Si′ren.
An instrument for producing musical tones and for measuring the number of sound waves or vibrations per second which produce a note of given pitch.
The production of these tympanic shocks by puffs of air was first realized by
Dr. Robinson, and his device was the first and simplest form of the siren.
A stop-cock was so constructed that it opened and shut the passage of a pipe 720 times in a second.
Air from the wind-chest of an organ being allowed to pass along the pipe during the rotation of the cock, a musical sound was smoothly uttered.
It was much improved by Cagniard de la Tour in 1827, who gave it its present name, and it has since been improved by Opelt, Seebeck, and
Dove.
In its original form it consists of a cylindrical box, into which a blast of air is introduced through an aperture in the bottom, while the top is perforated by a series of equidistant apertures arranged in a circle.
A circular plate, similarly perforated, turns freely on its center, nearly in contact with the top of the box, alternately opening and closing the airpassages in the latter.
The apertures are so inclined as to most effectually impart a rotary motion to the disk; and when air under pressure is admitted to the box, its escape through these apertures produces musical tones rising in pitch according to the velocity of the disk's rotation.
The axis of the disk carries a screw-thread, operating a set of toothed wheels, by means of which the number of revolutions made in a given time is shown on a series of dials.
This may be thrown in and out of gear.
When it is desired to ascertain the number of impulses per second required to produce a note of given pitch, the wind pressure must be maintained at a uniform degree.
The recording apparatus is thrown into gear and allowed to run for a given time; when it is detached, the register indicates the number of revolutions of the disk, which, multiplied by the number of perforations, gives the total number of impulses; this being divided by the number of seconds during which the observation was made, shows the number of vibrations or sound-waves per second corresponding to the given pitch.
In the siren of Seebeck the rotation is produced by mechanism, and not by the blast.
The double siren of
Helmholtz consists of two sirens, one placed in inverted position over the other.
The upper one may be rotated on its axis, while the instrument is in operation, by means of a crank.
This instrument is adapted for the investigation of the interferences of unequal sound-waves, of combination tones, and many other acoustic phenomena.
 |
|
Siren. |
In its most elementary form the siren is simply a perforated rotating disk, against which a current of air is directed, producing sounds of higher or lower pitch according to the velocity of rotation.
The limits of audible sound are the rates of vibration of 16 and 38,000 in a second, which embrace more than 11 octaves; but the range of effective musical sounds is much less.
The sounds used in music are almost never lower than 32 vibrations in a second (the lowest C on the piano-forte), thus comprising seven octaves.
From the deep, piercing nature of the sound which the siren emits, it is well adapted for fog signals or alarms.
In this case two disks rotating with great velocity in opposite directions are employed.
They are driven by a steam-engine, which also forces a blast of steam through their apertures, when those in the two disks come in apposition.
The device is placed at the smaller extremity of a large trumpet, which intensifies the sound.
The rate of rotation of the disks is about 1,000 per minute, and the cost of the device is about $3,500.
The doctrine of the different sounds of vibrating strings of different lengths, and the communication of sounds to the ear by the vibration of the atmosphere, is ascribed to
Pythagoras, about 500 B. C.; mentioned by Aristotle, 300 B. C.; explained by Galileo, A. D. 1600; investigated by
Newton, 1700.
“ Discoursed with
Mr. Hooke about the nature of sounds, and he did make me understand the nature of musical sounds made
[
2191]
by strings, mighty prettily; and told me that having come to a certain number of vibrations proper to make any tone, he is able to tell how many strokes a fly makes with her wings, by the note that it answers to in musique.
This I suppose is a little too much refined; but his discourse in general of sound was mighty fine.” — Pepys'
Diary, 1668.
It has been ascertained by means of the siren that the wings of the mosquito move at the rate of 15,000 times a second.
An ingenious method of registering the vibrations of the wings of insects was adopted by
E. J. Marey, and is described in the “Comptes Rendus.”
The author of the article sought for a mode of exhibiting, in an unmistakable manner, each of the beats of the wing of an insect, and the graphic method answers very well for determining their frequency.
 |
|
Steam-siren. |
He grasped, with a fine pair of nippers, the hind part of the abdomen of an insect, and when it sought to fly, directed one of its wings in such a way that it rubbed by its point against the surface of a smoked cylinder, which revolved with a known velocity.
The wing, at each of these revolutions, carried away a little of the black of the smoke which covered the cylinder, and left a trace of its passage.
This experiment gives a diagram exhibiting the varied forms that are periodically reproduced with the same characteristics, and, consequently, correspond to one revolution of the wing.
By means of a chronographic diapason, the exact number of the revolutions of the wing which are effected in a second were precisely determined.
That which he used gave a graphic delineation of five hundred simple vibrations per second.
 |
|
Sirup-stand. |
A continual rubbing of the wing on the cylinder presents a resistance to this organ, which retards its frequency; so in order to have the nearest approach to the truth, those drawings were selected in which the contact of the wing with the cylinder was at a minimum, so that the diagrams were reduced to a series of points.
The following results were obtained: Common fly, 330; drone, 240; bee, 190; wasp, 110; hawk-moth, 72; dragon-fly, 28; cabbage-butterfly, 9.
Experiments with the
siren show that the extreme limits of the human voice in males vary from 384 to 1,266 vibrations per second, and in temales from 1,152 to 3,240.
The highest note in music is about the 14th C (five octaves above the middle C of the piano-forte, and this is due to 8,192 vibrations per second; but much higher tones can still be heard.
Savart has produced tones due to 48,000 vibrations per second.
