The remaining pulses contain data using pulse position amplitude modulation. RF impedance is a resistance inversely proportional to the signal amplitude. Rectifiers are also used for detection of amplitude modulated radio signals. Sound is measured based on the amplitude and frequency of a sound wave. The fastest four modes use OFDM with forms of quadrature amplitude modulation. Fiber optic systems can be upgraded by the use of quadrature amplitude modulation. The intensity is defined as the square of the amplitude of the oscillations. Each species has a range of normal hearing for both amplitude and frequency. Oscillators that use limiting for amplitude control often have significant harmonic distortion. There are no markers as in the amplitude modulated time code. The form and waviness (i.e. both amplitude and frequency) must also be considered. This is a case where both amplitude and frequency are very important. The fundamental AC output amplitude is equal to . For instance, a signal with a peak amplitude of has a of . The amplitude and phase of the current density varies with depth.