Electrical properties include conductivity, resistance, impedance and capacitance. Video cables are 75 ohm impedance, low in capacitance. Electrical impedance, like electrical resistance, is measured in ohms. Multiplying a phasor current by an impedance produces a phasor voltage. A per-unit system provides units for power, voltage, current, impedance, and admittance. This prototype filter can be scaled for other values of impedance and frequency. The resonance frequency is defined in terms of the impedance presented to a driving source. The delta form uses resistors that are equal to the system impedance. Consequently, newer amplifiers are based on designs with relatively low output impedance. Speaker wire is a passive electrical component described by its electrical impedance, Z. Only the resistive part of the loudspeaker impedance is used. Line inputs present a much higher impedance, typically or more. Each class has an allowable maximum phase error for a specified load impedance. Electrically this appears to be a very high impedance. Scanning acoustic microscopes use sound waves to measure variations in acoustic impedance.