DSP midrange directivity control aka kinda cardioid - AVS Forum
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post #1 of 42 Old 07-26-2014, 10:03 PM - Thread Starter
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DSP midrange directivity control aka kinda cardioid

I had a really long post all typed out and then accidentally deleted it. The gods of brevity sending me a message

My goal here is to use my current midwoofers (TD15m) from 80-1000ish and have controlled directivity from 300hz up. In my room I'd like to have a 60° beamwidth and I'd make elliptical os waveguides to match, but I won't settle for anything more than 90°.

There's really three ways that I see to do this: dipole, cardioid(or variations thereof), or some big horns. I don't like dipole because that's a 120° pattern and I don't like the rear energy bouncing off the front wall....been there and done that. Horns will be another project. So that leaves cardioid. Face and RyanCo have passive resistance boxes for their midbass. The rear wave of the driver is down in level and delayed to narrow the pattern and create a null at the rear. I suspect that there isn't a perfect null because the damping needed varies with frequency but that's a close enough approximation. IIRC Face said his box had issues over 4-500hz, and this is something I've seen with other passive implementations online.

Being the pc dsp dork that I am I thought I would try an active cardioid using smaller drivers on the rear or sides of the cab. Linkwitz says a cardioid can be made with 2 monopoles back to back, with the rear driver out of phase and delayed the time it takes to reach the front baffle plane. I'm using some junk 2cu ft sealed cabs, old (early 80's...older than me!) 15" Eminence woofers and 8" Vifa hifi drivers. The first implementation was a single 8" on the back. I tried different delay settings, levels, as well as hi and low pass on the 8" but could only get good results up to 400hz. Above that things get goofy. Sorry didn't save the data.

I wasn't surprised considering the cab is 15" deep which is more than a wl at 1khz where I want to xo.....so I put an 8" on each side of the cab about 3.5" from the front baffle plane. I ended up delaying the main 15" by .25ms (which I thought was odd). The 8's are out of phase, -9dB in my dsp and have a first order low pass at 700hz. Here's 0-90°:



And a polar plot:



Sonogram? Why not:



Not bad! Right around 90° or less from 300 up. As you can see it's really only cardioid at 1khz with a good bit of rearward energy as you go down in freq. I'd like to see some better rear attenuation, and that 90° measurement is funny. These cabs already had a 12"x12" hole in the back that I covered with mdf, so I opened it up and stuffed them with bonded logic:



Here you can see that it didn't really affect the forward response, but attenuated the rear a good bit and tends toward a hyper-cardioid. Nice.

I did a lot of playing around with this. I could get a 60° beamwidth by shaping the side drivers with hi and low shelving filters, but it would create some serious lobing at 90° and beyond. Ideally I would have a driver on the back as well. That would need a channel of it's own for a different delay and that's getting too complex. We're talking 5 channels at that point for a 2.5 way speaker

I've got enough data at this point to feel comfortable moving forward with this. Any ideas on suitable 8" drivers for the directivity control? They're down quite a ways in level, and since they aren't really contributing to the direct sound they don't need to be anything special. I'm thinking the Dayton PA's. Also, any thoughts on the closed vs. open back? I'd like more attenuation to the rear but the open back does have more lf roll-off.
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post #2 of 42 Old 07-26-2014, 10:05 PM - Thread Starter
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I should add that these are ground plane measurements done in my garage with a 5ms gate, so the lf resolution isn't great. I think it's good enough to see the general trend though. I was hoping to do this outside but it kept threatening rain. Hopefully tomorrow I can get out and get some better data.......
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post #3 of 42 Old 07-27-2014, 07:15 AM
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Great stuff Nate. Lots of complexity though. Do you plan to live with a system like that?
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post #4 of 42 Old 07-27-2014, 07:15 AM
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I am unfamiliar with the Cardioid enclosures, and have spent the last 10 minutes trying to search and figure out what they do?

