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Discussion Starter #21

JBL uses these "RBI waveguides" on their prosound line these days









I decided to give this a try on my system.


Here's the frequency response and distortion of the 8NDL51 in my small sealed box


Here's the polar response of the woofer. (this is with no waveguide.)


Here's the horizontal polars of the woofer, using my crude approximation of JBL's RBI waveguide.
yowzers, this is impressive performance.
Look at that! The RBI basically makes this 8" woofer behave as if it was just 2" in diameter. The off-axis response is virtually identical to the on-axis response, all the way to 7000Hz!


There's no free lunch though. Here's the vertical polars. To me, it looks like the RBI makes a HUGE improvement in the horizontal polars, but it also has a negative impact on the vertical polars.

I have a feeling you could probably come up with a compromise between the two by making the aperture shape closer to circular, instead of elliptical.

 

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If the SB on the waveguide can play mostly flat down to 400hz or so, why worry about pushing the midbass up to 1000hz? I pondered this for a good long time when planing my midbass, which led me into full stupid with bandpass midbass. Four 3$ buyout per box and I can hit nearly 114db peaked at 300hz. But I need the midrange to carry from 400hz on up, so something like your waveguides seem like a good way to go. Any reason to not push the waveguides that low?

I totally agree with the visual aspect. That was the biggest issue with the dash mounted waveguides a built with the Whisper. It was so obvious. I did horizontal arrays under the dash, and they worked great. However, I am fairly tall and I found that vertical head location was a huge factor with that location. It was good in the normal seated position, but if I scrunched down 2" or so in the seat, it was amazing. Does anybody know if that is inherent to under dash placement, or is there something that would have rectified the vertical head position issue? With all the effort I have put into hiding the locations of both the subs and the midbass, I would hate to destroy the illusion with obvious midrange placement. So, i am a bit stuck here.

Any issues or caveats you can think of is using a square midrange in a waveguide? Got a bunch of square BMRs that I really like.

I like that after trying a lot crazy stuff, you came back to a fairly simple solution...for now! Which is cool.

Often more things are learned from failure than success.

You abandoned your midbass arrays!?
 

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Patrick did you listen to the speaker? Did you hear any distortion or artifacts from the wave guide?

And I don't want to be a downer but you didn't copy the JBL waveguide.

1-Your waveguide has points and is not symmetrical on the horizontal or vertical axis. This may seem minor issue but we don't know.

2-Your waveguide has points the JBL doesn't have any points. Is this important. We don't know.

3-Your wave guide is flat the JBL waveguide seems t have a minor horn shape. Is this important. We don't know.

4-Also what does the inside of that waveguide look like? Is it flat or does it have some geometry like the Nexus waveguide
 

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The biggest impact of that JBL wave guide is the use of technique for diffraction. That's the characteristic he wanted to mimic and test. Creating this for diffraction, the points aren't all that critical. I remember reading up on another created by I think an JBL Engineer?, and he created a multi point star that looked like an explosion. It worked extremely well and was VERY simple.
 

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Discussion Starter #25
The biggest impact of that JBL wave guide is the use of technique for diffraction. That's the characteristic he wanted to mimic and test. Creating this for diffraction, the points aren't all that critical. I remember reading up on another created by I think an JBL Engineer?, and he created a multi point star that looked like an explosion. It worked extremely well and was VERY simple.
I agree. I think JBL swiped this idea from Doug Winker. He was working for JBL when he was competing with this design:



Forum member Kevin K references it here, looks like Mr Winker was competing with this about fourteen years ago:

http://www.diymobileaudio.com/forum/539215-post10.html

Kevin and I are in the same city, might be fun to talk shop sometime? Looks like he used to work for Speakerworks?

[/font]
 

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I've got a neat story about Doug Winker. We held an IASCA show at our shop in 97. Doug was here for a couple of days. We did some fun experiments and one was a station that was setup where you had 2 speakers in front of you. There were dual 31 band eq's on the table, one for each channel. One by one we had people sit there listening to pink noise. The goal was to eq it by ear to be as flat as possible. Across from the listener was the person conducting the test that could see the rta screen. The person being tested could not. It was a ridiculous experiment but it was basically just something fun to mess with.

