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

June 1931, Vol. V. No. 9 Blasts and Reverberation But whether or not a speaker is able to radiate the lower frequencies there is a common failing in the faithfulness of reproduction due to one or more peaks or valleys in the frequency character- istic, resulting in individual notes sounding con- siderably louder or softer than they should. Per- haps one note of a phrase blasts or fades while the others are of equal intensity. By matching the pitch of the offending note on the piano it may be located definitely. Let us say for example that there is a scale passage of five notes beginning with middle “C”. The third note in the passage, “E”, stands out unpleasantly above the rest. Now play a different record and if the same phenom- eon occurs with the “E” above middle “C”, the chances are that the trouble is in the phonograph and that this particular frequency is not being faithfully reproduced. If there is no. blasting of “E” in the second record, the fault is most like- ly in the original record and not in the instru- ment. It may be checked up again by playing the original record at a higher speed, raising the pitch of the music exactly one whole tone. If the loud note of the scale passage is now the second of the five and still matches the “E” of the piano, the phonograph is to blame; whereas if it is still the third—now matching the “F sharp” of the piano—the record is at fault. Another but more obvious complication is the possibility that the extra volume at a particular frequency may be caused by reverberation, or sympathetic vibration, with something in the room or within the cabinet or loud speaker com- partment of the instrument. Room reverberation may be reduced by transferring the instument to another position or eliminated by proper padding of the walls or ceiling. Failing that, the phono- graph must be transferred to another room en- tirely. Reverberation within the cabinet is a fault in the construction of the instrument, and may be corrected by padding, tightening the parts, or sometimes merely by leaving the back of the cabinet open. Constructing a Baffle Board As some of the more experimentally-minded of my readers may be sufficiently interested, I ap- pend a few hints on the proper construction of a baffie board which will enable the ordinary dy- namic type of loud speaker to radiate the lower frequencies so essential to well balanced repro- duction. My first baffie board, which proved very satisfactory, was made of two pieces of 3/4 inch five ply veneer glued together with a feathered joint, the joined board measuring six by six feet (obtained at a cost of less than ten dollars), (Note 5). A hole of the same size as the loud speaker opening was cut before gluing and the 261 Cross Sectional View of Dynamic Speaker and Baffle Board loud speaker frame was screwed onto the back with a felt packed air tight joint, (Note 6). An- other time I took out a window pane and fitted a smaller board (with a loud speaker similarly at- tached) into the window, allowing the remainder of the window and the house wall to act as the baffle board. This method of course is less sight- ly and requires some protection for the loud speaker against the weather, but it has the great advantage that extraneous sound radiated from the back of the diaphragm never gets into the room to cause standing waves, reverberations, or reactions back on the diaphragm. The volume of sound was noticeably less from the same setting, but the music was much clearer. Another man has used a tightly fitting closet door as his baffle board with excellent results. When this is done it is best to pick a closet of about 120 cubic feet in capacity and to hang it full of old clothes or other soft materials to prevent the same annoying res- onances and reverberations which are so likely to mar the performance of the ordinary cabinet loud speaker installation, (Note 7). Undoubtedly the average phonograph or radio user will be inclined to accept his individual in- strument and all its inherent weaknesses “as is”, and to dismiss my suggested tests and corrective meaures as troublesome experimentation by no means essential to the pleasure he obtains at pres- ent from records and broadcasts. But the lay- men’s ear is too ready to accept musical shadow for the substance. The musician and theorist realize that the reproduction of even the best of modern instruments covers only a portion of