The View Up Here

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Posts Tagged ‘EM172’

Microphone Self-Noise vs. Recorder Equivalent Input Noise

Posted by Tom Benedict on 25/08/2016

Yet another attempt at combining math and sound recording… Ye have been warned!

A number of threads on a number of field recording forums revolve around a simple question: I have X amount of money. Where do I throw it to improve my recording?

An obvious and common answer is, “upgrade your pre-amps!” This can be done a couple of different ways: The first is to trade out your recorder for one with better pre-amps. The second is to send your recorder to a shop to have the pre-amps changed out for better ones. The third is to buy an external pre-amp like the Sound Devices MicPre or MicPre-D, and plug it into the Line-In jack on your existing recorder.

But is that always the right approach?

A bunch of head-scratching, web-searching, and number-crunching led me to the conclusion that it’s not as obvious as it might appear. A number of factors come into play: noise level, sound quality, build quality, ergonomics and convenience, useful features of the gear, battery life, etc. Of these, the easiest to tackle from a quantitative standpoint is noise, so that’s where I’m starting.

Most of the calculations I’m doing are spelled out in an article on the RANE web site titled, Selecting Mic Preamps. The first set of calculations help you determine the maximum pressure levels a particular mic/pre-amp combination can handle. Since the field recording I’m doing involves quiet sources I skipped that bit and went to the second set of calculations. These help you determine the level of self-noise a given combination of mic and pre-amp will have.  To run the calculations you need information about the mics as well as the pre-amps.

(If you’re recording loud sources that first set of calculations may be of use to you! You don’t have to skip them just because I did.)

Right now all of the mics I own are based off of Primo capsules: BT-EM172, BT-EM158, and BT-EM184. The data I used for the mics all comes entirely from the Primo datasheets.

I currently own two recorders: a Tascam DR-05 and a Tascam DR-70D. In the spirit of this question I’m looking at two competing solutions: one is to buy a new recorder, a Tascam DR-680 MkII, and the other is to buy a used Sound Devices MicPre to use as an external pre-amp. The data I used for the recorders comes from a mix of sources, the most important being the Avisoft Bioacoustics Microphone Input Noise Comparison website. The rest came from the manufacturer’s datasheets.

The RANE calculations require the self-noise and sensitivity of the mics in question. From these you can use Table 3 in their article to calculate the mic output noise. For all of these I’m using A-weighted noise values for the mics and recorders. A-weighted noise levels are scaled for the auditory response of a normal human. They tend to be about 5dB more optimistic than their non-weighted counterparts. So long as I stick to A-weighted for both, I’m comparing apples to apples. The numbers for my mics and for the DPA 4060 omni by way of comparison are:

  • DPA 4060
    • Self Noise 23dBA
    • Sensitivity -34dB
    • Mic Output Noise -103dBu A-weighted
  • EM172
    • Self Noise 14dBA
    • Sensitivity -28dB
    • Mic Output Noise -106dBu A-weighted
  • EM158
    • Self Noise 20dBA
    • Sensitivity -32dB
    • Mic Output Noise -106dBu A-weighted
  • EM184 Cardioid
    • Self Noise 22dBA
    • Sensitivity -39dB
    • Mic Output Noise -110dBu A-weighted

The RANE article says that when you compare the output noise of the mic to the equivalent input noise of the pre-amp, you really want to see a factor of -10dB lower noise in the pre-amp or better. A -10dB lower noise in the pre-amp means it’s only contributing 0.4dB of noise to the final signal. Looking at the recorders I’m using, along with the two I’m considering, their EIN levels are:

  • Tascam DR-05 EIN -109dB A-weighted
  • Tascam DR-70D EIN -120dB A-weighted
  • Tascam DR-680 MkII EIN -127dB A-weighted
  • Sound Devices MixPre -126dB A-weighted

Here’s how I’m reading this:

If I plug any of these mics into my DR-05, the noise from the recorder’s pre-amps will be the limiting factor. Getting a better mic won’t improve my sound with that recorder.

My DR-70D is -12dB lower noise than the EM172 that my go-to mics are built around. In this case the mic’s own self-noise is the limiting factor. Switching to a DPA 4060 won’t help from the standpoint of noise, either. (I’m not mentioning any improvements in the character of the sound, mind you.) This does imply that I’m coming up on the limits of my pre-amps with the EM184 cardioid mics.

Switching to either a DR-680 MkII or a MixPre certainly wouldn’t hurt, and the higher quality amplifiers on either device may improve the sound in other ways, but it probably wouldn’t help the noise much overall because the mics would still be the limiting factor. At most I could improve my noise levels by a tenth of a dB.

Conclusion:

Unfortunately what this means is that to make any substantial improvement in the noise level of my recordings, I need to upgrade both my recorder and my microphones. Upgrading either one without the other really won’t buy me that much.

The Real Conclusion:

This leads to the next obvious question: Have I reached a point from which the only way to improve my gear is to throw orders of magnitude more money at it than I already have? (Or to word that only slightly differently, more money than I have at all?)

In short, is this it?

(Or is this the excuse I need to stop improving the gear I’m using and start building parabolic mics?)

Tom

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Posted in Audio, Engineering | Tagged: , , , , , , , , , , , , , , | 2 Comments »

DIY Microphone: EM172 Capsule and XLR Plug

Posted by Tom Benedict on 05/03/2016

This is the last in a four part series about powering the Primo EM172 microphone capsule. Part 1 outlined the problem of how to provide 5-10v to the capsule and predicted some results. Part 2 shared some results and pointed out that the gain differences between inputs on my recorder invalidated my predictions. Part 3 discussed my reasons for going with XLR connectors on all my microphones, and some of the details of that. This last part puts it all together into a step-by-step DIY for building microphones with Primo EM172 capsules, powered by 48v phantom power on an XLR plug.

If you need to build a microphone based around the EM172 capsule that plugs into the 1/8″ mic jack on your recorder, or a laptop, tablet, whatever, there are already several excellent tutorials out there. Rather than adapt this one to your needs, refer to one of the existing tutorials. The two I used when I first started building EM172 microphones were the ones on Zach Poff’s page and the one on Wild Mountain Echoes.

In this DIY I’m going to assume you already have a plan for making a mic body. I made mine out of Delrin bar stock on a lathe. Others have used Sharpie pen caps, which also provide a nice clip for clipping the mic to things (see the tutorial on Wild Mountain Echoes), PVC pipe, brass tubing, etc. When mounting the mic in the mic body, make sure the front of the capsule is flush with or slightly proud of the mic body. Don’t recess it. I made that mistake with my first set of mics and wound up with mics that sounded like they were inside a sewer pipe. If in doubt experiment by wiring up the mic completely, plugging it in, and listening to it as you slide it in and out of the mic body you plan to use. After all, this is DIY. Experimentation is part of the deal.

Primo BT-EM172 to P48 XLR Wiring

Credit for the circuit goes entirely to David McGriffy, and credit for the component choice goes entirely to David McGriffy and Ricardo Lee. Ricardo Lee’s writeup, SimpleP48wm61, goes into the theory of the circuit and the reasons for the component choices in depth. It’s the real reference for this. (In order to use that link to download Ricardo’s file, you may need to be a member of the micbuilders group on Yahoo!. If you’re doing this DIY you’re a mic builder, so it’s not a stretch.)

EDIT: A couple of weeks ago Akira So brought to my attention that I had the capacitor poloarity reversed from how David McGriffy and Ricardo Lee have it in SimpleP48. I’ve since corrected the schematic here. Credit where credit’s due.

EDIT: Akira also pointed out that my value for R (120k) resulted in something like 1.3-1.5V at the capsule. I experimented with a number of resistors to see what value of R would produce 7.5V at the capsule on my recorder, and for a Tascam DR-70D, R=40k produces just over 7.5V. When you do this build, you will have to find what works best for your equipment.

EDIT: I also swapped the supplier for the EM172 from Frogloggers to Micbooster (FEL Communications). I haven’t heard from Gene at Frogloggers in a while. Hoping he’s doing ok.

For my build I used the following:

I also used some metal tape (copper in my case, from the local gardening center), heat shrink of various sizes, and the solder I found on the bench in the lab. (My Alphametals solder I’ve been using for the past 20 years isn’t ROHS certified, so I can’t say “use this stuff, it’s great!”)

Not including the tools necessary to fabricate the mic bodies, you’ll also need:

  • Soldering iron (temperature regulated if possible)
  • Source of heat for heat shrink (heat gun, lighter, etc.)
  • Assortment of wire cutters, strippers, fine tip pliers, etc.

Since most of the bodies people use for these require the mic to slide in  from the front end of the housing, we’ll start with the mic capsule.

