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Surround Speaker Design for a Small Home Theater - Page 3

post #61 of 99
Thread Starter 
I made a pair of test boxes this weekend. I hope to get them mounted on the wall Wednesday with a simple adjustable mount so I can start testing them in position. The internal volume came out at 0.41 ft^3 which is a little small (12"x12"x8.5" external dimensions), but it won't have too much impact. My Winisd model suggests 0.5 ft^3 is about optimum in terms of keeping woofer excursion below Xlim.

I'll run some room curves at the listening positions later in the week. wink.gif

Mike
post #62 of 99
Hey Mike, I have the the v8 for rear, sides, and front heights. Are you saying the front heights don't need to be angled. I have my rears and sides planned to angle just past the lp. The fronts are still unplanned.
post #63 of 99
Thread Starter 
Quote:
Originally Posted by d_c View Post

Hey Mike, Are you saying the front heights don't need to be angled. I have my rears and sides planned to angle just past the lp. The fronts are still unplanned.

I'm not saying that at all. Heights probably need to be angled. In fact, they may need toe-in in the horizontal plane and they may also need to be angled downward in the vertical plane

Side surrounds, however, are not toed in or out since they typically fire straight across the listening positions at a 90 degree angle to the viewing axis. They probably need to be angled downward in the vertical plane, like the Heights. In my room, the outer listeners, those next to the wall, are nearly sitting right underneath the speaker. By adjusting the downward angle with my temporary boxes, I am hoping to find a down angle that facilitates Time-Intensity Trading. I want to maximize the breadth of the stereo image created by the surrounds while minimizing the listeners' awareness of a speaker directly overhead.

I am hoping to accomplish Time Intensity Trading in the vertical plane the same way we do with our Left and Right main speakers in the horizontal plane. We'll see what happens..... wink.gif

Mike
Edited by mhutchins - 1/21/14 at 7:52am
post #64 of 99
Ok I see. Thanks for the explanation. It seems like every time I get something figured out, there is a new angle on it (pun intended) wink.gif

I look forward to your impressions of them
post #65 of 99
I angle my surrounds to get the same time/intensity trading to get a wider listenhing area as for as fronts
post #66 of 99
Thread Starter 
Well, I made a few measurements using a temporary enclosure for my V10. Here is a picture:



I mounted the enclosure so I could adjust the down angle, to see what impact, if any, it has on the frequency response and SPL in the listening area. I used Room EQ Wizard ("REW") with the microphone that came with my Denon receiver for use with Audyssey. I moved the microphone to each of 3 different listening positions, progressively farther from the test speaker. Recall that my room is pretty narrow, so there is only room for 2-3 chairs, with the outer chairs pretty close to the wall.

The frequency sweeps were not very informative to me. They may have shown a slight tendency for rolloff of the frequencies above 10kHz, at angles > 20-30 degrees, but I am not experienced enough with REW to make that claim. Next I tried Pink Noise, and this seemed to produce some repeatable numbers. I charted the results in MS Excel:



Down Angle refers to the angle of the speaker from vertical. For example, in the photo above, the speaker is tilted down 20 degrees.

The reason I performed these measurements was that I was having a hard time perceiving a stereo image with these speakers mounted above my head. I suspect it is related to some psychacoustic phenomena of localization and our perception of sound from above and to our sides...

Anyway, based on the above, can you make any informed observations related to speaker angle and Time-Intensity-Trading and my quest for the perfect surround image?

Thanks in advance,
Mike
post #67 of 99
Very cool Mike - I'm glad you posted.

Somewhere, there are equations that can be used to anticipate the SPL level and delay level that can be matched to obtain a stereo image. I don't have those equations. I did some searching a few months back when you started this thread and came up empty - at least I couldn't find the equations themselves, but I think I saw reference to the papers where they are expressed. I don't have any of that reference information now.

That said, we can know a few things from reading here and there. The longer the delay, the greater the intensity difference that will be required to maintain a stable center image - as you know, the delayed sound must be higher SPL. There is a limit to this - measured in msec, but I don't know what it it. I suspect that in small rooms like ours, the delay will not be large enough to be insurmountable, at least theoretically.

So the challenge, again as you know, is to make the late arriving sound - the sound from the farthest speaker - or in the terms you have used here, the SPL at the farthest seat louder than at the nearest seat. None of your measurements show that this can be accomplished, as distance is the dominant factor in determining the SPL at any given seat aside from the nearest. The off-axis roll-off is not strong enough - that is to say, the dispersion is too wide. This is my interpretation: there may be other ways of looking at this.

Still, there must be a good compromise to make here. I personally expect that 20 degrees is a good compromise, but listening will be the way to know. Some well controlled listening tests could be very informative. Set up two at the same angle and listen in each seat to real music; listen carefully for the stability of the stereo image as you move from center. Give it a shot! You've done all the hard work!

Obviously, someone out here knows more about this than I do, so I hope they can share.
post #68 of 99
Thread Starter 
Quote:
Originally Posted by HopefulFred View Post

Very cool Mike - I'm glad you posted. Thanks Fred!

