My conclusion would be that there is no gain in due to the slot loading at low frequency. The result that 0 and 180 degree amplitude are different for the OB system is more likely due to the lack of truly omnipolar response due to baffle effects and phase/delay differences. Remember the front and rear sources have different characteristics in both amplitude and phase with frequency and those differences carry over to off axis angles.
The phase response of either of the sources when isolated is the sum of the propagation delay and the minimum phase response consistent with the amplitude response at the observation point. Thus, even if the propagation delay is made constant, the amplitude response varies with off axis angle therefore the minimum phase component changes as well.
Also regarding the baffle, the slot is like a flush mounted source but the rear is a group of drivers mounted sideways and standing off the baffle. I would not be surprised to see different blockage effects for the rear.
In any event, I have no problem with the observation that the OB system may have greater on axis SPL response at low frequency than at 180 degrees. It is a highly complex, asymmetric, 3D system. However, the observation that the 0 degree SPL is greater than the180 degree SPL does not lead to the conclusion that the slot loading leads to greater efficiency at low frequency. If you want to say the axial sensitivity of the system is greater at 0 degrees than at 180, fine. You could say that for a cardioid too, but the cardioid is formed by equal strength sources. So I think you just have to be careful about what the system response implies about the behavior of the individual sources which make up the system.
Ultimately it is all academic because the system does what the system does.
On the slots. Back side was also open so there was no blockage form the outer layer of plywood (1/4" ply was used).
After all these discussions, I would have one simple question for Nelson: Do you prefer the sound of this Open Baffle compared to the baffle with a simple, single Beta 15 woofer? (I think you had that one on the last year's BAF)
Personally, I don't care about the measurements. If it sounds better to my ears, I will like it
Personally, I don't care about the measurements. If it sounds better to my ears, I will like it
Glad to see the discussion turning away from sensitivity - no sense going back there.
I think it's worth discussing all the benefits of slot loaded open baffle woofer versus plain open baffle though. It's clear the front slot loaded system as presented differs from a vanilla OB as follows -
Fs is lowered - do we have any guidelines on how this follows the cone area to slot ratio?
Qts is lowered - same question - guidelines?
Front / rear nearfield SPL response for 3:1 area ratio is 9dB - this may create smiles for people listening semi-nearfield. As mentioned in another referenced thread, this can also create advantages for those who love deep bass, with neighbors...
From most listeners perspective, it may also be interesting to discuss more what can influence the near field to far field transition. If it is based on relative *effective* baffle size, you may find interesting listening room situations where the near field extends a lot farther than expected by theory for an anechoic baffle.
The floor, for example, will significantly extend the effective baffle size for floor mounted systems. So will large amps stacked between and butted against the inside of baffles, as seen in Nelson's system (copied below).
Not only the frontal area of the amps, but also the depth, directly to the side of the woofer stack, will increase the effective size of the baffle.
Am I on track here - can a larger effective baffle possible account for part of the +2.5 Nelson is measuring?
Bob
I think it's worth discussing all the benefits of slot loaded open baffle woofer versus plain open baffle though. It's clear the front slot loaded system as presented differs from a vanilla OB as follows -
Fs is lowered - do we have any guidelines on how this follows the cone area to slot ratio?
Qts is lowered - same question - guidelines?
Front / rear nearfield SPL response for 3:1 area ratio is 9dB - this may create smiles for people listening semi-nearfield. As mentioned in another referenced thread, this can also create advantages for those who love deep bass, with neighbors...
From most listeners perspective, it may also be interesting to discuss more what can influence the near field to far field transition. If it is based on relative *effective* baffle size, you may find interesting listening room situations where the near field extends a lot farther than expected by theory for an anechoic baffle.
The floor, for example, will significantly extend the effective baffle size for floor mounted systems. So will large amps stacked between and butted against the inside of baffles, as seen in Nelson's system (copied below).
Not only the frontal area of the amps, but also the depth, directly to the side of the woofer stack, will increase the effective size of the baffle.
Am I on track here - can a larger effective baffle possible account for part of the +2.5 Nelson is measuring?
Bob
Attachments
Speaking as a non-technical intuitive, wouldn't the degree to which the Fs is lowered (compared to the open air Fs number) be a function of the quality of the coupling to the surrounding air mass, in that the air mass then becomes effectively added to the moving mass of the driver?
Could the increased output at 0º vs 180º not be completely explained by the superior air coupling of the front of the drivers, thanks to what amounts to a very rudimentary horn produced by the slot and the front baffle. "Badly drawn" as this horn is, it is obviously superior to what the backsides of the drivers' cones see.
Could the increased output at 0º vs 180º not be completely explained by the superior air coupling of the front of the drivers, thanks to what amounts to a very rudimentary horn produced by the slot and the front baffle. "Badly drawn" as this horn is, it is obviously superior to what the backsides of the drivers' cones see.
Qts is increased under slot loaded in my case, from 0.25 to 0.34.
