Signal transfer properties of a spring-suspended platform?

[interestedperson]

TL;DR Summary

E-drum raiser platform against coupling kick drum pedal sound into the appartment floor: How to approximately model a solution e.g. with springs, to get a rough transfer function, to not just guess and build (another) dud?

Ok, the last time I did something with physics, save for simple in-game simulation of gravity, was many moons ago in high school.
I certainly did not learn to do this in high school, although two subjects I had there at a basic level, are touched: Waves and Classical mechanics.
I have no idea how to model or calculate something like this, and if I look around of how to calculate anything with springs, it never (unsurprisingly) touches anything "signal", ... as that's not what most people are intersted in or are most common topics.

I had the intuition, though, that springs and mass must be able to made work as something like the electrical equivalent of a low pass filter.
E.g. the Hammond (tonewheel) Organ drives the tone wheels with a rather pulsating torque delivered by a 1-phase AC motor, followed by a spiral spring, and a flywheel, which smoothes the jerky drive from the motor into a more steady one.
That seems like a low pass filter to me.

So here it goes:

Say you have a sturdy plywood plate of dimensions 190 x 110 cm² (finger thick with some stiffening reinforcements underneath in a grid pattern - but let's keep it simple).
That surface would rest on a bunch of springs or pogo pins with springs or something like that.
The number /placement pattern, size and stiffness of such springs are to be determined.
(and how to buy parts then, hah. My idea that you get springs in "stiffness" stems from game programming articles - no idea about the real world and what one can buy at a reasonable price)

The goal is to have a signal transfer function from the top of the plate, through the springs (and maybesome sort of feet for them), into the floor,
which strongly dampens anything starting in the low end of the audible frequency range.

This directly stems from a practical application scenario: A drum raiser for an electronic drumkit but very mechanical double bass drum pedals and transfer of contact bump noise through the stand of the "base drum" into the floor.
Some people glue dozens of old tennis balls under such a wood plate, might work, but not for me, for the smell. I only have the living room for this. I.e. I need to live there ;) One, even old, tennis ball smells horrible, let alone 100 of them or so.
100kg of fine grained sand in bags under the thing did not help. (I thought, when boxing into a sandbox, this seems to absorb quite a bit of energy, but apparently not enough - and the bags enclosing the sand limit sideways expansion when hit from above, I guess.)

The requirements do not include my whole body weight resting on the platform while the lowpass effect is at a maximum. (the drum throne sits right before the platform, only my feet on the pedals depress the platform vs. resting force by the drumkit's mass)
I.e. the springs don't necessarily need to be so numerous and/or stiff that they carry my whole weight without being compressed to the maximum. There might even be some sort of guide bushing and pin that come in contact at a certain compression level of a spring, to then create a somewhat stable platform to walk on - while the springs could be very "floppy" when not standing on the platform - if that helps the primary goal.

So it would be neat to be able to model this and build something that will perform roughly as calculated, as opposed to trying stuff out willy nilly.

 

[jrmichler]

So you want to play drums without annoying people in the apartment underneath yours. You have a vibration isolation problem. The Lord Corporation is a world leader in vibration isolation systems: https://www.lord.com/products-and-solutions/vibration-and-motion-control/industrial/isolators. And this page is an excellent discussion of vibration isolation: https://www.lord.com/products-and-s...dustrial/isolators/vibration-isolation-theory.

If the theory is more complicated than your desire to study it, a good rule of thumb is that the vibration isolators should deflect 10 to 15 mm under the weight of the equipment being isolated. In many cases, less deflection (stiffer isolators) will give good isolation. The best vibration isolation system is a balance between soft, to get isolation, and stiff to keep it from bouncing around. You will need to experiment to find the best system.

You may want to make a larger platform that you stand on. The vibration isolation is purely a function of the static deflection, and a larger platform will just need stiffer isolators than a smaller platform. For the same static deflection, the vibration isolation will be the same either way.

Have you considered hanging it from the ceiling to completely isolate it from the apartment below? Small diameter nylon cord is stretchy, and can make good springs. Another possibility is several small inner tubes from, for example, wheelbarrow tires laid flat underneath the platform.

Avoid guide bushings - they will conduct vibrations.

 

[berkeman]

So you want to play drums without annoying people in the apartment underneath yours.

It looks like his drum set is electronic, and the only vibration is from the kick pedal hitting the sensor (not a drum). At first I missed the e-drum part...

E-drum raiser platform against coupling kick drum pedal sound into the appartment floor

 

[interestedperson]

That's right, electronic drum kit, hence the spring-resting platform as an idea - it is solely about (not) coupling sound into the the floor, nothing else.. Alas I can't edit the topic anymore, or I'd make it stand out more.

 

[berkeman]

I can still edit it, but I think it's fine as-is for now. Do you have any pictures of your setup, especially the kick drum part?

 

[Tom.G]

For low-cost approaches:
Try folding a heavy bed blanket to the appropriate size and placing the offending hardware on that. Or try bed pillows or throw pillows. Both supply some isolation and are lossy, that is they dissipate some of the sound energy as heat.

And the idea of @jrmichler of using softly inflated inner tubes is also a good low cost approach.

Cheers,
Tom

 

[interestedperson]

(it seems I don't get how quoting here works ...)
As for what this stuff looks like... Well, I have the plywood thing, resting on 4 sandbags of 25kg each, and a carpet on top. I also tried adding thick multi layers of cardboard directly under the pedals to no avail.
Some people just put massive amounts of foam material from home depot under the platform, but again, that sort of stuff tends to emit aggressive (to my airways) gases into the room air, so not for me.

I will read the so far suggestions and linked to sites this weekend, have been too tired the past evenings.

 

[berkeman]

(it seems I don't get how quoting here works ...)

To quote a sentence or two, click-drag with your mouse to highlight the selection, and then click the "Reply" option that pops up when you complete the click-drag.

To quote an entire post, just click the "Reply" link in the lower right of the post.

 

[sophiecentaur]

(I thought, when boxing into a sandbox, this seems to absorb quite a bit of energy, but apparently not enough - and the bags enclosing the sand limit sideways expansion when hit from above, I guess.)

I would think that this statement contains the right approach. You need to be dissipating Energy and a spring, on its own, just stores it. Also a spring plus whatever mass is involved, will have a resonant frequency. The sort of anti-vibration mountings you can get for machinery tend to be based on rubber cones or diaphragms (your tennis balls idea). The optimum system always depends on details of the particular vibration source. Bigger is nearly always better - problem for your living room, probably.

Your friends with the platforms, sandbags and foam seem to be working on the right lines. Their solution seems to be based on high mass, damping and resilience, which is the basis of a lot of sound / vibration insulation methods. Have you looked at and heard the results (from the downstairs flat)? If it seems to work then why not just modify its by putting the foam in industrial thickness plastic bags, to eliminate the smell? If that system works then use the results of their 'research'.

If you haven't done a lot of Googling then it may be worth while spending a few hours, starting with this Wiki link.

 

[interestedperson]

What I ended up doing is, using a readily made damped spring thingy, and added the mass on top.

I.e. used an old mattress, put the plywood layer on it, and since the mattress is a bit smaller than the platform, mounted some stub legs with hard rubber feet on the outer edges of the platform - but keeping those feet 1cm above ground.

That way, when I walk over the thing close to an edge, the closest-by foot would contact the ground (due to compression of the mattress) and not allow the whole thing to catapult the stuff parked on the platform through the room, i.e. by restricting the tilt angle. But in "operational mode", those legs won't transfer extra sound to the floor.

Since my own weight is not on the whole thing, but the drum throne is right before the platform, this seems to work well.
No new complaints so far.