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Old 18th February 2013, 12:02 PM   #11
wowo101 is offline wowo101  Germany
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John and Markus, can you elaborate a bit on the frequency ranges where the effects you mention come into play?

Up to now, i thought that
  • dipoles and cardioids should be considered as multiple monopoles only in the modal region of the room,
  • a cardioid pattern should emerge as wavelengths become short compared to the wall distance(s).

But in a_tewinkel's case, the wavelengths are still large compared to the wall distance, and the frequency region the OP is focusing (midbass) is – at leat partly – well above the Schröder frequency of modestly sized rooms.

I guess I'm missing something important here…
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Old 18th February 2013, 01:32 PM   #12
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Quote:
Originally Posted by a_tewinkel View Post
Probably. I use it from 120 Hz up which does involve somewhat long wavelengths, but it's not really that low I guess. I've noticed an absence of a sensation of pressure or a pressure gradient between the ears when I put my head directly behind the speaker. I'm curious what a free-field measurement at different distances would show.

I want to do a better measurement some time, and perhaps a comparison with a closed box speaker. I cannot hear or measure the bass increase a normal box speaker has when you move it towards the wall.
The thing to understand is that dipoles and cardioids only achieve the theoretical radiation pattern in the free field. Free field means 3 to 10 wave lengths away from the source. If you move much closer then the difference in distance between front and rear sources means that there is a significant difference is source strength at the measurement point due to path length differences. See Dipole on axis. In a room, there is not dipole or cardioid radiation at low frequency. It's all modal.
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Old 18th February 2013, 01:46 PM   #13
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Originally Posted by john k... View Post
In a room, there is not dipole or cardioid radiation at low frequency. It's all modal.
...unless you're in the near field
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Old 18th February 2013, 01:51 PM   #14
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Originally Posted by markus76 View Post
...unless you're in the near field
Yes, that is a point I have made before as well. But you know that.
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Old 18th February 2013, 02:15 PM   #15
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Quote:
Originally Posted by john k... View Post
The thing to understand is that dipoles and cardioids only achieve the theoretical radiation pattern in the free field. Free field means 3 to 10 wave lengths away from the source. If you move much closer then the difference in distance between front and rear sources means that there is a significant difference is source strength at the measurement point due to path length differences. See Dipole on axis. In a room, there is not dipole or cardioid radiation at low frequency. It's all modal.
Is the wavelength the most important parameter or the effective path distance between the two sources? In my case, the former would be < 2.85 meters, the latter would be about 0.16 m.
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Old 18th February 2013, 04:39 PM   #16
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Quote:
Originally Posted by a_tewinkel View Post
I have a cardioid midrange with > 20 dB front/back ratio and I cannot measure the front wall reflection, even though the speakers are nearly directly in front of the wall (baffle is about 30 cm from the wall). Guess it does work rather good.
Can you illustrate or show photos of the cab's here? I'm interested in the mechanics behind what polar(s) are being produced compared to what a cardioid pattern would normally show. I'm having some difficulty imagining a true cardioid rotation not being affected (ie. no reflection) 11" off a wall.
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Old 19th February 2013, 02:57 AM   #17
Pallas is offline Pallas  Pakistan
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Originally Posted by a_tewinkel View Post
I have a cardioid midrange with > 20 dB front/back ratio and I cannot measure the front wall reflection, even though the speakers are nearly directly in front of the wall (baffle is about 30 cm from the wall). Guess it does work rather good.
Cardoid down to 120Hz? Do you have a thread about the build/design?
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Old 20th February 2013, 02:38 PM   #18
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No, I haven't. The design was based mainly on the experiments of some fellow DIYers here and the ideas on this site.
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Old 20th February 2013, 03:02 PM   #19
wowo101 is offline wowo101  Germany
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Maybe you could post some pictures and/or graphs here? I would also be very interested in a few details of your solution.
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Old 20th February 2013, 05:31 PM   #20
Pallas is offline Pallas  Pakistan
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Maybe you could post some pictures and/or graphs here? I would also be very interested in a few details of your solution.
Ditto.
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