HK1149411B - Waveguide electroacoustical transducing - Google Patents

Description

BACKGROUND

This specification describes an improved acoustic waveguide. Acoustic waveguides are described generally in U.S. Pat. 4,628,528 . Some specific aspects of acoustic waveguides are described in U.S. Pat. 6,771,787 and in U.S Pat. App. 09/753,167 . US 5,740,259 discloses a loudspeaker assembly comprising an acoustic waveguide, an acoustic driver mounted to the waveguide, and an acoustic volume acoustically coupled to the acoustic waveguide through an opening.

SUMMARY

The invention consists in a loudspeaker assembly according to claim 1. Further details are presented in the dependent claims.

Other features, objects, and advantages will become apparent from the following detailed description, when read in connection with the following drawing, in which:

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

• Figs. 1A and 1B are geometric objects useful in understanding some of the other figures;
• Fig. 2 is a diagrammatic view of a waveguide assembly;
• Figs. 3A and 3B are diagrammatic views of waveguide assemblies;
• Figs. 3C and 3D are diagrammatic cross-sectional views of waveguide assemblies;
• Figs. 4A - 4G are diagrammatic views of waveguide assemblies;
• Figs. 5A and 5B are diagrammatic views of a waveguide assembly;
• Figs. 6A and 6B are diagrammatic views of a portion of a waveguide assembly; and
• Figs. 7A - 7D are drawings of a practical implementation of loudspeaker systems with waveguide assemblies including features shown diagrammatically in other figures.

DETAILED DESCRIPTION

FIGS. 1A and 1B show some geometric objects useful in understanding some of the figures that follow. FIG. 1A is an isometric view of two waveguides 6 and 7. Waveguides 6 and 7 are depicted as structures having rectangular cross-sections in the Y-Z plane and an X-dimension longer than both the Y- and Z- dimensions. The area dimension in the Y-Z plane (hereinafter the "area dimension") of waveguide 6 is A and the linear dimension along the Y-axis is h. In the specification, there are references to changes in the area dimension. In the corresponding figures, changes to the area are depicted by changes in dimension in the Y-direction, holding the dimension in the Z-direction uniform. So for example, a waveguide 7 with an area dimension of 2A would be depicted in the corresponding figure by a doubling of the linear dimension h along the Y-axis to 2h. FIG. 1B shows the waveguides of FIG. 1A as cross sections in the X-Y plane and includes some additional elements. Except where otherwise specified, the waveguides in the following figures are shown as cross-sections in the X-Y plane, with the longest dimension in the X-dimension. Except where otherwise specified, "length" refers to the length of the acoustic path through the waveguide. Since waveguides are frequently bent or curved, the length may be greater than the X-dimension of a device incorporating the waveguide. Acoustic waveguides typically have at least one open end 18 and may have a closed end 11. An acoustic driver 10 is typically mounted in the closed end 11 as shown, but may be mounted in one of the walls 13 as represented by the dashed line. In the figures that follow, the acoustic driver is shown as mounted in closed end 11.

Fig. 2 shows a first waveguide assembly 100. An acoustic driver 10 is mounted in one end of a waveguide 12A that is low loss and preferably substantially lossless through the frequency range of operation of the waveguide. The waveguide 12A has a cross-sectional area A and an effective acoustic length l. The waveguide has a tuning frequency which is determined principally by the effective acoustic length of the waveguide, which is the physical length plus end effect corrections. End effect corrections may be determined using estimation techniques or empirically. For simplicity, in the figures the length l will be shown as the physical length and the term "length" will refer to the effective acoustic length. The waveguide 12A has a volume given by lA.

