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Multi-Way Conventional loudspeakers with crossovers

An Improved Array
An Improved Array
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Old 27th December 2016, 10:53 PM   #21
Patrick Bateman is offline Patrick Bateman  United States
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Quote:
Originally Posted by wesayso View Post
I was thinking of the steering effect in the straight array and not using an out of phase driver on the front end, just wondering out loud if that's the beam steering they used as they have about 3 big modes, selfish (where they optimise for one listener), normal, where the beam width would be used to supply a couch and party where it would behave more like an Omni.

Now these 3 modes aren't their real names, but you get the drift.

B&O would be able to change the phase and level of each driver. That's a lot of freedom right there.
However it may just be level and delay. Though I expect they do use some extra tricks in the selfish mode with mid/side EQ.
I'm still trying to process how this thing works, but here's some thoughts:

1) We can't get too aggressive with delay or EQ, because both will affect the directivity. If you look at the hornresp sims, you'll notice that the wavefront shape is almost perfectly cardioid. So if you were to EQ one of the drivers differently than the other, that would change the directivity pattern.

This is something that I learned with cardioid midbass arrays in my car. When they work, they're unbelievable. But the 'trick' is that each driver in the array has to be equalized to have the same response shape. IE, if you had four drivers in an array, and one driver was 6dB louder at 100Hz than another driver in the array, then the directivity would change at that frequency. The trick is to keep the response shape of each driver identical across their bandwidth. Note that this DOESN'T mean that they have to play at the same SPL level; the curve itself just needs to have the same shape. Varying the SPL level of the elements in the array can be used to steer the wavefront.

Naturally, this can be used to our advantage. For instance, if the measured pattern wasn't perfect, EQ could be used to 'smooth out' the polar response.
Click the image to open in full size.
That may explain how they're getting these epically smooth polar curves. Admittedly, waveguides can get close to this. But as noted a few posts back, waveguides don't sound the same. (I think this speaker sounds better.)

2) I would speculate that the most powerful tool at our disposal is the shape of the enclosure. If a perfectly hexagonal enclosure yields a cardioid, then what does a pentagon yield? What happens if you elongate the enclosure? (Note that the patent shows a hexagon, but the real speaker appears to be 'squished' in the X axis.)

3) I would also speculate that the second most powerful tool would be varying the intensity level. I *really* wish Hornresp had the ability to vary the output levels in the wavefront simulator.

Last edited by Patrick Bateman; 27th December 2016 at 10:57 PM.
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Old 27th December 2016, 11:01 PM   #22
wesayso is offline wesayso  Netherlands
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The mid-side EQ I was getting at is for the stereo pair, not on one loudspeaker.
mid/side as in phantom centre and (Left and right).

They use more than one trick to optimise the listening spot.
Quote:
First of all, as KR described in his review, the BeoLab 90 system uses digital signal processing (DSP) to correct the room's acoustic. However, it differs from conventional correction solutions in applying not just individual filters for the Left and Right speakers, but also filters to correct the speakers' summed (Mono) output and the difference between their outputs (Side). The responses of these filters, as set up by B&O's Geoff Martin, are shown in fig.1 (Mono and Side) and fig.2 (Left and Right). Both sets of filters are applied simultaneously; you can see that the amount of correction is relatively small, covering a range of +5 to –6dB.
Read more at Bang & Olufsen BeoLab 90 loudspeaker Measurements | Stereophile.com
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Old 27th December 2016, 11:02 PM   #23
SunRa is offline SunRa  Romania
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@mitchba great find!


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Old 27th December 2016, 11:06 PM   #24
wesayso is offline wesayso  Netherlands
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Quote:
Originally Posted by mitchba View Post
Don't know if you have seen this article: B&O Tech: Beam Width Control – A Primer – earfluff and eyecandy

Happy Holidays guys!
Hi Mitch, happy Holidays to you too!

Yes, I've been following that blog for quite a while, very interesting stuff.
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Old 28th December 2016, 12:04 AM   #25
Patrick Bateman is offline Patrick Bateman  United States
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I believe I have the Beolab 90 sorted out. I know this is a long and complex thread; if you're going to read one post in this thread, this is it:

First off, why do we want directivity control? I believe that we want directivity control because it reduces the room's influence on the sound of the recording. For instance, with a Danley SH-50, what you hear is what's on the recording. If there's a big soundstage on the recording, you'll hear it. It's true that many speakers can produce a BIG stage, but many that do make *everything* sound big. The SH-50 isn't like that; it's basically an x-ray on a recording.

