tions. In lungs under normal tension the pulmonary septa act as a load, and tend to localize and delay the vibrations. The relaxed lung in which the septa are no longer under tension yields a tympanitic note of much higher pitch than the normal organ, and vibrates as a whole, not merely where it is percussed, as does the normal lung.

FIG. 56. The production of overtones. Opposite the drawings are reproduced (from actual photographs obtained by means of the improved Koenig manometric flame apparatus) the vibrations of the fundamental note and the overtones. American.)

(From the Scientific

Resonators. Sound waves are transmitted readily from solid to solid bodies, as from a tuning fork to the skull, but not so readily from vibrating bodies to the air-a violin string vibrating between the fingers produces little sound. To increase the volume of sound, therefore, we resort to the use of sound boxes or resonators, as when a tuning fork rests on a table. The resonators used for musical instruments readily assume all varieties of simple and compound vibrations which are im

parted to them, that is they vibrate in unison with the source of the sound (sympathetic vibration) and thus amplify the waves. Indeed, it is in part due to certain vibratory habits which they have acquired that old violins owe their mellow richness of tone.1

Resonators are also used to analyze sounds-to resolve a complex sound into its component elements to a degree which is not otherwise possible. Such resonators are hollow bodies, with a smooth lining, stiff walls and two openings. The most powerful types are spherical in shape, but many forms are used, as in case of the violin. For the purpose of analyzing the sounds met with in physical diagnosis, simple telescopic tubes 2 meters in length have proved sufficient. Resonators are characterized by the fact that when a sound is produced they cause the fundamental note to predominate, while the overtones remain weak and unharmonic.

The large end of the resonator is brought into proximity with the sound to be analyzed, the small end is connected with the ear by means of rubber tubing. By varying the length of the tube to the point which produces the loudest sound for a given note, the wave length of that note can be computed and its position in the musical scale established.

In the human body the thorax, a distended stomach, a pneumothorax, etc., may act as resonators. The more elastic the material of which the resonator is composed, the longer its vibration. Thus a bell sounds longer than a glass, and a glass longer than a wooden bowl. A resonator 15 to 30 cm. in length is sufficient to amplify the sound of pure bronchial breathing (little "D" to "E" on the musical scale), but for normal vesicular breathing a length of from 1 to 2 meters ("A" to "F") is required. The latter is, therefore, from two to three octaves lower in pitch than the former.

While inferior to the resonator in point of accuracy, very complex sounds can be resolved by the human ear into their component elements. The expert leader can volitionally single out the notes of any individual instrument of his orchestra, and it is the unconscious analysis of sound-the separation of the fundamental note from the overtones-which enables us to distinguish individual voices, and to recognize tympany, resonance, metallic sounds, etc., in percussion and auscultation. With percussion of equal force, the high notes are more acutely perceived by the human ear, than the low ones, which are apt. to be overshadowed by their overtones. It is evident, therefore, that the individual who is the possessor of a good musical ear will much more readily acquire efficiency in auscultation and percussion, and will derive much more information from the employment of these methods, than he who is more or less tone-deaf.

THE QUALITIES OF SOUND

The qualities of sound are as difficult to describe as are colors to the blind, hence we are forced to use similes and comparisons. Sounds are classified according to their component qualities, which are intensity, duration, quality and pitch.

Wooden resonators, especially those made of spruce (which is made up of very long, straight, regular fibers, and stretched taut like a string) are chosen for most musical instruments, since their vibrations result in much richer, mellower tones than do those of other wood or metal.

* MUELLER, F.: Zeitschr. f. Aertzliche Fortbildung, ix, 1912, No. 14.

Intensity. This depends upon the amplitude of the sound waves; not only on the force of the blow, but also on the number of air columns set in vibration. Hence the importance of percussion of equal force.

Duration. The more air in a vibrating column the longer the duration of the sound. The "fullness" and the "leerness" of Skoda, terms which are still occasionally employed in German literature, although compound perceptions, depend mainly upon the duration of the vibrations. Resonant and tympanitic notes are long; dull and flat notes are short.

Quality. By this term we mean that a sound either is or is not musical. If it has a musical quality, we say that the sound possesses timbre or klang, attributes which depend upon the number and quality of the overtones. It must, of course, be remembered that in physical diagnosis we shall find no pure musical notes, but only varying degrees of tone dulling. The difference between resonance and tympany, between a wooden bowl and a glass, is due to the fact that the last named in each instance possesses many and harmonious overtones.

