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Full range cardioid
Full range cardioid
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Old 8th May 2014, 10:17 AM   #21
john k... is offline john k...  United States
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I don't mean to burst anyone bubble but dipoles and cardioid woofers are free field systems. You don't get a dipole or cardioid pattern at a few feet from a wall. As was pointed out, by virtue of the differing distance to a wall form each source, each source of a dipole or cardioid will reflect off of all walls. When you are in a room all of these systems come down to multiples sources distributed around a room with each source having, perhaps, different magnitude and phase and delay. Specific arrangements and alignments will excite or fail to excite different modes but for analysis the modal response of each source can be considered separately and then the net result for all the sources is simply the superposition of the response of all sources. You can not discuss dipoles or cardioids without discussing source separation and orientation. Woofer systems which exhibit dipole or cardioid radiation in the free field will have certain behavior when placed in a room provided they are positioned with specific orientation. But in general they behave only as multiple sources.
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Old 8th May 2014, 10:58 AM   #22
speaker dave is offline speaker dave  United States
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You certainly won't burst my bubble but you should read my responses carefully.

Distance to the wall is not an issue. Only the apparent distance to the observer. Since we want to cancel a reflection across a boundary the actual distance is to the virtual image source, equivalent to the source distance plus 2 source to wall spacing. This should be more than far enough to be a far field condition.

I was doing some careful simulations a few weeks back regarding using a second driver on a wall surface to cancel the wall bounce from a system a few inches off the surface, say a typical hanging on-wall system. Interestingly, you had to consider the canceling driver as a 2 pi unit but the unit being cancelled is in 4 pi even though it is only inches off the surface. For complete boundary cancelation you adjust for equal contribution and opposite phase from your vantage point. (And it is easy to get complete cancellation.)

If you are not directly on axis to the source and reflection, then the proper delay can still give cancelation at your position, turning the cardioid into the appropriate hyper cardioid.

Yes, it is only one cancelation of one reflection, and many others exist, but if it is the earliest reflection this is a very good thing.

David
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Old 8th May 2014, 01:47 PM   #23
john k... is offline john k...  United States
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Quote:
Originally Posted by speaker dave View Post
You certainly won't burst my bubble but you should read my responses carefully.

Distance to the wall is not an issue. Only the apparent distance to the observer. Since we want to cancel a reflection across a boundary the actual distance is to the virtual image source, equivalent to the source distance plus 2 source to wall spacing. This should be more than far enough to be a far field condition.

I was doing some careful simulations a few weeks back regarding using a second driver on a wall surface to cancel the wall bounce from a system a few inches off the surface, say a typical hanging on-wall system. Interestingly, you had to consider the canceling driver as a 2 pi unit but the unit being cancelled is in 4 pi even though it is only inches off the surface. For complete boundary cancelation you adjust for equal contribution and opposite phase from your vantage point. (And it is easy to get complete cancellation.)

If you are not directly on axis to the source and reflection, then the proper delay can still give cancelation at your position, turning the cardioid into the appropriate hyper cardioid.

Yes, it is only one cancelation of one reflection, and many others exist, but if it is the earliest reflection this is a very good thing.

David
There is a difference between making the contribution form a given mode null at a given point in a room and preventing that mode from being excited all together.
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Old 8th May 2014, 03:12 PM   #24
speaker dave is offline speaker dave  United States
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Originally Posted by john k... View Post
There is a difference between making the contribution form a given mode null at a given point in a room and preventing that mode from being excited all together.
Which is why I was careful to discuss the effects of a single boundary reflection, i.e. the nulling of the rear wall bounce.

With regard to standing waves, there are a number of papers out there that show significant reduction of standing waves in one direction through carefully aimed dipoles.

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Old 16th May 2014, 07:17 AM   #25
keyser is offline keyser  Netherlands
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Any news here?
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Old 1st June 2014, 11:10 PM   #26
Jag768 is offline Jag768  Netherlands
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New measurements outdoor measurements today! Much better weather than last time, low winds and sunny

I concentrated on the (cardioid) sub again. I started off with a more easy option of equal response from monopole and dipole subs. And as a next step i added a high pass to the dipole and corrected the phase of the monopole using an allpass.

