Standing waves

[bifcake]

Could someone please explain to me what standing waves are, what causes them and why they're bad?

Thanks

 

[usc goose]

as i understand it, standing waves are what happens either inside the speaker cabinet or in the listening room there are undampened parallel surfaces where sound can reflect back and forth. this back and forth reflected sound i think is the standing wave which is bad because the waves coming from each side cancel each other out.

that's how i understand it anyways.

anyone else?

 

[ServinginEcuador]

One wave is where the reflected wave off of some surface is out of phase with the sound coming from the speaker. This can diminish or outright kill the sound in that region where the two meet.

The other end of the spectrum is the wave that is reflected and in phase with the speaker's output, thus reinforcing the sound and making it sound a bit boomy.



Subs are notorious for having problems with reflected sounds can ben easily heard, and if you walk around the room you can find spots where the bass is totally gone, and other spots where it is really boomy, and even others where it sounds just right.

 

[bifcake]

What causes standing waves and how do you prevent them? Am I correct to assume that standing waves are not an issue with headphones?

 

[usc goose]

Quote:

Originally posted by usc goose as i understand it, standing waves are what happens either inside the speaker cabinet or in the listening room there are undampened parallel surfaces where sound can reflect back and forth. this back and forth reflected sound i think is the standing wave which is bad because the waves coming from each side cancel each other out.

did you read my post?

eh whatever, yeah, not really an issue with headphones as long as the housings are well designed. you can prevent with sound dampening and avoiding parallel walls.

 

[Wilson M.]

Honestly, no one reads your posts after all they are useless.

Sound is energy transmitted as waveforms. Waveforms have certain types of behaviour: reflection, refraction, superposition, interference, diffraction and polarization. Some of these are related to the others. Interference is the superposition of waves from two different wave sources such that there are cancellations of waveform energy in certain locations and there are waveform reinforcements in other locations. (It's a bit more complicated that that but we don't need to go there).

All waves are progressive. That is they travel through space. A standing or stationary wave is the illusion that the waves are not traveling through space. You will hear the waves oscillate in place but there seems to be no progression through space.

When 2 waves of equal amplitude, frequency and speed traveling in opposite directions superpose, a stationary wave is formed. A node in a stationary wave will be an area where the combined waveform does not oscillate (hence no sound). An anti-node is where the waves have superposed and oscillate with more energy than a single wave.

As far as I know, it actually has nothing to do with the waveforms being out of phase. Superposition of two identical waveforms that are 180 degrees out of phase will result in absolutely no sound anywhere because the out of phase nature necessarily causes all of the waveform energy to cancel out. That is obviously not true for the case of standing waves because there are spots that will still oscillate and those spots are "stationary".

What is important is that the waveforms are travelling in the opposite direction but in phase with each other.

SIE mentioned the "boomy" thing. That's the the effect of a standing wave as described above. The superposition of two waves will reinforce the most at the anti-nodes while there is no energy at all at the nodes. That's why the sound will sound boomy at the anti-nodes because there is more energy there than there should be.

I would post diagrams but I don't have a web server to stick them on and the file attachment thing is lame.

 

[soupy]

and to think..after two semesters of hard as hell physics, i don't remember a damn thing...

 

[wallijonn]

i always thought that the eggo shapes were done to diminish standing waves (which are more likely in headphones that are completely circular).

 

[Budgie]

In any closed area standing waves will be present. It's unavoidable. You can try to reduce it's amplitude by absorbtion, but you can't make it go away, since no surface absorbs 100% of the energy that impacts it. If you change the shape of the room, it will still have standing waves, but at different frequencies.

 

[bifcake]

Thanks guys.

 

[Whitebread]

hehe, I have no standing waves, cause I have no room...............

 

[Dusty Chalk]

Quote:

Originally posted by usc goose as i understand it, standing waves are what happens either inside the speaker cabinet or in the listening room there are undampened parallel surfaces where sound can reflect back and forth. this back and forth reflected sound i think is the standing wave which is bad because the waves coming from each side cancel each other out.

Close -- cancelling each other out is one problem, but reinforcing each other is the other, causing resonance, which is the real problem. The problem is only if you have large, parallel flat surfaces -- these will cause resonances at certain frequencies. The fewer parallel flat surfaces you have, the better. And yes, it is mostly a problem with rooms, and in speaker cabinets some. Rooms, for example, usually have one or two major resonance frequencies (and their harmonics), based on the length and width of the room (floor to ceiling, for some reason, isn't as much of a problem). Quote:

Originally posted by bifcake What causes standing waves and how do you prevent them? Am I correct to assume that standing waves are not an issue with headphones?

Only if your head is hollow. (J/K...)

 

[dabblerblue]

The way I was taught to calculate standing waves (axial: reflecting off 2 surfaces only) was to do this:

1) measure the length, width, and height of a room in feet,

2) divide half the speed of sound (565 feet per second) by the length of the room. i.e. 565/12 = ~47

3) add the result to itself continually until you reach 300. i.e. 47+47 = 94, 94+47 = 141...)

4) repeat steps 2 & 3 for width and height.

5) take the list of numbers you got from all of the dimensions (length, width and height) and arrange them in a single list in order from lowest to highest (up to 300)

These are the standing waves in this room.

Any difference between two consecutive numbers that is less than 5 means there is a peak at that frequency. Any difference greater than 20 indicates a dip.

Standing frequencies are generally not a problem above 300Hz for some reason, and this was never really explained to me.

Hope this wasn't completely useless!!

 

[Whitebread]

Quote:

Originally posted by dabblerblue The way I was taught to calculate standing waves (axial: reflecting off 2 surfaces only) was to do this: 1) measure the length, width, and height of a room in feet, 2) divide half the speed of sound (565 feet per second) by the length of the room. i.e. 565/12 = ~47 3) add the result to itself continually until you reach 300. i.e. 47+47 = 94, 94+47 = 141...) 4) repeat steps 2 & 3 for width and height. 5) take the list of numbers you got from all of the dimensions (length, width and height) and arrange them in a single list in order from lowest to highest (up to 300) These are the standing waves in this room. Any difference between two consecutive numbers that is less than 5 means there is a peak at that frequency. Any difference greater than 20 indicates a dip. Standing frequencies are generally not a problem above 300Hz for some reason, and this was never really explained to me. Hope this wasn't completely useless!!

Isn't sound faster, at sea level at least?

 

[dabblerblue]

yeah probably, i just know that on average, the speed of sound is 1130 feet per second