[Geezer (OP)]

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?

[neilep]

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.

[Geezer (OP)]

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?

[neilep]

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 !!

[neilep]

...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 !  []

[RD]

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].

[LeeE]

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

[Geezer (OP)]

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?

[RD]

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

[LeeE]

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.

[Geezer (OP)]

RD's submarine case is interesting too. Hopefully there is not too much air entrained in the steel hull []

[neilep]

Can I just say that I told LeeE and RD to write all that klevur stuff !!

[JP]

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.

[Geezer (OP)]

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.

[JP]

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? 

[Geezer (OP)]

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!

[tommya300]

http://en.wikipedia.org/wiki/Anechoic_chamber

[Geezer (OP)]

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.

[tommya300]

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.

[tommya300]

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