EM172 Back End

The first step is to strip one end of the cable, trim back the red and white wires to a workable length, and still leave plenty of shield exposed. The red and white wires are then soldered onto the appropriate pads on the capsule.

Warning: The EM172 capsule is sensitive to heat. These two photos were made with a capsule I’d killed using an unregulated soldering iron, which is why the capsule looks a little ugly. If you have access to a regulated iron set your iron no higher than 735C and don’t hold the iron on a pad for more than a few seconds. If you don’t have access to a regulated soldering iron, be sure to get EM172 capsules with stub leads already soldered in place. The tutorial on Wild Mountain Echoes uses capsules with stub leads, so you can see how she did it. Do all your work on the stub leads. Don’t fry your microphones!

EM172 With Wires

Now we build the shielding around the capsule itself. Insulate the sides and back of the capsule with some heat shrink.

Capsule Isolated

Be sure to account for every strand in the shield as you bring it up and over the heat shrink. Wrap with foil tape and trim back the shield so no wires protrude. Be sure no wires cross over the heat shrink and touch the front of the capsule.

Making a Shield

Apply a second layer of heat shrink over the foil tape. I like to apply a short length of colored heat shrink to help me identify which mic is which when I’m running wires and plugging things in out in the field.

Heat Shrunk Ready To Go

At this point go ahead and run the mic cable through your mic body, but don’t mount the capsule just yet. Once you’ve soldered the connector end of the cable, it’s a good idea to test everything to make sure you didn’t make any soldering mistakes, and to make sure the capsule didn’t get damaged during soldering. Strip the other end of the cable, leaving a little more wire to work with than on the capsule end. Thread the wire through the end cap for the XLR connector and set it aside. Since the XLR connector provides its own shield you don’t have to do any metal tape trickery on this end. Gather the wires from the cable’s shield, twist into a bundle, and cover with heat shrink tubing. This is also a good time to apply a length of colored heat shrink to match the capsule end of the cable.

Cable Prepped With Shell

Grab the XLR connector body in a vise or some other holding fixture. If you don’t have a vise, a set of vise-grip pliers with tape over the serrated part of the jaw works well. Just don’t grab it so hard that the connector body is damaged or distorted. Another way to hold these connectors that works great is to have the mating connector screwed into a board. Plug the connector you’re working on into its counterpart and solder to your heart’s content. (I used a vise.)

Trim back the leads on the capacitor and resistor to something reasonable that’ll fit inside the XLR connector. Save the snipped off bits of the leads. One of these works well to bridge from pin 1 to the ground tab.

Resistor and Capacitor

Solder a leftover component lead from pin 1 to the ground tab. Next, solder one end of the resistor to the ground tab as well. Next, solder the (+) end of the capacitor to pin 2. Finally, tie the two free ends of the capacitor and resistor together.

XLR Plug with McGriffy Components

All that’s left is to solder the cable onto the plug. Red goes to pin 3, white goes to the (+) lead of the capacitor as well as the free end of the resistor, and the cable’s shield is soldered to the ground tab. (In this photo the connector is rotated 180 degrees from how it’s drawn in the schematic, but that’s how the solder cups are oriented. Flip it around in your mind and it’ll make sense.)

XLR Plug with Cable

At this point your microphone’s electronics are finished. Put the connector together and screw things tight.

This is a good time to test the mic to make sure nothing went wrong. Plug it into your recorder, turn on phantom 48v power, and dial up the gain. If all went well you should have a low noise microphone ready to be installed in its mic body. If not, go back and check each step to find out what went wrong.

Finished Mic

Have fun recording!

Tom

Posted in Audio, Electronics, Engineering | Tagged: , , , , , , , , , , | 88 Comments »

Powering the EM172 Capsule – Part 3: Capitulation

Posted by Tom Benedict on 23/02/2016

I made up my mind about powering my EM172 microphones. Ultimately this decision had less to do with how I was powering the microphones than how I was plugging the mics into the recorder. One of the things I discovered when I wrote my last post was that the Tascam DR-70D uses completely different amplifiers for the XLR inputs and the 1/8″ inputs. Different form factor, obviously; different impedance; different gain. It’s that last part that really drove this decision.