It seems that you are trying to get directivity from 300hz to 1000hz, but, I am confused. Does your LCR mains not have directivity at or below 1000hz? Are you building these strictly as mid bass cabs, or is this the mid cab of a 2-way speaker design?
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post #5 of 42 Old 07-27-2014, 10:24 AM - Thread Starter
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Tux - it's complicated for sure. The only real hold up at this point is I need another amp. I have some that will do for now but I'd want something balanced and I'm going to add a couple surrounds in the future so I'll look for a 4 channel job. Maybe I'll just build another one. The complexity is really a non issue once everything is setup. My dac and amps are connected to a power conditioner with 3 delay banks so I can fire everything up in the correct sequence with a single switch, and I have a single 8-pole cable running from the amp stack to the main speaker.

Marty - here's a set of measurements of Geddes' old Summa speaker:



It's a waveguide and a 15" midwoofer crossed around 900hz. As you can see below 900hz the directivity is quickly widening and by 200hz is omni - meaning that the sound is radiating 360° around the speaker with little to no decrease in SPL around the back side. This is typical of a direct radiator in a box. It will be omni to a point where the driver and baffle begin to constrain the output based on the dimensions of the device, and the directivity will continue to narrow to a point that we call beaming.

My current waveguide and 15" midwoofer is the same deal. My goal as you said is to have constant directivity from 300hz and up, thus the cardioid-like enclosure. The cardioid will xo to the waveguide around 1khz. In my opening post I gave a brief description of a cardioid.
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post #6 of 42 Old 07-27-2014, 01:09 PM
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Super cool. Do you have any pictures?
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post #7 of 42 Old 07-27-2014, 01:25 PM
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Nice setup with the relays and 8 pole wiring. I'm far from that organized.

So, what's the underlying objective here? The obvious states objective is constant directivity from 300hz and up. But why? To keep sound off the walls? Is there any other reason? I'm just thinking, is there research to support this is a good idea and well worth the effort? I was impressed with Face's work, and also with this. I've never attempted such a thing. So, what do we know about this method?

Also, if you have less LF sound power, I think you might have to raise the low end response of this speaker to make it sound balanced. Thoughts on that?
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post #8 of 42 Old 07-27-2014, 05:06 PM - Thread Starter
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Thanks guys.

HopefulFred - no pics of my fugly prototypes at this time.

Tux, yeah obviously the objective is cd down to the transition region. This is one of those topics that's debated endlessly over on DIYA. I think if you really wanted to you could probably find a paper or some research supporting your opinion....whatever your opinion may be . My opinion is that it is important, just that it's given less importance by most due to the complexity/difficulty of getting it done depending on your design constraints.

Geddes says 700-7khz is the most important, and if I remember right his thinking is in line with David Griesinger http://www.davidgriesinger.com/

Yeah, the lf output of a cardioid/dipole rolls off at 6dB/octave below the point where the dipole spl=monopole spl. Linkwitz calls this point "fequal". No free lunch here like usual. I'll apply some low shelving, but I'm also using a sealed flanking woofer up to 300hz that will help balance this region.

Here's something I tried this afternoon. Someone over on DIYA gave me the idea:



It's a dipole in front of a monopole with a minimum distance between the two, 4.5" in this case is the best I could do. They are in phase and no delay but the monopole is about -10dB. The nice thing here is I get greater cancellation to the sides and back. There is a bit of a cavity resonance between the dipole and the monopole though and this absolutely will not work over 1khz....I'd like a nice even response at all angles up to 1.3khz or so to make the xo easier/better. My pc was acting up so I couldn't test it as much as I would have liked and I didn't save the data. I whipped up a quick sealed box for my 8" woofer and I'll try that as the monopole instead of the 15". Hopefully I'll have less resonance issues but the distance between the two will be greater so it might be a dud.
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post #9 of 42 Old 07-27-2014, 05:48 PM
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What I am not understanding is what you are doing as far as driver alignment and how that changes the potential constant directivity? How would adding a rear woofer to the cabinet, delayed with reversed polarity do anything to help constant directivity? Same with side firing 8" drivers?

Would you simply just build a box for the AE TD15 and then add either a rear mounted woofer or side mounted 8" woofers and play with the delay and phase via DSP until you get the directivity you desire?

