LOUDSPEAKER SYSTEM HAVING A BASS-REFLEX PORT

Abstract: A loudspeaker system comprises an enclosure (1) which accommodates a loudspeaker device (3) and a bass-reflex port (5) having two open ends (5b, 5c). The port has a longitudinal axis (5a) and a length (L) defined by the two open ends and has a passage which flares towards the two open ends. This provides flared portions (5d, 5e) which portions extend over a substantial part of the length of the port. In order to minimize undesirable noises and distortions said flared portions, in a longitudinal section of the port, have bounding lines (7) which extend at an angle alpha having a value of between 3 DEG and 12 DEG with respect to the longitudinal axis of the port.

Description: Loudspeaker system having a bass-reflex port. The invention relates to a loudspeaker system comprising an enclosure which accommodates a loudspeaker device and a bass-reflex port having two open ends, which port has a longitudinal axis and a length defined by the two open ends and has a passage which flares towards the two open ends, flared portions being present, which portions extend over a substantial part of the length of the port. A bass-reflex port is essentially an open duct by means of which an internal volume of a loudspeaker enclosure communicates with the medium outside the enclosure, a first open end being situated inside the enclosure and a second open end being situated outside the enclosure. Such a port enhances the sound reproduction in the lower range of the frequency spectrum. The effect of the bass-reflex port is based on the Helmholtz resonator principle, which is known per se, the frequency of the reproduced sound being dependent on the volume of the enclosure, the length and cross-section of the port and the velocity of sound. Small loudspeaker enclosures require a port of comparatively small cross- section. However, in order to obtain the same sound pressure at or near the Helmholtz frequency in a small enclosure as in a large enclosure, it is necessary that per unit of time the same amount of air flows through the port. This means that the flow velocity of the air in the port is comparatively high for a small enclosure. It has been found that when known bass-reflex ports are used, which ports have a constant cross-section, a high sound level at or near the Helmholtz frequency is attended by noises, acoustic losses and distortions of the sound. From WO-A 90/11668 a loudspeaker system is known, which comprises a bass-reflex cabinet with a loudspeaker and a duct. The duct provides the communication between the interior and the exterior of the cabinet and constitutes an adapter of converging- diverging longitudinal section. Shapes proposed for the adapter are conical, exponential and hyperbolic profiles, the asymptote to the adapter being required to extend at an angle of between 30° and 50° with respect to the longitudinal direction. The adapter used in the known loudspeaker system serves to take the place of a duct of constant cross-section. It is an object of the invention to propose measures which result in an improvement of the loudspeaker system of the type defined in the opening paragraph. To this end, the loudspeaker in accordance with the invention is characterized in that in a longitudinal section of the port said flared portions have bounding lines which extend at an angle having a value of between 3° and 12° with respect to the longitudinal axis of the port. Thus, the port used therein has inner walls which are slightly inclined with respect to the longitudinal axis of the port. Surprisingly, it has been found that the loudspeaker system in accordance with the invention has a higher sound reproduction level at low frequencies and produces considerably less noises than the known loudspeaker systems. A reason found for this is that the air stream can follow the slightly inclined inner walls of the port for a long time without the passing air being separated from the wall. As a result of this, annoying vortex motions of the air and noises, acoustic losses and distortions attending these can be minimized. The port used, in which the flared portions extend to the proximity of the open ends, can be of round or rectangular or any other suitable cross-section. An embodiment of the loudspeaker system in accordance with the invention is characterized in that said angle is maximum 6°. Experiments have shown that a value of between 3° and 6° produces only minimal noises and acoustic losses at the Helmholtz frequency, particularly if the bass-reflex port has a length dimension and a smallest diameter of the order of magnitude of 13 cm and 2 cm, respectively, while the volume of the enclosure is comparatively small, for example 2.5 dm 3 . An embodiment of the loudspeaker system in accordance with the invention is characterized in that in a longitudinal section the port has bounding lines of parabolic shape. In this embodiment the bounding lines of said flared portions consequently have a parabolic shape. It has been found that, in the case of the gradual increase in cross- section towards both open ends obtained in this specific manner, the air which passes through the port follows the walls of the port. An embodiment of the loudspeaker system in accordance with the invention is characterized in that both open ends of the port have end portions which are radially rounded towards the exterior. The rounded end portions in conjunction with the measures defined in Claim 1, 2 or 3 ensure that, as it leaves the port, the air remains on the wall for such a long time that separation does not take place until an area is reached where the air velocity has already decreased considerably. For realistic air velocities in the port this only results in comparatively slight turbulences near the ends of the port, which only give rise to minimal noises and acoustic losses. Since the air stream is pulsating, i.e. constantly changes direction, it is important that both open ends are rounded. It has been found that the most favorable