DIY Projects – Page 2

Primo capsules – for lav mics and DIY

February 7, 2021

A pair of the tiny Primo EM272 omni capsules: add two wires and a 3.5mm plug and you are ready to record!

Primo microphone capsules are much-loved by the DIY crowd, especially the omni EM172 (now replaced by the EM272) capsule: it is a 10mm capsule with 14dBA self-noise. As such it offers similar scope to a lavalier (lav) mic for times when you want something much smaller than a small diaphragm condenser (SDC) mic, but has much lower self-noise: lav mics typically have around 22-5dBA self-noise, which can be pretty noticeable for many recording uses, such as ambience and nature recording. The sound quality is on a par with capsules used in mid-range handheld recorders, but, of course, once you have external mics you can be much more flexible. For example the omni mics on the much-loved Sony PCM10 are too close together for stereo, so plugging in a pair of EM272 mics allows for a proper spaced pair: you can clip those to your hat, the sides of your specs or, to look slightly less silly and to avoid your head movement giving odd shifts in the stereo image, to the sides of a bag or rucksack.

And the best thing about the Primo capsules is that they are cheap. Currently in the UK an individual bare capsule is £12.78 from FEL Communications Ltd (Micboosters): FEL/Micboosters also offers various versions (e.g. pre-soldered), other Primo capsules (including cardioid and figure 8: see below), matched pairs and, for those not into DIY, capsules already made up into finished mics – their Clippy and Pluggy models. There are a couple of other manufacturers who use Primo capsules in affordable mics, the most well-known of which is LOM in Slovakia, but I think Micboosters is the only one that also sells the bare capsules: and it is one of those great small British companies, run by the very helpful Nick Roast, who has worked as a BBC sound engineer for over 30 years.

I have a pair of the Clippy mics (together with Rycote furry covers that are made specifically for them), which are useful for discreet or minimalistic stereo recordings and as lower noise lav mics for dialogue. And I have used bare EM172 capsules for what Curt Olson – who inspired my experiments – calls a ‘head-spaced parallel barrier array’, albeit in my case small enough to fit inside a Rode Mk1 blimp and with some of the mic placement attributes of SASS arrays: the circular baffles are c.90mm diameter and the mics are 160mm apart. Surprisingly effective. I’ve also used an EM172 to make a boundary mic, with the capsule set off-centre in a disk of perspex (150mm diameter and 5mm thick). Perhaps next I should buy some silicon ears (I see Micboosters sell them too) and make a binaural head with a pair of EM272s, as others have done.

Primo also make a single diaphragm figure 8 capsule (the EM283), again 10mm diameter, but this is not normally available via retailers. I noticed that Micboosters had started selling them and, as I had never heard of the capsule nor could find anything about it via the internet, I bought one for the princely sum of £19.68 for fun/curiosity. The specs are not as attractive as the EM172/EM272 and the 22dBA self-noise might be rather too high for many, but it is fine when used as part of a mid-side pair for louder sources such as music or some street ambiences. I found that the EM283 capsule needs better RFI screening than the EM172 and EM272, but that’s nothing unusual. I just bunged it in an old sawn-off shotgun mic tube for testing and, though this makes the mic unnecessarily large, it is fine – all hum eliminated. At some point I’ll get around to making a smaller housing with the fine mesh screening I have bought for the job. Not entirely sure if I’ll make much use of this capsule, unlike the EM172 and EM272 ones, but it has proved useful in an odd way: I had been thinking of acquiring an AKG CK94 figure 8 mic to provide the side mic for a mid-side pair with my CK93 hypercardioid, but was worried about self-noise (the CK94 is also 22dBA) and this convinced me that for my intended use I really do need something a lot quieter (so will need to save for an MKH30). UPDATE 26.5.2021: despite my conclusions about the EM283, I did buy a used AKG CK94 after all (a bargain came along) and its theoretically identical self-noise of 22dBA is not at all problematic. Lesson learned? That not all self-noise specs are equal! I’ve written a blog post about the new mic here.

So the final word: I’d really recommend playing around with bare Primo capsules as a cheap way to learn about arrays, and, even if DIY isn’t your thing, I’d recommend a pair of ready-made Clippy mics with these capsules as a great and very affordable alternative to a pair of lav mics for those occasions when SDC mics (and P48 power) aren’t feasible.

Primo EM272 capsules as delivered: matched pair with measured sensitivity a bit better than the published specs.

A head-spaced parallel barrier array – with the influence of SASS arrays – built to fit inside a Rode Blimp.

A boundary mic with an EM172 capsule: so simple and cheap, but better performing than so many commercial offerings.

A size comparison of a Clippy mic and a miniature lav (in this case my Rode lav). The Clippy’s EM172 capsule makes it rather chunkier, but for many uses this isn’t a problem, and it is still small enough to hide under much clothing.

Experimenting with a Primo EM283 fig 8 capsule: mounting it inside part of an old shotgun mic body.

And using the Primo EM283 fig 8 mic as the side mic in a mid-side mic pair with a Rode NT55.

