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16 January 2017

Acoustic baffle

A CONDENSER MICROPHONE has a diaphragm that’s open to the air, a backplate hidden out of view behind it, and a voltage across them. Sound waves impact on the diaphragm and make it move nearer to and further from the backplate, causing fluctuations in electrical capacitance.

In an ideal world sound waves alone would make the diaphragm move, but that’s not how it works out in real life. Sudden jolts, tapping the mic’s casing, and vibrations propagated along the cable can all produce unwelcome noises. So too can the wind and, more specifically, the turbulence in the air stream which batters away at the diaphragm. The solution is to absorb as much of that energy as possible before it can be transferred to the diaphragm, like the way a harbour breakwater is there to absorb the energy of the sea’s waves.

Any mic used outdoors needs some kind of protection from the wind. Furry windcovers are made for the mic pairs at the business ends of pocket-sized recorders. A larger mic can be mounted inside a mesh windshield equipped with a handle or stuck on the end of a boom pole: a standard arrangement for many professional field recordists.

I’m more of a hobbyist who’s become wedded to using omnidirectional mics in pairs. These can produce an appealing stereo image when used with some sort of acoustic baffle between them: this old blog post goes into more detail about that. One of the problems of this approach is that it’s not easy to protect the mics against wind because one side of them is up against your head or some head-substitute, like a wooden or closed-cell foam block.

A way around this presented itself when Rycote introduced their new Cyclone windshields. I noticed that they split lengthways into two and thought you could make an acoustic baffle out of a wooden block with one half of a windshield shell attached to each side, covering and protecting the mics under them. However, the Cyclone halves are asymmetric: the port side (relative to the mic facing forward) has the whole rear end-cap, and the starboard side has the larger front end-cap.

Rycote kindly agreed to supply me with two starboard sides, thus saving me the expense of buying a pair of complete windshields. I then set about making the baffle, starting with a wooden chopping board as the core, with most of the baffle’s volume comprised of balsa wood, given about six coats of varnish to toughen it up. This is what it looks like from the front.

The back shows (a) no great prizes for woodwork skills and (b) how the cables are carried beneath removable side blocks to exit at the rear. At its widest point the baffle is about one foot or 30cm across.

With the windshield covers removed, you can see how the mics are held in place with tool clips, and the covers are positioned with the help of metal pegs (cheap drill bits embedded and glued into the wood). Black sticky-backed foam makes for a more draught-tight surface for the edges of the Cyclone halve to come into contact with.

Tony at the Wire magazine asked if I wore it on my head, perhaps hoping for an amusing photo opportunity. No, it goes on top of a tripod. Obviously. Here it is with some camo scrim wrapped round it at an RSPB nature reserve in Suffolk. Without the camo it looks maybe a bit alarming to birds, and several people have pointed out that it also resembles a big pair of stupendous breasts and, even though form strictly follows function, this is true. It’s not a bad thing in my view and, in fact, I’m proud to be doing my part to help redress the balance against countless consumer products which have a distinctly phallic appearance. But maybe sometimes modesty’s the better policy.

The tool clips allow a small degree of latitude in sliding the mics backwards and forwards. If they’re too far back, then the recording might suggest an acoustic hole-in-the-middle, with plenty of sound at each side but not much sense of anything happening directly in front. This seems to be happening with the first trial recordings I made in Suffolk.

The mics used on the baffle are Sennheiser MKH 8020s which have low self-noise, making them suitable for very quiet environments. Here’s a later attempt along the Thames estuary, with the mics moved forwards perhaps half an inch.

Neither of those recordings address the issue of wind, as the Suffolk one was made on quite a still day, and the Shornemead Fort one was made during blustery weather, but where the fort’s structure made an effective windbreak. The other day I saw that the weather forecast was predicting gusts of up to 37 miles per hour along the Thames estuary, so I went to Rainham to see how things would work out. It really was windy that day and only the baffle’s weight prevented the tripod from toppling over.

There is wind noise in that recording, but it’s produced more by the wind’s action on and around the baffle than on the mic diaphragms themselves. The Rycote Cyclone windshield halves seem to work well. Later, I moved the baffle to beside the long concrete wall bordering the Tilda rice factory nearby, and here the wind speed increases, perhaps by being funnelled along the wall. Close up I could see that the baffle was starting to shake and vibrate, and on that recording it began to produce brief audio drop-out where the diaphragms had likely hit their backplates. So 40mph gusts probably mark the maximum limit beyond which the baffle’s no good.

All sorts of improvements suggest themselves: carrying the rear-exiting cables beneath the surface of the wood, various ways to cut down on the baffle’s nearly six pounds (2.6kg) of weight, using Rycote lyre suspensions to deal with vibration, and making a fake fur cover to fit over the whole thing.

But, in the meantime, it is quite satisfying to have made something myself, and that does go against the grain of many hobby activities, which increasingly seem to be about simply buying the right components to fit together.