How does sound travel?

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I just realize I dont know how sound travel in air. What is the form of sound wave in air?
If I has a flashlight in front of me, and block it with thick book, I cannot see the light, because it is blocked by the book.

But sound is different. If I has a speaker infront of me and block the whole surface with thick newspaper, I still can hear the sound. Or I hide the speaker in other room, I still can hear the sound. Where does this sound come from, as the direct path to my face has been blocked? Through reflecting surfaces (Walls, everything else in the room?)

But I doubt it is from reflection. I imagine if I stand in center of a very big room, and floating in the middle of the room, far away from any reflecting surface, I think I still can hear sound from the blocked speaker.

So, how does the sound travel? In what form? Changing air pressure or has its own form?
 
The key here is a phenomena called "edge diffraction". Basically it means that when a wavefront (be it mechanical as sound or electromagnetic as a radio wave, light or x-rays) hits the edge of a surface (or a slot) it "bends" around it. This is related to the wavelength of the wavefront and the slot diameter.

The angle of deflection can be approximated by:

sin v = 1.2 *lambda/D

whee v is half of the "opening angle", lambda is the wavelength (in meter) and D is the diameter of the slot (in meter).

For visible light, the wavelength is very small (400-800 nm) so the diameter of the slot has to be very small for this effect to occur. The diffraction of a laser beam by a human hair is one popular example usually demo'd in high school physics classes.

For sound, the wavelengths are very large large for bass notes (>10 m) which means that the slot (paper in your example) has to be VERY large to effectively block the sound wavefront. In the top octave on the other hand the wavelengths are a few centimeters (about the same as electromagnetic microwaves) so this effect will be more obvious. Try blocking a tweeter with something and you will see.

If you want further information, look in a book about acoustics or in a physics book with the key word "diffraction".
 
lumanauw, i never took physics either. Which will be soon evident to anyone who has. But this is the (half scientific) way that I've been able to rationalize it. Leaving aside the particle/wave debate, look at sound as waves of varying frequency. Once it becomes obvious that bass notes are audible in the other room while treble is not, you can see that longer (lower) waves penetrate objects better than shorter ones. This has, partly, to do with the fact that the lower freq is able to excite the object in its way, a wall, a mountain, the ocean, a thick newspaper - so it continues to move forward. That's why all the wattage is needed to the woofer, and less the tweeter. The Navy uses ultra low freqs for undersea communication for this reason. High freqs, unable to excite the mass of the newpaper, or much else besides thin air, are simply reflected. Extrapolate. Light waves are extremely high compared to sound. Take a look at a wavelength chart - it should be easy to find one on the net. If you can imagine visible light as an EXTREMELY high wavelenth of sound (we're being very unscientific here) you can see why you can block it with a thin piece of cardboard.
Now as to how xrays (ultra high freq) can penetrate objects, that's probably where particle physics comes in. I hope i didn't just confuse you more.
 
Diffraction is indeed important, and is why you can hear around corners, but in addition, sound can travel through any matter.

Sound is a longitudinal wave. It consists of alternate compressions (high pressure) and rarefactions (low pressure). A way to visualize this is to imagine holding a loose spring vertically and moving it up and down. A wave will travel down the spring, with the motion of the spring being parallel to the motion of the wave (which is why it's called a 'longitudinal' wave, as opposed to a transverse wave where the motion of the medium is perpendicular to the wave).

Another way of thinking of it is like a Newton's cradle, where the balls (like molecules of air) hit each other in turn, passing the force along. when the air particles hit the wall, the particles in the wall will move. The particles in the wall will hit other particles in the wall and thus the sound continues on through the wall in the same manner as it did in the air.

Of course not all materials pass sound equally, some have higher losses and these are frequency dependant, and there are losses associated with the wave passing through a boundary of materials with different densities (which is why horns increase speaker efficiency - they match a large volume of low density air with a small, high density speaker).

Also, the speed of sound depends on how far a molecule has to travel before it hits another, so sound will travel faster in some materials, from very slowly in a near vacuum where all the particles are very far apart, to the speed of light in a neutron star where all the particles are exactly touching.
 
random comments

Particle-wave duality is pretty much a non-issue in sound propagation!

One thing to keep in mind when drawing analogies between sound and light is that sound is a longitudinal wave, light is a transverse wave.

The Newton's Cradle or billiard ball picture is pretty accurate on a small scale, though thinking of air as a continuous and compressible fluid is probably more appropriate for the scale of audible sound.

Olson's "Music, Physics, and Engineering" has wonderful pictures and explanations. You ought to own it.
 
Right, air is compressible fluid. It flows in a room and around obstacles. Thats why hermetical enclosures give best acoustical separation (block your ears by hands airtightly).

Sound is pressure changes. Waves occur due to air inertia - it takes time to compress and propagate that through compressible fluid. Then comes difraction into the play.

Much like water waves from a splash sound waves emanate from the source outwards in concentrical rings, just in 3D, ie. spherically.