Fig. 3A shows a second waveguide assembly. An acoustic driver 10 is coupled to a waveguide 12B that is low loss and preferably substantially lossless through the frequency range of operation of the waveguide. Waveguide 12B has a physical length βl and a cross -sectional area βA, where β is a factor <1. The volume of the waveguide 12B is β²lA. Acoustically coupled by opening 34 to the waveguide 12B is an acoustic volume or chamber 22. The volume of the chamber 22 is lA - β²lA, so that the volume of the waveguide 12B plus the volume of the chamber 22 is the same as the volume of the waveguide 12A of Fig. 2. An effect of the chamber 22 is that the waveguide 12B has essentially the same tuning frequency as the waveguide 12A of FIG. 2 despite having a shorter length. An advantage of the waveguide of Fig. 3A is that (except as described below in the discussion of Helmholtz resonators and in the discussion of FIGS. 6A and 6B) the chamber 22 can be many shapes so long as the chamber 22 has the correct volume dimension. So, for example, as shown in Fig. 3B, the walls of chamber 22 can form a gradually curved surface 31 which forms the walls of the waveguide 12B. A waveguide having a gradual curve causes less turbulence and undesirable noise than waveguides with a more abrupt curve or change in direction and also use space efficiently. As long as the intended volume is maintained, the dimensions of chamber 22 may have a wide range of values, except as discussed below in the discussion of FIGS. 6A and 6B.

FIGS. 3C and 3D show cross-sections of a waveguide assembly in the Y-Z plane, so that the x-dimension (the longest dimension of the waveguide) is perpendicular to the sheet of the drawing. In the waveguide of FIG. 3C, the chamber 22 has a dimension in the Y direction and the Z direction that is larger than the Y and Z dimension of the waveguide 12B so that the chamber partially or completely envelops the waveguide. If desired, for example for ease of manufacture, a barrier 46 or a barrier 48 or both may be placed in the waveguide 12B or the chamber, respectively (so that there are two waveguides 12B-1 and 12B-2 or two chambers 22A and 22B or both), and achieve the same acoustic result as if there were no barriers. Sight lines 52, 54, and 56 will be referenced below. To eliminate high frequency peaks, there may be a small amount of acoustically resistant material in accordance with U.S. Pat. 6,278,789 in the waveguide of FIG. 3A and in the waveguides of all subsequent figures.

The concepts of reducing the cross-sectional area and length of a waveguide and adding a chamber to the waveguide as shown in Figs. 3A and 3B can be applied to portions of waveguides, for example stepped portions of stepped waveguides, as well as whole waveguides, for example stepped waveguides. Fig. 4A shows a stepped waveguide 12C according to U.S. Pat. 6,771,787 . An acoustic driver 10 is mounted in one end of the stepped waveguide 12C. The stepped waveguide 12C has four sections 24 - 27 along the length of the waveguide, with section 24 adjacent the acoustic driver and section 27 adjacent the open end 18 of the waveguide. The sections are of substantially equal length l. Section 24 has a cross sectional area A₁ , section 25 has a cross sectional area A₂ , which is larger than A₁ ; section 26 has a cross sectional area A₃, and section 27 has a cross sectional area A₄ which is larger than cross sectional area A₃ . The volume V₁ of section 24 is A₁l, the volume V₂ of section 25 is A₂l, the volume V₃ of section 26 is A₃l and the volume V₄ of section 26 is A₄l. In conventional waveguides, radiation from a surface of the acoustic driver that faces the environment (hereinafter the exterior surface) is out of phase with radiation from the surface of the acoustic driver that faces into the waveguide. At wavelengths equal to the effective acoustic length of the waveguide, the radiation from the waveguide and the radiation from the exterior surface of the waveguide destructively interfere, reducing the combined radiation of the waveguide and the acoustic driver. In a waveguide system according to FIG. 4A, the radiation from the waveguide is greater than the radiation from the exterior surface of the acoustic driver, and therefore the dip in the combined radiation from the waveguide and the exterior surface is eliminated. In one example, the waveguide assembly of FIG. 4A, A₁=A₃, A₂=A₄, and The operation of the waveguide assembly of FIG. 4A is described in U.S. Pat. 6,711,787 .

