Fundamental frequency

The fundamental frequency is the lowest main frequency at which a vibrating system naturally repeats. For a sonic horn, it is the nameplate frequency at which the driver, resonator and horn geometry are designed to produce the main acoustic output.

In a sonic horn

A horn marked 75 Hz produces its strongest intended tone around 75 cycles per second. The real sound also contains harmonics at integer multiples such as 150 Hz and 225 Hz, plus broadband mechanical and flow noise. The fundamental is the frequency used for selection, wavelength estimates and comparison between low- and high-frequency cleaners.

Why it matters

The fundamental frequency determines wavelength, horn size and coupling to the process volume. A low fundamental creates a long wavelength that can propagate through large boilers, ESPs or silos. A higher fundamental creates a shorter wavelength that can work better in compact or dense geometries such as bag rows and catalyst faces.

Design limits

Changing supply pressure may change sound pressure and stability, but it does not turn a 75 Hz horn into a 350 Hz horn. The frequency is set mainly by the mechanical and acoustic design. If a horn sounds off-frequency, unstable or weak, likely causes include diaphragm wear, incorrect compressed-air pressure, damaged resonator parts, fouled bell, loose mounting or poor air quality.

Selection notes

A horn's fundamental should be matched to the cleaning volume before purchase. Large vessels usually need lower fundamentals because the wave must travel further and interact with large deposit fields. Compact targets can use higher fundamentals. The selection should also consider external noise, available compressed air, mounting space and the ability to maintain the driver. Two horns with the same nominal frequency can still differ in sound pressure, air consumption and durability, so frequency is a starting point rather than the full specification.

Acoustic-cleaning context

For a horn or resonant cavity, the fundamental frequency is the main tone around which the device is designed to operate. Harmonics may also be present, but the fundamental usually defines wavelength, penetration and how the sound couples into a vessel. A low fundamental gives a long wavelength that can work across larger gas volumes. A higher fundamental gives shorter wavelengths that may interact better with compact passages or fine dust layers.

Field measurement is normally done with a sound analyser or microphone positioned safely outside or at an access point. The measured peak can shift if compressed-air pressure, diaphragm condition, temperature or horn damage changes. If a horn no longer cleans as expected, confirming the fundamental frequency and sound pressure is a useful check before assuming the deposit has changed.

Related terms

• Frequency
• Harmonic
• Resonance
• Diaphragm horn