A Question for all you Physicists out there....

Discussion in 'Off-Topic Chat' started by TuTuKu, Dec 2, 2004.

  1. TuTuKu

    TuTuKu Active Member

    As part of my A2 coursework in Physics, we have to do some sort of experiment.. idea left entirely up to you. I had the harebrianed idea of blowing down various lengths of tubing and trying to find a relationship between frequency of the sound and length of the tube (and then possibly using this to find out how much tubing is added and taken away from a cornet each time a valve is depressed... :-? ). After playing around with various different types of tubing, I rekon that the bunsen burner stuff is the easiest.... a soft ish rubber tubing, diameter around 8mm (It is also the nearest size to a cornet)

    The only problem is that I can't get a sound out of it unless I stick a mouthpiece in the end and blow through that... will this effect the overall result and give false readings? I also used hose pipe, but it's more difficult to cut and doesn't produce such a clear sound - but i can produce a range of sounds without a mouthpiece as the diameter is larger.


    A rather confuddled TuTuKu
  2. ronnie_the_lizard

    ronnie_the_lizard Active Member

    Not a physicist, but someone with reasonable experience of planning scientific studies and of peer reviewing papers for a variety of scientific journals:

    If the aim is to correlate lengths with those in a brass instrument then a mouthpiece should be used in order to more accurately mimic the final article.

    In any case, as with any experiment, the design is pretty much optional as long as it is standardised every time it is used - i.e. it would be wrong to compare 100cm of "small rubber tube with a mouthpiece" to 100cm of "large rubber (hosepipe) tube without a mouthpiece" as you have then changed 2 variables and your experiment will not tell you which change made the differences.

    Comparing 50cm of small tube with a mouthpiece with 100cm of small tube with a mouthpiece would be a valid comparison BUT the results may not give you a nice simple linear regression because the length and construction of the mouthpiece will be a constant factor additional to the linear alteration in tube length.

    The other problem I would see, from aplaying point of view, is that the flexibility of the rubber may make it difficult to get a single clear pitch (or clear set of harmonics) - when you blow through the mouthpiece you might well be able to easily alter the pitch of the notes simply by 'lipping' them up or down a tone or two. This might add a significant amount of 'observational error' into the process of determining the actual pitch of the note produced by the tube.

    You may want to consider reading :

    The Physics of Inner Brass and the Acoustical Effects of Various Materials and Their Treatment
    http://www.dallasmusic.org/schilke/Brass Clinic.html

    and also


    ......or possibly not......... ;)

  3. DublinBass

    DublinBass Supporting Member

    I see this as a big problem as well. I would recommend buting something like 2 meters of PVC pipe, and then after repeating each trial 5 times (or until you get consistent results within 10% of the mean), methodically cut off 0.20 meters at a time.

    You also need to decide which patial you are going to play each time and keep that consistent. I would choose the second partial because that means there is one full wavelength in the tube. (that would be the C one ledger line below the staff - instead of the first partial, pedal C which is only half a wavelength)

    You may also want to do some background research into Bach's equal temperment system for your planning. I think the formula is something like frequency of a given note = frequency of 'C' * 2 ^(x/12) (Where x is the number of half steps up from C the note is)
  4. Nuke

    Nuke Active Member

    arnt i glad i do biological science
  5. brasscrest

    brasscrest Active Member

    You also need to take into account the geometry of the tube - curves can change the internal physics of the sound, so you would either need a straight, rigid tube or some way to keep the flexible tubes bent in exactly the same way for each test.

    If you could control temperature and humidity that would also help - these can also be factors (barometric pressure also, but you'd have to have hyperbaric chamber in order to control that).
  6. 2nd man down

    2nd man down Moderator Staff Member

    You people are far too clever by half :-?

    I'll see you in the pub.
  7. Big Twigge

    Big Twigge Active Member

    What is puzzling me is why, when I saw the title of the thread I still looked to see if I'd know....given my advanced physics knowledge :-?

    (which pub? ;) )
  8. BoBo

    BoBo Member

    Fancy thinking anyone on this site would know about blowing down tubes :shock:

    Good luck with the experiment.
  9. super_sop

    super_sop Supporting Member

    perhaps you thought it said psychics??;)
  10. DublinBass

    DublinBass Supporting Member

    I think the easiest way to do this is not to find some high tech isolated chamber, but rather look at the weather forecast and find a day that looks relatively stable. Then record the temp/ humidity/ air pressure, etc.... of the room before and after all your trials (try to get them all in within an hour or so).

    There should be little variation on a good day to help ensure that you do indeed have a "fair" test.
  11. TuTuKu

    TuTuKu Active Member

    Thanks for all your replies.

    I didn't realise that the curvature of the pipe would have any effect.... but now I do I will endeavour to keep it straight... cheers!

    I did think I might have to measure the accuracy by seeing how far I could bend each note with my lips, although this would probably get less as I got into the higher frequencies (could be a whole seperate experiment!) but I don't see it as a major problem, gives me something to write about in the conclusion/evaluation and gives me the chance to utilise the 'error bar' function in excel (can hear the marks rolling in...)

    I do have some PVC pipe and could use it, but its diameter is larger than that of the bunsen pipe, or a cornet. The main reasons I thought the bunsen pipe would be better was because it was the nearest diameter to a cornet, it produced a clearer sound, particularily in the lower freqs... and the mouthpiece fitted in snugly, and no air leaked out of the connection. But I can see the problem with it being a softer material than the PVC or hose pipe.

    As for the temperature/humidity, I'll get a set time that I can lock myself in the physics 'dark room' so I can't really pick a time to suit the atmospheric conditions.. although I will try and see if they have any equipment to monitor it.

    Thanks a lot for all your suggestions, they've been really helpful!

    A less confuddled TuTuKu !!
  12. brasscrest

    brasscrest Active Member

    I'm not sure it would be measurable - but it would be an uncontrolled variable, which you should avoid if possible.

    If you do all of the experiment in one sitting, you're probably OK. If you can, note the temp/humidity at the beginning and at the end, just in case someone questions it.
  13. six pints

    six pints Active Member

    just a very drunken thought..
    if ya has to say why this type is useful (cos i av to in my labs reports know) then talk about how when a instrument warms up it gets sharper, which is very obvious looking at v=f lamba dont have greek letters.
    im going to bed.
    that makes a bit more sense now anyway.
    Last edited: Dec 3, 2004
  14. DublinBass

    DublinBass Supporting Member

    Actually, seeing as how it is generally accepted that wavelength and frequency are inversely related...six pints has a good idea.

    What if you were to use the same length tube, but only vary the temperature of the room to see how it affects pitch? That might be a more practical experiment. Not only that but if you get a good cold day (say about 5C outside) and you can crank up the heat in your house/flat (to say 30 C) you might be able to get some incredible useful data.
  15. TuTuKu

    TuTuKu Active Member

    It's a good idea! but there's no way of varying it at college, and haven't got the computer software and equipment to do it at home... and also our heating's broken here, brrrr it's freezing, so we're sat in huge jumpers and a scarf!

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