How to add sound to amateur films (1954)

acoustics to some extent by absorbing sound, but it produces competitive sounds in the form of coughing, rustling and so on. In a full hall therefore you need far more volume to reach the back. Unless you allow for this, the amplifier output may be inadequate when you have an audience, even though it was more than enough when the hall was empty. To get a rough idea of your requirements, use the following equation: A,. . „.. M , _ x Volume of room in cubic feet Minimum amplifier output (watts) = — — ! — /QQQQ ! — So for a hall measuring 100 x 40 x 20 feet high, your amplifier must be rated at no less than 8 watts. This figure is conservative and may be influenced by a variety of factors such as the shape of the hall and the material used for walls and ceiling. Loudspeakers vary in efficiency also — the equation above assumes a good loudspeaker in a small box cabinet. Back Radiation Most loudspeakers produce sound from the back as well as the front. If this back radiation strikes a hard wall behind the screen, it will be reflected forward and confuse the sound received directly. You can greatly reduce this trouble by hanging a curtain, or even a coat, behind the speaker to absorb the sound from the back. Do not put anything close against the back of the cabinet or you will stifle the action of the speaker and possibly increase the tendency to box resonance. One radiogram manufacturer provides an ingenious solution to this problem by closing the back of the cabinet with stacked sheets of corrugated paper. The back radiation enters the hundreds of tubes so formed without any reflection. The sound is absorbed as it passes down the tubes, so none escapes from the back. Whenever you set up your speaker, take these simple 149