Electrical audio signal processing systems and devices – Electro-acoustic audio transducer – Having acoustic wave modifying structure
Reexamination Certificate
1999-05-24
2001-08-14
Le, Huyen (Department: 2643)
Electrical audio signal processing systems and devices
Electro-acoustic audio transducer
Having acoustic wave modifying structure
C381S349000, C381S353000, C181S156000
Reexamination Certificate
active
06275597
ABSTRACT:
BACKGROUND OF THE INVENTION
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.
U.S. Pat. No. 5,109,422 discloses a loudspeaker system, 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.
SUMMARY OF THE INVENTION
In accordance with the invention 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 ½&lgr; resonant frequency of the port and harmonics of this frequency. Undesired sounds produced at the ½&lgr; resonant frequency are reduced effectively by the measure taken in the bass-reflex port of the loudspeaker system in accordance with the invention. The ½&lgr; 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 ½&lgr; 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 ½&lgr; resonance is suppressed most effectively.
An embodiment of the loudspeaker system in accordance with the invention also suppresses noise by means of a non-central area, outside the central area, provided with perforations. The use of this embodiment also leads to a reduction of undesired sound caused by further resonant frequencies, particularly harmonics of the ½&lgr; 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 a foam material in and/or on the perforations in the non-central area. The use of this embodiment makes it possible to achieve an optimum reduction of undesired sound caused by the ½&lgr; 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.
U.S. Pat. No. 5,892,183; 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,
Hirschberg Abraham
Roozen Nicolaas B.
Van Eck Pascas
Le Huyen
U.S. Philips Corporation
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