A sonic horn (also called an acoustic horn or acoustic cleaner) produces and amplifies low-frequency sound vibrations at high intensity to fluidize and remove material buildup. Because the sonic horn doesn’t apply mechanical vibration, it doesn’t compact material or segregate it by particle size and is less likely to cause vessel failure than a continuously operating mechanical vibrator.A sonic horn is also used to start bulk

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Sonic horns are used to
reduce dust accumulation in areas that are difficult to access, such as this baghouse.

solids flowing from vessels and to ensure vessels are completely emptied to prevent batch-to-batch contamination in some applications.Unlike an air cannon, which directs a narrow localized air blast into one vessel area, the sonic horn provides fluidization throughout the vessel. The horn also uses less compressed air than an air cannon, eliminating a need for a large compressed-air reservoir, which can be a source of potential moisture contamination. The sonic horn also eliminates the labor required for injecting fluidizing air into a vessel with a manual air lance. Sonic horns have widespread application in dust collectors, where they can
fluidize dust collected in hoppers to aid discharge and can be used in conjunction with the filter-cleaning system — even replacing a baghouse shaker system — to dislodge dust from filters. Among the major benefits are reducing pressure drop across the collector and extending filter life.

How the sonic horn works
The sonic horn consists of three main sections: a driver, which includes a compressed-air inlet and a steel diaphragm; a cone-shaped chamber called a bell with a round or square cross section; and a larger- diameter horn outlet. The horn can be equipped with a mounting flange at any of

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A typical sonic horn uses
compressed air to produce acoustic energy to fluidize materials. This horn is installed in the baghouse of a coal-fired power plant.

various points along the horn, depending on how it will be installed.

In operation, an automatic timer linked to the sonic horn driver’s compressed-air inlet releases a burst of 60- to 80-psig compressed air into the driver. Typically, each air burst lasts several seconds (up to about a 20-second maximum), with the intervals between bursts depending on the application. The air burst entering the driver causes the diaphragm to vibrate. This produces sound waves that are amplified as they move out through the bell, which functions much like a handheld megaphone.

The sound waves move through the horn outlet into the vessel and displace the air, producing low-frequency sound vibrations at a high-pressure acoustic energy level. Because sound waves are pressure waves, they cause pressure fluctuations that break the bond structure in the material buildup. This fluidizes the material and facilitates its flow from the vessel.

The vibrations’ acoustic energy level falls within a fixed frequency band determined by the bell’s shape and size.

Where horns are installed
The sonic horn is typically top-mounted in a storage vessel, usually in the access hatch on the vessel’s roof, so the sound vibration is directed down into the materials. A second sonic horn can be bottom-mounted on the vessel’s cone section, just above the discharge or at the cone-body transition point, with the horn outlet facing inward to break up material bridges above the discharge. In some cases, the bottom-mounted sonic horn is installed on a small fluidizing chamber mounted below the vessel discharge.

On a hopper truck or railcar, the sonic horn can be top-mounted in the roof’s loading hatch to fluidize compacted material and break up ratholes. A second horn can be bottom-mounted at the
truck or railcar’s discharge to fluidize compacted material and bridges above the discharge.

Sound pressure level
The sonic horn’s ability to remove material buildup depends on its sound vibration intensity, or sound pressure level, and is measured in decibels. To fluidize most materials, the sound pressure level must be above 120 decibels. The greater the sound pressure level, the more effective the material removal.

Low-frequency limits
The sonic horn’s sound frequency is typically between 125 and 250 Hz. Frequencies above 250 Hz begin to enter the audible spectrum and may annoy nearby plant workers. Frequencies lower than 60 Hz not only lose their power to remove material buildup but can damage solid structures, such as silo walls and support legs, and mechanical connections, such as feeders and dischargers.

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