Nelson Pass: The Slot Loaded Open Baffle Project

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Well, I put together a web page with some preliminary data. There is a lot going on here and things are not perfectly clean in my mind yet. Next step is to follow Tom's suggestion and build a box to allow the front and rear sources to be measured independently. You can look at the progress at my web page, Nelson Pass' slot loaded OB. Please excuse and grammar or spelling errors.

P.S. I can measure distortion but it is not going to be a priority. I probably won't get back to this until next week when I will have time to build a box.

Then maybe add a tweeter and made a speaker for DIY NE if DRL hosts it this year. :rolleyes:
 
@DLR
I am not talking about differences between loading on the rear and the front, but about differences in acoustic loading between inward and outward cone movement on the front side of the cone, driving the slot.
The two are inextricably related. If there is asymmetry of loading, by whatever means, then there would be asymmetry of movement if you are correct. The mechanism is the same, the difference is the magnitude of the results.

Dave
 
The two are inextricably related. If there is asymmetry of loading, by whatever means, then there would be asymmetry of movement if you are correct. The mechanism is the same, the difference is the magnitude of the results.

Dave

Yes, but it is just like sealed box loading. It is generally a small effect unless excursions are great. But suspension nonlinearity is generally more significant.
 
Well, I put together a web page with some preliminary data. There is a lot going on here and things are not perfectly clean in my mind yet. Next step is to follow Tom's suggestion and build a box to allow the front and rear sources to be measured independently. You can look at the progress at my web page, Nelson Pass' slot loaded OB. Please excuse and grammar or spelling errors.

John,

That's very nice work and very helpful.

I know it fundamentally changes the design, but I guess we would have to add a small horn (suggested by phase_accurate) to the front to preserve more of the asymmetry in the far field.

Jeremy
 
The one and only
Joined 2001
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Thanks very much for your efforts, John.

A note with regard to the null on the polar plots, I found it helpful to
simply plot the frequency response at and near the apparent null area,
as it gave me a more continuous range of samples, allowing me to try
to better evaluate the data.

:cool:
 
The one and only
Joined 2001
Paid Member
BTW, I think the higher spread above 300 Hz is not basket blockage, as I
see a similar effect above 100 Hz on mine, and interestingly the spread
starts looking more like the 4.5 dB I expected. This also corresponds to
the frequencies where the acoustic loading is becoming resistive. I speculate
that perhaps the Heil effect behaves as predicted in this region, and not
as well in the regions where the acoustic load is largely reactive.

:cool:
 
P.S. I can measure distortion but it is not going to be a priority. I probably won't get back to this until next week when I will have time to build a box.

Then maybe add a tweeter and made a speaker for DIY NE if DRL hosts it this year. :rolleyes:
I've been remiss in posting queries about a date. Any weekend Saturday in Oct is good, but my wife will be out the 1st and 3rd (to be confirmed), so those are the probable dates to consider. :)

Dave
 
BTW, I think the higher spread above 300 Hz is not basket blockage, as I see a similar effect above 100 Hz on mine, and interestingly the spread starts looking more like the 4.5 dB I expected. This also corresponds to the frequencies where the acoustic loading is becoming resistive. I speculate that perhaps the Heil ffect behaves as predicted in this region, and not as well in the regions where the acoustic load is largely reactive.

:cool:

Hi Nelson,

Perhaps I was not clear. I see two major effects: 1) Asymmetry of the front and rear response due to cavity resonance. In my model this is around 800 Hz, but the effect extends down to 300 Hz or so. You can also see from my last figure that above resonance the slot response roll off sharply. (Mounted on a flat baffle this driver has very flat on axis response to 10K Hz. Here is a link to a pdf for the driver specs.) 2) Baffle blockage, i.e. the same as the baffle step in a conventional speaker. Above 200 or 300 Hz the baffle influences the interaction between front and rear waves, blocking the "wrap around effect".


My initial comment about basket blockage on the rear axial response was made rather quickly and is incorrect. (I'll need to correct that.) The basket affects the rear response primarily above 1k Hz. At lower frequency the dip is more due to what is discussed with respect to the last figure. When the front axial response is eq'ed for flat response around 800 Hz the rear response is further attenuated.

In any event, based on my scaling my results below 300 Hz should be representative of your system below 100 Hz. The difference may be in the frequency of the cavity resonance since it will depend on cavity depth.

I never followed Heil but I was of the mind that his argument was more along the lines of getting higher volume velocity from a unit with small frontal area. That is, if I look at the Heil from an AMT 1 the frontal area is about 1/2" wide by 4" high. So by folding the element and squeezing the Heil can get much high volume velocity over that 1/2"x 4" area than would be possible with a 1/2 x 4" flat radiator moving in the axial direction.

Anyway, once I build the box so I can measure the front and rear independently we will have more insight.
 
Hi John
Boy you work fast! Build measure document and then make a web page that fast is something and evidence of a very keen interest.
I am pretty used to looking at horn responses where the systems are more elaborate than usual.

I would offer that “if” the rear were enclosed like a simple horn I would offer that to me what I see is;

The peak around 700Hz is the Helmholtz resonance made from the trapped air volume and mass in the exit port AND the onset of horn gain. One can see the increased loading at that point by examining the dip in the response from the back side. An impedance curve would also show a “feature” or dip at this frequency.

