Murphy Corner Line Array (MCLA)

[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

[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)

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

[Enigma]

Rays are not Waves

All the research and theory put into this has been truly fascinating! Looks like a great project! And I hope it does work out like you think in most situations.

However...

I hate to say this but your theories are somewhat flawed. The entire project is based on the concept of reflection, however, your concept of reflection seems to revolve around thinking about audio and speaker placement as if they reflect like light waves. Audio waves and light waves are very different in form and where light tends to travel and act as a straight beam, audio waves are more analogous to wave rings that radiate out. This means that your presumptions using the mirror examples are all wrong since they do not take into account wave reflection, refraction and interference. A much more accurate setup for your experimentation would be to have a large rectangular box filled with water. Create waves at different points within the box to see how they are reflected back to the center (listening position). This would more accurately represent a true audio wave sound stage in a room.

The net affect means that your setup will end up sounding much more "airy" and non-directional than it needs to be since you are thinking as if you are addressing rays not waves. Bose tried something akin to this using multi-directional speakers but they just don't work well for the same reason.

Might be worth some thought. ;)

[audiorasp]

Re: Murphy Corner Line Array (MCLA)

I've had the MCLA's running for 4 days now but my first impressions have not been that great, mainly because of the distance between corners in my room. My corners are 17 ft. apart along my front wall, and my listening position can be 12 ft. max back from the front wall. As a result I'm getting too much separation between the left and right channels.

In the MCLA article, I can't see where John mentions anything about optimum spacing between speakers. I don't have any experience with using equalizers. Would optimum equalization settings minimize this problem?

Geo

[marvin]

Re: Murphy Corner Line Array (MCLA)

I've had the MCLA's running for 4 days now but my first impressions have not been that great, mainly because of the distance between corners in my room. My corners are 17 ft. apart along my front wall, and my listening position can be 12 ft. max back from the front wall. As a result I'm getting too much separation between the left and right channels.

In the MCLA article, I can't see where John mentions anything about optimum spacing between speakers. I don't have any experience with using equalizers. Would optimum equalization settings minimize this problem?

Geo

Your room configuration may be simiar to mine. My front wall is wider than my listening distance from it, but my speakers and listening position are to one end of the front wall.

If you can correct it with EQ, great!

If not, you might consider what I thought might be a solution for a proportionally too wide front wall. I thought erecting a four foot wide partition wall at a 90 degree angle could provide another corner reflecting surface at a more desirable location. I would do a temporary install for test purposes before committing to it permanently.

Even before that, I would try placing the MCLA's at the narrow end of the room, to hear if the performance improves.

Hope you can get them to work up to expectations.

Marvin

[captainjack115]

Re: Murphy Corner Line Array (MCLA)

I tried to look over the construction plans as carefully as I can.

I haven't seen anything about encloser stuffing. I realize that a triangular shape cuts down on reflections, but shouldn't there be some sort of poly foam inside.

Also, I wonder about some program material with low frequency sustained bass creating audible Doppler distortion at loud listening levels.

[johnlmurphy]

Re: Rays are not Waves

Enigma Wrote:

<snip> I hate to say this but your theories are somewhat flawed. The entire project is based on the concept of reflection, however, your concept of reflection seems to revolve around thinking about audio and speaker placement as if they reflect like light waves. Audio waves and light waves are very different in form and where light tends to travel and act as a straight beam, audio waves are more analogous to wave rings that radiate out. This means that your presumptions using the mirror examples are all wrong since they do not take into account wave reflection, refraction and interference. <snip>

Fear not, the image method I employ in the analysis of the corner line array is actually well documented in the scientific literature on acoustics. Sound and light waves DO behave exactly the same way with respect to specular reflection: the angle of incidence equals the angle of reflection. Enigma's post does succeed in making me realize that I only gave one reference to the scientific literature concerning the image method in acoustics. Because it is so critical to understanding the benefits of corner placement a line array, I have expanded the discussion of the image method (also called the image-source method) at the project's Design Concepts page here:

http://www.trueaudio.com/array/MCLA_design.htm

Below is the new text with added references and links to so that you can now download several of the references in .pdf form:

Image analysis is a powerful technique used in the field of acoustics to study reverberation and room reflections. In this analysis the walls of the room are considered to be mirrors and reflected images of sound sources can be readily located. Before proceeding let's review the image analysis method as it applies to loudspeakers in rooms.

