advice for sound installation

[Brenda Bach]

Hello

I'm wondering if any of you can help me. Apologies if any of these are stupid questions - I'm new to the world of vacuum technology:

I'm based in the UK and I'm trying to source a bell jar, vacuum plate and electronically driven pump that I can use to create a classic 'bell jar' acoustic effect, where sound peters out to (almost) silence as the bell jar pressure becomes (almost) a vacuum. This is hopefully something I'm going to use in the centre of an installation for a sonic arts festival in the UK. I'm looking for a sizeable jar - ideally 30cm diameter (although I'm flexible on this) - as I want to put something quite chunky into it. I've found a site that seems to be selling jars second-hand for £50 or so - that looks good. But I'm not sure where to source the other equipment: the plate, pump etc. Second-hand is always good as I'm working on a tight budget.

As for the quality of the vacuum: all I need is to evacuate the jar well enough to get a very noticeable, classic bell jar effect when I put a noisy object in there (i.e. as close as reasonably possible to a vacuum - but doesn't need to be laboratory standard).

Finally, I need to find a vacuum pump that's electrically driven and that I can hook up to, say, the output of some relays so I can switch it automatically. I want to find a system that will create the vacuum from 1 atmosphere (or vice versa) in, say, 20 seconds. I'm fine with all the electrical side of things (I can make up a circuit to suit) - I'm just stuck on where to find the right kind of pump.

If the pump is noisy, I'll also want to have quite a long tube between the pump and the fan, so the pump will need to be powerful enough to overcome that.

Finally, I'd like to experiment with mixing air with another gas (e.g. nitrogen) in varying amounts to get some interesting acoustic effects. Any tips here on pump systems that might act as good mixing systems?

...does this sound feasible? Any ideas how much such a system is likely to cost? Any tips on companies to try?

Any advice gratefully received

Thanks!

Brenda

[Steve Hansen]

Hi Brenda,

The "sound in vacuum" demonstration is a favorite for showing properties of vacuum.

You don't need an industrial quality pump. A single stage rotary pump of one or two stages and 5 or so CFM will work. A vacuum (as measured by a mechanical dial (Bourdon) gauge of at least 28 inHg (70 cmHg) will do. I'm not sure what pump brands are available to you but the type of pump used by HVAC repair people is appropriate to the task. Brands include Robinair (see http://www.absolute-vacuum.com/manufactures_robinair.php for example) and many others who distribute Chinese made HVAC pumps.

Of the jars you are looking at, are they rated for vacuum? The base plate could be a piece of 1.2 cm aluminum plate and a piece of rubber gasket sheeting. Clear plastic (thick acrylic) will work. Whatever the chamber is, have a safety shield in case it breaks.

The bell should have a broad frequency - some of the little piezo buzzers are high pitched and don't show the effect very well. Make sure that the bell is suspended or otherwise acoustically insulated from the jar and plate. You want the sound transmission to be through the air only.

Hope this helps.

[Steve Hansen]

Hi again Brenda,

Should have thought of this before but a ready-made plastic chamber and baseplate is made by Nalgene and is probably available worldwide. The largest is 12 inches diameter x 12 inches tall. Wires can be fed through epoxied binding posts installed in the base and there is a ready-made pump out port. Needless to say something similar could be improvised from plastic tubing and plate. See http://www.nalgenelabware.com/products/productDetail.asp?product_id=221&subcategory_id=&category_id=168&brand_name=NALGENE+Labware&category_name=Vacuum+Equipment&subcategory_name=