WO2007054709A2

WO2007054709A2 - Acoustic horn waveguides

Info

Publication number: WO2007054709A2
Application number: PCT/GB2006/004197
Authority: WO
Inventors: Ambrose Thompson
Original assignee: Martin Audio Limited
Priority date: 2005-11-09
Filing date: 2006-11-09
Publication date: 2007-05-18
Also published as: GB0810537D0; GB2446547B; GB2432289A; GB0522876D0; WO2007054709A3; GB2446547A

Abstract

An acoustic horn waveguide for use in a curved line array and having a predetermined acoustic operating frequency range, comprising an inlet (20) and an outlet (22) connected by a diverging passage, an obstructer (26) disposed towards the inlet end of the passage and around which sound waves from the inlet are directed, and a mixing region (50) downstream of the obstructer, wherein sound waves which have passed around the obstructer have a concave wavefront and merge within the waveguide in a mixing region (50) downstream of the obstructer before propagating from the outlet with a convex wavefront such that when the waveguide is assembled with at least one further such waveguide as a curved line array, the combined wavefront from the assembled line array has a desired curvature.

Description

Acoustic Horn Waveguides

This application relates to acoustic horn waveguides for sound delivery systems. In particular but not exclusively it relates to such waveguides for use in curved line arrays.

A line array is a series of like acoustic horns connected together side by side or stacked one on top of the other. The array can be mounted on the wall of an auditorium or (when arranged vertically) suspended from a mast at an outdoor event so as to be deployed in a vertical curve.

Waves generated by acoustic transducers can contain more than one frequency and can have more than one direction. One consequence of this is that the sound radiated has relatively low directivity when the dimension of the source is small compared to the radiated wavelength. Line arrays can be used to improve this situation, as is known in the art; sound emanating from the sources in such an array interferes to produce a more directional waveform. Directivity is the ratio of radiated sound intensity on the source axis to the intensity of a simple spherical source of the same strength. This quantifies how efficiently the source concentrates the available acoustic power in a preferred direction.

In a typical vertical-oriented line array the desirable total wavefront will be dictated by the coverage that is required. Generally the desirable wavefront will increase in curvature from top to bottom, which provides greater directivity at the top (to reach furthest audience member) and less at the bottom (to ensure coverage of the nearer audience members) - in this manner a relatively even sound pressure level (SPL) over the audience can be achieved. However, if all the audience members are roughly the same distance away then the desirable wavefront is different, being more constant in curvature and therefore having more uniform directivity.

Wavefronts from individual horn waveguide sources within the array are combined so that a wavefront approximating to the desired wavefront is produced. Hitherto, individual wavefronts approximating a flat shape as closely as possible have been advocated, but in experiments we have found that a wavefront curved to follow the physical curvature of the array can give superior sound delivery to the audience.

According to one aspect of the invention there is provided an acoustic horn waveguide for use in a curved line array and having a predetermined acoustic operating frequency range, comprising an inlet and an outlet connected by a diverging passage, an obstructer disposed towards the inlet end of the passage and around which sound waves from the inlet are directed, and a mixing region downstream of the obstructer, wherein sound waves which have passed around the obstructer have a concave wavefront and merge within the waveguide in the mixing region before propagating from the outlet with a convex wavefront such that when the waveguide is assembled with at least one further such waveguide as a curved line array, the combined wavefront from the assembled line array has a desired curvature.

The waveguide may comprise two opposing pairs of walls, the obstructer extending between one pair of said walls and defining between itself and the other pair of said walls passage sections through which sound waves from the inlet are directed.

In another aspect, the invention provides an acoustic horn waveguide for a sound delivery system having a predetermined acoustic operating frequency range, comprising an inlet and an outlet connected by a diverging passage defined by two pairs of opposing walls, an obstructer disposed towards the inlet end of the passage and extending between one. pair of said walls, the obstructer defining between itself and the other pair of walls passage sections through which sound waves from the inlet are directed, and a mixing region downstream of the obstructer, the arrangement being such that sound waves leaving the passage sections have a concave wavefront and merge within the waveguide downstream of the obstructer before propagating from the outlet with a convex wavefront.

Preferably the transmission of higher-order propagation modes, which are propagation paths where the wavefront is not substantially perpendicular to the horn axis, are impeded by the presence of the obstructer. Thus the passage sections may be adapted substantially to impede the transmission of higher-order propagation modes of the sound waves.

A said passage section may have a first zone adapted to impede the propagation of higher order modes and a second zone adapted to expand the sound waves passing therethrough.

The first zone may be defined by substantially parallel or minimally diverging walls.

Facing walls of the horn and the obstructer in the divergent zone may diverge at substantially different angles relative to a longitudinal propagation axis of the zone. For example, the difference in the angles may be of the order of twenty to thirty degrees. Thus, the facing wall of the horn may be parallel to the propagation axis of the divergent zone, and the facing wall of the obstructer may diverge from that axis by between twenty and thirty degrees. In a preferred embodiment the obstructer wall diverges from the axis at 23 degrees. Alternatively expressed, the horn facing wall makes an angle of 10 degrees to the main longitudinal axis of the horn, and the obstructer wall makes an angle with it of 13 degrees in the opposite sense.

The mixing region may be a portion of the waveguide defined only by the said opposing pairs of walls.

The length of the mixing region may correspond to substantially one to three, preferably substantially two, wavelengths of the upper frequency of the operating range.

The obstructer may define two said passage sections.

Preferably the obstructer is substantially within the half of the waveguide that is closer to the inlet.

Preferably the first zone starts at from substantially one to substantially two times its smallest transverse dimension from the inlet. The obstructer may diverge from a wedge shaped upstream-end.

The obstructer may converge to a wedge shaped downstream end.

The obstructer may have at least two inlet-facing surfaces and at least two outlet- facing surfaces.

Preferably the inlet-facing and/or outlet-facing surfaces are planar.

Respective inlet-facing surfaces may join respective outlet-facing surfaces at an obtuse angle.

The longitudinal section of the obstructer may be a regular polygon and is preferably a kite-shape.

The waveguide may be of rectangular cross section in the region of the obstructer.

The said one pair of walls may be parallel and non-divergent in at least a portion of the waveguide.

The passage may comprise a flare region at the outlet thereof, which diverges more rapidly than upstream regions of the passage.

The divergence of the waveguide passage along its longitudinal axis may be linear.

The outlet and/or the inlet may be rectangular.

Alternatively the inlet may be of circular cross-section.

The passage may change in cross-sectional shape between the inlet and the obstructer.

The desired curvature of the combined output wavefront may follow the physical curvature of the line array. The invention also provides a line array comprising a plurality of waveguides as set forth above.

In another aspect the invention provides a method of generating an acoustic wavefront from a horn waveguide in a predetermined operating frequency range that is suitable for combination with wavefronts from other such waveguides in a curved line array to produce a combined output wavefront, the method comprising: directing sound waves passing through the waveguide around an obstructer so as to produce a concave wavefront within the waveguide, and mixing and expanding the sound waves within the waveguide to produce a convex wavefront at the outlet of the waveguide such that when the waveguide is assembled with at least one other such waveguide as a curved line array, the combined wavefront from the assembled line array has a desired curvature.

The invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 shows schematically mode pressure magnitude plots in a theoretical horn at 6755Hz;

Figure 2 shows schematically mode pressure magnitude plots in a theoretical horn at 6955Hz;

Figure 3 shows schematically mode pressure magnitude plots in a theoretical horn at 7755 Hz;

Figure 4 shows schematically a perspective view of a known horn;

Figure 5 a-c show schematically pressures on the surface of the horn of Figure 4 at 6800 Hz, 8000 Hz and 10000Hz respectively;

Figure 6 a-c show schematically outward normal velocity magnitudes at the exit plane of the horn of Figure 4 at 6300 Hz, 8000 Hz and 10000 Hz respectively; Figure 7 a-c show schematically outward normal velocity phases at the exit plane of the horn of Figure 4 at 6300 Hz, 8000 Hz and 10000 Hz respectively;

Figure 8 a and b show schematically an embodiment of a horn according to the invention;

Figure 9 a-c show schematically pressures on the surface of the horn of Figure 8 at 6800 Hz₁ 8000 Hz and 10000Hz respectively;

Figure 10 a-c show schematically outward normal velocity magnitudes at the exit plane of the horn of Figure 8 at 6300 Hz, 8000 Hz and 10000 Hz respectively;

Figure 11 a-c show schematically outward normal velocity phases at the exit plane of the horn of Figure 8 at 6300 Hz, 8000 Hz and 10000 Hz respectively;

Figure 12 shows Polar SPL at 8 m for the horn of Figure 4;

Figure 13 shows Polar SPL at 8 m for the horn of Figure 8;

Figure 14 shows measured 8m Polar data and off axis frequency response with fifteen degrees interval for the horn of Figure 4; and

Figure 15 shows measured 8m Polar data and off axis frequency response with fifteen degrees interval for the horn of Figure 8.

