Topic: Does Size Matter? (J-Disk Question)

[stevetoney]

OK, I've been telling my wife for many years now that size indeed matters  , but taking this very important subject in a different direction, I was wondering if people that are experienced with applying the J-Disk concept can comment on whether or not the size of the disc matters alot with respect to creating separation. 

The reason I'm asking is that I'm now in the process of 'designing' my four mic setup, which will include omni's.  So, I'm contemplating a more compact setup at the top of my stand that includes a fairly closely spaced pair of omni's separated by a modestly sized piece of fur/foam.  Obviously, compact is a good goal, but the reason I'm asking this is that my first priority is sound, so if I have to sacrifice sound/effect for size, I want to err in the direction of sound/effect. 

PS:  People that have experimented with this, you don't have to re-post the pictures of your setup.  I've seen the pictures and I know how and what this looks like...I'm more interested in hearing what you have to say about your results.

Thanks!

[boojum]

I have used my J-disk with DPA 4006 TL's, the mics it was designed for.  I built it to spec, or very close to spec.  I was not overly impressed with it but used it only in one venue on one group, a chorale.  Perhaps I should try it some more.

I would not try to re-invent this thing.  Jecklin is pretty sharp.  Other similar efforts work but I wonder how well.  I  know of no comparison tests.

L8R

Addendum: FWIW an LP is almost the exact size spec'ed by Jecklin.  And that is what I used.  I used spray on contact cement to attach foam. I added a foam strip around the edge and then covered the whole thing in fake lamb's wool.  One of the Japanese groups, NHK I think, used the lambs wools.  I kind of thought that it looked cool.  My LP was an Oral Roberts recording.       

[stevetoney]

I have used my J-disk with DPA 4006 TL's, the mics it was designed for.  I built it to spec, or very close to spec.  I was not overly impressed with it but used it only in one venue on one group, a chorale.  Perhaps I should try it some more.

I would not try to re-invent this thing.  Jecklin is pretty sharp.  Other similar efforts work but I wonder how well.  I  know of no comparison tests.

L8R

Thanks Boojum.  Any others have similar experiences?

[Brian Skalinder]

Like boojum, I built mine very close to spec.  Only unlike boojum, I was very impressed with the results.  (Recording a ~12 member a cappella group.)  I now have a proper retail version that's much nicer than my DIY jobber (though it got the job done), but still need to cobble the gear back together to put it to use.

It makes sense to me that the size should matte.  Bigger the disc, greater the blocking of soundwaves from the opposite side.  I'm just not sure at what point any given size fails to produce, or over-produces, the desired stereo separation (which I think is your question).  I suppose it depends on what degree of stereo separation one finds desirable.  To a point, you can move the mics closer to the disc to increase the stereo separation, so even if the disc is smaller you may be able to maintain the same separation as a larger disc.  But as for what's ideal...dunno, other than the standard disc size.  FWIW, for the couple recordings I've done, I've preferred the mics slightly closer to the disc (~5 cm) than the "official" OSSR mounting of ~8 cm.

I have a vague recollection Moke has tried a couple different sizes/shapes.  If he doesn't jump in, maybe PM him directly?

[stevetoney]

Like boojum, I built mine very close to spec.  Only unlike boojum, I was very impressed with the results.  (Recording a ~12 member a cappella group.)  I now have a proper retail version that's much nicer than my DIY jobber (though it got the job done), but still need to cobble the gear back together to put it to use.

It makes sense to me that the size should matte.  Bigger the disc, greater the blocking of soundwaves from the opposite side.  I'm just not sure at what point any given size fails to produce, or over-produces, the desired stereo separation (which I think is your question).  I suppose it depends on what degree of stereo separation one finds desirable.  To a point, you can move the mics closer to the disc to increase the stereo separation, so even if the disc is smaller you may be able to maintain the same separation as a larger disc.  But as for what's ideal...dunno, other than the standard disc size.  FWIW, for the couple recordings I've done, I've preferred the mics slightly closer to the disc (~5 cm) than the "official" OSSR mounting of ~8 cm.

I have a vague recollection Moke has tried a couple different sizes/shapes.  If he doesn't jump in, maybe PM him directly?

Thanks Brian.  The closer, smaller is what I was wondering.  One of the concerns I have is that back a couple years ago I watched Peter King struggle with his J-Disc setup at an outdoor festy where the disc acted like a wind sail.  One of the reasons I was giving this some thought was just to go for a more compact overall 4 mic arrangement and my initial thoughts were that bringing the mics quite close together with a smaller disc might work.  So far, seems like both of you guys are thinking that may not be good logic, although I guess as you both suggest, the only way to know for sure it to do some testing on my own.  Shouldn't be too hard to make a rudimentary test setup down in the basement with the stereo.  Thanks for the responses guys.

