GB2476477A

GB2476477A - Loudspeaker uses movable panels to re-direct rearward sound waves

Info

Publication number: GB2476477A
Application number: GB0922448A
Authority: GB
Inventors: Malcolm Hyde
Original assignee: Individual
Current assignee: Individual
Priority date: 2009-12-22
Filing date: 2009-12-22
Publication date: 2011-06-29
Also published as: GB0922448D0

Abstract

The loudspeaker 1 comprises a mounting board 4, a speaker driver 8 mounted in the mounting board, and at least one acoustically reflective component 10, 11 which may be curved. Rearward acoustic waves from the speaker driver 8 impinge upon the acoustically reflective components 10, 11. The acoustically reflective components are movably mounted at pivots 12, 13 relative to the speaker driver to allow the direction of reflection of the rear acoustic waves emitted by the speaker 20 driver to be varied.

Description

Improvements to loudspeakers

Background of the Invention

The present invention concerns improvements to loudspeakers. More particularly, but not exclusively, the invention concerns loudspeakers with at least one acoustically reflective component to reflect and diffuse rear acoustic waves emitted by the speaker driver, to improve the performance of open-baffle dipole loudspeaker systems.

Loudspeaker drivers typically comprise a paper or synthetic material cone diaphragm, which is vibrated by means of an electromagnetic coil in response to an electrical signal. The vibration of the cone diaphragm causes the air adjacent to the cone to vibrate, creating acoustic waves.

Such a speaker driver is generally intended to emit forward acoustic waves, by which is meant acoustic waves generated by the interior of the cone, which travel in the direction of increasing diameter of the cone. However, such a speaker driver will also emit rear acoustic waves, which are by the exterior of the cone and travel in the direction of decreasing diameter of the cone (in other words in the opposite direction to the forward acoustic waves). The forward and rear acoustic waves will be 180° out of phase with each other. This is because when the interior of the paper cone moves towards the adjacent air when creating a forward acoustic wave, it must necessarily be the case that the exterior of the paper cone moves away from the adjacent air (and vice versa). Thus, whenever a forward acoustic wave is generated by the interior of the paper cone, a corresponding rear acoustic wave at 180° out of phase is generated by the exterior of the paper cone. Loudspeaker systems that emit acoustic waves from both forwards and backwards, particular in the case that the acoustic waves form two lobes of equal radiation, with no waves being emitted perpendicularly, are known as dipole loudspeaker systems.

Such rear acoustic waves are generally considered to be undesirable, as their difference in phase can cause them to interfere with the forward acoustic waves and cancel them out.

A well-known solution to this problem is to mount the speaker driver in a wall of an enclosed cabinet, so that any rear acoustic waves are contained within the cabinet. (This arrangement is sometimes called a box enclosure loudspeaker arrangement.) However, such an arrangement will cause the rear acoustic waves to cause undesirable acoustic effects from the rear acoustic waves, due to the energy of the rear waves contained within the cabinet; for example standing waves may form, which can affect the vibration of the speaker driver thus having a detrimental effect on its performance. (A standing wave, also known as a stationary wave, is a wave that remains in a constant position. This phenomenon can occur because the medium through which the wave travels is moving in the opposite direction to the wave, or it can arise in a stationary medium as a result of interference between two waves travelling in opposite directions. In the second case, for waves of equal amplitude travelling in opposing directions, there is on average no net propagation of energy. Standing waves in resonators are one cause of the phenomenon called resonance.) A known solution to the problem of standing waves is to line the interior of the cabinet with an acoustically absorbent material such as foam, but this only reduces and does not completely eliminate any undesirable effects.

A loudspeaker arrangement that attempts to mitigate this problem is disclosed in US 2002/0011379 Al (Taylor), published.31 January 2002. The speaker driver is mounted in a cabinet that has an opening at the rear, in which is mounted a "venturi expander". The expander is an acoustically reflective insert that reflects the rear acoustic waves emitted by the speaker driver around the sides and front of the cabinet. By allowing the rear acoustic waves to de dispersed from the back of the cabinet, the problems caused by the use of a fully enclosed cabinet are reduced.

A further problem with conventional box enclosure loudspeaker arrangements is that the overall pattern of acoustic waves that they produce does not accurately mimic those produced during the real-world experience of a "live" performance. During a live performance, acoustic waves will in general be emitted in all directions from the source of the sound, and as a consequence the acoustic waves transmitted to a listener will be a combination of acoustic waves that travel directly to the listener (the forward acoustic waves), and indirect acoustic waves that travel to the listener by reflecting off one or more surfaces to reach the listener (the rear acoustic waves).