My method in steps:
1. Correct amplitude of monopole sub to linear from 40 - 150Hz, at 180 degrees
2. Correct amplitude of dipole sub to linear from 40 - 150Hz, at 180 degrees
3. Set optimum delay for dipole sub for equal phase (0.5ms)
4. Save DSP settings (full cardioid operation)

5. Set high pass @ 50Hz on dipole
6. Add 2nd order allpass filter (Q 0.70) to monopole @50Hz
7. Check for equal phase
8. Save DSP settings (monopole - dipool operation)

9. Reverse polarity of the dipole (resulting in cancellation @ 180 degrees). Polar patterns of both DSP presets.

The result was 15dB+ of suppression at the rear over the range of operation (30 - 150Hz) using the full cardioid preset. The polar pattern quite closely resembles a cardioid. Still, the prototype has some flaws: the (not so) sealed enclosure is too leaky causing 2 notches at the rear. And it's not symetrical, resulting in an "off axis" polar . However, this should suffice for some listening tests at home. Than i can also choose wheter i prefer monopole - cardioid operation or full cardioid for the sub.
Attached Images
File Type: jpg Meten.jpg (377.0 KB, 695 views)
File Type: png Full range cardioid amplitude.png (9.9 KB, 681 views)
File Type: png Full range cardioid phase.png (15.1 KB, 643 views)
File Type: png Monopole - cardioid phase.png (15.2 KB, 634 views)
File Type: png Full range cardioid Polar.png (37.8 KB, 639 views)
File Type: png Monopole - cardioid Polar.png (39.7 KB, 208 views)
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Old 2nd June 2014, 10:59 AM   #27
speaker dave is offline speaker dave  United States
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Mad science at its best. I especially like the grassy ground plane.

That looks like a very good result. Similar to my experiments, you have to consider phase and amplitude in the rearward direction. Match them well and you will get good broadband cancellation, as you have.

I don't see the double dip you mention. Is this like a hypercardioid pattern? Note that if you want to cancel a rear wall bounce at an angle, rather than straight back, then hypercardioid may give greater cancellation.

I'm not sure which is better, or if the difference is significant, but it can be played with via the rear element electrical delay.

I hope you will end up with in-room measurement, because the potential cancelation of standing wave/improvement of the room curve is the objective.

Good stuff,
David
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Old 2nd June 2014, 05:57 PM   #28
Jag768 is offline Jag768  Netherlands
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Quote:
Originally Posted by speaker dave View Post
Mad science at its best. I especially like the grassy ground plane.
Haha, it felt a bit mad as well . The people hosting my measurement sessions on their field, found it fascinating as well .

Quote:
That looks like a very good result. Similar to my experiments, you have to consider phase and amplitude in the rearward direction. Match them well and you will get good broadband cancellation, as you have.
I was quite pleased with the result as well. We measured at 180 degrees, i could have possibly found a deeper null if measured slightly at a different angle. I ran out of time unfortunately .

Quote:
I don't see the double dip you mention. Is this like a hypercardioid pattern? Note that if you want to cancel a rear wall bounce at an angle, rather than straight back, then hypercardioid may give greater cancellation.
The dips i referred to are not in the polar response but in the magnitude reponse at 180degrees. You are still slightly visible in the magnitude response of the monopole sub @ 196Hz and 255Hz. Before fastening the side panel, the dips were far more pronounced and somewhat lower in frequency.

Quote:
I'm not sure which is better, or if the difference is significant, but it can be played with via the rear element electrical delay.
Does this work over a wide range?

Quote:
I hope you will end up with in-room measurement, because the potential cancelation of standing wave/improvement of the room curve is the objective.
Sure thing. Im going to finish the other prototype subwoofer (in 4 days actually). I'll do some more outdoor measurements of the woofer and compression driver and then ill move the set into my house and do indoor measurement of the indoor low frequency response.

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Good stuff,
David
Thank you .
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Old 3rd June 2014, 07:39 AM   #29
Koen K is offline Koen K  Netherlands
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That's better

Adding a delay should be effective over a wide range. I'd also expect a hypercardioid to be pretty effective at reducing irregularities in room.

Good job!
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Old 3rd June 2014, 07:59 AM   #30
keyser is offline keyser  Netherlands
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Great stuff! I especially like the combination of a high-pass and an all-pass to cancel the phase rotation. Good thinking, Batman!
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