[0001] The present invention relates to an apparatus for generating in particular low-frequency
sound, comprising a drive unit which is adapted to generate sound by the supply of
compressed air, and an open resonator which is connected to said drive unit and preferably
is in the form of a straight pipe.
[0002] It is known to remove soot from furnaces and to clean for example filters by means
of low-frequency sound generated by means of low-frequency generators provided with
resonators. For efficient soot removal and cleaning, considerable sound pressures
are required, and this in turn means that large and cumbersome units must be used.
Frequently, several such units must be used together. The low--frequency generators
normally are of the pneumatic type, the frequency generation being integrated with
the oscillator (the diaphragm) and the frequency control being integrated with the
horn. The sound transmitter is self-adjusting and operates on the system's own resonance
frequency, and the frequency cannot be changed. Existing pneumatic sound transmitters
with controlled frequency are highly sophisticated. The air supply valve conforms
slavishly to changes of optional signals. The control is, so to speak, an "analog"
control. Coupling in parallel of such known devices may result in phase shifts reducing
the efficiency. It may also happen that entire systems or at least parts thereof are
subjected to heavy resonant vibration.
[0003] It is the object of the present invention to eliminate the above mentioned disadvantages
of prior art technique and to provide an extremely simple apparatus for generating
low-frequency sound, said apparatus being provided with means for controlling the
frequency of the sound generated. In this manner, it is possible without difficulty
to couple in parallel a desired number of apparatuses of relatively small size. This
object is achieved in that the drive unit comprises a housing having a chamber with
an aperture opening in the resonator, a member oscillating within said chamber and
having a first actuating surface which is located within a first space of said chamber
and by means of which said member is movable, by supply of compressed air to said
first space, in a direction away from said aperture, such that said aperture is opened,
and a second actuating surface which is located in a second space and by means of
which said body is movable, by supply of compressed air to said second space, in a
direction toward said aperture, such that said aperture is closed, that said second
actuating surface has a larger area than said first surface, and that said second
space communicates with the atmosphere via a passage which is opened and closed by
means of an electrically operated valve.
[0004] The invention will be described in more detail below, reference being had to the
accompanying drawings illustrating embodiments. Fig. 1 is a lateral view, partly in
section, of an apparatus designed in accordance with the invention and adapted to
generate low-frequency sound, and Fig. 2 illustrates an modified embodiment of such
an apparatus. Fig. 3 illustrates how a number of apparatuses according to the invention
can be coupled in parallel for cleaning the combustion space of a furnace. The apparatus
according to the invention as illustrated in Fig.
1 comprises a drive unit generally designated
10 and having means for generating low-frequency sound. Connected to said drive unit
10 is a resonator 11 which may have any shape suitable in the context, for example
flaring exponentially, although it preferably is in the form of a straight pipe. The
drive unit 10 has a chamber 12 which is connected to the resonator 11 via a duct or
aperture 13. The chamber 12 houses an approximately cylindrical member 14 provided
at one end with a sealing means 15 which, in Fig. [, as its slightly raised peripheral
edge engaging a portion around the aperture 13. Nearest the sealing means 15, the
member 14 has a smaller cross-sectional area than the chamber 12, such that a first
space 16 is formed. At some distance from the aperture 13, the member 14 widens to
the cross-sectional area on the chamber by means of an inclined annular portion 35
forming an actuating surface. The cylindrical member terminates at a distance from
the rear end of the chamber 12 to form a second space 17, and it appears that the
side of the member 14 facing the space 17 has a much larger area than the transitional
portion 35. The member 14 is provided in its rear side with a bore housing a spring
19 which strives to urge the member 14 toward the aperture 13. Extending between said
first space 16 and said second space 17 is a duct 18 having a relatively small cross-sectional
area and opening in the bore of the member 14. In the rear side of the drive unit
10, which is constituted by a removable lid, two passages 20, 22 are provided, the
first of which extends from the space 17 and opens in a cavity in a valve 21 mounted
on said drive unit 10. This cavity of the valve 21 communicates with the atmosphere
through the second passage 22. The valve 21 is a solenoid valve having a coil 23 which
is connected to a current source by means of wires 24 and within which. an armature
26 is movable back and forth in conventional manner in response to the activation
of the coil 23. The armature 26 is connected with a valve member 25 which, in its
advanced position, closes the entrance to the passage 22 communicating with the atmosphere.