Fig. 4B illustrates a waveguide system using chambers acoustically coupled to the waveguide so that the waveguide is shorter than a corresponding conventional waveguide. An acoustic driver 10 is mounted in one end of a waveguide 12D. Waveguide 12D, and waveguides in the subsequent figures, is low loss and preferably substantially lossless through the frequency range of operation of the waveguide. The waveguide 12D has a cross sectional area equal to the cross sectional area A₁ of sections 24 and 26 of the waveguide of FIG. 4A. Sections 25 and 27 of Fig. 4A have been replaced by sections 25' and 27', respectively. Sections 25' and 27' have a length of βl and a cross-sectional area A'₂ equal to βA₂ where is a number 0<k<1. In this example, so that the waveguide of Fig. 4B has a uniform cross-sectional area A throughout the length of the waveguide. Sections 24' and 26' have a cross-sectional area of A and volumes (V₁ and V₃ respectively) of lA. Sections 25' and section 27' have a cross-sectional area of A'₂ and volumes (V' ₂ and V'₄ respectively) of β²A₂l. At a distance d₁ (where l<d₁<l+βl, in one example ) from the acoustic driver end of the waveguide, a chamber 22 is acoustically coupled to the waveguide through an opening 34. At a distance d₂ (where l+βl+l<d₂ <l+βl+l+βl, in one example ) from the acoustic driver end 11 of the waveguide, a chamber 29 is acoustically coupled to the waveguide through an opening 38. Chamber 22 has a volume dimension V_c of A₂l(1-β²) so that V'₂ +V_c=V₂, and chamber 29 has a volume dimension V_D of A₄l(1-β²) so that V'₄ +V_c = V₄ , so that the total volume occupied by the assembly of FIG. 4B and the total volume occupied by the assembly of FIG. 4A are substantially equal. As stated above, so long as the chambers have the correct volume, the volume can have any shape, orientation, or linear dimensions of the chambers, except as shown below in FIGS. 6A and 6B and discussed in the corresponding portion of the specification.

The opening 34 or 38 may have an area such that it may form, with the chamber 22 or 29, respectively, a Helmholtz resonator which could have adverse acoustic effects on the operation of the waveguide system. Helmholtz resonators are described in, for example, http://www.phys.unsw.edu.au/jw/Helmholtz.html, a copy of which is attached as an appendix. However, the dimensions of the opening 34 and of the chamber 22 can be selected so that the Helmholtz resonance frequency is at a frequency that does not adversely affect the operation of the waveguide system or that is outside the operating frequency range of the waveguide. Selecting dimensions so that the Helmholtz resonance frequency is outside the operating frequency of the waveguide can be done by making the width of openings 34 and 38 to the chambers 22 and 29 respectively, close to (for example >50% of) the width of the chambers.

The tuning of the waveguide 12D of FIG. 4B is essentially the same as the tuning of the waveguide 12C of FIG. 4A. Sections 24' and 26' of FIG. 4B have the same effect on the tuning of the waveguide as sections 24 and 26 of FIG. 4A. Sections 25' and 27' of FIG. 4B have the same effect on the tuning of the waveguide as sections 25 and 27 of FIG. 4A, even though the physical length of sections 25' and 27' of FIG. 4B is βl which (since β<1) is shorter than the physical length l of sections 25 and 27 of FIG. 1.

The figures disclosed above are merely illustrative and not exhaustive and many variations are possible. For example, the waveguide may have more than four sections; sections such as sections 25' and 27' may have different lengths; the volume dimensions of sections such as 25' and 27' may have different volume dimensions; the combined volume dimensions such as V₃ and V₄ may not be equal to V₂; and as will be seen below, different configurations of the chambers are possible (for example, there may be different numbers of chambers, and the chambers may have different volume dimensions, shapes, and placements along the waveguide as will be described below).

