LOUDSPEAKER SYSTEM HAVING A BASS-REFLEX PORT

Abstract: A loudspeaker system comprises an enclosure which accommodates a loudspeaker device and a bass-reflex port (105). The port has two open ends (105b, 105c) and means for the suppression of noises. These means comprise a central area (c) of the port provided with perforations (109), as well as a foam material (111) provided in and/or on the perforations.

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, which port has two open ends and means for the suppression of noises. A bass-reflex port is basically an open tube or pipe by means of which an internal volume of a loudspeaker enclosure communicates with an environment outside the enclosure, one open end being situated inside the enclosure and one open end being situated outside the enclosure. Such a port enhances the reproduction of sound in the lower range of the frequency spectrum. The operation 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 the cross-sectional dimension of the port and the velocity of sound. Small loudspeaker enclosures require a port of comparatively small cross- section. However, in a small enclosure, it is necessary that per unit of time the same amount of air flows through the port in order to obtain the same sound pressure at or near the Helmholtz frequency as in a large enclosure. 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, a high sound level at or near the Helmholtz frequency is attended by noises, i.e. undesired sounds. From EP-A 0 361 445 a loudspeaker system is known, which comprises an enclosure with a loudspeaker and a bass reflex port made up of a plurality of parts. This bass reflex port comprises a visco-elastic part formed by two rubber outer cylinders disposed in line with one another and an air-permeable part formed by a felt inner cylinder, the inner cylinder extending between and for a substantial part in the outer cylinders, to which it is connected. The bass-reflex port used therein serves to counteract undesired sounds, particularly duct resonant sounds. However, the multi-part port also forms a drawback of the known loudspeaker system because the manufacture of such a bass-reflex port is rather laborious owing to the necessity of positioning various parts relative to one another and subsequently securing them to one another. It is an object of the invention to improve the known loudspeaker system so as to counteract undesired sounds in an effective and simple manner. In order to achieve this object, the loudspeaker in accordance with the invention is characterized in that the means for the suppression of noises comprise a central area of the port, which area is situated centrally with respect to the open ends and has been provided with perforations, as well as a foam material provided in and/or on the perforations. For the sake of completeness, it should be noticed that the bass-reflex port has a circumferential wall, the perforations being consequently situated in this wall. Undesired sounds arise in that air vortices are produced at the two open ends of the port. This effect occurs particularly in the case of small and slender ports which are usually employed in small enclosures. The vortices arising at the open ends of the port produce sound dominated by certain frequencies. These frequencies correspond to the so-called l/2λ resonant frequency of the port and harmonics of this frequency. Undesired sounds produced at the l/2λ resonant frequency are reduced effectively by the measure taken in the bass-reflex port of the loudspeaker system in accordance with the invention. The l/2λ resonance exhibits a maximum pressure level in the center of the port but as a result of the perforations present in the central area the sound pressure remains limited owing to leakage of air through the perforations, the presence of the foam material providing the necessary damping. Experiments have shown that by the simple measures thus taken a substantial reduction of undesired sounds can be achieved, while the desired sound level can be maintained unchanged. Furthermore, it has been found that the measures taken have a favorable effect on the so-called Q factor of the port. For the sake of completeness, it is to be noted that this quality factor is a measure of the damping of sound. In the case of a favorable, i.e. comparatively low Q factor the decay time is comparatively short, as a result of which both desired sounds and undesired sounds are sustained for a shorter time. The bass-reflex port, which essentially has the shape of a tubular body, can in principle be manufactured from any regular solid material such as a hard plastic, for example polystyrene. The basically hollow port can be of round or non-round cross-section. In principle, the foam material can be any regular porous air-permeable foam plastic, such as polyurethane foam, polyether foam and PVC foam, or air-permeable rubber. It is to be noted that from JP-A 4 114598 a loudspeaker system of the bass- reflex type is known, which employs a pipe having a communication opening halfway its length. In this known system relatively many undesired sounds are produced, while the damping is comparatively low (high Q factor). Moreover, the known system only the 1/2 λ resonance is suppressed. In a practical embodiment of the loudspeaker system in accordance with the invention perforations are situated along the circumference of the central area of the port. In this embodiment the l/2λ resonance is suppressed