GB2535844A

GB2535844A - Speaker unit

Info

Publication number: GB2535844A
Application number: GB1600102.6A
Authority: GB
Inventors: Willems Stefan
Original assignee: PSS Belgium NV
Current assignee: PSS Belgium NV
Priority date: 2015-02-27
Filing date: 2016-01-04
Publication date: 2016-08-31
Also published as: GB201503384D0; GB2535790A; GB201600102D0; WO2016134861A1

Abstract

The speaker produces high directivity sound over a broad range of frequencies, including at low frequencies. The speaker unit including a speaker drive unit 102, a phase plug and an acoustic tube 104 having an open first end acoustically coupled to the phase plug. The tube has a series of apertures 106 with the centres of the apertures lying on a generally a straight line along the length of the tube, and the size of the apertures increases with distance from the speaker.

Description

SPEAKER UNIT

Field of the Invention

The present invention relates to a speaker unit including a perforated acoustic tube for use with a speaker.

Background of the Invention

The directivity of a speaker unit relates to the distribution of the acoustic output (sound) from that speaker. Speaker units with a high directivity project sound preferably in a given direction or directions, whilst speaker units with a low directivity tend to project sound isotropically (equally in all directions).

In many situations, for example in a room of a house or in a car, it is preferable for a speaker unit to have a high directivity, such that sound is projected towards an area where the sound is needed (for example to a driver of the car), rather than wasting energy by projecting the sound to places where it is not needed.

Traditional speakers, such as cone speakers, have some directivity by virtue of the coned speaker diaphragm, and owing to the fact that they are often housed in a way that prevents sound from escaping from a back of the speaker.

The directivity of a speaker assembly can be greatly improved over that of a single speaker, by using a series of speakers in combination. In particular, by driving the series/array of speakers with similar sound signals, but where the signals fed to the speakers may be filtered or delayed with respect to each other (for example where the speaker signals are phase-shifted with respect to each other), it is possible to achieve a speaker assembly output with a high directivity, for example which projects sound primarily in a given direction.

Implementing a series/array of speakers to produce sound of a high directivity can be impractical for some applications. For example, in some cars space may be at a premium and therefore may not permit the use of a large number of speakers.

In an alternative method for increasing the directivity of a speaker, a perforated acoustic tube can be attached to a speaker, forming a speaker unit. Fig. 1 shows such a speaker unit. A series of identical perforations (holes) along the length of the tube allow sound from the speaker to exit through them, and can therefore be considered as individual acoustic sources/speakers. Due to their spacing, the series of holes behave like a series of individual speakers, fed with similar but phase-shifted sound signals. The phase relationship between these individual 'speakers' is such that the sound coming from the speaker unit has a high directivity.

Such a perforated tube for a speaker unit is disclosed in: Fahy, "A low cost end-fire acoustic radiator", JAES Vol. 39, Issue 7/8, pp. 540-550, July 1991.

Speaker units containing an acoustic tube with uniformly sized holes, as described above, cannot create high directivity acoustic outputs at low frequencies.

Also, in smaller/more compact speaker systems in particular (as are often found in cars, home cinemas and computer electronics), the sound output tends to attenuate at low frequencies due to the small size of the speaker drivers.

Hence, a speaker unit able to produce a high directivity acoustic output at low frequencies is desirable.

Summary of the Invention

In general terms, the present invention uses a hollow acoustic tube which has apertures along its length and where those apertures increase in size from one end of the tube to the other.

In a first aspect, the present invention provides a speaker unit for producing high directivity sound at a broad range of frequencies, including at low frequencies, the speaker unit including a speaker, a phase plug and an acoustic tube having an open first end, the first end being acoustically coupled to the phase plug, the tube having a series of apertures, and the centres of the apertures lying on a generally straight line along the length of the tube, wherein the size of the apertures increases with distance from the speaker.

The increase in the size of the apertures with increasing distance from the speaker of the speaker unit is seen to increase the directivity of the sound from the speaker unit at low frequencies, and hence to increase the range of frequencies for which the speaker unit has a high directivity.

Optionally, a horn may also be included, located between the phase plug and the first end of the tube.

Preferably, the speaker unit may have at least at least 6 apertures, more preferably at least 10. In practice, it is believed that at least 11 apertures produces the most desirable results, and the invention may include more than 11 apertures e.g. at least 12 or 15.

