DSP midrange directivity control aka kinda cardioid - Page 2 - AVS Forum
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post #31 of 42 Old 08-13-2014, 05:32 AM - Thread Starter
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Originally Posted by Martycool007 View Post
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?


If you want I can send you a file to play with in REW. I always learn best by getting hands-on.
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post #32 of 42 Old 08-13-2014, 11:05 AM
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Nate, what are your thoughts about using the corners of the room to accomplish the directivity that you seek?

That ~2dB hit in sensitivity is a bit less elegant than the elegant polars you're getting. Have you experimented with moving the slots further back - maybe using the rear corners instead? I would think the frontal lobe attenuation would be reduced the further back you put the rear-facing attenuator.

Also, what about a variable width slot? What I'm thinking is a way of making the Q of the "port resonance" wider in such a way as to only allow low frequencies to come out the back - and maybe in the process you can get some extra LF boost (although that will be minimal with a low Q design).

I've been trying to write my own directivity prediction software, but I haven't found a free version of MATLAB that implements Jacobian functions properly. Sci-Lab has been the best one I've found thus far. You might be able to use Akabak to simulate the Q of a variable width port (you could just do it with multiple ports of different diameters to approximate the idea - or you could just build it that way too).

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post #33 of 42 Old 08-13-2014, 11:35 AM - Thread Starter
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If I had good room corners is build corner horns in heartbeat! It was something I was considering when I was house shopping last year.

Unfortunately the efficiency loss is a necessary evil of dipole/cardioid systems due to the way they get the directivity by cancellation. This is mitigated to a point by using higher eff drivers. No free lunch as usual. In my system I use flanking woofers up to 300 hz so I won't be applying the full 6dB of roll-off compensation.

I'm fairly sure that rear corner vents won't work due to the greater distance to the baffle plane. It would work to a certain point (my rear vent was only good to 200hz or so) then the pattern would widen before the piston diameter takes over. Varying the slot position on the side is something I ought to investigate so thanks for pointing that out.

I'll have to think on the variable width slot.
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post #34 of 42 Old 08-13-2014, 03:27 PM - Thread Starter
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Here's the plot of the rear vent cardioid to give you an idea of what I was talking about before. Really not very directive at all in the mid-bass and lower midrange. Basically similar to the monopole response but with greater rear attenuation. The vent itself is on the back of the cab and is about 15" square with a 19" path-length to the baffle plane:



Looks like rain this weekend, but if not I'll be out annoying my neighbors as usual . I'm going to rebuild my test box so it's narrower (22" wide right now for the corner vents). It'll be 19" wide and I'll have removable side panels to test different front to back slot positions. I may even drag a TD out and see how it does. My hope is this will be the final test and be close to the final box so I can start building. I'm thinking the final box will have 2" radius roundovers on the baffle.

One thing I haven't addressed, but have been thinking about, is directivity in the vertical plane. Everything I've done so far has basically ignored this. It would be interesting to see if the side vents have any effect. I'm sure it won't be as dramatic as what we see horizontally but I wonder if it changes things at all. At this point I can't really think of a good way to measure it without impeding the side vents. I might add a vent on the bottom of the cab (it'll be up off the floor about 12" so I'll have to build stands to accommodate). Maybe increased directivity in the vertical plane can help with the floor bounce...
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post #35 of 42 Old 08-14-2014, 02:59 AM
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Quote:
Originally Posted by Nate Hansen View Post


If you want I can send you a file to play with in REW. I always learn best by getting hands-on.
Nate, thanks for the reply! It is my understanding that a Low Shelf Filter is used to either boost or cut frequencies below the Fc, and conversely, a High Shelf Filtr can boost or cut frequencies above the Fc, is this correct?

When is it beneficial to use a Low or High Shelf Filter? I mean, if you have access to a DSP/EQ-able setup like I have, iNuke1000dsp's for LCR's, can you not just use traditional boost or cuts to the frequencies in question? What does a Low or High Shelf filter offer over these methods?
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post #36 of 42 Old 08-14-2014, 11:33 AM
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1) What frequencies are you referring to when you say mid-bass? We have data here, it'd be easier to communicate if we use actual number. Or maybe I'm just being a stubborn engineer

2) What is that dip at ~220Hz showing up in every measurement? I would recommend adjusting the measurement configuration to guarantee you're not dealing with reflections at these frequencies. How do you know they're not affecting what you think the polars are doing?

3) I think minimizing front lobe attenuation should be a major focus of the approach - or at least a means of comparing the merit of different approaches. What is the relative weighting of the polar width versus distortion? I think it should be possible to avoid front lobe attenuation. I know I've seen subwoofer measurements getting as low as only 0.5dB on the front lobe - there was a small expense at the polar pattern, but I personally would prefer that versus 2dB on the front lobe with perfect polars. The power response in the room can be adjusted via other means, but distortion can't...

4) You'll improve the SNR of your measurements if you start by EQ'ing the on-axis response to be flat. In REW I'll take that mostly flat EQ'd response and then make it my calibration file - now everything is referenced to that on-axis response and I don't have to twist the response plots in my head to understand the attenuation for each axis.

5) I noticed that you're increasing the distance between angles as you get wider out. Clover-leafing usually happens at the wider angles, so I would recommend keeping a 10 degree step all the way through. I built myself a lazy susan to make this go faster (I cut holes for each angle and used a pin).