Second, what's so great about the Beolab 90? It's great because it offers the virtues of something like an SH-50. But it's more attractive, it doesn't use compression drivers, and I find that it combines the rock solid imaging of an SH-50 with the pleasant ambience of a good dipole. The reason is because it offers the directivity control of a waveguide while using conventional drivers.

It *is* possible to get good results with a simple Cardioid like Linkwitz's LX Mini. For instance, in an online review, the LX Mini was described like this:

"Overall, the resulting soundstage from the LXmini presents as a soundstage plane, with exciting width and depth and depth acuity that go far beyond what most speakers can accomplish.

The description above is consistent with what I hear, and the reason that I've enjoyed cardioids like the LX Mini and the Gradient Helsinki. I'd definitely love to hear Juhazi's AinoGradient.

Click the image to open in full size.
Although the LX Mini sounds very good, it's response curves are not exceptional. This isn't a knock against Linkwitz; there's only so much you can accomplish with a resistive cardioid.

Click the image to open in full size.
So let's take a minute and think about why this is. In a dipole speaker, we have positive radiation in the forward lobe, out-of-phase radiation to the back, and nulls to the left and the right where the two wavefronts interfere.

Click the image to open in full size.
In the LX Mini, acoustic damping is used to absorb a fraction of that back wave. So we end up with more energy going FORWARD than going backward.

But why does the response of the LX Mini vary so much? The reason is because acoustic damping varies with frequency. IE, six inches of damping will have a larger effect on a wavelength that's three inches long than a wavelength that's 30 inches long. Due to that, a resistive cardioid will never have the same directivity across it's bandwidth.

Click the image to open in full size.
This is an 'ideal' cardioid response pattern.

Click the image to open in full size.
This is the predicted response pattern of a hexagonal loudspeaker enclosure, similar to the array in the Beolab 90. In the predictions, you can see that it's possible to get very close to cardioid response, particularly at high frequency.

The reason why it's easier to get cardioid response at high frequency than low is because the enclosure needs to get bigger and bigger as the wavelengths get longer. For instance, if you want directivity control down to 1000Hz, you need an enclosure that's approximately 34cm in width. (13.5")

Click the image to open in full size.

Although the midrange array in the Beolab is under a foot wide, the measurements indicate that directivity control goes all the way down to 200Hz. So what gives?

Click the image to open in full size.
Click the image to open in full size.
I believe the 'trick' is that it is also an end-fire array, but a very interesting one. I believe it is a end-fire array but only at certain frequencies.

This next part makes my head hurt, so I hope that I'm explaining this correctly:

If you look at the sims of the hexagonal array, you'll notice that the wavefront shape is almost perfectly cardiod at high frequency. As the wavelengths get longer, the directivity control begins to collapse.

The concept behind a end-fire array is very simple. In an endfire array, we put our speakers in a line. Then we delay the speaker at the front so that the array becomes directional.

Click the image to open in full size.
Click the image to open in full size.
Here's a comparison between one speaker's radiation (spherical) and an end-fire array of twelve. Obviously, the end fire array is far more directional, at the expense of efficiency. (Thanks to Peter Morris for the sims.)




Okay, if you've made it this far, congratulations. In summary we have a hexagonal loudspeaker which produced a cardioid wavefront, particularly at high frequency. To make the speaker more directional than it's small size would indicate, an end-fire array is also used.

But we're not done yet.

I'd hoped that the end-fire array would work across the board. But it doesn't. The reason that the end fire array doesn't work across the board is because it creates severe lobes, particularly at high frequency.

So we have a conundrum here. We have one thing that works well at high frequency (the hexagonal enclosure) and another thing that works well at low frequency (the end-fire array.)

Click the image to open in full size.
If we look at the Beolab 90 from overhead, we can see why the end-fire isn't a cure all. For instance, 1000Hz is 34cm long. If we delay the midranges at the FRONT of the enclosure so that the wavefronts from the BACK of the enclosure line up in FRONT of the enclosure, we'll get a strong lobe facing forward of the speaker. That's exactly what we want, that's one of the reasons we're getting directivity control without the use of a waveguide. But at 500Hz, that same delay will generate a null on-axis. I hope that makes sense; the delay that improves our directivity at 1000hz will screw up our frequency response at 500Hz.

