Expanded polystyrene as a muffler

[atferrari]

I was asked to help in finding a simple/cheap way to muffle the whine coming from four small (really small) inverters driving electroluminiscent panels used in PC "moding".

Snooping with a scope found that output frequency goes from 400 to 2000 Hz depending the units.

Since they are commercial (sealed) units the only mean that came to my mind was to pack them in expanded polystyrene (the one used for protective packaging).

My capability to hear high pitches was lost years ago so I can not judge by myself and have to relay on what younger people says.

The power involved is minimal. Temperature of the inverters doesn't change when in use.

My questions:

a) is this a valid means whitout using a bulky packaging of polystyrene? There is little space available.

b) may I be sure that I am not creating another unexpected problem? Not being hygroscopic, my last concern is electrical isolation or ...?

Suggestions will be appreciated. :!:

Agustín Tomás

 

[stevez]

Some things to think about -
1. Do what you can to reduce the generation in the first place. That option may not be available to you but it's worth mentioning.

2. You could enclose the invertors completely with a sufficiently dense material that will stop the sound. 'Completely' means just that - completely - no little holes or cracks. 'Dense' can mean something like lead or a leaded plastic - or thicker alternative materials that will not transmit any sound. Alternative materials will do a good job - and may be good enough. Sometimes a sealed metal or plastic box attenuates the noise enough.

3. You could attempt to absorb the sound so it can't reflect off of surfaces. Some materials are better than others at doing this.

4. Quite often a combination of stopping the sound and absorbing it is the way to solve a problem. You might enclose the invertor so that there is no direct path from the invertor to the exterior of the enclosure then line the enclosure and any pathway to the exterior with an absorbing liner. This is often done when a pathway for cooling air is required. Note that a fan or some other means may need to be employed for cooling.

5. The acoustic performance of materials is frequency dependant. As frequency goes up it's easier to attenuate the sound. The big HOWEVER is that our ears do not respond uniformly across the frequency spectrum. It's easier to attenuate higher frequencies but we hear those better too.

6. Distance can be your friend - as you increase the separation between source and receiver the reduction or attenuation is fairly dramatic.

7. Back to the generator in the first place - is there a component or two on the invertor that is the bad actor?? Can you pot or otherwise cover the bad guy(s) without fouling up the cooling or other characteristics?

 

[Oznog]

A problem that comes to mind is an EL driver makes some heat. Polystyrene is of course a good thermal insulator and can create signficant heat buildup, which is bad for the device.

Now keep in mind polystyrene is flammable and melts at only moderately high temps. So be careful where you put it! I'd have more than a little concern about that heat buildup inside the foam too.

Also, be aware that not all the EL noise actually comes from the inverter itself. The wire itself does vibrate and radiate sound at the inverter freq.

 

[Pyroandrew]

I know the perfect solution. Have you ever heard of silicone, that's polymerised silicon, the stuff fake tits are made from. Silicone is one of the most versatile resistant materials available today. It comes in a variety of forms. It is considered something of a super material. It comes in gels of various viscosity and stickiness etc, it comes in hard and soft rubber and plastic forms. The list is endless. I would suggest using a gel and casting you components in it, a bit like potting systems into boxes. Instead of using potting compound, use a silicone gel.

I have used something called SILGEL 612 before, it is one of a very wide range of silicone products made by Wacker. This one is probably a little too unviscous for your application but it's a good start. It has a very high thermal conductivity and dielectric strength. It is sticky and soft, yet extremely resistant to plastic deformation. Here are the two best properties, it is probably the best, if not one of the best materials ever for mechanical isolation (especially audio frequencies), this means that you will not even hear a peep, and it comes in two parts, you just mix them and pour into your potting container (with your components of course, making sure your board is not touching the side and the noisy one has the most material around it) and it will vulcanise within 8 hours at room temp (I think). It's not that expensive, and if you play your cards right you might be able to get a free sample, Wacker usually send out 1L samples, PM me if you want to know telephone numbers and distributors.

Hope this helps

 

[Pyroandrew]

Oh, it's not flamable either.

 

[JimB]

Beware of polystyrene, it can build up static charge which can destroy semi-conductors, paticularly CMOS.

JimB