Figure 16a shows a combination of flat sources making up a combined wavefront of a line array, and figure 16b is the polar output of that array at 8000Hz;

Figure 17 shows schematically a continuous arc source;

Figure 18 shows schematically polar output of the continuous arc source of figure 17 at 8000Hz; Figure 19 shows schematically a compound arc source;

Figure 20 shows schematically polar output of the compound arc source of figure 18 at 8000Hz;

Figure 21 shows schematically some excessively curved wavefronts arranged in an arc;

Figure 22 shows schematically polar output at 8000Hz of some excessively curved wavefronts arranged in an arc;

Figure 23 shows schematically polar output of an objective continuous 40 degree arc source at 8000Hz;

Figure 24 shows schematically polar output at 8000Hz of optimally curved wavefronts arranged in a 40° arc;

Figures 25a and b respectively shows polar outputs at 8000Hz of flat and excessively curved way fronts arranged in a 40° arc;

Figure 26 shows schematically SPL distribution for standard and truncated horns at 6300 Hz;

Figure 27 shows schematically comparison of normal exit velocity for (a) the standard horn and (b) the shortest horn at 6300 Hz;

Figure 28 shows velocity phase comparison of different shape source wavefronts for the shortest horn at 8000 Hz (a) flat source, (b) moderately concave (c) very concave;

Figure 29 shows schematically pressure phase using two source funnels as sources to the short horn at 6300 Hz;

Figure 30 shows velocity magnitude (figure 30(1)) and phase (figure 30(2)) for the short horn driven by two source funnels at (a) 6300 Hz, (b) 8000 Hz and (c) 10000 Hz respectively;

In order to understand the context of the invention it is helpfui to review some of the mathematical analysis involved in determining the propagation of waveforms in horns. Such horns have an axis along which waves propagate and a cross section which varies along this axis. In order to analyse the propagation of sound along such horns mathematically, it is usual to make some assumptions. In the analysis that follows it is assumed that the walls that form such horns are rigid and the assumptions of linear acoustics are made. Although of limited practical application for horn design, it is instructive to look at the behaviour of an infinite horn of constant (rectangular) cross section. The mechanisms of sound transmission through such a horn have some similarities to non-constant cross section finite horns.

The solution of an acoustic field within a rectangular horn is well known. The solution describes possible paths a wave can travel in the horn, each path being termed a mode. A mode can be classified as a propagating mode or an evanescent mode depending on the frequency of the incident wave and the so- called cut-off frequency of that mode. The cut-off frequency is the limiting value above which a so-called separation constant for the propagation axis is real and is a function of two other separation constants associated with the two other dimensions. Waves at frequencies above the cut-off value can propagate and those below decay at an exponential rate along the axis. For further information, see Kinsler and Frey, Fundamentals of Acoustics (ISBN: 0-471-09410.2). Suitable eigenfunctions for the rectangular horn where the propagation axis is z and the direction positive consist of plane wave components in two dimensions and a travelling wave along the axis. This can be expressed as follows.

j>_lm = A^ coβdfcn) co_S(fc_vmy)e^*-^k"⁾ (D

After the usual substitution into the wave equation and then separating the variables the separation constants can be related as follows.

where the allowed values for k_x| and ky_m assuming rigid walls are

Iπ kxt = I = 0, 1, 2 (3)

rearranging for k₂ and noting k = ω/c, ω being the angular driving frequency

where k|_m = ^{xi 5}^* represents the transverse components of the propagation vector, then the condition that k_z is real and thus the wave

propagates along the axis is satisfied for all frequencies above the so call cutoff frequency given by

Wm = ^chm (6) below this value implies a purely imaginary value for k₂ and since the horn is infinite there is no positive component of k₂ so taking the negative imaginary k_z back into the original solution indicates that the wave decays exponentially with z. As frequency increases more and more higher modes can be propagated along the axis. The effective angle θ made with the axis of the plane wave components of a particular mode also change with frequency. At the cutoff value the propagation is transverse to the axis, as frequency rises the angle reduces and tends to 0. Although the speed the plane wave components in its propagating direction c the effective speed along the z axis is not, this given by the component of the plane wave c along the axis and is termed group speed.