The famous acoustician/physicist Carl F. Eyring commented on the image method in 1930 as follows:

"This necessary analysis is aided by the method of images. Just as a plane mirror produces an image of a source of light, so also will a reflecting wall with dimensions large as compared with the wave length of the sound wave produce the image of a source of sound. An image will be produced at each reflection. In a rectangular room, the source images will be discretely located through space. This infinity of image sources may replace the walls of the room, for they will produce an energy density at a point in the room just as if they were absent and the walls were present." [2-1]

Here Eyring is saying explicitly that the image sound sources can be substituted for the room and the resulting sound field will be the same. The assembly of reflected images is exactly equivalent to the effect of the room. A sound source in a room is exactly equivalent to that same sound source in free space along with an infinite array of sound sources representing the mirror image reflections of the single sound source in the room.

A rigorous mathematical foundation for the image method can be found in the 1978 paper by Allen and Berkley titled: "Image method for efficiently simulating small-room acoustics" [2-2]

It is also worth mentioning that in 1957 the well known acoustician Richard V. Waterhouse also used the image method to analyze the effect of reflecting surfaces on sound sources. [2-3]

Floyd Toole in his recent (2008) book titled "Sound Reproduction, The Acoustics and Psychoacoustics of Loudspeakers in Rooms" [2-4] writes:

"Explanations of sound fields in concert halls begin with notions of ray (geometrical) acoustics, showing direct sound and discrete reflections from large surfaces. The rules are simple: the angle of incidence equals the angle of reflection." [2-4, p. 43]

The simple rule of equal angles of incidence and reflection is shared with the field of optics, a sister field of acoustics falling under the fundamental science we call physics. This shared rule is why acoustical reflections are located in exactly the same positions as the visual images of the sound source that we would see if the walls were mirrored.

Here are links to the referenced .pdf files:

[2-1] http://www.trueaudio.com/array/downl...ad%20Rooms.pdf

[2-2] http://www.trueaudio.com/array/downl...ley%201978.pdf

[2-3] http://www.trueaudio.com/array/downl...rce%201958.pdf

[2-4] Floyd D. Toole, "Sound Reproduction, The Acoustics and Psychoacoustics of Loudspeakers and Rooms.", Focal Press, 2008

I realize that I am taking the image method further than most past applications of the method to speaker placement, so it is important that the reader be confident in the validity of the image method. Most prior discussion of reflected images is limited to acknowledgement of just one or two reflections. A speaker designer might address "floor bounce" but then fails to consider "ceiling bounce", "front wall bounce", "left wall bounce", "right wall bounce" or "rear wall bounce" much less the ongoing reflections of reflections. In my opinion the image method lets us see the problem of speakers in rooms more clearly than any other method of analysis. It is only by approaching the speaker and the room as a "system" that we can hope to make progress addressing the biggest problem facing speaker designers: the speaker-room interface.

Floyd Toole's book (the 4th reference above) is highly recommended by the way.

Regards,

John

[johnlmurphy]

Re: Murphy Corner Line Array (MCLA)

I've had the MCLA's running for 4 days now but my first impressions have not been that great, mainly because of the distance between corners in my room. My corners are 17 ft. apart along my front wall, and my listening position can be 12 ft. max back from the front wall. As a result I'm getting too much separation between the left and right channels.

In the MCLA article, I can't see where John mentions anything about optimum spacing between speakers. I don't have any experience with using equalizers. Would optimum equalization settings minimize this problem?

