Projection engineering (Sept 1929-Nov 1930)

Page 20 Projection Engineering, April, 1930 Roosevelt Field to speed the departure of planes and as a public safety factor. Roosevelt Field is studded todaj with outdoor type exponential horns. Pilots are called to the main offices by this far-reaching open air publicaddress system. Late plane arrivals are announced. Departures are announced. When mechanics or special persons are needed at the flying field's main offices, they are paged over this extensive field without the need of runners or messenger boys. At times, it clears the landing field so that arriving planes may have sufficient landing space cleared of spectators and attendants. Power amplifiers have now become a factor in public safety. In New York The city which is known as Greater New York actually comprises five boroughs named Manhattan, Bronx, Richmond, Brooklyn and Queens. In this area are about seventeen city-owned public parks. The largest and most popular of all the parks is located in the borough of Manhattan and is known as Central Park. Here the city gives evening concerts in the summer. Everyone cannot come to listen in Central Park but power amplifiers now make it possible to sit in any one of the other parks and listen to the music played in Central Park, emanating from powerful speakers and horns situated out of doors in each of these parks. This is a rather elaborate installation, but in principle it is nothing other than a public-address system because the original music is picked up by microphones and transmitted by power amplifiers over the fire alarm wires of New York to each park where other amplifiers are connected to the line and to the speakers. Coin Operated Systems The idea of coin-operated phonographs with power amplifiers has already become an accepted furnishing for roadside restaurants, small inns, A dynamic reproducer of powerful volume. Electrically operated phonograph pickup. etc. Baseball parks, race tracks, summer and winter indoor and outdoor swimming pools are now regularly equipped with music distributing systems which can be instantly changed over to public-address systems. The Allerton House in Chicago has just been equipped with a central radio receiving set which feeds into a power amplifier and every room of the Allerton House in Chicago supplies radio to its guests through power amplification. Modern hotels no longer send pages through the halls and lobbies seeking some particular man who is wanted on the telephone. The telephone girl simply speaks into a microphone and conveniently placed radio speakers in the hotel lobby, dining room halls, etc., announce the name of the person called on the telephone. We might add that these speakers are not the old cumbersome and awkward looking so-called radio "loudspeakers." They are the modern flush type wall recessed speaker, with a well-designed cloth covering the speaker opening to keep out dust and yet be of such design as to harmonize with the room's decorative scheme. Here again a simple switch makes this system distribute music from phonograph records or a specially important radio event that may be broadcast. As a safety factor in case of fire, it means but one operator and one switch. This is of vital importance in large hotels, schools, hospitals, etc. In hospitals it can be said that power amplification acts as a healer with music routed over many pairs of conveniently located headphones. It locates doctors and nurses when needed elsewhere, and as a safety factor, we know of no quicker carrier of news. The Engineering Many persons know that this all involves a great deal of specialized knowledge and engineering ability. Quite true, but the manufacturing of the components of these systems and not their installation requires engineering ability. As with radio, a good radio tube must always be employed. Extras should always be on hand, as a bad tube will of course make the entire system inoperative. The amplifier must be properly "engineered" to operate perfectly, not for an hour but for eight to twenty hours at a time if necessary. In this respect the writer wishes to point out that cafes and restaurants in Switzerland open for business early in the morning and keep their music playing in many instances until 11 :00 p. m. And this goes on day after day until Sunday, when it may even play longer. A poorly-designed amplifier will not stand this sort of usage. It will break down due to heating, defective transformers, or poorly insulated condenser blocks. A poorly-designed speaker cannot stand the heavy drive of the output of a well-designed amplifier unless it is constructed correctly. A pickup must be correct in terms of ohms impedance. There are low impedance, high and intermediate impedance pickups and each has its particular type of amplifier input impedance rating. A phonograph electric motor and its turntable seem simple enough, but they also must be correctly "engineered." If it is a brush type motor, the pickup will carry the sound of sputtering brushes to the amplifier and with amplification it is -easy to judge the clatter which will come forth from the speakers. A great many people will accept a vacuum cleaner with a so-called rating of 100/120 volts 50/60 cycles a-c. Not so in sound amplification. If your city is served with 125 volts 50 cycles a-c, order motors of that exact voltage and frequency. If you don't, well, the motor will either overheat and kill its life efficiency by 50 per cent or your turntables will revolve unevenly regardless of the position of the speed regulator. If the reader has ever listened to an ordinary hand, spring-wound phonograph revolving at a variable speedwell, he can imagine how a loudspeaker with power amplification will sound, with motors revolving at a variable speed, the better the amplifier the (Concluded on page 28)