Acoustic horns
Our use of the acoustic horns system is based on co-operation with CONTROL CONCEPTS, INC. who have been operating in this field for many years. Official company representatives in other countries.
Acoustic horns are wideband sound sources of low frequency (64 – 272 Hz) and high sound pressure Lp (137 - 147 dB with reference r = 1 m).
Sound of the necessary parameters is transmitted into the equipment which needs cleaning. Most of the energy created by the sound pressure is used for cleaning the equipment; it causes sections of the air to vibrate and this is able to dislodge dust particles which have settled in the equipment.
Construction and functioning principles of acoustic horns
Looking at the construction of the acoustic horn, it is composed of a sound generator and a bell section. A titanium sheet membrane is located inside the sound generator, which is made of a flexible tube connected through a reducing valve to a compressed air distribution system (0.50 – 0.65 MPa). The compressed air vibrates the membrane thus creating a turbulent current of air at the edges of the membrane and inside the bell section itself.
The movement of the membrane and the air turbulence become the source of the sound waves which are emitted through the bell section. The bell section functions as a quarter-wave resonator and also as a directional radiator (its directional properties are displayed mainly at higher frequencies). Different sized bell sections mainly influence the spectral qualities of the radiated sound and only in a limited way can influence the actual acoustic power.
Acoustic properties of acoustic horns
Acoustic horns radiate a wide spectrum sound composed of a large number of discrete elements. The radiated sound is produced at the first harmonic of frequency f1 and higher harmonics of whole integers of the first harmonic f1. The harmonic levels gradually fall with vibrational number. Although the majority of the acoustic power (dB) is composed of the first few harmonics, in practice even much higher harmonics are important.
At the first harmonic frequency, the horn radiates almost equally in all directions. This is because the mouth of the bell has a relatively small "d" and because at the low frequencies which acoustic horns work at, the condition d << l (where 1 is the sound’s wavelength) is met. The narrow directional properties of acoustic horns occurs at higher frequencies, where the condition d ~ l or d > l is met.
With increasing distance „r“ from the acoustic horn the level of the sound pressure Lp falls – see picture 1.
Picture 1. Dependence of sound pressure level Lp on distance r from the mouth of the bell section in the axis of the horn (ACL-17220 acoustic horn).
Acoustic horn connection diagram
An acoustic horn system is composed of the acoustic horn itself, a compressed air distribution system and a control system, see picture 2. Compressed air is transferred to the acoustic horn through a flexible tube attached to the compressed air distribution system through a pressure regulator, a round seal and a solenoid valve. The solenoid valve can be controlled by the operator’s current system or it can be equipped with a local control unit.
Picture 2. Acoustic horn connection diagram
Operational conditions
For the whole acoustic horn system to work properly, it is necessary to ensure a constant pressure value of the compressed air – min. 0.50 MPa. For this reason the acoustic horn system includes a pressure regulator so that it can be precisely set. If the working air pressure pp drops below the minimum value of 0.50 MPa, the level of the radiated sound pressure Lp falls and the efficiency of acoustic horn is lowered – see picture 3.
Picture 3. Dependence of the sound pressure level Lp on the acoustic horn pressure pp
Examples of acoustic horns in use
Boiler - the active area of the pipe exchanger, superheater and flue-gas preheater where ash and dust have caused deterioration in the heat conversion and thus lowered the boiler’s efficiency.
Fibre filter - removal of dust from the cloth tube increases its lifespan.
Electrical precipitator - elimination of dust created by flue gases, replacement for mechanical cleaning.
Silos and storage bins - prevention of clogging of fine materials, eliminating bridging.
Ventilators - cleaning the fan wheel prevents its imbalance.