In addition to providing the same tuning frequency with a waveguide of shorter length, the waveguide system of FIG. 4B has the same advantage of FIG. 4A with regard to eliminating the dip in the combined output of the acoustic driver and the waveguide at frequencies at which the corresponding wavelength equals the effective length of the waveguide. At these frequencies, the acoustic output of the waveguide is greater than the acoustic output radiated directly to the environment by acoustic driver, so the combined radiation from the waveguide and the acoustic driver is greater than the combined output from a conventional waveguide system. The waveguide assembly of FIG. 4B is also less prone than the waveguide assembly of FIG. 4A to wind noises that can occur at abrupt area discontinuities.

FIG. 4C shows a variation of the waveguide assembly of FIG. 4B. In the waveguide assembly of FIG. 4C, the chamber 22 of FIG. 4B is replaced by chambers 22A and 22B with a total volume equal to the volume of chamber 22. The entrance to chamber 22A is placed at distance d₁ such that from the acoustic driver, in one example and the entrance 34B to chamber 22B is placed at distance d₂ such that from the acoustic driver, in one example Chamber 29 of FIG. 4B is replaced by chambers 29A and 29B with a total volume equal to the volume of chamber 29. The entrance 38A to chamber 29A is placed at distance d₃ such that from the acoustic driver, in one example and the entrance 38B to chamber 29B is placed at distance d₄ such that from the acoustic driver, in one example The effect of the tuning of the waveguide assembly of chambers 22A and 22B is substantially the same as the effect of chamber 22 of FIG. 4B, and the effect of on the tuning of the waveguide assembly of chambers 29A and 29B substantially is the same as the effect of chamber 26 of FIG. 4B and have the same beneficial effect of alleviating the dip in the output of the waveguide assembly at the frequency at which the wavelength equals the effective length of the waveguide. Generally, using multiple chambers permits the tuning frequency to more closely match the tuning frequency of the equivalent stepped waveguide such as the waveguide of FIG. 4A.

Aspects of FIGS. 4A, 4B, and 4C can be combined. For example, the waveguide assembly of FIG. 4D has a chamber 32 coupled to the waveguide 12E in the first section at distance d₁, where l<d₁ <l+βl and a stepped section 27 beginning at distance d₂=l+βl+l. The waveguide assembly of FIG. 4E has a waveguide 12F with a stepped section 25 beginning at distance d ₁ = l and a chamber 29 at a distance d₂ >l+l+l. Aspects of FIGS. 4A, 4B, and 4C can also be implemented in a tapered waveguide if the type shown in FIG. 1 of U.S. Pat. 6,771,787 , as shown in FIG. 4F. For use in a tapered waveguide, the size of the chambers and the location of the openings from the waveguide to the chambers may be determined by modeling. A waveguide such as the waveguide with substantially continuous walls such as the waveguide of FIG. 4F may be less subject to wind noises that may occur at abrupt area discontinuities. The waveguide assembly of FIG. 4G is a diagrammatic view of a practical waveguide assembly incorporating elements of FIGS. 4A - 4E. The implementation of FIG. 4G has six 2.25 inch acoustic drivers 10A - 10F and dimensions as shown.

FIG. 5A shows an implementation of the waveguide assembly shown schematically in FIG. 4B illustrating walls of chambers 22 and 29 forming multiple curved surfaces 31A and 31B which also forms walls of the waveguide resulting in less turbulence than would occur with a more abrupt curve, while using space efficiently. The reference numbers in FIG. 5A indicate similarly numbered elements in the corresponding waveguide system of FIG. 4B. FIG. 5B shows an implementation of the waveguide shown schematically in FIG. 4E illustrating walls of chamber 29 and stepped section 25. The reference numbers in FIG. 5B indicate similarly numbered elements in the corresponding waveguide system of FIG. 4E.

FIGS. 6A and 6B illustrate another feature of a waveguide assembly. In FIG. 6A, waveguide 12B is acoustically coupled to a chamber 22 through an opening 34. Acoustic waves enter the opening 34 and propagate into the chamber 22 along a number of acoustic paths, for example path 66A until the acoustic waves encounter an acoustic boundary. There may be many acoustic paths along which the acoustic waves propagate; for simplicity only one is shown.

Generally, it is desirable to configure the chamber so that the lengths of all acoustic paths are significantly shorter than one-fourth of the effective acoustic length of the waveguide 12B. If the length of one of the acoustic paths is not significantly shorter than one fourth (for example, not shorter than 10%) of the effective acoustic length of the waveguide, output dips may occur at certain frequencies. In one example, a waveguide assembly similar to waveguide assembly of FIG. 4B is tuned to 44 Hz, so that it has an effective acoustic length of 1.96 m. (6.43 feet). A chamber 22 with a volume of 1851.1 cc (114 cubic inches) is coupled to waveguide 12B at a position 39.6 cm (15.6 inches) from the closed end 11. Chamber 22 has an acoustic path 66A (see FIG. 6A) that has a length of 40.6 cm (16 inches), that is of the effective acoustic length of the waveguide assembly. An undesirable dip in the frequency response may occur at about 200 Hz. Depending on factors such as the distance of the chamber 22 from the closed end 11, the dip in the frequency response may occur when the length of acoustic path 66A is as short as 25.4 cm (10 inches), which is of the effective acoustic length of waveguide 12B.

One way of eliminating the frequency response dip is to reconfigure chamber 22 so that acoustic path 66A has a length shorter than 10% (in this case 19.6 cm) of the effective acoustic length of the waveguide system. However in a practical waveguide, it may be difficult to reconfigure the chamber so that acoustic path 66A has a length of less than 10% of the effective acoustic length of the waveguide system.

Another way of eliminating the frequency response dip is to add structure to the chamber 22 that changes the length of an acoustic path such as 66A to a length that does not cause a frequency response dip. FIG. 6B shows the waveguide system of FIG. 6A with baffles 42 inserted into the chamber so that the length of acoustic path 66B is 50.8 ± 1.3 cm (20 ± 0.5 inches). The waveguide system of FIG. 6B does not have the frequency response dip of the waveguide system of FIG. 6A. The path length dimensions at which dips may occur and the range of path lengths at which dips do not occur, and the variance of the path length with regard to the placement of the chamber opening relative to the ends of the waveguide can be determined by modeling or experimentation. If the situation shown in FIGS. 6A and 6B occurs, it is generally desirable to shorten the path length because the tolerance (the range of path lengths that result in no dip) is wider. In the example above, any length shorter than 25.4 cm is suitable, but the tolerance of the longer acoustic path is only ± 1.3 cm.

FIGS. 7A and 7B show a practical implementation of an audio reproduction device incorporating a waveguide assembly having features shown diagrammatically in previous figures. The elements in FIGS. 7A and 7B correspond to similarly numbered elements in the previous figures. The dashed lines in FIGS. 7A and 7B illustrate the boundaries of the chambers 22 and 29. FIG. 7A is a cross section in the X-Z plane of the audio reproduction device. The waveguide assembly 12B has the form of the waveguide assembly of FIG. 3C and the cross section is taken along a sight line corresponding to sight line 52 or 54 of FIG. 3C; the cross sections taken along sight lines corresponding to sight lines 52 and 54 are substantially identical. There is a barrier 46 (of FIG. 3C, not shown in this view) resulting in the waveguide assembly having two waveguides. FIG. 7B is a cross section in the X-Z plane, taken along a sight line corresponding to sight line 56 of FIG. 3C. The acoustic driver 10 (of previous figures), not shown in this view is coupled to the waveguide 12B. Compartments 58 and 60 are for high frequency acoustic drivers (not shown), which are not germane to the waveguide assembly. In the implementation of FIGS. 7A and 7B, volume V₁ of chamber 22 is about 1861 cm³ (114 cubic inches); the volume V₂ of chamber 29 is about 836 cm³ (51 cubic inches); the physical length of the waveguide is about 132.1 cm (52 inches); the center of opening 34 to chamber 22 is located about 39.6 cm (15.6 inches) from closed end 11 and the width of opening 34 is about 3.8 cm (1.5 inches); the center of opening 38 to chamber 29 is about 11.7 cm (4.6 inches) from the open end 18 of the waveguide and the width of opening 38 is about 3.8 cm (1.5 inches); and the waveguide is tuned to about 44 Hz.

The waveguide assembly of FIG. 7C has two low frequency acoustic drivers 10A and 10B. The elements in FIG. 7C correspond to similarly reference numbered elements in the previous figures. The second section of the waveguide 12 has coupled to it two chambers 22A and 22B by openings 34A and 34B, respectively. The fourth section of the waveguide 12 has coupled to it a single chamber 26 by opening 38. The walls of the waveguide 12 form walls (which for the purposes of this application includes following substantially the same outline as the walls) of chambers 22A and 22B and substantially enclose chambers 22A and 22B. Chambers 22A and 22B are "teardrop" shaped to provide large turning radii for the waveguide, providing a lessening of turbulence than would occur with smaller turning radii or with sharp bends. Chamber 26 provides a large chamber with low air velocity that provides a convenient location for electronics components 36. The low velocity air causes less turbulence when it encounters the electronics 36. The irregular, multiply curved shape of chamber 26 permits the assembly to be fit efficiently into a small device enclosure 34. High frequency acoustic drivers do not radiate into the waveguide 12.

The waveguide assembly of FIG. 7D is a practical implementation of the waveguide illustrated schematically in FIG. 4F. The elements of FIG. 7D correspond to similarly reference numbers in FIG. 4F.

Claims (10)

1. A loudspeaker assembly, comprising:

   an acoustic waveguide (12B,12D);

   an acoustic driver (10) mounted to the waveguide so that a first surface of the acoustic driver radiates sound waves into the waveguide so that the sound waves are radiated from the waveguide, and a second surface of the acoustic driver radiates sound waves directly to the environment; and

   an acoustic volume (22), acoustically coupled to the acoustic waveguide through an opening (34), which operates to increase the amplitude of the sound waves radiated from the acoustic waveguide at a wavelength equal to the physical length plus end effect corrections of the acoustic waveguide (12B, 12D) to a level greater than the amplitude of the sound waves radiated by the second surface of the acoustic driver, characterised in that dimensions of the opening and of the acoustic volume are selected so that a Helmholtz resonance frequency is at a frequency that is outside an operating frequency range of the waveguide.
2. A loudspeaker assembly according to claim 1, the acoustic waveguide having curved walls forming walls of the acoustic volume.
3. A loudspeaker assembly according to claim 2, the waveguide walls forming walls of another acoustic volume coupled to the acoustic waveguide.
4. A loudspeaker assembly according to claim 2, further comprising electronic components positioned in the acoustic volume.
5. A loudspeaker assembly in accordance with claim 1, wherein the dimensions of the opening and of the acoustic volume are selected so that a width of the opening exceeds 50% of a width of the acoustic volume.
6. A loudspeaker assembly according to claim 1, the waveguide comprising multiple curved sections substantially defining the acoustic volume.
7. A loudspeaker assembly according to claim 6, the acoustic waveguide substantially defining another acoustic volume.
8. A loudspeaker assembly according to claim 6, wherein the acoustic volume is teardrop shaped.
9. A loudspeaker assembly according to claim 1, the waveguide having a substantially constant cross-sectional area.
10. A loudspeaker assembly according to claim 1, wherein a closed end of the waveguide adjacent the acoustic driver has a larger cross-sectional area than an open end of the waveguide.