At normal incidence on a plane boundary between two media the split of sound between reflection and transmission is fixed entirely by the two acoustic impedances Z = ρ·c. The pressure reflection coefficient is R = (Z₂ − Z₁)/(Z₂ + Z₁) and the pressure transmission coefficient is T = 2Z₂/(Z₂ + Z₁), where medium 1 is the one the wave arrives in.

Two things surprise people. First, R is negative when the second medium is the softer one (steel into air, steel into a resin-filled defect): the reflected pulse comes back phase-inverted, which is how a flaw echo can be distinguished from a backwall echo on an RF display. Second, the pressure transmission coefficient can exceed 1 — pressure amplitude in the stiffer medium is higher than in the incident medium — without violating energy conservation, because the particle velocity is correspondingly lower.

Energy (intensity) coefficients are the ones to quote for sensitivity budgets: Rᵢ = R² and Tᵢ = 4Z₁Z₂/(Z₁+Z₂)² = 1 − Rᵢ. Tᵢ is also the two-way pressure factor for a pulse that crosses the interface, reflects deeper in, and crosses back — so the dB figure it gives is the real transmission loss you lose from a wedge-to-steel or water-to-steel boundary on every scan.

Typical impedances (MRayl, compression): air 0.0004, water 1.48, perspex/acrylic 3.22, Rexolite 2.45, aluminium 17.1, ferritic steel 46.3, copper 41.6, tungsten 100. This calculation assumes normal incidence and a smooth, clean, unbonded interface; at oblique incidence mode conversion redistributes the energy and these figures no longer apply.

Compression wave

Worked example

Velocity, medium 1 (incident side)	2730 m/s
Density, medium 1	1180 kg/m³
Velocity, medium 2 (transmitted side)	5900 m/s
Density, medium 2	7850 kg/m³
Impedance Z₁	3.221 MRayl
Impedance Z₂	46.315 MRayl
Pressure reflection coefficient R	0.8699
Pressure transmission coefficient T	1.8699
Energy reflected	75.68 %
Energy transmitted	24.32 %
Reflection loss	-1.21 dB
Two-way transmission loss	-12.28 dB

Perspex wedge (1180 × 2730 = 3.2214 MRayl) onto ferritic steel (7850 × 5900 = 46.315 MRayl). R = (46.315 − 3.2214)/(46.315 + 3.2214) = 0.86994, so R² = 0.7568 → 75.68 % of the energy is reflected back into the wedge and only 24.32 % gets into the steel. Two-way loss = 20·log₁₀(0.24321) = −12.28 dB.