Frequency

Frequency is the number of cycles per second in a repeating wave, measured in hertz (Hz). In acoustic cleaning it describes how many pressure oscillations a horn produces each second and therefore how the sound couples into a vessel, duct, hopper or tube bank.

Frequency and wavelength

For sound in air, wavelength equals speed of sound divided by frequency. At ordinary industrial temperatures, a 75 Hz tone has a wavelength of several metres, while a 350 Hz tone has a wavelength around one metre. Hot gas changes the speed of sound, so the actual wavelength inside a boiler or HRSG differs from a room-temperature estimate.

Frequency bands in acoustic cleaning

The right frequency is not simply the loudest horn. It depends on target geometry, deposit type, gas temperature, wall openings, attenuation and whether the cleaning target is a large open volume or a dense matrix.

Design implications

Lower frequencies travel further and diffract around obstacles, but the horns are larger. Higher frequencies can deliver stronger local agitation in compact spaces, but they attenuate faster and may not penetrate deep vessels as well. Firing duration and interval are selected with frequency because deposit response depends on repeated cycles, not just peak sound pressure.

Commissioning notes

Frequency should be checked in the installed condition if performance is uncertain. A handheld sound analyser or plant acoustic test can confirm whether the expected fundamental appears during firing. The measured spectrum will include reflections and casing response, so it should be interpreted with the horn supplier's data rather than as a laboratory purity test. If the frequency content changes over time, inspect the driver, diaphragm or piston, air pressure, nozzle blockage and mounting bolts before changing the process cleaning schedule.

Measurement and plant context

Frequency appears in several plant systems at once. Electrical systems use 50 or 60 Hz supply frequency. Rotating equipment has running speed and blade-pass frequencies. Acoustic cleaners have operating frequencies tied to wavelength and sound propagation. Vibration analysts use frequency spectra to separate imbalance, misalignment, looseness, gear defects and resonance. The same unit, hertz, can therefore describe very different physical events.

In acoustic cleaning, frequency affects how sound couples into a space. Low frequencies have longer wavelengths and can project through large boiler passes, hoppers or ESP chambers. Higher frequencies have shorter wavelengths and can be useful in compact passages or fine-dust applications, but they attenuate more readily and may be more audible to workers. Frequency selection should be checked against vessel dimensions, deposit type, gas temperature, horn power and noise exposure, not chosen from a generic range.

Commissioning and troubleshooting

Frequency should be measured under normal operating conditions because the value can shift with air pressure, temperature, load, mounting stiffness or wear. In acoustic-cleaner commissioning, the measured frequency spectrum helps confirm that the horn is producing the intended tone and that nearby panels are not adding unwanted resonance. In rotating equipment, a new frequency peak can identify a developing mechanical fault before amplitude alone becomes alarming.

Plant teams should avoid mixing frequency with level. A sound can have the correct frequency but too little pressure to clean a deposit. A vibration can occur at a known frequency but still be harmless if amplitude and phase are stable. Frequency gives the pattern; severity comes from the rest of the measurement.

Related terms

• Wavelength
• Sound pressure level
• Fundamental frequency
• Low-frequency acoustic cleaner
• High-frequency acoustic cleaner
• Infrasonic cleaner