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Old 13th November 2004, 07:05 AM   #11
Enzo is offline Enzo  United States
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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.
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Old 13th November 2004, 12:52 PM   #12
markp is offline markp  United States
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Play with this: http://www.falstad.com/ripple/ site to get some idea of difraction.
Also, the reason high freqs dont go through walls is that there is not enough energy in them to excite the mass of the wall to vibrate at those freqs and they are also absorbed by the wall.
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Old 13th November 2004, 03:26 PM   #13
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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?
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Old 13th November 2004, 04:37 PM   #14
sam9 is offline sam9  United States
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Default 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.
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Old 13th November 2004, 04:42 PM   #15
markp is offline markp  United States
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Default Re: sam9

Quote:
Originally posted by 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.
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.
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Old 13th November 2004, 06:31 PM   #16
Mr Evil is offline Mr Evil  United Kingdom
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Quote:
Originally posted by lumanauw
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.
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Old 13th November 2004, 11:14 PM   #17
sam9 is offline sam9  United States
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Quote:
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!
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Old 14th November 2004, 01:19 AM   #18
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Quote:
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?

Quote:
that is a very cool website and applet
Yes, It's cool. I can make a "sound shadow" if the wall is wide enough.
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Old 14th November 2004, 04:33 AM   #19
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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 ), 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
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Old 14th November 2004, 11:06 AM   #20
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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.

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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