Comb filtering (?) question.....

[Coytee]

If you have a double MWM bass bin and stack a second double MWM bin on it....  my understanding is the bass will couple and add verses be destructive.

 

I've seen a picture (here I think https://www.flickr.com/photos/klipsch_audio/sets/72157635139488333/

 

that shows some of this.

 

This however, got me wondering.....

 

If you have a double MWM bass bin and (magically) hold another one above it say, 12 feet, I understand that NOW, you might have some comb filtering and or some destructive issues with the sound.

 

Leave the floating MWM in the air and now add other bins between the two so you now have a stack of MWM bins 12 feet high.

 

As I understand it, we are back to summed bass verses destructive comb filtering (or lobes??)

 

 

If this is correct, what makes filling in the middle keep it from being destructive?

 

 

I'll admit I don't know enough to be sure that I'm even articulating the question properly.

Edited August 26, 2015 by Coytee

[Coytee]

Well cripes, the link isn't working.

 

Delete the closing paragraph when it gives you an error.  I just tried to edit it out and it still didn't work for me

[WMcD]

We had ripple tanks in high school to experiment with.  I guess they were part of some package of lab experiments.  Suffice to say, you could see the effects of two sources at the surface of the shallow tank when an overhead light shown though the glass bottom and onto a piece of paper a foot or so underneath.

 

But, back to the question.

 

Lets get back to speakers placed apart in the horizontal plane, lets say by one foot.

 

You put your ear at a location equidistant from each speaker.  Say at the midpoint of the two speakers but 5 feet away.  Play a sine wave.  Now the sound from each reaches you ear with the same delay.  So the sine waves reach you ear with the same phase.  They add.  They add no matter what the wavelength from the sources. That is called constructive interference, which is to say no interference.

 

But move off axis to the left.  Suppose you have tape measures so you can judge your distance from each driver.  At some point as you're off axis, there is a difference in path length, and therefore a different delay comparing the two.  When the difference is a half wavelength for that sine wave frequency (or wavelength), one signal arrives 180 degrees out of phase from the other at your ear.  They are out of phase and cancel. This is because of the relative delay.

 

Roughly, if the sine wave is 1000 Hz the wavelength is 1 foot and half that is 6 inches.  So if the difference is 6 inches, this is where you get a null.

 

The issue is, though, what happens if you change the frequency.  Now that 6 inches is not a half wavelength.  So the null is not as strong.

 

It take math, or imagination, to realize that if you stand in one spot and vary the wavelength, different wavelengths (frequencies) from the two drivers will add sometime or subtract sometimes.  That is the combing (peaks and valleys) when you stand is one place and vary the wavelength.

 

The other thing is that if you keep the wavelength constant, and move around the room, there are spots where the difference in distances varies.  And any difference in distance which is a multiple of half a wavelength will have the comb effect. 

 

Overall, it is a mess you can detect with your ear.

 

The room walls cause reflections and we can use just one speaker.  It is an ear opening experiment to play one upper bass tone though one speaker and move around the room.  At some locations it sounds like someone turned off the amp.  But a couple of feet away, the sound is strong.

 

We had a report of that from a fellow here years ago.  He had an RS SPL meter and played a sine wave and wanted to measure the level. But as he moved around the room the level change radically.   If you want to avoid wall reflections, you need a chamber with not echoes of the walls because they are made of absorbing material. That is an anechoic chamber. (Addition by Edit.  This means without echo.)

 

Back to the question.  When you have multiple drivers and they are located close to each other, it is more difficult to get different lengths (in terms of wavelengths) to your ear. 

 

At long bass wavelengths you have to get very far off axis to create the half wavelength difference.

 

Of course you are talking about stacked bass drivers, but at some off set up and down, there will be the same effect.

 

Overall, when we consider the constructive and destructive interference from two sources (or more), we have to think in wavelengths (rather than frequencies) and also think in terms of "differences in distance" as compared to wavelength sent out by the source.

 

WMcD

Edited August 27, 2015 by William F. Gil McDermott

[JohnA]

Nice job.