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DIY Projects

Speakers – exponential tapered horns

January 30, 2021

Though not strictly part of filmmaking or sound recording, ultimately playback is on hi-fi speakers (well for those not limiting themselves to earbuds!) and having something decent on which to check mixes other than nearfield studio monitors is critical. To me, speakers means DIY and I have been playing around with them since childhood: an early speaker project around undergrad days was a long-throw horn using WEM drivers, and then a few years later, when a thick cold meant I wasn’t feeling much like working on the thesis, I made my first exponential tapered horn speakers, loosely inspired by Laurence Dickie’s famous Nautilus B&W speakers. The rationale of the Nautilus design (and subsequent speakers using this approach) is to use tapered tubes to absorb the rearward sound produced by the drivers, to stop it being reflected back (as in a conventional speaker cabinet) through the driver again, adding unwanted resonances and colour.

The 1990s old snake speakers, with B&W drivers and that chipboard aesthetic

These first tapered horn speakers weren’t snail shaped like the Nautilus (too complex and derivative) but, rather, vaguely snake shaped, built simply out of chipboard, and had more of the WEM PA speakers: they weren’t very hi-fi at all. But modifications quickly followed. I was given some spare B&W drivers (I think for their 802s, but they are marked ‘prototype’ on the back), so modified the snakes to take them, and then some backroom boffins at B&W kindly stuck them in their anechoic chamber, stuffed them properly (I was too tentative with my wadding), added plasticine under the tweeters to take out a nasty reflection, and designed and built crossovers to suit: this might seem unusual, but I was brought up in the small Sussex market town of Steyning, where B&W’s R&D centre was located, so enthusiastic and generous research staff were thick on the ground.

I’d always meant to house the speakers in something rather nicer than chipboard, but the decades slipped by (actually over 25 years) and it took the Covid-19 lockdown for me to get round to it. Somehow the project rather developed: instead of re-housing the early 1990s B&W drivers (or, actually, similar: I had a spare set), I was offered some rather more modern and better (indeed, world-class) drivers (including metal-coned pistonic bass/mid drivers) actually designed for this type of speaker, and, again, some technical support (e.g. on crossover design and build). I’ll draw a discreet veil over which (different) manufacturer gifted them this time (those interested can probably guess), as they don’t supply for DIY use: again, these were some spares knocking around – acoustically fine, but not quite pristine looking.

For the new speakers I decided to go for laminated plywood construction. I like the aesthetic of varnished laminated plywood sculptures and it seemed an achievable way of building a complex curvaceous shape: there would be no flat sides this time round. Also, with laminated construction I could create fairly thick and inert walls. The overall form is similar, nonetheless: a rearing snake (or a large number 2 if you want to be more prosaic) and a two-way design with passive crossovers. And the underlying principle was the same. That is, the form was defined entirely by eye and a gut feeling that the exact taper of the tube isn’t critical, so there is no scientific analysis behind it! Work began with drawing in AutoCAD and was followed by the production of a 0.3 scale half-model – shades of boat building. After sanding this so it looked right the laminated sections were each scanned on a flat-bed scanner, and then inserted into the CAD drawing to allow tweaking of the shape of the templates.

Then construction for real started, with endless hours of cutting plywood just with a circular saw and, mainly, a jigsaw: with 30 pieces of plywood per speaker I was soon cursing the frugal approach that precluded handing it all over to a CNC-equipped workshop (partly due to lockdown-derived impoverishment and partly due to the fact that I wanted to build them myself). Then gluing the plywood sections together to make half-speakers: it sounds easy, but the awkward shape and slippery glue were a difficult combination, resolved by adding a few laminations at a time and holding things in place with piles of books. Fiddlier still was making the front plywood baffles for mounting the drivers: the drivers sit in rubber O-rings with fixings via threaded bolts out of the back of the speakers, so millimetre precision was needed (achieved by spinning my router round a nail). Then back to hard graft: heavy sanding – mainly with a belt sander – to get into something near the final shape. I must have clocked up more hours with a mask on in my own garden than anyone else in lockdown. With initial sanding done it was on to stuffing the lower half of the tapered tube with woollen jumpers (almost solid at the bottom), add cables, and then, without stray bits of wool mucking things up, glue the two halves together: more wool and then – for the top part – speaker wadding was added through the bass driver hole.

Setting out paper templates, and working how to fit to the 8ft x 4ft plywood sheets

Plywood laminations cut for one half-speaker

The not-so-sophisticated gluing process: lashings of Titebond and precariously-balanced books.

The first half-speaker nearly finished, with drivers loaded. The tweeter has its own (plastic) exponential tube fitted, and both drivers are held in place by bolts through the back of the cabinet – requiring careful alignment and rubber fittings to avoid resonance issues. The interior was intentionally left with a stepped effect since sanding (which would have been difficult) would not have improved the performance.