Geo

Geo,

The wide placement compared to your listening distance is likely destabilizing the image causing it to quickly suck to one side as you move off axis. Is there any way you can rearrange the room to place the arrays at the narrow end of the room? I think that would stabilize the image compared to the wide placement. In my case I have a "shoe box" shaped room with the arrays at the narrow end about 12' apart.

captainjack115 asked about filling. I have not discussed this in the official project discussion, but yes, I did use filling in the enclosure. I prefer the white fluffy lose polyester such as typically sold at fabric stores. I used about 3/4 of a 1 pound bag (12 oz.) to fill each enclosure. I spindled some of it to wrap the wiring harness and prevent buzzing. The rest was carefully placed to fill all the spaces in the enclosure. As the rear was put in place the overflowing poly was compressed about 20%.

Are you using the Behringer DEQ2496 EQ? I expect my settings will work well above about 150-200 Hz but below that you may need custom adjustments for your room. Hopefully they will only be small adjustments. Which voicings have you tried? Flat? X-curve? Are you feeding the EQ with a healthy signal level as indicated by the input level meter on the EQ? How is the EQ inserted into the system?

How did you mount the arrays to the wall? Be careful that the enclosure does not buzz against the wall. You might use small rubber bumpers on the corners of each rear surface to control the contact with the wall. A custom made base can be used to set the height and bear the weight while the enclosure is drawn into to the corner using a tie wrap through screw eyes in the enclosure top/bottom and the corner area of the wall.

I think that with just a little fine tuning we can have them sounding great. Keep us updated as you can.

Regards,

John

[johnlmurphy]

Re: Murphy Corner Line Array (MCLA)

Your room configuration may be simiar to mine. My front wall is wider than my listening distance from it, but my speakers and listening position are to one end of the front wall.

If you can correct it with EQ, great!

If not, you might consider what I thought might be a solution for a proportionally too wide front wall. I thought erecting a four foot wide partition wall at a 90 degree angle could provide another corner reflecting surface at a more desirable location. I would do a temporary install for test purposes before committing to it permanently.

Even before that, I would try placing the MCLA's at the narrow end of the room, to hear if the performance improves.

Hope you can get them to work up to expectations.

Marvin

I like Marvin's solution of adding a 4' partition wall to create a corner where there was none. But if the room can just be rearranged to place the arrays at the narrow end, that would be the preferred solution.

John

[captainjack115]

Re: Murphy Corner Line Array (MCLA)

[QUOTE=johnlmurphy;1631689]Geo,

captainjack115 asked about filling. I have not discussed this in the official project discussion, but yes, I did use filling in the enclosure. I prefer the white fluffy lose polyester such as typically sold at fabric stores. I used about 3/4 of a 1 pound bag (12 oz.) to fill each enclosure. I spindled some of it to wrap the wiring harness and prevent buzzing. The rest was carefully placed to fill all the spaces in the enclosure. As the rear was put in place the overflowing poly was compressed about 20%.
Hi John,

Thanks for the info on the filling.

Some time ago before seeing your MCLA project, I built a pair of line arrays into my wall, taking the place of B&G RD-75's.
I used 16 four inch drivers per side and EQ'ed the response curve. They are cheap automotive drivers and the Xmax is unknown, I doubt they're more than 1mm. The drivers are enclosed behind the free standing wall, I'd say the encloser volume is about one cubic foot. The sound at low volume isn't bad, there is deep bass, but compression sets in at higher volume levels and I do hear doppler distortion on sustained bass. Having more Xmax will allow louder bass with less compression, but Doppler will only worsen.
Since you have not mentioned Doppler problems with the MCLA, I can only assume that corner placement increases low frequency acoustic output enough to make Doppler distortion a moot point.
I'm seriously considering the MCLA project as a way of scaling back my system. I have four 12inch drivers in my free standing wall as an infinite baffle, bass is outstanding but I'll be moving to a smaller house.