We have solved this problem to illustrate the pressure amplitude and phase inside the horn. As an example, horn with dimensions Lx = 0.025m, Ly = 0.15m is used below where we examine the first 0.18m along the z axis. For a horn of these dimensions the cut-off frequency for the / = 1 ([1, 1]) mode is 6974H₂ and, of course, OH₂ for the / = 0, m = 0 ([0, O]) mode. Figures 1 to 3 show the behavior of a combination of these modes at three frequencies; one below, one just below, and one above the cut-off frequency of the [1, 1] mode.

Starting with a frequency below cutoff (f = 6755H₂) it is apparent for Figure 1 that propagation is evanescent. Note there is one nodal plane between opposing walls which corresponds to the cancellation of the plane wave components at that position. For the [0, 0] mode the pressure amplitude is constant and when summed with the [1, 1] mode gives the pressure amplitude shown. Here, as expected, the fundamental mode dominates. Stepping up in frequency to just below the cutoff (f = 6955 H₂) the same two views of the pressure amplitude are shown in Figure 2. The [1, 1] mode is still evanescent though propagates further at this higher frequency. Finally at a frequency significantly above the [1, 1] cutoff frequency (f = 7755 H₂) both modes fully propagate. The pressure amplitudes are shown in Figure 3. The pressure amplitudes shown are only the sum of two particular modes, in reality all modes are summed potentially producing a very complex distribution. The amplitudes of the modes shown were chosen to illustrate the effect of higher modes, namely; the non-uniform distribution of pressure amplitude within the horn. The extent to which particular modes are excited and therefore their role in the final pressure distribution depends on various factors. Remaining with the infinite horn, a factor is the spatial design of the source and whether or not the walls of the horn have any absorption. Real horns have further attributes that excite higher order modes and complicate the distribution including changing cross-sections and the impedance discontinuity at the exit which creates a reflected component in the z direction.

Unfortunately it is more difficult to find solutions similar to the one we have detailed for the simple infinite rectangular horn for more realistic horns.

The horns or horns used in line array type products conform to a generic design differing in length and horizontal directivity between various products. This type of horn is shown in Figure 4 with various sections labelled. Rather than using an analytical approach as was done with the rectangular horn a numerical technique is needed. Such a technique, known in the art as Boundary Element Method has been implemented. This is described in "The Boundary Element Method in Acoustics" by Stephen Kirkup (ISBN: 0-9534031-0.6).

Using this method it is possible to examine the pressure or velocity anywhere in or outside the device.

The motivation for the following analysis was to improve the vertical directivity of the device. Polar SPL measurements indicated that there was too much energy generated in the side lobes; whilst still useful for the application an improvement is desirable. An analysis at these frequencies revealed some interesting patterns of pressure in the horn and velocity on the exit plane. Three frequencies are shown in Figures 5 to 7; 6300, 8000 and 10000 Hz with the lower two frequencies needing the most improvement.

Figures 5 a to c shows the pressure distribution at these frequencies respectively. Looking at the results indicates a non-uniform pressure distribution in cross sections down the horn axis. The patterns are reminiscent of those seen in the rectangular horn indicating that various modes and other reflections are present. This is particularly true for the lower two frequencies.

When considering the pressure generated by the horn at an exterior point it is more useful to look at the outward normal velocity on the exit plane. A view of the magnitude of this velocity at the three frequencies is shown in Figure 6. It can be seen that the two lower frequencies have a far from uniform amplitude distribution vertically which will result in a deterioration of the directivity. The other important variable on the exit plane is the phase of the outward normal velocity there, this gives an indication of the wavefront shape leaving the horn and thus, the directivity. Figure 7 shows this variable for the three frequencies, at all frequencies the phase is convex and well behaved. The amount of phase change or rather the curvature of the wavefront is more than what we consider optimum, we will return to this aspect later.

Through this analysis, therefore, we have discovered that the patterns of pressure, velocity and phase in known horns are not necessarily optimum.

Figure 8a and 8b show an acoustic horn waveguide 10 according to the invention. The horn comprises a diverging funnel-shaped passage 12 defined by top and bottom walls 14, 16 and side walls 17, 18. The side walls are parallel and do not diverge. The top and bottom walls diverge linearly, each at 10 degrees to the longitudinal axis of the passage 12, from an inlet or throat 20 to a rectangular slot-shaped outlet 22 although other divergence rules (eg. parabolic, or exponential) could be employed instead, as known in the art. At the outlet there is a short flared section 24, in which the side walls as well as the top and bottom walls diverge, again as known in the art, so as to provide control of horizontal directivity. The inlet 20 is circular in shape in order to match the shape of a driving transducer (not shown), but converts via a short (half-wavelength or less) morphing section to a rectangular cross-section. Thereafter the cross- section of the passage 12 is rectangular and increases uniformly from the inlet 20 to the flare 24 at the outlet.

