How Do They Work?
Both microphones and loudspeakers work on the same principle, only in reverse. What we hear is the result of sound waves carried in the air (imagine ripples on water…) Different frequencies have different sized waves (ripples). Higher frequencies produce shorter waves and lower frequencies, longer waves.
Many moons ago it was discovered that sound waves would cause a thin metal sheet (or diaphragm) to vibrate. If this diaphragm was magnetised and suspended close to another magnetised diaphragm, then the vibration would cause microscopic changes in the polarity between the two which would in turn produce tiny electrical emissions (voltages) which would vary relative to the amount of vibration. In simple terms, by creating a polarised capsule sound could be translated into voltages.
Briefly, these voltages must then be amplified (by your trusty mic preamp) and can then be processed (compressor, eq etc) and routed (via your desk) and then either stored (on your tape or hard disk) or converted back into sound via a power amplifier (required to convert the small voltages into large voltages sufficient to move loudspeaker cones or diaphragms) and then into the speaker.
And here the cycle comes full circle, as these large electrical currents transmit varying amounts of electrical energy to the magnets controlling the paper (or plastic) cones of the loudspeakers. This in turn causes the speaker cones to vibrate relative to the signal received, thereby converting the energy back into sound waves and … sound. It is a beautifully symmetrical process; sound is converted into electrical pulses which are then amplified, treated and processed, and either stored or reamplified and converted back into sound.
Types of Microphones
There are three main types of music recording microphone, dynamic, ribbon and condenser (or capacitor).
- Dynamic mics have a small diaphragm attached to a coil that vibrates in a magnetic field; this induces a voltage corresponding to the sound.
- Ribbon mics operate on a similar principle, but with a light metal ribbon suspended between two magnets; this makes ribbon mics more fragile but also more sensitive than dynamics.
- Condenser mics have a diaphragm placed very close to a backplate, which is charged with a high voltage. Sound vibrations change the distance between the plates, altering the capacitance of the circuit, producing fluctuations in voltage. These variations have the same shape as the sound waves that created them.
These differences mean that each type is suited to particular tasks.
Dynamic mics are generally small, robust and tolerant of high SPLs, with a frequency response peaking in the middle range. This makes them suitable for loud singers, guitar cabs or drums (and Prime Ministers; they use AKG D202s in the House of Commons.)
Ribbon mics were originally heavy and fragile, but now with small, powerful magnets and tougher ribbons they have a full range of applications. The ribbon moves in three dimensions, giving a detailed and natural sound, and their frequency response has a lift at the bottom and a roll off at the top giving warm, rounded results.
They now have much improved SPL tolerance which has given the ribbon mic a new lease of life in front of guitar cabs. They are ideal for all acoustic instruments, particularly strings and brass, choirs, organs, overhead miking and ambience. They don’t need phantom power, and may in fact be damaged by it.
Condenser mics are much the commonest type of studio mic. They come in all shapes and sizes, but all need powering, either by a power supply particular to the mic, by phantom power down the audio cable, or by battery. They have a rise in their response at around 8-12kHz, which gives them a present, bright sound and made them very popular for close miking of solo voices.
Since the success of the U47 and C12 in the early 1950s condensers have become the standard all-purpose studio mic throughout the world. They are suitable for recording all types of musical sources, and if you only need one mic, primarily for vocals, a condenser is a good choice.