Physics question

[Zylka]

so frequencies change as there is a transfer between medium.. though pressure is always changing with distance, regardless of whether medium has changed?

[KrazyHorse]

Originally posted by Jon Miller
the ambulance driving down the street effect is beats, I beleive

JM

?

It's just a doppler effect with a changing source velocity vector.

EDIT: more properly a changing projection of the source velocity on the source displacement

[KrazyHorse]

Originally posted by Az

Zylka, I would imagine oyu have been places, where if you move a little bit, because of the shape of the place you can hear somebody very well (At a position), but if you move a bit, even if it is closer, the shape of the space you share is such that you can't hear them at all (even though they have not moved)

That's not due to interference nodes, I think.

After all, sound waves of various frequencies are emitted, and they create a mishmash of maximums and minimums, interfering with each other along the way, just like visible light.

If there is only a single source and no reflections you do not get position dependent intensities (other than simple inverse-square law)

[Jon Miller]

Originally posted by KrazyHorse


?

It's just a doppler effect with a changing source velocity vector.

EDIT: more properly a changing projection of the source velocity on the source displacement

I thought that was beats?

I always forget definitions

JM

[KrazyHorse]

Definitions? Beats are simple movement across nodes/antinodes. The ambulance is a single source. There is no need to have a reflection to hear its pitch changing.

[Jon Miller]

bah

I never really cared much for the definition of beats anyway (apparently I forgot)

JM

[Zylka]

what's a node and antinode? Are those points of wave intersection?

[Jon Miller]

anti node is where two waves add constructively, this is a pressure maximum

node is where the two waves add deconstructively, this is a pressure minimum

Jon Miller

[Jon Miller]

but I am tired, and despite teaching this stuff don't have a good memory for definitions

so I won't attempt to handle further questions

night all

JM

[KrazyHorse]

Originally posted by Zylka
so frequencies change as there is a transfer between medium.. though pressure is always changing with distance, regardless of whether medium has changed?

Not really. Frequencies remain constant as you transfer/reflect through different media. What can happen is that some frequencies are preferentially transmitted or reflected, so the perceived pitch can change. Same thing can happen due to nodes/antinodes thing. There are some frequencies at certain points which will be loud and some which will be soft, so if you have multiple frequencies streaming in the loud frequencies "take over" and they're different at every spot, so you might get a perceived shift in tone from one point to the next. If there's a single frequency source (say, a flute playing A 440) then you will here A 440 everywhere, but it will get loud or soft depending on where you're standing relative to the walls.

"Pressure" as you're referring to it, is the height or amplitude of the sine wave representing the sound. In open space, the amplitude drops off as the square of the distance to the source. In an enclosed room, the function is much more complicated (like I said, there are loud spots and quiet spots; at A 440 the characteristic distance between a loud and a quiet spot is something like (1/4)*(330ms^-1)/(440s^-1) = 0.1875m = 7.5 inches or so

Higher frequencies will have shorter characteristic distances. Also, close to corners and edges there will be more complicated behaviour due to multi-wall echoing.

[Az]

Hey, I actually know this stuff. /me pats himself on the ass.

[KrazyHorse]

It's just trigonometry...

[Az]

yes. Shuddup

[Jon Miller]

yeah

I got that wrong, wavelength and speed within the wall cahnge, frequency does not

(can't sleep)

JM

[Raion]

Is this possible interference and distortion a negligible effect in terms of human hearing?

I would think -- no -- because before the power (amplitude) got to a point of doing all of that -- you would be deaf!

Go into a bathroom and play or sing some music --what happens there is bouncing around of the waves causing -- reverb and reflections of the original waves in the first place.

Why?
Because sound is absorbed even by the wood cabinet that hold the speakers -- wood -- something around 15% to begin with never leaves the box.

Now with a duct port, the bass is reinforced by the back wave of the speaker moving backwards in the cabinet, while the front wave comes out and starts to spread out.
By the time it gets to you, there will be the resonance of the room you are in -- the bathroom vs. another room like the living room.

In all -- the room -- will absorb sound in high frequencies if there is carpet or other absorbing material. Why? The wavelengths are at most - about an inch long.

Why is bass louder?
Because to fully develop the bass note frequencies -- you would need around 56 feet for the wave in the first place.

So, in a 16 foot room you never hear the bass frequencies so you boost up the bass, and also the high frequencies because simply your ear can not hear that good at soft volumes, but at louder volumes you can so you compensate according to what you hear with your ear which involves the --- power -- of the sound -- which means that for an increase in volume of twice as loud you need -- six times more power out of your amp.

Most of that power is needed for the bass notes and not for high frequencies. High frequencies are like bullets in a sense -- very directional. Move off axis and it is the high frequencies that disappear -- unless you have a very good hi-fidelity speaker system.

So what the room and the speakers do -- is add to the total sound you will hear with your ears. Thus the bathroom reverberates while another room does not.

What happens is that everything is added to make the sound have reverb and anything else -- which means that with all the stuff added-- there is always more distortion in the sound -- by simply adding reverb or echo or anything else -- like special effects.

So when designing a hi-fidelity stereo speaker system -- everything must be taken into account. Try Carnegie Hall -- and see why there are Sound Engineers that design places for only -- Sound Quality!

Of course the recording studio is a place where all that resonance of the room and anything else -- should be damped out as much as it can be -- or dead!

Thus the anechoic room (spelling may be wrong ) were simply any sound hitting the walls -- is damped out completly -- or a totally dead room. Of course the sound also sounds that way - - unless adding reverb and echo and anything else.

So to get back to your question -- everything affects the sound - from the distortion of the amplifier to the speaker cabinets to the room and all that adds up to what you will hear -- so the answer can be yes or no depending on how much money you are willing to spend to make the room ---- a Sound Designed Engineered Room!