Pitch. Pitch, as was first notably emphasized by Austin Flint, is from a medical standpoint probably the most important element in the analysis of sound. It depends on the rate of the vibrations--the more rapid the rate, the higher the pitch. The longer the air column set in motion, the slower the vibration and the lower the pitch. Long organ pipes or long instrumental strings of equal caliber give forth the lowest sounds. The adult chest yields a lower note than that of the infant. Increase in pitch, loss of resonance, and shortness of duration go hand in hand (pulmonary consolidation). Auenbrugger wrote: "Ubi sonus est altior, ibi est morbus." Our recognition of the degree of pulmonary consolidation depends largely upon the high-pitched note which such tissue yields when percussed.

All sounds possess the qualities of intensity (loudness) and duration, some in addition the qualities of pitch and tone (klang).

In medical parlance we speak of "tones" and apply the term to more or less distinct resonances, but it must be remembered that none of them are tones as defined by physics, nor in the musical sense. In the latter a tone is a sound of definite pitch which cannot be further resolved into simpler sounds. "No pure tone can have timbre (klang)." Percussion sounds are always more or less muffled, impure and dull. In a strict sense there is no sharp distinction between a noise and a tone. can be resolved by the ear into simpler tones.

Each

Extremes are easily recognized, but there are many gradations which cannot be definitely analyzed. Even the typical tympany of an air-distended abdomen is far from having the rhythmic relations between the fundamental note and the overtones which is met with in pure musical tones. Again tympanitic, and non-tympanitic notes often merge so gradually into each other that an absolute differentiation is impossible. In the case of a tympanitic sound, the fundamental note is readily appreciable-we can sing it, but with a non-tympanitic note this is not the case. Practically the boundary between the two lies at the point at which the ear fails to distinguish any one pitch, on account of the interference of the "overtones," and this in turn is largely a question of the individual ear.

For example, if we strike a key on the piano a tone results, but if we strike a number of adjacent keys, musical notes blend to form a noise. Certain of the modern "harmonics" are mere noises to the uneducated ear.1

1 GEIGEL: Deut. Arch. f. kl. Med., vol. lxxxviii, p. 598.

THE ORIGIN OF SOUNDS HEARD OVER THE CHEST

The act of speaking or breathing produces sounds in the upper respiratory tract-larynx, vocal cords, glottis, mouth, nares-which are conducted downward into the chest mainly by the air columns in the lumen of the bronchi. The moving column of air sets the pulmonary tissues into vibration, and vibrations of certain kinds produce audible sounds.

These sounds are conducted through the overlying tissues to the ear of the examiner, and the character of the sound we hear depends upon the character of the tissues in their effect upon the following acoustic factors: (1) diffusion, (2) absorption, (3) reflection, (4) resonance, and perhaps (5) interference.

The Diffusion of Sound.-By this is meant a loss of intensity due to conduction over a wide area-a dilution of energy. In spreading from a sphere of a given size to one double its size, the intensity on the surface of the second sphere, the vibratory energy per unit area, will be halved. The function of the stethoscope or of a megaphone is to diminish diffusion (lateral radiation). Diffusion appears to be the most important factor in reducing the intensity of sound heard over the chest (Montgomery).

The Absorption of Sound. By this term we refer to the annihilation of sound as such, and its conversion into forms of energy such as heat, of which the ear takes no cognizance. It plays but a small rôle if sounds travel such short distances as is the case in the chest. Its effects are separate and distinct from those of diffusion.

The Reflection of Sound.-Sound waves may be reflected backward toward the direction from which they come. This is especially apt to occur when vibrations pass through tissues of different densities, as for instance when air-borne vibrations pass from the lumen of a bronchus to its walls or from the alveoli to fluid or to the chest wall. The degree of loss of sound depends "upon the differences in density and elasticity of the two media, irrespective of whether the sound is passing from the heavier to the lighter, or from the lighter to the heavier medium, provided the sound passes from one medium to another at the normal incidence, and provided that plane surfaces come into contact or separate two media, and provided that the two media are homogeneous and infinite in extent" (Montgomery).

Resonance. By resonance we mean tone reinforcement, due to the fact that amplifying vibrations are set up in a second medium so that the two media vibrate in unison and a louder sound results.

Although playing a less important rôle in modifying the acoustic phenomena in the chest than do diffusion and reflection, yet resonance at times is an important factor. It is to be noted that whereas diffusion, absorption and reflection modify all sounds in like degree providing that intensity is constant, resonance is selective, and amplifies sounds of different pitch (vibratory rate) in very different degree. "Conditions favorable to the resonance of sounds of one pitch may actually diminish sounds of another pitch. In addition sounds from different sources may come together, and thus sounds will be superimposed upon one another, and a condition known as interference may result," causing blurring of sound. As an example of this we have the fact that vocal resonance may be diminished, and yet the whispered voice sounds present, over a pleural

effusion.