Phonograph Monthly Review, Vol. 5, No. 9 (1931-06)

June 1931, Vol. V. No. 9 259 Problems of Sound Reproduction By BEN DRISKO # A description of electrical phono-radio systems with special reference to loud speakers, and suggestions for the correction of some of their imperfections From Needle Groove to Sound Waves A Visualization of Electrical Sound Production W ITH the approach of perfection in phonographic recording and broadcast transmission, sealed with the practically complete approval of the greatest living artists, one must be either hypersensitive or wholly in- sensate not to find much to enjoy and to learn through sound reproducing media which have come to play so dominant a part in modern life. But while it is a very simple matter to place a rec- ord on a phonograph turntable and set it spin- ning, or to turn on a radio and tune in a broad- casting station, there are many details—mostly of a mechanical nature—which if improperly ar- ranged may so detract from the faithfulness of the reproduction as to neutralize or negate the pleasure and interest of the audition. Most phono- graph and rado owners are unaware of the nature of these imperfections and blame their individual instruments for faults which are generic. I shall endeavor to set forth some of these details in or- der that the reader may understand them better, and if he be of an experimental turn of mind per- haps also apply corrective measures. Limiting our subject to the reproduction only, we must assume a comparatively faultless source of sound—either good recorded or broadcast ma- terial. If it be music, we must have a competent orchestra or other talent, making music in a room that is acoustically correct—which is to say that it is absolutely free from extraneous sounds and excessive reverberation. And the recording (or broadcasting) apparatus should be efficient over a musical range of at least seven octaves. (Note 1 ). Factors; of Phonographic Reproduction Sound reproduction by the phonographic me- dium is roughly the reverse of the recording proc- ess in that mechanical vibrations of the needle are transformed into sound waves. To generate sound waves requires power. In a mechanical phono- graph (utilizing a sound-box with a diaphragm of mica, metal, or some other substance) this pow- er must all come from the motion which the ir- regular sound track on a spinning record imparts to the needle, causing the diaphragm to vibrate, and thereby generating the actual sound waves. In a phonograph equipped with an electrical pick- up the amount of actual power which the needle delivers is considerably less, and substantially all of the sound generating power comes from the vacuum tube amplifier. Consequently there is less physical wear both on the needle and the record, (Note 2), and if the record-engraving be rede- signed to take advantage of this difference in fav- or of the electrical process, the amplitude of the vibrations in the sound track (and hence its width) may be greatly reduced. Many more sound tracks can be put in the same space, resulting in a longer playing record. The essential features of an electrical phono- graph are a record turntable actuated by a motor; a needle; a pick-up; a vacuum tube amplifying system; and loud speaker. All have their limita- tions and are possible centers of sound repro- ductive faults. Since the speed of the turntable must be exactly the same at which the turntable was rotating when the recording was made, and since it is next to impossible to construct a motor which will run with undetectable variations in speed, we have at once a serious source of trouble: fluctuations in the pitch of the music played re- sulting from fluctuations in the speed of either the phonograph turntable or the original record- ing apparatus, (Note 3). If the phonograph mo- tor is to blame it may be corrected at least in part by having the motor overhauled or equipped with a better governor. If the record has been record- ed at some constant speed other than the standard 78 r.p.m., correct reproduction may be obtained by altering the speed of the phonograph corres- pondingly. Otherwise a symphony written and played in one key will be heard in one entirely dif- ferent.