Acoustics and physics
Acoustic impedance
Also known as characteristic impedance (acoustic), specific acoustic impedance.
Acoustic impedance is the resistance a medium offers to the flow of acoustic energy. It is the product of medium density and the local speed of sound and is measured in pascal-seconds per metre (Pa·s/m). When sound travels from one medium to another with different impedance, a fraction of the energy is reflected at the interface — the larger the mismatch, the more is reflected.
Why bell horns exist
A bare pneumatic diaphragm is small, stiff and presents a high acoustic impedance. The open volume inside an ESP or boiler is large and low-impedance. A direct coupling would reflect most of the diaphragm's energy back to itself instead of radiating it into the vessel. The bell horn is an impedance-matching transformer: its exponential flare gradually steps the impedance down from the throat to the mouth, letting acoustic energy escape efficiently into the gas.
Why air-to-metal interfaces reflect almost everything
Air has an acoustic impedance of roughly 410 Pa·s/m; steel is roughly 47 million Pa·s/m — a five-order-of-magnitude mismatch. Sound waves striking a metal tube reflect with essentially no transmission. Cleaning energy therefore couples to deposits via gas-borne pressure variation, not by transmission into the metal.
Related terms
Related terms
- Bell hornA bell horn is the conical or exponential flare that amplifies and projects sound from an industrial sonic horn's driver into the vessel being cleaned.
- Sound pressure levelSPL is the logarithmic measure of sound pressure in decibels relative to a 20 µPa reference. Industrial sonic horns operate at 140–180 dB SPL.
- Attenuation (acoustic)Attenuation is the loss of acoustic energy as a sound wave propagates. Higher frequencies attenuate faster, which is why low-frequency sonic horns reach further in industrial vessels.