The gain ranges on the 1/8″ plug are +3dB, +11dB, +26dB, and +38dB. The XLR gain ranges are +21dB, +36dB, +51dB, and +63dB. While I was performing side-by-side tests I kept having to crank back the gain on the XLR input to match the levels on the 1/8″ input. As I tested with quieter and quieter subjects it finally hit me: +38dB of gain just wasn’t enough to bring up the levels of some of the subjects I want to record. The XLR input gave me more gain to play with. The last test I ran was what finally convinced me. Even with the gain cranked all the way up on the 1/8″ input mics, I couldn’t get the sound levels over -25dBFS. The recording was just too quiet to use. I cranked up the gain on the XLR input, and was able to get -12dBFS with the same subject.

Good news is the mics really do perform better with the 9.6v bias voltage David McGriffy’s circuit provides. So this is a win-win.

The lavalier mics were no problem to convert. I bought a stash of Neutrik XLR connectors when I started this whole investigation, so it was just a matter of lopping off the 1/8″ connectors and soldering up the XLRs with the resistor and capacitor from McGriffy’s circuit.

XLR-Converted Lavalier

My SASS was another story. I really hate having things with cords that can’t be unplugged, so I wanted to connectorize everything and use extension cables. Only problem: I’m a beginner! So I had no idea how all the connectors worked.

After some Googling and image searching I learned that:

  • XLR extension cables are gender-inspecific. One end is male, the other is female.
  • Female XLR connectors are the ones with the latch. This is true of both panel and cable connectors. So female panel connectors have a latch, but male panel connectors don’t. (This confused me.)
  • Neutrik makes a crapload of XLR connectors you can choose from. It’s worth looking them up in multiple catalogs to find out which series were developed to fix the bugs in previous series. Though it’s really hard to go wrong, so long as you get all the genders right. These things are built like tanks.

I picked up a pair of pre-built 10′ extension cables for a little over the price of the connectors themselves along with some male panel jacks to install in the SASS. Installation meant cutting into the back of my SASS, but it went quite smoothly and the results look (and sound!) nice. (Yeah, this is an infrared photo. Ironwood trees look like Dr. Seuss trees in the IR, so I just had to play.)

SASS Back in the Field

Meanwhile I figured it was finally time to solve the issue of wind protection. A few months back I learned I’m really REALLY bad at sewing fake fur. I did some reading since then, so I think I know what I did wrong. But rather than getting stalled on my own lack of sewing skill I ordered a pair of lavalier windscreens from Cat Ears. They fit over my oversized mic bodies, but they’re too small to go over a foam windscreen. I probably needed the larger ones. They do a decent job by themselves, but in wind over 15-20kts the mics still suffer from wind noise. Good enough to use the lavs as tree ears, but not enough to use them at the beach in solid wind.

Cat Ears Windscreens

Now I just need to solve the issue of wind protection for my SASS. Back to learning to sew fur…

In any case my gear and I are off the soldering bench and back out in the field. Finally. YAAAAAAAY!

Tom

Posted in Audio, Electronics, Engineering | Tagged: , , , , , , , , , | 3 Comments »

Powering the EM172 Capsule

Posted by Tom Benedict on 16/01/2016

Chris Hass wrote a very nice article on building microphones around the EM172 capsules on her site, Wild Mountain Echoes. In it she mentions the issue of power. The datasheet for the EM172 specifies a supply voltage of 5-10v, but most handheld recorders supply something considerably lower than that. Chris and I compared notes, and her Sony PCM-M10 and both of my Tascams supply something closer to 2.3-2.7v. My question to her was how this affects performance, and what my options are for doing something about it.

Chris suggested bypassing the recorder’s own built-in power and using an external battery box to supply a higher voltage to the mic. She pointed me toward the boxes made by Church Audio. I followed her advice and bought a Bat 2B from them. It should be here in a couple of weeks.

Since my 70D has XLR inputs I decided to pursue another possibility as well. Most recorders can supply 24V or 48V phantom power on their XLR inputs. The only trick left is to drop that down to the 5-10V the microphones want. I ran across a thread on the Yahoo! micbuilder forum that referenced a circuit by David McGriffy called Simple P48 WM61 (referring to a simple circuit to power the Panasonic WM61 microphone from 48v phantom power). Richard Lee uploaded a document to the forum describing McGriffy’s circuit, along with modifications for using it with (you guessed it) the EM172 capsule. I still have a bunch of Mogami cable left over from building my earlier mics, so I ordered the remaining parts for McGriffy’s circuit from Mouser Electronics. he parts should be here in a couple of weeks as well.