Towards the inlet end of the passage 12 in an acoustically opaque wave-shaping obstructer or splitter 26. The obstructer extends across the passage 12 between the side walls, thereby splitting it into two passage sections 28, 30. The flow splitter or obstructer 26 is kite-shaped, being defined by two inlet-facing surfaces 31 ,32 which diverge from a leading edge 34 forming the apex of a wedge, and by two outlet-facing surfaces 36, 38 which converge to a trailing edge 40 forming the apex of a further wedge.

The inlet-facing surfaces define with adjacent portions of the top and bottom walls 14, 16, two upstream or first passage zones 42, 44, the cross-sectional dimensions of which are sufficiently small to ensure that the cut-off frequency for higher-mode propagation is beyond the top end of the operating frequency range of the horn waveguide. These passage sections preferably are of constant cross- section, but may be slightly divergent if preferred, in order to avoid the possibility of compression. The outlet-facing surfaces 36, 38 define with adjacent portions of the top and bottom walls 14, 16 two divergent downstream or second passage zones 46, 48. Each of these zones (which although not shown as such in figure 8 can be made longer than the upstream zones 42, 44) expands the wavefront passing through it. The divergence of the walls 14, 36 and 16, 38 defining the two expansion zones are chosen to be asymmetric relative to the centre line of those zones. Thus the longitudinal axes of zones 46, 48 are parallel to the adjacent walls 14, 16, and the walls 36, 38 diverge inwardly at 23 degrees to the respective axes of the zones 46, 48. The asymmetric divergence has the result that the two zones each produce a wave form leaving the downstream end of the obstructer which is concave. These wavefronts proceed to the region 50 downstream of the obstructer in which the wavefronts are expanded and mixed by the time they reach the flared outlet. In a conventional horn waveguide, the expansion through the length of the funnel would produce a convex waveform at the outlet 22. By interposing the obstructer 26 a concavity is achieved in the waveform approximately half way along the funnel. By suitable choice of the length and shape of the obstructer this concave waveform can wholly or partially compensate for the convexity imparted by on the wavefront by expansion in the downstream mixing region 50 of the funnel.

The upstream end of the obstructer 26 is positioned a distance from the phase of the inlet 20 equal to 1 to 2 times the smallest cross-sectional dimension of the damping zone passages 42, 44. The length of the mixing region is such that sound waves can propagate down it without material deterioration of pressure distribution across its cross-section. We estimate a suitable length to be around two wavelengths at the upper end of the operating frequency range of the horn. For example, for an upper frequency of 8KHz, the mixing section could be around 85mm in length.

The overall length of this type of horn depends on the required lowest frequency of operation and the included angle of the horn walls.

For the horn to be an efficient radiator ka>2 at the horn mouth where,

k=2π f/c (~ω/c) and

a is half the total height of the horn mouth. For example for the horn to a good radiator at 2000Hz, this implies that the mouth height would need to be greater than,

2 x 2/2 τr.2000/344

ie. greater than about 110mm. With this fixed, the length is determined by the included angle of the horn walls, such that the arc "curvature" is close to the desired curvature ie.

L = (am-at) / tan (θ/2)

where am=half mouth height, at=half throat height, θ=half included angle. The length is often limited by practical constraints and by the desire to keep the horn short to reduce distortion - here the use of an obstructer in accordance with the invention as described hereafter may be of assistance. In general terms for a lower frequency of 2kHz mouth heights are in the region of 100-150 mm and horn lengths 200-350 mm for an approximately correct curvature suitable for manipulation with an obstructer.

In Figures 9 to 11 the improvement generated by the splitter or obstructer 26 can be seen. Figure 11 shows that variation in the phase of the outward normal velocity on the exit plane has been reduced thereby incrementally improving the preferred directivity of the horn. The outward velocity distribution normal to the exit plane has been altered by the presence of the obstructer so that the maximum value is near the centre of the plane, whereas without the splitter a minimum value near the centre of the plane was observed for certain frequencies (see Figures 5 to 7). This is also a contributing factor to the improved directional response. We consider it likely that the causes of unfavourable exit plane velocity are high order modes and reflections within the horn which the splitter alters in a favourable manner.

