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Speaker cab gurus--help me clear something up

bongomania

bongomania

Commercial User
Oct 17, 2005
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PDX, OR
www.ovnilab.com
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owner, OVNIFX and OVNILabs
#1
I remember reading one or more threads over the last few years in which (IIRC) it was claimed that two identical woofers side by side would cause problems for each other, reducing their efficiency compared to a vertical stack. However I cannot remember the details, or what keywords would even be useful for a search. I do remember at least once somebody drew a parallel with a double-kick-drum setup versus a single drum with double beaters, saying that two drums would act as wave traps on each other. This is the point I especially want to know about--is this wave trap thing true, and if so, exactly how and why? How specifically does it play out in a speaker cab?

Mind, I'm not asking about cancellations from a simple phase reversal. :)

Any help in clearing this up is greatly appreciated. Links to technical documents or other trustworthy sources would be awesome.

Thanks!
 
#4
Well the loudspeaker that sits above another has better phase coherence when you stand off Axis
simply because there is no discrepancy between the distance or phase of either driver.
Kick drum problem is more to do with definition of a single kick being better because on beater hit it stops and restarts the head.
With two kicks the decay of one drum smears the attack of the other irrespective of the distance/phase issues.:-)
 
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#6
For subs, side-by-side isn't a concern because the distance between speakers is small compared to the wavelength. At higher frequencies, it's better to stack vertically or to crossfire drivers to minimize comb filtering in the horizontal plane caused by the distance between the drivers. This site has some good pictures from simulations of this effect at different frequencies: http://www.plantation-productions.com/Webster/AudioStuff/audioStuff.html

The bass drum analogy is flawed: since the two drums are not in sync with one another, phasing is not an issue.
 
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#7
Woofers side by side is not so much an issue as mid and high frequency ones. Woofers just need to not be separated, just close enough together to act as one (acoustically coupled), doesn't matter in what direction. But speakers producing mids and highs can't be close enough due to the short wavelength, so being side by side makes them beamy in the horizontal plane. Stacking them means beamy in the vertical, and since people tend to be dispersed horizontally rather than vertically, that's a much happier scenario, and limiting sound going to floor and ceiling is also win as reflections from there are trouble.
 
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#9
Don't know about some wave trap thing. The 2 drivers couple to make a more efficient and larger radiating surface than one.

This also makes it no longer a point source, so vertical/horizontal placement affects dispersion angle and comb filtering stuff in either the vertical or horizontal plane.

Keeping it as good as one can in the horizontal plane (thus vertical driver placement) is generally accepted as the default better placement seeing as an audience could be 100ft. wide, but the distance in ear height between a short person sitting and a tall person standing is still only 3 or 4 feet......unless you have a gig someplace like a ladder convention or escaltor engineering convention. :)
 
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#10
I could be completely out in left field with this one, so bare with me...

But what Bongo is describing sort of sounds to me like the effect that having unplugged cabinets in the room does... As in trying out one rig after another in a room full of mostly unplugged cabs...

Feel free to disregard if I am off base.
 
#11
Bongo.

I think this is BS.

The effect occurs with bass drums because only one is played at time. Play one drum and the drum beside it will act like a bass trap and absorb some of the energy. The closer the tuning between the two drums the worse the effect. This would also hold true for two identical speakers that are side by side providing only one of them is driven.

This effect does not occur with two speakers that are driven side by side. Providing the speakers are spaced within 1/4 wavelength of each other a the operating frequency they will sum coherently. As the space between the drivers increases beyond 1/4 wavelength summing will be degraded and eventually phasing occurs.

For speakers operating in free space the only difference between vertical or horizontal orientation is this determines which plane the phasing will occur in.

In a half space the orientation of the speakers can make a difference. Putting both drivers horizontally tightly packed on the floor will maximize low frequency output because of the reflection from the ground plane. If you align the speakers vertically the top speaker will be further from the ground plane and speaker output will sum less effectively with the reflected wave.

This is why when you elevate your speakers they get a less bassy sound. If you raise you speaker to a height of 1/4 wavelength off the floor, or place it at 1/4 wavelength from a nearby wall, it will actually cancel at the frequency associated with the 1/4 wavelength distance. See article from Peavey here. Invalid Link Removed

I am no expert on this, so I apologize if I have misapplied terms.
 
