Acoustics and physics
Attenuation (acoustic)
Also known as acoustic attenuation, sound attenuation.
Attenuation is the loss of acoustic energy as a sound wave propagates through a medium. It combines geometric spreading (the inverse-square law) with absorption losses to viscosity, heat conduction and molecular relaxation. Attenuation rises sharply with frequency, which is the physical reason low-frequency acoustic cleaners reach further into large industrial vessels than their high-frequency counterparts.
Implications for cleaning reach
A 60 Hz wave loses very little energy per metre of air travel; a 400 Hz wave loses substantially more. In hot flue gas the absolute losses change but the frequency dependence remains the same. The result is that a 60 Hz horn can clean fly-ash deposits 8–10 metres from the bell, while a 400 Hz horn is generally effective only within 3–4 metres at the same nameplate SPL.
Implications for noise control
The same physics that lets a low-frequency horn reach deep into a vessel also lets it travel further outside the vessel. Operator-station noise control is therefore harder for low-frequency installations, and sound-attenuation enclosures are sometimes added at the bell.
Related terms
Related terms
- WavelengthWavelength is the distance a sound wave travels in one cycle. At 60 Hz in air a wave is 5.7 m long; at 400 Hz it is 0.85 m. Wavelength governs how far a sonic horn's cleaning reach extends.
- FrequencyFrequency is the number of acoustic cycles per second, measured in hertz. Industrial acoustic cleaners operate at 12–30 Hz (infrasonic), 60–250 Hz (low) or 250–450 Hz (high).
- Inverse-square lawIn free-field conditions sound intensity falls as 1/r². Sound pressure level drops by approximately 6 dB for each doubling of distance from the source.
- Sound-attenuation enclosure (sonic horn)A sound-attenuation enclosure surrounds the sonic horn to reduce SPL at the operator station. Typical 10–25 dB reduction; required where horn proximity exceeds OSHA / EU action levels.