In contrast, with conventional box enclosure loudspeaker systems only forward acoustic waves are emitted, with there being no indirect waves heard by the listener as a result of acoustic waves transmitted in other directions.

(This is the case even for the loudspeaker arrangement of US 2002/0011379 Al, as the expander causes the rear acoustic waves to be dispersed and so there are no significant indirect waves transmitted to the listener.) The psycho-acoustic effect of this on a listener (i.e. the psychological effect on the perception of the listener as a result of the acoustic waves transmitted to them) is that the performance they experience can sound artificial, lacking the sense of "space" experienced during a live performance.

A generally accepted solution to this problem is to mount a dipole loudspeaker driver in an open mounting, rather than a conventional box enclosure; such that the loudspeaker driver is allowed to emit acoustic energy unobstructed in both directions. The rear acoustic wave is free to reflect off the rear wall behind the loudspeaker and radiate in a forward direction to the listener. This arrangement provides a psycho-acoustically satisfying listening experience comprising a mix of direct and reflected sound, similar to that experienced by a listener during a live performance. However, the open-baffle arrangement suffers the problems associated with the 180° out of phase rear acoustic waves cancelling the forward acoustic waves discussed above. The non-linear frequency response of open-baffles is a well understood limitation of this design, and is variously referred to as "baffle roll-off", "baffle-step cancellation" and other similar terms, and can result in -6dB roll-off in the lower frequency response.

The present invention seeks to mitigate the above-mentioned problems.

Summary of the Invention

In accordance with a first aspect of the present invention there is provided a loudspeaker comprising: a mounting board; a speaker driver mounted in the mounting board, the speaker driver being arranged to emit forward acoustic waves in a forward direction from a first surface of the mounting board, and rear acoustic waves in a rearward direction from the opposing surface of the mounting board; at least one acoustically reflective component mounted relative to the speaker driver to reflect at least a portion of the rear acoustic waves emitted by the speaker driver; wherein the acoustically reflective component is movably mounted relative to the speaker driver to allow the direction of reflection of the rear acoustic waves emitted by the speaker driver to be varied.

By reflecting the rear acoustic waves using the acoustically reflective component, the problems associated with trapping the rear acoustic waves within an enclosed cabinet are avoided. Further, by having the acoustically reflective component movable relative to the speaker driver, the path travelled by the rear acoustic waves can be changed. This allows a path to be selected such that the time taken for the rear acoustic waves to reach a listener at a desired listening position relative to the time taken for the forward acoustic waves to reach the listener mimics that experienced by a listener during a live performance. A path of suitable length can also be selected that means that the rear acoustic waves are not 1800 out of phase with the forward acoustic waves when they reach a listener.

Further, the use of a movable acoustically reflective component means that a suitable path can be selected for different sizes and shapes of room, and for different positions of the speakers within such rooms.

The invention is particularly advantageous when the loudspeaker driver is a dipole loudspeaker driver, though the skilled person will appreciate that the invention is applicable with any loudspeaker driver that emits both forward and rear acoustic waves.

Advantageously, the surface of the acoustically reflective component facing the speaker driver is flat or curved away from the speaker driver. Particularly when the surface, of the acoustically reflective component facing the speaker driver is curved away from the speaker driver, this diffuses the rear acoustic waves, giving a better acoustic experience to a listener as the desired combination of front acoustic waves arid diffused rear acoustic waves can be experienced over a larger area, increasing the area of optimum listening position (or "sweet spot"). Preferably, the surface of the acoustically reflective component curves in range of 00 to 20° a straight profile. The acoustically reflective component may comprise a curved panel of uniform thickness, in other words with a shallow C-shaped cross-section. Alternatively, the acoustically reflective component may.for example have a D-shaped cross-section, or any other suitable cross-section.

Preferably, the loudspeaker comprises a pivot on which the acoustically reflective component is rotatably mounted relative to the speaker driver. Preferably, a handle is mounted on the pivot for rotating the acoustically reflective component. In an alternative embodiment, the loudspeaker comprises a hinged arm on which the acoustically reflective component is movably mounted relative to the speaker driver. In a further alternative embodiment, a plurality of slots are provided in which the acoustically reflective component can be fitted, the different slots giving different positions for the acoustically reflective component relative to the speaker driver.

Advantageously, the loudspeaker comprises a first acoustically reflective component arranged to reflect a portion of the rear acoustic waves emitted by the speaker driver in a first direction perpendicular to the rearward direction, and a second acoustically reflective component arranged to reflect a portion of the rear acoustic waves emitted by the speaker driver in a second direction adverse to the first direction. This allows rear acoustic waves to be reflected and diffused rearwards to both sides of the loudspeaker, so that they can reach the listener from both sides, giving a better listening experience.

Preferably, the loudspeaker comprises first and second walls extending in the rearward direction from opposing sides of the mounting board. These walls help direct the rear acoustic waves towards the acoustically reflective component.

In accordance with a second aspect of the present invention there is provided a room for an acoustic performance, the room having a listening position from which the acoustic performance is to be experienced, wherein one or more loudspeakers as claimed in any of claims 1 to S are positioned in the room so that at least a portion of the forward acoustic waves emitted by the speaker drivers of the loudspeakers will travel directly to the listening position in a first time period; and wherein the acoustically reflective components of the one or more speakers are positioned relative to their respective speaker drivers so that at least a portion of the rear acoustic waves emitted by the speaker drivers will travel, via the acoustically reflective components and the walls of the room, to the listening position in a second time period after the first time period.

Such an arrangement of the loudspeakers within a room allows an improved listening experience to be provided to a listener positioned at the listening position.

Advantageously, the second time period is 10 to 40 milliseconds after the first time period. In this timefrarne reflected sound is not perceived as a discrete echo, but instead spatial and audio image effects dominate. These early reflections allow the human hearing mechanism to coordinate the source location and distance of a particular sound and so provide third-dimensional information to the brain. In this way reflected sound is essential to providing a realistic or plausible listening experience.

In accordance with a third aspect of the present invention there is provided a method of setting up a room for an acoustic performance, the room having a listening position from which the acoustic performance is to be experienced, comprising the steps of: positioning one or more loudspeakers as claimed in any of claims 1 to 6 in the room so that at least a portion of the forward acoustic waves emitted by the speaker drivers of the loudspeakers will travel directly to the listening position in a first time period; positioning the acoustically reflective components of the one or more speakers relative to their respective speaker drivers so that at least a portion of the rear acoustic waves emitted by the speaker drivers will travel, via the acoustically reflective components and walls of the room, to the listening position in a second time period after the first time period.

The moveable nature of the acoustically reflective components allows the loudspeakers to be "acoustically coupled" to listening rooms of different shapes or sizes, for different given speaker placements.

-10 -Advantageously, the second time period is 10 to 40 milliseconds after the first time period.

It will of course be appreciated that features described in relation to one aspect of the present invention may be incorporated into other aspects of the present invention. For eamp1e, the method of the invention may incorporate any of the features described with reference to the apparatus of the invention and vice versa.

Description of the Drawings

Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which: Figure 1 shows a perspective view of a loudspeaker according to an embodiment of the invention; Figure 2 shows a front view of the loudspeaker of Figure 1; Figure 3 shows a side view of the loudspeaker of Figure 1; Figure 4 shows a top view of the loudspeaker of Figure 1; and Figure 5 shows a room incorporating the speaker of Figure 1.

-11 -

Detailed Description

A loudspeaker 1 according to an embodiment of the invention is shown in Figures 1 to 4. As can be seen in particular from Figures 1 and 2, the loudspeaker 1 comprises a housing 2 mounted on a stand 3 (the stand is shown in Figure 1 only). The housing 2 comprises a mounting board 4, from which a left side wall 5, right side wall 6, and top wall 7 extend rearwards. The top edges of the left side wall 5 and right side wall 6 are respectively joined to the top wall 7, so that the housing is closed at the top and sides, but open at the bottom and rear. A dipole speaker driver 8 is mounted in the mounting board 4 so that the primary acoustic output from the speaker driver 8 is emitted in the forward direction relative to the mounting board 4, with the secondary acoustic output from the dipole speaker driver 8 being emitted in the rearward direction.

As can be seen from Figures 3 and 4, a bottom wall 9 extends rearwards from the mounting board 4. The left side wall 5 and right side wall 6 are respectively joined to the bottom wall 9. However, the left side wall 5 and right side wall 6 extend below the position at which they are joined to the bottom wall 9, so that the bottom wall 9 is generally hidden when the housing 2 is viewed from the front. The housing 2 comprising the mounting board 4, left side wall 5, right side wall 6, top wall 7, and bottom wall 9 together acts to direct the secondary acoustic output from the dipole speaker driver 8 in the rearward direction.

