The Naked Scientists

The Naked Scientists Forum

Author Topic: Why can't I block out low frequency sounds as easily as high frequencies?  (Read 28106 times)

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
When my neighbour is listening to music, I can keep the higher frequencies out of my house by closing the windows and doors, but I can still hear the thump of the low frequencies.

I think it's because the low frequencies exert considerable pressure on the walls and windows of my house so that they re-radiate some of the low frequency sound into my house, whereas the higher frequencies tend to be either reflected or absorbed by the walls and windows.

What do you think?
« Last Edit: 28/09/2010 22:38:38 by chris »


 

Offline neilep

  • Withdrawnmist
  • Naked Science Forum GOD!
  • *******
  • Posts: 20602
  • Thanked: 8 times
    • View Profile
High frequencies are very directional so they are relatively easy to deal woth whereas low frequencies are omni directional and thus radiate all over the place. This is why a sub woofer can almost be placed anywhere in a room without ewe knowing where the thumping bass is coming from. ....so..in addition to other elemets for which I am sure klevur peeps here will discuss.... I am sure this must have something do with it.
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
High frequencies are very directional so they are relatively easy to deal woth whereas low frequencies are omni directional and thus radiate all over the place. This is why a sub woofer can almost be placed anywhere in a room without ewe knowing where the thumping bass is coming from. ....so..in addition to other elemets for which I am sure klevur peeps here will discuss.... I am sure this must have something do with it.

That's very true! The LF stuff is more omnidirectional. I'm wondering how it can get through the walls and windows if they are all closed. Sound travels on air wavys, so if there is no way for the air to flow, how does the sound get in?
 

Offline neilep

  • Withdrawnmist
  • Naked Science Forum GOD!
  • *******
  • Posts: 20602
  • Thanked: 8 times
    • View Profile
High frequencies are very directional so they are relatively easy to deal woth whereas low frequencies are omni directional and thus radiate all over the place. This is why a sub woofer can almost be placed anywhere in a room without ewe knowing where the thumping bass is coming from. ....so..in addition to other elemets for which I am sure klevur peeps here will discuss.... I am sure this must have something do with it.

That's very true! The LF stuff is more omnidirectional. I'm wondering how it can get through the walls and windows if they are all closed. Sound travels on air wavys, so if there is no way for the air to flow, how does the sound get in?

hang on !!..ewe expect me to know more stuff ?....lol !!
 

Offline neilep

  • Withdrawnmist
  • Naked Science Forum GOD!
  • *******
  • Posts: 20602
  • Thanked: 8 times
    • View Profile
...i think ewe are right in the first place with the wall absorption thing !...the sound then travels through the walls...which is not nice !...

.....don't expect any science from me in the way of an explanation !  ;D
 

Offline RD

  • Neilep Level Member
  • ******
  • Posts: 8128
  • Thanked: 53 times
    • View Profile
Damping is proportional to the speed of oscillations (the frequency), so the rapid oscillations  (high frequencies) die-off first (temporally and  spatially), leaving the slow oscillations, the low frequencies.

Sound travels on air wavys, so if there is no way for the air to flow, how does the sound get in?

Sound also travels through solid (walls, floors, string).
[Surely you've seen the cowboy movies where the indian native American puts his ear to the railroad track to tell if the "Iron horse" was coming].
« Last Edit: 28/09/2010 20:27:59 by RD »
 

Offline LeeE

  • Neilep Level Member
  • ******
  • Posts: 3382
    • View Profile
    • Spatial
Low Frequency sound isn't actually omnidirectional - it just seems like that to us because we i.e. our ears, are very small in comparison to LF sound wavelengths.  In dry air, at 20°C a 20Hz sound will have a wavelength of 343m / 20 = 17.15m (conversely, a relatively high pitched sound at 15kHz will have a wavelength of just ~0.02m i.e ~2cm).

It is the long wavelengths of LF sound that make it so difficult to block.  I can't remember the exact formula, but to absorb a 20Hz sound, with its 17.17m wavelength, needs something with absorbent features that are several metres in size.

It is precisely because LF sounds are so hard to absorb that fog horns are low pitched - high frequency sounds, although possessing more energy for a given amplitude (because after the amplitude is taken into account the energy of a wave is dependent upon its frequency) have a much smaller wavelength and are absorbed more easily.

