1932 - Radio Talk on Echo Sounding by Dr. Herbert Grove Dorsey
The following talk was
presented by Dr. Herbert Grove Dorsey, chief of the Coast and Geodetic
Survey Radiosonic Laboratory, on September 16, 1932. Dr. Dorsey had worked
for Submarine Signal Corporation prior to 1925 when he chose to make a
career with the Coast and Geodetic Survey. Although not working in submarine
acoustics at the time of Reginald Fessenden’s submarine signaling
experiments in 1914, he was a colleague of Fessenden’s in the early
1920's. Dr. Dorsey presented this talk in a familiar folksy manner as his
audience was scientifically-minded people in the United States who listened
to the short science presentations given by Science Service over the Columbia
Broadcasting System (CBS).
Although Dr. Dorsey allowed himself a little whimsy in this talk, he fills a void in the history of the evolution of sounding instruments in the United States. Through his amplifier coupled with his invention of the Fathometer, an electro-mechanical sounding instrument, he was able to develop a means of measuring both very shoal water and very deep water with the same instrument. His fathometers increased the efficiency of hydrographic surveying for the Coast and Geodetic Survey and also proved to be of great value to the maritime shipping industry. His term, fathometer, was adapted by Submarine Signal Corporation and became a standard term for many different types of depth measuring instruments over the next 30 to 40 years. His instruments helped delineate much of the continental shelf and slope of the United States and its territories as well as much of the deep sea, in particular the northeast Pacific Ocean, the mid-Atlantic shelf and slope, and Gulf of Mexico.
Echoes Give Ocean Depths
Dr. Herbert Grove Dorsey
Principal Electrical Engineer,
United States Coast and geodetic Survey
A Radio Talk presented Friday, September 16, 1932, under the auspices of Science Service over the Columbia broadcasting System.
HELLO, EVERYBODY! Hello, Everybody! SOUNDS FUNNY, DOESN’T IT? Sounds Funny, Doesn’t It? ECHO! Echo! Yes, I am going to talk to you about echoes. Echoes, which are made by sounds reflected back to you, are heard on all sides and by everybody, you are so used to hearing them that they are seldom noticed unless the echo is heard at a considerable time interval after the sound is produced. In a room with bare walls we hear the echoes so quickly that the result is called reverberation and considerable money is spent preventing reverberation in a broadcasting studio.
Being so common, you would scarcely believe that echoes are useful and can save time and money as well as promote safety at sea.
Safety at sea was on everybody’s lips after the tragic disaster of the TITANIC, twenty years ago. How could the sea be made safer; how could we know when icebergs might be near enough to cause danger? Many minds were turned towards a solution of this problem and one mind, at least, though of the possibility of receiving a submarine echo from an iceberg by a sound produced in water. The experiment was actually tried by Prof. Reginald A. Fessenden and whether he received echoes from the iceberg seems to be omitted from the record. But he did succeed in getting echoes from the bottom of the ocean, which was perhaps more important, for the sea always has a bottom while you can’t find icebergs on every little summer’s outing at sea.
While you get echoes so easily in air you will be surprised to know that you can hear them more easily in water, for sound travels better in water than in air, travels faster and farther. It passes through water so much faster, about four and one-half times, that it is not easy to measure shallow ocean depths by the echo method because the time interval is so short. In water, sound travels about 4,800 feet per second and since a fathom is six feet the sound will go down and back through a depth of 400 fathoms in one second, or through a depth of one fathom in one four hundredth of a second, which is only two and one-half thousandths of a second. We think of a stop watch measuring to a fifth of a second as being very fast. It used to be the last word in timing horse races, but in this day and age, when we hear so much about split seconds, it is entirely too long even for races!
