New workshop & 'anechoic' booth DIY.

Hi!

I've spent the last week moving my microphone workshop into the basement of our studio building. The mics are getting the bit at the far end, away from the noisy / messy stuff. I had to do a bit of construction to subdivide the room and put some benches in.



Wall going up...



A door, window and a slap of paint



On the other side, I'm putting in separate benches for testing, building and another bench for the re-ribboning. I got a load of scrap offcuts of kitchen bench tops from the local timber yard. Bargain!



The door on the left is an isolation / semi-anechoic box for testing mics / punishing drummers. The sound treatment is on order, so more about that later.



I had help for a day putting the wall up, but otherwise it really has been DIY. But it's actually been rather fun with the circular saw and impact drivers for a change.

Anyway, loads left to do.... I'll pop up a couple more pix when I'm done.

Stewart

8) 8)

Cool!

Don't forget "machinery corner". I'd suggest somehow enclose that space and keep far from the ribbon test bench. The steel debries will be attracted by the magnets like crazy  .

Best, M

Hi Marik,

Actually that's why I'm doing this - the drill press, machinery, big storage and messy stuff like painting is all in the other room now. It will be a much easier space to keep tidy, and warm in winter.

I'm also paying a lot of attention to draughts.

Well done Stewart!
Semi-anechoic box on the left, Avo MKIII on the right... That's what I'd call : enticing working conditions!

Axel

mad.ax said:

Semi-anechoic box on the left, Avo MKIII on the right... That's what I'd call : enticing working conditions!

Click to expand...

Oh yes, lots of fun. Makes me very motivated!

Here's the chamber of secrets....



Or should we call it the iron Maiden, because of all those (foam) spikes?



I'm getting a fairly flat response from the booth, apart from a big dip around 250 - 350 Hz, which is about the length of the booth. Still more work to do!

zebra50 said:

Here's the chamber of secrets....

Click to expand...

very curious about the construction and how much isolation you managed to achieve thusfar... i want one!

zebra50 said:

mad.ax said:

Semi-anechoic box on the left, Avo MKIII on the right... That's what I'd call : enticing working conditions!

Click to expand...

Oh yes, lots of fun. Makes me very motivated!

Here's the chamber of secrets....

Click to expand...

Get out of here!!!

Stewart, you are officially banned from this site  8) 8) 8)

That's AWESOME!!!

Best, M

Hi Grant (or lack thereof...)

Construction is, from outside in:

Carpet
Roofing insulation (about 3 to 4 inches)
Acoustic foam tiles (3 inches - see photos!)

Before the build, one of the side walls and floor were brick / concrete. The wall support is pretty much like building a stud wall, but without the plasterboard.
The door is simply a stuffed frame, with carpet on the outside.

I'm not shooting for isolation here, although  The workshop is pretty quiet if I turn off the radio (and wait for the tractor to go away). The aim is to stop the sound bouncing around, so I've gone for non-reflective materials as much as I can.

The work now is to try a few different speakers to that I can identify 'room' and 'speaker' effects. Then I'm looking to EQ the remaining bumps out.

I have an old skool pink noise spectrum analyser, plus a some swept-sine software to play with, which should get me most of the way. I'm also looking for a good, affordable concentric speaker to avoid phase effects between woofer and tweeter.

zebra50 said:

Hi Grant (or lack thereof...)

Click to expand...

ah, the perils of having both first and last name being dictionary accessible...  i can thank my norfolk ancestry for the latter half  . 

i like the compactness of the setup.  have you noticed any resonant peaks from the other axial modes in the space, or does the directionality of the driver and microphones being tested make them less consequential?  as an aside: i assume with omni's you'd orient the driver such that the off-axis projection would clear the capsule plane before being absorbed, thus avoiding any false nulling?  admittedly, that might be a gross overthink.

zebra50 said:

I'm also looking for a good, affordable concentric speaker to avoid phase effects between woofer and tweeter.

Click to expand...

took the words right out of my mouth- or fingers, as it seems.  i'll be curious about future findings in the coax realm.

Marik said:

[ 8) 8) 8)

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Thanks Marik!  ;D

grantlack said:

i like the compactness of the setup. 

Click to expand...