The acoustic tube may have a generally constant cross section, for example the acoustic tube may have a constant cross-sectional area for the entirety of its length.

In embodiments, the ratio of the area of the smallest aperture to the area of the largest aperture may be between 1:4 and 1:36. For example, the ratio of the area of the smallest aperture to the area of the largest aperture may generally be 1:9.

The areas of the apertures may generally be between 3mm2 and 113mm2.

The cross-sectional area of the tube may generally be between 254mm2 and 804mm2. In one embodiment, there may be an optimal relationship between the length of the tube and the tapering ratio of the apertures.

The square of the length of the tube may be proportional to the cross-sectional area of the tube and may be inversely proportional to the tapering ratio of the apertures, where, in this case, the tapering ratio of the apertures may be the ratio of the area of the largest aperture to the area of the smallest aperture.

The acoustic tube may be of a length corresponding generally to at least twice the length of the wavelength of the sound that the speaker unit is provided to provide directivity for. For example, the tube might have a general length of at least 2*/' (where A is the wavelength of the sound).

The above-mentioned measurement restrictions are designed to maximise the directivity and range of directivity of the speaker unit. The exact measurements of the acoustic tube may be governed by the intended use of the speaker unit.

The tube may be filled with a low-density material for reducing the speed of sound waves within the tube. This low-density material may be a low-density mineral wool.

The second end of the tube, in particular the end of the tube not connected to the speaker, may be dampened to reduce reflections of sound travelling from the speaker. For example, the end of the tube may be closed at the second end, for example by a foam, mineral wool or other acoustically insulating material.

In some embodiments, the length of the tube may contain a low-density mineral wool for reducing the speed of sound waves in the tube, and the end of the tube may incorporate a higher density mineral wool for preventing reflection of sound waves at the end of the tube.

In some embodiments, the end of the tube may be dampened by a material which is packed more densely than the material with which the tube is filled. For example, the length of the tube may contain a loosely packed mineral wool for reducing the speed of sound waves in the tube, and the end of the tube may incorporate a more densely packed mineral wool for preventing reflection of sound waves at the end of the tube.

The apertures in the acoustic tube of the present invention may be oval, elliptical or circular. The cross-section of the acoustic tube may be oval, elliptical or circular.

In the case of a cylindrical tube and circular apertures, the length of the tube may be proportional to the radius of the tube and inversely proportional to the tapering ratio of the apertures, where; in this case, the tapering ratio of the apertures may be the ratio of the radius of the largest aperture to the radius of the smallest aperture.

The centres of the apertures of the acoustic tube may be equally spaced.

The ratio of the square root of the area of one aperture to the square root of the area of the adjacent aperture is generally constant for all apertures. For example, in the case of circular apertures, the ratio of the radius of one aperture to the radius of the adjacent aperture may be generally constant for all apertures.

In another aspect, the present invention may be considered as being an acoustic tube for the speaker unit as described above, for attachment to a speaker.

The speaker of the speaker unit of the present invention may be a cone speaker. Alternatively, the speaker may be a horn speaker or a dome speaker.

The first end of a first speaker unit of the present invention may be attached to the first end of a second speaker unit of the present invention, so that the second ends of the two speaker tubes face in different directions, in order that the two speaker units direct sound in different directions. For example, the first tube may direct sound in generally the opposite direction to the first tube. This arrangement may create a dipole behaviour speaker unit. When connected to small speakers (such as dome tweeters), the dipole behaviour speaker unit may be used to create a dipole behaviour speaker unit for high frequency sounds. The first acoustic tube of this arrangement may be of different dimensions to the second acoustic tube. For example, the radius of the first tube may be different to the radius of the second tube, or the apertures of the first tube may be of a different size to the apertures of the second tube. The dimensions of the first and/or second tube may be varied in order to create a cardioid or hypercardioid behaviour speaker unit. In such a dipole speaker unit, there may be no gap between the speakers and the respective acoustic tubes.

The speaker of the speaker unit may be a speaker assembly, wherein a speaker assembly is defined as containing more than one speaker. Such a speaker assembly may be configured for a specific use.

This speaker assembly may include at least one cardioid and/or gradient speaker system.

At least one acoustic tube may be used in combination with a speaker assembly. For example, at least one speaker of the speaker assembly may be connected to a respective acoustic tube. This combination may form the speaker unit.

Preferably, the directivity of the speaker assembly and the directivity of the at least one acoustic tube can be carefully matched/controlled in order to achieve a speaker unit of the desired directivity. For example, the directivity of the speaker assembly and the at least one acoustic tube may be configured to generally match at the frequency (or frequencies) at which they operate.

A speaker assembly including at least one cardioid, hypercardioid, and/or gradient speaker system may be used in combination with two acoustic tubes. Such a speaker assembly may be used in order to achieve a stereophonic effect for two observers situated at locations remote from each other.

A dipole behaviour speaker unit may be incorporated into a speaker assembly. Alternatively, a dipole behaviour speaker unit may be added to a speaker assembly. The directivity of the dipole speaker unit and the speaker assembly may be matched.

In some embodiments, a dipole behaviour speaker unit may be incorporated into a speaker assembly.

In other embodiments, a dipole behaviour speaker unit may be added to a speaker assembly.

For example, such a speaker unit may be used to direct sound to a driver and a passenger of a car, without the need for individual, remotely located speakers.

At least one speaker unit may be used in a car environment. For example, at least one speaker unit may be used to project sound to one or more areas of a car.

Two speaker units may be used in a car. For example, one speaker unit may be mounted on the driver-side dash board for projection of sound to the driver, and another speaker unit may be mounted on the passenger-side dash board for projection of sound to a passenger.

Brief Description of the Drawings

Fig. 1 shows a speaker unit incorporating a speaker and a prior art acoustic tube.

Fig. 2 shows a radial plot of the directivity of the speaker unit of Fig. 1 for a range of acoustic (sound) frequencies.

Fig. 3 shows a speaker unit incorporating a speaker and an acoustic tube according to an embodiment of the present invention.

Fig. 4 shows a radial plot of the directivity of the speaker unit of Fig 3 for a range of acoustic (sound) 20 frequencies.

Fig. 5 shows one embodiment of the present invention designed for use with stereophonic sound, the embodiment including two acoustic tubes connected in the middle.

Fig. 6 shows one embodiment of the present invention, where two acoustic tubes configured according to Fig. 5 are incorporated into a speaker with multiple acoustic outputs.

Fig. 7 shows the application of the speaker unit of Fig. 6 in an automotive interior environment.

Fig. 8 shows an embodiment of the invention incorporating a phase plug and a horn. Detailed Description and Further Optional Features of the Invention Speaker units are often used in applications such as in home cinema, consumer electronics and automotive interiors. In such applications, it is advantageous to be able to direct the sound from the speaker unit in a particular direction, for example towards a person listening to the sound from the speaker unit.

In such applications, it is also advantageous to be able to project the sound in a particular direction (i.e. to create sound of a high directivity), so that sound is not sent to areas where it is not needed. Furthermore, in applications where speakers are required to play music, TV audio or film audio, the sound to be projected to a listener is generally of a high audio range, for example incorporating a large range of sound frequencies. Hence, it is necessary for a speaker unit to produce high directivity sound over a wide range of frequencies, such that the full range of frequencies is directed to the listener.

The directivity of a speaker is broadly defined as a ratio between the power emitted by a speaker in a preferable direction (i.e. a desired output direction), to the power emitted in any one direction by an acoustic source which emits sound equally in all directions. Directivity is well understood by those skilled in the art of speaker design. In practice, speaker directivity is often a function of frequency. -4 -

Commonly, speakers used in applications such as home cinema, consumer electronics and automotive applications are small, for example due to space constraints and/or cost constraints. Speakers of a small size often suffer in that they are less effective at generating low-frequency sounds, owing to the small size of the speaker driver. Hence, the requirement to create highly directive sound at the low end of the audio frequency range is increased for small speaker units.

The general form of the speaker unit 100 of the present invention can be viewed as containing a speaker 102, from which an audio signal (sound) is played. A (hollow) acoustic tube 104 can be connected to the speaker 102, so that the sound from the speaker 102 is initially directed down the inside of the acoustic tube 104. The acoustic tube 104 contains perforations (or apertures) 106 along its length, these apertures increasing in size with distance from the speaker 102 to which the acoustic tube 104 is attached. The apertures 106 exist in a generally straight line along the length of the tube. Neither a phase plug nor the optional horn is shown, but both are described in more detail with reference to Fig 8. Figs 3 and 4 are used to describe and illustrate the function of the acoustic tube. In some aspects of the invention, an acoustic tube with the arrangement of holes described herein may be used without a phase plug or horn.

