CN1261002C

CN1261002C - Acoustic device

Info

Publication number: CN1261002C
Application number: CNB008078068A
Authority: CN
Inventors: 格雷厄姆·班克; 尼尔·哈里斯
Original assignee: New Transducers Ltd
Current assignee: Google LLC
Priority date: 1999-06-10
Filing date: 2000-06-07
Publication date: 2006-06-21
Anticipated expiration: 2020-06-07
Also published as: JP2003501982A; CN1356015A; DE60004045D1; WO2000078090A3; AU5089600A; NZ515328A; GB9913465D0; EP1186203A2; HK1040875A1; HK1040875B; TW479434B; EP1186203B1; WO2000078090A2; DE60004045T2

Abstract

An acoustic device has a plurality of resonant bending wave modes along the length of a member (1). The fundamental frequency of resonant bending wave modes in directions perpendicular to the length is much higher, so that the lower frequency resonant bending wave modes are substantially one directional. A plurality of transducers (5) may be spaced across the width of the member (1) at a preferred position (3) along the length of the panel (1).

Description

Sound device

Technical Field

The present invention relates to acoustic devices, and in particular to acoustic devices using resonant bending wave modes.

Background

A previous resonant bending wave device is described in WO 97/09842. This document describes a plate having a resonant bending wave mode in the region of the plate. A transducer may be provided on the board at a preferred location for exciting a resonant mode to produce a speaker or transducer. Such an arrangement is known as a distributed mode loudspeaker.

US 3347335 describes a loudspeaker in which bending waves are transmitted along a beam. In this arrangement, bending waves are excited at one end of the beam, and a non-reflective termination is provided at the other end of the beam. Because this termination is non-reflective, the bending wave will propagate along the beam and be absorbed, without being reflected back to form a resonant mode.

Disclosure of Invention

According to the present invention there is provided an acoustic device comprising an element having a modal axis and a plurality of non-modal axes. There are multiple resonant bending wave modes on the mode axis, with the non-mode axis perpendicular to the mode axis. Wherein the fundamental frequency of the resonant mode along each non-modal axis is five times the fundamental frequency of the resonant mode along the modal axis.

Preferably, the fundamental frequency of the resonant mode along each non-modal axis is at least ten times that along the modal axis. The higher the fundamental frequency along the non-modal axis, the more the acoustic device can be said to be "one-dimensional" compared to the fundamental frequency along the modal axis.

The element may be a plate with a modal axis along the length of the plate and a non-modal axis along the width of the plate. Such a plate need not be flat.

When a bending wave mode is excited in a plate, this bending wave mode will cause a small amount of displacement of the plate on its face. This displacement will vary with one direction in the board surface. When a bending wave mode is said to be along a particular direction, that direction refers to the direction in which the displacement changes, not the direction of the displacement itself.

The fundamental frequency along a particular axis is the frequency of the lowest bending wave mode along that axis. The mode density along an axis is related to the fundamental frequency along that axis: there will be more resonant modes along the axis with the fundamental frequency lower than along the axis with the fundamental frequency higher over a wide frequency range.

For comparison, prior art document WO97/09842 teaches interleaving of mode frequencies along the long and short axes, which requires that these fundamental frequencies be approximated. This document teaches the use of isotropic plates with an aspect ratio of 1.134 or 1.41. Aspect ratios 1.134 and 1.41 correspond to fundamental frequency ratios 1.285 and 2, respectively.

Fundamental frequency f along one axis of the plate_oBending stiffness B (winding) of the plateA vertical axis) and the length L of the plate along the axis are in the following proportional relationship (assuming constant mass per unit area):

(f_o)²∝B/L⁴

it can be seen that in order to achieve a high ratio of the fundamental frequency on the width-wise axis to the fundamental frequency on the length-wise axis, the width may be less than half the length, preferably less than one third of the length.

