calculating appropriate Sorbothane isolation discs

[almikel]

A recent thread on speaker spikes/isolation, and specifically a post by @svenr got me looking again at Sorbothane for speaker isolation

Last time I looked into it I was a bit daunted by the maths, and the fact that if you get it wrong, you can actually make things worse by introducing a resonant frequency.

At the time I sent a few emails to distributors seeking advice and didn't get far.

 

Today I had another look at the Sorbothane Engineering Guide...

http://www.sorbothane.com/Data/Sites/31/pdfs/product-guides/Sorbothane-EDG.pdf

...and I'm still daunted by the maths - their worked example for Vibration Analysis isn't quite "dummy proof" enough for this dummy....

 

I then downloaded the Sorbothane Design Guide Application and have been mucking with that this evening.

http://www.sorbothane.com/engineering-design-guide.aspx

 

I've also been looking at the catalog from one of the Australian distributors of Suredamp Sorbothane products.

http://www.mackayrubber.com.au/pdf/flex_I_pages/Suredamp.pdf

The disc pads on page 10 is what I'd plan to use - particularly as the Sorbothane Design app caters for this shape - the popular "hemi-sphere" shape is not catered for in the Sorbothane app.

Unfortunately the catalog doesn't include the durometer of the discs - I think this is where I got stuck last time...but with the Sorbothane Design app I'm better informed on what's going on.

 

For my specific case I'm trying to isolate my mid bass speakers from my floor. My PSE horns sit on top of the mid bass speakers, slightly increasing the total mass load.

Isolating the PSE's from the mid bass speakers is a separate calculation for a later time.

This is a paper based exercise currently, so I haven't weighed anything recently, but I think each mid bass speaker is about 55kg and I'll estimate the PSE at 5kg.

Total weight on the floor 60kg, which for this exercise I'll assume is evenly distributed.

I'll choose to use 4 discs under each speaker - so 15kg on each disc.

 

From the catalog, based on mass loading the options are:

• thickness 6mm diam 35mm
• thickness 10mm diam 35mm
• thickness 16mm diam 50mm
• thickness 25mm diam 50mm

At this point I'm thinking everything would be so much easier if the catalog included the durometer for each 

 

Mucking within the app and taking notice of "warnings", you can adjust Durometer, Assumed % deflection and the Excitation Freq.

Playing  with the excitation frequency can easily get negative numbers in % isolation - presumably a sign of issues/resonances.

 

Too many variables to test every combination...still experimenting...but if I got a -ve % isolation, I increased the Excitation Freq till the isolation went +ve

 

For 6mm/35mm - I could not avoid 1 or more "warnings" in the app - Shape Factor was always a warning

 

For 10mm/35mm

30 and 40 durometer - too much deflection

50 durometer gave 20% deflection and only 7% isolation @ 35Hz and -ve isolation below that

70 durometer gave 8% deflection and only 12% isolation @ 42Hz and -ve below that

 

I jumped to 25mm thick and 50mm diam

30 duro  - too much deflection (30%)

50 duro gave 10% isolation @ 21Hz and -ve below that

60 duro gave 12% isolation @ 25Hz and -ve below that

 

Let's assume we want positive and reasonable isolation (say >50%) above 20Hz.

Let's try 2 stacked discs of the same dimensions and duro

50mm thick and 50mm diam

30 duro too much deflection, same for 40 duro

50 duro gave 69% isolation @ 20Hz, 41% @ 15Hz, 16% @ 13 Hz, then goes -ve

60 duro gave 58% isolation @ 20Hz, 14% @15Hz then goes -ve

 

So 2 stacked 50 duro discs (total 50mm tall) looks a reasonable compromise - prob a bit wobbly, but my mid bass speakers are only 600mm tall with a wide base and the PSE on top is pretty stable.

 

Just need to source the discs in 50 durometer - back to talking to distributors again.

 

Using the Sorbothane app was definitely worthwhile, as it was easy to muck with the parameters to see where % isolation went negative indicating the creation of a resonance.

Clearly going on the weight guide only in the catalog won't provide the best outcome if targeting performance down to below 20Hz.

 

The next step is to calculate discs for isolation between the mid bass speakers and the PSE - a different scenario as I probably still want positive isolation above 20Hz but with a low mass speaker...

 

Cheers to @svenr for motivating me to look at this again.

 

On a side topic, I also took note of svenr's comment on the mass of the speaker needing to be around 200 times the mass of the driver to avoid intermodulation distortion (box moving around based on Newton's 3rd law)...

MMS of my mid bass driver = 165g 

Mass of the speaker = 55kg

Ratio of speaker to driver masses = 333 

 

cheers

Mike

 

 

 

[jamiebosco]

I am definitely NOT an expert on this but I do have several Sorbothane hemispheres and discs in 30, 50 and 70 Duro (these are the Sorbo durometers most commonly seen on ebay/Amazon)

 

Everything I've read on the subject has suggested that the thicker/softer the sorbo is ,the better.

 

If using the hemi's I would personally recommend under-loading them as opposed to trying to get exactly "in the sweet spot" or pushing them to the bleeding edge of their load capacity .

 

If the total weight of your speakers would be ~135lbs

These 2.5"x1.25" hemis have a loading "sweet spot" of 52.5lbs each

https://www.amazon.com/Sorbothane-Hemisphere-Non-Skid-Adhesive-Durometer/dp/B0042UA5WC/ref=sr_1_27?ie=UTF8&qid=1485775677&sr=8-27&keywords=sorbothane

 

4 of these would MORE than handle the weight of your midbass and PSE without over-compressing, 3 of them (2 at front 1 at the rear) would have you pretty close to the sweet spot, without over-loading

[Guest rondine]

This is all you need:

 

"The most effective static deflection for Sorbothane 
with a shape factor between 0.3 and 1.0 is in the 
range of 10-20%."

 

ron

 

[SandS]

Can you still get it from Bunnings, need some 50 durometer