The acoustic tube 104 may be securely connected to the chassis of the speaker 102, in a way which is mechanically stable. Mechanical stability of the connection is important to prevent vibrations, and/or other such effects which may damage the sound quality.

As sound travels from the speaker 102, and down the inside of the acoustic tube 104, it encounters the apertures 106. At each aperture 106, the sound is able to exit the tube. In effect, each aperture 106 can be considered as an individual speaker. Furthermore, because each aperture 106 is located at a different position along the acoustic tube 104, and because of the finite speed of sound, the sound coming out of each aperture 106 is effectively phase shifted with respect to the adjacent apertures. Hence, the phase relationship between the sounds exiting each aperture 106 at a given time is dependent on the spacing of the apertures 106 and the speed of the sound in the tube 104. In other words, the sound from each aperture 106 has a phase lag with respect to the previous aperture 106.

The above-described phase relationship between the sound exiting the apertures 106 of the present invention can create an end-fire array effect, causing a projection of the sound from the speaker 102 (and accordingly the apertures) to an observer who is positioned so as to view the acoustic tube 104 end-on, i.e. with the end of the tube 108 pointing towards him.

The following description describes optional, and in many cases preferable, features according to aspects and embodiments of the present invention, which may be used to further improve the function of the generally described speaker unit 100 mentioned above. Any feature described in the following description, for example in association with any particular embodiment of the present invention, can be incorporated in any other embodiment of the present invention as described below, unless the skilled person would understand the incorporation to be technically impossible.

In order to prevent sound from escaping/exiting from the end of the acoustic tube 108, the end of the tube 108 (i.e. the end of the tube not connected to the speaker 102) may be closed.

In order to increase the phase shift between the sound exiting adjacent apertures, or to achieve a desired phase shift between the sound exiting the adjacent apertures, it may be necessary to reduce the speed of sound along the length of the tube. This can be achieved by filling the length of the tube with a (preferably low-density) material. This material may be a loosely-packed mineral wool.

To maximise the performance of the acoustic tube 104, the end of the acoustic tube 108 may preferably be designed to reduce reflections of sound coming from the speaker. For example, the end of the tube may be dampened by use of a foam, mineral wool or other sound-insulating material. Any suitable method may be used for reducing the reflections of sound coming from the speaker. Reducing reflections of sounds from the speaker 102 is desirable because it reduces the chance of standing waves being established in the acoustic tube 104.

In embodiments, the acoustic tube itself may be open at both ends, and the end not attached to the speaker may be dampened as described above, for example by a foam or mineral wool.

In preferable embodiments, the tube may be filled with a loosely packed mineral wool, and the end of the tube may be dampened with a more densely packed mineral wool.

In order to create the maximum directivity at a given frequency, the maximum frequency range of directivity and/or a desired directivity pattern, the dimensions of the acoustic tube 104 and its apertures 106 may be accurately controlled.

Computational simulations or real-life testing of varying acoustic tube 104 dimensions may be used to determine the optimal dimensions for a particular application of the speaker unit 100.

In particular, our computational simulations have indicated that the directive performance of a speaker unit 100 may be improved for certain relationships between the tapering ratio (for example the ratio of the size of the smallest aperture 110 to the size of the largest aperture 112) and the length of the acoustic tube 104.

For a given directivity, the minimum frequency (fm,n) for which the given directivity is desired may be related to the ratio of the radius of the smallest aperture (rs) to the radius of the largest aperture (rL) as follows: fmt,, « 8. rs/ rL Preferably, the acoustic tube 104 has a constant cross-sectional area along its length. It may also have a constant cross-section profile.

Another optimal configuration may include a ratio of the area of the smallest aperture 110 to the area of the largest aperture 112 of between 1:4 and 1:36. For example, this ratio may be 1:9. The minimum area of the smallest aperture 110 may be at least 3mm2 and the maximum area of the largest aperture 112 may be 113mm2. The minimum cross-sectional area of the acoustic tube 104 may be 254mm2 and the maximum cross-sectional area of the acoustic tube 104 may be 804mm2.

The above noted configurations may be used individually, or in any desired combination, and may be viewed as optimal configurations of the acoustic tube. In particular, the inventor has used the above-noted configurations of the present invention to improve the directive performance over the most general form of the present invention, and importantly to further improve the performance of the present invention over the performance of the prior art.

The cross-section of the tube may be circular or oval. The cross-section of the apertures may be circular.

