Murphy Corner Line Array re-purposing as a sub

[ppataki]

Some time ago I built a pair of Murphy Corner Line Array (MCLA) speakers - the project was a success, I enjoyed them a lot
But then I moved to a 15" fullrange project instead and I wanted to sell my MCLAs which turned out to be Mission Impossible.....
They were resting in my garage but then I was thinking - that does not make any sense, let's do something with them!

So I have decided to convert them to a sub that will go on the floor below my couch (totally invisible)

Here is the cabinet plan:

The sealed box will have a height of only 14cm and there will be 24 speakers along both AB sides (so 48 speakers altogether) in a DOS configuration
The drivers are Dayton ND91-8 and they will be driven by a Sabaj A30A amp

This will give me almost 101dB SPL at 24Hz at 1 meter + room gain when driven with 360W of power (this is amp limited performance)

Max cone displacement will be a bit more than 3mm so I am not expecting a lot of distortion (5.1mm is the xmax)

Group delay looks fine too:

We will start the project with my neighbor (THE carpenter) hopefully this week but I guess we will not be able to finish this year
I will post pictures as we make progress and of course some measurements too
Needless to say there will be heavy DSP involved, as usual

Stay tuned
In case of any questions, comments do not hesitate

 

[mcdn]

Ooh, cool. I had a thread a while back for a shallow sub but never considered using small drivers.

 

[ppataki]

Ooh, cool. I had a thread a while back for a shallow sub but never considered using small drivers.

The reason why I chose to convert the MCLAs to a sub (apart from the fact that I got fed up with storing them in the garage...) is that the low end of the MCLA was really impressive! - pretty much beyond anything I imagined it will be
So hopefully I will get the same experience in this new configuration too - we will see latest by the end of January

 

[-Matt-]

No idea if it would make sense, just throwing the idea out there...

You could perhaps add an enormous DIY passive radiator to the upper surface?

 

[ppataki]

No idea if it would make sense, just throwing the idea out there...

You could perhaps add an enormous DIY passive radiator to the upper surface?

I always prefer a fully closed cabinet, no PR
A PR will make the phase and group delay curves really ugly

 

[-Matt-]

I thought you would probably say something like that!

Anyway, good luck with the build and testing. I look forward to seeing the photos and results.

 

[ppataki]

So here is some progress!

I have managed to remove all the 48 drivers from the original cabinets:

Then we transported the cabinets to my neighbor's workshop and put together the new cabinet
Basically we have re-used whatever we could from the old cabinets - the blue part is new
Aesthetics is not a concern since the cabinet will be under the couch, 100% invisible (so I will not do any paintjob, etc.)

Now the new cabinet is in my garage; I will spend the next few weeks finishing it
Will post more pictures as I make progress

 

[ppataki]

So the work with the hardware part has come to an end, the cabinet is (almost) finished
I will need to do the 'airtightness' test + see if there are any faulty drivers (can happen if you have 48 of them...)

Again, aesthetics is not a consideration since the cabinet will be under the couch, totally hidden

I will not be able to finish testing/listening/calibration/DSP/etc. before Christmas so I wish all of you a Merry Christmas!!

 

[ppataki]

I managed to get some time to play around with DSP and measurements

Here is the frequency curve of the sub alone (1/12 smoothing)

Then I added some pre-EQ before applying Dirac Live 3.8

Unfortunately distortion sky-rocketed.....apparently the placement of the sub is not ideal since there is a cutoff at around 40Hz and the room mode around 24Hz is not very much excited

For comparison see how the same speakers behaved when they were in the corner as MCLAs - I just had to flatten the peaks and it easily went down to <24Hz

Anyway, back to the sub under the couch, after applying Dirac and all the DSP goodies (min. phase crossover at 80Hz, 24dB/octave with SPL and delay compensation, all pre-Dirac so Dirac 'sees' the sub integrated already)

I am pretty satisfied with the frequency response but there are two things that are very annoying:
- the rumbling couch under my a$$ - I know I should have seen this coming but I had to give it a try to experience it - unfortunately I had to realize that this is not my cup of tea
- distortion is audible

So I will revert the whole setup and as a next step I will re-position the sub to the front to see how it measures/sounds there
Since this was a near-zero cost experiment I don't feel bad about it at all - I have again learnt something new = never put a sub under my a$$ anymore
Late next week hopefully I will be able to send measurements about the new position

 

[ppataki]

I had some time to move the sub to another position (in the front between the front speakers)

Here is how the FR curve looks like without any correction:

Now we are talking!

