Dipole/OB bass -small dipole woofers vs bigger woofers on same baffle

[wonderfulaudio]

Hi,

Consider the following 2 setups. Baffle width is fixed, lets say 20". Other things being equal. Low end extension, about 35-40Hz.

1) A dipole with a pair of 15" stacked drivers. The driver covers a large part of the baffle horizontally (2.5" left on each side) and hence would lead to a narrow 8 pattern.
2) A dipole with 4 of 10" stacked. The driver covers a small part of the baffle horizontally (5") and hence would lead to a wider 8 pattern.

For bass usage, how would the setups differ sonically?
Is narrow pattern a goal in dipole subwoofer systems?
Is narrow pattern a benefit for bass quality, by how much?

Thanks in advance,
WonderfulAudio

 

[TBTL]

Pattern is equal up to some frequency where the wavelength becomes comparable to the baffle width. It's p(angle) = cos(angle) with p the sound pressure.

The pair of 15" drivers makes more sense as it offers more displacement for less money.

 

[wonderfulaudio]

The pair of 15" drivers makes more sense as it offers more displacement for less money.

I meant if displacement is not an issue. If its an issue then it can always be increased with more drivers vertically stacked.

 

[wonderfulaudio]

Pls see the figure from John's excellent work at musicanddesign site. Though its plotted in terms of distance, i believe its also valid for distance that is as a result of baffle width/driver size.

An externally hosted image should be here but it was not working when we last tested it.

The red is for a driver thats much smaller than the baffle. The blue is for a driver that covers larger part of the baffle. Though both are dipoles and have figure 8 pattern, yet, the former has a wider pattern with more energy towards the sides. The blue has a very narrow pattern with very less energy towards the sides. How would they compare in sound?

 

[TBTL]

Do you have a link?

Those patterns seem to be about dipole spacing relative to wavelength. They show what happens when frequency goes up, such that wavelength becomes comparable to dipole spacing (which is not applicable for bass frequencies). They neglect the directivity of the woofer itself so they are not valid for real world dipole woofers.

Dipole spacing depends on baffle width, not on woofer size. For an equal baffle width, the dipole spacing remains approximately equal.

 

[wonderfulaudio]

Dipoles and Open Baffles
As per the diagram, the patterns apply to all lower frequencies.

 

[TBTL]

d = dipole separation
w = wavelength

For long wavelengths, (low frequency) d/W is small and the radiation pattern is the classical dipole figure 8. This pattern remains fairly constant up to about d/w = 0.25 [...]

d/w = 0,25 occurs at 169 Hz for a 20" separation. I do not know whether baffle width equals separation or that there is some in factor between.

 

[wonderfulaudio]

What the graphs are saying is that for off axis the dipole cancellation is different for a large driver vs a small driver in same baffle. It is this that produces wider or narrower figure 8 beam.

 

[scottjoplin]

No it doesn't. How do you come to that conclusion? TBTL is right

 

[wonderfulaudio]

Partially quoting John's post OB theory: Baffle width and roll-off

"Additionally, if you are interested in true dipole response and uniform polar response, the dipole response should only be used up to a frequency of about 1/2 to 1 octave below the peak. Pushing the response higher on a wider baffle generally leads to a response that broadens off axis and the -3dB point moves past the 60 degree point. "

 

[scottjoplin]

Are you confusing the null above the dipole peak with the 90 degree cancellation?