Sound Absorbing Foam

[Silicon]

Vintage equipment used a type of foam that degraded over time into a brittle foam that disintegrated into powder when it was disturbed.

Sometimes it was used to absorb sound in a reel to reel tape recorder.

In my Telefunken Magnetophon 85 it was wrapped around the EF86 preamplifier valve.

Foam sheets were also stuck onto the loudspeaker chassis.
It was also used to guide the airflow through the capstan motor and to act as ‘bump stops’ for mechanical linkages.

I am looking for a foam sheet with a thickness of about one centimetre.

What foam types do you recommend and where do you get them from?

Brendan

[brenellic2000]

I think it's a case of 'googling' "sound absorbing foam". Local upholsterers will stock a wide range, but their sound absorbing efficiency is another matter. RS will likely stock it.

Critical to their use is the type of glue; a water-based rubber solution is probably best.

[Goldie99]

The foam you've described sounds like a degraded flex Polyurethane foam, after 50+ yrs of ageing. Although flex PU foams are engineered for acoustic properties today, it's very unlikely that would have been the case when the Magnetophon was made in '67/68, only 10-12 yrs after the introduction of flex. PU foams, and particularly not at only ca. 10mm thickness. I suspect they were just conveniently available at the time...

I'd agree with 'brenellic2000', local upholsterers, or even eventually your local market, are likely to have suitable PU foam 'sheet' materials available. They'll typically vary by density and hardness, but 'hand's on feel' will probably be your best guide. Alternatively, another option may be some of the neoprene 'tapes'. I sometimes use various self-adhesive types, that are widely available on eBay in various widths, thicknesses, etc.

[G6Tanuki]

Also if you have a car audio place nearby ask them if they have any offcuts of the sound damping foam used to stop body panel resonances. Dynamat is one brand I am familiar with.

I used it in the past to damp resonances and drumming noises from rack mount data servers when they were used in an office.

[mark_in_manc]

As G6Tanuki says, gluing foam on will deaden mechanical resonance. If you want to have an acoustic effect (on airborne sound) then the foam must be open-cell - the 'interstices' (as they say...) interconnect. It tends to be a bit less mechanically stiff than furniture foams, which are usually closed-cell.

I have some acoustic foam 2" thick. If you can be bothered cutting it about and want some to play with, you are welcome FOC though postage will cost you something.

cheers
Mark

[DMcMahon]

Encountered the foam issue loads of times particulary in tape recorders when even if the foam looks OK, as soon as it is touched/distubed it often will crumble to dust, particulary common with the brown foam.

I normally replace with white flexible firm foam like (may not actually be foam) sheets that often comes as packing material (not Polystyrene), have no idea of its sound absorbant properites

David

[Silicon]

I was thinking of using some adhesive backed foam (possibly neoprene) for some of the smaller applications. I have some already.

I could attach packaging foam with double sided adhesive tape.

Some Yoga mats are made from EVA plastic. EVA seems to have some useful properties.

[McMurdo]

would a foam windshield from a microphone work around the valve? They come in all sorts of sizes

[vidjoman]

Speaker cabinets can be deadened with carpet underlay stuck on. Various types are available from soft rubber to dense foam. Often available as free offcuts.