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From Wikipedia, the free encyclopedia

A drum produces sound via a vibrating membrane.
A drum produces sound via a vibrating membrane.

In physics, sound is a vibration that typically propagates as an audible wave of pressure, through a transmission medium such as a gas, liquid or solid.

In human physiology and psychology, sound is the reception of such waves and their perception by the brain.[1] Humans can only hear sound waves as distinct pitches when the frequency lies between about 20 Hz and 20 kHz. Sound waves above 20 kHz are known as ultrasound and is not perceptible by humans. Sound waves below 20 Hz are known as infrasound. Different animal species have varying hearing ranges.

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  • ✪ Sound: Crash Course Physics #18
  • ✪ Sound Waves, Intensity level, Decibels, Beat Frequency, Doppler Effect, Open Organ Pipe - Physics
  • ✪ Sound Intensity Level in Decibels & Distance - Physics Problems
  • ✪ 3:00 PM - RRB ALP CBT-2 2018 | Physics By Neeraj Sir | Sound
  • ✪ Lesson 32 - Sound Waves - Sources of Sound - Demonstrations in Physics

Transcription

When you think about it, you probably receive hundreds -- even thousands -- of cues about what’s going on in your environment every day, strictly from sound. In addition to things like speech and music, there are other bits of auditory information that shape your day: an ambulance passing by, a baby crying in the next room, and of course [cell-phone style text ding goes off] -- -- sorry. Just got a text. But there’s a lot that we can learn, not just from what these cues MEAN, but from how Sound itself works. Studying sound waves has helped doctors learn more about our ears, and has allowed engineers to design things like microphones and speakers. Biologists have even used the science of sound to figure out how animals like elephants can communicate over long distances -- when we can’t even hear them doing it. It all comes down to the fact that SOUND is a wave, which travels through a medium like air or water. And knowing that sound is a wave is important, because it means that we can use the physics of waves to describe the qualities of sound. [Intro Music Plays] When you think of a wave, you probably think of the kind you see at the ocean, or the ones you made when you jumped on that trampoline last time. Those waves produce ripples that run perpendicular to the direction, that the wave is traveling in. But sound is the other kind of wave: it’s a longitudinal wave, meaning that the wave’s back-and-forth motion happens in the same direction in which the wave travels. Say you get a text message on your phone, and it makes a nice, bright little ‘ding!’ sound. What actually happened? Like, on a physical level? Your phone’s speaker contains a diaphragm -- a piece of stiff material, usually in the shape of a cone. When you got the message, the electronics inside the speaker made the diaphragm move back and forth, which vibrated the air around your phone. That made the atoms and molecules in the air move back and forth. Then, those moving particles vibrated the air around them -- and as the process continued, the sound wave spread outward. [ding!] Sorry! I’m just gonna turn this off now. Anyway, physicists sometimes describe sound waves in terms of the movement of these particles in the air -- in what’s known as a displacement wave. But by moving particles in the air, sound waves also do something else: They cause the air to compress and expand -- which is why sound waves are sometimes described as ‘pressure waves’. As the wave spreads through the air, the particles end up bunching together in some places, and ‘spreading out’ in others. Together, all that bunching and spreading-out causes areas of high pressure and low pressure to form and move through the air. It’s useful to describe sound waves as pressure waves, because we can build devices that detect those changes in pressure. That’s how some microphones work, for example: They use a diaphragm stretched over a sealed compartment, and as sound waves pass by, they create areas of lower or higher pressure in the compartment. The differences in pressure cause the diaphragm to move back and forth, which electronics then translate into audio data And your eardrums basically work the same way! As pressure waves pass through, they make your eardrum vibrate. Your brain then interprets those vibrations as sound. But not all sounds are the same. Even before we knew much about physics, humans were describing sound in terms of certain qualities: mainly, by things like ‘loudness’ and ‘pitch’. Our understanding of those qualities helped shape the development of music -- which we’ll talk more about next time. But there’s also a more physics-y side to those qualities of music. Pitch can be high or low, and it corresponds to the ‘frequency’ of the wave. So, air that’s vibrating back and forth more times per second will have a higher pitch, and air that’s vibrating fewer times per second will have a lower pitch. Humans hear sounds best when the vibrations are somewhere between 20 per second on the low end and 20,000 per second on the high end. As we get older and lose more of the cells that help us detect sound, we start to lose the ability to hear higher-pitched sounds. Some building security companies will take advantage of this, using devices that emit a high-pitched noise that most people over the age of 25 can’t hear. The idea is that since kids and teens can hear it, and it’s super annoying to them, they won’t hang out near the building. But some sounds are too high or low for any humans to hear. Sounds that are too high in pitch are called ultrasonic, and sounds that are too low are called infrasonic. Dog whistles, for example, use an ultrasonic pitch that’s too high for us, but is perfectly audible to dogs. Elephants, on the other hand, use INFRAsonic sound to communicate with each other across long distances. They can hear these calls from several kilometers away, but we can’t hear them at all. Another aspect that shapes sound is its loudness -- when you increase the intensity of a sound, you increase its loudness, and vice versa. We’ve talked about the intensity of a wave before: it’s the wave’s power over its area, measured in Watts per square meter. We’ve also said that the intensity of a wave is proportional to the wave’s amplitude, squared. And the farther you are from the source of a wave, the lower its intensity -- by the square of the distance between you and the source. And just as there’s a range of pitches that humans can hear, there’s also a range of sound wave intensity that humans can comfortably hear. Generally, people can safely hear sounds from about 1 picowatt per square meter, up to 1 Watt per square meter -- which is about as loud as a rock concert, if you’re near the speakers. The sound waves coming from a jet plane that’s 30 meters away, for example, probably has an intensity of around 100 Watts per square meter. Now, I don’t know if you’ve ever been that close to a roaring jet plane. But there’s a reason people who work on the tarmac at airports use those heavy-duty headphones. Below 1 picowatt per square meter, sounds are just too soft for us to detect them. And although we will HEAR sounds above a Watt per square meter, they tend to hurt our ears. But here’s a weird thing about loudness and intensity: it’s not a linear relationship. Generally, a sound wave needs to have ten times the intensity to sound twice as loud to us. This relationship holds true as long as the sound is toward the middle of the range of frequencies we can hear. So, instead of directly measuring the loudness of sounds by their intensity, we use units called ‘decibels’ -- which are based on bels. Bels convert a sound wave’s intensity to a ‘logarithmic scale’, where every notch on the scale is ten times higher than the previous one. The scale starts off with an intensity of 1 picowatt per square meter, corresponding to 0 bels. So a sound that’s 1 bel is ten times as intense as a sound that’s 0 bels. And a sound that’s 2 bels is 10 times as intense as a sound that’s 1 bel -- -- but 100 times as intense as a sound that’s 0 bels. Measuring everything in bels can be kind of annoying, because sometimes you want to talk about sounds that are, say, 3.4 bels without having to deal with decimal points. That’s why most of the time, you’ll hear the loudness of a sound described using the more familiar decibel unit -- a tenth of a bel. To find the loudness of a sound when you know its intensity, you take the base-10 logarithm of its intensity, over the reference intensity of 1 picowatt per square meter. Then, you multiply that number by 10 to get the sound’s decibel level. We can use this equation to convert the intensity of that noisy rock concert -- which we said was 1 Watt per square meter -- to decibels. First, we take the base 10 log of 1 Watt per square meter, over 1 picowatt per square meter. Now, 1 divided by 1 x 10^-12 is just 1 x 10^12. So what we really want to do is take the base 10 log of 1 x 10^12 -- or a trillion -- watts per square meter. What a logarithm asks you to do, is find the power that you would need to raise the base to in order to get the number in parentheses. In other words, we’re looking for the exponent of 10 that would equal 1 x 10^12. Which is just 12. To finish off the calculation of decibels from intensity, we multiply that value -- 12 -- by 10 to get the decibel level of the rock concert, where you were standing: 120 decibels. Ouch. You’ll notice that as the source of a sound moves closer to you, it gets louder, and as it moves away, it gets softer. That makes sense, since the closer you are to the source of a sound, the greater the intensity of the wave that hits your ear. But have you ever noticed that the pitch of the sound changes, too? It’s called the ‘Doppler effect’: As a source of sound moves toward you, the pitch of the sound you hear increases. And as the source moves away, the pitch decreases. To see why, imagine you’re standing on the sidewalk, when suddenly you hear an ambulance siren start up. It’s coming from down the road, and it seems to be moving toward you. The ambulance is continuously emitting sound waves at a certain frequency, in the form of that siren. But as the ambulance moves toward you, the ambulance is also driving toward those sound waves. So, the peaks that hit your eardrums are closer together -- even though they’re moving at the same speed -- and you get hit by them more often. Which means you hear a higher-pitched sound. At the same time, it keeps emitting more sound, which adds more peaks to those earlier sound waves that are heading your way. What you end up with, is a sound wave with a higher frequency than before. That’s what hits your eardrum, so you hear a sound that’s higher in pitch than the one you heard before the ambulance started moving. As the ambulance passes you and starts to drive away down the road, the opposite happens. The sound waves are still coming toward you, but the ambulance is driving away from them. So the peaks that hit your eardrum are farther apart, and you hear a sound with a lower pitch. The Doppler effect isn’t unique to sound waves, though -- it happens with light, too. Which means we can actually use it to measure the distance of stars -- but more on that much later. For now, you learned about sound waves, and how they move particles back and forth to create differences in pressure. We also talked about pitch, and how the intensity of a sound wave changes with amplitude and distance. Finally, we covered decibels, as well as the Doppler effect. Crash Course Physics is produced in association with PBS Digital Studios. You can head over to their channel to check out amazing shows like Gross Science, PBS Idea Channel, and It's Okay to be Smart. This episode of Crash Course was filmed in the Doctor Cheryl C. Kinney Crash Course Studio with the help of these amazing people and our equally amazing graphics team is Thought Cafe.

Contents

Acoustics

Acoustics is the interdisciplinary science that deals with the study of mechanical waves in gases, liquids, and solids including vibration, sound, ultrasound, and infrasound. A scientist who works in the field of acoustics is an acoustician, while someone working in the field of acoustical engineering may be called an acoustical engineer.[2] An audio engineer, on the other hand, is concerned with the recording, manipulation, mixing, and reproduction of sound.

Applications of acoustics are found in almost all aspects of modern society, subdisciplines include aeroacoustics, audio signal processing, architectural acoustics, bioacoustics, electro-acoustics, environmental noise, musical acoustics, noise control, psychoacoustics, speech, ultrasound, underwater acoustics, and vibration.[3]

Definition

Sound is defined as "(a) Oscillation in pressure, stress, particle displacement, particle velocity, etc., propagated in a medium with internal forces (e.g., elastic or viscous), or the superposition of such propagated oscillation. (b) Auditory sensation evoked by the oscillation described in (a)."[4] Sound can be viewed as a wave motion in air or other elastic media. In this case, sound is a stimulus. Sound can also be viewed as an excitation of the hearing mechanism that results in the perception of sound. In this case, sound is a sensation.

Physics of sound

Experiment using two tuning forks oscillating usually at the same frequency. One of the forks is being hit with a rubberized mallet. Although only the first tuning fork has been hit, the second fork is visibly excited due to the oscillation caused by the periodic change in the pressure and density of the air by hitting the other fork, creating an acoustic resonance between the forks. However, if we place a piece of metal on a prong, we see that the effect dampens, and the excitations become less and less pronounced as resonance isn't achieved as effectively.

Sound can propagate through a medium such as air, water and solids as longitudinal waves and also as a transverse wave in solids (see Longitudinal and transverse waves, below). The sound waves are generated by a sound source, such as the vibrating diaphragm of a stereo speaker. The sound source creates vibrations in the surrounding medium. As the source continues to vibrate the medium, the vibrations propagate away from the source at the speed of sound, thus forming the sound wave. At a fixed distance from the source, the pressure, velocity, and displacement of the medium vary in time. At an instant in time, the pressure, velocity, and displacement vary in space. Note that the particles of the medium do not travel with the sound wave. This is intuitively obvious for a solid, and the same is true for liquids and gases (that is, the vibrations of particles in the gas or liquid transport the vibrations, while the average position of the particles over time does not change). During propagation, waves can be reflected, refracted, or attenuated by the medium.[5]

The behavior of sound propagation is generally affected by three things:

  • A complex relationship between the density and pressure of the medium. This relationship, affected by temperature, determines the speed of sound within the medium.
  • Motion of the medium itself. If the medium is moving, this movement may increase or decrease the absolute speed of the sound wave depending on the direction of the movement. For example, sound moving through wind will have its speed of propagation increased by the speed of the wind if the sound and wind are moving in the same direction. If the sound and wind are moving in opposite directions, the speed of the sound wave will be decreased by the speed of the wind.
  • The viscosity of the medium. Medium viscosity determines the rate at which sound is attenuated. For many media, such as air or water, attenuation due to viscosity is negligible.

When sound is moving through a medium that does not have constant physical properties, it may be refracted (either dispersed or focused).[5]

Spherical compression (longitudinal) waves
Spherical compression (longitudinal) waves

The mechanical vibrations that can be interpreted as sound can travel through all forms of matter: gases, liquids, solids, and plasmas. The matter that supports the sound is called the medium. Sound cannot travel through a vacuum.[6][7]

Longitudinal and transverse waves

Sound is transmitted through gases, plasma, and liquids as longitudinal waves, also called compression waves. It requires a medium to propagate. Through solids, however, it can be transmitted as both longitudinal waves and transverse waves. Longitudinal sound waves are waves of alternating pressure deviations from the equilibrium pressure, causing local regions of compression and rarefaction, while transverse waves (in solids) are waves of alternating shear stress at right angle to the direction of propagation.

Sound waves may be "viewed" using parabolic mirrors and objects that produce sound.[8]

The energy carried by an oscillating sound wave converts back and forth between the potential energy of the extra compression (in case of longitudinal waves) or lateral displacement strain (in case of transverse waves) of the matter, and the kinetic energy of the displacement velocity of particles of the medium.

Longitudinal plane wave.
Transverse plane wave.
Longitudinal and transverse plane wave.

Sound wave properties and characteristics

A 'pressure over time' graph of a 20 ms recording of a clarinet tone demonstrates the two fundamental elements of sound: Pressure and Time.
A 'pressure over time' graph of a 20 ms recording of a clarinet tone demonstrates the two fundamental elements of sound: Pressure and Time.
Sounds can be represented as a mixture of their component Sinusoidal waves of different frequencies. The bottom waves have higher frequencies than those above. The horizontal axis represents time.
Sounds can be represented as a mixture of their component Sinusoidal waves of different frequencies. The bottom waves have higher frequencies than those above. The horizontal axis represents time.

Although there are many complexities relating to the transmission of sounds, at the point of reception (i.e. the ears), sound is readily dividable into two simple elements: pressure and time. These fundamental elements form the basis of all sound waves. They can be used to describe, in absolute terms, every sound we hear.

In order to understand the sound more fully, a complex wave such as the one shown in a blue background on the right of this text, is usually separated into its component parts, which are a combination of various sound wave frequencies (and noise).[9][10][11]

Sound waves are often simplified to a description in terms of sinusoidal plane waves, which are characterized by these generic properties:

Sound that is perceptible by humans has frequencies from about 20 Hz to 20,000 Hz. In air at standard temperature and pressure, the corresponding wavelengths of sound waves range from 17 m to 17 mm. Sometimes speed and direction are combined as a velocity vector; wave number and direction are combined as a wave vector.

Transverse waves, also known as shear waves, have the additional property, polarization, and are not a characteristic of sound waves.

Speed of sound

U.S. Navy F/A-18 approaching the speed of sound. The white halo is formed by condensed water droplets thought to result from a drop in air pressure around the aircraft (see Prandtl–Glauert singularity).[12]
U.S. Navy F/A-18 approaching the speed of sound. The white halo is formed by condensed water droplets thought to result from a drop in air pressure around the aircraft (see Prandtl–Glauert singularity).[12]

The speed of sound depends on the medium the waves pass through, and is a fundamental property of the material. The first significant effort towards measurement of the speed of sound was made by Isaac Newton. He believed the speed of sound in a particular substance was equal to the square root of the pressure acting on it divided by its density:

This was later proven wrong when found to incorrectly derive the speed. The French mathematician Laplace corrected the formula by deducing that the phenomenon of sound travelling is not isothermal, as believed by Newton, but adiabatic. He added another factor to the equation—gamma—and multiplied by , thus coming up with the equation . Since , the final equation came up to be , which is also known as the Newton–Laplace equation. In this equation, K is the elastic bulk modulus, c is the velocity of sound, and is the density. Thus, the speed of sound is proportional to the square root of the ratio of the bulk modulus of the medium to its density.

Those physical properties and the speed of sound change with ambient conditions. For example, the speed of sound in gases depends on temperature. In 20 °C (68 °F) air at sea level, the speed of sound is approximately 343 m/s (1,230 km/h; 767 mph) using the formula v [m/s] = 331 + 0.6 T [°C]. In fresh water, also at 20 °C, the speed of sound is approximately 1,482 m/s (5,335 km/h; 3,315 mph). In steel, the speed of sound is about 5,960 m/s (21,460 km/h; 13,330 mph). The speed of sound is also slightly sensitive, being subject to a second-order anharmonic effect, to the sound amplitude, which means there are non-linear propagation effects, such as the production of harmonics and mixed tones not present in the original sound (see parametric array).

If relativistic effects are important, the speed of sound is calculated from the relativistic Euler equations.

Perception of sound

A distinct use of the term sound from its use in physics is that in physiology and psychology, where the term refers to the subject of perception by the brain. The field of psychoacoustics is dedicated to such studies. Webster's 1936 dictionary defined sound as: "1. The sensation of hearing, that which is heard; specif.: a. Psychophysics. Sensation due to stimulation of the auditory nerves and auditory centers of the brain, usually by vibrations transmitted in a material medium, commonly air, affecting the organ of hearing. b. Physics. Vibrational energy which occasions such a sensation. Sound is propagated by progressive longitudinal vibratory disturbances (sound waves)." [13] This means that the correct response to the question: "if a tree falls in the forest with no one to hear it fall, does it make a sound?" is "yes", and "no", dependent on whether being answered using the physical, or the psychophysical definition, respectively.

The physical reception of sound in any hearing organism is limited to a range of frequencies. Humans normally hear sound frequencies between approximately 20 Hz and 20,000 Hz (20 kHz),[14]:382 The upper limit decreases with age.[14]:249 Sometimes sound refers to only those vibrations with frequencies that are within the hearing range for humans[15] or sometimes it relates to a particular animal. Other species have different ranges of hearing. For example, dogs can perceive vibrations higher than 20 kHz.

As a signal perceived by one of the major senses, sound is used by many species for detecting danger, navigation, predation, and communication. Earth's atmosphere, water, and virtually any physical phenomenon, such as fire, rain, wind, surf, or earthquake, produces (and is characterized by) its unique sounds. Many species, such as frogs, birds, marine and terrestrial mammals, have also developed special organs to produce sound. In some species, these produce song and speech. Furthermore, humans have developed culture and technology (such as music, telephone and radio) that allows them to generate, record, transmit, and broadcast sound.

Noise is a term often used to refer to an unwanted sound. In science and engineering, noise is an undesirable component that obscures a wanted signal. However, in sound perception it can often be used to identify the source of a sound and is an important component of timbre perception (see above).

Soundscape is the component of the acoustic environment that can be perceived by humans. The acoustic environment is the combination of all sounds (whether audible to humans or not) within a given area as modified by the environment and understood by people, in context of the surrounding environment.

There are, historically, six experimentally separable ways in which sound waves are analysed. They are: pitch, duration, loudness, timbre, sonic texture and spatial location.[16] Some of these terms have a standardised definition (for instance in the ANSI Acoustical Terminology ANSI/ASA S1.1-2013). More recent approaches have also considered temporal envelope and temporal fine structure as perceptually relevant analyses.[17][18][19]

Pitch

Figure 1. Pitch perception
Figure 1. Pitch perception

Pitch is perceived as how "low" or "high" a sound is and represents the cyclic, repetitive nature of the vibrations that make up sound. For simple sounds, pitch relates to the frequency of the slowest vibration in the sound (called the fundamental harmonic). In the case of complex sounds, pitch perception can vary. Sometimes individuals identify different pitches for the same sound, based on their personal experience of particular sound patterns. Selection of a particular pitch is determined by pre-conscious examination of vibrations, including their frequencies and the balance between them. Specific attention is given to recognising potential harmonics.[20][21] Every sound is placed on a pitch continuum from low to high. For example: white noise (random noise spread evenly across all frequencies) sounds higher in pitch than pink noise (random noise spread evenly across octaves) as white noise has more high frequency content. Figure 1 shows an example of pitch recognition. During the listening process, each sound is analysed for a repeating pattern (See Figure 1: orange arrows) and the results forwarded to the auditory cortex as a single pitch of a certain height (octave) and chroma (note name).

Duration

Figure 2. Duration perception
Figure 2. Duration perception

Duration is perceived as how "long" or "short" a sound is and relates to onset and offset signals created by nerve responses to sounds. The duration of a sound usually lasts from the time the sound is first noticed until the sound is identified as having changed or ceased.[22] Sometimes this is not directly related to the physical duration of a sound. For example; in a noisy environment, gapped sounds (sounds that stop and start) can sound as if they are continuous because the offset messages are missed owing to disruptions from noises in the same general bandwidth.[23] This can be of great benefit in understanding distorted messages such as radio signals that suffer from interference, as (owing to this effect) the message is heard as if it was continuous. Figure 2 gives an example of duration identification. When a new sound is noticed (see Figure 2, Green arrows), a sound onset message is sent to the auditory cortex. When the repeating pattern is missed, a sound offset messages is sent.

Loudness

Loudness is perceived as how "loud" or "soft" a sound is and relates to the totalled number of auditory nerve stimulations over short cyclic time periods, most likely over the duration of theta wave cycles.[24][25][26] This means that at short durations, a very short sound can sound softer than a longer sound even though they are presented at the same intensity level. Past around 200 ms this is no longer the case and the duration of the sound no longer affects the apparent loudness of the sound. Figure 3 gives an impression of how loudness information is summed over a period of about 200 ms before being sent to the auditory cortex. Louder signals create a greater 'push' on the Basilar membrane and thus stimulate more nerves, creating a stronger loudness signal. A more complex signal also creates more nerve firings and so sounds louder (for the same wave amplitude) than a simpler sound, such as a sine wave.

Timbre

Timbre is perceived as the quality of different sounds (e.g. the thud of a fallen rock, the whir of a drill, the tone of a musical instrument or the quality of a voice) and represents the pre-conscious allocation of a sonic identity to a sound (e.g. “it’s an oboe!"). This identity is based on information gained from frequency transients, noisiness, unsteadiness, perceived pitch and the spread and intensity of overtones in the sound over an extended time frame.[9][10][11] The way a sound changes over time (see figure 4) provides most of the information for timbre identification. Even though a small section of the wave form from each instrument looks very similar (see the expanded sections indicated by the orange arrows in figure 4), differences in changes over time between the clarinet and the piano are evident in both loudness and harmonic content. Less noticeable are the different noises heard, such as air hisses for the clarinet and hammer strikes for the piano.

Figure 3. Loudness perception
Figure 3. Loudness perception
Figure 4. Timbre perception
Figure 4. Timbre perception

Sonic texture

Sonic texture relates to the number of sound sources and the interaction between them.[27][28] The word 'texture', in this context, relates to the cognitive separation of auditory objects.[29] In music, texture is often referred to as the difference between unison, polyphony and homophony, but it can also relate (for example) to a busy cafe; a sound which might be referred to as 'cacophony'. However texture refers to more than this. The texture of an orchestral piece is very different to the texture of a brass quintet because of the different numbers of players. The texture of a market place is very different to a school hall because of the differences in the various sound sources.

Spatial location

Spatial location (see: Sound localization) represents the cognitive placement of a sound in an environmental context; including the placement of a sound on both the horizontal and vertical plane, the distance from the sound source and the characteristics of the sonic environment.[29][30] In a thick texture, it is possible to identify multiple sound sources using a combination of spatial location and timbre identification. It is the main reason why we can pick the sound of an oboe in an orchestra and the words of a single person at a cocktail party.

Sound pressure level

Sound pressure is the difference, in a given medium, between average local pressure and the pressure in the sound wave. A square of this difference (i.e., a square of the deviation from the equilibrium pressure) is usually averaged over time and/or space, and a square root of this average provides a root mean square (RMS) value. For example, 1 Pa RMS sound pressure (94 dBSPL) in atmospheric air implies that the actual pressure in the sound wave oscillates between (1 atm Pa) and (1 atm Pa), that is between 101323.6 and 101326.4 Pa. As the human ear can detect sounds with a wide range of amplitudes, sound pressure is often measured as a level on a logarithmic decibel scale. The sound pressure level (SPL) or Lp is defined as

where p is the root-mean-square sound pressure and is a reference sound pressure. Commonly used reference sound pressures, defined in the standard ANSI S1.1-1994, are 20 µPa in air and 1 µPa in water. Without a specified reference sound pressure, a value expressed in decibels cannot represent a sound pressure level.

Since the human ear does not have a flat spectral response, sound pressures are often frequency weighted so that the measured level matches perceived levels more closely. The International Electrotechnical Commission (IEC) has defined several weighting schemes. A-weighting attempts to match the response of the human ear to noise and A-weighted sound pressure levels are labeled dBA. C-weighting is used to measure peak levels.

Ultrasound

Approximate frequency ranges corresponding to ultrasound, with rough guide of some applications
Approximate frequency ranges corresponding to ultrasound, with rough guide of some applications

Ultrasound is sound waves with frequencies higher than 20,000 Hz (or 20 kHz) . Ultrasound is not different from "normal" (audible) sound in its physical properties, except in that humans cannot hear it. Ultrasound devices operate with frequencies from 20 kHz up to several gigahertz.

Ultrasound is commonly used for medical diagnostics such as sonograms.

Infrasound

Infrasound is sound waves with frequencies lower than 20 Hz. Although sounds of such low frequency are too low for humans to hear, whales, elephants and other animals can detect infrasound and use it to communicate. It can be used to detect volcanic eruptions and is used in some types of music.

See also

References

  1. ^ Fundamentals of Telephone Communication Systems. Western Electrical Company. 1969. p. 2.1.
  2. ^ ANSI S1.1-1994. American National Standard: Acoustic Terminology. Sec 3.03.
  3. ^ Acoustical Society of America. "PACS 2010 Regular Edition—Acoustics Appendix". Archived from the original on 14 May 2013. Retrieved 22 May 2013.
  4. ^ ANSI/ASA S1.1-2013
  5. ^ a b "The Propagation of sound". Archived from the original on 30 April 2015. Retrieved 26 June 2015.
  6. ^ Is there sound in space? Archived 2017-10-16 at the Wayback Machine Northwestern University.
  7. ^ Can you hear sounds in space? (Beginner) Archived 2017-06-18 at the Wayback Machine. Cornell University.
  8. ^ "What Does Sound Look Like?". NPR. YouTube. Archived from the original on 10 April 2014. Retrieved 9 April 2014.
  9. ^ a b Handel, S. (1995). Timbre perception and auditory object identification. Hearing, 425–461.
  10. ^ a b Kendall, R.A. (1986). The role of acoustic signal partitions in listener categorization of musical phrases. Music Perception, 185–213.
  11. ^ a b Matthews, M. (1999). Introduction to timbre. In P.R. Cook (Ed.), Music, cognition, and computerized sound: An introduction to psychoacoustic (pp. 79–88). Cambridge, Massachusetts: The MIT press.
  12. ^ Nemiroff, R.; Bonnell, J., eds. (19 August 2007). "A Sonic Boom". Astronomy Picture of the Day. NASA. Retrieved 26 June 2015.
  13. ^ Webster, Noah (1936). Sound. In Webster's Collegiate Dictionary (Fifth ed.). Cambridge, Mass.: The Riverside Press. pp. 950–951.
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