Vibration table construction project for amp servicing use

[nevetslab]

I’ve got the permission to build a vibration table to assist in troubleshooting amps that seem to misbehave from speaker cabinet vibration coupling, or, just to try and shake loose something that you can’t find by hammering on it with your fist or rubber mallet. In our rental inventory, we have a number of Guitammer Co. Buttkicker Concert Vibration motors that are normally mounted to the bottom of a drum stool, and driven with LF signal to let you ‘feel’ the low end while you’re sitting at the drum kit.

So, my idea is to build a shock-mount isolated platform large enough to place Ampeg SVT’s, Fender Twin Reverb combo amps, Mesa, Vox, most any normal size combo amps on, as well as most anything we use in backline applications. Mount the Buttkicker vibration motor to the bottom of the platform, have it shock-mounted on supports attached to the inside walls of a sturdy open or closed-frame housing.

I’m just at the thinking stage of this, and began looking for shock-mount isolation pads, and found what might be suitable for loads up to 64 lbs from Tech Products, a vibration and shock control company, who make some plate-mount rubber-isolated pads that accept 1/4” mounting bolts and aren’t outrageously expensive. Their 62434 Series has a load range of 6 to 16 lbs, which, if I’m using four of them, would be 24 to 64 lbs, if my thinking is correct. Probably a 3/4” piece of Baltic Birch, say 30” x 15” with stiffening braces on the panel, suspended by the shock mounts, and set into the frame of the housing. Haven’t figured the housing details yet….could be open frame with 2 x 4’s, could be with 3/4” plywood 4-sided box, properly braced to suspend the vibration plate with suitable clearance. No doubt it would sit on the floor with suitable rubber bumpers, and could handle most amps without bottoming out. A Twin Reverb with JBL’s in it WOULD bottom out the 62434-16P mounts. So, the load capacity is one of those issues someone smarter than me would have suggestions on. They're mostly there to let the Buttkicker Shaker Motor do it's thing and not waste the energy by coupling it into the frame.

I’ve attached the data sheet of the Buttkicker vibration motor, along with Tech Products’ Plate Mount shock absorbers as a starting point.

NoVibes-Platemounts.pdf
ButtKicker_Concert_User_Guide.pdf
ButtKicker_Concert_Motor.pdf
62434 Plate Mount Vibration Suspension.pdf

The sort of drive signal I'd be using is Sine-Random noise from a Bruel & Kjaer 1027 or Filtered Pink Noise from the same generator with companion 1617 1/3 Octave Filter.



This might be the time to finally open up my Solid Works software and relearn 3-D drawing, having only done that in ACAD 13 & 14 years ago. I don't yet have any sketches drawn up to post, but that will be coming.

[Enzo]

Used to be they had think felt-ish pads for under typewriters, to isolate the machines from the desk tops. Cut down on noise in the office. Don't know what similar is out there. But back in the early 1960s on the wrestling team, the matches were held on slow footprint pads about 2" thick. By setting your UUT ON something like those instead of suspending, might be simpler?

For rubber suspensions, maybe something heavier, more like motor mounts from automotive land. In any case, seems to me it would be better to use compression rubber mounts rather than suspending things with rubber.

I used to have a couple wheel tables in the shop. A roughly two or two and a half foot square table a bit lower than bench/counter top. And on wheels, so I could roll it around. Set a Fender Twin on it, and I can sit there and spin it front to back as needed. Then pull the chassis, and set chassis on bench. SO instead of some big dedicated piece of furniture, consider a smalll piece of plywood say 18x24" and bolt your buttkickers to the underside. This would be large enough to sit an amp on. Give the board a few inches of legs to clear the buttkickers, and some sort of padding to isolate from the table top. Now, you get this thing out only when you need to run this test. Set it on a wheel cart, or on an empty bench, or wherever you have some horizontal surface to use. Otherwise it is out of the way.


Not sure I like your test signal. Scope says 20ms, and I see roughly two cycles per grid, so 100Hz? Modulated by 8-10Hz? The 100Hz seems un needed. You know what occurs to me as a signal? A drum machine set to a couple patters. Crank that through whatever drive amp you had going. That seems like a fairly reasonable test signal for the amp as it mimics music and the sorts of vibrations on stage.