At those frequencies where resonant bending wave modes are excited along the modal axis rather than the non-modal axis, the sound emitted by the panel is anisotropic. In such a frequency range, sound is preferentially emitted into a plane passing through the mode axis and perpendicular to the panel, while there is less sound in a plane passing through the non-mode axis and perpendicular to the panel. In this way, sound in the plane through the mode axis can be enhanced at these frequencies. Such a plate may therefore be particularly suitable for use with a piezoelectric transducer. The frequency response of the piezoelectric transducer becomes smaller at low frequencies. The increased low frequency sound output can compensate for this trailing of the excitation to produce an overall more uniform sound.

In some specific applications, it may also be useful to preferentially radiate sound into a single plane. Such as directing sound into a horizontal plane in a room to avoid sending too much sound to the ceiling or floor of the room.

It is the sound that is preferentially emitted into a plane that is most meaningful for a flat panel, rather than a bar. And this preferential emission increases with increasing width. However, this is the case assuming that this one-dimensional property can be maintained and that no mode is excited in the non-mode axis of the plate. The latter condition requires a narrow width. In order to be able to meet the conflicting requirements of one-dimensional properties but large plate widths, highly anisotropic plates may be used.

Such a plate may be more rigid about the mode axis than about the non-mode axis. The bending stiffness of the plate about the modal axis may be at least 1.5 times, more preferably at least 2 times as stiff, as the bending stiffness about the non-modal axis. Since resonant bending wave modes along one axis cause bending about a perpendicular axis, the number of modes along the non-modal axis will be reduced if the plate is more rigid about the modal axis.

The plate with anisotropic bending stiffness may be made of a material with a corrugated or grid-like structure, the grid or corrugations being distributed in the plate surface along a non-modal axis.

In an embodiment, a transducer may be employed to excite a resonant bending wave mode. The locations where the transducers may be preferentially placed are separated from nodes of lower modes along the mode direction. To this end, the transducer may be positioned at a preferred location along the length of the element, for example, at about 4/9, 3/7, or 5/13 along the mode axis. These positions are similar to those taught in WO97/09842 except that in this document, these preferred positions have coordinate values in two directions. The transducer need not be positioned on the mode axis, but may be positioned to one side of the mode axis.

Multiple transducers may be provided. In order to provide multiple transducers at a preferred location, multiple transducers may be juxtaposed across the width of the board. This arrangement can increase the output. Alternatively, a single transducer may extend across the width of the plate at a preferred location. Such transducers are effective even if they only cause bending along one axis.

When used on a two-dimensional panel where the spatial extent cannot be too large, a curved transducer across the width of the panel can provide more power than a single point transducer.

The plate may also be excited at a less preferred position, for example a position closer to one end than the preferred position. The bending stiffness may be varied along the mode axis such that other positions than those described above become preferred. Alternatively, the plate may be damped or clamped in some way, which improves the effect of the plate even when it is excited in a less preferred position.

Drawings

For a better understanding of the invention, an embodiment thereof will now be described, by way of example only, with reference to the accompanying drawings. Wherein,

FIG. 1 illustrates an audio device according to the present invention;

FIG. 2 shows the output of the panel of FIG. 1 as a function of frequency in three directions in a plane perpendicular to the panel, in the direction of the modes;

fig. 3 shows the output of the panel of fig. 1 as a function of frequency in three directions in a non-modal direction in a plane perpendicular to the panel.

Detailed Description

As shown in the figure, the rectangular plate 1 is substantially flat in the x (longitudinal) and y (width) directions. The plate is anisotropic with respect to bending stiffness and is much narrower with respect to its length. The plate is also much more rigid about the x-axis than about the y-axis. Thus, the fundamental frequency on the x-axis, i.e., the mode axis, is much smaller than the fundamental frequency on the non-mode y-axis. Thus, there are more resonant bending wave modes along the x-axis than along the y-axis.

A plurality of transducers 5 are arranged spaced apart from one another in the y-direction along a line 3 across the width of the panel. This straight line 3 is four ninth of the length from one end of the plate in the x-direction along the plate length direction. More power can be input by multiple transducers than with a single transducer. The use of multiple transducers in the device of the invention is less restrictive than the use of multiple transducers in a distributed mode board according to the teachings of WO97/09842, because in such a two-dimensional board the location of the transducers is limited to a preferred area, whereas in a one-dimensional board such an area is limited to only a preferred distance along the length of the board.

