Sonic Horns: A User’s Introduction

Issue 11 and Volume 112.

By Glenn Smith, CEO, Industrial Accessories Co.

A sonic horn can get material flowing in your storage vessel or hopper truck or railcar by directing low-frequency sound vibrations into the vessel to fluidize stagnant material and buildup. This article introduces storage applications for sonic horns, and explains sonic horn benefits, operations and selection.

A sonic horn (also called an acoustic horn or acoustic cleaner) is one of many types of flow aids you can use to get bulk solid materials flowing from vessels and ensure complete discharge to prevent batch-to-batch contamination. Such aids are commonly used to remove ratholes, bridging and sidewall buildup caused by fine materials.

Unlike flow aids that depend on continuous mechanical vibration, air blasting or manual air lancing, a sonic horn 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. 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 the need for a large compressed-air reservoir that 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.

How the Sonic Horn Works

The sonic horn consists of three main sections, as shown in Figure 1: 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 is available in carbon steel and other materials for special applications, including stainless steel for resistance to reactive chemicals and aluminum to make it easy to transfer from one truck or railcar to another.

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The sonic horn can be used on silos, bins and hoppers and on hopper trucks or railcars. On a storage vessel, as shown in Figure 2, the sonic horn is typically top-mounted, that is, mounted in the access hatch on the vessel’s roof. In this way the horn can direct sound vibrations down into material in the vessel. (In some applications, chains are used to suspend the horn in the vessel.)

A second sonic horn can be bottom-mounted, installed 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 mounted at the truck or railcar’s discharge to fluidize compacted material and bridges above the discharge.

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

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

The sonic horn’s sound frequency is typically between 125 and 250 hertz (i.e., cycles per second). Frequencies above 250 hertz are more audible and are likely to annoy nearby plant workers. But this doesn’t mean that the lower the frequency, the better. In fact, sound vibrations at frequencies below 60 hertz 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.

Sonic Horn Selection

Working with a sonic horn supplier can help you determine which horn features are best for your application. Typically, the supplier will ask you to complete a technical data sheet describing your current flow problems; your material’s flow properties; your vessel type, size, and operating details; the maximum operating temperature; and other details about your application. Based on this data, the supplier will recommend a sonic horn with the dimensions, sound pressure level (in decibels) and frequency level (in hertz) that can get material flowing from your storage vessel, truck or railcar. The supplier will also help you choose the horn’s construction materials. Once you’ve selected a sonic horn, the supplier can provide advice on where and how to install it and how often to actuate the horn to promote flow from your vessel.

Sonic horns also 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. Reducing pressure drop across the collector and extending filter life are two of the major benefits of this sonic horn application. Sonic horns can also be used in duct and pipe elbows to reduce dust accumulation in these areas, in fans to dislodge dust from the fan vanes and eliminate fan balance problems, and in spray dryers to dislodge buildup from the dryer walls.

Author: With 35 years of OEM and after-market experience, Glenn Smith is CEO of IAC Group Cos., responsible for corporate operations including sales, equipment applications, product development and engineering.