Somewhere, there are equations that can be used to anticipate the SPL level and delay level that can be matched to obtain a stereo image. I don't have those equations. I did some searching a few months back when you started this thread and came up empty - at least I couldn't find the equations themselves, but I think I saw reference to the papers where they are expressed. I don't have any of that reference information now. Outdoors, sound diminishes 6dB/double distance, but indoors, where boundaries play a big role, the rate is more like 3dB/double distance. In my graph, the 45degree line shows this behavior almost perfectly. The relative distances between the Near position and the Far position are close to double, and the SPL loss is 3dB.

That said, we can know a few things from reading here and there. The longer the delay, the greater the intensity difference that will be required to maintain a stable center image - as you know, the delayed sound must be higher SPL. There is a limit to this - measured in msec, but I don't know what it it. I suspect that in small rooms like ours, the delay will not be large enough to be insurmountable, at least theoretically. In my room, the maximum difference in arrival times for the left vs. right surrounds is 4-5msec. This is well within the range preferred by most listeners, according to Toole.

So the challenge, again as you know, is to make the late arriving sound - the sound from the farthest speaker - or in the terms you have used here, the SPL at the farthest seat louder than at the nearest seat. None of your measurements show that this can be accomplished, as distance is the dominant factor in determining the SPL at any given seat aside from the nearest. The off-axis roll-off is not strong enough - that is to say, the dispersion is too wide. This is my interpretation: there may be other ways of looking at this. You reached this conclusion before I did... cool.gif

Still, there must be a good compromise to make here. I personally expect that 20 degrees is a good compromise, but listening will be the way to know. Some well controlled listening tests could be very informative. Set up two at the same angle and listen in each seat to real music; listen carefully for the stability of the stereo image as you move from center. Give it a shot! You've done all the hard work! So far, my attempts at this have not yielded a stable image anywhere except the midpoint between the two surrounds... mad.gif

Obviously, someone out here knows more about this than I do, so I hope they can share.
post #69 of 99
I have often wondered how to go about finding the perfect amount of delay, or how to properly dial in the time domain aspects of EQ'ing your speakers in a typical small room listening environment.
post #70 of 99
I still don't have the equations, but here's a really cool demonstration.

http://auditoryneuroscience.com/topics/time-intensity-trading

The way I believe that demonstration is set up, the diagonal line from top left to bottom right represents combinations that should trade pretty evenly. My casual headphone listening supports this, but my ears don't have very high similarity in sensitivity. Transcribing: 0.22msec can be offset by 3dB, 0.44msec by 6dB, and 0.66msec by 9dB. That seems like an insurmountable SPL differential, but maybe I'm misreading.
post #71 of 99
Thread Starter 
Those gain figures are huge! My Cheap Thrills have a very broad and stable stereo image, but there is no way I am seeing a 6dB difference in SPL as I move between the Left and Right speakers. I wonder if that is relative to the opposite channel or relative to the same channel. By this I mean,if I maintain the same SPL with a doubling of distance, I am effectively 3dB louder than expected when indoors, and 6dB louder than expected when outdoors.
post #72 of 99
Quote:
Originally Posted by mhutchins View Post

Those gain figures are huge!
That's what I thought. Did you listen to them? I assume that they are labeled correctly and that -6dB means 6dB less than the late arriving signal.

Maybe measuring with pink noise with the cheap thrills would give a good reference. If you're happy with the way that image stabilizes, you need to mimic that.
post #73 of 99
It's clear that the 20 deg angle gives the most uniform coverage, but I'm surprised how little the SPL changes for even the worst case.
post #74 of 99
What happens if the speakers are brought closer together? I know that sounds stupid - you have a narrow room - why would you bring the speakers closer together? Three things may/will happen: one you get a more similar distance at all the seats; the time difference will be more similar; you can get the center seat more off-axis, which might improve the SPL pattern you observed.

May be worth a shot?
post #75 of 99
Thread Starter 
Quote:
Originally Posted by noah katz View Post

It's clear that the 20 deg angle gives the most uniform coverage, but I'm surprised how little the SPL changes for even the worst case.

I'm sure part of what we are seeing in the SPL graph is the result of first reflections off the opposite wall. These are pretty easy to see in the impulse response plots. I have some absorption on one wall in the form of blackout drapes covering a sliding glass door, but there is still 6' of wall uncovered on both sides of the seating area.

I was also surprised by the lack of high frequency roll-off for the off-axis plots I measured earlier. These were measured by setting my test enclosure on top of my Cheap Thrills and progressively toeing-in the driver in front of the listening position:


post #76 of 99
I'll say, I don't see how that could be true (so little HF rolloff)
post #77 of 99
Thread Starter 
Part of the problem is that I don't know what I don't know about measuring speakers... eek.gif
post #78 of 99
Thread Starter 
After a lot of cursing and swearing at Sketchup, I was able to produce a reasonable facsimile of my intended surround enclosure. It is a sealed box just over 0.5 ft^3. The speaker baffle is 12"x18" at a 20 degree down angle from vertical. The current design has a brace on each side of the woofer between the front and back panels. I will use 3/4" MDF for the front baffle, and 1/2" plywood for the rest of the enclosure.



