Abstract:
An acoustic transducer provided with at least one sound source for generating an acoustic centre and a predetermined construction for guiding sound generated by the acoustic centre, which acoustic transducer can be fixed to a fixing wall, wherein the predetermined construction is so designed that, during operation, the generated sound is displaced by the predetermined construction to a displaced acoustic centre at a location that is on the fixing wall ( 5 ), when the acoustic transducer ( 10 ) is fixed to the fixing wall ( 5 ).

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to an acoustic transducer provided with at least one sound source for generating an acoustic centre and a predetermined construction for guiding sound generated by the acoustic centre, which acoustic transducer can be fixed to a fixing wall.  
         [0002]     State of the Art  
         [0003]      FIG. 1  shows a transducer  2  that is known from the state of the art, wherein the known transducer  2  is mounted in a manner known from the state of the art. The known transducer  2  consists of a compression driver and a horn (both not shown), which according to the state of the art convert electrical signals into an acoustic wave front and disseminate this through the area. The known transducer can, however, also be another transducer known from the state of the art, such as, for example, a cone loudspeaker.  
         [0004]     If such a known transducer  2 , suitable for reproducing acoustic signals, is mounted some distance away from a fixing wall, that is to say a sound-reflecting surface  5 , some of the sound energy radiated will travel directly from the known transducer  2  to the sound-reflecting surface  5  and be reflected from the latter. Such reflections are termed primary reflections. This means that an observer  1  hears both sound that originates directly from the known transducer  2  and sound that originates from the reflection from the sound-reflecting surface  5 . The direct sound and the reflected sound have both travelled a different distance between leaving the known transducer  2  and reaching the observer  1  and thus a phase difference is produced between the direct and the reflected sound at the location of the observer  1 . This phase difference results in location-dependant destructive interferences in the listening plane that can degrade the sound quality in the listening position. This effect can also be modelled using, instead of reflections, the addition of a virtual sound source  3 , which is the same distance away on the other side of the sound-reflecting surface  5 . This virtual sound source  3  can be defined as the sound source associated with the reflected sound.  
         [0005]     The production of destructive interference is, in particular, a problem in the case of known transducers  2  that are installed in relatively low and acoustically hard environments, such as tunnels and multi-storey car parks, where the walls are made of concrete or hard plating and where the walls reflect the sound well.  
         [0006]     The aim of the present invention is to provide a transducer that, by means of correct fixing to a wall, is able to prevent primary reflections, which results in a substantial reduction in the destructive interferences in the listening plane. To this end the invention relates to a transducer of the type mentioned in the preamble, characterised in that the predetermined construction is so designed that, during operation, the generated sound is displaced by the predetermined construction to a displaced acoustic centre at a location that is on the fixing wall, when the acoustic transducer is fixed to the fixing wall. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0007]     The invention will be discussed with reference to a few figures, which, however, are intended merely to illustrate the invention and in no way whatsoever have a restrictive effect on the scope of the invention, which is determined solely by the appended claims.  
         [0008]      FIG. 1  shows a diagrammatic overview of the functioning of a known transducer according to the state of the art;  
         [0009]      FIG. 2  shows a diagrammatic overview of the functioning of a transducer according to the present invention;  
         [0010]      FIG. 3   a  shows a diagrammatic side view of a first preferred embodiment of the invention;  
         [0011]      FIG. 3   b  shows a diagrammatic bottom view of the first preferred embodiment of the invention;  
         [0012]      FIG. 4  shows a few components of  FIG. 3  from a different viewpoint;  
         [0013]      FIG. 5 , shows, diagrammatically, a second embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0014]     It will be clear that the embodiment described below is described merely by way of example and not with any limiting significance and that various changes and modifications are possible without going beyond the scope of the invention and that the scope is determined solely by the appended claims.  
         [0015]     In  FIG. 2  an observer  1  can be seen who hears sound originating from a transducer  10  according to the present invention that has been mounted in the immediate vicinity of a sound-reflecting surface  5 .  
         [0016]     The sound can be either a spoken message or a warning signal, music or any other acoustic signal.  