With one speaker roughly made, it was time for loading the drivers for testing. With no access to a handy anechoic chamber this time, the garden on a windless day was the next best thing – getting away from any nasty indoor reflections. The neighbours must have thought it odd as I fed sine-wave sweeps through the speakers: aliens in the village? Measurement (bass driver and tweeter separately) was via a Dayton Audio EMM-6 measurement mic, using its individual calibration data and REW software (useful not just for measuring room EQ). The results were looking hopeful, so data was sent to the crossover designer (a renowned loudspeaker designer, so in safe hands), then the crossovers were assembled for me (using hand-wound inductors, made on a slowly spinning lathe), and the tests run again, this time with both drivers in unison: superb results, with impressive waterfall plots revealing negligible resonances and reverberation. During the to-ing and fro-ing of crossover design and build, I ground on with construction of speaker no. 2, and then both speakers were given the finer finishing sanding, crossovers were fixed, and then the speakers were treated to several coats of high-gloss yacht varnish.

Gluing the two half-speakers together: not the easiest shape…

Measuring a speaker in the garden: the poor man’s anechoic chamber.

Almost there: soldering connectors to one of the varnished speakers.

And the sound? After teething troubles with one bass driver (an elusive intermittent distortion caused by a loose connector in the driver – as I said, these were spares, and had been knocking around on a shelf for a few years) were sorted, absolutely superb! REW results indoors were exactly as anticipated and aimed for. But, more to the point, listening reveals the sound to be uncoloured, responsive, clear, with a good stereo image, and easily sufficient bass (something I was wondering about) for the size of room. At some points the complexity of utilizing the new drivers seemed daunting (compared to the old B&W ones), but, in the end, it was worth it: not only was there an aesthetic upgrade (the original plan), but an equal acoustic one. For the cost of several sheets of ply, a lot of glue and jigsaw blades, some crossover components, a tin of varnish and countless hours (which were going free during lockdown anyway), I’ve ended up with some unique-looking speakers that to better performance-wise would cost upwards of £20k. And such a satisfying feeling. Given that the mark 1 snakes served over 25 years, these ones will see me out.

Ah, I forgot to mention, though, that there is one drawback: this time round you can’t balance a mug of tea on the top of the speakers…

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Audio Gear DIY Projects

Windshield for LDC mics

January 7, 2021

TIG welded cage

Sometimes, however impractical it seems, it is useful or at least tempting to get a low-noise large-diaphragm condenser (LDC) microphone pair outside. Various set-ups have been tried over the years by nature recordists, often taking advantage of a pair of the affordable and low-noise Rode NT1a mics. I have been particularly impressed by Magnús Bergsson’s recordings with NT1a mics, not least as he often runs these in parallel to Sennheiser MKH20, 30, 40, 8020 and 8040 mics: see his website at HLJODMYND – SOUNDIMAGE.

For use when I need lower self-noise than provided by my usual small-diaphragm condenser (SDC) options, I wanted a mid-side pair of LDC mics (i.e. a coincident stereo pair comprising a figure-of-8 mic for the sides and, in this case, a forward facing cardioid mic), so having a Rode NT1 (the more neutral successor to the NT1a: 4dBA) and a Rode NT2a (7dBA) to hand I have put together an oversize windshield or blimp for a vertical mid-side set up.

For better stiffness than the usual plastic, I have gone for TIG-welded stainless-steel wire (2mm diameter), with the blimp cage incorporating (isolated) spigots to fit a Manfrotto 154 stereo bar. The blimp disassembles into two halves, but, in reality, I just leave it assembled and insert the mics through the spaces in the cage. For the covering, I have gone for Rycote’s red lining cloth and Rycote long fur (all supplied by the metre direct from Rycote: amazingly helpful people there), with the usual elasticated drawstring tightly closing the side opening. The cage was built to my design by a friend of mine, who works with stainless steel wire for rolling-ball sculptures – all for a few pints of beer – and the fur covering was made by another friend with professional sewing skills.

Blimp with fur

Initial testing met expectations, not least with the better windnoise attenuation resulting from a larger diameter windshield than those usually designed with necessary compromises for boom-pole use: it’s the distance from the sound generating surface (the outer side of the blimp) that matters, with the inverse-square law applying.

Needless to say, I am by no means claiming this as a sensible/feasible option for most usage (and I have much more practical alternatives for most projects): it is heavy and I wouldn’t want to carry it and its stand (I use a Manfrotto 1004BAC) more than half a mile or so. There are, of course, many lighter, more robust and less humidity-sensitive microphone solutions that will be preferable for most projects (e.g. a Sennheiser MKH 30/40 pair).

However, this DIY approach might be of interest to anyone else was thinking along similar lines with LDCs (and there is no need to be afraid of getting large studio mics outdoors): LDC mid-side arrays are feasible for such use and it makes good sense to consider (affordable) purpose-built windshields as alternatives to shoe-horning LDC mics into undersized windshields or adaptation of less than ideal items from the local DIY store!

In action, recording musicians in the grounds of Mannington Hall

And for anyone really keen, here’s my design

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