The theorem that, in an acoustic system consisting of a fluid medium with boundary surfaces and subject to no impressed body forces, if p₁ and p₂ are the pressure fields produced respectively by the components of the fluid velocities V₁ and V₂ normal to the boundary surfaces, then the integral over the boundary surfaces of p₁V₂ - p₂V₁ vanishes.

1. The electric potentials V₁ and V₂ produced at some arbitrary point, due to charge distributions having total charges of q₁ and q₂ respectively, are such that q₁V₂ = q₂V₁.

2. In an electric network consisting of linear passive impedances, the ratio of the electromotive force introduced in any branch to the current in any other branch is equal in magnitude and phase to the ratio that results if the positions of electromotive force and current are exchanged.

Given two loop antennas, a and b, then I_ab/V_a = I_ba/V_b , where I_ab denotes the current received in b when a is used as transmitter, and V_a denotes the voltage applied in a; I_ba and V_b are the corresponding quantities when b is the transmitter, a the receiver; it is assumed that the frequency and impedances remain unchanged.

The sensitivity of a reversible electroacoustic transducer when used as a microphone divided by the sensitivity when used as a source of sound is independent of the type and construction of the transducer.

In general, any theorem that expresses various reciprocal relations for the behavior of some physical systems, in which input and output can be interchanged without altering the response of the system to a given excitation.