A microphone is essential for teachers who must project their voices in a noisy gymnasium or over music. The signal strength of your voice is reduced by six decibels (dB) every time you double the distance from the teacher to the students. For comparison, 0 dB is near total silence; a sound 10 times more powerful is 10 dB, and a sound 100 times more than 0 is 20 dB. Some common sounds and their decibel ratings include: near whisper (15 dB), normal conversation (60 dB), and a car horn (110 dB). Microphones are defined by the directional pattern in which they pick up sound. Three common types of microphones are unidirectional, bidirectional, and omnidirectional. The unidirectional microphone picks up sound from one direction, the bidirectional microphone picks up sound from two directions, a