The loudspeaker 1 further comprises acoustic reflecting components or fins 10 and 11, which are mounted vertically -12 -behind the speaker driver 8 on pivots 12 and 13 respectively. The pivots 12 and 13 are attached to the acoustic reflecting components 10 and 11 at a distance from either vertical edge of the components. The pivot 12 is attached to the acoustic reflecting component 10 roughly equidistantly from either vertical edge, while the pivot 13 is attached to the acoustic reflecting component 11 similarly. The pivots 12 and 13 are attached to the rear surfaces of the acoustic reflecting components 10 and 11, so that the acoustic reflecting components 10 and 11 lie entirely between the pivots 12 and 13 and the speaker driver 8.

* The top and bottom ends of the pivots 12 and 13 are rotatably attached to the top wall 7 and bottom wall 9 * 15 respectively. The pivots 12 and 13 extend through the bottom wall 9, and at their lower ends have knobs 14 and 15 respectively by which the acoustic reflecting components 10 and 11 can be rotated.

As can be seen from Figure 8 in particular, the acoustic reflecting components 10 and 11 have a slightly curved horizontal cross-section, so that the surfaces of the acoustic reflecting components 10 and 11 facing the speaker driver 8 are curved away from the speaker driver 8. In alternative embodiments the surfaces of the acoustic reflecting components facing the speaker driver can be straight or can be have greater or lesser curvature. The more curved the surface is in relation to the rear acoustic wave, the more acoustically diffuse will be the rear reflection which reaches the listener.

-13 -The acoustically reflecting components 10 and 11 are made from an acoustically reflective material such as rosewood, acacia koa or mahogany, though the skilled person will appreciate that any other material or combination of materials that provides an acoustically reflecting surface to reflect the rear acoustic waves emitted by the dipole speaker driver 8 would be suitable for the invention.

A room incorporating a loudspeaker 1 in accordance with an embodiment of the invention is shown in Figure 5. As can be seen a room 100 has located in it first and second loudspeakers la and lb. of the same type as the loudspeaker 1. The room 100 has a left side wall 101, right side wall 102 and rear wall 103 (the fourth wall is not shown). The arrangement of the room 101 and the loudspeakers la and lb is symmetrical about a central line C equidistant between the left side wall 101 and right side 102. The loudspeakers la and lb themselves are also symmetrical, in other words the construction of one is a mirror-image of the construction of the other. Consequently, in the following only the behaviour of the first loudspeaker la will be described.

A listening position L is arranged on the central line C. The listening position L is the position at which any acoustic performance provided by the loudspeakers la and lb is intended to be experienced.

In use, the loudspeaker 1 will produce sound by means of the dipole speaker driver 8. The dipole speaker driver 8 will emit acOustic waves from the front of the speaker driver 8, in a forward direction relative to the mounting -14 -board 4. A portion of these front acoustic waves will travel directly to the listening position L along the path F shown in Figure 5. This portion of the front acoustic waves will take a time TF to travel from the speaker driver 8 to the listening position L. As well as the speaker driver 8 emitting the front acoustic waves, as discussed above the speaker driver 8 will emit a corresponding set of acoustic waves from the rear of the speaker driver 8. These rear acoustic waves will be 1800 out of phase from the front acoustic waves.

A first portion of the rear acoustic waves emitted by the speaker driver 8 will be reflected by the acoustically reflective component 10 at an angle O. onto the rear wall 103, which will in turn reflect them onto the left side wall 101, which will in turn reflect them to the listening position L. Thus, this first portion of the rear acoustic waves will travel from the. speaker driver 8 to the listening position L along the path R1 shown in Figure 5. This first portion of the rear acoustic waves will take a time TR1 to travel from the speaker driver 8 to the listening position L. A second portion of the rear acoustic waves emitted by the speaker driver 8 will be reflected by the acoustically reflective component 11 at an angle 02 onto the rear wall 103, which will in turn reflect them onto the right side wall 102, which will in turn reflect them to the listening position L. Thus, this second portion of the rear acoustic waves will travel from the speaker driver 8 to the listening position L along the path R2 shown in Figure 5. This first portion of the rear acoustic waves will take a time TR2 to -15 -travel from the speaker driver 8 to the listening position L. By rotating the acoustically reflective component 10, the angle 9. can be varied, which will in turn vary the path R1 along which the portion of rear acoustic waves travel to the listening position L. (In fact, the exact portion of rear acoustic waves which reach the listening position by reflecting off the rear wall 103 and left side wall 101 will itself be varied.) By varying the length of the path R1, the time TR1 it takes the rear acoustic waves to travel from the speaker driver 8 to the listening position L can be varied. Similarly, the time TR2 can be varied by rotating the acoustically reflective component 11.

