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05 December 2014

The Block: making a simple stereo baffle

OVER THE PAST few months I’ve been doing up my flat prior to moving. I don’t really have any great insights to share from that time except: you can do almost anything with bonding plaster (a huge bag of which is only a fiver), everyone should own an SDS+ drill, and those rated tradesmen-type websites are rubbish.

The DIY mindset is contagious and quickly spreads to unrelated tasks, so I took some time out to make a stereo baffle. The idea of a baffle is to help create a convincing stereo image by sticking a sound-absorbing barrier between two mics, typically ones with an omnidirectional pattern. The first person to do this was Alan Blumlein in 1931, and you can read more about him and stereo images in general on this blog post from 2012.

The established design for stereo baffles nowadays is called a Jecklin Disc after its inventor Jürg Jecklin. In its most recent specification, the Disc must be 35cm in diameter and made from a wooden or other solid core, covered on both sides with a sound-absorbent material, typically closed-cell foam. The mics are then positioned on either side of the Disc so there’s a gap of 36cm between them. At least one firm sells Jecklin Discs online but most people seem to make their own. Here’s a photo from the SEA Nature Sound Workshop of Mike Wall demonstrating his own set-up, reproduced here from Flickr under the terms of its Creative Commons licence:

Mike Wall demonstrates his Jecklin Disc setup at the 2013 SEA Nature Sound Workshop

Mike’s Disc array looks to be smaller than the standard spec and the use of two Rycote windshields is a good move for outdoor work. However, I wanted to make something a bit different and began by sawing a £3 wooden chopping board to make a 25cm square:

Sawing a wooden chopping board to make the core of an acoustic baffle

Then I paid Pentonville Rubber to cut me some closed-cell foam tiles to the same dimensions so I could glue layers of them onto the sides of the wooden board, making a block some 17cm thick, roughly the width of a human head. I screwed a camera sliding clamp onto one side and the Block was ready to go.

The Block acoustic baffle in use outdoors

The Block’s sliding clamp means it can be fitted onto the tripod in a few seconds. A photographer’s stone bag fits onto the tripod’s legs and holds the Sound Devices MixPre-D preamp and whatever pocket-sized recorder I’ve got that still works.

You can see that the Block holds the headworn setup I’ve been using since I began compiling the Waterways sound map. This simply consists of a Beyerdynamic headband with two mics attached using plumber’s jubilee clips. A foam and a furry windshield then go on top of each mic.

This way I can use the mics in headworn mode or, if I want to make a longer recording, I can put them on the Block instead. One of the drawbacks of wearing mics on your head is that they’ll pick up any sound you make: breathing, stomach rumbles, and so on. It becomes hard work after about four or five minutes and having drool coming out of your mouth because you daren’t swallow isn’t a good look. Especially if you’re a bloke. And you’re recording in a park. By yourself.

Fortunately, the Block is there to save you from such embarrassment. Also, it lets you monitor what you’re recording over headphones, which you can’t do when you’ve got a mic in its furry pelt covering each ear. I’m not one for monitoring my recordings much, preferring to get home and listen to what’s in the box, but that’s because I’m an amateur.

Talking of parks, here’s a test recording I made using the Block with two DPA 2006C mics in my then-local neighbourhood green space: