소리

소리. 드럼을 치면 드럼 헤드가 진동하고 진동이 파도의 형태로 공기를 통해 전달됩니다. 그들이 귀를 때릴 때, 이 파동은 소리의 감각을 생성합니다. 그러나 들을 수 없는 소리도 있습니다. 인간의 가청 범위보다 낮은 초저주파와 인간의 가청 범위를 넘는 초음파.

소리 연구에 사용되는 용어

음향학은 소리와 그것이 사람에게 미치는 영향에 대한 과학입니다.

결로 현상은 음파의 음파가 정상보다 밀도가 높은 영역입니다.

데시벨(dB)은 소리의 강도를 측정하는 데 사용되는 단위입니다. 0dB의 3,000Hz 톤은 정상적인 사람의 귀가 들을 수 있는 가장 부드러운 소리입니다.

소리의 주파수는 초당 주어진 지점을 통과하는 음파의 수입니다.

헤르츠는 음파의 주파수를 측정하는 데 사용되는 단위입니다. 1 헤르츠는 초당 1 주기(진동 또는 음파)와 같습니다.

소리의 강도는 파도의 힘을 측정한 것입니다.

라우드니스는 소리를 들었을 때 소리가 얼마나 강하게 들리는지를 나타냅니다.

소음은 불쾌하고 성가시며 주의를 산만하게 하는 소리입니다.

피치는 우리가 들을 때 소리의 높거나 낮은 정도입니다.

희박은 음파의 음파의 밀도가 정상보다 작은 영역입니다.

공명 주파수는 물체가 방해를 받으면 자연스럽게 진동하는 주파수입니다.

음파는 음파가 이동하는 물질입니다. 예를 들어 공기는 소리 매체입니다.

음색이라고도 하는 음질은 음악 소리의 특성입니다. 음질은 다른 악기에서 생성되는 동일한 주파수와 강도의 음표를 구별합니다.

초음파는 인간이 들을 수 있는 범위 이상의 주파수, 즉 20,000 헤르츠 이상의 소리입니다.

파장은 파동의 임의 지점과 다음 파동의 해당 지점 사이의 거리입니다.

기술적으로 소리는 변형된 후 원래 상태로 돌아가려는 경향이 있는 재료인 탄성 매체를 통해 이동하는 기계적 교란으로 정의됩니다. 매체는 공기일 필요는 없습니다. 금속, 나무, 돌, 유리, 물 및 기타 많은 물질은 소리를 전달하며, 그 중 많은 물질이 공기보다 좋습니다.

소리의 소스는 매우 많습니다. 친숙한 종류에는 사람의 성대의 진동, 진동하는 현(피아노, 바이올린), 진동하는 기주(트럼펫, 플루트), 진동하는 고체(누군가가 두드리는 문) 등이 있습니다. 그것들을 모두 나열하는 것은 불가능합니다. 왜냐하면 탄성 매체에 방해를 주는 모든 것(예를 들어, 폭발하는 폭죽이 주변 공기에 영향을 미치는 것과 같은)이 소리의 원천이기 때문입니다.

소리는 음높이(먼 천둥의 낮은 우르릉거림에서 모기의 고음 윙윙거림에 이르기까지)와 크기로 설명할 수 있습니다. 그러나 음높이와 음량은 주관적인 특성입니다. 그들은 부분적으로 청자의 청력에 의존합니다. 소리의 객관적이고 측정 가능한 품질에는 음높이 및 크기와 관련된 주파수 및 강도가 포함됩니다. 이러한 용어와 소리를 논의하는 데 사용되는 다른 용어는 음파와 그 거동을 조사하여 가장 잘 이해할 수 있습니다.

다양한 매체에서의 음속
중간	초당 피트의 속도	초당 미터의 속도
화씨 59도(섭씨 15도)의 공기	1,116	340
알류미늄	16,000	5,000
벽돌	11,980	3,650
화씨 77도(섭씨 25도)의 증류수	4,908	1,496
유리	14,900	4,540
화씨 77도(섭씨 25도)의 바닷물	5,023	1,531
강철	17,100	5,200
우드(메이플)	13,480	4,110

내용물

1. 음파
2. 소리의 속도
3. 음파의 거동
4. 소리 품질
5. 역사

공기는 모든 물질과 마찬가지로 분자로 구성되어 있습니다. 아주 작은 공기 영역에도 엄청난 수의 공기 분자가 포함되어 있습니다. 분자는 일정한 운동을 하며 무작위로 빠른 속도로 이동합니다. 그들은 끊임없이 서로 충돌하고 반발하며 공기와 접촉하는 물체에 부딪히고 반발합니다.