Anyways, just thinking out loud - as you've mentioned I've seen this pop up very infrequently, but it's something I think is worth investigating a little bit....especially if it's as simple as cutting holes in the box. I've decided on going the full-sized horn approach, but I may give this another look since the solution size should be smaller.

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post #37 of 42 Old 08-14-2014, 02:04 PM - Thread Starter
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1.) Mike - sorry, when I say midbass I'm speaking about 100-300hz. Might as well use numbers!

2.) I'm fairly sure that dip at 220 hz is a reflection due to the distance of the woofer to the ground. It shows up in every measurement where I was using the old modified box. If you look at the measurements of the front corner vent cab it's gone because the driver center is lower. Good point about it affecting the polars as it very well could, though I'm mostly concerned about 300hz and up.

3.) I'd like to have my cake and eat it too , but I just don't see how this is possible. The whole point of this is the increased directivity and the lower sensitivity is the trade off. I'm willing to trade that for the polar pattern to be honest. I really don't see how this can be improved with this kind of design....it's the nature of the beast. My version with the 8" side drivers did well to control the pattern from 300hz+ while leaving the region below that omni so there's little sensitivity lost where I believe it would matter most: at lower frequencies. The trade-off there is more $$ in drivers and amps. This was the direction I thought I'd go when I started all this but when I saw I could get close with the acoustic resistance box I switched gears.

I tend to listen at about 80-85dB, and you could figure 20-25dB for dynamic peaks on good recordings (to be honest the stuff I listen to isn't that good). Is that 2dB of attenuation really that huge of a deal with quality pro drivers used in the home? These will be high passed at 80hz.

4.) I didn't take the time to eq everything flat because I was working on 5-6 different iterations each time I was out. It would increase the SNR to be sure, but I felt that I was high enough above the noise floor that I could get a good picture of the polar pattern as is.

5.) I increase the angle as I get wider out for a few reasons. One - to spare my neighbors having to listen to 30 sweeps for each set of polars . Also it saves time. I don't think it's as critical to be as detailed at the wider angles since it's far out of the listening window, but it would be worthwhile to check it out once I get close to a final design. I have a thread about some diy waveguides (quiet now) going over on DIYA, and Geddes mentioned that this is the measurement scheme he uses. To me his reasoning is sound.

I have a lazy Susan turntable I built so doing this is all pretty easy once I have everything setup.
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post #38 of 42 Old 08-14-2014, 03:21 PM - Thread Starter
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Marty - a shelving filter is useful because it raises the entire spectrum above or below the corner, depending if it's a high or low shelf. A peaking filter only affects a small range of frequencies depending on the q of the filter which we talked about before. If you look at my graphic again I think you will see how it works with this in mind.

A low shelf of about -8dB centered around 800hz would be useful for the response shown in post 34.
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post #39 of 42 Old 08-15-2014, 03:51 AM
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Quote:
Originally Posted by Nate Hansen View Post
Marty - a shelving filter is useful because it raises the entire spectrum above or below the corner, depending if it's a high or low shelf. A peaking filter only affects a small range of frequencies depending on the q of the filter which we talked about before. If you look at my graphic again I think you will see how it works with this in mind.

A low shelf of about -8dB centered around 800hz would be useful for the response shown in post 34.
So based on the plot in post #34 , a low shelf filter of around -8db at 800hz would take down everything below that 800hz and make the response more flat? It is my understanding that by using a Low or High Shelf Filter you are either cutting, ie: a low shelf filter, or boosting, ie: a High Shelf Filter over the entire range either above or below the number specified by the user? That makes sense, only thing I am unsure of is when to use a High or Low Shelf Filter, instead of a traditional filter, ie: boost or cut at "x" or "y" frequency with what ever Q you need? Are the shelf filters mainly beneficial when the entire response needs to he adjusted above or below a specific area?
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post #40 of 42 Old 08-15-2014, 08:46 AM
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Marty, a high shelf will increase from the chosen frequency and above. So say you want to boost the treble, you will high shelf at 2000 hz by 2 dB. If you want to lower the treble, high shelf at 2000 hz -2 dB. Low shelf goes the opposite way in the frequency spectrum. If you want to boost from 0-30 hz, you low shelf at 30 hz + 5 dB.

A traditional filter will just increase or decrease the SPL at a specific frequency or over a small range. A shelf can do it over a large range. You are correct in saying shelf filters are beneficial when the entire response needs to be adjusted above or below a specific frequency.


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post #41 of 42 Old 08-15-2014, 10:14 AM
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cool project.


just thought that I'd kick in that this method works well for controlling the rear wave for bass applications.
the front wave summation though isn't perfect because (obviously) the rear driver running on both a distance and electronic delay doesn't sum the leading edge of the front firing wave.
also, the summation is frequency dependent -- some frequencies will be running with more or less summation than others.
might be best to try a large physical horn that just doesn't go quite as low. not sure...

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post #42 of 42 Old 08-15-2014, 11:03 AM - Thread Starter
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Ltd - my intention before I started building these waveguides and "cardioid" boxes was to build large Synergy horns. As you can see I got a little sidetracked.......at this point I intend to take the waveguide and directive 15 as far as I can. I'll likely tackle the big horn project next year.
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