I would assume that to achieve constant directivity at any frequency above 100hz or so, the mid and/or woofers would need a waveguide of its own. I would love to hear more as to this topic and my above questions! Thanks!
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post #10 of 42 Old 07-27-2014, 06:31 PM
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Ok fair enough. I haven't read enough on the subject because I'd probably sit in the "it's to hard for me" camp. But I like the idea of keeping energy off the walls.
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post #11 of 42 Old 07-27-2014, 06:40 PM - Thread Starter
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Marty - while you could build a waveguide to get directivity that low ....it would be huge. The wl at 100hz is 135" and that's how wide the mouth would be. That's why I'm trying cardioid. That said below 300hz or so room modes dominate the response and I don't think directivity will gain anything here.

Cardioid/dipole works by cancellation. When the rear wave meets the front wave out of phase there's cancellation of varying degrees depending on what kind of response you want. A waveguide/horn works by increasing spl in a narrow beam. Cardioid/dipole cancels at the sides to create a narrower beam. It's more complicated than that but that's the gist.

This has been a lot of fun - seeing how the drivers interact while varying level, delay and fr to influence the pattern. I have a few more things I want to try. I'll take some pics and post the data.
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post #12 of 42 Old 08-08-2014, 06:43 PM - Thread Starter
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I'm still plugging away at this. I've been getting some really nice (too nice) results with different setups that in theory shouldn't work......the garage ground plane measurements aren't cutting it. So, I took this stuff outside. Measurements done at about 12' from the baffle with a 60ms gate to get a rid of a reflection I think from my garage. The 220hz null is not in the driver but a reflection.

I measured the monopole (about 1.5cu ft sealed with a Eminence Delta 15" LFA) out to 180° to get a baseline. Very little eq, I think the highs roll off because I didn't have the cab angled towards the mic correctly:



edit: I added the sonogram. Eq post processing done to mimic a usable speaker response and make comparison easier:



Next is an acoustic resistance box (same box) that has 2 large slots cut on each side of the cab. Total area of the slots is about 150% of the driver sd. The baffle on these is 19" wide. The first slot is about 2" back from the baffle and the cab and slots are stuffed with Bonded Logic:



edit: Sonogram added:





Fr tracks quite well over most of the range with around 100° beamwidth and good rear attenuation. I'm liking this but ideally I'd like a bit narrower directivity. You can see the pattern widen around 1khz (where I would probably xo) then start beaming soon after due to the driver size. I think the narrowing at 1khz is due to the slot spacing and the baffle width.

Next is the same cab, but instead of slots on the side I have an 8" woofer on each side with about 3.5" from the woofer frame to the baffle. Each 8" woofer has it's own enclosure within the main cab. The 15 and the 8s were eq'd relatively flat, and the 8s are -8dB, delayed .2ms, and low passed 1st order at 700hz:





The fr tracks pretty well here too, with maybe a bit less than 100° beamwidth. I'm sure I can narrow the pattern with delay, level, and fr changes to the side drivers. Monopole response below 250hz which I don't like. I don't think that directivity is important below 300hz or so, but an attenuated rear response might help with the front wall notch in the mid-bass.

Last, here's the cab with the side drivers again, only I have about a 15" square opening on the back side of the box as well. Still stuffed with BL and I was hoping to get some rear output cancellation:



edit: Sonogram added and post-processed:



The response out to 90° is mostly unchanged but you can see significant rear attenuation.

Last edited by Nate Hansen; 08-11-2014 at 04:21 PM. Reason: Added pics of the enclosures and sonograms. Both cabs have a removable rear panel
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post #13 of 42 Old 08-08-2014, 06:51 PM - Thread Starter
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I tested some other configurations but these were the best.

If anyone is interested I have directivity sonograms and full polar plots as well.

The reason I'm posting this (other than feedback) is because I haven't seen any data like it (other than a couple guys over on DIYA). There's been several that show a cardioid or cardioid-like response up to 4-500hz but that's it. This guy has a lot of good data: http://kimmosaunisto.net/ but there again most of it is bass to mid-bass. He hints at how to get midrange cardioid response but doesn't show anything.

My favorite at this point is the acoustic resistance box with the slots. It's a touch wider pattern than I would like but I won't need to buy any more drivers and amps so it'll be mucho cheaper just to build cabs. Honestly I think the wide bandwidth cd response is pretty impressive for a 15. I'm going to build some new test boxes with maybe a 2" roundover on the baffle and get the slots up as close to the baffle plane as I possibly can. I'll test these with my TD's which is what I'll be using.
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post #14 of 42 Old 08-08-2014, 06:59 PM
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This is very cool. Thanks for sharing.