Measuring ocean depths is probably as old as the art of sailing ships, for Herodotus some four hundred years B.C. mentions putting grease on the bottom of the sinker to bring up a sample of the bottom. No practical improvements were made for 23 centuries until Lord Kelvin utilized the idea of pressures tubes, which was still further improved in accuracy by Commander G. T. Rude of the Coast and Geodetic Survey. While Fessenden’s method would work in deep water , it was cumbersome, requiring a skilled observer to use head phones and listen intently to distinguish the difference between echoes and water noise, and it was absolutely impossible to measure shallow depths with it. Ship captains were not interested in it and nothing was done commercially. Although several attempts were made to produce something better, Fessenden’s model lay practically dormant until 1922.At that time the Submarine Signal Company put two of its engineers on the problem, R.L. Williams and myself. Williams was a mechanical engineer unfamiliar with radio, while I had just completed experiments on the radio loud speaker, now so extensively used. Naturally, we attacked the problem along different lines, he holding to mechanical devices while I tried to tune everything and amplify the weak echoes. This desire to amplify was ridiculed because it was thought impracticable to attempt to use amplifying tubes on a ship unless under the constant supervision of the wireless operator. Of course amplification helped and proved to be the only way to success. On an experimental trip on the Ship CALAMARES, the captain asked me if I could measure shallow depths and said “When you can measure six to ten fathoms, you will be doing something!” It was not long after that until I had a visual method worked out in the following manner.
In one part of equipment, called the indicator, there is a small motor with a governor which, through a system of gears, rotates a black disc four times per second. Attached to the back side of the disc is a tiny neon tube, just a small edition of the same as is used in advertising signs, and when it is illuminated its red light shines through a slot in the disc but is lighted only now and then. In front of the disc is a sheet of glass on which is painted a circular scale marked in fathoms form zero to 100. Every time the neon tube passes the zero point of the scale, an electric current passes through a sounder bolted to the bottom of the ship and a sound is produced as a short whistle blast, two octaves above middle C, like this: (Illustrate by whistling the short blasts for two seconds), only these sounds pass into the water. No wire is lowered, nothing is dropped, no connection with the bottom is made. The sounds themselves do the work by being reflected from the bottom of the ocean as echoes.As the echoes return to the ship they are “heard” by a receiver of submarine sounds, or electric ear as it might be called, and are amplified using thermionic tubes, similar to those by which you are now hearing my voice, increases the loudness of the echoes so that the electrical energy will cause the tiny neon tube to make a single brilliant instantaneous red flash of light as the tube whirls around with the disc. This flash will shine through the glass, opposite some mark on the scale, six fathoms for example, if that happens to be the depth of water through which the ship is passing. Four times a second the red light flashes at six fathoms, thus measuring a time interval of only fifteen thousandths of a second, and you read the depth as easily as you read time on a clock.. Now, as the ship travels through deeper water, the red flashes will occur at later intervals, making the indications move along the scale to show increasing depths. In going from deep water to shallow, of course the red flashes will follow the scale backwards just as well as forwards. If the depth increases to more than a hundred fathoms a handle is turned shifting to a slower speed and another scale, so that, while the indications come less often, the depths can be measured to 3,000 fathoms or more, nearly 31/2 miles of water.
What to you suppose the fish think about all this whistling? Well, by watching them, in clear water, when the fathometer is first started, they appear frightened and rush away from the sound; but after ten or fifteen seconds they get used to it and swim around the ship as usual. Dog fish might try to bark at it–who knows?
Herodotus probably knew
about echoes as well as about grease on the bottom of the sinker, but
I would like to watch his reaction to a fathometer while sailing the
Citation: Dorsey, Herbert Grove, Principal Electrical Engineer, United States Coast and Geodetic Survey. 1932. Echoes Gives Ocean Depths: A Radio Talk presented Friday, September 16, 1932, under the auspices of Science Service over the Columbia Broadcasting System. Science Service, Columbia Broadcasting System, Washington, D.C. Unpublished Mimeograph. Held by NOAA Central Library.
Note on Transcription: Typed transcript prepared by NOAA Central Library staff April 2002.