In the case where the cross-section of the acoustic tube 104 is circular and of constant cross-section, preferably the cross-sectional diameter is between 18mm and 32mm.

In the case that the apertures 106 are circular, the ratio of the diameter of the smallest aperture 110 to the diameter of the largest aperture 112 may be between 1:2 and 1:6. For example, the ratio of the diameter of the smallest aperture to the diameter of the largest aperture may be 1:3.

Preferably, the diameter of the smallest aperture 110 is minimum of 2mm. Preferably the diameter of the largest aperture is a maximum of 12mm.

For arbitrarily shaped apertures, the ratio of the square root of the area of one aperture to the square root of the area of the adjacent aperture may be constant for all apertures. For example, in the case of circular apertures, the ratio of the radius of one aperture to the radius of the adjacent aperture may be constant for all apertures.

In the case of circular apertures and a circular cross-section tube, the relationship between the length of the tube, L (m), radius of the tube. R (m), tapering ratio of the apertures, T, speed of sound in air, c (m.sec-1) and density of air, pa (kg.m-3) may be as follows: -6 -T = (C.R) / (po.L) Where, in this case, the tapering ratio of the apertures is the ratio of the radius of the largest aperture. n (m), to the radius of the smallest aperture, re (m), and the tube has a circular cross-section. In the case of an elliptical tubular cross-section, R=V(a*b), where a and b are the major and minor axes.

The centres of the apertures 106 may be equally spaced along the length of the acoustic tube 104.

One or more of the above-noted configurations may combine with the general form of the present invention to form a speaker unit 100 with the ability to produce sound with a high directivity across a range of frequencies. The performance of such a speaker unit is shown in Fig. 4. Each of the polar plots in Fig. 4 shows the directivity of the speaker unit from an aerial view, for a range of sound frequencies. The orientation of the acoustic tube is shown on each plot. The thick lines on the polar plots indicate the distribution of sound energy, where the distance of the thick line from the origin of each polar plot is indicative of the sound energy emitted in that direction. Such a polar plot would be well understood by a person skilled in the art. The sound frequency that each polar plot represents is shown.

Accordingly, the present invention provides a speaker unit with the ability to create high directivity sound over a wide range of audio frequencies, using as few as one speaker. In particular, it provides a speaker unit with the ability to create high directivity sound at the low end of audio frequencies.

Comparatively, Fig. 2 shows corresponding polar plots of the acoustic output of a prior art speaker unit 200 (this speaker unit containing uniformly sized apertures 206 along the length of an acoustic tube 204, which is connected to a speaker 202). It should be noted that for the purpose of the polar plots in Figs. 2 and 4 respectively, speaker 102 is identical to speaker 202, and the cross-sectional profile of acoustic tube 104 is identical to acoustic tube 204. Similarly, the size of apertures 206 can be considered as being similar to the average size of apertures 106. Accordingly, the polar plots of Figs. 2 and 4 can be considered as being a fair comparison of the performance of the speaker units of Figs. 1 and 3.

Plots 120 and 220, 122 and 222, 124 and 224, 126 and 226, 128 and 228, 130 and 230 represent the directivity of the respective speaker units at the same respective frequencies. Accordingly, the directivity of the present invention is dramatically improved over the prior art, especially at lower frequencies, as is shown by Figs. 2 and 4.

The acoustic tube 104 may be connected to a cone speaker, a dome speaker, a horn speaker,or any other audio source.

It is noted that an aspect of the present invention could be considered to be the acoustic tube on its own. The tube may include any or all of the features described in this application. In other words, the acoustic tube 104 may be provided without a speaker, and may be user-attachable to a speaker 102, in order to form a speaker unit 100.

In the remaining description of this section, possible applications of an acoustic tube 104 according to one or more of the above embodiments in combination with the general form of the acoustic tube will be discussed. These applications are not to be taken as limiting, but rather are intended to disclose potential applications and/or uses for an acoustic tube 104 of the present invention.

With reference to the following, a dipole directivity corresponds to a directivity having a dipole shape, a cardioid directivity corresponds to a directivity having a cardioid shape and a hypercardioid directivity corresponds to a directivity having a hypercardioid shape. Accordingly, a cardioid speaker is a speaker with a cardioid-shaped directivity. The inventor notes that the terms dipole, cardioid and hypercardioid are known by those skilled in the art. The inventor also notes that cardioid and gradient speaker systems (as discussed below) are known in the art.