Can you tell (or speculate) about the fr above 250-300, where I would describe the on-axis response as rising, while off-axis is flat? Should I have expected that? Would you leave it that way or shape the input signal to get back to flat on axis?
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post #15 of 42 Old 08-08-2014, 10:10 PM - Thread Starter
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Thanks.

I'm not sure what you're seeing.....are you talking about the monopole response? It's directivity does gradually narrow due to the driver diameter, which could be interpreted as a rising response. I would say the off axis response of my cardioid-like setups tracks the axial response quite well out to about 60° up to about 1khz.
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post #16 of 42 Old 08-09-2014, 07:51 AM - Thread Starter
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Here's what I'm thinking about trying........corner vents:

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post #17 of 42 Old 08-09-2014, 01:02 PM
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Impressive directivity down to 250 Hz.

How wide is the box exactly?

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post #18 of 42 Old 08-09-2014, 05:21 PM - Thread Starter
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Thanks. The cab I was using is 19" wide. I built a new test box per my model above. The baffle is 18" wide with 3" wide by 20" tall slots at the corners. Hopefully I'll be able to test it tomorrow.
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post #19 of 42 Old 08-10-2014, 02:33 PM
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Do you have pictures of the various designs to correspond with the measurements?

I've seen data on this for subwoofers in PA applications, which I'd expect should scale well to different wavelengths (higher frequencies).....although subs tend to be narrower bandwidth devices.
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post #20 of 42 Old 08-10-2014, 02:35 PM
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Btw, are you set up to do distortion measurements? Would be curious if there are any hits in distortion for any of the approaches?
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post #21 of 42 Old 08-10-2014, 06:29 PM - Thread Starter
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I can get some pics. Did some more testing today with different variations of a front corner vent....I'll get the data up in the next couple days.

To tell you the truth I've never really focused on distortion measurements. REW has the capability but I don't know what's required of the measurement process to get good data here. I would assume it should be done in a quiet environment (obviously) at 2.84v to be well away from the noise floor?

I've saved everything if it's of use. My measurements were at about 75dB.
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post #22 of 42 Old 08-10-2014, 08:19 PM
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Very cool, I love reading this thread and the research involved.
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post #23 of 42 Old 08-11-2014, 01:26 PM - Thread Starter
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I edited post #12 to include pics of the enclosures and added the directivity sonograms. More to come.....

Last edited by Nate Hansen; 08-11-2014 at 02:48 PM. Reason: why does avs have hash tags?
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post #24 of 42 Old 08-11-2014, 03:02 PM - Thread Starter
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Here's the results of my Sunday geek session:







If you compare this result with my monopole result from earlier I think it's safe to say that there's little difference in directivity at 500+hz. My thought with the front corner vents was to get them closer to the baffle plane and actually narrow the pattern in this region but it appears that the baffle size dominates. Below 500hz the pattern is a fair bit narrower on my corner vent cab than the monopole. My goal is to maintain a 90ish or better degree pattern from 300 up........so this one didn't work out.

I tried narrowing the baffle, making the vent wider to the back, making it narrower, more stuffing, less stuffing, and I even removed the sides completely. The effect on the direcitivity of these changes was negligible. It only increased or decreased the sensitivity in the 800-1khz region.

At this point my side vent version from post 12 looks the best when comparing the sonograms. It maintains a roughly 100° beamwidth from 300hz and up and it looks like I'll have a bit of room to play in the xo region around 1khz.

Last edited by Nate Hansen; 08-12-2014 at 03:25 PM.
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post #25 of 42 Old 08-11-2014, 07:06 PM
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I'm not clear on the cabinet construction.

Are there internal partitions to increase the path length between the woofer and the vents?

If not, how can the bass go so low?

Either way, I'd like to see what could be accomplished in a box the same width as one for a SEOS12.

Even if directiviity could only be held to 500 - 600 Hz, that's an octave better than with a 12" woofer.

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post #26 of 42 Old 08-12-2014, 03:36 AM - Thread Starter
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The box with the partitions is the version with the dual 8" drivers on the side to control directivity. I suppose it's confusing because the 15" isn't mounted in the box when I got the pic. The partitions are sealed enclosures for the 8" drivers so there's no interference with the 15 (the main forward driver) inside the cab.