         [0017]     The transducer  10  is so constructed that the acoustic centre is located in the sound-reflecting surface  5 , the acoustic centre having to be regarded as the point or the set of points from which, according to the observer  1 , the sound appears to originate. In this way the observer  1  hears only sound that originates directly from the transducer  10 . In this way destructive interferences in the listening plane are prevented, which makes better detection of the sound reproduced by the transducer  10  possible.  
         [0018]      FIG. 3   a  shows a preferred embodiment of the transducer  10 , which consists of a compression driver  11 , a neck-shaped transition  12  and a horn  13 . Furthermore, the transducer  10  comprises a flange  14  and a removable horn mouth  15 . The compression driver  11  is connected to the neck-shaped transition  12  and the neck-shaped transition  12  is connected to the horn  13  for transmitting a sound signal. There is a slight kink  17  at the location of the connection between the neck-shaped transition  12  and the horn  13 . The transducer  10  has a mounting surface  6  for fixing to the sound-reflecting surface  5 .  
         [0019]      FIG. 3   b  shows a bottom view of  FIG. 3   a , the horizontal angle between the side walls of the horn  13  being indicated as angle  30 .  
         [0020]     A cross-sectional line IIIc-IIIc is indicated in  FIG. 3   a . A cross-section of the transducer  10  along said line gives a rectangular cross-section, as shown in  FIG. 3   c , which increases in the direction away from the neck-shaped transition  12 .  
         [0021]      FIG. 4  shows part of the transducer  10  in  FIGS. 3   a  and  3   b  from a different viewpoint. In this figure only the compression driver  11  and the neck-shaped transition  12  are shown. An outlet  18  of the compression driver  11  and an outlet  19  of the neck-shaped transition  12  are also shown in this figure.  
         [0022]     The compression driver  11  can be, for example, an electrodynamic loudspeaker that provides for the conversion of an electrical input signal into an acoustic wave front. The input of the electrical input signal is not shown. The way in which this takes place or can take place is known to those skilled in the art. There are no further restrictions in this regard.  
         [0023]     The compression driver  11  can be, for example, an 80 watt compression driver  11  having an outlet  18  with a diameter of 5.08 cm, which in the embodiment shown generates a circular flat wave front with a low amplitude and a relatively high pressure at the outlet  18  of the compression driver  11 . As is known to those skilled in the art, a first order high pass filter (not shown) can be used to restrict the power that is supplied to the compression driver  11  beyond its operating range. Similarly, pass filters of higher order can be used, as is known to those skilled in the art.  
         [0024]     However, other compression drivers  11  known from the state of the art can also be used.  
         [0025]     The function of the neck-shaped transition  12  is to transform the wave front originating from the compression driver  11  into a shape that corresponds to the shape of the horn  13 , as is shown in  FIG. 4 . The compression driver  11  generates an acoustic centre.  
         [0026]     In the example described and shown here, the circular wave front at the outlet  18  of the compression driver  11 , originating from the acoustic centre, is transformed into a rectangular wave front having a small dimension of typically 2.5 cm in a first direction and a dimension in a second direction, perpendicular to the first direction, of approximately 16 cm. In use, the first direction will usually be vertical and the second direction horizontal. Other directions are, however, possible. Depending on the intended use, design requirements and the like, other dimensions can also be used.  
         [0027]     The shape of the neck-shaped transition  12  is such that the acoustic centre is displaced from the outlet  18  of the driver  11  to a displaced acoustic centre that is located at the outlet  19  of the neck-shaped transition  12 , which is so positioned that this outlet  19  is in contact with the sound-reflecting surface  5  after fixing to surface  5 . In this case the direct sound and reflected sound are coincident and they are no longer able adversely to interfere with one another at the location of the listener  1 . In order words, the position of the acoustic centre has been displaced to the reflecting surface  5 .  
