Music and sound science fair project

[jua]

My science fair project, which has to be completed by Monday, is on how the length of a column of air of my trombone slide affects the pitch. My hypothesis is that the longer the slide the lower the pitch because there is a longer column of air. I've had problems with the design of my experiment. The experiment I did first was blowing a straw in a glass water cup filled up 25%, 50%, 75%and 100%. I cut a plastic straw through about 3/4 of the center (down about 3/8ths of the length) and bent it at a 90 degree angle. This either didn't produce a great sound that could be picked up on my chromatic tuner or it wouldn't give me a consistent note. Then I decided not to cut the straw and just blow over the opening of the straw when it was in a glass of water. This produced better notes that could be picked up on a tuner but it still wasn't consistent. (I did three trials for each water level.) Two trials showed that the longer column of air (25% water volume) produced a higher pitch, the opposite of my hypothesis. (I compared each note to a piano note to get the frequency.) So I decided to do the same experiment except with a plastic bottle. It gave me a consistent note for each of the water levels. Are the two experiments the same concept? Also, with the straw I couldn't control my air speed for each trial so I wonder if the speed of air changed the pitch? Sometimes the water level moved around when I blew, too. With the bottle I could blow at a pretty constant air speed and the water wasn't displaced in the bottle. Do you have another explanation? What is the point of cutting a straw and bending it? I've read the Science Buddies experiments, Do-Re-Mi with Straws, Blowing Bottletops and Singing Wine Glasses all by Andrew Olsen. I hope you can answer me as soon as possible. Thank you for your help.

[deleted-71691]

Hi Jua,

The straws and bottles both test the vibration of a column of air so the concept is the same. I agree with your hypothesis, and am therefore confused with your results, where the longer column of air produced a higher pitch. To my knowledge the more water that is in the bottle, the shorter the column of air, and the higher the pitch.

[deleted-71588]

Congratulations, you have wandered into some geometries where scientific simplified formulas no longer explain everything.

Assuming you are using a small diameter thin wall plastic straws and adjusting the effective length and playing it like a flute by passing air over one end, there are several things that can be happening to affect the simple length / frequency relationship.

Any slit can act as a reed and vibrate independently at its own resonant frequency independent of any standing wave air column resonance. Any bend alters the structural integrity of the tube.

The thin wall of the straw can deform and vibrate in sympathy with any other vibration (standing air wave resonance or reed vibration) and produce harmonics. Look up harmonics of string vibrations to get a simple understanding of this property. A hollow thin wall pipe/straw can behave both as a simple string and a more complex object particularly if there are any bends or slits that alter the uniformity of its structure. Fully understanding what is happening in any specific complex set of conditions is extremely difficult and often not worth the effort even when you have the measurement equipment and knowledge to pursue investigating it.

I suspect that you wandered into an area where you are trying to use too thin and too small a diameter straw for the length and secondary effects are suppressing the desired/expected base resonance of the
column.

The design and construction of musical instruments has hundreds of years of practical experience with materials and geometries to reliably produce instruments. When something didn't work other approaches were used and refined and it evolved as an art of building and playing instruments.

[jua]

Wow, thanks for the explanations.