physics/audiology question (samoth, where are you?)

[p0ink]

I need your expert opinion on this, so my fiance doesn't get this question wrong. she sent it to me, but i have no freakin clue.

Question:

A loudspeaker is vibrating at 20 Hz and the amplitude of vibration is very high. I take a light piece of paper and hold it a foot or so in front of the loudspeaker. The paper vibrates in my hand. Why is it vibrating and at what frequency is it likely to be vibrating? (You might want to try this sort of "experiment" with your stereo system. Just make sure your neighbors are tolerant.)

 

[Mr. dB]

If your stereo system will reproduce 20 Hz, especially at high amplitude, you're doing pretty well.

Sound is moving air molecules.

 

[samoth]

I need your expert opinion on this, so my fiance doesn't get this question wrong. she sent it to me, but i have no freakin clue.

Question:

A loudspeaker is vibrating at 20 Hz and the amplitude of vibration is very high. I take a light piece of paper and hold it a foot or so in front of the loudspeaker. The paper vibrates in my hand. Why is it vibrating and at what frequency is it likely to be vibrating? (You might want to try this sort of "experiment" with your stereo system. Just make sure your neighbors are tolerant.)

The why it's vibrating would be because the speaker is sending sound waves out towards the paper. Sound propagates in air, water, paper, and most other media, so these vibrate as the sound's travelling through them.

I don't get if this is aimed at the atomic level or not, if you're looking at wavelength/frequency equations, or if someone's actually trying to measure vibrating paper for some reason.

 

[5150guy]

I need your expert opinion on this, so my fiance doesn't get this question wrong. she sent it to me, but i have no freakin clue.

Question:

A loudspeaker is vibrating at 20 Hz and the amplitude of vibration is very high. I take a light piece of paper and hold it a foot or so in front of the loudspeaker. The paper vibrates in my hand. Why is it vibrating and at what frequency is it likely to be vibrating? (You might want to try this sort of "experiment" with your stereo system. Just make sure your neighbors are tolerant.)

Assuming that your stereo loudspeakers can actually produce a significant amount of decibels at 20Hz, the paper is going to be driven by the compressions and rarefactions in the air. The paper theoretically should vibrate or move at the same fundamental as the cone on the loudspeaker driver if you have good solid accoustic coupling between the air that is forming the compressions and rarefactions.

I forget how long a 20Hz wave ends up being, it is been too long since i had accoustical physics. However, as is the case with true subwoofers, it takes a some distance and time for the wave form to assemble.

Are you using a pure sine wave at 20Hz? At 20 Hz you are not really hearing the sound as much as you are feeling the sound.

Let us know what the answer is. There seems to be a lack of information to get into a more complex analysis and to use math to figure out what is going on.

 

[PICK3]

If your hearing isn't perfect you maynot hear 20Hz.

The range of hearing is from 20 to 20k Hz.

 

[Mr. dB]

If your hearing isn't perfect you maynot hear 20Hz.

The range of hearing is from 20 to 20k Hz.

People tend to lose their hearing in the high-frequency end of the audible spectrum.

A 20 Hz tone, assuming it's a perfectly-reproduced sine wave with no harmonics, is too low to have an audible pitch. You won't hear it as a note on the musical scale.

 

[redsamurai]

I need your expert opinion on this, so my fiance doesn't get this question wrong. she sent it to me, but i have no freakin clue.

Question:

A loudspeaker is vibrating at 20 Hz and the amplitude of vibration is very high. I take a light piece of paper and hold it a foot or so in front of the loudspeaker. The paper vibrates in my hand. Why is it vibrating and at what frequency is it likely to be vibrating? (You might want to try this sort of "experiment" with your stereo system. Just make sure your neighbors are tolerant.)

The paper isn't really "vibrating" is it? It's just getting buffeted by the air coming out of those speakers. The frequency it's vibrating at can't really be determined because of factors such as A) how tightly are you holding the paper B) is there any interference in the room in the form of other sounds or an open window that could be causing air currents that could slightly change the frequency of the sound.

I dunno...........I felt the same way as samoth, is this some sort of question dealing with the atomical properties of "paper"?? what kind of paper? Paper comes from various kinds of trees mixed with certain chemicals.......all which put their own properties into the mix. A vexing question. I don't remember my physics well enough in this category.........is 20hz a deep or high pitched sound? The deeper the sound, the lower frequencies...........tend to travel farther distances. Higher frequency sounds travel much less and don't hold their, what is the word for the horizontal difference between the waves??............well.

 

[5150guy]

The paper isn't really "vibrating" is it? It's just getting buffeted by the air coming out of those speakers. The frequency it's vibrating at can't really be determined because of factors such as A) how tightly are you holding the paper B) is there any interference in the room in the form of other sounds or an open window that could be causing air currents that could slightly change the frequency of the sound.


