Friday 3 June 2011

Silence and Simplicity

what could pass as some sort of advert slogan is really just my attempt to improve the treadmill.

Prof. Hans-Jürgen Dahmen, one of the main people who designed the very first rodent virtual reality system (Hölscher 2005) has given me a lot of advice and information since I started the project and I'd like to thank him at this point for that. Part of his advice was that having 8 air inlets is unwise, which, after some thought, made perfect sense. Here is why: according to Bernoulli's Principle a larger tube diameter means slower fluid/gas speed. This in turn means less noise which is beneficial as it causes less stress in the animal.

So instead of 8 inlets with an outer diameter of 6mm I now have one single inlet at the southpole with 12mm outer diameter. The surface area of a 12mm crossection is still smaller than the surface area of eight 6mm tubes, however one has to consider that wall thickness on 6mm tubes is the same as on 12mm tubes, which makes a significant difference.

Single 12mm inlet at the southpole instead of 8 inlets.

Regardless of all that though, I found out that by far the biggest source of noise is the valve which regulates air-flow from the compressor. After disconnecting the treadmill and just opening the valve I realised that the noise produced is almost deafening and for a mouse that must be the equivalent of having a jet engine blowing right into it's ear - continuously.

The company that sells the connectors and tubing also sells noise silencers but after a brief test I can say that the efficiency of these units is questionable. However, another useful tip from an engineer worked a lot better: put a generous amount of tubing between the source of the noise and the output point. In my case that is between the valve and the treadmill:
A 20m coil of 8mm tubing between the valve and the treadmill reduces noise significantly. The valve with the red handle is the main source of noise.
The result is a much quieter treadmill, here are two videos to compare the amount of noise, first the loud one:

And here the dampened one:

It's still not exactly quiet, but it certainly has taken off a lot of noise. Should this amount still be a problem I can still think of other ways to reduce the noise.Using a 12mm or 16mm inlet tube and 20 metre 'buffer tubing' of the same diameter for example would probably take noise down another notch. The ball starts turning by itself in the videos but that is purely because I haven't inserted the tube into the cup straight, it just needs some clamping down of the tube to make and keep it straight.

The single inlet at the south pole works well. Because the surface of the cross section is if anything a bit smaller than that of eight 6mm tubes there isn't much difference in terms of performance as far as I can tell. Dr. Dahmen noted that 3-4cm would probably be ideal for me and I might do that at some point should there be a need to improve this treadmill design, but for now I like the simplicity and efficiency of a single inlet.

Single inlet treadmill. Works very well and is much more efficient than the complicated 8-inlet system. I like efficiency.

With this improvement of the treadmill I hope that a mouse will have the pleasent experience of a cool summer breeze stroking its fur rather than the feeling of sitting on a jet engine.

Hölscher, C., Schnee, A., Dahmen, H., Setia, L., & Mallot, H. A. (2005). Rats are able to navigate in virtual environments. Journal of Experimental Biology, 208(Pt 3), 561-569. Co Biol. Retrieved from

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