I have a few questions for those who may have experimented with the concept.
1) By "loading" the driver are you also protecting it from possible over excursion; are you effectively changing the compliance or springiness of the driver and Is this another possible advantage?
2) It's suggested that by increasing the velocity from the front of the driver, you now have less cancellation occurring as the front wave is no longer such a close reciprocal of the rear. But isn't the speed of sound a constant, regardless of the speed in which a driver pushes air?
3) It's also suggested that as a result of the front loading, the diffusion pattern is also altered from the traditional figure-of-eight dipole radiation pattern. In a traditional OB alignment (single flat baffle) there is a strong null on the left and right side of the baffle. Has this null been shifted in SLOB, and where is it likely moved to?
I'm still trying to wrap my head around this concept so thanks for any comments you might have.
1) By "loading" the driver are you also protecting it from possible over excursion; are you effectively changing the compliance or springiness of the driver and Is this another possible advantage?
2) It's suggested that by increasing the velocity from the front of the driver, you now have less cancellation occurring as the front wave is no longer such a close reciprocal of the rear. But isn't the speed of sound a constant, regardless of the speed in which a driver pushes air?
3) It's also suggested that as a result of the front loading, the diffusion pattern is also altered from the traditional figure-of-eight dipole radiation pattern. In a traditional OB alignment (single flat baffle) there is a strong null on the left and right side of the baffle. Has this null been shifted in SLOB, and where is it likely moved to?
I'm still trying to wrap my head around this concept so thanks for any comments you might have.
At low frequency the slot loading does nothing to any of the three items you mention. There is no protection form over excursion; the front and rear waves remain symmetrical; and the radiation pattern will still be a figure 8. AS the frequency rises the slot resonance does affect the front and rear radiation and they no longer are symmetric and the radiation pattern is no longer a figure 8.
I did extensive testing of an acoustically scaled version of Nelson's original configuration, with polar response plots, which you can view here. The exuberance over the concept seems to ignore the reality.
Hello John,
Thanks for taking the time to post all those measurements! If we propose building an open baffle loudspeaker and we prioritize system efficiency and space efficiency, would this arrangement give some advantages since 4 15" drivers can be used on a baffle that is perhaps something like 24" wide and 32" tall?
I was considering going with the original design that had two slots: one on each vertical edge, then doing a decoupled full range in the center with the subs operating below 200Hz (at least 2nd order active filter, maybe as high as 4th).
Thanks for taking the time to post all those measurements! If we propose building an open baffle loudspeaker and we prioritize system efficiency and space efficiency, would this arrangement give some advantages since 4 15" drivers can be used on a baffle that is perhaps something like 24" wide and 32" tall?
I was considering going with the original design that had two slots: one on each vertical edge, then doing a decoupled full range in the center with the subs operating below 200Hz (at least 2nd order active filter, maybe as high as 4th).
Hi from Argentina, I have a question about the slot area.
I have read "slot area is approximately 1/3 to 1/4 of
the driver total Sd, depending of xmax of the speaker" but... this is for one or for both speakers?
example: a 10" speaker with 340 sd need a slot area of 85 or 170 cm2?
thanks in advance and excuse my poor english
I have read "slot area is approximately 1/3 to 1/4 of
the driver total Sd, depending of xmax of the speaker" but... this is for one or for both speakers?
example: a 10" speaker with 340 sd need a slot area of 85 or 170 cm2?
thanks in advance and excuse my poor english
I'm interested in SLOB for the space saving aspect, and also because I understand the mounting of the woofers provides force cancellation, so you could mount the front baffle directly to the woofers without worrying about it being shaken like you get with an H-frame.
Yes, loading the driver with air mass would lower the Fs and thus it would also affect the system resonance of the BR, but it might be quite hard to predict how much exactly. Here are some quidelines how air loading changes the Fs in U-frame and H-frame: http://www.quarter-wave.com/OBs/U_and_H_Frames.pdf
It was calculated using 60% of the total cavity volume. With SLOB the compression of the air in the slot should also be accounted for, so the gross Fs drop might be substantially larger than what the 60% cavity volume rule says.
In near field the efficiency could be somewhat larger due to the directed air pushing from the slot, but the effect diminishes fast when going away from the slot. If the slot is made tall (line array subs) or deeper (air is directed more in deep cavity) the near field effect reaches further. Beware of the cavity resonances (not necessary a problem with a sub).
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Excellent. I'm thinking this would be a good way to provide a stable base for a simple baffle for the BG RD50s I just bought, or even for a variation of the Linkwitz LX521.
I don't think there is a formula for calculating it, but it depends mainly on the compression ratio. I've measured a driver in a 1:10 ratio SLOB, and Fs was lowered from 37 Hz (free-air) to 24 Hz. That's a 35% reduction in Fs.
You get lowered Fs with regular H-frames as well, but not as much as with a SLOB. I use drivers with Fs=33 Hz. In the H-frame, Fs is lowered to 26 Hz, or ~20%.
If slot-loading the driver in a BR box will result in lowered Fs or not...? I have no idea.
It can be simulated quite exactly with Akabak.
That is one part. But you have to do it right. The basic design flaw of the SLOT is to have the compression on ONE side of the driver only. This will result in a highly asymmetric load on the driver. What you want is comparable compression on both sides of the cone. That design is known as a "ripole". Basic design guidelines are available by googling.
How much reduction of Fs is possible? By making the front (and back!) "slot" area 1/3-1/4 of driver Sd, you should get reductions roughly around 10 Hz. In general the reduction will be larger for drivers with high Vas (with regard to cone size) than with low Vas.
Rudolf
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