Directional microphones are a sort of sound collesting devices able to focus on a single sound source while eliminating unwanted signals comming from nearby noise sources. Principles on which directional microphones are based are similar to those used in optics - both sound and light are forms of waves and consequently share many physical properties.
Types of directional microphones
There are basically three major groups of these devices in use. The most compact one looks as a tube with multiple holes along its length. The microphone capsule itself is placed at the bottom of the tube. Sounds comming from in the front of the tube go directly to the microphone capsule sensitive surface while sounds comming at a certain angle go through numerous holes along the tube intefring destructively in the process and producing practically no impact at the capsule membrane. Thus in theory, only sounds comming from the sources in front of the microphone get picked up and fed to the preamplifier. This is of course doubtful in practice as it is not easy to achieve strong destructive interference of stray sounds over the whole audio frequency band. Another problem arises from the fact that there is no acoustical amplification of sounds of interest which means that such microphones are not particularly suitable for recording of faint distant sounds which most hobbysts actually have on their minds when thinkering about directional microphones. Although not ideal, the compactness and the abbility to reduce stray sound noise to a certain degree makes these microphones the type of choyce for radio and TV studio work.
The most sophisticated type of directional microphones is based on "phased array
" principle. Numerous sound sensitive components are placed over a rigid 2D surface and then by controlling the phase difference of individual signals they collect, stray sounds can be destructively while wanted sounds can be constructively interfered in an electronic sound processing unit. As phase differences vary enormously over the audio band, interfering of initial sound signals is usually carried out inside a highly sophisticated digital signal processors. An advantage of this system over the competing technologies is its flexibility - by varying phase differences, one can pick out sounds from any desired direction and not simply from an axis perpendicular to base 2D surface. Changing the favoured incident angle can be done extremely quickly, much faster than in mechanical systems. Modern submarines are usually equipped with a lot of underwater microphones planted into their external hulls which enable them to analyse ocean sounds in order to virtually "see" through the ocean in any direction. What's important is that this is being done without powering up any active sound sources such as sonars so the listening submarine position cannot be detected by its opponents. Similar signal processing is done in phased array radars and "smart antennas
" with radio- instead of soundwaves, while hologram images
represent an application of phasing effect in optics. Unfortunately, audio phasing technology is yet far out of reach or average DIYers.
But there is yet another type of directional microphones which homebrewers can easily build and use - parabolic dishes. The pronciple they exploit is sound focusing using a paraboloid reflector, exactly the same as used in satellite of WiFi parabolic antennas. A single sound sensing element i.e. a microphone capsule is placed in the focal point of the reflector in which sounds incomming along the reflector symmetry axis can be amplified thousands of times! Since this amplification takes place before the signal has even been transformed into its electronic form, no thermal noise is introduced in the process. The only condition that must be satisfied in order for a parabolic microphone to perform well is that the diameter of its reflector be large enough to be wider than the wavelength of sounds in interest. For sound frequencies above 600 Hz, the wavelength is shorter than 340 m/s / 600 Hz = 0,5 m. Thus, if bulkiness of the reflector can be dealth with in use, a dilligent hobbist will be able to make high performance device for a fraction of the cost of profesional systems.
Now, the problem arises of how to actually make a parabolic reflector. There are several successfull DIY parabolic microphone projects on the Internet but they most often don't deal with the making of the reflector. It is true that some gadgets such as umbrellas can be used for this purpose with some success, but wouldn't it be fantastic if a quality reflector with strong focusing power could be built at home? Well, of course it's possible if a pinch of mathematics is added into the brew.