[stevetoney]

The size of the disc determines the cutoff frequency of absorption--when the wavelength gets longer, it effectively just travels around the barrier.  Of course, that is also dependent upon the effectiveness of the absorptive material, so there is a practical lower limit irrespective of the size of the disc.

mshilarious...thank you for your response, but you lost me. 

Perhaps I'm thinking too simplistically or perhaps my thinking has been shaped too much from repeated reading of the Stereophonic Zoom article, but my thinking was that the concept of the J-disc was simply to create a sound buffer of sorts between the two omni mics in order to simulate spacing the omni's apart from each other by distance.  In more detailed terms, I was thinking that the J-disc causes sound waves coming from the left to travel a longer and more diffuse path to reach the right microphone...which is analogous to a spaced omni configuration where the right mic were...say...4 or 5 feet distant from the left mic.  In that manner, the stereo effect is created similar to the spaced array described in the SZ article...IOW via the combination of SPL difference and time difference.

With this in mind, I don't understand 'cutoff frequency' or 'absorption of the disc' in terms of creating stereo effect.  Can you please provide a more technical explanation?

Thanks!

EDIT:
mshilarious...I thought about this some more.  I think I just figured out your response.  The J-Disc must absorb sound waves to keep sound from traveling THROUGH the disc.  Obviously, if the disc doesn't absorb the direct sound coming from the left from reaching the right mic, then there will be no delta SPL or delta time created by the disc.  IOW, the less absorptive the J-disc, the more it would be as if there was no disc at all.

Perhaps I could clarify your previous response, if you don't mind, where you stated...

The size of the disc determines the cutoff frequency of absorption--when the wavelength gets longer, it effectively just travels around THROUGH the barrier.  Of course, that is also dependent upon the effectiveness of the absorptive material, so there is a practical lower limit irrespective of the size of the disc.

I would have to comment though that it's probably the material that determines the absorptive factor, not the size.  However, the size would seem to matter in that it contributes to delta SPL and delta time in creating the stereo effect, as I mentioned in first response (prior to the edit).

Thanks man!

[stevetoney]

Interesting!  Thanks so much for the response!  I'm not sure at this point that I totally understand, but your more detailed explanation does help.  I'll have to give the linked page some quiet time and I'd bet that at that point, I'll have more questions.  LOL.  Thanks again for taking the time.

[boojum]

Interesting!  Thanks so much for the response!  I'm not sure at this point that I totally understand, but your more detailed explanation does help.  I'll have to give the linked page some quiet time and I'd bet that at that point, I'll have more questions.  LOL.  Thanks again for taking the time.

You don't have to understand electricity to turn on the lights.  Just try it.    ;o)

[Massive Dynamic]

Here's a related thread. http://taperssection.com/index.php?topic=110646 I am also wondering if my tiny disc (photos at bottom of thread) really provides any real stereo separation. Isn't that the point of the j-disk? I've also assumed that as long as the disk came between the left mic and right PA and vice versa, I'd set it up right. Does distance from mic to source also figure into this equation (closer [steeper angle] = more separation)?

[Tedley]

I am exploring a j-disk idea while waiting for my mics to come. I just got an M-10 a few days ago and was disappointed with the sound stage (to say the least), but found the mics sort of pleasant sounding. I made a 6" disk of 1/2" styrofoam and cut a notch into the center so it slips onto the front of the M-10, putting the mics at center. As the M-10 mics are less than 1" from the disks I thought a smaller disk might give better results. I cut a bit off the bottom back edge of the disk so I could put on a mini flexible stand. I put some fake fur under this setup on the table and glued some fur on the disk. I have only recorded two of us playing acoustic instruments at home thus far, but the results are encouraging. In an A-B test with and without the disc I found the stereo effect definitely improved with the disc. I will experiment with other size discs and mounting it on a stand.  Of course this is a bit of a silk purse from a sow's ear, but it may be useful for someone and seems a cheap improvement to the M-10 as a stand alone recorder.

[boojum]

You would be well advised to do your homework on the Jecklin disc before you build one.  Jecklin arrived at his disc size after much experimentation.  He is specific in its dimensions.  It is not just a disc of approximate size with mics at approximate distances and angles.  This is all about physics.

Close counts in horseshoes and A-bombs only.     