         [0028]     The outlet  19  of the neck-shaped transition  12  merges into the start of the horn  13 . As can be seen in  FIG. 3   a , the wave front makes a slight kink  17  at this point. Depending on the frequencies of the sound used, this kink  17  will or will not have an influence on the reproduction of the sound. For frequencies for which the wavelength is greater than the first dimension of the neck-shaped transition  12  this kink will have no effect. In the case described here, by way of example, the neck-shaped transition  12  has a first dimension of 2.5 cm at the location of the horn  13 . This means that the kink  17  will have no effect on frequencies lower than approximately 13,000 Hz. For these relatively low frequencies the outlet  19  of the neck-shaped transition  12  will behave as a line source located in the sound-reflecting surface  5 .  
         [0029]     The second dimension of the neck-shaped transition  12  is such that the latter closely adjoins the horn  13 .  
         [0030]     The horn  13  can have a wide variety of shapes depending on the requirements in respect of the horn  13 , as is known to those skilled in the art. The shape of the horn  13  determines both the radiation characteristic and the acoustic impedance as a function of the frequency of the sound signal for the acoustic centre displaced to the fixing surface  5 .  
         [0031]     Various shapes of the horn  13  are known from the state of the art.  
         [0032]     In  FIGS. 3   a  and  3   b  a horn  13  having a rectangular frontal cross-section IIIc-IIIc is shown, the surface area of this rectangular frontal cross-section IIIc-IIIc increasing exponentially towards the outlet of the horn  13 . It is known that this shape gives the flattest acoustic impedance curve. Typical values for this design of the vertical opening angle are 40° for a sound signal having a frequency of 250 Hz and 15° for a sound signal having a frequency of 1000 Hz.  
         [0033]     These values are obtained by determining the curve for the sound strength for specific frequencies of the sound, along a vertical arc around the source and determining the angle associated with the line section on said arc on which the value of the sound strength is less than 6 dB weaker than the maximum value on that line section.  
         [0034]     In order to optimise the sound pressure distribution on the listening plane a relatively short distance away from the transducer  10 , the horn mouth  15  can be positioned at an angle with respect to the sound-reflecting surface  5 , as is the case in  FIGS. 3   a  and  3   b.    
         [0035]     Since the horn  13  described here has a rectangular frontal cross-section IIIc-IIIc, the horizontal radiation characteristic is determined by the angle  30  between the side walls of the horn  13 . However, this applies only for frequencies for which the wavelength is less than the dimensions of the horn mouth  15 .  
         [0036]     If the transducer  10  is used, for example, in a road tunnel, it is then advisable to keep the angle  30  small in order to reduce reflections at vertical side walls of the tunnel and the like and to bundle all sound energy in a relatively narrow strip. In the preferred embodiment shown in  FIGS. 3   a  and  3   b , where angle  30  has a value of 36°, the following were found as typical values for the horizontal opening angle: 60° for a sound signal having a frequency of 250 Hz and 26° for a sound signal having a frequency of 1000 Hz.  
         [0037]     These values are obtained by determining the curve for the sound strength, for specific frequencies of the sound, along a horizontal arc around the source and determining the angle associated with the line section on said arc on which the value of the sound strength is less than 6 dB weaker than the maximum value on that line section.  
         [0038]     The mounting surface  6  is the part of the horn  13  that is placed against the sound-reflecting surface  5 . This mounting surface  6  can be provided with fixing means for fixing the transducer  10  in an intended location on the sound-reflecting surface  5 .  
         [0039]     The end of the horn  13  consists of the flange  14  to which the horn mouth  15  can be fixed. This offers the possibility of stretching a fine mesh steel gauze in front of the horn outlet, which can offer protection against, for example, the ingress of water, as can be the case with high pressure cleaning, or other possible sources of damage.  
         [0040]     If the transducer  10  is used on ceilings of road tunnels or multi-storey car parks there is a risk of damage from collision by a vehicle. By virtue of the construction with the removable horn mouth  15 , it is possible, if the damage is restricted to the horn mouth  15 , to replace the horn mouth  15  instead of the entire transducer  10 .  
         [0041]     It is also possible by omitting the horn mouth  15  to obtain a transducer  10  which has a limited dimension in the vertical direction. This can also be advantageous in car tunnels and multi-storey car parks.  
         [0042]     The transducer  10  described above has the additional advantage that the construction has a low height, which is an obvious advantage when used in contact with reflecting surfaces in road tunnels and multi-storey car parks and the like. A typical dimension for the height of the embodiment of the transducer  10  described here is 32 cm.  
