Author Topic: Very Early Reflections  (Read 9128 times)

Offline richidoo

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Very Early Reflections
« on: May 31, 2010, 04:47:57 PM »
What delay is considered the threshold for early reflections?  Reflected sounds that arrive after the direct sound but before this time threshold are processed with the direct sound to tell the brain about the shape and texture of the room boundaries.  If you want to minimize the awareness of the room while listening, kill these 1st reflections and you will experience the recorded ambience more purely.

But what about very early reflections, like 1mS? Are these so close to the direct sound that they are not noticed? Or are they so close to the direct sound that they screw up higher mid frequencies with comb filtering at almost the same amplitude as the direct sound?

The longer the delay, the easier it is to ignore as reflections become diffuse ambient "reverb." Reflections also attenuate while moving through the air so later reflections are also quieter than direct.

I'm thinking about building line arrays that mount in the corner. Drivers will be only a 4" from side walls and front wall, so 1st reflections will be <1mS. What should I expect to hear (or not) hear acoustics-wise? I could kill these reflections fairly easily since the reflection patch is small at that short distance, but it would not look as nice as without treatment. I'm curious what audible effect it would have, if any. Thanks for any opinions.

Rollo has put me at ease that different arrival times between individual drivers is not an issue, and I think these wall reflections are even less than max delay between closest and farthest drivers, maybe <2mS. But lots of drivers and lots of reflections is peaking my nervosa.
Thanks!!
Rich

Offline BobM

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Re: Very Early Reflections
« Reply #1 on: June 01, 2010, 06:23:06 AM »
With line arrays it's more about tapering the relative levels of the drivers in the stack. Otherwise you get hugely oversized images, like a 3 foot wide and 3 foot high nose from the vocalist in the middle of your soundstage, 12 foot tall saxophones, etc.
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Offline bpape

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Re: Very Early Reflections
« Reply #2 on: June 01, 2010, 06:33:20 AM »
While the reflections from the close wall will be fairly short in difference, the ones off the opposite wall (left speaker reflecting off of right wall) will still be fairly long.  Corner mounting will also maximally excite all of the room modes though the line source will minimize height related issues.

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Offline richidoo

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Re: Very Early Reflections
« Reply #3 on: June 01, 2010, 07:10:58 AM »
Bryan, it will be digitally EQed for bass correction. Opposite wall reflections will be 25' path to ears and they will be treated at that location, but putting the treatment right up next to the speakers is difficult. Thanks

Interesting about the imaging, Bob. I have thought about that, and people have told me that images are bigger, but more like the real thing, not small as from pseudo point source (MidTweet). These things will be 9 feet tall 30 drivers per side, so if image height is an issue I will know it. How is the tapering of level per driver done to tune the height? Resistors?   If you're gonna taper the volume of individual drivers in the stack, then what remains as the advantage of line array? Seems like point source is more technically correct in small rooms. If I was 50 feet away the line array benefits would be more evident and its weakness less evident. Thanks for your input.

I keep thinking that even if they don't work out as primary speakers, they would work well as bass reinforcement for satellites, while doing double duty as serious party speakers.  But it's a big project, even if not so expensive.

Offline _Scotty_

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Re: Very Early Reflections
« Reply #4 on: June 01, 2010, 09:18:26 AM »
Rich,you might do an experiment to assess the effect corner placement has on the sound by mounting a small loudspeaker on a board and locating it in the corner. I built a couple speakers that were prism shaped
and was fairly happy with how they turned out. I copied Roy Allison's design. What I did notice is that the stereo image stopped at the wall boundary. Which is to say that the image was no wider than the room, period. I don't know why it turned out that way. The rooms that the loudspeakers were in were 12ft. and 14ft.wide so I wasn't disappointed with the image size. I did not treat the walls near the loudspeakers and in one case in my fathers living room it wouldn't have been possible due to WAF considerations. When you locate a speaker right in the corner of the room you have basically placed it in a 90 degree Horn with everything that implies.
Scotty

Offline rollo

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Re: Very Early Reflections
« Reply #5 on: June 01, 2010, 10:01:59 AM »
 Since I am not technically literate about speaker design corner placement just may have too many reflections to control. Further as brian pointed out room nodes will excite more than if out into the room.
  Line arrays like reflections, controlled reflections that is[ Brian jump in here]. A reflective rear wall with first and second wall reflections treated accordingly [ Brian] worked out best for my application.
  I also found that the more the speakers were AWAY from the rear wall the depth and imaging is solid 3D and realistic sounding. As in life a powerfull center image with all the depth and width of the real deal.
  About 7 to 8 ft. apart [ listening seat as well], 10 to 12 ft away from rear wall and a large room to boot. Some insist on a low clg. hgt. others a high clg hgt.
  I believe corner placement is a mistake, however you can always move them when completed. Casters are a wonderful thing.  :thumb:


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Offline bpape

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Re: Very Early Reflections
« Reply #6 on: June 01, 2010, 10:32:41 AM »
Corner placement is a lot about understanding the room gain you'll get from that location and at what frequency ranges.  What you're effectively doing is turning the walls into a huge horn.  As long as you can live with the image depth being in front of the speaker plane, the width effectively stopping at the boundaries on the side, and select drivers/design the xover to work with the horn loading instead of against it, they can work out OK.  Probably the most famous of those designs were the original Klipschorns. 

