Acoustics – Diaphragm and enclosure – Reflex baffle
Patent
1990-04-19
1991-10-15
Brown, Brian W.
Acoustics
Diaphragm and enclosure
Reflex baffle
H05K 500
Patent
active
050566164
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a loudspeaker comprising a loudspeaker cabinet or box with at least one loudspeaker element provided with a loudspeaker membrane means, and with a sound passage arrangement which in association with a wall of the cabinet defines a cavity which via apertures is acoustically coupled with the membrane means and with the surroundings of the cabinet.
The principles on which today reproduction of low frequencies of music is based, have existed in approximately 20 years. Within the area of electronics, on the other hand, there has recently been an enormous development. The new CD players thus have their best properties in the bass region. The loudspeakers are therefore the most limiting factor in today's bass reproduction.
The object of the invention has been to provide a loudspeaker design which more fulfils today's requirements on bass reproduction and simpler tuning of the loudspeaker system.
This object has been attained in that, in the loudspeaker according to the invention, one of the apertures communicates with other ones of the apertures via sound passages, the sectional area of which varies from the first mentioned aperture to the other apertures, and in that at least two of the sound passages have mutually different lengths.
The invention can be stated to be based on the so called Helmholtz-resonator and can therefore be regarded as a further development of the conventional bass reflex loudspeaker.
More in detail the invention in a practical case can involve the following.
In front of an aperture or opening in a loudspeaker cabinet an outer baffle is mounted on a small distance from the otherwise closed cabinet. The loudspeaker element can be mounted in the outer baffle via the opening, or in a wall at a distance from the opening, e.g. the same wall as the opening wall, but outside the area of the baffle. Between the cabinet and this outer baffle there is thereby formed a cavity in the form of a narrow gap from the opening to the outer edge of the baffle. In this outer edge there are a number of small side apertures through which the sound can pass. The sound thus passes outwardly partly through the side apertures and partly through the loudspeaker membrane. Tuning is carried through by varying the distance between the box wall and the baffle and by varying the width, depth, length, and location of the side apertures with respect to the opening.
By locating the small side apertures on different distances from the opening the frequency response becomes very uniform. No blowing sounds (air noises) appear since the sound channel from the opening to the side apertures has a varying sectional area. Thereby a linear impedance transformation is obtained between the inner of the cabinet and the surroundings.
The impedance curve of the loudspeaker system becomes uniform without great variations and the loudspeaker membrane deflections become very moderate, also at the lowest frequencies.
The power capacity of the loudspeaker is increased at the same time as it requires less power to be able to produce a high sound pressure at low frequencies, and the cabinet volume can be made smaller as compared with known technique.
Due to the fact that the side apertures are distributed on different distances from the opening a very smooth frequency response can be obtained, as mentioned, down to the lowest spectrum around 20 Hz. The different distances to the side apertures furthermore results in the Q-value of the Helmholtz-resonance being low. This together with the smooth impedance curve results in a very good transient reproduction and low distortion also at very low frequencies.
Despite of the low Q-value of this resonator design an increase of the efficiency of 10 to 12 dB can be obtained in the lowest bass, as compared with a loudspeaker according to "the closed box" principle. This is possible due to the fact that the resonator action has here been provided in two steps. First the sound is conducted from the inner of the cabinet via the opening to the gap cavity. From this small volum
REFERENCES:
patent: 2670053 (1954-02-01), Doms
patent: 2718932 (1955-09-01), Benjamin
patent: 3122214 (1964-02-01), Robbins
patent: 3938617 (1976-02-01), Forbes
patent: 4131179 (1978-12-01), Pope
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