How does sound travel?
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 12th November 2004, 02:22 PM #1 lumanauw   diyAudio Moderator Emeritus     Join Date: Oct 2002 Location: Bandung How does sound travel? 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?
 12th November 2004, 02:53 PM #2 Swedish Chef   diyAudio Member     Join Date: Jun 2004 Location: Stockholm 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". __________________ "Knowing what to do but not why is no use in a changing world" - The Art of Sound Reproduction
 12th November 2004, 03:29 PM #3 peace brainerd diyAudio Member   Join Date: Nov 2004 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.
 12th November 2004, 03:37 PM #4 Mr Evil   diyAudio Member   Join Date: Aug 2004 Location: Behind you 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. __________________ https://mrevil.asvachin.eu/
 12th November 2004, 04:50 PM #5 SY   On Hiatus     Join Date: Oct 2002 Location: Chicagoland 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. __________________ "You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
 12th November 2004, 05:52 PM #6 wimms   diyAudio Member   Join Date: Mar 2004 Location: home 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.
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Re: How does sound travel?

Quote:
 Originally posted by lumanauw 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.
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 13th November 2004, 02:37 AM #8 lumanauw   diyAudio Moderator Emeritus     Join Date: Oct 2002 Location: Bandung Has any clever man invented a device to see sound traveling? Can see where the reflacting surfaces, how the sound gets to the ear? If one of these device has invented, I can make an accoustic consultant /sound room designer/ concert hall designer with 100% success guaranteed
 13th November 2004, 03:51 AM #9 Sch3mat1c   diyAudio Member     Join Date: Jan 2003 Location: Milwaukee, WI 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 __________________ Seven Transistor Labs, LLC Projects and Resources / Electronic Design and Consultation
 13th November 2004, 04:17 AM #10 lumanauw   diyAudio Moderator Emeritus     Join Date: Oct 2002 Location: Bandung 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.

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