This enclosure is very similar to the enclosure Erich previewed a few days ago in this thread.

Just need a few warm days to make a pair.

Mike
post #79 of 99
Looks good Mike. I would think the braces in a cabinet that small are not terribly important - but they can't hurt. Did you share Erich's concern about the round over on the bottom edge? Any what are you doing for terminals and mounting? Have you included any structural provision that would relate to that?
post #80 of 99
Quote:
Originally Posted by mhutchins View Post

After a lot of cursing and swearing at Sketchup, I was able to produce a reasonable facsimile of my intended surround enclosure. It is a sealed box just over 0.5 ft^3. The speaker baffle is 12"x18" at a 20 degree down angle from vertical. The current design has a brace on each side of the woofer between the front and back panels. I will use 3/4" MDF for the front baffle, and 1/2" plywood for the rest of the enclosure.



This enclosure is very similar to the enclosure Erich previewed a few days ago in this thread.

Just need a few warm days to make a pair.

Mike

Which part was difficult in sketchup ? The angle ?
post #81 of 99
Thread Starter 
Everything! I'm used to 2D Autocad LT and Sketchup just seems the opposite. I used follow me for the bevel, but at the last corner it didn't finish right, then there were missing segments in the roundover. . . . .The speaker shading would not work right, so I have multiple areas that are see through. Ever tried to push-pull into a corner where 2 of the 3 dimensions are getting smaller.....somehow my planes would twist and then I could not create a surface without an extra diagonal line....My axes would be off so I would have to use the CTRL-Orbit combo to orient my model. If I forget to release the Orbit button first, the model reverts back to some disoriented position...

For terminals, I will use the airtight Neutrik Speakon NRL4MP connectors mounted to the ends of the box.





I'm Still planning an acoustically transparent soffitt with bass traps and surround speakers behind the AT fabric. For wall mounts, I was thinking simple angle brackets into simple screw anchors, but I'm open to ideas!

Mike
post #82 of 99
Quote:
Originally Posted by mhutchins View Post

Everything! I'm used to 2D Autocad LT and Sketchup just seems the opposite. I used follow me for the bevel, but at the last corner it didn't finish right, then there were missing segments in the roundover. . . . .The speaker shading would not work right, so I have multiple areas that are see through. Ever tried to push-pull into a corner where 2 of the 3 dimensions are getting smaller.....somehow my planes would twist and then I could not create a surface without an extra diagonal line....My axes would be off so I would have to use the CTRL-Orbit combo to orient my model. If I forget to release the Orbit button first, the model reverts back to some disoriented position...

For terminals, I will use the airtight Neutrik Speakon NRL4MP connectors mounted to the ends of the box.





I'm Still planning an acoustically transparent soffitt with bass traps and surround speakers behind the AT fabric. For wall mounts, I was thinking simple angle brackets into simple screw anchors, but I'm open to ideas!

Mike

Along with teaching woodwork , I teach sketchup . You need some plugins that will make it much easier. If you want them pm me. If you run into problems in the future just post the skp file and I can help. Before I taught sketchup I taught cad lt . Sketchup is much better , kids like it a lot more. cool.gif
post #83 of 99
If I build similar cabs, I'll extend the sides an inch or two so that the back can be recessed to hide mounting and wires.
post #84 of 99
Thread Starter 
I plan to hide my enclosures in an acoustically transparent soffit that runs the perimeter of the room. It will hide cable, speakers and bass traps. Somewhat similar to what d_c is doing in his build, but AT rather than drywall.
post #85 of 99
Thread Starter 
Thanks, Chaluga.

I'll pm you later this week for advice on plugins. For now, I'm done with Sketchup...

Mike
post #86 of 99
Thread Starter 
I did a dry fit for one of my V10 surround enclosures today after I finished cutting out the driver and Speakon recesses. A few adjustments were required, but I hope to complete the assembly by this weekend.


post #87 of 99
Thread Starter 
Got 1 finished this evening, just need Duratex and some stuffing:


post #88 of 99
Quote:
Originally Posted by mhutchins View Post

Got 1 finished this evening, just need Duratex and some stuffing:


Looking good cool.gif
post #89 of 99
+1
post #90 of 99
Thread Starter 
Great progress lately. biggrin.gif

I completed both side surrounds and installed them. This past weekend I started building enclosures for my rear surrounds and the V10 kits arrived today!! Woohoo. I got one of my rear surrounds installed tonight and will install the other one tomorrow. I went with a 30 degree down angle for the rear surrounds as I am not so worried about sound stage and side-to-side balance and localization.

I tried a magic marker on the woofer to turn purple into black. I think I need to draw on the rest of my surrounds.... cool.gif



I pushed the brightness a bit to make the details more visible.
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