It will be seen that, by displacing the valve member 25, the communication between
the space 17 and the atmosphere is opened and closed. It should be emphasised in this
connection that the valve 21 is but schematically shown and may be replaced by other
valves; the important thing is that the communication between the space 17 and the
atmosphere can be opened and closed electrically at the desired frequency. A transverse
bore provided in the drive unit 10 and opening in the space 16 houses a connection
27 for connecting a compressed air line 29 to said drive unit 10.
[0005] Sound is generated by the apparatus according to Fig.
1 by supplying compressed air to the drive unit 10 via the line 29 and the connection
27. The compressed air first enters the space 16 and then goes to the space 17 via
the duct 18 and the bore. Since the compressed air acts on a much larger area of the
rear side of the member 14 than on the front side of said member, and since this force
furthermore is supported by the force of the spring 19, the member 14 will be held
with the seal 15 i engagement around the aperture 13, as shown in Fig. 1. If an electric
voltage is applied to the coil 23 which thus moves the armature 26 away from the drive
unit 10, the valve member 25 is lifted from its seat surface, thereby exposing the
opening of the passage 22 in the valve cavity so that the space 17 will communicate
with the atmosphere via the passages 20, 22. As a result, the pressure in the space
17 is reduced, and by the action of the compressed air on the actuating surface 35
the member 14 is moved to the right in Fig. 1, whereby the aperture 13 is opened toward
the chamber 12. When the valve member 25 again closes the passage 22, the pressure
in the space 17 is again built up so that the member 14 is moved to the left in Fig.
1 and the aperture 13 is closed. This movement of the member 14 in the drive unit
10 generates the low-frequency sound which is amplified by the resonator 11. It will
be appreciated that the reciprocating movement of the member 14 is controlled by means
of the valve member 25 whose movement is dependent upon the movement of the armature
26 and, thus, by the activation of the coil 23. The movement of the member 14 and
thus the frequency of the sound generated may be readily controlled electrically by
connecting the lines 24 of the coil 23 to a suitable current source which is capable
of supplying pulses of a frequency readily adjustable, preferably a square wave voltage.
[0006] Fig. 2 illustrates a modified embodiment of the drive unit, a diaphragm
14 forming the reciprocable member. As will be seen, an annular ridge 28 is provided
around the aperture 13 to the resonator 11, and this ridge is engaged by one side
of the diaphragm 14 in the state corresponding to the one shown in Fig. 1. In this
embodiment, the space 16 is formed by the annular space between the ridge 28 and the
periphery of the diaphragm 14, and the space 16 communicates with the space 14 on
the rear side of the diaphragm 14 via the duct 18 which here is formed in the diaphragm
proper. The diaphragm 14 is mounted such that it engages the ridge without actuation
by compressed air, and when compressed air is supplied via the passage 29 and the
connection 27, the air will flow, as in the embodiment previously described, into
the space 16 and from there via the duct 18 into the space 17 which is vented in the
same manner as in the earlier embodiment, the diaphragm 1
4 being urged away from the ridge 28 by the compressed air in the space 16.
[0007] As has been indicated above, the resonator preferably is a straight pipe 11 because
this will give a resonance curve having a relatively wide peak. lf, on the other hand,
the hom flares exponentially or in accordance with a Bessel curve, the degree of efficiency
will be affected, and the resonance curve will be far more narrow. The resonance frequency
is determined by the length of the horn. With low frequencies, the horns must be of
considerable lengths and therefore may be curved, for example helically. For optimation
of the horns at low frequencies, the horns may also comprise several different sections,
such as straight pipes with conical sections.
