Noise escaping out of window

[fairy!]

If I'm in my room using my hairdryer & I don't want the noise of it to wake everyone else in my house, if I open the window in my room, will some of the noise escape out of the window & therefore mean there's less noise travelling around the inside of my house??

[Clanad]

Interesting question, fairy!  Since sound radiates in all directions as a wave, simply opening a window would not let more waves escape in that direction.  Somethings, such as metal, wood and even water transmit the waves more efficiently than others.  The walls of the room would tend to deaden the sound only because they may not transmit the wave vibrations effectively.  Even more so if a sound deadening material, such as insulation were added between the walls.  However, letting the sound escape from the open window would only acerbate the amount of noise in that direction, not lessen it in other directions...

[Tim Baxter]

Whilst Clanad's reply seems to be well argued, it has a flaw.  If sound energy is allowed to escape through the window there must be a dimunition in total sound energy inside the building.  If this energy is not reflected back into the building by the closed window it must escape through the open window thus making the volume less inside the building. 

To answer the question less pedantically, the amount of noise reduction to the occupants of the house would be infinitesimal, you'd  be far better off using your drier on a lower speed and taking longer.

[fairy!]

Clever!  Thanks guys! :-)

[Clanad]

Hmmm...I don't think so Tim... consider this: ( From Colorado College Oval Project) When a sound starts on its way, the molecules near the vibrating source of the sound are momentarily forced close together. A moment later these nearby molecules spring apart and in doing so push on molecules slightly farther away, pushing those closer together. This process continues and the sound wave propagates away from the original source of the vibrations. In air, molecules have to move a considerable distance before they are likely to hit an adjacent molecule and this takes a bit of time. The speed of sound in air is about 330 meters per second. In contrast, in liquid water, molecules are shoulder to shoulder touching one another and hence the time it takes for one molecule to influence its neighbor is much less than in air. The result is that the speed of sound in water is about five times that in air (typically 1500 meters per second). It's likely to be even higher in a more dense medium, such as wood or other building materials... I think the sound would actually be less escaping the window into air than being propagated through the building....

[Hamish]

Hi Folks, interesting argument. I'm afraid I have to disagree with you Clanad though. I think you have made too many assumptions. One is that energy transfer from air to material is 100% efficient, this is unlikely to be the case and there is probably a large amount of refelction of the pressure wave from the glass in the window. this means that less energy escapes and therefore a higher volume.

Imageine being in a room with another room next door with a car in it. Will the noise from next door be louder if it has four wall or if the only wall is the one separating you?

[Hamish]

Sorry forgot to answer Fariys question. You could hang some more curtains in you room, exp over the door (ie dressing gown on a hook!). These abosorbe the sound and therefore reduce the noise.

[woofgang]

what if the people who you want not to be disturbed also have their window open??

[Hamish]

You claim that it was the cold air that woke them and not your hairdryer

[Jay70]

I guess without knowing the shape and other parameters of your room it's a job to answer this accurately. Maybe the quickest and most effective way to answer your own question would be to carry out a test.

[Clanad]

Hamish, I'm trying to follow you here... but I don't understand your statement that less energy equates to greater volume.  The point of my answer was that sound propagates less efficiently through air than through solids (or water) and therefore, opening the window and thereby allowing air to air propagation would be less efficient than through a solid. If the window were a singlepane, then the sound wave striking the window would have been transmitted to the air outside prior to opening.  If it were a doublepane insulated type, then the vacuum between the panes would have greatly inhibited the propagation and it would then be louder when opened.  But that doesn't equate to the original question i.e., would there be less noise in the house simply by opening the window.  I think not, but that could be only intuitive and not based on mathmatical formula... Interesting question though...

[Tim Baxter]

OK guys it's time we started to read and listen to what others are saying.

Energy (at least in the Newtonian world) cannot be destroyed or created.  Therefore, if sound is allowed to pass (through an open window) rather than being reflected back (as in against a closed window) less sound energy must remain in the room.  To say that because glass is a better conductor of sound energy means that it can 'transmit' or 'suck' more sound energy out of a room than the medium through which the sound is travelling is arrant nonsense!  If the sound energy 'escapes' (for want of a better word) through a window then there must be less sound energy left in the room.

[Tim Baxter]

Further to the above because it posted itself too soon.....

The suggestion that sound will be diminished by hanging a dressing gown up is entirely valid.  Sound energy is 'lost' as heat when it strikes a medium.  Therefore any substance which will slow down or absorb the compression waves (already slightly alluded to) will lessen the sound energy.  A dressing gown/blanket/egg boxes ( see http://www.isvr.co.uk/faciliti/lg_anech.htm ) will reduce the sound energy transmitted outside your walls.  The walls' linings will get slightly hotter but that's the way of energy conservation!

[Freg]

fairy!, if you were asking a question to start a debate, you have one. If, on the other hand, you wanted an answer that was more practical, I would suggest not using your hairdryer when other people are sleeping. This would solve the argument easiest. There is surely a time when this is possible. If not, I should think that you would have to do more that just open the window to have a significant effect on noise levels. The others have suggested ways to help.

[tony1941]

Interesting bit about curtains etc for sound absorption! This will definitely work. Have you ever looked at the bottom of the seats in concert halls?  They have rows of holes so that if the seat is empty (and hence upright on its spring), more sound will be absorbed.  This is because it's important to calculate for absorbent material (or bodies!) to get the concert hall right.  My old acoustics lecturer used to tell us to use a rule of thumb :one human body is equivalent in absorption to 8 square feet of open window. There, I knew I'd get back on track eventually - -  why don't you just invite lots of freinds in while you're drying your hair and then the neighbours probably won't notice at all!

[3Styler]

just tell everyone else to wear earplugs.  alternatively wear earplugs yourself and then you dont have to hear them complainig about being woken up.