The solution is simple: we need a variable delay. For instance, if we have seven midranges cover the frequency band of 350Hz - 2500Hz, we want virtually no delay at 2500hz, but at 350Hz we want as much as 1-2ms of delay. The net result is to reinforce the forward lobe at low frequency (where the enclosure is too small to do it) but not to screw up the polar response at higher frequencies, where the enclosure is physically large enough to control directivity on it's own.
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Old 28th December 2016, 01:06 AM   #26
mitchba is offline mitchba  Canada
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The reason I linked the article in post 20 is that is describes how BeoLab 90 achieves its beamwidth control:

...

"The total result is a careful balancing act where some things are known or given:
  • the frequency range of the music being played through a given loudspeaker driver (this is limited by the crossover)
  • the natural directivity patterns of the loudspeaker drivers on the loudspeaker enclosure within that frequency range
  • the locations of the loudspeaker drivers in three-dimensional space
  • the orientations of the loudspeaker drivers in three-dimensional space (in other words, which way they’re pointing)
Note that these last two have been calculated and optimised based on a combination of the natural directivity patterns of the loudspeaker drivers and the desired beam widths we want to make available.
As a result each individual loudspeaker driver gets its own customised filter that controls
  • the level of the signal at any given frequency
  • the phase (which includes delay and polarity – sort of…) of the signal at any given frequency
By controlling the individual output levels and phases of each loudspeaker driver at each frequency it produces, we can change the overall level of the combined signals from all the loudspeaker drivers in a given direction. If we want to be loud at 0º (on-axis) and 20 dB quieter at 90º (to the side), we just apply the filters to all of the drivers to make that happen. If we want the loudspeaker to be only 10 dB down at 90º, then this just means changing to a different set of filters. This can be done independently at different frequencies – with the end goal to make all frequencies behave more similarly, as I talked about in this posting and this posting."

Also, since the filters are merely settings of the DSP (the digital signal processing “brain” of the loudspeaker), we can change the beam width of the loudspeaker merely by loading different filters – one for each loudspeaker driver in the system.

The end result is a loudspeaker that can play many different roles, as was shown in the different plots in this posting. In one mode, the beam width can be set to “narrow” for situations where you have one chair, and no friends and you want the ultimate “sweet spot” experience for an hour or two. In another mode, the beam width can be set to “wide”, resulting in a directivity that is very much like an improved version of the wide dispersion of the BeoLab 5. In yet another mode, the beam width can be set to “omni”, sending sound in all directions, making a kind of “party mode”.
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Old 28th December 2016, 01:23 AM   #27
Patrick Bateman is offline Patrick Bateman  United States
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Oh I've read that already

I'm going to CES next week, had some time to kill due to the holidays, thought I'd reverse engineer it.

IMHO, B&O can't possibly be making much money on the Beolab 90. The cost of the drivers is something like $6000, the cost of the amplifiers is in that ball park, the cost of the DSP also. The enclosure is a solid chunk of aluminum.

So either this is a showboat designed to show off what they can do, or they're going to take technology from the B90 and trickle it down to something more affordable.

If it's the latter maybe I can come up with something that doesn't require 38 channels of DSP for a stereo pair.
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Old 28th December 2016, 01:31 AM   #28
mitchba is offline mitchba  Canada
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LOL! Well R&D is expensive... Given the power of DSP, I expect this to be trickle down with less drivers and cost. Will be interesting to hear your thoughts on what you... hear ;-) at CES.

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Old 28th December 2016, 10:24 AM   #29
Juhazi is online now Juhazi  Finland
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Thank you Patrick for the brainstorm! It would be nice to know excactly what 90 does when settings are changed, I believe that both delay and spl are changed.
Judging from Stereophile's measurements, it is a 4-way system
Click the image to open in full size.

I believe that 4-way is needed for wide-range directivity control, and dsp/multidriver tricks help a lot to achieve that specially in low and low-mid registers. Variable directivity patterns set another extra challenge for dsp, but that is just a matter of parameters when one has multiple drivers and enough channels available.
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Old 28th December 2016, 12:06 PM   #30
Navy Guy is offline Navy Guy  United States
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Very interesting discussion.
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