What are "standing waves"?

This article explains standing waves:

Small room treatment

See the sidebar that explains it in detail. The same thing happens in electrical wires at radio

frequencies.

--Ethan

That is some  good info.

Quote:

Originally Posted by EthanWiner 

This article explains standing waves:

 

Small room treatment

 

See the sidebar that explains it in detail. The same thing happens in electrical wires at radio

frequencies.

 

--Ethan

Surely you mean resonance frequencies, not radio?
 

I believe he meant radio frequencies.  Standing waves do occur at resonance frequencies, regardless of the medium.  It just so happens that in an acoustic space/room, the length scale and speed of sound conspire to allow standing sound waves at audio frequencies.  In a wire, the length scale and transmission speed (approximately the speed of light) allows standing electromagnetic waves to occur at radio frequencies.

Yes, I meant radio frequencies. See the last part of this article:

Comb Filtering

The same reflections that cause acoustic interference and standing waves in rooms also happens in wires and inside microwave ovens etc.

--Ethan

There is a lot of information on this topic, and room treatment on the RealTraps site, and several videos that Ethan made.  Great way to start learning about this issue, and several other room related audio concerns.  They have put a lot of effort into demonstration and education.

Not relevant for headphone listening, but I watched them all.  I hope to make my "man cave" a good listening room when I build a new house.

Standing waves or stationary waves are simple harmonic waves which actually "stand" or don't move.  They are closed loops of energy which oscillates between two mediums. An example

These waves are "standing" or just bouncing back and forth between the two mediums.

imagine it something like a ball bouncing between two walls.

Why are they important?

Standing waves can sometime resonate (increase in their amplitude) which often is trouble for a speaker system in room as they get superimposed on other waves (add up with other waves) which causes distortion and "rough" sound.

P.S "google is your best friend" 

Rohan

Quote:

Originally Posted by MrGreen 

 

 

That's one of the best standing waves I've seen in a long time. 

Think of standing waves like ripples of water in a pond.  If you drop two stones in the water they will both create ripples.  The ripples will eventually reach each other depending upon how far apart they are from one another.  When the ripples colide, some ripples will reinforce one another, and some will cancel each other out depending upon whether they meet peak to peak, trough to trough, or peak to trough.

So it is with sound in a confined space.  As sound waves bounce around the enclosed space, the reflections eventually meet one another.  The meeting point phase determines whether or not the reinforce one another, or cancel each other out, or simply commingle.  This phenomenon is also true of electrons reflecting off of impedance boundaries.

Those meeting of waves and the resultant mutual cancellation or reinforcement of those waves is commonly known as the standing wave phenomenon.

A standing wave is where a waveform is established and any single point on the wave does not move, it only oscillates up and down (antinode).  It happens when the room or space has a certain dimension that is particular to the frequency of the wave.  Think of a wave on a lake and the waves match up perfectly to the shore.  The bobber on the end of your fishing lines only moves up and down (antinode), or if it is in the right spot, does not move at all (node)!  

What ends up happening is that the waves form a pattern that interfere with other waves in the same room, and effectively cancel out each other at certain spots.  This results in "dead zones" where technically no sound might appear.  These are the minimum points in an interference pattern in the waves (minimum pressure).  The points where two peaks intersect are amplified twice as much (maximum pressure).  

Nice diagrams!