One by one we take our turns listening and adjusting, only to be shown in the end how terrible we ended up doing. Then Doug gets in the seat. For those of you that have never met him, Doug is a large, awkward looking behemoth of a man, but one of the coolest people you'll run into. The pink noise gets turned on, he closes his eyes, and then puts a hand on each eq. I'm pretty sure every finger was in use. He slowly moved those fingers on the sliders, adjusting, and readjusting, and doing both eq's simultaneously. It was the strangest thing to see. If the X-Men had an audio specialist, it would be Doug. He stopped fiddling with it and said he was done. Lo and behold, that rta was nearly flat. Close enough to consider him a clairvoyant. It was amazing. When it comes to hearing, he's definitely a savant.
 

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Reminds me of what ESB is doing with their tweeters and dome midranges in their 9000 series. Well, if they ever actually release them. It would be interesting to see the FR graphs on these with and without the grills.

 

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There was a link explaining Doug's take on the diffraction ring on the old ECA forums. I thought, from memory, that he used a random pattern to spread the diffraction artifacts across a broader spectrum. This is in stark contrast to all of the OEM diffraction devices that I have seen. They all seem to be highly symmetrical. I wonder if this is more a cosmetic consideration rather than a performance issue... If you can't sell 'em it doesn't matter how good they sound.
 

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To the question about "points" in the diffraction ring - I believe points tend to be a negative feature of the diffraction ring. It would seem to concentrate the diffraction artifacts at specific frequencies whereas a more smooth shape (without slope discontinuities) would more evenly spread the diffraction artifacts across a broader spectrum thereby making them much less audible. I always wanted to talk to Doug about this and see if he tried any non-pointed diffraction rings. Who knows you may be able to use symmetric shapes if you eliminate the points. The first use of diffraction rings that I saw was just a simple circle that was smaller than the diameter of the speaker. These seem to work fine and symmetrically in both the horizontal and vertical dimensions however most noted they heard the diffraction artifacts and reported some reduction in output vs the driver with no diffraction ring. The bigger the difference in diffraction ring opening vs speaker size the greater the reduction in output (using the circular diffraction ring). Most noted the diffraction ring had the effect of a mechanical high pass filter. Giving up low freq output is not generally a trade most are willing to make... IME if you go non-symmetrical in the diffraction ring you can locate the attenuation where you want it geometrically. This has a TON of advantages in a car environment but you have to tailor the ring to the car.
 

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Doug explains it the the 'Reducing Speaker Beaming' thread I started in May 2010.
:
Here is the text from my post. Let's see if it works.

I see from this thread that people were asking questions about the setup in my BMW. Here are some answers to your questions.

First, my dissertation was on a constant beamwidth, wide bandwidth array and had nothing to do with my car. JBL was nice enough to send me a bunch of speakers to use in the array though. I think it is cool that some of your have actually found and downloaded it btw. :)

No, I don't work for Harman. I work for this company ETS-Lindgren | EMC and RF Microwave Testing, Measurement and Shielding We build test chambers and I do all of the chamber design for the acoustics side of the business - anechoic chambers, reverberation rooms, noise control rooms, studios, audiometric rooms, etc.... So I live primarily in the industrial test and measurement world. If you are ever in the Austin area, send me a PM and I can give you a tour of our factory and our lab. We have a very large hemi-anechoic chamber with a >0dBA noise floor. Nice and quiet in there. Anyway, on to the 2118 setup I ran.

So the 7 series is no more. It was in a fire at Audio FX in Georgetown, Texas. Some of the equipment was recovered and I was able to save the diffraction rings for the mids. I gave them to someone in Florida with a Civic as long as he promised to be nice to them. That was a few years ago.

No this was not part of a thesis and was never published. I lost all of my test data when my old school pc died and don't have a need/desire to test again. I have the math and my notes somewhere but no promises on finding that and posting.