Fig. 5 shows two acoustic tubes 104 with their respective first ends being connected to each other at a centre 140, to form the pictured acoustic tube arrangement 146. Such an acoustic tube arrangement can be configured such that the first end of each acoustic tube 104 is attached to a -7 -respective speaker. In other words, Fig. 5 can be considered as containing two speaker units of the present invention, connected to each other at their respective first (speaker) ends. In the embodiment shown, each speaker directs sound down the respective tube to which it is mounted.

The two speakers in the tubes of Fig. 5 may be fed with a stereophonic sound. For example, the left speaker may be fed with the left-channel of a stereophonic signal, and the right speaker may be fed with the right-channel of the same stereophonic signal. Such a speaker unit can create a high directivity acoustic output by controlling reflections in a domestic living room or automotive interior environment. Such a speaker unit arrangement is known to create a high directivity of sound for a large range of frequencies, and can be used in combination with other speakers, speaker systems or speaker assemblies.

By altering the dimensions of the respective tubes of the acoustic tube arrangement 146, the directivity of the output of the tube arrangement 146 can be made to have a dipole, cardioid or hypercardioid shape.

The speakers in the acoustic tube arrangement of Fig. 5 may be dome tweeters, for producing sound of a high frequency.

Fig. 6 shows a possible use for the acoustic tube arrangement 146 in combination with a speaker assembly 150. For example, the speaker unit of Fig. 5 may be added to a speaker arrangement as in Fig. 6. In embodiments, the speaker assembly 150 may include a cardioid and/or gradient speaker system.

The speaker assembly 150 shown in the embodiment of Fig. 6 forms, with the acoustic tube arrangement 146, a speaker unit 152. The output of the speaker unit 152 creates a left-facing cardioid output and a right-facing cardioid output, so can be viewed as producing the same output as could be created by two individual cardioid speaker systems. The left and right-facing cardioid outputs may be configured to project sound to respective observers, for example to a driver and a passenger of a car.

By careful matching of the directivity of the speaker assembly 150 with the acoustic tube arrangement 146, one can obtain a convincing stereo, full-range, and directive acoustic experience from a single speaker unit of relatively compact dimensions. For example, if the acoustic tube arrangement 146 were to contain dome tweeters, it could be used to generate high directivity sound of a high frequency, whilst the speaker assembly 150 could be used to direct sound at lower frequency.

In the specific example shown in Fig. 6, the acoustic tubes situated on the top of the speaker assembly are connected to respective speakers, as is explained for the speaker unit of Fig. 5. Hence, the speaker unit of Fig. 6 contains 6 speakers in total. The left-hand speakers of the assembly shown in Fig. 6 are fed with the left-hand channel of a stereophonic signal, and the right-hand speakers fed with the right-hand channel, in order to produce the stereophonic output. This particular embodiment may be best suited to use in an automotive interior environment.

The embodiment shown could also be used in home cinema, with the ability to create a stereophonic effect for multiple observers, using a single loudspeaker.

Fig 7.shows a schematic diagram of a possible use of a speaker unit, similar to the shown in Fig. 6, in an automotive interior environment. In embodiments, the speaker unit of Fig. 6 may be used in an automotive interior environment, and may be placed in the centre of the dash board, similarly to the embodiment of Fig. 7. In the application pictured, the speaker unit 152 is placed at the centre of the dashboard of a car, in order to project sound towards both the driver and a passenger of the car, and to create a stereophonic effect. Each acoustic tube directs sound at the door/window of the car, so that it is reflected back towards the driver/passenger. This function of the tubes eliminates the need to have separate speakers mounted in the doors of the car, or on the outer edges of the dashboard.

Fig. 8 shows an embodiment of the invention in which the perforated acoustic tube is shown with the attached optional phase plug and optional horn. Starting from the left hand side of the figure, the loudspeaker is connected to a phase plug which in turn is connected to a horn. The horn is connected to the acoustic tube. -8 -

The function of the phase plug is to improve the equalisation of sound wave path lengths from different parts of the loudspeaker driver to the input to the horn (or the input to the acoustic tube, if a horn is not present). In normal operation, the waves produced by a loudspeaker are substantially spherical in nature and the function of the phase plug is to convert the spherical wave front that it receives into a substantially planar wave front; or at least a more planar wave front.

