Acoustic Impedance Calculator
Calculate acoustic impedance from density and speed of sound, or solve for density, speed, or impedance.
What Is Acoustic Impedance?
Acoustic impedance measures how much a medium resists the propagation of sound waves. It relates particle velocity to acoustic pressure and determines how sound reflects at boundaries between different materials.
Acoustic Impedance Formula
$$Z = \rho c$$where \(Z\) is acoustic impedance in pascal-seconds per meter (Pa·s/m), \(\rho\) is density (kg/m³), and \(c\) is speed of sound (m/s). Water at room temperature (\(\rho = 1000\) kg/m³, \(c \approx 1480\) m/s) has impedance about \(1.48 \times 10^6\) Pa·s/m.
Applications
Acoustic impedance matching reduces reflections in ultrasound imaging, speaker design, and architectural acoustics. A large impedance mismatch between air and tissue is why ultrasound gel is used in medical imaging.
Frequently Asked Questions
What are the units of acoustic impedance?
SI units are Pa·s/m, equivalent to kg/(m²·s). Rayl (1 Rayl = 1 Pa·s/m) is also common in acoustics literature.
How does acoustic impedance differ from electrical impedance?
Both describe resistance to flow, but acoustic impedance relates pressure to particle velocity, while electrical impedance relates voltage to current.
What is the impedance of air?
At 20 °C, air has density about 1.2 kg/m³ and sound speed about 343 m/s, giving \(Z \approx 415\) Pa·s/m — much lower than water or tissue.
Why does impedance matter for ultrasound?
Sound reflects strongly at interfaces with large impedance differences. Matching layers and coupling gel improve energy transfer into the body.
Can I solve for density or sound speed?
Yes. Rearrange \(Z = \rho c\) to get \(\rho = Z/c\) or \(c = Z/\rho\) when the other two quantities are known.
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