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How to make a reed out of a straw?

(Last Updated On: July 12, 2022)

This project uses low-cost materials to educate students in grades 6-12 and the general population about acoustics and the science of sound. This is a great choice for diverse groups and budgets. Explanations can be modified depending on the educational background of the target audience.

MATERIALS:

Plastic soda straws and scissors (two per person suggested).

INSTRUCTIONS:

Gently flatten the last 3 to 5 centimeters of straw as shown in Figure 1. Cut the straw’s sides in an angle, like in Figure 2.

Gentle blowing through the cut side on the straw, until you hear a sound heard. If you’ve done it correctly you’ll be able to hear a broad-band buzzing sound, with many harmonics. That’s because it’s the sound that comes from the ends working as a double reed exactly like the oboe.Before your outreach event, try this two-step process until you’re able to construct the straw-oboe, without an issue. It’s best to have a practice model.

If you have time, let participants alter their instruments by adding an additional straw or making tiny holes in the straw. The straw’s length will decrease the pitch of your instrument by cutting holes, you can raise the pitch (unless the straw is covered with fingers) through the process of reducing the resonance tube.

PHYSICS:

The straw is an enclosed circular resonator, with sound generators at its top (the triangle-shaped end). The length of the tube defines the frequency of resonance for the straw, and the triangular cut-off end functions as an acoustic generator with double-reeds, that produces sound at a variety of frequency. The frequencies generated by the double reed which coincide with the resonance frequency of the pipe are reflected and create the sounds we hear. The base frequency is connected with the size of the straw using the generalized wave relation that is u=f L Eq (1)

where the u value is the speed of the wave where u is the speed of the wave, f is speed of sound and the wavelength is l. In this instance the speed of sound in the air is about 340 milliseconds. The audible human hearing range is approximately 20 Hz to 20 KHz. Effective length determines the wavelength l. This is the frequency that we hear.

The straw has one end which is exposed to atmospheric pressure. the end that has the reeds is closed. Similar to an oboe this straw has resonance frequency that is multiplications of odd whole numbers because the pressure boundary requirements require that one pipe’s ending be a node and the other pipe’s end has to be an Antinode. The open end may allow air to flow between the pipes (longitudinal waveforms of air) but it cannot permit pressure variations since it is open to the air. The closed triangular end (reeds) do not allow air to move when the reeds are sealed together, but it can allow atmospheric pressure variations. Numerous references have been published that provide the physics of the process in greater detail and are worth studying for those who want to understand the acoustics on a deeper understanding, such as the reasons why a straw can have resonance frequency. 1-3

EXPLANATION:

Make sure you are addressing the level of experience and age of the people that you’ll be working with. Children who are younger will be able to comprehend that the V-shaped tube functions like a reed, since they sense it’s vibrating. It is important to note that the diameter of the pipe is a factor in the timbre or frequency of the sound created. You should have several straws of various lengths available to illustrate this idea.

If you’re working with older people Spend some time explaining that either lengthening the straw or making tiny holes make the straw in effect shorter or longer and causes higher or lower pitches.

If you’re working alongside college kids, you must work through a back-of the-envelope math calculation of what pitch base should emerge out of the straw the length you have chosen.

COMMON QUESTIONS TO EXPECT:

Does the size of the hole impact the volume of the sound?

It’s true. Small holes can make the tube shorter, while a bigger hole makes the hole the end to the tubing. Find out more about how this can be done on this page. 4

Why doesn’t my work?

It requires some practice and effort to create an sound when you first try it, because you require the triangular-shaped plastic ends to be able to resonate. Take extra straws with you so the participants can try it again!

Can this be used in conjunction with different materials?

Yes! It appears that you could use a reed for nearly any pipe and it’ll work similarly. This is a great experiment because it allows you to make the pipe and reed as one thing in a short time. An interesting demonstration is to make a hollowed-out carrot and place the straw or reed at the bottom of the carrot to create the carrot-oboe.

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