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The decibel (dB) is a logarithmic unit used to measure sound level, widely applied in electronics, signals, and communication to describe power, sound pressure, voltage, or intensity ratios. For instance, a 3 dB increase signifies a doubling of power, while a 10 dB increase represents a tenfold power multiplication. Sound pressure level is calculated as 20 log (p2/p1) dB, reflecting the square relationship between power and pressure. Standard reference levels, such as 20 micropascals for sound pressure in air, establish absolute dB values, where 0 dB corresponds to the reference level, not an absence of sound. Human hearing sensitivity varies with frequency, necessitating A and C weighting filters (dBA, dBC) in sound meters to approximate the ear's response. The phon unit relates to dB via psychophysically measured frequency response, with 1 kHz readings being identical. The sone unit quantifies perceived loudness, where 1 sone equals 40 phons, and a 10 dB increase above 40 phons roughly doubles perceived loudness. In radio, dBi measures antenna gain relative to an isotropic radiator, crucial for anisotropic radiation patterns.