A vibrating object will produce sound waves in the air. For example, when the head of a drum is hit with a mallet, the drumhead vibrates and produces sound waves. The vibrating drumhead produces sound waves because it moves alternately outward and inward, pushing against, then moving away from, the air next to it. The air molecules that strike the drumhead while it is moving outward rebound from it with more than their normal energy and speed, having received a push from the drumhead. These faster-moving molecules move into the surrounding air. For a moment, therefore, the region next to the drumhead has a greater than normal concentration of air molecules—it becomes a region of compression. As the faster-moving molecules overtake the air molecules in the surrounding air, they collide with them and pass on their extra energy. The region of compression moves outward as the energy from the vibrating drumhead is transferred to groups of molecules farther and farther away.

Air molecules that strike the drumhead while it is moving inward rebound from it with less than their normal energy and speed. For a moment, therefore, the region next to the drumhead has fewer air molecules than normal—it becomes a region of rarefaction. Molecules colliding with these slower-moving molecules also rebound with less speed than normal, and the region of rarefaction travels outward.

The wave nature of sound becomes apparent when a graph is drawn to show the changes in the concentration of air molecules at some point as the alternating pulses of compression and rarefaction pass that point. The graph for a single pure tone, such as that produced by a tuning fork. The curve shows the changes in concentration. It begins, arbitrarily, at some time when the concentration is normal and a compression pulse is just arriving. The distance of each point on the curve from the horizontal axis indicates how much the concentration varies from normal.

Each compression and the following rarefaction makes up one cycle. (A cycle can also be measured from any point on the curve to the next corresponding point.) The frequency of a sound is measured in cycles per second, or hertz (abbreviated Hz). The amplitude is the greatest amount by which the concentration of air molecules varies from the normal.

The wavelength of a sound is the distance the disturbance travels during one cycle. It is related to the sound's speed and frequency by the formula speed/frequency = wavelength. This means that high-frequency sounds have short wavelengths and low-frequency sounds long wavelengths. The human ear can detect sounds with frequencies as low as 15 Hz and as high as 20,000 Hz. In still air at room temperature, sounds with these frequencies have wavelengths of 75 feet (23 m) and 0.68 inch (1.7 cm) respectively.

Intensity refers to the amount of energy transmitted by the disturbance. It is proportional to the square of the amplitude. Intensity is measured in watts per square centimeter or in decibels (db). The decibel scale is defined as follows: An intensity of 10-16 watts per square centimeter equals 0 db. (Written out in decimal form, 10-16 appears as 0.0000000000000001.) Each tenfold increase in watts per square centimeter means an increase of 10 db. Thus an intensity of 10-15 watts per square centimeter can also be expressed as 10 db and an intensity of 10-4 (or 0.0001) watts per square centimeter as 120 db.

The intensity of sound drops rapidly with increasing distance from the source. For a small sound source radiating energy uniformly in all directions, intensity varies inversely with the square of the distance from the source. That is, at a distance of two feet from the source the intensity is one-fourth as great as it is at a distance of one foot; at three feet it is only one-ninth as great as at one foot, etc.

Pitch

Pitch depends on the frequency; in general, a rise in frequency causes a sensation of rising pitch. The ability to distinguish between two sounds that are close in frequency, however, decreases in the upper and lower parts of the audible frequency range. There is also variation from person to person in the ability to distinguish between two sounds of very nearly the same frequency. Some trained musicians can detect differences in frequency as small as 1 or 2 Hz.

Because of the way in which the hearing mechanism functions, the perception of pitch is also affected by intensity. Thus when a tuning fork vibrating at 440 Hz (the frequency of A above middle C on the piano) is brought closer to the ear, a slightly lower tone, as though the fork were vibrating more slowly, is heard.

When the source of a sound is moving at relatively high speed, a stationary listener hears a sound higher in pitch when the source is moving toward him or her, and a sound lower in pitch when the source is moving away. This phenomenon, known as the Doppler effect, is due to the wave nature of sound.

Loudness

In general, an increase in intensity will cause a sensation of increased loudness. But loudness does not increase in direct proportion to intensity. A sound of 50 dB has ten times the intensity of a sound of 40 dB, but is only twice as loud. Loudness doubles with each increase of 10 dB in intensity.

Loudness is also affected by frequency, because the human ear is more sensitive to some frequencies than to others. The threshold of hearing—the lowest sound intensity that will produce the sensation of hearing for most people—is about 0 dB in the 2,000 to 5,000 Hz frequency range. For frequencies below and above this range, sounds must have greater intensity to be heard. Thus, for example, a sound of 100 Hz is barely audible at 30 dB; a sound of 10,000 Hz is barely audible at 20 dB. At 120 to 140 dB most people experience physical discomfort or actual pain, and this level of intensity is referred to as the threshold of pain.