Yes, it what Gary used in his Regal, but due to the crossover points and size of his mids, it was more for points that sound. The technique I used only works when trying to mate a large speaker with a tweeter. If I had the room, I would have run the 2118 with a 2105 and then the Scan tweet, but I did not have the room. So unless you are going to run an 8 in a 2 way setup, don't waste your time trying this.

Now for the educational portion of the program....(no math)

If you look at sound radiation from a baffled piston (loudspeaker), you will see a relationship between loudspeaker diameter and wavelength as they relate to beamwidth. As the wavelength approaches the diameter of the piston, the main lobe narrows and side lobes appear. Now that is a simple algebraic relationship and nothing in acoustics starts out as a simple algebraic equation. It starts with differential equations with certain boundary conditions. One of the boundary conditions is that the edge of the loudspeaker in not clamped, which is what you have with a loudspeaker that moves freely at the edge. The other boundary condition is that the distance from the center of the loudspeaker to the edge of the loudspeaker is constant. It is this condition that determines loudspeaker beam patterns. If you are at a point directly in front of a speaker (say 1 m) and measure to the center of the dust cap, you get a distance of 1m. Now if you measure from the same point to the edge of the cone, you get a longer measurement. The difference between the two is the key because that difference if the same for any point at the edge of the cone. When the speaker moves (assuming pistonic motion), a wave front appears about the surface of the cone and travels toward you. The wave motion from the center of the cone gets there first and the wave from the edge (called the edge wave) gets there a little later. This difference or delay is also a phase shift. When you combine both waves at your position, they can combine constructively and destructively depending on the amount of delay. If the wave length is very large, there is little or no change in the directivity. If the difference is on the order of a wavelength, then you get interference and the beam pattern changes.

Now, if the edge of the loudspeaker is random, then the interference is reduced and the beamwidth stays omni-directional at higher frequencies. Also, the side lobes, when they appear, are 27 dB below the main lobe in amplitude. Since it is hard to build a random edge woofer and make it work well, you build the next best thing – a lens or diffraction ring as I call it. Of course, if you do the math you will see that an 8” speaker is good to about 1 kHz before you need this. My 8s play to almost 3 k so I need a ring. When you get to a 6”, you are at ~3k before this would do you any good so it is not necessary. They work for me because I go from an 8 to a tweeter. If you put them on anything smaller, they are not worth the effort.

I have to give credit where credit is due on these. Gary Biggs helped me make mine. Actually he made them because my aluminum skills at the time were not so hot. I did polish them myself though.

So....who wants to see a picture? I would like to post one.

Feel free to ask questions, but I don't spend much time on forums. So send me a PM if I don't reply.

-Doug
http://www.diymobileaudio.com/forum/technical-advanced-car-audio-discussion/82067-diy-reducing-speaker-beaming-2.html

So that is what we knew in 2010. Have there been any further developments since then? I would think so. Doug's phase plugs look vastly different form the JBL phase plugs. Are the differences purely cosmetic or are they functional?

I have been and R&D engineer for 40 years and can't tell you how many times engineers (including myself) have said 'I don't think that feature shape matters' and have been 100% wrong. My question always is, 'Why do you say that? Do you have any data to back up what you say'. Things alway seems simple at a distance.

So why is the JBL phase plug have a slight horn shape? Could this also help control wave propagation or is it purely cosmetic. I don't know how that can be brushed aside without data.
 

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Discussion Starter #33


Here's Winker's phase plug, from fourteen years ago


I believe that JBL used a similar contraption, starting twelve years ago. Theirs covers up much more of the speaker. Although it might seem that covering more of the cone will filter the sound, it doesn't actually work like this, for the most part. What filters the sound is the air trapped between the cone and the phase plug. So if you can reduce that as much as possible, you can mask off quite a bit of the cone. Don't go too crazy; if you use a compression ratio of more than four-to-one, you might start hearing issues. Also, this will be dependent on the BL of the driver. More motor strength allows for higher compression.


