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Characteristics of Sound

When a body vibrates, it communicates the energy of vibration to the surrounding medium, usually the air, where the wave travels and is detected as sound. The particles of the medium transmitting the wave vibrate with certain amplitude and the audibility of sound depends on the amplitude of vibration. Sound may also be communicated in liquids like water and solid like wire or rod. In general, sound may be defined as any vibrating disturbance in an elastic medium capable of producing an auditory sensation. In this sense, three things are necessary to have sound effect.

First, there must be a vibrating body (sources of sound); second, there must be a medium to transmit vibration (air); and third, there must be a receiver to detect sound (ear). Discussion 1. Characteristics of Sound Sound may be classified according to form and frequency of vibration as musical sound and as noise. Noise is sound of irregular vibration or that has no definite range of frequencies. It is annoying to the ear. Musical sound is sound that is regular in vibration. It may consist of one or more frequencies whose combination becomes pleasant to the hearing sensation.

However, the distinction between musical sound and noise may be psychological because what might be musical to someone may be noise to another or vice versa. There are three physiological characteristics of sound which depends on physical form. One characteristic is pitch, the “highness” or “lowness” of a note or tone. Pitch depends on the frequency. A note of high frequency has a high pitch, and one low of frequency has a low pitch. Another characteristic of sound is intensity or loudness. It depends on the amplitude of vibration or energy. Energy of vibration of a particle is proportional to the square of its amplitude.

A big amplitude will produce loud sound, and a small one will produce low sound. Intensity also depends on the distance of the observer from the source, provided the spreading of the waves is uniform in all directions. Intensity of sound varies inversely as the square of the distance from the source which can also be shown in the case of light. The third characteristic of sound is quality or timbre. It depends on the mode of vibration of the source, and therefore, on the wave form emitted by the vibrating body. A string that vibrates as the first harmonic or one segment will emit a simple tone.

If it is made to vibrate as a fundamental together with two or more overtones, the tone is of better quality because the wave form is complex. Quality or timbre depends on the predominance of overtones over the fundamental. It is this attribute of tone quality that distinguishes the sound of one instrument from another. From the sound an instrument emits, a listener can determine what instrument is playing. There are other factors that affect the quality of a note like density of the medium where sound is propagated, and the strength of the blow. 2. Sound intensity

The intensity of sound depends on the amplitude of vibration. Intensity is expressed as the amount of energy received per unit area per unit time. It can be expressed in ergs/cm2-sec or watts/cm2 or in equivalent units. The relative unit of intensity that is used in acoustics is the bel. The bel is based on a certain zero level intensity taken to be equal to 10-16 watt/cm2. This intensity is considered to be the smallest intensity for hearing and any intensity may be expressed in terms of this zero level, or in bels, by the equation, where (I) is any intensity or absolute intensity and (IO) taken as 10-16 watt/cm2.

The bell is a big unit so that a smaller unit is used, the decibel, which is 1/10 of a bel, which can be expressed as, The absolute intensity (I) is a function of pressure, amplitude of the wave, the density of the medium, and the velocity of the sound. Where (p) is the pressure amplitude of the wave in dynes/cm2, (? ) is the density of the propagating medium in g/cm3, and (u) the speed of sound in the medium in cm/sec, and (I) in ergs/cm2-sec. Table 1 shows examples of sound pressure and sound pressure level. 3. Echo and Reverberation In sound, if reflection takes place in a plane surface, sound image is produced.

If this sound image can be differentiated by the ear from the original sound, it is heard as echo. Echo is heard when the reflecting surface is far enough so that the time interval between the original sound and the reflected sound is about 1/15 sec or more. In an ordinary room, the effect of reflection is to increase the intensity of sound. In an open space, where nothing will reflect sound, a speaker cannot be heard very well, but if there is a reflecting surface, which will reflect sound at an interval of about 1/15 sec from the original sound, two separate sounds are distinguishable.

Echoes are produced by reflecting surfaces like cliffs, woods, clouds and even large buildings. Sometimes, succession of echoes can be heard, and this is called reverberation. Long reverberation time in large room and concert halls are objectionable. On the other hand, music is slightly affected if reverberation is entirely eliminated. Experiments in acoustics have produced ways and means to control echoes and reverberation for the best results. In a medium, an echo travels with the same speed as the original sound, or incident waves. 4.

Limits of Audibility Experiments in acoustics have shown that human ear cab hear a certain frequencies. Vibration of about 20 per second us the lowest to which the ear can respond to an intensity level of 10-16 watt/cm2. At 15,000 vps, the sound becomes a hiss. Higher than 20,000 vps sound becomes inaudible. The range from 20-20,000 is called audio frequency, and frequencies higher than 20,000 per sec are called radio frequencies. Longitudinal vibrations of about 20,000 per sec or more are called supersonics, which are also radio frequencies.

Supersonic waves, being longitudinal, are produced in matter like gas, liquid, and solid. They differ from radio waves which are produced in electromagnetic medium by oscillations of electrical charges. 5. Resonance In general, resonance is defined as the intensification of sound by the union of waves under proper condition. When two bodies slightly differ in frequencies, the difference in frequencies is called beats. 6. Doppler Effect Doppler Effect is concerned with the change in frequency of a note or sound heard by an observer or listener due to relative speed of the source and the observer.

The pitch of the surrounding whistle of a train appears higher when the train is approaching a stationary observer in the station, and appears lower when it is receding from the observer. The same phenomenon occurs when the sound and source is stationary and the observer is moving toward or away from the source. There are three cases in connection with Doppler Effect: ? When the source is moving and the observer at rest; ? Observer moving toward a stationary source; and ? When source and observer are both moving. Picture 1 further illustrates Doppler Effect.

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