacks complexity.

The AKG Spring reverbs sound pretty decent, for a compact alternative to a plate.

 

[Sounddesign84]

Similar to what JR said, the problem is more in the time domain than in the frequency domain. Let's say you could shift the frequency up and down again, if you are processing in real time the reverberation time will be the time the plate keeps ringing, and that will be pretty much the same for higher or lower frequencies, or more like lower at HF. For processing offline could be implemented, let's say you get your track recorded sampled at 48kHz and then you get that signal and put the output at 384kHz so it's 8 times faster and the frequency is 8 times higher. Then you record that back into 384kHz and reproduce it down to 48kHz, that way you have a plate that's 8ish times smaller the one you are listening. Instead of doing that I'd go for digital processing anyway.

Trying to make a plate reverb pocket size, is as trying to make a room verb pocket size. It takes some space. The process described earlier is used in a similar way to reproduce ambience to replace simulations when designing big complex rooms, frequency is shifted and size is reduced to be able to use a model of the room.

The more compact alternative to a physical reverb is the spring reverb, I haven't heard one sounding really good in anything but a guitar, lacks complexity. But maybe building a net of springs with multiple transducers instead of having few springs directly in parallel may allow a bit more complexity. Also combining different springs types and tensions may help. I'm thinking 4 springs in diamond shape supported by other 4 springs, that makes a cell with 2 path, which allows, in a 3D expansion, add as many pairs of springs as wanted (within size limitations) and then the pattern may be repeated. Good thing here is that resonant frequency of springs can be really low and long times in a small size, less than a foot may do the trick. Each node (where 3 or more springs meets) could be dampened or driven individually allowing for tuning of time and path, with different sources and receivers positions possible. That may worth experimentation, I don't know if that would fit the size you are aiming for but I guess than in a box of 2'x1'x1' there's more than enough space to fit few diamond cells and have a lot of fun. I don't know which kind of transducers are the best for this application, for driving something similar as the plate but with a wire connected to a node. For the receiver maybe a rod to a piezo. All mounted on a modular mecano-like structure to allow moving the holding points and experiment around. May worth playing with...

JS

This was very interesting.