Seven years ago, Alexander Voishvillo of JBL took the 'sunburst' shape and changed it to an eight pointed star. This is a pic from his patent, for the compression driver in the JBL M2


On Sunday, I copied Voishvillo's phase plug. Here's the Saturday version versus the Sunday version


This time around, I designed it using Xara, instead of 'eyeballing it.' A laser cutter would come in really handy here. I can't believe I've had mine for 1.5yrs and I still haven't set it up :(


If anyone makes one of these, pay attention to sealing the phase plug so it's air tight. Any leaks will create dips.

 

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Discussion Starter #34


Here's the frequency response and distortion of the speaker without the phase plug, and with the 8 point star phase plug. There's no real increase in distortion. There's a rolloff of the highs, as expected.





Here are some polars of all devices:

1) the first graph is the 8NDL51 in a small sealed box, no phase plug
2) the 2nd graph is the horizontal polars with the butterfly shaped phase plug
3) the 3rd graph is the vertical polars with the butterfly shaped phase plug
4) the 4th graph is the 8 pointed start phase plug


IMHO, the 8 pointed star phase plug works the best. The butterfly phase plug offers some absolutely GORGEOUS horizontal polars, but they come at the expense of the vertical polars, which aren't very good. You'll notice the same issue if you look at the horizontal and verticals of a ribbon, their verticals just aren't good. But the 8 pointed star phase plug offers good polars, and it is symmetrical. This is important since we're listening off axis in both the vertical and horizontal axis. For a speaker mounted at ear height, you might prefer the butterfly phase plug. Then again, there's nothing stopping someone from shrinking the horizontal width of the eight point phase plug; that might offer superior polars than the butterfly phase plug.


There's an obvious improvement with the 8 point phase plug...

 

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So that makes me curious as to what the vertical and horizontal polar response might look like with a more symmetrical RBI waveguide. More along the lines of what ESB has done with their tweeters. I.E. how much of the difference in FR response and polars is attributed to waveguide/diffraction ring vs phase plug compared to asymmetry vs symmetry.

You've got more wood to cut, my friend. ;)
 

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Discussion Starter #36
So that makes me curious as to what the vertical and horizontal polar response might look like with a more symmetrical RBI waveguide. More along the lines of what ESB has done with their tweeters. I.E. how much of the difference in FR response and polars is attributed to waveguide/diffraction ring vs phase plug compared to asymmetry vs symmetry.

You've got more wood to cut, my friend. ;)
I think I've got a fairly good grasp on how these work, so I can predict how the shapes will behave:


If you want the widest directivity possible, you want to funnel all the energy through a single hole. The dimensions of the hole will dictate the directivity pattern. For instance, a hole that measures 3/4" x 3/4" will be omnipolar up to 18,000Hz


The eight pointed star phase plug will give you even directivity that mostly behaves like an idealized radiator. Note that the polar measurements of the eight pointed star are more consistent than a "bare" driver.

Changing the aspect ratio will broaden the beamwidth along the narrower axis. Basically you want to make the shape skinnier to broaden the beam.

The butterfly shape will likely work the same way. The advantage of the eight pointed star phase plug is higher compression, yielding higher output. The easiest way to see this is to compare the output level of the JBL versus the BMS compression drivers. Their motor is all but identical, but the JBL adds this phase plug, and has higher output.


I've only seen one person use a Costas array, but this would be an option too. The Costas array should offer the narrowest beamwidth of all of these. Might be a neat option for up on the dash, to reduce radiation reaching the windshield, dash, and side windows.

 

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Discussion Starter #38
I hit a 'sticking point' on the design of this system. But I think I just solved it.

Here's the issue that I was having:


Mic Wallace (appears) to have cut into the firewall, and that allowed him to put big prosound midbasses *and* horns up front.


I haven't cut into the firewall, so I have to 'push' everything as far back and to the sides as possible.


The 'secret sauce' was this eight-pointed star phase plug, which allows me to put a tweeter in the center.