#12
#14
Wasnex said:
Bongo.

I think this is BS.

The effect occurs with bass drums because only one is played at time. Play one drum and the drum beside it will act like a bass trap and absorb some of the energy. The closer the tuning between the two drums the worse the effect. This would also hold true for two identical speakers that are side by side providing only one of them is driven.

This effect does not occur with two speakers that are driven side by side. Providing the speakers are spaced within 1/4 wavelength of each other a the operating frequency they will sum coherently. As the space between the drivers increases beyond 1/4 wavelength summing will be degraded and eventually phasing occurs.

For speakers operating in free space the only difference between vertical or horizontal orientation is this determines which plane the phasing will occur in.

In a half space the orientation of the speakers can make a difference. Putting both drivers horizontally tightly packed on the floor will maximize low frequency output because of the reflection from the ground plane. If you align the speakers vertically the top speaker will be further from the ground plane and speaker output will sum less effectively with the reflected wave.

This is why when you elevate your speakers they get a less bassy sound. If you raise you speaker to a height of 1/4 wavelength off the floor, or place it at 1/4 wavelength from a nearby wall, it will actually cancel at the frequency associated with the 1/4 wavelength distance. See article from Peavey here. Invalid Link Removed

I am no expert on this, so I apologize if I have misapplied terms.
Click to expand...
No. The upper cab couples with the lower cab, which is coupled with the floor, no loss of bass in the room. More mids for the player close to the stack doesn't equal less bass.
 
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#16
Downunderwonder said:
No. The upper cab couples with the lower cab, which is coupled with the floor, no loss of bass in the room. More mids for the player close to the stack doesn't equal less bass.
Click to expand...

I am actually talking about out in the room rather than next to the speaker. In my experience vertically stacking subs and bass speakers slightly decreases bass output over tight packing on the floor. However, I actually prefer vertically stacking because of phasing and dispersion issues in most situations. With a bass cabinet I want the high frequencies as close to my ear as possible and the widest possible dispersion (IE vertical stacking). In a PA setting, vertically stacking the subs so they are right under the mains mitigates frequency response anomalies in the low mids that occurs when you have subs on the floor and mains seven or more feet in the air. A shortcoming of this this vertical alignment is it reduces the output of subs and bass speakers. Yes the top woofer couples with the bottom woofer but you also have to consider how both woofers couple with the floor. The top woofer's coupling with the floor is less effective so there will be slightly less output from the total system. For example, wavelength of 140 Hz is approx 8 feet; so quarter wavelength is 2 ft. If your vertically aligned speakers are 2 feet tall. The top speaker will sum coherently with the bottom speaker but cancel with the floor reflection at 140 Hz and thus reduce output at the frequency.

Here's an article that explains the frequency response anomalies. Invalid Link Removed
 
#18
The sensitivity ratings pretty much tell the story. Two woofers together are twice as efficient as a single woofer. Among the many ways of thinking about it, one is that each speaker in the pair only has to fill half the room. This is neglecting diffraction effects.
 
#20
Here we are!
CL400Peavey said:
Bongo, I think I remember the thread you are referencing, but it was talking about multiple cabs side by side at gc. I don't remember which is the case, but shorted/unshorted inactive cabs can absorb your low end.
Click to expand...
Bmorefoozler said:
what Bongo is describing sort of sounds to me like the effect that having unplugged cabinets in the room does... As in trying out one rig after another in a room full of mostly unplugged cabs.
Click to expand...
Wasnex said:
The effect occurs with bass drums because only one is played at time. Play one drum and the drum beside it will act like a bass trap and absorb some of the energy. The closer the tuning between the two drums the worse the effect. This would also hold true for two identical speakers that are side by side providing only one of them is driven.
Click to expand...
This is the idea I was after! I had forgotten the key part about one of the speakers being shorted or unused. Apparently I had mixed it up with other posts/threads about beaming and comb filtering, obviously not related issues.

Do any of you happen to know where I could look for more authoritative details on this point? Thanks all!
 
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