The speed of sound depends on the elasticity and density of the medium through which it is traveling. In general, sound travels faster in liquids than in gases and faster in solids than in liquids. The greater the elasticity and the lower the density, the faster sound travels in a medium. The mathematical relationship is speed = (elasticity/density).

The effect of elasticity and density on the speed of sound can be seen by comparing the speed of sound in air, hydrogen, and iron. Air and hydrogen have nearly the same elastic properties, but the density of hydrogen is less than that of air. Sound thus travels faster (about 4 times as fast) in hydrogen than in air. Although the density of air is much less than that of iron, the elasticity of iron is very much greater than that of air. Sound thus travels faster (about 14 times as fast) in iron than in air.

The speed of sound in a material, particularly in a gas or liquid, varies with temperature because a change in temperature affects the material's density. In air, for example, the speed of sound increases with an increase in temperature. At 32 °F. (0 °C.), the speed of sound in air is 1,087 feet per second (331 m/s); at 68 °F. (20 °C.), it is 1,127 feet per second (343 m/s).

The terms subsonic and supersonic refer to the speed of an object, such as an airplane, in relation to the speed of sound in the surrounding air. A subsonic speed is below the speed of sound; a supersonic speed, above the speed of sound. An object traveling at supersonic speed produces shock waves rather than ordinary sound waves. A shock wave is a compression wave that, when produced in air, can usually be heard as a sonic boom.

The speeds of supersonic objects are often expressed in terms of Mach number—the ratio of the object's speed to the speed of sound in the surrounding air. Thus an object traveling at Mach 1 is traveling at the speed of sound; at Mach 2 it is traveling at twice the speed of sound.

Like light waves and other waves, sound waves are reflected, refracted, and diffracted, and exhibit interference.

Reflection

Sound is constantly being reflected off many different surfaces. Most of the time the reflected sound is not noticed, because two identical sounds that reach the human ear less than 1/15 of a second apart cannot be distinguished as separate sounds. When the reflected sound is heard separately, it is called an echo.

Sound is reflected from a surface at the same angle at which it strikes the surface. This fact makes it possible to focus sound by means of curved reflecting surfaces in the same way that curved mirrors can be used to focus light. It also accounts for the effects of so-called whispering galleries, rooms in which a word whispered at one point can be heard distinctly at some other point fairly far away, though it cannot be heard anywhere else in the room. (Statuary Hall of the United States Capitol is an example.) Reflection is also used to focus sound in a megaphone and when calling through cupped hands.

The reflection of sound can pose a serious problem in concert halls and auditoriums. In a poorly designed hall, a speaker's first word may reverberate (echo repeatedly) for several seconds, so that the listeners may hear all the words of a sentence echoing at the same time. Music can be similarly distorted. Such problems can usually be corrected by covering reflecting surfaces with sound-absorbing materials such as draperies or acoustical tile. Clothing also absorbs sound; for this reason reverberation is greater in an empty hall than in one filled with people. All these sound-absorbing materials are porous; sound waves entering the tiny air-filled spaces bounce around in them until their energy is spent. They are, in effect, trapped.

The reflection of sound is used by some animals, notably bats and toothed whales, for echolocation—locating, and in some cases identifying, objects through the sense of hearing rather than the sense of sight. Bats and toothed whales emit bursts of sound of frequencies far beyond the upper limits of human hearing, as high as 200,000 Hz in the case of whales. Sounds with short wavelengths are reflected even from very small objects. A bat can unerringly locate and catch even a mosquito in total darkness. Sonar is an artificial form of echolocation.

Refraction

When a wave passes from one material to another at an angle, it usually changes speed, causing the wave front to bend. The refraction of sound can be demonstrated in a physics laboratory by using a lens-shaped balloon filled with carbon dioxide to bring sound waves to a focus.

Diffraction

When sound waves pass around an obstacle or through an opening in an obstacle, the edge of the obstacle or the opening acts as a secondary sound source, sending out waves of the same frequency and wavelength (but of lower intensity) as the original source. The spreading out of sound waves from the secondary source is called diffraction. Because of this phenomenon, sound can be heard around corners despite the fact that sound waves generally travel in a straight line.

Interference

Whenever waves interact, interference occurs. For sound waves the phenomenon is perhaps best understood by thinking in terms of the compressions and rarefactions of the two waves as they arrive at some point. When the waves are in phase so that their compressions and rarefactions coincide, they reinforce each other (constructive interference). When they are out of phase, so that the compressions of one coincide with the rarefactions of the other, they tend to weaken or even cancel each other (destructive interference). The interaction between the two waves produces a resultant wave.