But here's the problem:

If you look at the pic of my car, you'll notice that the enclosure is already sticking out quite a bit. Although I can mount a tweeter coaxially, if I add a waveguide, it's going to stick out another 3-4" That's getting to the point where it's unusably large.

The obvious solution is to put the horns where Mic put them. But I'm not too keen on that; coaxials solve SO MANY problems. Coaxials give you the ability to achieve flat phase response, at nearly every point in the car. Coaxials improve the polar response.

It's true that you can achieve similar results with a conventional two-way, but it would require a really REALLY low crossover point. For instance, if I wanted to put my tweeters where Mic has his tweeters, and I wanted to keep them within one-third of a wavelength at the crossover point, then I'd be looking at a crossover point of around 642Hz. (This assumes that the woofer and tweeter are seven inches apart.)

And it's true that you could cross them over at one wavelength, instead of 1/3rd. That moves the crossover point from 642 to 1926Hz. But if you use one wavelength spacing, your polars go to hell.

No doubt, I'm being a little obsessive here, but polars matter and coaxials fix them.

But waveguides aren't the only way to control directivity. A ribbon or planar has controlled directivity too.

The obvious candidate is the BG NEO 8. At 8" in height, it's a perfect match for the 8NDL51.

So we have a winner, right?

WRONG...

As detailed here, I just can't find a way to get the NEO8 to play past 10Khz : Phase Plug for Planars - diyAudio

Another problem with the NEO8 is that it plays lower than I need it to. I don't need a 500Hz crossover point, 1500Hz-2000Hz is fine. The ideal tweeter for my application here would be about 6" in diameter, with an ability to play down to 1500Hz.

But I had a bit of a 'eureka' moment!

It occurred to me that there's no reason that the NEO8 and the 8NDL51 have to point the same direction. The great polars of a coaxial are due to the fact that the two drivers are very very close together. But they don't have to POINT the same direction.


So picture something like a Linkwitz LX Mini. But instead of the woofer facing UP, the woofer is in the kick panels. And the tweeter is pointed towards the listeners in the car.

That "tweeter" will probably be a BG NEO8, not a waveguide, but the jury is still out.
 

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Inner fender was cut so the horn motor physically sat inside the fender. The outside edge of the mouth was 1/2" from the actual inner fender.
BMW e36 couple have stock 5.25 in kicks but opening is large enough to house most 8s without sacrificing any interior legroom space. Original 8s were mounted on a 1/4" birch baffle to keep them as wide as possible. The genesis scan revelator mids were mounted to 1/4" birch and sits in the rocker panel

Personally stage width is huge for me. I do not enjoy listening to vehicles with narrow stages. I'll sacrifice some tonality for Staging but at high levels of competition its all about trading points to reach a desired outcome

Any question about my car, just ask instead of hypothesizing or guessing what or why I did it. Both BMW are gone so there are no secrets
 

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Discussion Starter #40
Inner fender was cut so the horn motor physically sat inside the fender. The outside edge of the mouth was 1/2" from the actual inner fender.
BMW e36 couple have stock 5.25 in kicks but opening is large enough to house most 8s without sacrificing any interior legroom space. Original 8s were mounted on a 1/4" birch baffle to keep them as wide as possible. The genesis scan revelator mids were mounted to 1/4" birch and sits in the rocker panel

Personally stage width is huge for me. I do not enjoy listening to vehicles with narrow stages. I'll sacrifice some tonality for Staging but at high levels of competition its all about trading points to reach a desired outcome

Any question about my car, just ask instead of hypothesizing or guessing what or why I did it. Both BMW are gone so there are no secrets
Thanks Mic! I usually don't ask people because I figure a lot of this stuff is kept on DL.

But now I'm more confused than ever :(


I thought you had a two way, with some type of prosound woofer on or in the firewall. (That circular shape, I though it's a woofer?)


But you're saying you had some type of eight in the stock location, similar to what's pictured above, plus horns, plus a ScanSpeak 5.25"?

Where was the ScanSpeak then? I googled "rocker panel" and that seems to be the panel that runs along the bottom of the car door. Which would be an odd location for a midrange.

 
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