Acoustics – Anatomic or prosthetic relation – Ear and mouth
Patent
1980-07-16
1983-07-26
Schreyer, Stafford D.
Acoustics
Anatomic or prosthetic relation
Ear and mouth
181172, H04R 904
Patent
active
043955983
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electro-acoustic transducer of the loudspeaker-type comprising a membrane which is flexible or rigid before assembling and of which the front portion has an aperture with a predetermined half-angle .alpha. and is the site of vibrations responsible for sound emission which are transmitted in the material of the membrane from the motor of the transducer with a velocity V.sub.m such as V.sub.m #V.sub.0 /cos .alpha., where V.sub.0 is the velocity of sound waves in air.
2. Description of the Prior Art
In the conventional loudspeakers, the membrane is the seat of standing waves depending upon the frequency to be reproduced. For certain frequencies, the position of nodes and antinodes involves a maximum acoustic radiation in conjunction with resonances and a tone colouration. For other frequencies, the membrane behaviour is inverse and the acoustic radiation is minimum. In this case, if it is considered that the membrane is excited with a same energy, the amplitude of the membrane displacement increases as a total loss; the acoustic radiation impedance becomes wholly reactive. This leads to a low sound output and amplitude distorsions. In this type of loudspeaker, if the excitation in red noise ceases, the energy stored by the membrane is dissipated non-uniformly as a function of the frequency; the loudspeaker displays an acoustic tailing for certain frequencies.
The above-mentioned drawbacks have for consequence an amplitude-frequency response curve undulating at least in the high frequency range of its transmission band and the phase curve ceases to be at minimum. The sound power output of the loudspeaker is relatively low, because a considerable proportion of stored energy in the membrane is not radiated acoustically but dissipated in the form of heat, in particular by negative electrical feedback in the moving voice coil.
Moreover, in the known loudspeakers it is necessary to achieve a compromise between the flexibility of the membrane suspension, the choice of membrane material and the mass of the movable assembly of the motor, on one hand, and the width of the frequency range to be reproduced by the loudspeaker and its electro-acoustical output on the other hand. For the reproduction of frequencies up to medium frequencies, the membrane suspension is generally flexible and the moving emitting mass is large, in order to enable the membrane to move with a large amplitude. For reproducing medium to high audible frequencies, the membrane may by contrast be mounted rather rigidly on the loudspeaker chassis and be of small dimensions, because only the membrane area to the apex of the cone vibrates. Associated with the requirements in this second case, the mass of the movable assembly of the motor must be low, because the vibrational velocity in the membrane becomes high. In order to reproduce sound waves without non-linear amplitude distortions within a wide frequency range, it appears that it is necessary to constrain the geometrical deformations of the membrane to be much smaller than the amplitudes responsible for acoustic radiation, in analogy with the operation for which the membrane moves as an ideal rigid acoustic piston radiator, which is certainly not achieved, at least above a pulsation .omega..sub.0 =2 V.sub.0 /a, where V.sub.0 is the velocity of sound in air and a is the radius of the emitting portion of the loudspeaker. Consequently, the construction of a loudspeaker capable of reproducing a wide frequency range is difficult to achieve. This also results from the fact that sound waves generated at any two points along a generatrix of the conical membrane are not in phase at any listening point and that this phase shift is all the more pronounced as the frequencies to be reproduced become higher.
In order to avoid the above-mentioned drawbacks, Lincoln Walsh teaches in U.S. Pat. No. 3,424,873 a loudspeaker such as defined in the first paragraph of the present description. According to this patent, the membrane o
REFERENCES:
patent: 1726105 (1929-08-01), Harrison
patent: 2956636 (1960-10-01), Boersma
patent: 3032136 (1962-05-01), Hegeman
patent: 3125647 (1964-03-01), Rouy
patent: 3424873 (1969-01-01), Walsh
Journal of The Audio Engineering Society; vol. 2, No. 4; Oct. 1954, H. F. Olson; Recent Developments in Loudspeakers; pp. 219-227.
Saffitz A. A.
Schreyer Stafford D.
Societe Audax
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