Murphy Corner Line Array (MCLA)

[johnlmurphy]

Re: Murphy Corner Line Array (MCLA)

Could the back of the cabinet be omitted, and the rest directly caulked onto wallboard? Is standard wallboard strong enough? It feels pretty solid in the corners.

Sure, that would work fine. My only concern would be the possible need for future access to replace a failed/buzzy driver. If you carefully screened each driver before it was installed you would reduce the chances of needing access to the drivers in the future.

By building the enclosure into the corner you would eliminate the potential for buzzes that can occur between the enclosure and the walls.

Regards,

John

[Mr. Wizard]

Re: Murphy Corner Line Array (MCLA)

Could the back of the cabinet be omitted, and the rest directly caulked onto wallboard? Is standard wallboard strong enough? It feels pretty solid in the corners.

[johnlmurphy]

Re: Murphy Corner Line Array (MCLA)

Finally making some progress. Here's a pic of cabinet #2.

Geo

Geo, Looking good! Thanks for keeping us informed...

John

[richidoo]

Re: Murphy Corner Line Array (MCLA)

Looks good Geo!

[audiorasp]

Re: Murphy Corner Line Array (MCLA)

Finally making some progress. Here's a pic of cabinet #2.

Geo

[richidoo]

Re: Murphy Corner Line Array (MCLA)

Thanks John, good point about the internal volume.

[johnlmurphy]

Re: Murphy Corner Line Array (MCLA)

The cabinet height is 85.5 inches, not 96", so it doesn't fill up the 8 foot tall corner. I assume this is because of the drivers' electrical impedance limiting us to 24 drivers, to get an overall impedance between 4 and 8 ohms, and all drivers playing same SPL.

If we can use two amps per speaker, the input level of one amp can be padded to adjust for different driver wiring combinations. eg. 3x5 on one amp and 2x6 or 3x4 on the other amp to get 27 drivers to fill up the full 96" height. With the middle drivers taking one amp and the other amps driver split top and bottom, maybe the two groups could be time aligned with digital delay. You have to add in a DCX2496 to accomplish that. But now electronics for the project are becoming expensive and complex.

Is the impedance of the full range driver flat enough to allow using a single 8ohm 12W power resistor to simulate an additional driver to make 4.5 ohms total impedance with 4x7 "drivers?" 27 drivers and one resistor.

Or we can just build the cabinet 96" tall and just center the 24 drivers on it, still 3.5 inches apart. This avoids the pockets that would be created at top and bottom if we hung the short cabinet centered on the corner height. I think the reflection from those corner pockets would be rather nasty.
Rich

Rich,

As you saw in the previous post the next practical stop after 25 drivers is 28 drivers. While it would be possible to implement multiple amplifiers to drive the array I think you could use a 27 driver array configured as 7 groups of 4 for a nominal 2.67 Ohm impedance. Just make sure you use a power amp rated to drive 2 Ohm loads.

A custom enclosure with the ends extended to span the complete the floor to ceiling distance makes sense for a customized built-in look, especially for home theater. To maintain the specified enclosure volume you could use an internal baffle to close off the ends. Using the larger enclosure would also be fine except that it might change the bass response enough that you would want customized equalization.

Regards,

John

[johnlmurphy]

Re: Murphy Corner Line Array (MCLA)

Hello everyone, first post!

Hello John,

My father and I are going to be building a pair and I have a few questions. My father is going to be building his to spec, I on the other hand live in an apartment so I don't believe I can attach these monsters to my walls (my neighbors above me and beside me would not be too happy!). If I made them free standing and enlarged the cabinet volume could I possibly regain some of the lost low end output? How about if I tweak the eq a bit more? I also put the driver specs into bassbox pro and it recommended a ported enclosure, could that help me out a bit?

These things are going to basically be in active storage until I can get a place that isn't a shared living space. If I built your cabinets to spec and then added foam isolators to the back so that it wouldn't resonate against the walls should I be ok? Will I still get the same bass response if the cabinet is 1/2" away from the walls? Would the rear reflections in any way be more audible to my neighbors than say the reflections coming off of free standers? Basically I am trying to avoid building the cabinets twice.

Lastly you mention using 25 drivers for 8 foot ceilings, calculating that out I get roughly 7 feet 4 inches leaving 8 inches. My ceilings are 8 feet 4 inches, would you recommend going with 26 or 27 drivers per side? If so, what final impedance could I get?

I am so sorry for all of the questions! I just want to make sure that I can get this done the right way the first time.

Thanks,
Dan

Hello Dan and welcome to the thread...

