The PET-O-Phone

WARNING: Pressurizing PET bottles and playing around with them may be dangerous. Especially if you hit them, jump on them etc. You could possibly get hurt (your ears because of the bang if they explode and the rest of you because of the pieces which may fly around). As usual I'm not responsible for any injuries of yourself (or other people) or damages to your equipment and property (or the property of other people) caused directly or indirectly by anthing on this website. YOU HAVE BEEN WARNED.

The PET-O-Phone

What the hell is that?

Well. This is the product of an idea I had while sitting in the tram. We were talking about unusal musical instruments and I had the idea that PET bottles filles with different pressures of air may be used to like a xylophone. (Everyone who has pressurized an empty PET bottle with an air-pump knows that it makes a nice sound if you hit it.)

How does it work?

The principle is rather simple. The most complicated part is to persuade the air to get into the bottle and stay there. Additionally you need to be able to add and remove air from the bottles to tune the bottles. What's needed is a check valve in the cap of every bottle.

The check valve

The valve needed to be simple and cheap because I needed it several times. Basically it works like the check valves in old bicycles which are simply a small pipe which is closed on one end and has a small hole in the side. A rubber hose is slipped over the pipe and the pressure in the tire presses the hose against the pipe which seals the hole.

If the pressure from the outside is bigger than the pressure in the tire, the hose is pushed away from the hole and the air can enter.

In my case the pipe is a pice of PVC cable holder pipe with 16mm outer diameter. (The one which is used to clip cables onto the wall.) I closed one end by heating it and bending it together. You can use a hairdryer if it is hot enough, a hot-air gun or a small flame (e.g. a candle). But becareful not to burn the PVC because this will cause toxic fumes. Of yourse, this is not airtight. That's why I put some hot glue into it from the inside. I drilled a small (1mm) hole in the side of the pipe. It has to be small because otherwise the force onto the hose is too strong and it can rupture. Carefully chamfer the hole. I had no rubber hose so I used latex surgery gloves which I washed to remove the powder which prevents them from sticking together (but we want it to stick to the pipe). You can use a whole finger of the glove (the simplest and maybe most durable solution) if the diameter of your pipe fits the finger diameter. Mine didn't so I cut off small strips and fastened them with adhesive tape. The whole contruction was hot glued into the bottle caps and a hole was drilled into the cap from the outside to fill in the air. This hole should match your air pump nozzle.

I made 11 bottles and found that most of them are quite tight. Most leak about 1 small bubble (of I hold them into water) every 3 seconds or so at a pressure of 3bar. Such a bottle could be about 0.1ml thus we loose about 120ml air (at room pressure) every hour. At 3bar we pumped in 4.5l root-air. I estimate that they loose about 3% per hour. One of them looses much more and one bottle seems to be completely thight. 3%/h is not that much if you use the bottle as an air tank but if you want to play on them you must expect to retune them every hour or so.

Playing

I decide to play "Popcorn" from "Hot Butter". It's not easy because the bottles which are mounted via a string on the one side and a rubber band on the other start oscillating very strong once you hit them. Additionally I found out that the grooved bottles don't sound as well as the groveless ones because the groves stabilize the bottle and thus dampen its oscillations. Additionally between nearly no pressure and about 3.5bar there is about one octave so don't expect a pitch range of more than one and a half octaves because it is dangerous to hit bottles which hold high pressures with a drumstick (and even if you don't hit them hard, you'll still hit them often).

Pictures

The PET-O-Phone setup. Three broomsticks hold two rows of bottles in between.
Check valve front
Check valve top
Check valve bottle cap

Audio files

What else?

Yes, I made a video where I play on the PET-O-Phone which I won't publish. (I'm against pusblishing videos if they are not necessary to understand something. The reason is, that once you've posted a video you think that everything can be seen in it and should be selfexplanatory, but that's not the way it is. Usually publishing videos leads to even worse documentation, because all the details are missed.)

You can think yourself of lots of interesting things to do with pressurized PET bottles. E.g. you could use them to lift things by squeezing them flat and put them under something before you inflate them. (Such a bottle can lift a considerable amount of mass. I've lifted the wheel of a car which stood on one years ago. That is not surprinsing, think about it: 4bar on lets say 100cm^2 area result in 4kN.) Or you could use one as an air tank for your Leg0. Drill a second hole where you insert a Leg0 hose and pump the bottle up with a normal air pump. (Using a Leg0 pump takes veeery long.)

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