One thing I would definitely recommend though is to experiment with high frequency damping on the face of the baffle.

Bryan
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Offline richidoo

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Re: Very Early Reflections
« Reply #7 on: June 01, 2010, 11:17:32 AM »
Great ideas fellas. I really appreciate your comments. I have to consider the limitations carefully. Loss of depth is one thing I would miss, even though it is an acoustic illusion. But it's fun.

Offline bpape

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Re: Very Early Reflections
« Reply #8 on: June 01, 2010, 11:42:40 AM »
You can still get some front to back, it's just all from the speakers forward so it's basically usually limited to approx 1/2 the distance from you to the speakers.  It's not the 'wow, that was 30 feet back' type of thing.

Bryan
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Offline StereoNut

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Re: Very Early Reflections
« Reply #9 on: June 01, 2010, 12:22:02 PM »
With line arrays it's more about tapering the relative levels of the drivers in the stack. Otherwise you get hugely oversized images, like a 3 foot wide and 3 foot high nose from the vocalist in the middle of your soundstage, 12 foot tall saxophones, etc.

Uhhmmnnn... what if you like 12 foot tall saxophones!?  :rofl:

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Offline richidoo

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Re: Very Early Reflections
« Reply #10 on: June 01, 2010, 12:37:42 PM »
Here you go Bill!  :thumb:

Offline StereoNut

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Re: Very Early Reflections
« Reply #11 on: June 01, 2010, 01:18:20 PM »
Aaaaaaaaaaahhhhhhhhh!!!!!!!!!!

 :rofl: :rofl: :rofl: :rofl: :rofl: :rofl:

You've managed to make my whole day 12 foot TALL with that pic!!!  :thumb:

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Offline richidoo

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Re: Very Early Reflections
« Reply #12 on: June 03, 2010, 09:05:39 AM »
I found a good description of a speaker similar to the one I'm considering.

"Once the loudspeakers are positioned correctly, they create a large spatial image with big dynamic shades. A sort of wall of sound filled with subtle details and nuances. If pin-point imaging is your thing, then maybe a different speaker would be your choice, but even so, this loudspeaker creates a realistic 3D picture in which all the individual instruments and voices can easily be pointed out. With good orchestral recordings there is not only good left to right imaging but also there is depth in front of and behind the speakers. "  (These are not corner mounted.)

"The top-end of the spectrum is direct but never harsh, it reveals heaps of detail without getting over bright. It lets you take a deep look into the recording and takes you on a musical discovery journey. This time the frequency curve is measured at 3 metres distance instead of the standard 1 meter measurement because otherwise the comb-filter effect would be visible above 4kHz. This 3 metre corresponds with the perceived response at the listening seat. It shows a well balanced response with about 85dB efficiency. There is a slight roll-off towards the top-end, but that is normal when measuring at larger distances."

"As with any loudspeaker system, a line-array also has it's limitations. Seeing that each driver of the array produces the same frequency range, the theory predicts a cylindrically shaped sound source that only because spherical at a very large distance. In practice (in a normal living-room) you listen to a line-array in the near-field in which you have one large problem: due to the difference from each individual driver to your ear, you get cancellation at high frequencies. "

Supposedly if you sit at a distance >2x the height of the array then the cancellations are less noticable. That doesn't make sense, seems like the distance between drivers is more of a factor than the overall height.

Offline _Scotty_

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Re: Very Early Reflections
« Reply #13 on: June 03, 2010, 09:21:38 AM »
Rich, you might try emailing Danny Richie a note with some of your questions. He might be willing to point out some of the pitfalls you may encounter when designing your own line array.
Scotty

Offline tmazz

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Re: Very Early Reflections
« Reply #14 on: July 14, 2010, 07:49:56 PM »
Rich,

I found an excellent book on acoustics at the library called "Master Handbook of Acoustics" by F. Alton Everest. (published by McGraw-Hill)



The beginning of the book has all kinds of info on acoustics from a theoretical basis while the later chapters talk about practical applications of that theory (including chapters on home listening rooms, room acoustics software and room optimization.  My local library had the fourth edition but  I understand that a fifth edition has been published. An excellent reference book. You should check it out if you have a chance.

Tom
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