And there was I thinking it really needs to be bigger!

have you noticed any resonant peaks from the other axial modes in the space, or does the directionality of the driver and microphones being tested make them less consequential? 

Click to expand...

Not really yet, but I'm working on the bigger issues / bumps first. The plan was to have a sort of 1-dimensional box. I'm sure I'll find them if I start looking, along with the modes for the larger workshop area in which it sits.

If I worried about all the problems at once, I would never get started!

that might be a gross overthink.

Click to expand...

I'm still at the 'move stuff around and see what happens' stage.

What I am please about is that within the booth, positioning is not ultra-critical. I tested a dozen or so mics today with two different monitors, and

1) The booth has a profile
2) Each monitor has a profile
3) Each mic has a profile.

It doesn't seem to matter if you move the mic 10 cm in each direction, you still get the same bumps, although the magnitude may change a little.

Still early days. By the time I've finished this, I'll have the knowledge to build a good one... if not the budget!

I hate you!!! ;D ;D ;D That is sooo cool!!!

Did you build the room with something dense or is it just standerd 2x4's and drywall?

Eric

Hi Eric,

The big wall that divides the workshop (pics at  the top of the thread) is just like a drywall, but I used shuttering board (a tough compressed wood product) instead of plasterboard.

The walls for the microphone booth were built using 2x4 in the same way as you would do a stud wall, but with no boards. Instead of board I used thick carpet on the outside, then filled with dense roofing insulation (actually recycled wool!), then the acoustic foam tiles on the inside. The door and ceiling are the same.

Here's a picture during the construction, which probably explains better than words.




So on three sides at least, the booth is very non-reflective, even at low frequencies. It's quiet here anyway, and total isolation was not the main goal.

Nice!

I set our mic test station in the largest studio room, because it has 12 feet+ ceilings. We're using gated impulse (so no anechoic chamber), and can do a little over 5 ms window.

What I do find is a lot of little response wiggles from nearby mic stands/holders particularly with the B&K pressure calibration mic.

Wrapping 2" cotton thermal insulation on all the stands knocks most of that out, at least at HF.

Getting pretty good results. Managed to automate ARTA to do the whole gated test sequence including reference mic correction.
Put the mic on, enter serial #, one mouse click, and test is done with charts printed/saved.

Bet you get some variation with minor vertical placement of the mics? We do, due to the non coincident drivers. I'm making
a point source test speaker soon using a very rounded shape to control edge diffraction.

It take great care to get good measurements of a pressure gradient mic using a pressure mic standard!

Is that a big pedestal grinder in the first pic? Grinding ribbons to proper thickness, huh?

Anyway, looks great!

Les

Hi Les,

It sounds like you're way ahead on the learning curve on this, so thanks for your observations.

I do have the facility to post-gate the impulse response in the software, although I haven't gone too far with that side of things, beyond observing that it cleans the bottom end up. I need to convince myself that it isn't giving me false confidence.

As I understand it, the principal is simply to gate off the reflections. It's been a while since I've seen Hamming / Hanning windows!  

I've got foam wrapped around the speaker and mic stands.

It takes great care to get good measurements of a pressure gradient mic using a pressure mic standard!

Click to expand...

Yes! It's a big factor isn't it? I'm starting to realise that the omni reference mic doesn't really tell the whole story. But now who makes reference fig-8 ribbon microphones?

Seriously - is there another option?



The tape measure is my friend!

I'm getting better results with the Acoustic Energy speaker, which has smaller spacing and the drivers are mounted diagonally. And really it is just a better monitor that the Absolute 4P, which is really quite kinky. I know coincident is the way to go, but haven't found the right one at the right price yet.

Is that a big pedestal grinder in the first pic? Grinding ribbons to proper thickness, huh?

Click to expand...

Well spotted! The workshop & studio is an old brick cow shed, and the grinder was here when I moved in.  It does actually work, but I doubt it meets modern safety standards.

Cheers!

Stewart

Hi Stewart

Yes! It's a big factor isn't it? I'm starting to realise that the omni reference mic doesn't really tell the whole story. But now who makes reference fig-8 ribbon microphones?

Seriously - is there another option

Click to expand...

It's a good subject for discussion.

Gated impulse works. At Shure in the old days we used the musty old anechoic for HF, and a plane wave tube for LF.
Now they use Soundcheck gated impulse in an ordinary room, as do I with ARTA.