IF you have solid accoustical coupling, B is irrelevant,


I dunno...........I felt the same way as samoth, is this some sort of question dealing with the atomical properties of "paper"?? what kind of paper? Paper comes from various kinds of trees mixed with certain chemicals.......all which put their own properties into the mix. A vexing question. I don't remember my physics well enough in this category.........is 20hz a deep or high pitched sound? The deeper the sound, the lower frequencies...........tend to travel farther distances.

20 Hz is a low frequency in terms of the audible human range. Lower frequencies DO NOT tend to travel farther distances. They are just lest directive



Higher frequency sounds travel much less and don't hold their, what is the word for the horizontal difference between the waves??............well.

The friction incurred as the small wave lenghts travel through the air and the fact that smaller waves lengths can not bend around objects is more is more relevant.

 

[redsamurai]

friction?? ok, now I'm really scratching my head trying to remember my phyics courses. Sound is basically air being pushed along at a certain frequency..........with what would air create friction with as it travels??

The friction incurred as the small wave lenghts travel through the air and the fact that smaller waves lengths can not bend around objects is more is more relevant.

 

[Nathan]

So disappointed in everyone is this thread, especially you Samoth.

My guess:

It is vibrating because of resonance and it is likely doing so at 20Hz.

 

[samoth]

So disappointed in everyone is this thread, especially you Samoth.

My guess:

It is vibrating because of resonance and it is likely doing so at 20Hz.

I often have a trees/forest problem.

Wouldn't there be multiple resonances depending on where one held the paper? (Ahh! More trees!) I kinda took the "experiment" part of the OP to mean visually counting vibrations or something. I dunno. I suck at why questions because I never know what kind of answer a person is looking for.

 

[5150guy]

friction?? ok, now I'm really scratching my head trying to remember my phyics courses. Sound is basically air being pushed along at a certain frequency..........with what would air create friction with as it travels??

High Frequency Attenuation in Air

First you have the general principal of the inverse square law which models how sound pressure levels decrease generally as you move away from the sound source. The second factor that comes into play assuming the high frequency waves are allowed to propigate in air without colliding with objects is in fact friction produced when the compressions and rarefactions that comprise a sounds wave are forced through the air particles as the wave propigates from its source.

http://books.google.com/books?id=7Q...RQ6O&sig=FK_L5dlHCi8M2I7DslfYr7xEQjs#PPA54,M1

 

[Nathan]

I often have a trees/forest problem.

Wouldn't there be multiple resonances depending on where one held the paper? (Ahh! More trees!) I kinda took the "experiment" part of the OP to mean visually counting vibrations or something. I dunno. I suck at why questions because I never know what kind of answer a person is looking for.

I imagine the resonances are a function of both where you are holding the paper, material, etc. I could always be wrong as well.

And yeah, that was always my problem with quantum mechanics. I knew the answer would always be zero or pi/2 but could never tell which one the prof wanted with which question.

 

[5150guy]

So disappointed in everyone is this thread, especially you Samoth.

My guess:

It is vibrating because of resonance and it is likely doing so at 20Hz.

Are you so sure about this? While resonance plays a part in the picture, how different is this than the diaphram of an expensive Beyer ribbon mic placed infront of (or any decent mic for that matter) of one of the celestions in Hendrix's Marshall cab?

We dont know the size of the paper being held, whether the paper is being held at both ends, and we dont know anything about the thickness and tension of the paper. Too many variables here to really give specific answers.

If it is truly resonance causing the paper to "vibrate" then what would prevent the paper from vibrating at a harmonic of 20 Hz???

 

[samoth]

Are you so sure about this? While resonance plays a part in the picture, how different is this than the diaphram of an expensive Beyer ribbon mic placed infront of (or any decent mic for that matter) of one of the celestions in Hendrix's Marshall cab?

We dont know the size of the paper being held, whether the paper is being held at both ends, and we dont know anything about the thickness and tension of the paper. Too many variables here to really give specific answers.

If it is truly resonance causing the paper to "vibrate" then what would prevent the paper from vibrating at a harmonic of 20 Hz???

You're treading into Mr. dB's territory there, lol.

I think Nate was just giving a concise and correct answer to the question posed. Resonance might not be the right word, but I think I know what he means. The original question implies a rather simple case with a generalized answer, and whether you look at it from a viewpoint of acoustical sound, classical mechanics or as atoms being displaced could bring tons of details into play that probably weren't meant to be there in the first place. (I just want to know what one is supposed to visually observe in such an experiment.)

I think we're both looking into the question way too much. (Hey, look! I think I see a forest!)

 

[Mr. dB]

I think the paper is just flapping in the wind because the air is moving.

 

[Nathan]

I think the paper is just flapping in the wind because the air is moving.

As good an answer as any.