Time Constants

In electronics (and therefore in audio), a time constant τ is defined by the time it takes to charge a condenser C to approximately 63% of its full value by a feeding resistor R. The values of the condenser (unit: Farad) multiplied by the value of the resistor (unit: Ohm) yield time (unit: Second). The combination of the condenser and the resistor forms a filter. The cross-over frequency f₀ of that filter is defined by 1 / 2×p×f₀.

Examples:

R = 5 kΩ, C = 10 nF;
τ = 5 10³ * 10^-9 = 50*10^-6 s = 50 µs

Often applied time constants:

τ = 50 µs  =>  f₀ = 3180 Hz
(this applies to European radio transmission, including wireless systems, for pre-and de-emphasis).

τ = 75 µs  =>  f₀ = 2120 Hz
(this applies to radio transmission in the US for pre- and de-emphasis).

τ = 120 µs  =>  f₀ = 1325 Hz
(this applies to pre- and de-emphasis of CC cassette tape recording on iron oxide coating).