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Old 26th January 2015, 11:24 AM   #1
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Default R16 constant directivity dipoles

Based on S16 protoype I did in 2010, I'm now commissioning this one for a research project.

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Details to follow as they go.
This is is my first speaker build after almost two years.
gainphile.blogspot.com.au/2015/01/r16-constant-directivity-loudspeakers.html

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They will be dipoles up to 1.5khz and the forward directivity goal should be very similar to the prototype.
I've had mixed feelings about rear-firing tweeters.

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Old 26th January 2015, 12:00 PM   #2
keyser is offline keyser  Netherlands
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Dipole bass, dipole mid, forward-radiating highs... What's the rationale behind it?
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Old 26th January 2015, 01:14 PM   #3
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Quote:
Originally Posted by keyser View Post
Dipole bass, dipole mid, forward-radiating highs... What's the rationale behind it?
Controlled Directivity I would think. Both proper implemented dipoles and waveguides accomplish this, but through different methods.
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Old 26th January 2015, 05:10 PM   #4
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Quote:
Originally Posted by keyser View Post
Dipole bass, dipole mid, forward-radiating highs... What's the rationale behind it?
Obviously I cannot comment on gainphile's rationale, but I can state my own reason: backwards radiation >2kHz is a pain in the butt when speakers are less than 1m from the front wall.

Initially, my dipoles were dipoles all the way, including highs - at first with back-to-back tweeters, then planar dipole. All nice and controlled and even power response and all that, until one day I've discovered that imaging improves A LOT if backwards radiation is rolled off (not necessarily suppressed) above 2kHz such that to avoid a big primary reflection off the front wall in that range. The rest can stay dipole. I have no good explanation why this is so, one hint being this kind of reflection being very specular.

If you have plenty of space behind the speakers, this is likely not to be an issue. But not all of us have. You might - or might not - be able to address it using diffusers/absorbers, but treating it at the source is also an option.
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Old 27th January 2015, 09:03 AM   #5
Juhazi is offline Juhazi  Finland
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Turning dipoles 30-45¤ in helps with front-wall reflections. Alas they should sit a little wider than conventionals, often on the long wall.

Linkwitz says that reflections coming in less that 6ms should be eliminated, that means 1m to front-wall (shortest reflection path, means 2m longer path than direct radiation)
AS_creation

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Old 27th January 2015, 04:04 PM   #6
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Quote:
Originally Posted by Juhazi View Post
Turning dipoles 30-45¤ in helps with front-wall reflections. Alas they should sit a little wider than conventionals, often on the long wall.

Linkwitz says that reflections coming in less that 6ms should be eliminated, that means 1m to front-wall (shortest reflection path, means 2m longer path than direct radiation)
AS_creation

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Now that is almost regarded as common wisdom and I certainly don't intend to contradict it.

However, I do not have 1m. And below 1m there seems to be a frequency-dependent effect that makes the lower frequencies reflections okay-ish (from an imaging point of view) while high-frequency reflections are much more disturbing. Perhaps it's Haas effect related (don't remember if Haas effect is frequency-dependent though). Also, toe-in does not help as much when the distance to front wall is small.

I'll recheck Toole on this. I vaguely remember he's stating that early reflections actually increase ASW (apparent source width) but I also no longer remember if that is frequency-dependent.
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Old 28th January 2015, 07:25 AM   #7
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My 'rationale' at that time was 90° dispersion of the waveguide could produce a very smooth transition to 120°, given a correct crossover frequency. That showed in the sonogram. Of course this is for forward projection only.

They should make some music tonight if I'm lucky
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Old 28th January 2015, 08:08 AM   #8
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Controlled Directivity and Directivity Index. For home audio, horns are often selected because they control the directivity pattern and remove undesirable room reflections. There is good science supporting using a dipole woofer mated to a wide bandwidth horn when a bass horn with good pattern control is too large. The directivity index of the dipole woofer (DI=4.77) is closer to the DI of the horn than the DI=1 of a box woofer. The Unity/Synergy horn has the wide bandwidth to partner up with a dipole woofer.

I found an old photo. Probably a 200Hz Xover to the counter-force dipole pair.
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Old 28th January 2015, 10:08 AM   #9
keyser is offline keyser  Netherlands
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If matching directivity is what this is about, then I'd rather combine a waveguide and a cardioid.
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Old 28th January 2015, 10:29 AM   #10
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Perhaps slightly off-topic, but still relevant to the discussion.

Found some support for my finding above (if distance to front wall is small, backward radiation should have rolled-off or suppressed highs) related to the Haas effect:

Precedence effect - Wikipedia, the free encyclopedia
"A reflection with attenuated higher frequencies expands the time span that echo suppression is active"
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