December 4, 1918
Just finished two examinations which took all morning. I got 3.8 on the first -- which is 95 (the Navy grades on a scale where 4 = 100%) and 90 on the other, which is considered good. I had the best log book of the whole 1st Division -- took pains, and as there were lots of drawings I made them better than most. We have two more examinations today, and one I especially dread -- the one with the K-tube. Am enclosing the exam -- on the theory of sound -- first part. The part that has the markings of the theory of sound and the listening devices together they won't let go out. Of course, this is just what anyone learns in physics. I can get the mechanical tubes and devices very well, but am having a harder time with the electrical ones. The K tube is an electric device.
It's almost chow time so this can only be a note. Got two dandy pairs of socks from Aunt Hattie yesterday afternoon.
Mrs. Weldon, I don't want you to get the impression that your son is whiling away his idle hours all the time -- have only been to town one night in the last several. However, after today's examinations we won't have to study as hard -- it will be more practical work. Here's hoping I get by in that as well as I have in the theory.
By the time this reaches you I will have had a birthday. Will send you some pictures we took when we were out in the sound the other day if they are good.
Enclosed examination paper:
Theory of Sound
1. (a) A region of rarefaction is one in which the particles are further apart than average. (b) A region of condensation is one in which the particles are closer together than average. (c) Frequency or pitch is the number of vibrations per second of the source of the sound. Pitch that you hear in the stethoscopes may not be the cause as that given off by the submarine because of reflection of sound. Loudness is the sensation experienced due to the violence of disturbance in the medium. Wave length is the distance between two corresponding parts of a sound wave. Rhythm is the periodic variation of the body in vibration. Quality is that characteristic of a sound by which sounds of the same intensity can be distinguished from one another.
2. Phase is the part of a sound wave at a given point at a given instant of time. Phase difference is the distance between corresponding parts of a sound wave. Phase is one principle of the listening tubes -- for instance -- in the MB tube, when the sound wave strikes all the disc receivers at the same time, the sound will not be in phase, because they do not meet at the central intersection of the air passage. Consequently there is a phase difference and the sound is partially destroyed.
3. The ratio of the velocity of sound in water and in air is .23. Velocity of sound in water is 4530' per second at 32 degrees F and air is 1090' per second at 32 degrees F. By dividing the velocity in the water by the velocity in the air, we get .23 or the working ratio between the two.
4. Frequency of sound vibration determines to a large extent the speed of the submarine. A propeller that makes many revolutions when frequency is great naturally makes the pitch higher.
5. (a) Loudness is due directly to intensity. Loudness is merely the sensation in the ears -- while intensity denotes a violence of disturbance, whether heard or not. (b) By rhythm we can tell a submarine from any other boat or water noise -- in fact we can tell any two sounds from one another with the aid of quality. A submarine has an extremely even rhythm due to its motors.
6. As a submarine approaches from a great range, the pitch decreases and the intensity increases. When at distance only the rhythm may be detected and the submarine has a very high pitch, but as it comes closer the knocks and squeaks of the engines are plainly heard, and the pitch is noticeably lower and the intensity is much greater.
(Postmarked U.S. Receiving Barracks, Connecticut, December 4, 1918)