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

262 The Phonograph Monthly Review the musical range; even within that range there may be many imperfections. Until one has heard a perfectly balanced instrument equipped with a loud speaker able to radiate faithfully fre- quencies over a range of at least from thirty to five thousand cycles, one can have no conception of the full realism and effectiveness of modern sound reproduction. Even the untrained ear can recognize the tremendous difference between the half-good and the good, once it has a criterion of performance. And while theoretical perfection is still an unattained goal, it is possible even for the amateur to correct the deficiencies of the ordin- ary good radio or phonograph so as to attain re- sults that are infinitely more faithful technically as well as incomparably more pleasing to the ear. NOTES Note 1 . The recording or transmitting apparatus should have a flat (i.e. impartial) frequency characteristic from say thirty cycles to five thousand cycles. Middle “C” being 250 cycles; this range would extend approximately from three octaves below middle “C” to four and a quarter above. More would be better, particularly on the high frequency end in or- der to get the higher harmonics or overtones of the various instruments in the orchestra. There is a very good possibility that this upper range may be extended in the case of broad- cast transmitters, (in a few instances it already has been) but in phonographic recording needle scratch and surface noise become objectionable if much over four or five thous- and cycles is allowed to come out. 2. There is less physical wear on needle and record with a properly constructed pick-up. Some pick-ups, however, are either so heavy a weight on the record or so stiff in opera- tion as to cause even more record wear than mechanical phonographs. 3. Pitch fluctuation may also be due to incorrect centra- tion of the record. The fairly common wavering of the pitch of sustained notes near the end of a record is usually due to this purely mechanical fault. Imperfectly centered records may be corrected, but the method is tedious and at the best a make-shift. One should rather take pains to check up in correct centration before purchase of the record. 4. Many readers will protest that they have heard all kinds of low notes from much smaller horns than the above. Here again mathematics and laboratory experiment reveal some surprising things. The “C” three octaves below middle “C” on the piano is nominally thirty-two cycles in frequency, but the note also contains harmonics or overtones all the way up to perhaps as high as one thousand cycles. The human ear is so constituted that the fundamental (in this instance thirty-two cycles) and some of the lower harmonics may be entirely missing from a sounded note and yet the tone will be easily recognizable as to pitch and identity (piano, organ, string, etc.) If too many of the lower harmonics are missing the naturalness or depth of the tone is lost and the result is an unpleasant squeaking or rasping quality. An upright piano is practically incapable of radiating any- thing below about fifty cycles in sufficient quantity to be heard; all that we do hear of such notes is the overtones. A concert grand, however, due to its greatly increased radiating surface actually radiates the fundamental frequencies. A radio set or electrical phonograph which goes down to say one hundred fifty cycles sounds quite fair to the inexperi- enced ear, but when it is compared with one going down to thirty cycles even the untrained ear can detect the far greater depth and substance of tone in the latter. 5. The size of the baffle board is determined by the low- est frequency one wishes to radiate. Letting “N” represent this frequency in cycles per second, and “ij” the diameter in feet to be determined for the baffle board, the equation which expresses the relationship between these factors is (1100 is the distance in feet covered in one second by any sound wave—regardless of its pitch.) Conversely the lowest frequency a given instrument can radiate can be found. In this case the cabinet itself acts as a baffle board and “D” is the distance of the shortest path from the front of the diaphragm around the edge of the cabi- net and back to the base of the diaphragm. 6. In mounting the speaker in a baffle board the connect- ing wires will generally have to be lengthened by splicing. These wires are four in number—two to the field coil (carry- ing direct current) and two carrying voice or music frequency. If the length of the latter two wires is to be over ten feet or so they should be two separate wires (i.e. not a double con- duit) and of fairly large size, say No. 14 B. & S. gauge, and *■ should not be run closely together. Otherwise there is likely to be a capacity leakage of the higher frequencies. 7. Reverberation is present to some extent in any room or hall and too much of it can certainly ruin any kind of sound reproduction. Probably we have all noticed how much more beautiful music sounds over water, although I dare say com- paratively few have figured out that the reason lies in the fact that here there are no walls or other sound reflecting surfaces nearby to cause reverberation. The same effect may be accomplished indoors by padding the walls and ceiling with some nonreflecting material. So far as I know, Gimco Rock- wool Flexfelt (applied over battens to leave an inch or so of dead air space next to the wall) is the best material for this purpose at present. This type of sound-proofing has been used for years in talking film, broadcast, and recording studios. Another source of distortion lies in mechanically transmit- ted vibrations from the speaker back to the vacuum tubes, resulting in an abnormal reinforcement of certain tones. This of course is eliminated when the speaker is removed from con- tact with the amplifier. PLAN A DAY IN PHILADELPHIA THIS SUMMER You will enjoy browsing through our stock of imported discs. New ship' ments are arriving every week and you will surely find some items of interest. Make your choice from over 17,000 different records on our shelves. Visitors are always welcome at “The World’s Record Shop”. We will ap' preciate it if our mailorder clients will make themselves known when they call. H. ROYER SMITH CO. 10th & Walnut Streets Philadelphia, Pa., U.S.A. Wflthln walking distance of all railroad stations & ferries