[stevetoney]

You would be well advised to do your homework on the Jecklin disc before you build one.  Jecklin arrived at his disc size after much experimentation.  He is specific in its dimensions.  It is not just a disc of approximate size with mics at approximate distances and angles.  This is all about physics.

Close counts in horseshoes and A-bombs only.     

So if I build a disc that's, say 2 inches smaller diameter than Jecklin's, the sound will be hosed and there will be no stereo effect?!?  Seems like an extreme statement to me, but I've never experimented with a disc before.

[Will_S]

You would be well advised to do your homework on the Jecklin disc before you build one.  Jecklin arrived at his disc size after much experimentation.  He is specific in its dimensions.  It is not just a disc of approximate size with mics at approximate distances and angles.  This is all about physics.

Close counts in horseshoes and A-bombs only.     

So if I build a disc that's, say 2 inches smaller diameter than Jecklin's, the sound will be hosed and there will be no stereo effect?!?  Seems like an extreme statement to me, but I've never experimented with a disc before.

Of course not.  Jecklin found, after experimentation, what sounded best to him.  You may find something else you prefer, or need to make other compromises due to the "sail" effect outside, etc.

Keep in mind that we all perceive the stereo illusion differently, even given the exact same speaker setup (which we don't use).  There's no absolutely correct configuration.  The experiences of others are useful guides to relative changes (e.g. how will a large disk compare to a smaller disk in perceived separation, how will NOS compare to XY-90) but there is no objective exactly correct amount.

Add:  Also, you talk about a 4 mic setup in the original post.  Is the idea to mix all 4 mics down to stereo at some point, or have separate recordings to pick from later?  If you are going to mix the two stereo sources, it makes even less sense to worry about matching some theoretical ideal for one set of mics because any delicate perfection of that stereo image will be changed when you mix in the second source.

[stevetoney]

You would be well advised to do your homework on the Jecklin disc before you build one.  Jecklin arrived at his disc size after much experimentation.  He is specific in its dimensions.  It is not just a disc of approximate size with mics at approximate distances and angles.  This is all about physics.

Close counts in horseshoes and A-bombs only.     

So if I build a disc that's, say 2 inches smaller diameter than Jecklin's, the sound will be hosed and there will be no stereo effect?!?  Seems like an extreme statement to me, but I've never experimented with a disc before.

Of course not.  Jecklin found, after experimentation, what sounded best to him.  You may find something else you prefer, or need to make other compromises due to the "sail" effect outside, etc.

Keep in mind that we all perceive the stereo illusion differently, even given the exact same speaker setup (which we don't use).  There's no absolutely correct configuration.  The experiences of others are useful guides to relative changes (e.g. how will a large disk compare to a smaller disk in perceived separation, how will NOS compare to XY-90) but there is no objective exactly correct amount.

Add:  Also, you talk about a 4 mic setup in the original post.  Is the idea to mix all 4 mics down to stereo at some point, or have separate recordings to pick from later?  If you are going to mix the two stereo sources, it makes even less sense to worry about matching some theoretical ideal for one set of mics because any delicate perfection of that stereo image will be changed when you mix in the second source.

Theoretical ideal always seems to be true when it comes to mic placement discussions.  That's a common conclusion when discussion gets really technical and detailed about the Stereophonic Zoom article.  That's also why I thought the statement that the J-Disc must be the size that Jecklin concluded because of physics and that statements such as 'close only counts in horseshoes and h-bombs' to be a fairly extreme opinion with respect to being able to hear and differentiate material differences in the music that's recorded.

[mfrench]

I've played with baffled omnis probably as much as Jecklin, and probably far more than any other live music recordist.
Back when I started experimenting with them, there was little to no information about the technique, just a few stringent technique parameter roadblocks.

Jurg Jecklin gave us a wonderful technique, then he dropped the ball.
Baffle size has effect but the effect is also very much related to size and distance of mics from the disc face.
I've had outstanding results with discs ranging from 4" -> 15", by setting the mic span accordingly to the size of the baffle; Ex: The 4" baffle had the mics within a fraction of an inch from the disc face, with the DPA 406x micros.  The 15" baffle (square baffle) was used with LD's.  Both of these mic types were supposed to be no-no's according to Jecklin. Guess what? They work perfectly fine if you approach with an open mind that thinks away from fixed stringent unwavering parameters.
They can also range up as large as 18' -> 20' in diameter as studied by a gent in Utah
No, there is no such thing as a fixed disc size parameter unless you like limiting yourself.