Jack

[broncosaurs]

Re: Murphy Corner Line Array (MCLA)

Hi John, first I would like to thank you for all the projects over the years. I have always been interested in the line arrays. Reading anything I could find on them, I even have your book. My love is in ribbons and a line of bg 3s and 10s (I dont think bg is going to let the 10 to the diy people) and Dayton rs150s was going to cost me to much and also be to large. So to a question about the MCLAs; the room is 18' wide where the speakers will go, 30 ' long but the problem I will have is the height of the ceiling is 18'. I don't think wife will be OK with 120 speakers going up the two walls. So what would you think the overall difference in sound quality and sound reinforcement not having the ceiling reflection? The speakers will be used for both HT and rock, jazz and country.

Thanks again, Brad

[John L. Murphy]

Re: Murphy Corner Line Array (MCLA)

<snip>... My love is in ribbons and a line of bg 3s and 10s (I dont think bg is going to let the 10 to the diy people) and Dayton rs150s was going to cost me to much and also be to large. So to a question about the MCLAs; the room is 18' wide where the speakers will go, 30 ' long but the problem I will have is the height of the ceiling is 18'. I don't think wife will be OK with 120 speakers going up the two walls. So what would you think the overall difference in sound quality and sound reinforcement not having the ceiling reflection? The speakers will be used for both HT and rock, jazz and country.

Thanks again, Brad

Thanks for the kind comments Brad. Your 18' x 30' room could work well with the lines 18' apart across the short wall. With an array that is less than the ceiling height you will still get ceiling reflections but the "equivalent system" (consisting of the direct sound source and its reflections) will have gaps in the infinite array due to the "missing" drivers. Short of simulating the system along with its first few reflections it is hard to say how it would perform. My intuition says to expect some loss of low end output and a somewhat more variable response around the room. But if you were to build the system and then custom EQ it to flat (X-curve, or other) I have no doubt it would sound quite fine. My system EQ would be a good starting place but would probably need some adjustments.

Regards,

John

[Hearing Specialist]

Re: Murphy Corner Line Array (MCLA)

Hello Mr. Murphy, my brother Brent use to work for Ford Audio and Video in Tulsa back in the 90's. I read all the above posts and find it very interesting. I'm sure there are some like myself that don't have that perfect room setup. I love the science behind the build and would love to hear them. I'm also interested in your listening space, i.e how far you are from them...you get the point. Were you at Ford on the 90's?

[John L. Murphy]

Re: Murphy Corner Line Array (MCLA)

Hello Mr. Murphy, my brother Brent use to work for Ford Audio and Video in Tulsa back in the 90's. I read all the above posts and find it very interesting. I'm sure there are some like myself that don't have that perfect room setup. I love the science behind the build and would love to hear them. I'm also interested in your listening space, i.e how far you are from them...you get the point. Were you at Ford on the 90's?

I did serve as the Chief Engineer at Ford Audio and Video in Oklahoma City from 1978-1981...just a bit earlier. Back then I was just beginning to plot enclosure responses one point at a time using a (programmable) hand-held calculator (TI59?). I kept the program on a little magnetic card that I would feed into the calculator at the start of each session. Back then I used to write the monthly "Hands-On Report" for Modern Recording magazine where I would perform product reviews based on both a field test and a bench test. I also built my first line arrays during that period.

Currently I have one pair of MCLAs in my music studio/listening room. The room is about 12' x 25' x 8'. The arrays are in the corners of the 12' wall and so are 12' apart. I probably spend more time playing guitar through them than listening to recorded music so I tend to listen very close at about 4 ' from each line. For music playback I like listening a bit further back unless I am trying to figure out a part in which case I find it easier to hear the finest "detail" loud and up close. My room is rather reflective with a bare hardwood floor and two triple and three single windows. When the room is quiet a handclap will reveal audible reverberation with minimum coloration and a quick decay. I think the room sounds nice for straight acoustic performances, such as my buddy and me on acoustic guitars accompanying his vocals.

Soon I will have a second pair in my home theater room which is just slightly larger at 14' x 24' x 8'. They will serve as the Front Left-Right mains.

Regards,

John