Acoustic impedance (symbol Z) is the property that lets an ultrasonic density meter read slurry density without a radioactive source. This explainer covers the units, the formula, and how Pisonics turns it into an online density reading.
Definition & formula
Specific (characteristic) acoustic impedance is the product of a medium's density and its speed of sound:
Z = ρ × c
where ρ is density (kg/m³) and c is the speed of sound in the medium (m/s).
Units of acoustic impedance
- SI: pascal-second per metre, Pa·s/m, equivalently kg/(m²·s).
- The rayl: 1 rayl = 1 Pa·s/m (SI). Practical values are large, so the megarayl (MRayl) is common — water is about 1.48 MRayl, steel about 45 MRayl.
How it measures density
At the boundary between a known reference (a wetted window) and the slurry, the fraction of an ultrasonic pulse that reflects depends on the impedance mismatch: R = (Z₂ − Z₁) / (Z₂ + Z₁). Measuring that reflection gives the slurry's Z; combined with the measured sound speed, the meter solves for density ρ. Because it reads a material property and not a radioactive count, an acoustic-impedance density meter like the PS7000 needs no source, licence or disposal.
Why it suits slurry
Impedance contrast is strong between liquid and suspended solids, so the method is sensitive to solids concentration — ideal for mining slurry, FGD slurry and tailings. See the ultrasonic density meter range or the measurement-principles overview.