Sounds like a place for an old Crown 300 - those worked all the way down to DC, hence "DC300".

[R.G.]

It's hard to get a vibration stage to go to other frequencies than it's natural/resonant frequency (including the shift caused by the mass of the equipment on it), so you might want to think of a "whacking" table. Set up a table that floats on supports or some such and contrive a motor or something to whack it with a lever every so often. The impacts are good at excitations other than the resonance of the equipment/table/supports, although not perfectly so.

Big vibration tables are HUGE affairs, with multi-KW amplifiers forcing them. They have to be so big/massive/powerful that they can ignore the equipment being vibrated as affecting them.

[nevetslab]

It's hard to get a vibration stage to go to other frequencies than it's natural/resonant frequency (including the shift caused by the mass of the equipment on it), so you might want to think of a "whacking" table. Set up a table that floats on supports or some such and contrive a motor or something to whack it with a lever every so often. The impacts are good at excitations other than the resonance of the equipment/table/supports, although not perfectly so.

Big vibration tables are HUGE affairs, with multi-KW amplifiers forcing them. They have to be so big/massive/powerful that they can ignore the equipment being vibrated as affecting them.

You bring up a very valid point. I've found that prime resonance to be different on various combo amps in the past, but forgot to apply that to this initial thinking stage. Still, having sat on a number of the Buttkicker and Porter-Davies vibrating drum stools that we have here in our rental inventory, with the Buttkicker motor being readily available, my first thought was a shock-mount isolated platform. Using that vibration motor still seems like it might offer a troubleshooting solution sometimes.

Instead, perhaps the way to make use of that motor would be to come up with a clamping system that would allow me to clamp it onto whatever gear I'm looking to excite, when the need arises, to see if that can help find a culprit that normal methods have failed to find. Still thinking out loud on how to use what's available to me. It is a 2 ohm motor, so Crown DC300's (have one here in the shop) is lower impedance than it likes to see. I've a different amp in mind for that.

Though, as we all have found, there's nothing so satisfying as giving a troubled amp who isn't revealing it's fault a good thumping to make it work again, for however long it will.

[nevetslab]

Having stopped off at my local industrial surplus hardware store to see what I could find in large washers for curing bent Fender SF front panel escutcheons (and found some aluminum washers suitable for dirt cheap), I also found a tub full of some Robinson Technical Products MET-L-FLEX vibration shock mounts, having a load range between 12-24 lbs, and bought four for $5 ea, just to have a look at them.



Robinson has been absorbed into some other business interests, while Hutchinson Aerospace Industries appears to have taken over the MET-L-FLEX products in their vibration shock mount products. This item makes use of a metal mesh shock mount spring, has a patent that I looked up just to see what else I could learn about it. I found a similar product in their current product line, indicating it's natural frequency is 15-20Hz, transmissibility @ resonance is 3-4 (whatever that means), weighs about the same as what I bought (2.4oz). Dimensions of this shock mount I found is 1.75" sq, suspended mtg bushing 1.675", ID is 0.255" (good for 1/4-20 mtg hardware). The current data sheet similar to this is here:

bcdi_prod_metal_mesh_1200.pdf

I also brought in my Roland TD-10W V-Drums controller, with which I can dial up suitable kick drum and other drum sounds to place in a loop for both running thru bass amps under test or used to excite a shaker motor. Means of coupling that to whatever combo amps or amp heads still being considered. Maybe even strapping one down tightly with truck securing straps could be a solution?

[tedmich]

Viscoelastic urethane (marketed as Sorbothane in the past) is wonderful for dampening small amplitude vibrations in a variety of frequencies, McMaster sells 1/2" pads that work wonderfully to isolate studio monitors for example.

Shaker platforms! Many vibration transferring speaker units out there, Bass shakers for the low frequency (Nissan 300Z had vibrasonic seats!)
https://www.parts-express.com/cat/ex...transducers/18
and many now will do wide frequency range

this guy does not know audio and is painfully dry but a decent empiricist.

My Dad told me about seeing a 1M watt tube amp used for vibration testing of BIG equipment