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The sounds are generated by a generic generator G like the shoot of a gun or a clap of the hands. When this generator is activated it triggers a vibratory phenomenon that swings the particles of material, in which it is immersed, from their state of equilibrium. These molecules will vibrate against the others nearby. This phenomenon continues until the energy isn’t all consumed. The displacement of the particles from their equilibrium position gives rise to an increase or a decrease in density. At the equilibrium point, the molecules form the zones of high and low pressure which are directly proportional to the density. WHAT IS THE SOUND?
This phenomenon is called “pressure wave”, which within certain limits of amplitude and frequency, relative to medium atmospheric pressure, is perceived as sound by the human ear. The waves described above are called sound waves and their speed of propagation is called “ speed of sound”.
A microphone is an example of a transducer, a device that changes information from one form to another. Sound information exists as patterns of air pressure; the microphone changes this information into patterns of electric current. There are a hundreds of designs for microphone with thousands of applications. HOW THEY WORK
A variety of mechanical techniques can be used in building microphones. The two most commonly encountered in recording studios are the: • The dynamic mic. • The condenser mic.
THE DYNAMIC MICS. • In the magneto-dynamic, commonly called dynamic microphone, sound waves cause movement of a thin metallic diaphragm and an attached coil of wire. A magnet produces a magnetic field which surrounds the coil, and motion of the coil within this field causes current to flow. • The principles are the same as those that produce electricity at the utility company, realized in a pocket-sized scale. • It is important to remember that current is produced by the motion of the diaphragm, and that the amount of current is determined by the speed of that motion. This kind of microphone is known as velocity sensitive.
THE CONDENSER MIC • In a condenser microphone, the diaphragm is mounted close to, but not touching, a rigid backplate. A battery is connected to both pieces of metal, which produces an electrical potential, or charge, between them. The amount of charge is determined by the voltage of the battery, the area of the diaphragm and backplate, and the distance between the two. This distance changes as the diaphragm moves in response to sound. When the distance changes, current flows in the wire as the battery maintains the correct charge. The amount of current is essentially proportional to the displacement of the diaphragm, and is so small that it must be electrically amplified before it leaves the microphone.
SPECIFICATIONS • The most important factor in choosing a microphone is how it sounds in the required application. The following specifics are the basic lines to make the right choise. • SENSITIVITY • OVERLOADCHARACTERISTICS • NOISE
This is a measure of how much electrical output is produced by a given sound pressure. This is a vital specification if you are trying to record very tiny sounds. More sensitivity equals less noise, but also means more distortion at high level pressures. SENSITIVITY
Any microphone will produce distortion when it is overdriven by loud sounds (heavy air pressure). With a dynamic, the coil may be pulled out of the magnetic field; in a condenser, the internal amplifier might clip. Loud sounds are encountered more often than you might think, especially if you place the mic very close to instruments. OVERLOAD CHARACTERISTIC
Microphones produce a very small amount of current, which makes sense when you consider just how light the moving parts must be to accurately follow sound waves. To be useful for recording or other electronic processes, the signal must be amplified by a factor of over a thousand. One of the most important characteristic to choose a mic is their signal to noise ratio specific. This specific directly proportional to the efficiency of the mic. NOISE
Another important characteristic of the Mics is the ability to capture sounds in a 3D space. There are 5 principal polar patterns: OMNI-DIRECTIONAL BI-DIRECTIONAL (OR HEIGHT FIGURE) CARDIOID HYPER-CARDIOID SHOTGUN POLAR PATTERNS
maximum sensitivity at 360° OMNI-DIRECTIONAL
BI-DIRECTIONAL • maximum sensitivity on axis 180°
CARDIOID • maximum sensitivity on front 120°
HYPER-CARDIOID • 90° on front • and • 30° rear
SHOTGUN • 50° on front, • 30° rear • and • 15° lateral
The knowing of those characteristics give us the ability to make a right choice of the microphones to use in a particular recording environment. For example: We want to record a classical string quartet.
At first we’ll analyse were the musicians are positioned in the space. Second, the characteristics of shape of sound outgoing from their instruments. Third, were to place the mics and at last their numbers.
the instruments are: two violins, one viola and one cello. • we need to capture every instrument in a separate channel and we need also capture a stereo image of the surrounding sound (ambience). • we need four cardioid mics for instruments and two omni-mics for ambience.
Another example could be a piano recording in a concert hall. • we need two hyper-cardioid mics for the high and low frequencies of the piano. • we need two cardioid mics pointing to the audience sits. • one omni mic on the stage for the early reflections of the concert hall.