In auditoriums, destructive interference between sound from the stage and sound reflected from other parts of the hall can create dead spots in which both volume and clarity of sound are poor. Such interference can be reduced by use of sound-absorbing materials on reflecting surfaces. On the other hand, interference can improve an auditorium's acoustical qualities. This is done by arranging the reflecting surfaces in such a way that the level of sound is actually increased in the area in which the audience sits.

Interference between two waves of nearly but not quite equal frequencies produces a tone of alternately increasing and decreasing intensity, because the two waves continually fall in and out of phase. The pulsations heard are called beats. Piano tuners make use of this effect, adjusting the tone of a string against that of a standard tuning fork until beats can no longer be heard.

Sounds of a single pure frequency are produced only by tuning forks and electronic devices called oscillators; most sounds are a mixture of tones of different frequencies and amplitudes. The tones produced by musical instruments have one important characteristic in common: they are periodic, that is, the vibrations occur in repeating patterns. The oscilloscope trace of a trumpet's sound shows such a pattern. For most non-musical sounds, such as those of a bursting balloon or a person coughing, an oscilloscope trace would show a jagged, irregular pattern, indicating a jumble of frequencies and amplitudes.

A column of air, as that in a trumpet, and a piano string both have a fundamental frequency—the frequency at which they vibrate most readily when set in motion. For a vibrating column of air, that frequency is determined principally by the length of the column. (The trumpet's valves are used to change the effective length of the column.) For a vibrating string, the fundamental frequency depends on the string's length, its tension, and its mass per unit length.

In addition to its fundamental frequency, a string or vibrating column of air also produces overtones with frequencies that are whole-number multiples of the fundamental frequency. It is the number of overtones produced and their relative strength that gives a musical tone from a given source its distinctive quality, or timbre. The addition of further overtones would produce a complicated pattern, such as that of the oscilloscope trace of the trumpet's sound.

How the fundamental frequency of a vibrating string depends on the string's length, tension, and mass per unit length is described by three laws:

1. The fundamental frequency of a vibrating string is inversely proportional to its length.

Reducing the length of a vibrating string by one-half will double its frequency, raising the pitch by one octave, if the tension remains the same.

2. The fundamental frequency of a vibrating string is directly proportional to the square root of the tension.

Increasing the tension of a vibrating string raises the frequency; if the tension is made four times as great, the frequency is doubled, and the pitch is raised by one octave.

3. The fundamental frequency of a vibrating string is inversely proportional to the square root of the mass per unit length.

This means that of two strings of the same material and with the same length and tension, the thicker string has the lower fundamental frequency. If the mass per unit length of one string is four times that of the other, the thicker string has a fundamental frequency one-half that of the thinner string and produces a tone one octave lower.

One of the first discoveries regarding sound was made in the sixth century B.C. by the Greek mathematician and philosopher Pythagoras. He noted the relationship between the length of a vibrating string and the tone it produces—what is now known as the first law of strings. Pythagoras may also have understood that the sensation of sound is caused by vibrations. Not long after his time it was recognized that this sensation depends on vibrations traveling through the air and striking the eardrum.

About 1640 the French mathematician Marin Mersenne conducted the first experiments to determine the speed of sound in air. Mersenne is also credited with discovering the second and third laws of strings. In 1660 the British scientist Robert Boyle demonstrated that the transmission of sound required a medium—by showing that the ringing of a bell in a jar from which the air had been pumped could not be heard.

Ernst Chladni, a German physicist, made extensive analyses of sound-producing vibrations during the late 1700's and early 1800's. In 1801 the French mathematician Fourier discovered that such complex waves as those produced by a vibrating string with all its overtones consist of a series of simple periodic waves.

Much work on waves in general was done during the 19th century. Thomas Young, an English physicist, did research especially on diffraction and interference. Christian Johann Doppler of Austria formulated the mathematical relationship between the actual and perceived frequencies of waves when the source of the waves is moving relative to the observer.

An important contribution to the understanding of acoustics was made by Wallace Clement Sabine, a physicist at Harvard University, in the late 1890's. Sabine was asked to improve the acoustics of the main lecture hall in Harvard's Fogg Art Museum. He was first to measure reverberation time—which he found to be 5 1/2 seconds in the lecture hall. Experimenting first with seat cushions from a nearby theater, and later with other sound-absorbing materials and other methods, Sabine laid the foundation for architectural acoustics. He designed Boston Symphony Hall (opened 1900), the first building with scientifically formulated acoustics.

20세기 후반에 현대 사회, 특히 도시 지역에서 증가하는 소음 수준은 인간에 대한 소음의 생리적, 심리적 영향을 대부분 다루는 완전히 새로운 일련의 조사를 촉구했습니다.