Obstacles on the path of sound cause air to both flow around them, and pressure reflect back and interact with sound waves. In addition, obstacles are reacting to changing pressure by vibrating mechanically, transfering these vibration though material and causing sound on opposite sides (walls). No walls are perfectly rigid. If there were, such wall would not transfer any sound through it, and pressure wave (sound) would bounce back from it completely without any attenuation.
 
lumanauw said:
I just realize I dont know how sound travel in air. What is the form of sound wave in air?
If I has a flashlight in front of me, and block it with thick book, I cannot see the light, because it is blocked by the book.

But sound is different. If I has a speaker infront of me and block the whole surface with thick newspaper, I still can hear the sound. Or I hide the speaker in other room, I still can hear the sound. Where does this sound come from, as the direct path to my face has been blocked? Through reflecting surfaces (Walls, everything else in the room?)

But I doubt it is from reflection. I imagine if I stand in center of a very big room, and floating in the middle of the room, far away from any reflecting surface, I think I still can hear sound from the blocked speaker.

So, how does the sound travel? In what form? Changing air pressure or has its own form?
Sound is changing air pressure, the rate of change determines the frequency. Get in a pool of water and float a 2 foot long piece of wood and from about 3 feet away(perpendicular) use your hand to make a small wave at the wood. Watch the ripples as they come off the corners of the wood. It looks just like they are the source of the wave, circles come off of them, not just a continuation of the original wave with a blank spot where the wood blocks the wave. Sound does the same thing. It also reflects off surfaces like the back wave from the wood.
 
If you use a sufficiently strong source in a very humid room, the regions which are reduced in pressure will have reduced temperature, causing condensation - cloudiness. Such can be seen on sonic aircraft in special conditions (clouds forming on upper wing surfaces and around the shockwave) and powerful explosions (nukes most famously, but a good old 1000 pound bomb will do it too).

Tim
 
After reading all the explenation, I wanted to write this. Please correct me if I'm wrong.

1. The sound is just a moving energy. It travels in air or other media by moving the energy to the nextdoor particle (like the Newton's ball or billiard ball analogy). It travels in the air in the form of changing pressure. (Because sound needs media to travel, it cannot travel in vacum space)

2. If sound encounters an edge, it will difract/bending direction. If it hits a surface there are 2 possibilities, 1=the sound is reflected, 2=the sound passing through inside the surface. This depends on the surface material and the frequency of the sound.

3. The sound travels quite different than a laser pointer. The laser pointer travels in a straight line from the source, but sound travels like an expanding ball (imagine an expanding fireball when a star exploded). So we can hear it every where around the source---->what's the energy relation for this kind of expansion? Surely not linear to distance.

I once heard that putting 2x power amp rating is not equal to 2x SPL. It takes more than 2x power to get 2xSPL. So, it is slight difference between 25W power amp and 50W power amp.
 
Yes, I would agree with your three points.

2x power results in 3db higher SPL, all things being equal otherwise. 10x power results in 2x SPL. The relation is logarithmic.

The reason you can take your speaker in the giant room and put a cover over it and still hear the sound is that the speaker makes the cover vibrate, and the cover itself then re-radiates the sound, though a lot less efficiently. Same thing with hearing sound through the wall. The wall itself is conducting the sound. If the wall were perfectly rigid, then there would be no sound heard. Byt in the real world, the wall is not perfect, so it is made to vibrate on one side by the sound, then the other side also vibrates which sends soundwaves into the quiet room.

The reason the high freq sounds do not survive is that the wall is less efficient at high freq than at low freq in transmitting sound. The wall would have to move very quickly to make high freq sound, and it has too much inertia to do so.

SOme companies make transducers you can glue to a plate glass window that make them sound emitters. The glass is hard and stif, unlike the wood in your wall, so it makes a better hgih freq emitter. Reasonable quality sound can be produced from your patio sliding doors.

People who live in glass houses should not throw stones, but they may have good hifi.
 
SOme companies make transducers you can glue to a plate glass window that make them sound emitters. The glass is hard and stif, unlike the wood in your wall, so it makes a better hgih freq emitter. Reasonable quality sound can be produced from your patio sliding doors.

Enzo, I posted a related question in the subwoofer forum last week having to do with this and got no response. Partsexpress has aurasound tactile inducers on sale for thirty bucks. What might be the result using this sort of device to on a long oak board attached to the floor joists? A stiff panel of sheet metal/sheet rock/plate glass? Would it produce a frequency anwhere within the range of hearing? How would it perfom compared to a typical subwoofer for extremely low frequency?
 
sam9

All the writen descriptions in the world will not take the place of playing with a wave tank for a few hours. Yo can make one from stuff you find in the kitchen. Start with a large cookie pan that you can fill with 1/2 inch of water. Use different objects to generate waves that correcpond to different spaker types, point source, line source etc. Then place various objects in the path of the waves and see how the interact - bend, diffuse, reflect. etc.

If you get turned on by all this, you should be able to find some kind small of battery powered motter that with the help of some plastiic gears and arms can be made into a wave generator whose frequency (motor speed) is controlled by a pot. Then you can explore standing waves, wave length with respect to room (i.e, cookie pan!) dimensions. Etc etc.