The transformer action of a horn has a “high pass” corner based on how far the area is expanding. For example, a simple exponential horn made for a 30Hz cutoff can only expand so fast, doubling it’s area every 24 inches or so. At 300Hz, the area is doubling every 2.4 inches and so on.

A horn also has to be about a quarter wavelength at it’s low cutoff (a half wl to be optimally efficient) In your experimental test model, you could estimate how fast your area is expanding within the passageway and then get a feel for where the corner for that gain is weighted against the effective length. In this case it is a little more involved as your trapped air volume also makes the required horn length to be shorter than normal (sound velocity and frequency).

The dip around 2500 can be several things, in the horns I make, the mid and low drivers are mounted forward of the horn’s apex. This accommodates the variable expansion rate of the conical horns we use.
A graph on page 4 at the link shows how the expansion rate is a function of position and so governs where the high mid and low drivers must be mounted in order to couple into the horn.

http://www.danleysoundlabs.com/pdf/danley_tapped.pdf

A limiting thing about drivers that are not at the acoustic end of a horn (or as in your experiment a port) is that some energy is radiated to the closed end and it bounces back, when that combined path length is ½ wl, there is a cancellation notch and in our case is just past the upper limit of that band passes operation. In your test, I would also suspect it might be mode coupling where the energy going “in and out” couples to a mode running crosswise, this also puts a notch in the response.

This is usually what sets the upper frequency limit in a simple bass horn and in that as it is the mode which occurs when the spacing of the parallel walls is one half wavelength.
If your parallel walls are around 2.5 to 2.75 inches, I would guess this is the cause of the dip. Obviously in a horn that operates that high, one can’t have parallel walls.

Note too though that even with the horn gain and Helmholtz resonant gain, that the HF response rolls off at a steeper angle than the rear due to the acoustic low pass filter.

Part of the fun working on these more complex acoustic systems is that a given lump of air can often serve as a mass and a compliance both depending on the circumstances.
Computer modeling can be very useful but must be built around the measured results and best guess so far as what does what. I use Akabak and while very powerful, it also requires that you guess, model, build, measure and modify the model in a loop.

So far as the Heil effect, by examining the dimensional requirements for a horn that operates at say 5Khz and above, it is pretty likely I think that the pleats themselves in membrane would be sufficient to place your 700Hz peak in the top of the range. Consider that with a 1 inch HF compression driver, ALL of the impedance transformation has been accomplished well before the exit at 20KHz and the exit is large enough to be limiting the dispersion to about 60 degrees maximum.
Best,
Tom Danley
Danley Sound Labs
 
Interesting idea. Of course i am wondering if size of slot is optimal and if there are even ways of figuring it out? I also can't help but wonder how two 15" drivers per baffle would sound simply facing forward compared to the slotted system.

I wonder if the speaker can be slimmed down by bending the sides of those wide baffles? May not help the Lowther's response but would it hurt the 'bass system's' response? I can picture 8 or more woofers from floor to ceiling in corners filling a room with incredible bass while maintaining a slim footprint.

Thanks for sharing. It's probably a good idea. Hopefully others will wrap their minds around this and move the idea forward.

I need coffee!

The arrangement forms a low pass acoustical filter. I wouldn't expect a lot of bandwidth out of it.
 
Karlson couplers above a certain size seem to throw out a high velocity shockwave - U could build one for Lowther - for something less rich, that limited production 8" Fostex at Madisound for $600 a pair might be fun -on the cheap side FE206EN (stock - not P10's hotrod driver) will do
- - it should be fun to run Nelson's setup with FE206EN/etc. wonder what's good value today and appropriate for woofers?
 
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I was wondering about the imaginative part. In other words, my OB's as they are now, are 50x120 cm dimensions, and are comprised of one Eminence Beta 15 as a bass and Visaton B200 (fullrange). Now I am contemplating about the possibility of rebuilding the thing so it will look more like Nelson's OB. So, due to the WAF, the baffle dimensions must stay the same, but I could mount Beta 15 like Nelson did with his woofers. The problem is that there is only a single woofer. So I guess that the slot should be placed a bit off-center...
 
Vix,

I don't think there is any magic here. The slot area needs to be 1/3 the area of the driver(s), more ore less.
Without 2, the rear output will be asymmetrical. I would place the slot somewhat asymmetric on the baffle, and have the pair mirror image.
Its not perfect, but what is?

Let us know how it turns out.
 
It seems to me that any asymmetry that may exist due to velocity differences could be if any air surrounding the higher velocity stream is 'entrained'. This would presumably occur within a certain distance of the slot before the peak velocity energy reduces to a value less than or comparable to the static sound pressure.

I don't know in reality how effective any such mechanism might be in converting velocity energy into additional SPL but if it did exist, that is how I could see it happening.

I'm not sure this is all that comparable to the situation of a restricted BR port at higher SPLs, since the BR port is most directly driven by the cabinet air compliance spring which is different than the case of the slot which is more or less directly driven by the driver cones so the behavior should be quite a bit different between the two at higher (audible) VLF SPLs where the restricted BR port compression and potentially rectification would be expected to be much more significant than for more or less direct slot loading of drivers.
 
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Horn loaded slot or slot loaded horn?
 

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