         [0043]     In the loudspeaker according to the invention the compression driver  11  can optionally be connected to a matching transformer, by means of which the transducer  10  can easily be matched to an available input signal, for example the widely used 100 volt installations.  
         [0044]     The compression driver  11  can be covered by a cap, which optionally offers space for fitting the matching transformer.  
         [0045]     The horn  13  can be made of polyester, but also of other plastics, such as, for example, polyurethane, and metals, such as, for example, steel or aluminium. The transducer  10  can also be made of wood or concrete or other materials suitable for this purpose. It is also possible to integrate the horn  13  in the construction of the sound-reflecting wall  5  and thus to leave space for fixing the neck-shaped transition  12 , the compression driver  11  and any other components.  
         [0046]     The material can be sandwiched composite material or adapted in some other way to increase the rigidity and damp any resonance. The material used is preferably fire-retardant.  
         [0047]     Tests on the transducer  10  described above have clearly shown the good performance that the transducer  10  is able to deliver. The tests were carried out in a room where the RT60 time was more than 6 seconds. The RT60 time is defined as the time needed for the sound intensity to decrease by 60 dB, measured from the point in time when the sound source is switched off.  
         [0048]     These tests have demonstrated that the transducer  10  is capable of generating a virtually constant sound pressure of approximately 100 dB(A) and that the transducer  10  has a speech transmission index of 0.55 or more for distances of up to 75 metres, in conditions with characteristic background noise level. The speech transmission index was determined in accordance with a method which is known to those skilled in the art.  
         [0049]     Furthermore, the transducer  10  has a high directivity. The term directivity denotes the ratio between the sound pressure measured at the direction point of the transducer  10  and the sound pressure at that point as if the transducer had been a transducer  10  purely radiating all round.  
         [0050]     The transducer  10  furthermore has a very high yield of 136 dB at a distance of 1 metre from the horn  13  when using an input power of 50 watt.  
         [0051]     If the present invention is used in tunnels, reflections also take place at walls other than the wall on which the transducer  10  is mounted, specifically the side walls of the tunnel. These reflections at the side walls of the tunnel can be reduced by modifying the angle  30  of the horn  13 , as has been described above. However, reflections at one side wall  9  can be prevented by having the neck-shaped transition  12  generate a displaced acoustic centre in the form of a point source that is located both in the plane of the ceiling  5  and in the plane of the side wall  9  instead of having it generate a displaced acoustic centre in the form of a line source. A possible embodiment where this is the case is shown in  FIG. 5 .  
         [0052]     Here the transducer  10  is placed in the corner between the side wall  9  of the tunnel and the ceiling  5  of the tunnel, such that the displaced acoustic centre is entirely coincident with the intersection between the side wall  9  of the tunnel and the ceiling  5  of the tunnel. Primary reflections at two walls are prevented in this way.  
         [0053]     The angle  30  of the horn  13  can have a wide variety of values and it is even possible to construct a horn  13  with an angle  30  of 360°.  
         [0054]     It is also possible to construct a transducer  10  where more than one neck-shaped transitions  12  and/or compression drivers  11  are combined. It is then possible to use different compression drivers  11  and neck-shaped transitions  12 , each of which functions in an optimum manner for a specific frequency range, for various frequency ranges. The different neck-shaped transitions  12  can, for example, all adjoin the same horn  13  or adjoin different horns  13 . In this way it is possible, for example, to construct a two-way reproduction system or to construct a multi-way reproduction system.  
         [0055]     The horn  13  of the transducer  10  can be constructed in various ways known from the state of the art. In a preferred embodiment the horn  13  is of non-folded construction, as is the case in the transducer  10  shown in the Figures. However, it is also possible to construct the horn  13  as a horn with three folds in order to achieve a reduction in depth. However, especially at high frequencies this gives rise to internal reflections and, as a consequence thereof, standing waves in the folded horn of the known transducer  2 . This has, in particular, an adverse effect on the acoustic performance of the transducer  2  above 2000 Hz, which frequency range specifically makes an important contribution to the understandability of the acoustic announcement.