In the meantime I figured it would be a good mental exercise to try to predict what each of these approaches would buy me in terms of performance. All of this ties back to a set of graphs on the micbuilder forum. It’s in Files/EM172/Primo EM172 Sens Noise vs RL VL.pdf. The graphs show the performance of the EM172 capsule as a function of supply voltage and input impedance. Using a battery box or McGriffy’s XLR circuit will let me change the supply voltage, but the input impedance is a function of the recorder. Here are some cases:

 

Tascam DR-05 and DR-70D 1/8″ Inputs:

Both the Tascams supply just under 3v for plug-in-power. The input impedance on the DR-05 is 25k ohms, and the DR-70D is 10k ohms. The graphs only go up to 10k ohms, so I’m using that number for both cases. Bumping the supply voltage from 3v to 9v should have the following effect:

Sensitivity: -38.6dB -> -36.7dB (smaller negative numbers are better)
Noise Floor: -112.7dB -> -116.1dB (bigger negative numbers are better)
S/N: 74.1 -> 79.4dB (bigger numbers are better)

In reality the DR-05 should get a bigger bump since its baseline performance will be lower than at 10k ohms, judging by the trend in the graphs. But the preamps on the DR-05 are noisier than those on the DR-70D, so I may not be able to hear the improvement.

 

Tascam DR-70D XLR Input:

The DR-70D’s XLR inputs have an input impedance of 2k ohms. Since I’m starting at 5V it should have the following performance:

Sensitivity: -38.3dB
Noise Floor: -116.8dB
S/N: 78.5dB

The noise floor is better than on the 1/8″ input, but the sensitivity won’t be quite as high. If I re-sized the resistor in the McGriffy circuit to provide something closer to 10v I’d get the following results:

Sensitivity: -37.7dB
Noise Floor: -116.8dB
S/N: 79.1dB

No change in the noise floor, but the sensitivity would improve by another 0.6dB. I’m not sure I can hear that, so it’s probably not worth dinking with.

 

Sony PCM-M10:

I also ran the numbers for Chris’s recorder. The Sony has an impedance of 3.9k ohms. Bumping from 3V to 9V should have the following effect:

Sensitivity: -37.8dB -> -37.2dB
Noise Floor: -115.1dB -> -116.5dB
S/N: 77.3dB -> 79.3dB

Almost 1.5dB improvement in noise floor, and 2dB overall improvement in signal to noise.

 

Sony PCM-D100:

The input impedance of the higher-end companion to the M10, the PCM-D100, is 22k ohms. It should see a similar performance bump to the Tascam DR-05, but since the preamps on the D100 are so much better than the DR-05, this will likely make for an audible improvement in the performance of the mic.

 

From the standpoint of mic performance, both approaches provide a clear gain. Whether my ear is sensitive enough to tell the difference remains to be seen (or heard!) From the standpoint of convenience, additional gear complexity, etc. each one has its pluses and minuses.

On the up side, the Church Audio battery box supplies 9V and will work with any recorder with a 1/8″ input, so I can use it on both of my recorders. Another up side for me, personally, is that so far I’ve built all my EM172 mics with 1/8″ plugs, so it requires the least re-work in order to test. On the down side it means I have to add a 9v battery, battery box, and cable to my setup. Velcro will go a long way toward making this a non-issue (mostly), but I wish this kind of thing could be designed in from the get-go. (Recorder manufacturers take heed! Being able to dial in a particular plug-in-power voltage would be nifty!)

The up side with the XLR approach is that from the standpoint of gear it amounts to changing the plug at the end of the cable. All of the circuitry fits inside the XLR plug. As an added bonus I’ll be able to plug EM172 mics into all four XLR inputs on my 70D, which is pretty darned cool. (The 70D only has one 1/8″ plug, which is tied to channels 1 and 2 only. Up until now I’ve only been able to do two channel recording on my four channel recorder.) The down side is that the 48v phantom supply on the 70D is a battery hog. So even if it works it means I’ll have to pack extra batteries or an external battery pack.

Good news is neither approach was all that expensive, and even with the Bat 2B or the external battery to compensate for the extra load from the 48v phantom power, neither adds too much bulk to my bag. For the moment I’m looking at it as having more options rather than having to choose between one approach or the other. In the extreme case it would give me the ability to plug two mics into my DR-05 with the Bat 2B, and another four into my DR-70D using XLR plugs. Six channels at once!