Figure 10 shows the outward normal velocity phase on the exit plane and we can see that a desirable reduction in wavefront curvature has been attained. Note the absence of the central dip shown in Figure 10 compared to Figure 6. In addition to this, as shown in Figure 9, there has been a radical change in the pressure magnitude distribution within the horn. This is reflected in an entirely different and much improved outward normal velocity magnitude on the exit plane. Without the splitter device the two lower frequencies suffered an increased magnitude concentrated at the horn edges (see Figure 5), however, with the device the peak magnitude is located at the centre.

To investigate the far field directional response, a number of exterior points were included in both numerical calculations. Points on an 8m arc centred at the centre of the exit plane were defined, the SPL (20*log P/Preference) at these points are plotted in Figures 12 and 13. These figures show a significant improvement in the directional response. The amplitude of the side lobes is significantly reduced resulting in a more consistent directivity with frequency.

In order to assess the performance of the design more thoroughly, prototypes were constructed and a 1 degree polar measurement was performed at 8 metres. The results are displayed in Figures 14 and 15 as a plot of SPL for all frequencies. It is apparent that the directional response has been improved from 4 kHz to 13.5 kHz which coincides with the useful bandwidth of the device. The asymmetry at very high frequencies might be due to misalignment either of the horn and/or of the splitter within the horn or both; but is relatively insignificant compared to the improvement obtained. It will be useful to consider how a more ideal waveshape, for a line array can be generated. We wish to approximate as closely as possible the output of a continuous arc radiator or other gentle curve (any splined set of points), though the example here is the arc. Let us consider an arc source where the chord length is 0.624m and the included angle is 20 deg at 8000Hz. A diagram of this source is shown in Fig 17. Note that we are investigating the vertical curvature, an arbitrary constant curvature has been used in the horizontal plane. A polar plot of the sound pressure level (SPL) with the centre of rotation coincident with the implied centre of the arc radius at 15m is shown in Fig 18. This then describes the objective response we are attempting to simulate with a number of horns.

We can model each horn in a similar way to the continuous source and then sum the outputs to arrive at the total output of the array. Such an arrangement with 4 horns according to the invention is shown in Fig 19, notice that there are small gap between the sub-wavefronts. In this example we have made the individual degree of curvature of the wavefront of each horn the same as the required curvature of the combined array wavefront. Note how the figure lighting shows a lack of discontinuity in shape over the total wavefront. The output of this array is shown in Fig 20 and unsurprisingly there is close agreement between the two polars. The small deviation well off axis and most likely due to the gaps mentioned earlier.

Flat wavefronts have hitherto been thought of as the preferred shape for vertically arrayed sources. However, we consider that this is not true for most ideally simulating a curved source. The same arrangement as in figure 19 but with fiat subwavefronts is shown in Fig 16a, notice here that clear discontinuities of shape can be seen aided by lighting. The output approximates less well to the objective compared to that of figure 19, as can be seen by comparing figure 16b with figure 20.

Since it is possible that the wavefronts can be too flat, is it possible that they can be too curved. Some excessively curved wavefronts in the same array are shown in Fig 21, here we have seen very clear discontinuities in the shape. The output shown in Fig 22 reflects this by the increased level of sidelobes. So far we have shown that to best approximate the objective wavefront with an array of sub-wavefronts then each sub-wavefront needs to closely match the degree of curvature of the objective wavefront.

We have also considered an objective wavefront for a line array having a 40 deg included angle arc of the same chord length as above, for the three cases; optimum curvature, fiat and excessive curvature. Fig 23 shows the polar output of the continuous objective wavefront.

The polar plot obtained using horns having optimally-curved wavefronts is shown in Fig 24, Fig 25a shows the result with horns having fiat wavefronts and Fig 25b the results with horns having excessively curved ones.

We now see that the errors become more significant with increased target curvature when using non-ideally curved wavefronts. This is particularly so for the flat wavefronts since importantly, the most significant deviation is within the enclosed angle, this is the region that radiates most usefully to the audience and therefore should be as consistent as possible.