This will be a much closer analogy than any comparrison to light. Since air and water are both fluids, it's almost not even an anology but the same phenomoa in a more observable media.

Of course you can do allthis with math if that's how your brain works, but the Mr.Wizard appoach is usually more fun.
 
Re: sam9

sam9 said:
All the writen descriptions in the world will not take the place of playing with a wave tank for a few hours. Yo can make one from stuff you find in the kitchen. Start with a large cookie pan that you can fill with 1/2 inch of water. Use different objects to generate waves that correcpond to different spaker types, point source, line source etc. Then place various objects in the path of the waves and see how the interact - bend, diffuse, reflect. etc.

If you get turned on by all this, you should be able to find some kind small of battery powered motter that with the help of some plastiic gears and arms can be made into a wave generator whose frequency (motor speed) is controlled by a pot. Then you can explore standing waves, wave length with respect to room (i.e, cookie pan!) dimensions. Etc etc.

This will be a much closer analogy than any comparrison to light. Since air and water are both fluids, it's almost not even an anology but the same phenomoa in a more observable media.

Of course you can do allthis with math if that's how your brain works, but the Mr.Wizard appoach is usually more fun.
Gee, that sounds just like one of my previous posts in this thread. You might also want to try the link I left, it is a virtual wavetank.
 
lumanauw said:
2. If sound encounters an edge, it will difract/bending direction. If it hits a surface there are 2 possibilities, 1=the sound is reflected, 2=the sound passing through inside the surface. This depends on the surface material and the frequency of the sound.

3. The sound travels quite different than a laser pointer. The laser pointer travels in a straight line from the source, but sound travels like an expanding ball (imagine an expanding fireball when a star exploded). So we can hear it every where around the source---->what's the energy relation for this kind of expansion? Surely not linear to distance.
2) Diffraction depends on the wavelength relative to the thing being diffracted around. Diffraction happens when the two are of similar size, which is why listening to a sound around a corner will be muffled. As the frequency increases, sound will behave more like light, casting acoustic shadows from large obstacles instead of diffracting.

3) It is mostly true that sound radiates out spherically, but not always. All waves behave according to similar rules, so it is possible to beam sound in a straight line for instance. Sound radiated spherically decreases in intensity with the inverse square of distance.
 
sounds just like one of my previous posts in this thread. You might also want to try the link I left, it is a virtual wavetank.

I just thought that since this is a DIY forum, building a real one might be more interesting. However, that is a very cool website and applet. Completly appart from the original post's issue, I was really impressed by looking at a monopole vs a dipole simulation. It shows visually Siegfried Linwitz's contention about the value of orthagonal nulls with dipole speakers!
 
so it is possible to beam sound in a straight line for instance.
The higher the sound frequency, it will behave more like light. Is this why tweeter angle usually critical compared to subwoofer angle (some subwoofer even face the floor). I can hear the difference if the tweeter angle is tilted. Does a tweeter always have to face the listener?

that is a very cool website and applet
Yes, It's cool. I can make a "sound shadow" if the wall is wide enough.
 
Yeah, pretty much.

For the woofer, consider the distance from the cone to the floor, or even the width of the box itself, compared to the wavelength. You can say the whole perimeter (approximately) is radiating the low frequencies evenly, and in phase. Also, soft materials like carpet have little effect on slow pressures like LF, while damping HFs more (because more of the wave appears inside the woven fiber). (Damping can also be considered as a function of more motion; low, slow frequencies simply don't move as much (in terms of friction) as the same SPL of a higher frequency.)

For midrange speakers, the whole cone radiates, but because it is smaller than a wavelength, it looks more like a hole in a box and thus follows the rules of diffraction - hence the 1.2 * something or other I saw earlier in this thread that reminds me of a similar Raleigh Criterion in physics class. Uh, *checks thread* this thing: sin v = 1.2 *lambda/D. Applies to all waves passing through a circular apeture.

For small tweeters with an apeture comparable to the wavelength, they will tend to radiate evenly as well. For larger ones, they look more like a phased array as the cone flexes at a different rate than the air in front of it, creating a range of intensities and phases coming off different parts of the cone. This is either directional or not (I don't care to draw it out myself and see :p ), but it seems to me you'll be best off pointing them at the listener. After all...if it works....it must be working...

Tim
 
Yes, and this poses a problem in loudspeaker design. While the on-axis frequency response of the speaker might be ruler flat, the off-axis response rarely is. Take a closed-box 3-way system for example. Look at it like this: The bass notes are way bigger than the drivers and box so they will pretty much be all over the place. The problem here is reflections in the room giving nulls in some places and very strong bass in others. I am quite sure you have experienced this. As the frequency goes up the "sound beam" will become narrower until the midrange takes over where it widens again. Moving up through the midrange the sound will again become narrower until the tweeter takes over and it widens again. This means that the off-axis response will look a little bit like a cartoon Christmas tree. :xmastree:

The off-axis response is what creates much of the reverbant field (sound reflected by the walls in your room) and a funky frequency response here leads to listening fatigue.
In some designs you will even see an additional tweeter facing backwards to create a smoother reverbant field adding some "space" to the listening experience.

/M
 
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