Now all I need is a subject that actually needs six channel audio. But that’s for another day.

Tom

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Pseudo-SASS Array

Posted by Tom Benedict on 03/09/2015

Following the successful testing of the new mics, I turned my attention toward building a pseudo SASS array. My prototype design was a copy of a copy of a copy of… well… a copy. This becomes important later, because it’s lessons learned from one of those copies that drove part of my final design. First, a bit of history, mostly gleaned from Vicki Powys’s web site:

The SASS, or Stereo Ambient Sampling System, was designed by Michael Billingsley in 1987 for Crown International. It was (and still is) aimed at recording music performances, audiences, and other subjects that lend themselves to stereo recording. It offers relatively strong stereo separation in a small package. Crown sold the SASS with microphones built into the system, which worked well for its intended purpose. But the original microphones exhibited a high noise floor, and weren’t well suited for quiet subjects. Lang Elliott and Michael Billingsley modified a Crown SASS to work with higher-end microphones, and Walter Knapp took that concept and offered re-tooled Crown SASS units that would take, among other mics, the Sennheiser MKH20. This made it a viable choice for recording quiet ambient sounds and field recording.

More variations tailored to field recording continue to be made. Rob Danielson’s PBB2N, built out of wood and PVC pipe along similar lines to the Crown SASS, takes the same range of microphones as the units made by Walter Knapp, and offers better bass response. Vicki Powys, concerned about the weight of a wood array, took that concept and made her own version out of dense closed-cell foam (pool floaties) using Primo BT-EM172 capusles. After building her foam SASS, she did side-by-side tests with it against a Crown SASS with MKH20 microphones. The performance of the two were remarkably close.

The idea behind the Crown SASS, Rob Danielson’s PBB2N, and Vicki Powys’s SASS-LN2, is to baffle the microphones so that the array acts as a pseudo-binaural stereo pair similar to the human head. The wings act as boundary surfaces, and the foam baffle in the center partially absorbs sound from unwanted angles. The end result is a higher gain than a bare microphone, and considerably better separation than two microphones arranged as an X-Y pair. This photo of a Crown Audio SASS-P MkII was a later model that used PZM mics, but the overall shape didn’t really change much from the original SASS:

I built my prototype based off of measurements I took from a photo of an original Crown SASS, scaled to the overall size of Vicki Powys’s SASS-LN2. I wanted to test the idea before leaping in and building an airborne unit, so I built the prototype out of 1/2″ foam core board. The unit provided quite good stereo separation, and had considerably higher gain than the built-in mics on the Tascam DR-05, but it lacked bass punch. I wanted to figure out why before going from prototype to final design.

One clue came from another of Rob Danielson’s designs: PBMB2. His design calls for significantly larger wings than the original Crown SASS. The larger wings provide a larger boundary surface for the microphones to work with, and therefore provide gain at lower frequencies than the original from Crown. Since I’m planning to use this in the air, cross-section is an important design consideration I need to take into account: larger objects are more easily affected by the wind than smaller ones. Rather than using the larger wings from Rob Danielson’s PBB2N array, I stuck with the dimensions of the Crown SASS.

Another clue came from a set of posts on the micbuilders Yahoo group: Electrets mics need to be mounted flush with the end of their enclosures. Mine aren’t. They’re recessed several millimeters into their tubes and hide behind a layer of stainless mesh. Recessing the mics this way colors the sound they pick up.

Mono Mic Assembled

Unfortunately I learned this late in the game, after I’d already built all five of my mic enclosures as well as my airborne pseudo-SASS. Before going out and designing all new enclosures I decided to test this for myself. I disassembled one of my mics and tried sliding the mic deeper into the tube. I found that the more recessed it was, the more mid-range gain I got, and the less bass. Finally I pulled it out entirely, bare to the world, and tried it that way. I could easily tell the difference. There was a lot more bass, and the mic sounded a lot less tinny. (Hey, if I can tell the difference, it’s HUGE!) Time to design all new mic enclosures!

The last clue came from the folks at DIY Boundary Mics. They ran some tests on the array built by Vicki Powys and the modified Crown SASS with Sennheiser MKH20 microphones. Rob D. (Rob Danielson?) from DIY Boundary Mics noted that Vicki’s foam array lacked some of the lower frequency response the Crown SASS / MKH20 combination had. He attributed it to the soft nature of the boundary surface (foam). Paul Jacobson at DIY Boundary Mics ran a comparison between Vicki Powys’s array and a similar one made of untreated wood, similar to Rob Danielson’s array. The untreated wood array recovered some of the bass lost in the foam array. This agrees with Rob D’s conclusions about the hardness of the boundary surface.