most effectively. An embodiment of the loudspeaker system in accordance with the invention has the characteristic features as defined in Claim 3. The use of this embodiment also leads to a reduction of undesired sound caused by further resonant frequencies, particularly harmonics of the l/2λ resonant frequency. In a practical embodiment perforations are present along the circumference in one or more non-central areas situated outside the central area. An embodiment of the loudspeaker system in accordance with the invention has the characteristic features as defined in Claim 5. The use of this embodiment makes it possible to achieve an optimum reduction of undesired sound caused by the l/2λ resonant frequency and harmonics of this frequency. In a simple to realize yet effective embodiment of the loudspeaker system in accordance with the invention the foam material takes the form of a layer surrounding the port and deposited on an outer wall of the port. Such a layer is obtained, for example by the provision of a collar or cuff of a foam material. The invention further relates to a bass-reflex port intended for use in the loudspeaker system in accordance with the invention and constructed as defined in any one of the Claims 1 through 6. It is to be noted that the European Patent Application which bears the application number 97202356.4 (PHN 16.477; which is incorporated herewith by reference) and which has not yet been published at the date of filing of the present Application, describes a loudspeaker system of the bass-reflex type, in which measures have been taken inter alia to counteract acoustic losses and distortions of the reproduced sound. Applying these measures in the present loudspeaker system in accordance with the invention leads to further surprising improvements of the system. Thus, it has proved to be highly favorable to provide the port with a passage which flares towards the two open ends, flared portions being provided, which portions extend over a substantial part of the length of the port and, in a longitudinal section of the port, 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. Such a port consequently has inner walls which are slightly inclined with respect to its longitudinal axis. The use of this measure leads to a higher sound reproduction level at low frequencies and a further reduction of noises. This is caused in particular by the fact that the air stream in the port 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. Furthermore, 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, in the case of a round port, 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 . Moreover, it has appeared that bounding lines of parabolic shape this promotes that the air which passes through the port in operation follows the walls of the port for a prolonged time. It has further been found that it is favorable if both open ends of the port have end portions which are radially rounded towards the exterior. Such rounded end portions promote that the air, as it leaves the port, 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. The rounded end portions at both open ends, which portions adjoin the slightly inclined inner walls, may change into flanges situated outside the port, for example flanges which are oriented transversely to the longitudinal axis of the port. This measure may yield a further improvement of the loudspeaker system at certain sound levels. One of the flanges can be integrated in a wall portion of the enclosure. The invention will be described in more detail, by way of example, with reference to the drawings, in which Figure 1 diagrammatically shows an embodiment of the loudspeaker system in accordance with the invention, Figures 2 through 4 diagrammatically show variants of bass-reflex ports suitable for use in the loudspeaker system in accordance with the invention, and Figures 5 is a graph representing the sound pressure as a function of the frequency. 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 dm3. The port 5, which provides open communication between the chamber and an environment of the enclosure, has a wall 5g and a longitudinal axis 51. The port 5 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 an outer side, particularly 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 together with the enclosure forms a Helmholtz resonator, but also acts as a so-called l/2λ pipe, the first resonant frequency having a wavelength which is substantially equal to twice the port length. The port 5, by way of example, has a passage which flares towards the two open ends 5b and 5c, while the port may have bounding lines 7 of parabolic shape in longitudinal section. A flared portion 5d and a flared portion 5e are present 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 51. In a central area c situated near the center M the port has perforations 9 formed in the wall 5a, which perforations are shown diagrammatically in broken lines in Figure 1. In or on these perforations a foam material is present. Furthermore, perforations 9 may be provided in one or more non-central areas d between the center M and the open ends 5b and 5c. The perforated areas in combination with the foam material form an effective means for the suppression of noises. The embodiments 105 of the bass-reflex port in accordance with the invention shown in Figure 2, which is intended for use in a loudspeaker system in accordance with the invention, comprises a tubular body 120 of a hard