It is believed that some whales use infrasound (iirc down to just a couple of Hz) to communicate over hundreds of km.  Once again, it is the very long wavelengths that allow the waves to travel so far without being absorbed.

Low frequency EMR is also similarly hard to block and VLF/ELF (Very/Extra Low Frequency) radio is used to communicate with submerged submarines at depths between 10m-40m (30ft - 130ft).  However, such low frequencies also have a very low bandwidth and are not capable of high-speed communications.  They also require correspondingly large aerials/antennae, both for transmission and reception: the transmission arrays can cover several square km and the subs have to trail a correspondingly long aerial behind them.

http://en.wikipedia.org/wiki/Vlf#Details_of_VLF_submarine_communication_methods
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
Thanks Lee!

Assuming a rather substantial brick wall, does the sound "travel through" the wall, or does it cause to wall to move slightly in sympathy?
 

Offline RD

  • Neilep Level Member
  • ******
  • Posts: 8128
  • Thanked: 53 times
    • View Profile
Sound even travels through a bank vault door or submarine hull ...


http://www.bbc.co.uk/news/science-environment-10701182


And pregnant women ...


http://en.wikipedia.org/wiki/Ultrasound
« Last Edit: 28/09/2010 20:46:29 by RD »
 

Offline LeeE

  • Neilep Level Member
  • ******
  • Posts: 3382
    • View Profile
    • Spatial
I think that the brick wall must move slightly, but not as much as you'd think because the bricks and mortar are porous, so the air can move within the pores of the wall material.  Even if you painted the wall, to make it airtight, the film of paint would flex and transmit the pressure to the internal air cavities within the material.

I've got to confess that this isn't an area I'm overly familiar with - I'm just remembering bits of stuff I looked into a long time ago - but the key thing I do remember is the relationship between the wavelength and the feature size of the absorbing or reflecting material, which iirc, pretty much dictates whether the sound is absorbed or reflected.
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
RD's submarine case is interesting too. Hopefully there is not too much air entrained in the steel hull :D
 

Offline neilep

  • Withdrawnmist
  • Naked Science Forum GOD!
  • *******
  • Posts: 20602
  • Thanked: 8 times
    • View Profile
Can I just say that I told LeeE and RD to write all that klevur stuff !!
 

Offline JP

  • Neilep Level Member
  • ******
  • Posts: 3366
  • Thanked: 2 times
    • View Profile
Good points, Lee.  I wonder if the thickness of the wall also has to do with setting up standing waves to cause reflection.  Another point is that the absorption coefficient of an object generally has a part that's proportional to its thickness, measured in wavelengths.  Since bass wavelengths are large, the wall won't absorb terribly well.
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
I think the low frequency oscillating air pressure couples energy into the wall to produce a sympathetic oscillation in its outer surface. The amplitude of the sympathetic oscillation is very small, but it sets up an acoustic wave that propagates through the wall by compressing/decompressing the material.

When the acoustic wave terminates at the inner surface of the wall, it produces a small deformation at the surface that couples acoustic energy into the air inside the room.

Shorter wavelengths tend to be reflected from the outer surface of the wall if it is hard, or dissipated into the material if it is soft.

Shorter wavelengths will propagate through a rigid wall reasonably well if the energy is effectively coupled into the wall (e.g. tap a solid wall with a hammer) but shorter wavelengths in air can't couple much energy into a dense wall.
 

Offline JP

  • Neilep Level Member
  • ******
  • Posts: 3366
  • Thanked: 2 times
    • View Profile
Shorter wavelengths will propagate through a rigid wall reasonably well if the energy is effectively coupled into the wall (e.g. tap a solid wall with a hammer) but shorter wavelengths in air can't couple much energy into a dense wall.

Good point!  So is there an easy explanation why tapping on a wall with a hammer is so much more effective than blasting high frequency noise at it from a stereo?  Does it just have to do with the density of the hammer being so much greater than the density of the air or is there something else at work? 
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
Shorter wavelengths will propagate through a rigid wall reasonably well if the energy is effectively coupled into the wall (e.g. tap a solid wall with a hammer) but shorter wavelengths in air can't couple much energy into a dense wall.

Good point!  So is there an easy explanation why tapping on a wall with a hammer is so much more effective than blasting high frequency noise at it from a stereo?  Does it just have to do with the density of the hammer being so much greater than the density of the air or is there something else at work? 

