DYNAMIC SPEAKERS

Sun (Auckland), Volume II, Issue 557, 9 January 1929, Page 14

DYNAMIC SPEAKERS

HOW THEY OPERATE THE LAST WORD IN REPRODUCERS The tremendous enthusiasm that has greeted the general acceptance of the dynamic loud-speaker as the last word in tone reproduction has caused many to ask “What is this new principle of reproduction that has so changed our ideas of radio tone values?” As is well known, the ordinary telephone receiver is the invention of Alexander Graham Bell. It has been in use since 1576, and with many improvements in design, but still using the same principle, it is the receiver in use to-day. In 1911, Mr. E. S. Pridham, who is chief engineer of the Magnavox Company, was engaged in laboratory research work in Napa, California. Mr. Pridham and his associates discussed the limitations of the Bell type of reproducer, and the fact that the principle involved had remained unchanged since its inception in 1876. . In this type of receiver there is an iron diaphragm, the pull on which is strengthened or weakened by current flowing through the coils. The stronger the current the greater the pull—but not in exact proportion to the current. The diaphragm must be very close to the pole of the magnet in order that the device be sensitive. For ordinary telephone work this device has been satisfactory, but the two points of deficiency—lack of uniform force with current and the inability of the diaphragm to vibrate with comparative wide amplitude—has practically eliminated this device from the field of loud-speaking telephones. The Magnavox engineers discussed these shortcomings and determined to find out whether or not a better reproducer could be invented. A DISCOVERY During their research they discovered a new and more powerful telephone receiver. The principle involved is of extreme importance, as it has led to the success of the dynamic speaker. When a current is passed through a wire around a piece of soft iron that iron becomes magnetisc|L It is then called an electro-magnet. It has the power of attracting other pieces of iron to it. We also know that there is a field of force around the end of the magnet. If the bar of iron is bent so that the two ends are close to each other it will be found that there is a very strong field of magnetic force in the space between the ends. This space is called the air gap. In the first magnavox dynamic speaker invented in 1911, a single turn of copper wire was suspended free to move in a strong magnetic field. A diaphragm was attached to this turn of wire. When voice currents passed through this wire the diaphragm moved in exact accordance with the strength of the currents. The reproduction of the voice was beautifully clear and strong, and was a revelation as compared to the character of speech over the ordinary type of telephone. When current passes through the wire which is suspended in the strong magnetic field a force acts upon the wire. The wire moves in a direction parallel to the pole faces and not toward them. The force is exactly proportional to the current. If the current reverses, the force reverses. The movement of the wire can be as great an amplitude as desired, as there are no pole pieces to hit. BUILT ON SOUND BASIS In reality the dynamic type of reproducer is a sound motor built upon the same fundamental principle as the great motors of the electric industry. As with every invention, much development and research were necessary before the Magnavox dynamic re producer became a commercial success. Magnavox engineers were not slow to take advantage of the new principle and bent all their efforts toward the making of an instrument to be used on telephone lines. Instruments which had marvellous reproduction we ye made in 1914 and 1915, but tile difficulties of trying to supply the strong electro-magnet were well-nigh insurmountable. To produce this electro-magnet a source of direct current such as a storage batr tery or direct current generator was required. There were at that time no rectifying tubes or dry contact rectifiers such as we have to-day. The receiver itself was very heavy, some models weighing nearly one hundred pounds. The listening was done with ear tubes. So while the reproduction was wonderful and the instrument very sensitive, there were still tremendous difficulties to be overcome. At about this stage of the development (1915) a very remarkable discovery was made which has had a far-reaching effect upon the use of the dynamic reproducer. Up to this time the instrument was only used with listening tubes on telephone lines. One of the models had a rather large opening in the sound box and one aftei non the Magnavox engineers chanced to fit a large horn to this sound-box outlet.

The voice that came out was of

great volume. They were much es cited and put the new instrumen through many tests. A proper box was made to fit the horn. A strong magnetic field was supplied and a powerful microphone transmitter using two amperes was devised. The instrument caused a sensation. MUSIC HEARD FOR MILES Music played by a gramophone could be heard for miles. The name suggested for this new instrument was “Magnavox,” or “Great Voice.” There is no limit to the power that a properly designed speaker can handle. The exact proportion of the force acting on the diaphragm to the current in the movable coil gives the lifelike reproduction of the original that is of the highest importance from an acoustic standpoint. The enthusiasm which has greeted the Magnavox dynamic reproducer is proof that at last the dynamic principle is recognised as the ultimate for the most faithful reproduction of sound.