This invention relates to an arrangement at a boiler plant or other cavity where an essentially pure tone of a low frequency sound of a certain frequency, preferably infrasound; and with a high intensity, is generated. For instance, such a low frequency sound is obtained when cleaning with infrasound, during combustion by infrasound, or in connection with any other equipment where infrasound generators are used.

Through EP, A, 0 006 833, an infrasound generator is previously known which, among other things, is used for the cleaning of heat exchanger surfaces in a boiler plant through exposing its surfaces to the effects of infrasound. Infrasound generators can also be used for the soot cleaning of big industrial boilers as well as along with other types of equipment used with industrial boilers, such as economizers and air preheaters. The required acoustic power is of the order of 100 W and more. In order to achieve such a high acoustic power at low frequencies, a special resonator is required, which usually consists of a resonance tube with a length equal to a quarter of the wave length to be generated. This resonance tube is attached to the feeder unit of the infrasound generator and is open at the other end. The resonance tube is, in principle, of constant diameter and communicates at its open end with a cavity by means of an opening in one of the walls of the mentioned cavity. The cavity consists of the boiler itself, in the case of soot cleaning a boiler plant.

It has been shown that, in certain cases, some of the acoustic power generated by the infrasound generator can leak out through existing ducts for the evacuation of flue gases or similar tubes or pipes. In the first place, this has the result that an unwanted, low frequency sound is obtained outside the cavity, which in different ways can be considered disturbing for the surroundings. Secondly, it has the result that the desired high sound intensity inside the cavity in question, is decreased.

The purpose of the present invention is to prevent leakage of low frequency sound at an installation of the above mentioned type.

It is previously known from i. a. the book "Fundamental of Acoustics" by Kinsler & Frey (2nd edition, Wiley 1962), to employ, what can be described as a side branch resonator, as an acoustic filter in connection with pipes. Said side branch resonator causes all the acoustic energy, which is transmitted into its cavity, to be redirected back into the main pipe with such a phase displacement that it is reflected back to the source of the sound.

The purpose of the present invention is to apply said known technique to low frequency sound in particular, and especially at installations of the above mentioned categories where infrasound generators are used.

The purpose of the invention is achieved by arranging a separate resonator, functioning as a sound reflector, by the side of and in

connection with a duct or a pipe, as described above. The acoustic power leaking out through the duct/pipe will, through the influence of the sound reflector, be reflected back into the original cavity, being the boiler or similar.

The invention will now be described with reference made to the attached drawing which shows one embodiment of the invention.

The infrasound generator 1 can e. g. be of the type described in the mentioned EP, A, 0 006 833, or EP, A 0 077 364. It consists of a feeder unit 2, called the Exigator, and a resonance tube 3. It is connected to a cavity 4, which for instance can be a boiler, via the resonance tube 3. Through the duct 5, flue gases are evacuated. A sound reflector 6 is connected to the duct 5; said sound reflector consisting of a resonator being a hollow box 7, as well as of a neck part 8, which connects said box with the duct 5. If any losses of viscosity can be considered negligible (regarding the conditions for this assumption, see below), no net dispersion of energy from the pipe 5 into the sound reflector 6 can be observed.

The transmission coefficient for sound, valid for the part of the duct 5 which is situated beyond the sound reflector, can be expressed as a function of the volume of the sound reflector; the cross section area of the duct 5; the sound speed; the frequency of the sound in the duct 5; and the natural frequency of the sound reflector. Mathematically, it can be shown that the said transmission coefficient will be zero (i.e. no sound will be propagated in the duct beyond the sound reflector)

when the sound frequency of the sound in the duct 5 is equal to the natural frequency of the sound reflector. From the basis of this knowledge the sound reflector can be dimensioned. An important condition for the above to be valid, and in order to be able to consider viscosity losses, if any, to be negligible, is that the neck 8 of the sound reflector must not be made too narrow.

By means of the above described manner of using a side branch resonator as a sound reflector in connection with boiler plants or other cavities where infrasound is generated, i. a. for soot cleaning purposes, any inconveniences for the surroundings, which can be caused by leaking sound, are avoided; and, at the same time, the favourable result is achieved that a larger part of the generated acoustic power can be utilized in the cavity itself.