Dynamic Loudspeaker Research Articles

Discrimination of Sound Quality in changing the Lowest Resonance of the Dynamic Loudspeaker

Discrimination of Sound Quality in changing the Lowest Resonance of the Dynamic Loudspeaker

Loudspeakers

A loudspeaker is an electroacoustic transducer intended to radiate acoustic power into the air, the acoustical waveform being essentially equivalent to that of the electrical input. Electrodynamic, electromagnetic and electrostatic driving systems have been used for loudspeakers operating in air. However, during the past two decades the electrodynamic has overwhelmingly predominated in all applications for loudspeakers operating in air. The almost universal use of the direct-radiator dynamic loudspeaker in radio receivers, phonographs, magnetic-tape reproducers, television receivers, announce and intercommunicating systems is due to the simplicity of construction, small space requirements and the relatively uniform response-frequency characteristics. For small-scale applications the low efficiency of the direct-radiator loudspeaker is not a handicap. However, for large-scale high-power sound applications, the high-efficiency horn loudspeaker is particularly suitable because the amplifier requirements are reduced by an order of magnitude. For the future, the promising developments in loudspeakers appear to be in the field of motion control.

A New Line of High-Fidelity Loudspeakers

Three direct radiator dynamic loudspeaker mechanisms have been developed for high-fidelity sound reproduction. These units are of three sizes, namely, 8-, 12-, and 15-inch mechanisms. The improvements include smoother response frequency characteristics, broader directivity patterns, improved transient response, and lower nonlinear distortion. These characteristics have been obtained by employing the proper configuration for the cone, a special pulp for the material of the cone and a damping ring in the outside suspension of the cone. In the case of the 15-inch duo cone loud-speaker an additional feature, namely, a series of conical domes are cemented to the low-frequency cone. The high-frequency response is improved by these domes because of the smaller solid angle into which the high-frequency cone operates. The directivity pattern is broadened due to the diffraction effects produced by the domes.

Non-Linear Distortion in Dynamic Loudspeakers Due to Magnetic Effects

Two sources of non-linear distortion in a dynamic loudspeaker are considered in this discussion; both are related to the magnetic characteristics of the driving mechanism. The first type of distortion arises due to a force of attraction between the voice coil, carrying a current, and the iron of the field structure. This force varies as the square of the current and produces second harmonic distortion. The force may be related to the space rate of change of self-inductance of the voice coil as it moves in the air gap. The magnitude of the distortion produced in this way may be several tenths of one percent, and is greater for low frequencies and large currents. This distortion may be reduced by proper proportioning of the voice coil and field structure, and by using a short-circuited winding on the field structure. The second type of distortion arises due to non-uniformity of the magnetic field in which the voice coil moves. This effect is well known qualitatively, but equations are given here for its quantitative evaluation. These equations indicate that the distortion characteristically is less than one percent, and is greater for large amplitudes of motion. If the voice coil is centered in a symmetrical field, only odd-order distortion is produced. If the field is not symmetrical about the voice coil, even-order distortion is also present. This distortion may be reduced by proportioning the voice coil and field structure so that the mean field in which the coil moves remains as constant as possible.

Non-Linear Distortion in Dynamic Loudspeakers due to Magnetic Effects

Two sources of non-linear distortion in a dynamic loudspeaker are considered in this discussion; both are related to the magnetic characteristics of the driving mechanism The first type of distortion arises because of a force of attraction between the voice coil, carrying a current, and the iron of the field structure. This force varies as the square of the current and produces second harmonic distortion. The force may be related to the space rate of change of self-inductance of the voice coil as it moves in the air gap. The magnitude of the distortion produced in this way may be several tenths of one percent, and is greater for low frequencies and large currents. This distortion may be reduced by proper proportioning of the voice coil and field structure and by using a short-circuited winding on the field structure. The second type of distortion arises due to non-uniformity of the magnetic field in which the voice coil moves. This effect is well known qualitatively, but equations are given here for its quantitative evaluation. These equations indicate that the distortion characteristically is less than one percent, and is greater for large amplitudes of motion. If the voice coil is centered in a symmetrical field, only odd-order distortion is produced. If the field is not symmetrical about the voice coil, even-order distortion is also present. This distortion may be reduced by proportioning the voice coil and field structure so that the mean field in which the coil moves remains as constant as possible.

Mechanical Impedance Bridge

A mechanical impedance bridge based on the mechanical analog of Flanders method for measurement of acoustic impedance is described. The unknown impedance is attached to a bar rigidly connected to the voice coils of two dynamic loudspeaker driving units. One coil acts as the driver; the other serves as a detector. The unknown impedance is obtained in terms of three balances of an a.c. potentiometer connected across the coils. Two of the balances are for known mechanical impedances attached to the bar. Results of measurements of the mechanical impedance of phonograph needles are presented.

Exhaust Valve for Pneumatic Cloud Chambers

An air valve designed for the purpose of triggering the expansion of a pneumatic cloud chamber utilizes Bridgman's ``unsupported area'' seal. The valve is controlled by a magnet constructed from the field coil of an inexpensive dynamic loudspeaker. This design allows vacuum tube control of forces up to 75 pounds. High speed of operation (∼1/300 second) recommends its use with counter control, and its extreme simplicity is advantageous in general work.

Recent Development in Dynamic Loud Speakers

THE reproduction of speech and musical sounds has received very careful study by engineers associated with the radio industry. As a result of such study, acoustical reproduction by broadcast receivers and their associated loud speakers has reached a fairly high degree of perfection. To be sure, this is not generally true for all loud speakers and all sets. Nevertheless, the improvements in reproducers and sets which have been brought about during the last three years, have made possible a new appreciation of radio broadcast and reception.