[0008] Since the apparatus according to the invention can be controlled with the greatest
ease, a plurality of apparatuses may be readily coupled in parallel without causing
phase difficulties. If a plurality of apparatuses are coupled in synchronism (in phase),
a considerable sound amplification may be achieved. Fig. 3 illustrates an arrangement
in which a combustion chamber 30 in a boiler is cleaned by means of several apparatuses
according to the invention coupled in parallel and provided with a resonator
11 in the form of a long straight pipe projecting into the space 30 with its end facing
away from the drive unit 10. The drive units 10 of the apparatuses are connected to
a compressed air line 29 and to a control wire 24 which is connected to a current
source 31. The current source 31 produces a square wave. voltage of readily controlled
frequency, as indicated at 32. It is also possible to provide on a resonator 11 a
sensor 34 which senses the resonance frequency of the resonator and is connected,
via a wire 33, to the current source 31 for automatic control of the frequency of
the square wave voltage.
[0009] With the arrangement according to Fig. 3, all drive units 10 are supplied with a
control voltage of the same frequency and the same phase. For optimation purposes,
this control voltage is manually adjustable by means of a knob on the current source
31. It is, of course, also conceivable to supply each driving unit with current from
a current source of its own and separately to adjust the oscillation frequency of
each drive unit
1. Apparatus for generating in particular low-frequency sound, comprising a drive
unit (10) which is adapted to generate sound by the supply of compressed air, and
an open resonator (11) which is connected to said drive unit - (10) and preferably
is in the form of a straight pipe, characterised in that the drive unit (10) comprises
a housing having a chamber (12) with an aperture opening (13) in the resonator (11),
a member (14) oscillating within said chamber (12) and having a first actuating surface
(35) which is located within a first space (16) of said chamber and by means of which
said member (14) is movable, by supply of compressed air to said first space (16),
in a direction away from said aperture (13), such that said aperture (13) is opened,
and a second actuating surface (36) which is located in the second space (17) and by means of which said body (14) is movable, by supply of compressed air to
said second space (17), in a direction toward said aperture (36), such that said apertures
is closed, that said second actuating surface (36) has a larger area than said first
surface (35), and that said second space (17) communicates with the atmosphere via
a passage (20, 22), which is opened and closed by means of an electrically operated
valve (21).
2. Apparatus as claimed in claim 1, characterisedin that the said electrically operated
valve (21) is a solenoid valve having an armature (26) which is reciprocable in a
coil and connected to a valve member (25) opening and closing said passage (20, 22),
and that a current source (31) is connected to said coil (23) for generating pulses
of a manually or automatically controllable frequency.
3. Apparatus as claimed in claim 1 or 2, characterisedin that there is provided in
said second space (17) a spring - (19) for actuation of said member (14) toward said
aperture (13).
4. An apparatus as claimed in claim 1, 2 or 3, characterised in that a sensor (34)
is mounted on said resonator (11) for sensing the resonance frequency thereof and
is connected to said current source (31) for frequency control.
5. An apparatus as claimed in any one of the preceding claims,characterised in that
said member (14) is an approximately cylindrical member axially movable within said
chamber (12) and provided with a sealing means (15) at its end facing said aperture
(13).
6. An apparatus as claimed in any one of claims 1-4, characterised in that said member (14) is a diaphragm clamped in said chamber (12)
and having its one side facing said aperture (13), said diaphragm being movable into
sealing engagement with a ridge (28) provided around said aperture.
7. An arrangement for removing soot in furnaces or the like, characterised in that
it comprises a plurality of sound generating apparatuses of the type stated in any
one of the preceding claims, said apparatuses being coupled in parallel and connected
to a common current source (31), and said apparatus further being operable in synchronism,
i. e. with the same frequency and phase.