A long horn has a greater capacity for supporting high order mode transmission than a shorter one. To investigate this, we simulated a number of horn, comprising a known standard horn, truncated in two places, from the throat end, and sound pressure level distribution is calculated for both lengths. The result at 6300 Hz is shown in Fig 26, the throat being driven uniformly over its extent. This is a very interesting result. The pressure distribution in the shortest horn is very much improved, whilst the normal and moderately shorter horn share very much the same pattern of distribution. Therefore, both the standard horn and the moderately shorter horn are such that their dimensions prevent acceptable pressure distributions. To confirm that that this improvement in pressure distribution has resulted in a corresponding improvement in normal velocity magnitude in the exit plane, Fig 27 displays the results of the standard horn (a) against the shortest horn (b).

Thus, considering the horn as a kind of waveshape transformer, which imposes a convex characteristic on source wave-front, then a concave source wavefront can be utilised to counteract this. Using the shortest horn from the last section the outputs for a flat source and two progressively more concave source wavefronts, in a piecewise linear sense, are calculated. The results are displayed in Fig 28. The results indicate that the exit wavefront curvature can be reduced by using a concave wavefront at the source to the horn.

With both magnitude and phase of the exit velocity thereby under control it only remains to create a source that approximates the driving source in the above horn. We have realised that it is possible to approximate such sources with the divergent funnel waveguide portions 46, 48. Each is a funnel waveguide where the angled walls defining it are not at the same angle to the propagation axis. In the limiting case if one wall is parallel to the propagation axis and the other angled away then we would expect there to be only divergence on one side, the angled side. Simplistically the waves on the angled side have space in which to expand whereas on the non angled side no expansion can take place. Thus an asymmetrical wave-front is produced at the exit plane of the funnel.

The effect of using two such funnel waveguides as sources on the smallest horn are shown in Fig 29. The plot shows the phase of the pressure which will indicate wavefront shape for positions substantially within the horn. A compromise has been made so that the two funnel sections 46, 48 and the short horn funnel forming the mixing section 50 can share the same wall; this places the source funnel propagation axis parallel with the short horn funnel wall. If the common wall constraint were dropped then a potentially more ideal exit wavefront of the source funnels could be achieved. Despite this the pressure distribution, velocity magnitude and velocity phase, shown in Fig 30 are all still usefully improved over a simple funnel.

It can be seen that the presence of a splitter improves the uniformity, velocity and phase of the waveform at the outlet of a horn. Surprisingly this is achieved without adversely affecting the pressure distribution. The pressure distribution may even be improved, unlike in previously known horns.

Each feature disclosed in this specification (which term includes the claims) and/or shown in the drawings may be incorporated in the invention independently of other disclosed and/or illustrated features. Statements in this specification of the "objects of the invention" relate to preferred embodiments of the invention, but not necessarily to all embodiments of the invention falling within the claims.

The description of the invention with reference to the drawings is by way of example only.

The text of the abstract filed herewith is repeated here as part of the specification. An acoustic horn waveguide for use in a curved line array and having a predetermined acoustic operating frequency range, comprising an inlet and an outlet connected by a diverging passage, an obstructer disposed towards the inlet end of the passage and around which sound waves from the inlet are directed, and a mixing region downstream of the obstructer, wherein sound waves which have passed around the obstructer have a concave wavefront and merge within the waveguide in a mixing region downstream of the obstructer before propagating from the outlet with a convex wavefront such that when the waveguide is assembled with at least one further such waveguide as a curved line array, the combined wavefront from the assembled line array has a desired curvature.

Claims

1. An acoustic horn waveguide for use in a curved line array and having a predetermined acoustic operating frequency range, comprising an inlet and an outlet connected by a diverging passage, an obstructer disposed towards the inlet end of the passage and around which sound waves from the inlet are directed, and a mixing region downstream of the obstructer, wherein sound waves which have passed around the obstructer have a concave wavefront and merge within the waveguide in the mixing region before propagating from the outlet with a convex wavefront such that when the waveguide is assembled with at least one further such waveguide as a curved line array, the combined wavefront from the assembled line array has a desired curvature.

2. An acoustic waveguide according to Claim 1 or Claim 2 wherein the diverging passage is defined by two opposing pairs of walls, the obstructer extending between one pair of said walls and defining between itself and the other pair of said walls passage sections through which sound waves from the inlet are directed.

3. An acoustic horn waveguide for a sound delivery system having a predetermined acoustic operating frequency range, comprising an inlet and an outlet connected by a diverging passage defined by two pairs of opposing walls, an obstructer disposed towards the inlet end of the passage and extending between one pair of said walls, the obstructer defining between itself and the other pair of walls passage sections through which sound waves from the inlet are directed, and a mixing region downstream of the obstructer, the arrangement being such that sound waves leaving the passage sections have a concave wavefront and merge within the waveguide downstream of the obstructer before propagating from the outlet with a convex wavefront.