Let's see how it looks with the crossover at 80Hz:

Pretty good! (=no crossover dip)

Now let's see with Dirac:

Distortion:

There is a nasty peak between 55Hz and 75Hz but nothing to really worry about

However the resonances and the decay have become much more prominent:

There are frequencies that resonate beyond my tolerance margin
I am not sure what is causing it but I cannot really live with it
I tried manual EQ instead of Dirac but ended up with the same results too

I guess I will need to terminate this project....anyway it was a great learning exercise and since it was near zero-cost I am not definitely not sad about it

Next year I might try a more traditional approach with a 15 inch DOS subwoofer, just for fun - will see
Happy New Year Everybody!!

 

[audiofooled]

Looks like a fun project! I'm only speculating here, but could it be that your room doesn't like the particular layout? I mean, cutting it in half and standing each one upright could make a difference? So many drivers near the floor over a large area could create some unwanted resonances in ways that just one big driver wouldn't? Just a thought.
Happy New Year.

 

[ppataki]

Looks like a fun project! I'm only speculating here, but could it be that your room doesn't like the particular layout? I mean, cutting it in half and standing each one upright could make a difference? So many drivers near the floor over a large area could create some unwanted resonances in ways that just one big driver wouldn't? Just a thought.
Happy New Year.

Yes, I agree, I came to the same conclusion.
To be honest I will not spend more time/effort with this but rather focus on the 15" DOS sub project that I have in mind - just for fun and more learning

 

[norman bates]

Rumble under couch = fun movies !!!!!..

 

[neRok]

I have only just seen this thread, and it's an interesting test you did. The performance with this "sub" against the wall is decent. Actually I think a good idea would be to try this sub on the back wall and use it as a double bass array (ie. timed to cancel the reflections from your main speakers). It might work in your rectangle box in the middle of the wall, or in the original corner boxes?

Alternatively I wonder if it could work as a cardioid sub, with the speakers one one face timed to cancel the other face. This might not work with them firing in to a common chamber though? Perhaps you could divide it?

Edit: In these results and your new 15" sub results, you can see the room modes ringing because you basically just have a single "sub" playing in to the room. You need something that is timed and/or phased differently to actually get some cancellation happening. DBA will do that by cancelling the reflections at the rear wall, and various sub arrangements can help by reducing the forward reflections at the source. Here's a graphic I've posted before;

 

[ppataki]

I have only just seen this thread, and it's an interesting test you did. The performance with this "sub" against the wall is decent. Actually I think a good idea would be to try this sub on the back wall and use it as a double bass array (ie. timed to cancel the reflections from your main speakers). It might work in your rectangle box in the middle of the wall, or in the original corner boxes?

Alternatively I wonder if it could work as a cardioid sub, with the speakers one one face timed to cancel the other face. This might not work with them firing in to a common chamber though? Perhaps you could divide it?

Edit: In these results and your new 15" sub results, you can see the room modes ringing because you basically just have a single "sub" playing in to the room. You need something that is timed and/or phased differently to actually get some cancellation happening. DBA will do that by cancelling the reflections at the rear wall, and various sub arrangements can help by reducing the forward reflections at the source. Here's a graphic I've posted before;
View attachment 342266

Thank you for the suggestions but this project is killed already so I will not spend any more time/effort with it

I had tried DBA already in my room a few years ago (I posted it here on ASR too) but since my room is not rectangular (it has six corners with an angled wall) DBA does not really work as intended unfortunately

 

[neRok]

Thank you for the suggestions but this project is killed already so I will not spend any more time/effort with it

I had tried DBA already in my room a few years ago (I posted it here on ASR too) but since my room is not rectangular (it has six corners with an angled wall) DBA does not really work as intended unfortunately

Interesting. But like another user posted in that thread, you wouldn't have had a plane wave;

Only 1 small sound source placed on the floor like this is not sufficient to create a plane wave even at low frequencies, and then the major requirement for a DBA/SBA is no longer met. Then the cancellation will not work as intended

I imagine this line array sub would have done much better in that regard. Maybe not though considering your normal speakers aren't doing a plane wave either.