The reason you see smoother bass response is basically because it's not there. The short window smears it away. At 5 ms gate I can go down to about
200. I ought to get much more time before first reflection in the room I have... but i'm geting a reflection off the speaker cab...it's an AR-90 big thing.

You don't need a plane wave tube anymore to do LF . You can do nearfield or impulse response reconstruction.
The trick is you must know the acoustic impedance (a complex number). It can be measured with a particle velocity and pressure mic or calculated.
Then proximity effect is compensated for.

I'm getting good repeatable results to a fraction of a dB if I watch temperature and humidity. FR of speakers change a bit with temp/humidity changes, especially paper cones. My calibration mic is a NIST certified B&K 4145, and it's pretty stable.

We're doing individual response chart for each microphone shipped, so it has to be fast and accurate, and work with various pattern units.

Right now doing SDC subcardioid instrument mics, soon we hope to have a polydirectional ribbon model. We're a new company.

Will talk about microphone measuring all the live long day if you want

Les

So, my initial testing is telling me that the box is indeed a little sensitive to positioning. I have found a 100W concentric driver which will hopefully be a bit better for this application, and will free up my Acoustic Energy monitors to go back into the studio, where they belong.

I'm wondering if it would be better to build a cab for the concentric, or to mount it directly on/into the wall? I will try it and see, but any thoughts in the meantime would be welcome.

Les said:

Right now doing SDC subcardioid instrument mics, soon we hope to have a polydirectional ribbon model. We're a new company.

Click to expand...

I saw some of your posts about your poly directional ribbon over at the mic builders group - sounds very cool and would love to see / hear that when it's ready. I still like those old Oktava ribbons that use labyrinths and rear baffles to achieve directionality.

Will talk about microphone measuring all the live long day if you want

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Sounds good! Although I have to get some work done too.

I was just about to say that the AE22, whilst it might get results as a monitor, is totally wrong for testing purposes! For some bizarre reason, they decided to give it a mid response like an NS10. There's quite a bit in Google about this. AE also state on their website that it's a pragmatic design for mixing, unintentionally not flat.

I would've thought a small Genelec might be the ticket here?

Nice - looks like the studio is coming along well. You don't want a coil winder to go in machinery corner, do you? I have a lovely, Swiss-made one that's designed for fine wire. Alas, I'm beginning to think I might be better off outsourcing wound components as it takes up space and my brain's hurting from doing so many duties.

Look forward to the progress shots.

Justin

Stewart.

If you have room for it a driver mounted in a large wall is  ideal... then you can pretty much forget about diffraction off cabinet edges and such that are within the gate window. It needs to be big and non resonant.

Next best is a small well rounded cab with concentric drivers.

The speaker does not have to be flat (the ref mic takes care of that) but needs to have a smooth polar pattern. Very unflat speakers tend to have
bad polar patterns though, so a smooth on axis response often has advantage.

I bought some 125 mm LF drivers and tiny 12mm neo domes to make a coincident testing cab, but haven't built it up yet. The AR-90 works well with their smooth response and gradient felt covered fronts, but a couple cm error up or down in mic placement can muck up the response due to the vertically spaced drivers. Horizontal errors are not so bad.

Thinking about an automated polar plot measurement as well. I could do it easily enough with USB and an allegro stepper motor driver and a little code.
Little rotating mount. Just need some time to make it.

Les

Thanks guys. The AE22 was just there because I could spare it for initial experiments.

If you have room for it a driver mounted in a large wall is  ideal...

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It is exactly what I did. I now have a concentric 8" driver in the back wall - actually from a 'pimp my ride' style website, but seems decent enough. The results are looking much better, or at least small changes in position make less of a difference. Results are, of course, best close up, within 40 cm of the centre of the driver.

Thinking about an automated polar plot measurement as well.

Click to expand...

Sounds like fun. I'm not nearly ready for something like that yet, beyond 0-90-180 degree tests turned by hand.

You don't want a coil winder to go in machinery corner, do you

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Justin, is that the one you drove down to Cornwall for recently? Although I have the space, I don't have the time to learn how to use one at the moment. And I need a milling machine first.

Actually I was hoping to outsource to you, when you get the hang of it, so keep at it - these things take time!

S.