The invention relates to a method and an apparatus for the generation of low frequency sound. In particular the apparatus according to the invention relates to a feeder unit, also called exigator or pulsator, to be used with a low frequency sound generator. In addition to the feeder unit a low frequency sound generator includes some type of resonator, and the purpose of the feeder unit is to excite a low frequency standing, gas bourne, soundwave inside the resonator. By low frequency sound is, for the purpose of this context, understood sound of a frequency of less than 60 Hz and even infrasound of less than 20 Hz.
The low frequency sound generators are typically used to excite a standing gas-bourne sound wave and it is the resulting oscillating movement of the gas which may be utilized for industrial purposes. Various types of low frequency sound generators for industrial use are previously known, for example through EP, B1, 0 006 833 and WO 88/07894. However, for some applications of low frequency sound, for example as described in WO 90/05275, it is desirable to use more than one low frequency sound generator. The operation of these low frequency sound generators must then be coordinated and synchronized in order to get maximum effect. As described in the mentioned WO 90/05275 this may be achieved by means of letting two motor driven feeder units be driven by a common motor. However, the apparatus according to the invention herein described offers a simpler solution to the problem of synchronization. Instead of using two feeder units, the apparatus herein described provides the possibility of employing only one feeder unit and this feeder unit is capable of servicing two resonators.
The present feeder unit has been designed in such a way that it may also be used to service only one resonator.
The basic principle for the method and the operation and design of the feeder unit (or pulsator) according to the invention is to generate periodic changes of the volume of a resonator and by having the period of the volume changes correspond to the natural frequency of the resonator a standing sound wave is excited in the resonator. The feeder unit comprises of three rotating parts, rotors, of basically cylindrical shapes and which are mounted in a casing. The rotors are driven by a common motor and their rotation is subsequently automatically synchronized. On two sides of the rotor complex a resonator is mounted. The rotation of the rotors in combination with their very special shape, together result in that a small volume of air or other gas is being transported from one resonator to the other resonator and then back to the first resonator. This back and forth transport of air is a continuous process in that as long as the rotors are rotating air is being transported from either of the resonators to the other resonator. By feeding a volume of air into either resonator and then by means of the movement of the rotors first decrease the volume of the resonator and then increase said volume and repeating this sequence with a certain periodicity, a standing, gas bourne, sound wave of a certain low frequency is excited inside the resonator corresponding to the first natural frequency of the resonator, which is determined by its dimensions. This is achieved by having the rotation frequency of the rotors, which is determined by the motor, correspond to the natural frequency of the resonator.