There's bass because I eq'd it.

I suspect a 12" driver would work even better up high due to the narrower baffle.....in fact I think that would be ideal. A AE LO12 would do nicely.
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post #27 of 42 Old 08-12-2014, 05:29 AM
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The polars above 500Hz are dominated by the driver itself....basically any 15" driver is 90deg at 1kHz.

How much EQ are you adding? This would be my distortion concern.
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post #28 of 42 Old 08-12-2014, 05:37 AM - Thread Starter
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I'll look at my dsp to see what kind of eq it took.

Last edited by Nate Hansen; 08-12-2014 at 03:32 PM. Reason: fixed the sonograms
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post #29 of 42 Old 08-12-2014, 03:58 PM - Thread Starter
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The driver I'm using is an Eminence Delta 15LFA and I doped the cone with PVA several years ago. The plot given by Eminence shows a rising response at 1khz.........and you can see in my data (specifically the monopole fr plot which was an un-eq'd measurement) that I have a high freq roll-off. Maybe the added mass from the PVA is causing this? I drew my measurement setup out in cad to determine what angle the 15 needed to be so it's on axis with the mic. This came out to 4.6°. I used a 2x4 under the back of the cab to angle it and that ended up being about 6°. The difference amounts to the axis shifting 3". At 12' this is not enough to cause that kind of roll off at hf I wouldn't think. It's my understanding that a groundplane measurement is +6dB over the entire bandwidth (if the dut is closely coupled to the ground) compared to a free field measurement.

Anyways..........that's my roundabout way of saying that there is no lf boost done (at least for the corner vent version and I believe the others) but rather I applied +5dB high shelving at 800hz and -5dB low shelf also at 800 to flatten the response. I think you could say then that my "kinda cardioid" cabs are around 4-5dB less sensitive than a comparable monopole so it does need nearly double the power to achieve the same spl. Next time I'm setup for measurements I'll do a near field of the Delta and see what I get.

Here's a monopole vs open back cardioid on axis measurement, unfortunately taken in my garage but I think it shows what you're looking for:



About 5-6dB lower spl for the cardioid at 200hz and -3dB from 400-2k. Theory says a dipole is going to be -6dB compared to a monopole at low freqs and I believe a true cardioid would be -3dB at lf.
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post #30 of 42 Old 08-13-2014, 03:13 AM
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Quote:
Originally Posted by Nate Hansen View Post
The driver I'm using is an Eminence Delta 15LFA and I doped the cone with PVA several years ago. The plot given by Eminence shows a rising response at 1khz.........and you can see in my data (specifically the monopole fr plot which was an un-eq'd measurement) that I have a high freq roll-off. Maybe the added mass from the PVA is causing this? I drew my measurement setup out in cad to determine what angle the 15 needed to be so it's on axis with the mic. This came out to 4.6°. I used a 2x4 under the back of the cab to angle it and that ended up being about 6°. The difference amounts to the axis shifting 3". At 12' this is not enough to cause that kind of roll off at hf I wouldn't think. It's my understanding that a groundplane measurement is +6dB over the entire bandwidth (if the dut is closely coupled to the ground) compared to a free field measurement.

Anyways..........that's my roundabout way of saying that there is no lf boost done (at least for the corner vent version and I believe the others) but rather I applied +5dB high shelving at 800hz and -5dB low shelf also at 800 to flatten the response. I think you could say then that my "kinda cardioid" cabs are around 4-5dB less sensitive than a comparable monopole so it does need nearly double the power to achieve the same spl. Next time I'm setup for measurements I'll do a near field of the Delta and see what I get.

Here's a monopole vs open back cardioid on axis measurement, unfortunately taken in my garage but I think it shows what you're looking for:



About 5-6dB lower spl for the cardioid at 200hz and -3dB from 400-2k. Theory says a dipole is going to be -6dB compared to a monopole at low freqs and I believe a true cardioid would be -3dB at lf.
Couple of quick questions, what does a High Shelf and/or Low Shelf filter do of accomplish? I keep seeing people talk about using a High or Low Shelf filter, just never way sure what that does, exactly?
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