material such as a hard plastic, in the present case in the form of a hollow cylinder, having a longitudinal axis 1051 and a wall 120a, in the present example a cylindrical wall. The port 105 further has two open ends 105b and 105c. In an area c which is situated centrally with respect to the open ends, the wall 120a has perforations 109 in the form of small through-holes in the wall, the perforations being uniformly spaced along the circumference of the port. A collar 111 of an air-permeable foam material, such as a polyurethane foam, surrounds the central area c. The embodiment 205 of the bass-reflex port in accordance with the invention shown in Figure 3 has a wall 220 which surrounds a longitudinal axis 2051 and which has inner surfaces 205dl and 205el, respectively, which are slightly inclined with respect to the longitudinal axis 2051. In the present example, these surfaces extend at an angle α of maximum 6° with respect to the longitudinal axis 2051. At both open ends 205b and 205c the port 205 has end portions 205f and 205g, respectively, which are radially rounded towards the exterior and which in the present example have a radius r of approximately 5 mm. A slightly smaller or greater radius of, preferably between 3 and 10 mm, is also suitable. The rounded end portions 205f and 205g smoothly adjoin the gently inclined surfaces 205dl and 205el, respectively. In a central area c the port has a perforated wall portion 220a around which a cuff 211 of a foam material, such as porous rubber, is arranged. The embodiment 305 of the bass-reflex port in accordance with the invention shown in Figure 4 has two open ends 305b and 305c.A central part c and two non-central parts d have perforations 309 and have a foam material 311, such as a polyester foam, in and/or on the perforations 309. In the present example, annular layers of a foam material have been provided around the port. The graph in Figure 5 successively shows results of measurements carried out on a loudspeaker system including a conventional bass-reflex port, a bass-reflex port as shown in Figure 2 and a bass-reflex port as shown in Figure 4. During the measurements the sound pressure was measured at a position situated at a distance of 1 meter from the system. The driving frequency was 45 Hz. The graph, which represents the air pressure as a function of the frequency, shows three measurement curves, namely a curve ci, which relates to measurements when the conventional port, i.e. a port without perforations, has been provided, a curve c 2 , which relates to measurements when a port as shown in Figure 2 has been provided, and a curve c 3 , which relates to measurements when a port as shown in Figure 4 has been provided. As is apparent from the graph, the noises are dominated by two port resonances, namely at approximately 1 kHz and approximately 2 kHz. When a port is used which has a perforated central area provided with a foam material at this location (curve c 2 ), the noise at 1 kHz appears to be reduced by approximately 20 dB and at 2 kHz by approximately 5 dB. When a port is used which, in addition, has perforated non-central areas provided with a foam material at this location (curve c 3 ), a substantially equal noise reduction is obtained at 1 kHz but a reduction of approximately 15 dB is obtained at 2 kHz. It is to be noted that the invention is not limited to the embodiments disclosed herein by way of example. For example, ports of non-round cross-section or ports having end portions of a foam material are possible. With regard to the Claims, it is to be noted that various combinations of measures defined in the dependent Claims are possible.

Claims: CLAIMS: 1. A loudspeaker system comprising an enclosure which accommodates a loudspeaker device and a bass-reflex port, which port has two open ends and means for the suppression of noises, characterized in that the means for the suppression of noises comprise a central area of the port, which area is situated centrally with respect to the open ends and has been provided with perforations, as well as a foam material provided in and/or on the perforations. 2. A loudspeaker system as claimed in Claim 1, characterized in that the perforations are situated along the circumference of the central area of the port. 3. A loudspeaker system as claimed in Claim 1, characterized in that the means for the suppression of noises further comprise a non-central area which is situated outside the central area, and has been provided with perforations. 4. A loudspeaker system as claimed in Claim 3, characterized in that perforations have been provided along the circumference of the non-central area. 5. A loudspeaker system as claimed in Claim 3, characterized in that the means of the suppression of noises further comprise a foam material provided in and/or on the perforations in the non-central area. 6. A loudspeaker system as claimed in Claim 1 or 5, characterized in that the foam material takes the form of a layer surrounding the port and deposited on an outer wall of the port. 7. A bass-reflex port constructed and evidently intended for use in the loudspeaker system as claimed in any one of the preceding Claims.

Inventors: HIRSCHBERG ABRAHAM, ROOZEN NICOLAAS B, VAN ECK PASCAL, HIRSCHBERG, ABRAHAM, ROOZEN, NICOLAAS, B., VAN ECK, PASCAL
Applicants: KONINKL PHILIPS ELECTRONICS NV, PHILIPS SVENSKA AB, KONINKLIJKE PHILIPS ELECTRONICS N.V., PHILIPS AB
Application year: 1999
Main classification: H04R1
IPC codes: H04R1