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1942 - VOICE TEST FOR TRANSFORMERS - engineer demonstrated the use of a radio microphone and a sound meter to check the hum of these electric voltage changers


E&MP 40.018

Electric Transformers

April 22, 1942

Microphone and Sound Meter Listen To Hum Of Voltage Changers to Make Sure They Will Be Good Neighbors --- SHARON, Pa., April 22 --

In a new sound-proof laboratory, so quiet you can actually hear a pin drop, electric transformers are given a "radio voice test" to make certain they will be good neighbors, it was disclosed today by Hans Fahnoe, consulting engineer at the Sharon Transformer Division of the Westinghouse Electric & Manufacturing Company.

The engineer demonstrated the use of a radio microphone and a sound meter to check the hum of these electric voltage changers. He said the tests are made to insure that the various types of transformers will not produce louder sounds than the normal noises of the locations in which they are to be installed.

"The average noise level of a residential district at night is between 25 and 30 decibels," explained Mr. Fahnoe.

"Therefore, a transformer in a substation near a residential area must be selected so that its sound level does not appreciably increase the sound level of the district if it is to be a good neighbor.

In a business district a transformer may be louder, since the noise of a busy city street is about 90 decibels."

Electric Transformers Receive'Voice Test' In Sound-Proof Room

Human Blood Also Makes Sound A decibel was described as a measurement of sound equal to the smallest difference of sound a normal human ear can distinguish.

Average conversation measures about 60 decibels. The purring of the family cat rates about 25 decibels.

Blood coursing through the human body makes a measurable sound -- about 10 decibels. At the other end of the noise scale, an average typewriter produces 70 decibels of noise; a printing press 80; a subway train 100; and a punch press 110 decibels. Above 120 decibels, sound becomes painful and can be felt as well as heard, the engineer explained.

"However, decibel values do not indicate, except to the trained engineer, the true differences between sound levels," Mr. Fahnoe added. "An increase of six decibels represents approximately doubled noise level.

For example, the 100 decibel voice of the subway train is not just four times as loud as the 25 decibel purring of the cat; it is about 10,000 times as loud.

And 106 decibels would be the equivalent of two subway trains."In the Westinghouse sound-proof laboratory, noise has been reduced to only 20 decibels, which Mr. Fahnoe describes as comparable to the quietness of a lonely woodland on a still night with only a few leaves rustling.

Three sound-proof doors shield the laboratory against noise from a busy factory building only five feet away.

The laboratory is a building within a building. Walls of the inside building are composed of layers of sound-absorbing materials, including mineral wool and wood. The outside building, totally enclosing the inner one, has eight-inch brick walls.

Meter Hears Pin Drop

A transformer to be tested is wheeled into the laboratory on a specially constructed car and connected to a power line, as it would be in actual service. A microphone, held just a foot from the humming transformer, is connected to an electric sound meter that is so sensitive it records the sound of a large pin dropping.

Electric Transformers Receive 'Voice Test' In Sound-Proof Room

"This apparatus is used to test all new designs of transformers and to keep a constant check on our established designs as they come off the production line," said Mr. Fahnoe.

The engineer explained that transformer hum comes from the rapid expansion and contraction of the steel core as it is magnetized 120 times a second by alternating current.

These vibrations are transmitted through the transformer's insulating oil and tank wall, escaping into the air as sound waves.

Transformer Hum Monotone "The hum of transformers is musical," the engineer added. "But it is a monotone, always 120 cycles a second, which corresponds with the second B below middle-C on a piano. "To eliminate transformer hum, it would be necessary to develop a new type of magnetic steel which would not expand and contract when magnetized.

So far, metallurgists have not been able to make such a steel. "However, it is possible to keep transformer hum at a minimum by careful design so that no part of the apparatus is the right size or shape to vibrate in resonance with the steel core.

Such resonance intensifies the hum, much the same as a loud speaker amplifies the sounds picked up by a phonograph needle."

The transformers tested at the Westinghouse laboratory are the voltage changers which produce usable low-voltage power for homes and factories from the high voltage electricity which is most economically transmitted over long distances between powerhouse and user

Electric Transformers Receive 'Voice Test' In Sound-Proof Room

Either or both of the following photographs, available to illustrate this release, will be sent upon request to Central News Bureau, Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa.


In this sound-proof laboratory at the Westinghouse Sharon, Pa., Works, engineers give an electric transformer a "voice test" to make certain it will not hum loud enough to disturb residents near the power substation where it is to be installed.

The man at left is holding a radio microphone which is connected to the sound meter at the lower right.

Standing at right is Hans Fahnoe, Westinghouse consulting engineer who directs the transformer sound tests

276981 - HOUSE WITHIN A HOUSE is this sound-proof laboratory at the Westinghouse Transformer Division in Sharon, Pa., where engineers measure the humming sound of electric transformers to make sure they are not too loud.

In this picture, a technician is connecting power cables to a transformer in preparation for a test with "listening" instruments which are sensitive{sic} enough to measure the sound of a large pin dropping.

Three sound-proof doors help make the laboratory as quiet as a lonely woodland. The laboratory itself has a double wall and roof of brick, wood and mineral wool and other sound-absorbing materials.

Photo ##65175##

Original Caption by Science Service
© Westinghouse Electric & Manufacturing Company

National Museum of American History


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