Help for 7th Grade Science Fair Project - How to Generate Sound Waves

[ScienceFairMom]

Hi Everyone,
My 7th grade son watched this video and is trying to make an acoustic fire extinguisher for his science fair project testing the effect of sound wave frequency on it's ability to put out fire.

https://www.cnet.com/news/engineering-s ... ith-sound/

He made a prototype with a SINE wave function generator, a VOX amplifier and a tube like in the video but it doesn't work. He was hoping to get in touch with experts to understand how to create the sound waves - don't know if the sine wave generator is strong enough or doesn't seem to be what they use in the video.


Expert advice would be appreciated.

Thank you,
ScienceFairMom

[vurt]

is he getting any sound at all?
is he using a similar frequency to the project you link to?

it could be he needs to increase the volume a little, making sure to be wearing ear protection!

[CHOOS]

Hi ScienceFairMom

First of all I wanna state upfront I'm a dumb-ass with no scientific knowledge of sound. (nor anything really)

"music is unsuitable -- the sound waves it produces are inconsistent."

"30 to 60 hertz -- seemed to be the so-called "Goldilocks zone" at which the waves were able to effectively keep the oxygen from the flames long enough to suffocate them."

Those two quotes tells us it probably is a sine wave although perhaps you could also play a "consistent" chord of sines between 30 - 60 Hz.
It also stated they used cardboard to direct the sound to the flames which I'm unsure how directional it really is but obviously better than nothing.
How strong is your amplification. I'm thinking this has to be pretty powerful. Also not having the tube made of cardboard means you could get a little closer to the flame.

If you can get a strong amp and a sine wave generator or even play a saw wave so you have lots of consistent harmonics
and play around with the pitch of the sound. One Hz up or down could make the difference.

the picture in the link is how a saw wave looks in individual harmonics.
A sine would be just the first harmonic.
https://www.google.com/url?sa=i&rct=j&q ... 3163429522

I'm probably not helping at all here but just in case you didn't know this info above...

GL and definitely keep me/us posted if you get it (or not)
Charles

[Mushy Mushy]

Thats a very small amp so if the sine is low the speaker wont be able to reproduce it.

[CHOOS]

Thats a very small amp so if the sine is low the speaker wont be able to reproduce it.

Yes wow I cant believe I overlooked that. you need a POWERFUL Speaker too as well as the Amp
A lot of subs (cheap ones) can kinda die off below 50Hz. usually 40 but a good one 10" to 12" ones should have the ability

[vurt]

Thats a very small amp so if the sine is low the speaker wont be able to reproduce it.

good point actually!
a bass subwoofer type thing would be ideal, for that low a freq.

[vurt]

"music is unsuitable -- the sound waves it produces are inconsistent."

i bet sunn0))) could extinguish a fire

[Mushy Mushy]

If you can change the wave to a saw or square it will generate some harmonics which will then confirm the speaker is working.

[Kwurqx]

A fun project. Might be effective. But practical? Then again...curiosity should be quenched

It is actually less about "sound" and more about "moving", "shifting" or even "shaking", thus disrupting the burning air/gas mixture. Since the optimal frequency seems to be some 30-60 Hz, the airpressure differences (waves) generated and used can be classified as sound. As with "sound" it is then about the wave shape, frequency and amplitude.

Low frequencies and high amplitudes (LOUD) will probably be the most effective, compressing and stretching the burning air mixture. Maybe disrupting the air/fuel mixture enough to end the oxidation/combustion process (fire). Intuitively i'd say a square wave will be the most effective. But I guess at low frequency (apparently some 30-60 Hz) and high amplitude any shape will be effective in disrupting the burning aircolumn.

Intuitively maybe even lower frequencies (only several Hertz) at high amplitudes might produce distinctive seperate pressure waves, pulses, blowing out the fire. But these pulses might be to directional and blow the fuel out of the container....

Be sure you protect your ears. Low frequencies at high amplitudes can be pretty destructive. But the ears are deceptively less sensitive to them (see Fletcher/Munson Equal Loudness contour).

[ScienceFairMom]

Thanks for all the great replies. I will share them with my young scientist and encourage some extra experimenting. Perhaps the Amp is too small although they say a "small amp" in the video. Perhaps cardboard is better than a roller Perhaps a different shape wave might work. The amplitude is as big as we could create. Sound was created and at a high frequency could bend a candle flame (but not put it out at low frequency). He has tried the 30-60hz range.

First step might be to use a stronger amplifier for trial even if it's not "portable". Will keep you all posted! Thanks again!

[Michael L]

If this is your son's project, he should be posting.
Just sayin'...

[Gruh]

What a cool project! The issue is surely with the amplifier your son is using. For a start, most guitar amplifiers have extremely uneven frequency response by design--they are meant to sound good with guitars, not to be full-range or particularly accurate--and part of that is typically a very significant bass rolloff. (Voxes have a reputation for being notoriously tinny, incidentally) I wouldn't be surprised if the low end response is meager-to-negligible beneath well over an octave above your target range. Furthermore, reproducing low frequencies even at relatively modest amplitudes requires a good-sized speaker--I'd want to be working with one about 2-3 times that size at minimum--and a lot of power. I would guess an amp that size probably weighs in between 5-10W; you really should be driving several hundred watts for this sort of thing. See if you can find a dedicated sub rated at north of 100W for this project; a decent bass guitar amp would probably do in a pinch--they are designed to handle low frequencies and typically have a much flatter frequency response than guitar amps. A final word of note--really be careful with input levels. I'm not particularly familiar with test equipment, but instrument amplifiers expect to "see" an instrument-level signal, which is a fairly tiny voltage that I would expect to be quite a bit smaller than the average output of a test oscillator. You probably won't damage anything, but I wouldn't temp fate. If your son is stuck using an instrument amplifier, it's probably safer if he patches into the effects return rather than the input, thereby bypassing the preamp and connecting directly to the power amp which (hopefully) is designed for a line level input and flatter frequency response. Best luck with the project.

[vurt]

If this is your son's project, he should be posting.
Just sayin'...

how old is 7th grade?
would you want your kids chatting to us?
have you seen some of the things we post?

[Mushy Mushy]

If this is your son's project, he should be posting.
Just sayin'...

how old is 7th grade?
would you want your kids chatting to us?
have you seen some of the things we post?

This. The dude will be 12 years old.

We would inadvertently tear him apart if we didn’t know.

[Michael L]

No.
I have seen students post questions here and and been treated kindly.
An important challenge of doing a science fair project is being able to communicate your hypothesis and methods in clear writing.