Many types and designs of phase plug are known, such as for example dome-type phase plugs with radial slits or ones with concentric ring slit(s) (also called annular or circumferential). The phase plug illustrated in Fig. 8 is of the annular type, but alternative types may be used instead. The annular phase plug in Fig. 8 includes a centre element and a ring spaced between the centre element and the outer wall of the phase plug. The ring creates two annular slits, labelled D and E. In this example; the ring is a teardrop shaped toroid, located between the centre plug and the outer wall (baffle cutout). The total throat area of this particular phase plug is the sum of the area of ring width D and the area of ring width E. This reduces the differential between the path lengths of waves produced by various parts of the loudspeaker. The net result is that the acoustic wave at the throat of the horn is preferably substantially planar.

Although the example phase plug shown includes two annular cut-outs, examples may include more than two or only one.

As can be seen in Fig. 8, the phase plug tapers inwards, such that the area of the face of the phase plug attached to the loudspeaker is greater than the area of the face of the face plug attached to the horn. The horn then preferably tapers in the opposite direction i.e. the area of the face of the horn attached to the phase plug is less than the area of the face of the horn attached to the acoustic tube. The incorporation of the horn further improves the production of planar waves.

In the acoustic tube, the gradient of the acoustic pressure before and after the holes is more constant with a plane wave rather than with a spherical wave. Thus the volume of displacement of air that escapes from the holes is higher. This means that the phase relationship between the holes is better defined and thus the directivity of the perforated tube is improved.

It should be noted that whilst the description of this section concerns the use of the present invention as a speaker unit, it would also be possible to use the present invention for applications in which the frequency of vibrations is not within the audio range. For example, the present invention could be used for ultrasound or infrasound applications.

Claims

Claims 1. A speaker unit, including: a speaker; a phase plug connected to the speaker, and an acoustic tube having an open first end, the first end being acoustically coupled to the phase plug; the tube having a series of apertures, and the centres of the apertures lying on a generally straight line along the length of the tube; wherein the size of the apertures increases with distance from the speaker.
2. The speaker unit of claim 1, wherein the tube includes at least 11 apertures.
3. The speaker unit of any one of the previous claims, wherein the tube has a generally constant cross-section.
4. The speaker unit of any one of the previous claims, wherein the ratio of the area of the smallest aperture to the area of the largest aperture is generally 1:9.
5. The speaker unit of any one of the previous claims, wherein the cross-sectional area of the tube is generally between 254mm2 and 804mm2.
6. The speaker unit of any one of the previous claims, wherein the areas of the apertures are generally between 3mm2 and 113mm2.
7. The speaker unit of any one of the previous claims, wherein the square of the length of the tube is proportional to the cross sectional area of the tube and inversely proportional to a tapering ratio of the apertures, wherein the tapering ratio of the apertures is the ratio of the area of the largest aperture to the area of the smallest aperture.
8. The speaker unit of any one of the previous claims, wherein the tube is filled with a material for reducing the speed of sound waves within the tube.
9. The speaker unit of any one of the previous claims, wherein the second end of the tube is dampened to reduce reflections of sound travelling from the speaker.
10. The speaker unit of claims 8 and 9, wherein the second end of the tube is dampened by a material which is packed more densely than the material with which the tube is filled.

-10 -
11. The speaker unit of any one of the previous claims, wherein the apertures are generally circular.
12. The speaker unit of any one of claims 1 to 10, wherein the apertures are generally elliptical.
13. The speaker unit of any one of the previous claims, wherein the cross-section of the tube is generally circular.
14. The speaker unit of any one of claims 1 to 12, wherein the cross-section of the tube is generally oval.
15. The speaker unit of any one of the previous claims, wherein the centres of the apertures are equally spaced.
16. The speaker unit of any one of the previous claims, wherein the ratio of the square root of the area of one aperture to the square root of the area of the adjacent aperture is generally constant for all apertures.
17. The speaker unit of claim 11, wherein the ratio of the radius of one aperture to the radius of the adjacent aperture is generally constant for all apertures.
18. The speaker unit of any one of the previous claims including a horn connected between the phase plug and the tube.
19. The speaker unit of any one of the previous claims wherein the speaker is a cone loudspeaker.
20. The speaker unit of any one of the previous claims; wherein the speaker has more than one acoustic output, at least one of the acoustic outputs being connected to a respective acoustic tube.
21. An acoustic tube as set out in of any one of claims 1 to 17.
22. A speaker unit as any one embodiment generally described herein with reference to and as illustrated by the accompanying drawings.