Which leads me back to the prototype I built out of foam core. The outer surface of the foam core is relatively hard, but the foam itself is acoustically thin, and the foam core board has a high natural frequency. I’m guessing that some of the lack of bass punch in my prototype can be traced to the material I used to build the array and the lack of damping material in the array’s inner cavity. I needed something better.

Years ago I made a kite aerial 4×5 film camera out of birch plywood. I never was completely happy with the photos it produced, but it turned out super pretty. Since Rob Danielson was making boundary array mics out of wood, and since the wood SASS had better bass performance than Vicki’s foam one, I figured I could build mine out of wood as well.

I already had some 0.200″ baltic birch plywood left over from the 4×5 camera, so that’s what I used for the array body. The woodwork came together relatively quickly, but I couldn’t finish sealing up the box until I had the damping material glued in place. Here’s one problem with living on an island: no one sells acoustic materials. Rob Danielson used carpet padding in his PBB2N, so I went that route. Here’s another problem with living on an island: stores that sell carpet padding don’t like breaking up rolls! I finally wound up at Home Depot. I waited patiently in the flooring department until someone could help me. I’d already been to several stores, and had received more than my fair share of blank stares when I asked for one foot of carpet padding. I wasn’t expecting much.

The folks at Home Depot surprised me! When I asked for such a small amount, the guy in flooring said, “You building a speaker box or something?” “A microphone array, actually, but it’s the same idea.” “Cool!” He was super helpful, and sent me home with my one foot roll.

Eventually my DIY-SASS came together. It’s shown here with my original mic enclosures, but in the next few weeks I’m planning to swap them out with flush mounts:

DIY-SASS

It uses the same Primo BT-EM172 capsules as Vicki Powys’s array, though she used four and I only used two. The covering for the baffle gave me fits until I finally bent to common wisdom and used sheet metal. (I’d wanted to make it out of the same plywood I’d used for the rest of the SASS for cosmetic reasons, but I ran into structural issues.) The hardware store sold 6″x12″ aluminum for almost the same cost as 6″x12″ polished stainless, so I went with the stainless. But I had to bead blast the outside of it to keep myself from going blind when I took it out in the sun.

DIY-SASS Front

There are a lot of screw holes on this, both to hold the baffle cover in place and to attach the array to a tripod (or a KAS rig!) I’ve seen too many wood screws strip out over time, so I epoxied T-nuts into each screw hole to provide machine threads. Since  I do a lot of my KAP along the coast and plan to record sound in that environment as well, I went with as much stainless hardware as I could. Even so, I’m going to have to open the unit periodically to check the wiring for salt contamination. (One more reason to be glad I used T-nuts!)

DIY-SASS Bottom

Since I’m planning to use this on the ground as well as in the air I didn’t want to wire in a dedicated cable. I’ll only need 2-3′ for aerial work, but on the ground there’s good reason to put some distance between a microphone and the recordist. Having a way to swap cables seemed like a good idea, so I wired it with a 1/8″ TRS jack so I can use the cable of choice, depending on what I’m doing.

I’ve now used my pseudo-SASS in the field several times. I was pleased to find that the heavier construction worked, and that I got back a lot of the bass punch I’d lost with the foam core prototype array. I’m looking forward to trying it with the flush mounted microphone enclosures to see how much more bass I can recover.

Meanwhile I’m facing yet another design problem. Like any microphone, my pseudo-SASS array suffers from wind noise. I learned this the hard way while trying to record the sound of waves crashing on rocks.

In Dire Need of a Windjammer

The wind buffeting was more than the mics could handle, so I wrapped the whole thing with my folded up t-shirt. Even that wasn’t enough to cut the wind, so none of the files were usable. Bummer!

Unfortunately the wind there was nothing compared to the wind I’ll get when I hang this thing from a kite line. And since Rycote and Rode don’t make windjammers for DIY mic projects I’ll have to build my own. My last act of the weekend was to order a yard of 2″ pile 100% polyester artificial fur with the loosest backing I could find. As I finished checking out I couldn’t help thinking yet again, “You’re getting in deep, man.” I fear I’ll learn how to sew fake fur before I learn how to make my own kites.

– Tom

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