4. An acoustic waveguide according to any preceding claim wherein the transmission of higher-order propagation modes of the sound waves are substantially impeded as the sound waves pass around the obstructer.

5. An acoustic waveguide according to Claim 4 when dependent from Claim 2 or Claim 3 wherein the passage sections are adapted substantially to impede the transmission of higher-order propagation modes of the sound waves.

6. An acoustic waveguide according to Claim 5 in which a said passage section has a first zone adapted to impede the propagation of higher order modes and a second zone adapted to expand the sound waves passing therethrough.

7. An acoustic waveguide according to Claim 6 in which the first zone is defined by substantially parallel or minimally diverging walls.

8. An acoustic waveguide according to Claim 6 or Claim 7 in which facing walls of the horn and the obstructer in the second zone diverge at substantially different angles relative to a longitudinal propagation axis of the zone.

9. An acoustic waveguide according to Claim 2 or Claim 3 or any claim dependent therefrom wherein the mixing region is a portion of the waveguide defined only by the said opposing pairs of walls.

10. An acoustic waveguide according to any preceding claim in which the length of the mixing region corresponds to substantially one to three, preferably substantially two, wavelengths of the upper frequency of the operating range.

11. An acoustic waveguide according to Claim 2 or Claim 3 or any claim dependent therefrom wherein the obstructer defines two said passage sections.

12. An acoustic waveguide according to any preceding claim in which the obstructer is substantially within the half of the waveguide that is closer to the inlet.

13. An acoustic waveguide according to Claim 6 or any claim dependent therefrom in which the first zone starts at from substantially one to substantially two times its smallest transverse dimension from the inlet.

14. An acoustic waveguide according to any preceding claim when the obstructer diverges from a wedge shaped upstream end.

15. An acoustic waveguide according to any preceding claim in which the obstructer converges to a wedge shaped downstream end.

16. An acoustic waveguide according to Claims 14 and 15 in which the obstructer has at least two inlet-facing surfaces and at least two outlet- facing surfaces.

17. An acoustic waveguide according to Claim 16 in which the inlet-facing and/or outlet-facing surfaces are planar.

18. An acoustic waveguide according to Claim 16 or Claim 17 in which respective inlet-facing surfaces join respective outlet-facing surfaces at an obtuse angle.

19. An acoustic waveguide according to any of Claims 14 to 18 in which in its longitudinal section the obstructer is a regular polygon and is preferably a kite-shape.

20. An acoustic waveguide according to any preceding claim being of rectangular cross section in the region of the obstructer.

21. An acoustic waveguide according to Claim 2 or Claim 3 or any claim dependent therefrom wherein the said one pair of walls are parallel and non-divergent in at least a portion of the waveguide.

22. An acoustic waveguide according to any preceding claim wherein the passage comprises a flare region at the outlet thereof, which diverges more rapidly than upstream regions of the passage.

23. An acoustic waveguide according to any preceding claim in which the divergence of the waveguide passage along its longitudinal axis is linear.

24. An acoustic waveguide according to any preceding claim in which the outlet and/or the inlet are rectangular.

25. An acoustic waveguide according to any preceding claim wherein the passage changes in cross-sectional shape between the inlet and the obstructer.

26. An acoustic waveguide according to Claim 25 wherein the inlet is of circular cross-section.

27. An acoustic waveguide according to Claim 1 or any claim dependent therefrom in which the desired curvature follows the physical curvature of the line array.

28. A line array comprising a plurality of waveguides according to any preceding claim.

29. A method of generating an acoustic wavefront from a horn waveguide in a predetermined operating frequency range that is suitable for combination with wavefronts from other such waveguides in a curved line array to produce a combined output wavefront, the method comprising: directing sound waves passing through the waveguide around an obstructer so as to produce a concave wavefront within the waveguide, and then mixing and expanding the sound waves within the waveguide to produce a convex wavefront at the outlet of the waveguide such that when the waveguide is assembled with at least one other such waveguide as a curved line array, the combined wavefront from the assembled line array has a desired curvature.

30. An acoustic horn waveguide substantially as herein described and/or as illustrated in figure 8.

31. A method of producing an acoustic wavefront substantially as herein described.