aerodynamic effects are achieved if the rounded end portions have a radius of between 3 and 10 mm. Moreover, it has been found that rounding at one end has hardly any effect. In an embodiment of the loudspeaker system in accordance with the invention the rounded end portions at both open ends, which portions adjoin the slightly inclined inner walls, change into flanges situated outside the port, for example flanges which are oriented transversely to the longitudinal axis of the port. The measure used in this embodiment may yield a further improvement of the loudspeaker system at given sound levels. One of the flanges can be integrated in a wall portion of the enclosure. An embodiment of the loudspeaker system in accordance with the invention has the characteristic feature as defined in Claim 7. As a result of the use of the element of a foam material one end or the ends of the port can be given a desired contour in a cheap manner. In principle, the foam material can be any regular foam plastic, such as for example polyurethane, or foam rubber. It has been found that the air permeability of foam plastic or foam rubber and the roughness of the surface structure of such a material have a favorable effect on the cancellation of acoustic losses and on the reduction of noises. As a result of the given roughness of the inner surface of the foam plastic or foam rubber element the fluid boundary layer formed in the case of air flow already becomes turbulent at low flow velocities. On the one hand, this gives rise to a small loss as a result of turbulence in the boundary layer but, on the other hand, the flow remains longer in contact than in the case of the completely smooth surface. Owing to the last-mentioned aspect the flow separates in the case of a larger local flow aperture and, consequently, in the case of a lower local air velocity, which results in a reduction of noises and acoustic losses. Moreover, as a result of the damping action of the foam material noises, particularly resonator-related noises, are reduced. An embodiment of the loudspeaker system in accordance with the invention has the characteristic feature as defined in Claim 8. In this embodiment the desired contour of the inner side of the port is obtained by means of the layer of a foam material. A foam material is to be understood to mean foam plastic and foam rubber. Such materials are slightly porous and have such a cell size that their surface structure is not perfectly smooth but exhibits a slight roughness. Owing to the afore-mentioned characteristics the presence of a layer of a foam material in the port leads to smaller acoustic losses and less noises. The non-perfectly smooth surface structure, leads to a later separation of air vortices in the boundary layer. The velocity of the air vortex motions is smaller owing to the larger local cross-section in the separation point, as a result of which acoustic losses and noises, which are both closely related to the separation velocity, are reduced. The slight roughness of the surface of the applied layer causes some acoustic loss but at higher levels this loss is small in comparison with the gain attainable owing to the later separation of the air vortices. A suitable foam plastic is, for example, polyurethane. The invention also relates to a bass-reflex port intended and specifically constructed for use in the loudspeaker system in accordance with the invention and defined as in any one of the Claims 1 to 8. The invention will be described in more detail, by way of example, with reference to the drawing, in which Figure 1 diagrammatically shows an embodiment of the loudspeaker system in accordance with the invention, Figure 2 diagrammatically shows a bass-reflex port used in the embodiment shown in Figure 1, and Figures 3 to 7 diagrammatically show variants of said port. The loudspeaker system in accordance with the invention shown in Figure 1 comprises an enclosure 1 which accommodates a loudspeaker device 3, taking the form of a cone loudspeaker known per se, and a bass-reflex port 5. The enclosure 1 forms a chamber 2 of a given volume, for example 2.5 dm 3 . The port 5, which provides open communication between the chamber and the medium outside the enclosure, has a longitudinal axis 5a and has two open ends 5b and 5c, of which one end 5b is situated inside the enclosure and the other end 5c is situated on or near a wall portion la of the enclosure 1. The port 5 in the present example has a length L of 13 cm and a round cross-section having a minimum diameter D of 2 cm. The port can be regarded as a resonator, a so-called Vih pipe, the first resonant frequency having a wavelength which is substantially equal to twice the port length. The port 5 in accordance with the invention has a passage which flares towards the two open ends 5b and 5c, the port in the present example having bounding lines 7 of parabolic shape in longitudinal section. In the present example this results in a flared portion 5d and a flared portion 5e at opposite sides of the center M of the port, which flared portions widen towards an open end 5b or 5c and extend over a substantial part of the port length L. The parabolic bounding lines 7 extend at an angle α.of maximum 12° with respect to the longitudinal axis 5a. Said flared portions 5d and 5e extend in areas where the angle α has a value of between 3° and 12°. The comparatively small area near the center M, where the angle α is smaller than 3°, is not relevant for the desired effect, i.e. particularly the reduction of noises and distortions. In the description of the bass-reflex port shown in Figures 2 through 5 parts and elements already described with reference to Figure 1 use the same reference numerals. Owing to the flared portions 5d and 5e the bass-reflex port 5 having a wall 8 and shown in Figure 2 has inner walls 5d j and 5e l 5 respectively, which are slightly inclined with respect to the longitudinal axis 5a. In the present example, these walls extend at a maximum angle α of 6° with respect to the longitudinal axis 5 a of the port 5. At both open ends 5b and 5c the port 5 has end portions 5f and 5g, respectively, which are radially rounded towards the exterior and which in the present example have a radius R of 5 mm. A slightly smaller or greater radius of, preferably between 3 and 10 mm, is also suitable. The rounded end portions 5f and 5g smoothly adjoin the gently inclined walls 5d j and 5e, , respectively, the end portion 5g in the present example changing smoothly into the wall portion la of the enclosure. The bass-reflex port 5 shown in Figure 3 differs from the port 5 shown in Figure 2 in that the rounded end portions 5f and 5g at the two open ends 5b and 5c change into flanges situated outside the port, one of the flanges being formed by the wall portion la of the housing 1. The other flange takes the form of a disc 9 having a central opening and is oriented transversely to the longitudinal axis 5a. The bass-reflex ports 5 shown in Figures 4 and 5, which are intended for use in the loudspeaker system in accordance with the invention, comprise two flared portions 5d and 5e, which in a longitudinal section of the port have bounding lines 7 extending at a constant angle α of 7° with respect to the longitudinal axis 5a of the port. A slightly greater or smaller angle of between 3° and 12° is also suitable. A cylindrical portion 5h extends in a central area situated between the flared portions 5d and 5e. In the present examples, each of the portions 5d, 5e and 5h extends over approximately one third of the port length. The two flared portions 5d and 5e are bounded by walls 5d j and 5e j , respectively, which at the open ends 5b and 5c of the port 5 change into rounded end portions 5f and 5g, respectively, having a radius R of, preferably, between 3 and 10 mm, in the present examples 6 mm. In the variant shown in Figure 4 the end portions 5f and 5g change smoothly into the flanges 9f and 9g, respectively. In the variant shown in Figure 5 the end portion 5g changes smoothly into the wall portion la of an enclosure. Furthermore, the end portion 5f changes smoothly into a flange 9. Possible methods of manufacturing the bass-reflex ports used in the loudspeaker system in accordance with the invention will be described hereinafter with reference to Figure 4. The port 5, which is essentially of a symmetrical nature, has its smallest cross-section halfway its length. The port 5 can be realized by using an injection- molding process and a die divided in the center of the port with an orientation transverse to the longitudinal axis 5a. Another possibility is to realize the port 5 in two production steps. In a first step an intermediate product is formed by means of an injection-molding process, which product has the characteristics of one half of the desired port 5, which half abuts on a temporary tubular port half. In a second step the tubular port half is given the desired shape by means of a hot-forming process. In comparison with the last-mentioned method the first- mentioned method has the drawback that the port may exhibit an irregularity as result of the necessary division of the die. The port 5 shown in Figure 6, which is intended for use in the loudspeaker system in accordance with the invention, has radially outwardly rounded end portions 5f, 5g at both open ends 5b, 5c, an end portion 5f being formed by a ring-shaped element 6 of polyurethane. This element 6 is secured to the wall 8 of the port 5, for example by clamping or by an adhesive joint. The port 5 shown in Figure 7 is lined with a layer 10 of foam rubber on its inner walls 5dl, 5el, the layer 10 in the present example extending into the open ends 5b, 5c and forming the rounded end portions 5f, 5g. It is to be noted that bass-reflex ports which differ from those shown but which meet the criteria defined the Claims are possible. For example, variants with ports of non-round cross-section are possible.

Claims: CLAIMS: 1. A loudspeaker system comprising an enclosure which accommodates a loudspeaker device and a bass-reflex port having two open ends, which port has a longitudinal axis and a length defined by the two open ends and has a passage which flares towards the two open ends, flared portions being present, which portions extend over a substantial part of the length of the port, characterized in that in a longitudinal section of the port said flared portions have bounding lines which extend at an angle having a value of between 3° and 12° with respect to the longitudinal axis of the port. 2. A loudspeaker system as claimed in Claim 1, characterized in that said angle is maximum 6°. 3. A loudspeaker system as claimed in Claim 1 , characterized in that in a longitudinal section the port has bounding lines of parabolic shape. 4. A loudspeaker system as claimed in Claim 1, characterized in that both open ends of the port have end portions which are radially rounded towards the exterior. 5. A loudspeaker system as claimed in Claim 4, characterized in that the rounded end portions have a radius of between 3 and 10 mm. 6. A loudspeaker system as claimed in Claim 4, characterized in that the rounded end portions change smoothly into flanges situated outside the port. 7. A loudspeaker system as claimed in Claim 4, characterized in that at least one of the end portions is formed by a ring-shaped element of a foam material secured to one end of the port. 8. A loudspeaker system as claimed in Claim 1, characterized in that the bass-reflex port is internally lined with a layer of a foam material. 9. A bass-reflex port suitable for use in the loudspeaker system as claimed in any one of the preceding Claims, and defined as in any one of the Claims 1 to 8.

Inventors: NIEUWENDIJK JORIS ADELBERT MAR, ROOZEN NICOLAAS BERNARDUS, VAEL JOZEF ERNEST MARIE, NIEUWENDIJK, JORIS, ADELBERT, MARIA, ROOZEN, NICOLAAS, BERNARDUS, VAEL, JOZEF, ERNEST, MARIE
Applicants: KONINKL PHILIPS ELECTRONICS NV, PHILIPS SVENSKA AB, KONINKLIJKE PHILIPS ELECTRONICS N.V., PHILIPS AB
Application year: 1998
Main classification: H04R1
IPC codes: H04R1