Suggestions for a metal for use in a aural/kinesthetic exhibition

[Hohohobo]

I'm working on a project to be exhibited early next year and I'm seeking some advice on one particular element.

The project explores the kinesthetic properties of sound, and the ability to transform an aural experience into a kinesthetic one.

A custom music track will play in the room around a series of metal plinths that the viewers will sit on. I'm seeking a metal that will resonate easily from the bass produced by the subwoofers.

I understand that a softer, thinner metal is going to resonate easier. I was wondering:

1) If I could get some suggestions on suitable (and cost-effective) metals to fashion the plinths from.

2) If there's a particular metal that can better transmit the vibrations from the subwoofer to or through the human body.

3) Whether anyone can recommend materials that will resonate from exposure to higher frequencies?

4) Whether there's information on how to use different frequencies to trigger physical sensations in the human body without the use of a conduit (i.e. the plinth).

Ideally I'd like to exhibit a series of plinths made of different metals fashioned at different thicknesses so that they're all unique and offer a unique kinesthetic experience.

Please excuse me if my terminology is inadequate or inaccurate.

 

[Bobbywhy]

Hohohobo, Welcome to Physics Forums!

You use the term “plinths” that people will sit on. The meaning of “plinth” is a base or support for a column or artwork, etc. Now, you have said you expect this structure to vibrate in sympathy with some bass frequencies. So what the people will sit on are not “plinths”; they are called “cavity resonators”.

Softer and thinner construction metals do not enter into the equations of acoustic resonance. To discover how a cavity resonator functions, see:
http://hyperphysics.phy-astr.gsu.edu/hbase/waves/cavity.html

Generally speaking, as the desired frequency of resonance increases, the volume of the resonator decreases. If the visitor is sitting on the resonator and it is driven by the correct resonant frequency, it will vibrate and she will probably feel that.

It’s really important to get your acoustic vocabulary correct. Using terms incorrectly only causes ambiguity and confusion. Study and learn the meaning of the terms you use, then everyone will understand your ideas clearly. One good website by Dan Russell, Ph.D., Professor of Acoustics at Penn State to study is: http://www.acs.psu.edu/drussell/demos.html

Your acoustic project sounds like fun and a great opportunity to educate folks at the same time. If you have any doubts or further questions, come back here and post them. Members here are always willing to assist any true searcher willing to learn more science.

Cheers,
Bobbywhy

 

[Hohohobo]

Hi Bobbwhy,

Thank you for taking the time to respond to my questions and for recommending these resources.

Unfortunately it's difficult to properly convey my requirements as a layperson in physics. When I mentioned the plinths, I used the term to give a sense of the shape and size of the object and because when the viewer sits on the plinth they become a part of the artwork.

I'll be sure to study the recommended resources and get back to you should I have additional questions.

Cheers.

 

[Studiot]

It is difficult to know what exactly you are attempting but,

I understand that a softer, thinner metal is going to resonate easier. I was wondering:

Not exactly.

When manufacturers make loudspeaker cones they have to balance two conflicting requirements.

The heavier the cone construction the stiffer it is. That is the whole cone is not floppy and moves in sympathy with the driving signal.

Unfortunately the penalty for this is the heavier the cone the harder it is to drive it at the signal frequency and the lower its resonant frequency so the less efficient it becomes as a generator of sound.

This is why manufacturers go for thin, yet stiff, materials.
Some more expensive solutions have included sandwich and honeycomb construction - to save weight yet make a stiff cone.

I fear that the thinner your metal sheeting the more likely it is to resonate at too high a frequency for your purposes. But the thicker it is the more energy you require to move it, due to its weight.

Auxiliary bass resonators are often made from thick polystyrene sheet or blocks for this reason.

Cavity resonators are of no value to you since you wish to affect the walls of the cavity, not the air inside.

 

[davidm1732]

I think Studiot and Bobbywhy are approaching this problem using somewhat different methodologies...
Bobbywhy is suggesting a cavity resonator. The source signal would excite the acoustic cavity at it's resonant frequency, and you'd see high internal acoustic pressures in the cavity at that frequency, which would then couple to the walls of the cavity, and transmit the vibration to the person sitting on it. A Helmholtz resonator works this way. The advantage of this approach is that the cavity resonance frequency is somewhat unaffected by the person sitting on the structure.

So we have an ambient acoustic field that excites the acoustic cavity resonance, which couples to the structure.

I believe Studiot is suggesting a direct couple between the ambient acoustic field and the structure. One potential problem could be that the person sitting on the structure will affect it's dynamic properties...ie: the modal response of that structure is going to change with the mass loading of the participant, so it could be hard to "tune", if that's the goal of the exercise. A physical example of this is to try to get a tuning fork to vibrate by exposing it to acoustic excitation... it's just the reciprocal of how we usually use a tuning fork, so it is possible... but we normally don't alter it's dynamic properties by mass loading it. Tuning forks also work better if you couple them to an efficient acoustic radiator... a larger surface that can drive more air and create a louder response.

The original question did ask for material recommendations...so the key here would be a material with very low structural damping properties. Think of a material you'd make a bell out of... brass, aluminum etc... Materials with high damping wouldn't work as well, plastic, wood, cast iron, copper. I'd think you'd want a material with very low damping to minimize energy losses as you transfer the acoustic pressure vibrational energy into structural vibration.

A hybrid solution could involve aspects of both suggestions... a cavity resonator that is excited through fluid/structure interaction, vs fluid/fluid interaction... IE: imagine a box with flexible side walls. The side walls are tuned structurally to have a first mode shape at some defined frequency. The cavity of the box is designed to have an acoustic mode that matches the structural mode of the side wall. When ambient acoustic energy hits the side wall, it resonates, and excites the cavity mode. The cavity mode then excites the top of the box, where the person is sitting. You'd have to do some math to figure out which method is most efficient at getting the area where the person is sitting to vibrate...and what shape you want for this...since it does have an artistic requirement I'd assume. I'm not sure the cavity/helmholtz resonator would need the large surface areas that the hybrid design would likely need...

to answer the last question, there is data available on human body parts resonance...ie: at what frequency do body parts resonate at... I think I've seen info on this in "Noise Control Engineering" by Beranek & Ver... usually we're trying to avoid those frequencies... not encourage them!