About the gain, how about a comparison of the lower end ? Lowered fs and higher Qts might help the output of lower end by common sense, no?
Of course this comparison should be done with flat OB vs sloted OB, instead of front/rear of the sloted one.
About the gain, how about a comparison of the lower end ? Lowered fs and higher Qts might help the output of lower end by common sense, no?
Of course this comparison should be done with flat OB vs sloted OB, instead of front/rear of the sloted one.
Just remember, the larger the driver the deeper the slot and the lower the cavity resonance. With a 15" driver you might have an 18" deep slot which would put the resonance at about 188 Hz. You will still need a relatively wide baffle to control the dipole roll off so why not just fold the baffle back and make an W frame dipole?
If you look at the response of my u-frame woofer when undamped, (upper right plot),
you can see that the polar response of the slot loaded woofer isn't much different from the undamped U-frame. Basically a dipole at low frequency degeneration to baffles monopole like response at higher frequency.
If you look at the response of my u-frame woofer when undamped, (upper right plot),
An externally hosted image should be here but it was not working when we last tested it.
you can see that the polar response of the slot loaded woofer isn't much different from the undamped U-frame. Basically a dipole at low frequency degeneration to baffles monopole like response at higher frequency.
Sorry, meant to say Qts is raised.
This is usually a good thing, as most big hefty drivers with large Xmax excursion don't have highish Qts numbers, which you want for open baffle operation. from what I recall from the past, isn't the target OB Qts in the 0.6-0.7 range?
Another thing, John's data makes one wonder if experimentation by adding/stuffing some light damping material to the OB front slot, (like in a transmission line) would be beneficial, especially to further raise Qts?
OTOH, eventually, Qts is not a keypoint. You may adjust it to whatever you want, if you are willing to leave the mainstream camp of "amps should have low output impedance".
Any woofer with decent Xmax and proper fs can be tuned to fit OB well. Low Qts (or Qes) of the driver is not a problem at all. Once it's working with high impedance amp, the system can be tuned to a proper Q.
Again, Nelon brought that up with a series of tests on fullrangers. I made a thread about that for OB bass purpose, too. Still largely ignored. Sigh~
Any woofer with decent Xmax and proper fs can be tuned to fit OB well. Low Qts (or Qes) of the driver is not a problem at all. Once it's working with high impedance amp, the system can be tuned to a proper Q.
Again, Nelon brought that up with a series of tests on fullrangers. I made a thread about that for OB bass purpose, too. Still largely ignored. Sigh~
I'm now working on another version with one driver facing foward...
For a quick experiment, why not just partially cover your woofer with anything convenient to you? A piece of whatever board with a hole (or window), in which the area is smaller than the Sd of your woofer.
Just like this:
http://www.emeraldphysics.com/index_files/cs2.3_finalweb.jpg
I remember someon mentioned that already, no?
It's now OK to go. (and please ignore my unfinished style of finish)
Rear mounting:
In position, with removable 'door' for an adjustable slot:
The slot in the picture above is 10cm wide, and about 35% of Sd. Later I tried various slot areas, including the largest (i.e., no) slot.
I settled at slightly larger slot then the above, 12cm wide, about 44% of Sd, for now.
When using the smallest slot (10cm, 35% of Sd), the bass sound tends to be "heavy" at the bottom end. It's not slow, but the effect of giant's steps is too much. Fun in the first few mimutes, gradually annoying.
OTOH, without slot, it's acting like a normal small OB. The slam is lighter, an open and easy going sound overall.
Between the two extreme, I settled at 44% Sd of slot for the time being, a comfortable compromise - bottom end with proper strength but not too heavy-footed.
These sound quality stuff might be nonesense to some. I understand. I can't do meaningful measurements for these, so no fancy chart whatsoever. The construction of 'door' shown above is super easy. Anyone can try that if interested.
BTW, measured by WT3, the smaller the slot, the lower the fs. It's now 15Hz or so.
OTOH, without slot, it's acting like a normal small OB. The slam is lighter, an open and easy going sound overall.
Between the two extreme, I settled at 44% Sd of slot for the time being, a comfortable compromise - bottom end with proper strength but not too heavy-footed.
These sound quality stuff might be nonesense to some. I understand. I can't do meaningful measurements for these, so no fancy chart whatsoever. The construction of 'door' shown above is super easy. Anyone can try that if interested.
BTW, measured by WT3, the smaller the slot, the lower the fs. It's now 15Hz or so.
I apologize if the question was raised before and I missed it...
I wonder if we could draw some insights/parallels from Ripole threads and builds, since this slot loading looks really similar to N ripol configuration that I'm experimenting with? Also, a lot of observations in this thread seem to corespond well with what i get from N ripole.
I wonder if we could draw some insights/parallels from Ripole threads and builds, since this slot loading looks really similar to N ripol configuration that I'm experimenting with? Also, a lot of observations in this thread seem to corespond well with what i get from N ripole.
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