Yes. I think the difference in density between a hammer and air has a lot to do with it. It's not so much that walls can't transmit high frequencies. I think they can. It's more a question of how effective the energy coupling method is.

There is something a bit different from RF/light going on too, I think. Higher frequency light has more energy than lower frequency light. Sound sort of goes the other way. There is a lot of energy in a LF sound wave. That's why LF sounds can travel for much greater distances - I think!
 


Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
http://en.wikipedia.org/wiki/Anechoic_chamber

Ah yes! We built an RF chamber for RF susceptibility and interference work, but it also worked quite well as an acoustic chamber.
 

Offline tommya300

  • Hero Member
  • *****
  • Posts: 655
    • View Profile
Shorter wavelengths will propagate through a rigid wall reasonably well if the energy is effectively coupled into the wall (e.g. tap a solid wall with a hammer) but shorter wavelengths in air can't couple much energy into a dense wall.

Good point!  So is there an easy explanation why tapping on a wall with a hammer is so much more effective than blasting high frequency noise at it from a stereo?  Does it just have to do with the density of the hammer being so much greater than the density of the air or is there something else at work? 

Excellent point is a hammer striking a wall actually is setting an energy node point, size of the hammer head, directly coupled on the surface of the wall.
While airborn noise is couples the total exposed area surface displacing the pressure to a wider area.
Think of this a pressure of lbs/in2
The node point is the point of origin of the energy to be spent.

What is easier to send cleanly through a board with the same reasonable instantanious pressure, a nail or a steel face plate

Maybe I should say Focal point not node point.
« Last Edit: 30/09/2010 07:21:46 by tommya300 »
 

Offline tommya300

  • Hero Member
  • *****
  • Posts: 655
    • View Profile
Just like RF? At the bottom of the URL page is a video that explains something of interest to audio low pitch sound

http://bbamusic.wikispaces.com/Audio+Standing+Waves+-+Room+Modes
« Last Edit: 30/09/2010 07:38:07 by tommya300 »
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
There may be another factor.

A higher frequency sound wave will interact with the wall in such a way that the maxima and minima of the wave encounter the wall at significantly different times - if you see what I mean.

So, the wall will receive a rather mixed signal as maxima and minima will be distributed across the surface of the wall. These will tend to interfere with, and cancel each other.

The lower the frequency, the greater the spatial separation between maxima and minima, so there will be less interference and canceling.
 

Offline tommya300

  • Hero Member
  • *****
  • Posts: 655
    • View Profile
http://en.wikipedia.org/wiki/Anechoic_chamber

Ah yes! We built an RF chamber for RF susceptibility and interference work, but it also worked quite well as an acoustic chamber.

I experienced a small full Anechoic/acoustic chamber. I felt a little bit dizzy at the first time entering and having the door shut. A bit claustrophobic?
 

Offline tommya300

  • Hero Member
  • *****
  • Posts: 655
    • View Profile
There may be another factor.

A higher frequency sound wave will interact with the wall in such a way that the maxima and minima of the wave encounter the wall at significantly different times - if you see what I mean.

So, the wall will receive a rather mixed signal as maxima and minima will be distributed across the surface of the wall. These will tend to interfere with, and cancel each other.

The lower the frequency, the greater the spatial separation between maxima and minima, so there will be less interference and canceling.

YES you hit it on the head. I seen this idea in my minds eye but could not put it to coherent words.

I believe the pyramid shape blocks of the acoustic chamber design enhances deflection not reflection  reducing the energy of the low freq longwaves
« Last Edit: 30/09/2010 07:52:22 by tommya300 »
 

Offline tommya300

  • Hero Member
  • *****
  • Posts: 655
    • View Profile
Sure would be something to use a targeting optics to be able to view the vibration at a long distance, convert and invert the audio signal duplicate through DSP and send it back at the same magnitude to its audiable source. Somewhere along the path of travel the should have some canceling affect.
I don't think it would be a cost effective idea
« Last Edit: 30/09/2010 08:19:19 by tommya300 »
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
Sure would be something to use a targeting optics to be able to view the vibration at a long distance, convert and invert the audio signal duplicate through DSP and send it back at the same magnitude to its audiable source. Somewhere along the path of travel the should have some canceling affect.
I don't think it would be a cost effective idea

I think you can do something like that to eavsedrop on conversations by pointing a laser "microphone" at a window. Not that I actually have one or anything!
 

The Naked Scientists Forum


 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums