Acoustics is the study of the physical characteristics of sounds. Its deal with things like the frequency, amplitude and complexity of sound waves and how sound waves interact with various environments. It can also be refer casually and generally to the over-all quality of sound in a given place. Someone might say in a non-technical conversation: “I like to perform at Smith Hall; the acoustics are very brights.”
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From the everyday sounds of speech, the hum of appliances, to the sounds caused by wind and water, we are immersed in an ocean of sounds. Yet, what is sound, and how do we “hear” it? Why do two instruments playing the same note “sound” different? In this lab you will learn the basics of the answers to these questions. To answer the later question, we will analyze sound as an audio engineer would, through a technique called harmonic analysis. Harmonic analysis allows sound to be understood from a quantitative perspective. Also, we will come to an understanding of why the way a computer analyses sound is similar to how our ears analyse sound.
I will start this genre presentation by introducing the genre acoustic music. It isn’t really a genre, as music played with acoustic instruments can sound very different, but I chose to call the post this, as acoustic music have many similarities. If you like these songs, you should really check out Bedtime Tunes, which is a site only with songs like these. So without further ado, here are 11 songs with acoustic guitars, pianos, strings and beautiful voices: First here is Antony Hearty with his band Antony and the Johnsons. Antony Hegarty is a very special person, he is transgendrous, and his voice is absolutely amazing. Unfortunately I haven’t seen him live, but I’ve heard that almost all of the audience comes out from the concert crying
Or Acoustics (from Greek pronounced acoustics meaning “of or for hearing, ready to hear”) is the science that studies sound, in particular its production, transmission, and effects. Sound can often be considered as something pleasant; an example of this would be music. In that case a main application is room acoustics, since the purpose of room acoustical design and optimisation is to make a room sound as good as possible. But some noises can also be unpleasant and make people feel uncomfortable. In fact noise reduction is a major challenge, particularly within the transportation industry as people are becoming more and more demanding. Furthermore ultrasounds also have applications in detection, such as sonar systems or non-destructive material testing.
2. History of acoustic
If he first mentioned the “Acoustique Art” in his Advancement of Learning (1605), Francis Bacon (1561-1626) was drawing a distinction between the physical acoustics he expanded in the Sylva Sylva rum (1627) and the harmonics of the Pythagorean mathematical tradition. The Pythagorean tradition still survived in Bacon’s time in the works of such diverse people as Gioseffo Zarlino (1517-1590), René Descartes (1596-1650), and Johannes Kepler (1571-1630). In Bacon’s words: “The nature of sounds, in some sort, [hath been with some diligence inquired,] as far as concerneth music. But the nature of sounds in general hath been superficially observed. It is one of the subtlest pieces of nature”.
Bacon’s “Acoustique Art” was therefore concerned with the study of “immusical sounds” and with experiments in the “migration in sounds” so that the harnessing of sounds in buildings (architectural acoustics) by their “enclosure” in artificial channels inside the walls or in the environment (hydraulic acoustics). Aim of Baconian acoustics was to catalog, quantify, and shape human space by means of sound. This stemmed from the echometria, an early modern tradition of literature on echo, as studied by the mathematicians Giuseppe Biancani (1566-1624), Marin Mersenne (1588-1648), and Daniello Bartoli (1608-1685), in which the model of optics was applied in acoustics to the behaviour of sound. It was in a sense a historical antecedent to Isaac Newton’s (1642-1727) analogy between colours and musical tones in Upticks (1704). Athanasius Kircher’s (1601-1680) Phonurgia Nova of 1673 was the outcome of this tradition. Attacking British acoustics traditions, Kirsches argued that the “origin of the Acoustical Art” lay in his own earlier experiments with sounding tubes at the Collegio Romano in 1649 and sketched the ideology of a Christian baroque science of acoustics designed to dominate the world by exploiting the “boundless powers of sound”
17th-century empirical observations and mathematical explanations of the simultaneous vibrations of a string at different frequencies were important in the development of modern experimental acoustics. The earliest contribution in this branch of acoustics was made by Mersenne, who derived the mathematical law governing the physics of a vibrating string. Around 1673 Christian Huygens (1629-1695) estimated its absolute frequency, and in 1677 John Wallis (1616-1703) published a report of experiments on the overtones of a vibrating string. In 1692 Francis Roberts (1650-1718) followed with similar findings.
These achievements paved the way for the 18th-century acoustique of Joseph Sauveur (1653-1716) and for the work of Brook Taylor (1685-1731), Leonhard Euler (1707-1783), Jean Le Rond d ‘Alembert (1717-1783), Daniel Bernoulli (1700-1782), and Giordani Riccati (1709-1790), who all attempted to determine mathematically the fundamental tone and the overtones of a sonorous body. Modern experimental acoustics sought in nature, a physical law of the sounding body, the perfect harmony that in the Pythagorean tradition sprang from the mind of the “geometrizing God.” Experimental epistemology in acoustics also influenced the studies of the anatomy and physiology of hearing, especially the work of Joseph-Guichard Duverney (1648-1730) and Antonio Maria Valsalva (1666-1723), that in the 19th century gave rise to physiological and psychological acoustics.
3. Fundamental concepts of acoustics
The study of acoustics revolves around the generation, propagation and reception of mechanical waves and vibrations.
The steps shown in the above diagram can be found in any acoustical event or process. There are many kinds of cause, both natural and volitional. There are many kinds of transduction process that convert energy from some other form into acoustic energy, producing the acoustic wave. There is one fundamental equation that describes acoustic wave propagation, but the phenomena that emerge from it are varied and often complex. The wave carries energy throughout the propagating medium. Eventually this energy is transduced again into other forms, in ways that again may be natural and/or volitionally contrived. The final effect may be purely physical or it may reach far into the biological or volitional domains. The five basic steps are found equally well whether we are talking about an earthquake, a submarine using sonar to locate its foe, or a band playing in a rock concert.
The central stage in the acoustical process is wave propagation. This falls within the domain of physical acoustics. In fluids, sound propagates primarily as a pressure wave. In solids, mechanical waves can take many forms including longitudinal waves, transverse HYPERLINK “http://www.answers.com/topic/transverse-wave”waves and surface waves. Acoustics looks first at the pressure levels and frequencies in the sound wave. Transduction processes are also of special importance.
4. Application of Acoustics
The science of sound and hearing. This treats the sonic qualities of rooms and buildings, and the transmission of sound by the voice, musical instruments or electric means. Voice is caused by vibration, which is communicated by the sound source to the air as fluctuations in pressure and then to the listener’s ear-drum. The faster the vibration (or the greater its ‘frequency’) the higher the pitch. The greater the amplitude of the vibration, the louder the sound. Mostly musical sound consist not only of regular vibration at one particular frequency but also vibration at various multiples of that frequency. The frequency of middle C is 256 cycles per second (or Hertz, abbreviated Hz) but when one hears middle C there are components of the sound vibrating at 512 Hz, 768 Hz etc (see Harmonics). The presence and relative strength of these harmonics determine the quality of a sound. The difference in quality, for example. between a flute, an oboe and a clarinet playing the same note is that the flute’s tone is relatively ‘pure’ (i.e. has few and weak harmonics), the oboe is rich in higher harmonics and the clarinet has a preponderance of odd-numbered harmonics. Their different harmonic spectra are caused primarily by the way the sound vibration is actuated (by the blowing of air across an edge with the flute, by the oboe’s double reed and the clarinet’s single reed) and by the shape of the tube. Where the player’s lips are the vibrating agent, as with most brass instruments, the tube can be made to sound not its fundamental note but other harmonics by means of the player’s lip pressure.
The vibrating air column is only one of the standard ways of creating musical sound. The longer the column the lower the pitch; the players can raise the pitch by uncovering hole in the tubes. With that human voice, air is set in motion by means of the vocal cords, folds in the throat which convert the air stream from the lungs into sound; pitch is controlled by the size and shape of the cavities in the pharynx and mouth. For a string instrument, such as the violin, the guitar or the piano, the string is set in vibration by (respectively) bowing, plucking or striking; the tighter and thinner the string, the fasters it will vibrate. By pressing the string against the fingerboard and thus making the operative string-length shorter, the player can raise the pitch. With a percussion instrument, such as the drum or the xylophone, a membrane or a piece of wood is set in vibration by striking; sometimes the vibration is regular and gives a definite pitch but sometimes the pitch is indefinite.
In the recording of sound, the vibration patterns set up by the instrument or instruments to be recorded are encode by analogue (or, in recent recordings. digitally) in terms of electrical impulse. This information can then be stored, in mechanical or electrical form; this can then be decoded, amplified and conveyed to loudspeakers which transmit the same vibration pattern to the airs.
The study of the acoustics of buildings is immensely complicated because of the variety of ways in which sound is conveyed, reflected, diffused, absorbed etc. The design of buildings for performances has to take account of such matters as the smooth and even representation of sound at all pitches in all parts of the building, the balance of clarity and blend and the directions in which reflected sound may impinge upon the audiences. The use of particular material (especially wood and artificial acoustical substances) and the breaking-up of surfaces, to avoid certain types of reflection of sounds, play a part in the design of concert halls, which however remains an uncertain art in which experimentation and ‘tuning’ (by shifting surface, by adding resonators etc.) is often necessary. The term ‘acoustic’ is sometimes used, of a recording or an instrument, to mean ‘not electric’: an acoustic recording is one made before electric methods came into use, and an acoustic guitar is one not electrically amplified.
4.1 Theory of acoustic
The area of physics known as acoustics is devoted to the study of the production, transmission, and reception of sound. Thus, wherever sound is produced and transmitted, it will have an effect some whereas, even if there is no one present to hear it. The medium of sound transmissions is an all-important, key factor. Among the areas addressed within the realm of acoustics are the production of sounds by the human sounds and various instrument, as like the reception of sound waves by the human ear.
5. Working concept of acoustic
Sound waves are an example of a larger phenomenon known as wave motion, and wave motion is, in turn, a subset of harmonic motion-that is, repeated movement of a particle about a position of equilibrium, or balance. In the case of sound, the “particle” is not an item of matter, but of energy, and wave motion is a type of harmonic movement that carries energy from one place to another without actually moving any matter.
Particles in waves experience oscillation, harmonic motion in one or more dimensions. Oscillation itself involves little movement, though some particles do move short distances as they interact with other particles. Primarily, however, it involves only movement in place. The waves themselves, on the other hand, move across space, ending up in a position different from the one in which they started.
A transverse wave forms a regular up-and-down pattern in which the oscillation is perpendicular to the direction the wave is moving. This is a fairly easy type of wave to visualize: imagine a curve moving up and down along a straight line. Sound waves, on the other hand, are longitudinal waves, in which oscillation occurs in the same direction as the wave itself.
These oscillations are really just fluctuations in pressure. As a sound wave moves through a medium such as air, these changes in pressure cause the medium to experience alternations of density and rarefaction (a decrease in density). It , in turn, produces vibrations in the human ear or in any other object that receives the sound waves.
5.1 Properties of Sound Waves
5.1.1 Cycle and Period
The term cycle has a definition that varies slightly, depending on whether the type of motion being discussed is oscillation, the movement of transverse waves, or the motion of a longitudinal sound wave. In the latter case, a cycle is defined as a single complete vibration.
A period (represented by the symbol T) is the amount of time required to complete one full cycle. The period of a sound wave can be mathematically related to several other aspects of wave motion, including wave speed, frequency, and wavelength.
5.1.2 The Speed of Sound in Various Medium
People often refer to the “speed of sound” as though this were a fixed value like the speed of light, but, in fact, the speed of sound is a function of the medium through which it travels. What people ordinarily mean by the “speed of sound” is the speed of sound through air at a specific temperature. For sound travelling at sea level, the speed at 32°F (0°C) is 740 MPH (331 m/s), and at 68°F (20°C), it is 767 MPH (343 m/s).
In the essay on aerodynamics, the speed of sound for aircraft was given at 660 MPH (451 m/s). This is much less than the figures given above for the speed of sound through air at sea level, because obviously, aircraft are not flying at sea level, but well above it, and the air through which they pass is well below freezing temperature.
The speed of sound through a gas is proportional to the square root of the pressure divided by the density. According to Gay-Lussac’s law, pressure is directly related to temperature, meaning that the lower the pressure, the lower the temperature-and vice versa. At high altitudes, the temperature is low, and, therefore, so is the pressure; and, due to the relatively small gravitational pull that Earth exerts on the air at that height, the density is also low. Hence, the speed of sound is also low.
It follows that the higher the pressure of the material, and the greater the density, the faster sound travels through it: thus sound travels faster through a liquid than through a gas. This might seem a bit surprising: at first glance, it would seem that sound travels fastest through air, but only because we are just more accustomed to hearing sounds that travel through that medium. The speed of sound in water varies from about 3,244 MPH (1,450 m/s) to about 3,355 MPH (1500 m/s). Sound travels even faster through a solid-typically about 11,185 MPH (5,000 m/s)-than it does through a liquid.
Frequency (abbreviated f) is the number of waves passing through a given point during the interval of one second. It is measured in Hertz (Hz), named after nineteenth-century German physicist Heinrich Rudolf Hertz (1857-1894) and a Hertz is equal to one cycle of oscillation per second. Higher frequencies are expressed in terms of kilohertz (kHz; 103 or 1,000 cycles per second) or megahertz(MHz; 106 or 1 million cycles per second.)
The human ear is capable of hearing sounds from 20 to approximately 20,000 Hz-a relatively small range for a mammal, considering that bats, whales, and dolphins can hear sounds at a frequency up to 150 kHz. Human speech is in the range of about 1 kHz, and the 88 keys on a piano vary in frequency from 27 Hz to 4,186 Hz. Each note has its own frequency, with middle C (the “white key” in the very middle of a piano keyboard) at 264 Hz. The quality of harmony or dissonance when two notes are played together is a function of the relationship between the frequencies of the two.
Frequencies below the range of human audibility are called infrasound, and those above it are referred to as ultrasound. There are a number of practical applications for ultrasonic technology in medicine, navigation, and other fields.
Wavelength (represented by the symbol Î», the Greek letter lambda) is the distance between a crest and the adjacent crest, or a trough and an adjacent trough, of a wave. The higher the frequency, the shorter the wavelength, and vice versa. Thus, a frequency of 20 Hz, at the bottom end of human audibility, has a very large wavelength: 56 ft. (17 m). The top end frequency of 20,000 Hz is only 0.67 inches (17 mm).
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There is a special type of high-frequency sound wave beyond ultrasound: hyper sound, which has frequencies above 107 MHz, or 10 trillion Hz. It is almost impossible for hyper sound waves to travel through all but the densest media, because their wavelengths are so short. In order to be transmitted properly, hyper sound requires an extremely tight molecular structure; otherwise, the wave would get lost between molecules.
Wavelengths of visible light, part of the electromagnetic spectrum, have a frequency much higher even than hyper sounds waves: about 109 MHz, 100 times greater than for hyper sound. This, in turn, means that these wavelengths are incredibly small, and this is why light waves can easily be blocked out by using one’s hand or a curtain.
The same does not hold for sound waves, because the wavelengths of sounds in the range of human audibility are comparable to the size of ordinary objects. To block out a sound wave, one needs something of much greater dimensions-width, height, and depth-than a mere cloth curtain. A thick concrete wall, for instance, may be enough to block out the waves. Better still would be the use of materials that absorb sound, such as cork, or even the use of machines that produce sound waves which destructively interfere with the offending sounds.
5.1.5 Amplitude and Intensity
Amplitude is critical to the understanding of sound, though it is mathematically independent from the parameters so far discussed. Defined as the maximum displacement of a vibrating material, amplitude is the “size” of a wave. The greater the amplitude, the greater the energy the wave contains: amplitude indicates intensity, commonly known as “volume,” which is the rate at which a wave moves energy per unit of a cross-sectional area.
Intensity can be measured in watts per square meter, or W/m2. A sound wave of minimum intensity for human audibility would have a value of 10âˆ’12, or 0.000000000001, W/m2. As a basis of comparison, a person speaking in an ordinary tone of voice generates about 10âˆ’4, or 0.0001, watts. On the other hand, a sound with an intensity of 1 W/m2 would be powerful enough to damage a person’s ears.
5.2 Real-Life Applications
5.2.1 Decibel Levels
For measuring the intensity of a sound as experienced by the human ear, we use a unit other than the watt per square meter, because ears do not respond to sounds in a linear, or straight-line, progression. If the intensity of a sound is doubled, a person perceives a greater intensity, but nothing approaching twice that of the original sound. Instead, a different system-known in mathematics as a logarithmic scale-is applied.
In measuring the effect of sound intensity on the human ear, a unit called the decibel (abbreviated dB) is used. A sound of minimal audibility (10âˆ’12 W/m2) is assigned the value of 0 dB, and 10 dB is 10 times as great-10âˆ’11 W/m2. But 20 dB is not 20 times as intense as 0 dB; it is 100 times as intense, or 10âˆ’10 W/m2. Every increase of 10 dB thus indicates a tenfold increase in intensity. Therefore, 120 dB, the maximum decibel level that a human ear can endure without experiencing damage, is not 120 times as great as the minimal level for audibility, but 1012 (1 trillion) times as great-equal to 1 W/m2, referred to above as the highest safe intensity level.
Of course, sounds can be much louder than 120 dB: a rock band, for instance, can generate sounds of 125 dB, which is 5 times the maximum safe decibel level. A gunshot, firecracker, or a jet-if one is exposed to these sounds at a sufficiently close proximity-can be as high as 140 dB, or 20 times the maximum safe level. Nor is 120 dB safe for prolonged periods: hearing experts indicate that regular and repeated exposure to even 85 dB (5 less than a lawn mower) can cause permanent damage to one’s hearing.
5.3 Production of Sound Waves
5.3.1 Musical Instruments
Sound waves are vibrations; thus, in order to produce sound, vibrations must be produced. For a stringed instrument, such as a guitar, harp, or piano, the strings must be set into vibration, either by the musician’s fingers or the mechanism that connects piano keys to the strings inside the case of the piano.
In other woodwind instruments and horns, the musician causes vibrations by blowing into the mouthpiece. The exact process by which the vibrations emerge as sound differs between woodwind instruments, such as a clarinet or saxophone on the one hand, and brass instruments, such as a trumpet or trombone on the other. Then there is a drum or other percussion instrument, which produces vibrations, if not musical notes.
5.3.2 Electronic Amplification
Sound is a form of energy: thus, when an automobile or other machine produces sound incidental to its operation, this actually represents energy that is lost. Energy itself is conserved, but not all of the energy put into the machine can ever be realized as useful energy; thus, the automobile loses some energy in the form of sound and heat.
The fact that sound is energy, however, also means that it can be converted to other forms of energy, and this is precisely what a microphone does: it receives sound waves and converts them to electrical energy. These electrical signals are transmitted to an amplifier, and next to a loudspeaker, which turns electrical energy back into sound energy-only now, the intensity of the sound is much greater.
Inside a loudspeaker is a diaphragm, a thin, flexible disk that vibrates with the intensity of the sound it produces. When it pushes outward, the diaphragm forces nearby air molecules closer together, creating a high-pressure region around the loudspeaker. (Remember, as stated earlier, that sound is a matter of fluctuations in pressure.) The diaphragm is then pushed backward in response, freeing up an area of space for the air molecules. These, then, rush toward the diaphragm, creating a low-pressure region behind the high-pressure one. The loudspeaker thus sends out alternating waves of high and low pressure, vibrations on the same frequency of the original sound.
5.3.3 The Human Voice
As impressive as the electronic means of sound production are (and of course the description just given is highly simplified), this technology pales in comparison to the greatest of all sound-producing mechanisms: the human voice. Speech itself is a highly complex physical process, much too involved to be discussed in any depth here. For our present purpose, it is important only to recognize that speech is essentially a matter of producing vibrations on the vocal cords, and then transmitting those vibrations.
Before a person speaks, the brain sends signals to the vocal cords, causing them to tighten. As speech begins, air is forced across the vocal cords, and this produces vibrations. The action of the vocal cords in producing these vibrations is, like everything about the miracle of speech, exceedingly involved: at any given moment as a person is talking, parts of the vocal cords are opened, and parts are closed.
The sound of a person’s voice is affected by a number of factors: the size and shape of the sinuses and other cavities in the head, the shape of the mouth, and the placement of the teeth and tongue. These factors influence the production of specific frequencies of sound, and result in differing vocal qualities. Again, the mechanisms of speech are highly complicated, involving action of the diaphragm (a partition of muscle and tissue between the chest and abdominal cavities), larynx, pharynx, glottis, hard and soft palates, and so on. But, it all begins with the production of vibrations.
6. Propagation: Does It Make a Sound
As stated in the introduction, acoustics is concerned with the production, transmission (sometimes called propagation), and reception of sound. Transmission has already been examined in terms of the speed at which sound travels through various media. One aspect of sound transmission needs to be reiterated, however: for sound to be propagated, there must be a medium.
There is an age-old “philosophical” question that goes something like this: If a tree falls in the woods and there is no one to hear it, does it make a sound? In fact, the question is not a matter of philosophy at all, but of physics, and the answer is, of course, “yes.” As the tree falls, it releases energy in a number of forms, and part of this energy is manifested as sound waves.
Consider, on the other hand, this rephrased version of the question: “If a tree falls in a vacuum-an area completely devoid of matter, including air-does it make a sound?” The answer is now a qualified “no”: certainly, there is a release of energy, as before, but the sound waves cannot be transmitted. Without air or any other matter to carry the waves, there is literally no sound.
Hence, there is a great deal of truth to the tagline associated with the 1979 science-fiction film Alien : “In space, no one can hear you scream.” Inside an astronaut’s suit, there is pressure and an oxygen supply; without either, the astronaut would perish quickly. The pressure and air inside the suit also allow the astronaut to hear sounds within the suit, including communications via microphone from other astronauts. But, if there were an explosion in the vacuum of deep space outside the spacecraft, no one inside would be able to hear it.
7. Reception of Sound
Earlier the structure of electronic amplification was described in very simple terms. Some of the same processes-specifically, the conversion of sound to electrical energy-are used in the recording of sound. In sound recording, when a sound wave is emitted, it causes vibrations in a diaphragm attached to an electrical condenser. This causes variations in the electrical current passed on by the condenser.
These electrical pulses are processed and ultimately passed on to an electromagnetic “recording head.” The magnetic field of the recording head extends over the section of tape being recorded: what began as loud sounds now produce strong magnetic fields, and soft sounds produce weak fields. Yet, just as electronic means of sound production and transmission are still not as impressive as the mechanisms of the human voice, so electronic sound reception and recording technology is a less magnificent device than the human ear.
8. How the Ear Hears
As almost everyone has noticed, a change in altitude (and, hence, of atmospheric pressure) leads to a strange “popping” sensation in the ears. Usually, this condition can be overcome by swallowing, or even better, by yawning. This opens the Eustachian tube, a passageway that maintains atmospheric pressure in the ear. Useful as it is, the Eustachian tube is just one of the human ear’s many parts.
The “funny” shape of the ear helps it to capture and amplify sound waves, which pass-through the ear canal and cause the eardrum to vibrate. Though humans can hear sounds over a much wider range, the optimal range of audibility is from 3,000 to 4,000 Hz. This is because the structure of the ear canal is such that sounds in this frequency produce magnified pressure fluctuations. Thanks to this, as well as other specific properties, the ear acts as an amplifier of sounds. Beyond the eardrum is the middle ear, an intricate sound-reception device containing some of the smallest bones in the human body-bones commonly known, because of their shapes, as the hammer, anvil, and stirrup. Vibrations pass from the hammer to the anvil to the stirrup, through the membrane that covers the oval window, and into the inner ear.
Filled with liquid, the inner ear contains the semi-circular canals responsible for providing a sense of balance or orientation: without these, a person literally “would not know which way is up.” Also, in the inner ear is the cochlea, an organ shaped like a snail. Waves of pressure from the fluids of the inner ear are passed through the cochlea to the auditory nerve, which then transmits these signals to the brain.
The basilar membrane of the cochlea is a particularly wondrous instrument, responsible in large part for the ability to discriminate between sounds of different frequencies and intensities. The surface of the membrane is covered with thousands of fibres, which are highly sensitive to disturbances, and it transmits information concerning these disturbances to the auditory nerve. The brain, in turn, forms a relation between the position of the nerve ending and the frequency of the sound. It also equates the degree of disturbance in the basilar membrane with the intensity of the sound: the greater the disturbance, the louder the sounds.
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