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When you stand in a field listening to a passing aeroplane with your eyes shut you are naturally aware of the direction from which the sound came. The big question is, why? Left to right is easy, you've two ears so sound arrives at your left ear fractionally before your right ear hears it. Your brain interprets this as sound coming from your left and vice virca for sound from the right. But what about sound directly in front, or directly behind you and not moving at all. Try it with your eyes shut and you will still know where it comes from. But how?
Everyone now understands the spacial awareness that modern surround sound systems provide ie. you would be conscious of whether a passing aeroplane was crossing in front of you or behind you or even directly overhead. Very sophisticated sound wave phase changes and wave delays are coded into surround sound systems to help achieve this. Multi speaker systems further enhance the illusion of natural spacial awareness by separating these coded streams and feeding them out to the appropriate speaker.
Before the arrival of Surround Sound technology many amature sound recorders had experimented with a technique generally referred to as holophonic or binaural sound recording.
This was simply a method of stereo sound recording which used a stereo microphone designed to emulate the position of the human ear on our own heads. It was intended that one should listen to the resulting sterophonic recording using headphones.
Conventional stereo sound recordists in the 1970's when recording a passing steam train would position two microphones about 20yds/metres apart and about 30yds/metres back from the railway track. This was generally very difficult to achieve in practise because of cuttings, embankments and the matter of rights of way. The result was generally impressive when listened to in your lounge at home with speakers 3yds/metres apart and about 3yds/metres away. However when listening on headphones the sound no longer passed in front of you but went instead straight through your head from one ear to the other.
The simplest way to record holophonic sound is to clip a miniature electret microphone to each of your ear lobes and record the sound to the appropriate left or right channel of your stereo sound recorder. For some people this gives excellant results as the sound reaching the electret mike is affected by your head mass in the same way your own ears are. The flap of your ear however is very important to the way you perceive sound, as anyone who has lost their outer ear flap can tell you. They have great difficulty telling where a sound came from until their brain adjusts and compensates for the physical change.
This approach has two major drawbacks for the steam train recorder. First it is very suseptible to wind noise and if you correct this by putting muffs over your ears and the mikes you will pick up your own heartbeat and a lot of rustling noises. Secondly you tend to turn and look at the approaching train which moves the sound from say the left field to centre field instantly, ruining the object of the excersize. If you are simultaneously operating a video camera then it becomes impossible to achieve your objective.
The answer is to make an artificial head. How far do you need to go, well the BBC in the UK experimented using a flat perspex disk of 9" diameter with a mike in the centre of each side very successfully. Some American enthusiasts have gone as far as having their outer ear flap and the passage to their eardrum replicated in soft rubber compounds and an accurate replica of their own head made with an appropriate sound density. The electret being positioned exactly where the eardrum would be. In theory they would be as spacially aware in their headphones as they were in the field. However I've always believed that replication of the passage to the eardrum would be pointless as the headphone earpiece is outside of this during sound replay.
My compromise answer is the microphone assembly now known as Webster (because like Websters Dictionary, its leather bound). Not my choice of name but that of a Hi Fi magazine reviewer and former colleague some 20yrs ago. I do not claim this gives full 360 degree spacial awareness but it does bring sound forward to the front and cause a drop in sound level as say an aircraft passes round behind you at an airshow. Not perfect, but vastly better than the quite good manufacturers stereo mike fitted to my video camera.
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It comprises two discs of plywood at an angle of 30 degrees to each other separated by sound deadening material. A microphone clipped to a post in the centre of each disc and a switch, batteries and a handle between them. The disc has a foam surface and the mikes are covered by a wire mesh and a nylon tights material wind silencing dome made from two flour sifters. The posts allow you to move the clip-on mikes closer to the centre to reduce stereo channel overlap or move them out to increase overlap. If you are close to the line of railway then the mikes need to be wide apart otherwise you will hear a dead spot as the train passes, farther back from the line of railway, mikes need to be closer in. People smile at my dead fly on a stick but it works and with better quality modern mike inserts it would probably be even better.
Its wind proof, its robust, it doesn't need two mikes 20yds/metres apart for other photographers to fall over and it stands still when I move about with my video camera. Also it can be within 2yds/metres of a passing train and still record sound successfully. It records spacial awareness better than any commercial mike I have used, and it cost only a few pounds to make. It is incredibly sensitive and picks up bird song from considerable distances when I'm quietly waiting for the next train. If you were making one now I believe you could use modern sub miniature electrets and a couple of stainless steel tea strainers and make it very much smaller without much loss of performance. If you have a stereo sound, video camera, with an external microphone connecting socket, then it really is worth trying this and making up your own holophonic mike. However good the mike fixed to your camera, it will be pointed at the locomotive all the time you are following its movement thus defeating the object of stereo sound. With a freestanding mike you will hear the locomotive approaching you from one side rising to a crescendo of sound and then disappearing of into the distance toward the other side.
The electrets can be bought from any electronic component supplier, mine came from Maplin in the UK, ( www.maplin.co.uk ) and only cost a few pounds. Mind you that was twenty years ago. I have since bought sub miniature electrets only 6mm in diameter from the above company and have been amazed by their performance when used as an headset microphone for the PC. I have by the way no connection with the above company and make the usual disclaimer if you choose to use them as a component supplier.
One last point to note. Railways are dangerous and the operating company may require you to have a lineside permit if you are on the railway property. In the UK this is an essential Health & Safety requirement and you will receive written instructions on lineside rules and behaviour. Therefore, as a permit is cheaply and readily available, there is no excuse for unauthorised trespass.
If you have a go at this, have fun and I'll see you by a railway somewhere, someday.
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