Concerning mounting the enclosure in your apartment...I would just use rubber bumpers at the back sides with a small attached "stand" at the bottom to raise the array to the appropriate height and bear the weight of the enclosure. The small spacing from the wall is normal. Place the enclosure in the corner and then fasten the enclosure top to the wall using a single screw eye (or similar) in the wall and a screw eye in the top of the cabinet. Tie the cabinet to the wall with a single tie wrap. (Thanks to Brian Myers at PE for suggesting this mounting scheme!)

I would not expect the arrays to be more audible to your neighbors unless you are compelled to play them louder than usual.

Using 25 drivers is very convenient because the series-parallel configuration is 5 groups of 5 for a net impedance the same as an individual driver, 8 Ohms. The problem with scaling up to 26 or 27 drivers is the wiring configuration

Using 26 drivers you could choose between 2 groups of 13 (52 Ohms) or 13 groups of 2 (1.2 Ohms) neither of which is practical.

The situation with 27 drivers is only slightly better with 3 groups of nine yielding 24 Ohms and 9 groups of 3 providing a 2.67 Ohm nominal impedance.

With 28 drivers the results become reasonable again. Configured as 4 groups of 7, the system would yield 14 Ohms while 7 groups of 4 would yield 4.57 Ohms. I would probably go for the 28 driver 4.57 Ohm configuration as the next step beyond 25 drivers. With 4 more drivers than my 24 driver enclosure the 28 driver enclosure would be extended to an overall outside height of 85.5" +14" = 99.5" or 8' 3.5". That is probably too tight a fit for your room depending on the exact setup. Since 28 drivers will probably not work your best choice would be to go with 25 drivers. The 25 driver system has the advantage of keeping you more flexible in case your next room has exactly 8' ceilings.

Keep us updated with your progress and your father's progress as well.

Regards,

John

[richidoo]

Re: Murphy Corner Line Array (MCLA)

The cabinet height is 85.5 inches, not 96", so it doesn't fill up the 8 foot tall corner. I assume this is because of the drivers' electrical impedance limiting us to 24 drivers, to get an overall impedance between 4 and 8 ohms, and all drivers playing same SPL.

If we can use two amps per speaker, the input level of one amp can be padded to adjust for different driver wiring combinations. eg. 3x5 on one amp and 2x6 or 3x4 on the other amp to get 27 drivers to fill up the full 96" height. With the middle drivers taking one amp and the other amps driver split top and bottom, maybe the two groups could be time aligned with digital delay. You have to add in a DCX2496 to accomplish that. But now electronics for the project are becoming expensive and complex.

Is the impedance of the full range driver flat enough to allow using a single 8ohm 12W power resistor to simulate an additional driver to make 4.5 ohms total impedance with 4x7 "drivers?" 27 drivers and one resistor.

Or we can just build the cabinet 96" tall and just center the 24 drivers on it, still 3.5 inches apart. This avoids the pockets that would be created at top and bottom if we hung the short cabinet centered on the corner height. I think the reflection from those corner pockets would be rather nasty.
Rich

[saabracer23]

Re: Murphy Corner Line Array (MCLA)

Hello everyone, first post!

Hello John,

My father and I are going to be building a pair and I have a few questions. My father is going to be building his to spec, I on the other hand live in an apartment so I don't believe I can attach these monsters to my walls (my neighbors above me and beside me would not be too happy!). If I made them free standing and enlarged the cabinet volume could I possibly regain some of the lost low end output? How about if I tweak the eq a bit more? I also put the driver specs into bassbox pro and it recommended a ported enclosure, could that help me out a bit?

These things are going to basically be in active storage until I can get a place that isn't a shared living space. If I built your cabinets to spec and then added foam isolators to the back so that it wouldn't resonate against the walls should I be ok? Will I still get the same bass response if the cabinet is 1/2" away from the walls? Would the rear reflections in any way be more audible to my neighbors than say the reflections coming off of free standers? Basically I am trying to avoid building the cabinets twice.

Lastly you mention using 25 drivers for 8 foot ceilings, calculating that out I get roughly 7 feet 4 inches leaving 8 inches. My ceilings are 8 feet 4 inches, would you recommend going with 26 or 27 drivers per side? If so, what final impedance could I get?

I am so sorry for all of the questions! I just want to make sure that I can get this done the right way the first time.

Thanks,
Dan

[Mr. Wizard]

Re: Murphy Corner Line Array (MCLA)

John, thanks for the analysis. I overlooked the in-room graphs or I would have done it myself.

103 dB is a little less that I guessed from you description, but it is respectable. Do you think at 20Hz the array is radiating more acoustic power at distance than a corner loaded point source, at the same SPL at 2m?

[johnlmurphy]

Re: Murphy Corner Line Array (MCLA)

John, the implied low frequency efficiency you describe is perplexing. The value of corner loading is not entirely lost on me, but I still cannot figure how it is enough. How do you get enough efficiency in the 14 Hz to 28 Hz band to shake the house, using only 150 watts, out of a box with one cubic foot internal volume (times 2)?

It is amazing (to me) the amount of bass this system puts out. But I will give you a more objective description of the actual 20 Hz capability of the system. The numbers play out something like this. One array plays at 76 dB SPL at 20 Hz with 1 Watt of input at a distance of 2 meters as seen in Figure 4-6 here:

http://www.trueaudio.com/array/MCLA_...st_results.htm

Next, for the increase from 1 Watt to 150 Watts of input add 21.7 dB ( 10*log(150) ) to get a single system SPL of 97.7 dB SPL. Finally add 6 dB for the coherent summation of the two line arrays and you get 103.7 dB SPL at 20 Hz with 150 Watts per side.

103 dB SPL at 20 Hz is enough to shake my house. I know that tastes, rooms and applications vary; some folks may require more output than this for their particular application.

By the way, the program material I used for the 14 to 28 Hz test was a "musical composition" consisting of a simple pattern of close spaced notes played on the bass guitar, recorded, then shifted down two octaves and low pass filtered below 30 Hz or so. I named the test track "Infra Terror"...

Regards,

John

[time]

Re: Murphy Corner Line Array (MCLA)

Okay, thanks for the explanation. I'm not necissarily a basshead myself but I have to admit I like the WOW! factor it produces with freinds. I'm in a rural location so no complaining neibors either.

I havn't done the math on it but I'm guessing the total cone area of all those drivers, corner loading, efficiency, and EQ could produce surprizing bass results.

[Mr. Wizard]

Re: Murphy Corner Line Array (MCLA)

John, the implied low frequency efficiency you describe is perplexing. The value of corner loading is not entirely lost on me, but I still cannot figure how it is enough. How do you get enough efficiency in the 14 Hz to 28 Hz band to shake the house, using only 150 watts, out of a box with one cubic foot internal volume (times 2)?

[johnlmurphy]

Re: Murphy Corner Line Array (MCLA)

I'm interested in your MCLA design. I like the theory behind it. I do have questions though.

You make this statement on your site: "A subwoofer is not required and would likely even degrade the performance."

Why would a sub(or pair) degrade the performance? I suppose I can think of two reasons on my own. Forgive my novice termonlogy.

Of coarse, there is the added crossover. With part of theory behind the array being 'no crossover', I can see where adding a sub would pose a problem.

There is also placement issues. My sub pair will be quite large and should be placed beside or behind the mains. I may have a way to do this in my room, but most will not.

I guess in simple terms I'm questioning the bass capabilities of the speakers. I wouldn't want to build the design and find I'm not happy with the lows and not be able to add a sub. After adding sub to my current system, I heard things in some of my music I didn't know was there.

When I wrote the comment about a subwoofer degrading the performance of the MCLA I was thinking only of the fact that the SPL falloff with distance is different for a point source vs. a line source. This means that if you set the subwoofer level to be correct at one location it will be different at other listening distances. For example, say you set the subwoofer level to match the MCLA level at 6' (~2m). At listening positions further back in the room the subwoofer would be lower in level than the MCLA. That's what I was thinking anyway.

With multiple subwoofers (especially placed front and rear) you would likely have less fall off with distance and the performance may be acceptable in terms of bass consistency around the room.

The amount of low bass energy that two MCLA systems can produce in my room is nothing short of amazing. And I am a dyed in the wool bass-aholic. So far I have been listening to CDs, my guitar and bass and live band performances on the system. I have yet to audition it for home theater but that is coming soon as I get ready to build my 2nd pair of MCLAs. My casual testing with specially recorded material having energy in the range from 14 Hz to 28 Hz shows good behavior even with material at these extreme frequencies. Nothing bad happens as I increase the level untill the power amp (~150 W per side ) eventually clips and adds an audible buzzing. By that point my wife and dog are announcing that the house is falling down and I stop the test. But the speakers never showed any sign of stress. When the power amp was clipping I was hitting peaks in the range of 150 Watts. Divided among 24 drivers that would be just 6.25 Watts per driver as the power amp clips (reaches its maximum output). With a power rating of 20 Watts and handling just 6 Watts the ND90 drivers are truly just loafing even as the house is shaking and I am ready to "hurl" from the extreme bass test.

Regards,

John