The transducer 5 is connected to a conventional amplifier by a wire 7. These transducers are conventional bending wave transducers. The transducers may be piezoelectric transducers.

The sound pressure level produced by such a panel as a function of frequency and in dB is measured. Figure 2 shows the sound pressure level on the "axis", i.e. perpendicular to the panel surface, and in two other directions offset by 45 deg., 60 deg. from this axis towards the x-direction. Figure 3 shows the sound pressure level on the "axis", i.e. perpendicular to the panel surface, and in two other directions offset by 45 °, 80 ° from this axis towards the y-direction. Thus, FIG. 3 shows sound pressure levels emitted laterally, while FIG. 2 shows sound pressure levels emitted along the length of the panel. The sound pressure level was measured at a distance of 1m from the plate.

The panels to be tested were made of corrugated polymer sold under the trademark "corex". The stiffness of such a plate about the modal axis is about 2.83 times stiffer than about the non-modal axis.

The acoustic energy is not very directional in the plane of the mode axis (as shown in figure 2). The high frequencies are radiated at very wide angles and the mid frequencies are only slightly reduced off axis. This curve is similar to that obtained from conventional distribution pattern boards as taught, for example, in WO 97/09842.

In contrast, in the plane of the non-mode axis, the sound pressure level is significantly reduced with off-axis at high frequencies, while the sound pressure level is maintained at mid-band (see fig. 3). The measurements show that a small amount of sound is emitted from the side.

To increase this effect, the width of the plate may be increased. When the plate is wide, the wavefront becomes cylindrical and the low frequency output rises by 3dB per octave with decreasing frequency. This can compensate for the decrease in output of the piezoelectric driver at these frequencies. However, as can be seen from the relationship given above, the width of the plate is limited so that the fundamental frequencies remain sufficiently distinct to achieve an effective one-dimensional characteristic.

Claims

1. An acoustic device capable of operating in a predetermined frequency range, comprising a plate (1) having a length and a width, the plate (1) having a mode axis along the length of the plate, whereby the plate (1) is capable of supporting a plurality of resonant bending wave modes in the predetermined frequency range along the mode axis, characterised in that the plate (1) further comprises a non-mode axis along the width of the plate, the non-mode axis being perpendicular to the mode axis, the fundamental frequency of the resonant bending wave modes along the non-mode axis being at least five times the fundamental frequency of the resonant bending wave modes along the mode axis, the sound emitted by the plate being anisotropic at frequencies at which resonant bending wave modes are excited along the mode axis but not the non-mode axis.

2. The acoustic device of claim 1, wherein the fundamental frequency of the resonant bending wave mode along the non-modal axis is at least ten times the fundamental frequency of the resonant bending wave mode along the modal axis.

3. The acoustic device of claim 1, wherein the width of the panel is less than one-half of the length of the panel.

4. The acoustic apparatus of claim 1, wherein the bending stiffness of the plate about the mode axis is at least 1.5 times greater than the bending stiffness of the plate about the non-mode axis.

5. Acoustic device according to claim 4, characterised in that the plate has a corrugated or grid-like structure, the grids or corrugations being distributed along the non-modal axis.

6. Acoustic device according to any of the preceding claims, further comprising a transducer (5) coupled to the plate (1) to excite bending wave modes.

7. An acoustic device according to claim 6, wherein the transducer (5) is positioned apart from the nodes of a predetermined plurality of low frequency resonant bending wave modes.

8. The sound arrangement as claimed in claim 7, characterised in that the transducer (5) is placed at a distance 4/9, 3/7 or 5/13 from one end of the panel (1) along the mode axis of the panel (1).

9. An acoustic device according to claim 6, characterised in that the transducer (5) is a piezoelectric transducer (5).

10. Acoustic device according to claim 6, characterised by comprising a plurality of transducers (5).

11. A sound device according to any of claims 3 to 5, further comprising a plurality of transducers (5) arranged across the width of the panel.