In use, the acoustically reflective components 10 and 11 of the loudspeakers la and lb are rotated to as to provide at the listening position L an ideal mix of direct and early reflected sound provided by the loudspeakers la and lb. In practice, this means that the acoustically reflective components are adjusted so that the periods TR1 and TR2 are roughly 10 to 40 milliseconds longer than the period TF, so as to mimic a live performance whereby a listener at listening position L hears a mixture of direct (front acoustic wave) and reflected (rear acoustic wave) * 25 sounds. The ability to adjust the rotation of the acoustically reflective components 10 and 11 means that the loudspeakers la and lb can be adjusted for use in different sizes and shapes of room and different positions of the loudspeakers la and lb in a room.

-16 -Another result of the rear acoustic waves taking a longer time to reach the listening position L is that the rear acoustic waves are when they reach the listening position L no longer 1800 out of phase with the front acoustic waves, so that the interference between and cancelation of the front and rear acoustic waves is reduced considerably.

The curved cross section of the surface of the acoustically reflective components 10 and 11 means that the rear acoustic waves are diffused as they are reflected, which results in a widened "sweet spot" of rear acoustic waves relative to the listening position L. This gives a better acoustic experience to a listener, as the desired combination of front acoustic waves and delayed rear acoustic waves can be experienced over a larger area, and generally provides a more enveloping psycho-acoustic experience.

Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein.

Claims

-17 -Claims 1. A loudspeaker comprising: a mounting board; a speaker driver mounted in the mounting board, the speaker driver being arranged to emit forward acoustic waves in a forward direction from a first surface of the mounting board, and rear acoustic waves in a rearward direction from the opposing surface of the mounting board; at least one acoustically reflective component mounted relative to the speaker driver to reflect at least a portion of the rear acousticwaves emitted by the speaker driver; wherein the acoustically reflective component is movably mounted relative to the speaker driver to allow the direction of reflection of the rear acoustic waves emitted by the speaker driver to be varied.
2. A loudspeaker as claimed in claim 1, wherein the surface of the acoustically reflective component facing the speaker driver is flat or curved away from the speaker driver.
3. A loudspeaker as claimed in claim 1 or 2, further comprising a pivot on which the acoustically reflective component is rotatably mounted relative to the speaker driver.
4. A loudspeaker as claimed in claim 3, further comprising a handle mounted on the pivot for rotating the acoustically reflective component.

-18 -
5. A loudspeaker as claimed in any preceding claim, comprising a first acoustically reflective component arranged to reflect a portion of the rear acoustic waves emitted by the speaker driver in a first direction perpendicular to the rearward direction, and a second acoustically reflective component arranged to reflect a portion of the rear acoustic waves emitted by the speaker driver in a second direction adverse to the first direction.
6. A loudspeaker as claimed in any preceding claim, further comprising first and second walls extending in the rearward direction from opposing sides of the mounting board.
7. A room for an acoustic performance, the room having a listening position from which the acoustic performance is to be experienced, wherein one or more lpudspeakers as claimed in any of claims 1 to 6 are positioned in the room so that at least a portion of the forward acoustic waves emitted by the speaker drivers of the loudspeakers will travel directly to the listening position in a first time period; and wherein the acoustically reflective components of the one or more speakers are positioned relative to their respective speaker drivers so that at least a portion of the rear acoustic waves emitted by the speaker drivers will travel, via the acoustically reflective components fins and the walls of the room, to the listening position in a second time period after the first time period.-19 - 7. A room as claimed in claim 6, wherein the second time period is 10 to 40 milliseconds after the first time period.
8. A method of setting up a room for an acoustic performance, the room having a listening position from which the acoustic performance is to be experienced, comprising the steps of: positioning one or more loudspeakers as claimed in any of claims 1 to 6 in the room so that at least a portion of the forward acoustic waves emitted by the speaker drivers of the loudspeakers will travel directly to the listening position in a first time period; positioning the acoustically reflective components of the one or more speakers relative to their respective speaker drivers so that at least a portion of the rear acoustic waves emitted by the speaker drivers will travel, via the acoustically reflective components fins and walls of the room, to the listening position in a second time period after the first time period.
9. A method as claimed in claim 8, wherein the second time period is 10 to 40 milliseconds after the first time period.
10. A loudspeaker substantially as herein described with reference to any of Figures 1 to 4 of the accompanying drawings.
11. A room substantially as herein described with reference to Figure 1 to 5 of the accompanying drawings.
12. A method of setting up a room substantially as herein described with reference to any of Figures 1 to 5 of the accompanying drawings.