Hair Cells

In the last section, we saw that higher pitches vibrate the basilar membrane most intensely near the oval window, and lower pitches vibrate the basilar membrane most intensely at a point farther down the cochlea. But how does the brain know where these vibrations occur?

This is the organ of corti's job. The organ of corti is a structure containing thousands of tiny hair cells. It lies on the surface of the basilar membrane and extends across the length of the cochlea.

Until a wave reaches the fibers with a resonant frequency, it doesn't move the basilar membrane a whole lot. But when the wave finally does reach the resonant point, the membrane suddenly releases a burst of energy in that area. This energy is strong enough to move the organ of corti hair cells at that point.

When these hair cells are moved, they send an electrical impulse through the cochlear nerve. The cochlear nerve sends these impulses on to the cerebral cortex, where the brain interprets them. The brain determines the pitch of the sound based on the position of the cells sending electrical impulses. Louder sounds release more energy at the resonant point along the membrane and so move a greater number of hair cells in that area. The brain knows a sound is louder because more hair cells are activated in an area.

The cochlea only sends raw data -- complex patterns of electrical impulses. The brain is like a central computer, taking this input and making some sense of it all. This is an extraordinarily complex operation, and scientists are still a long way from understanding everything about it.

In fact, hearing in general is still very mysterious to us. The basic concepts at work in human and animal ears are fairly simple, but the specific structures are extremely complex. Scientists are making rapid advancements, however, and they discover new hearing elements every year. It's astonishing how much is involved in the hearing process, and it's even more amazing that all these processes take place in such a small area of the body.

Fluid Wave

The cochlea is by far the most complex part of the ear. Its job is to take the physical vibrations caused by the sound wave and translate them into electrical information the brain can recognize as distinct sound.

The cochlea structure consists of three adjacent tubes separated from each other by sensitive membranes. In reality, these tubes are coiled in the shape of a snail shell, but it's easier to understand what's going on if you imagine them stretched out. It's also clearer if we treat two of the tubes, the scala vestibuli and the scala media, as one chamber. The membrane between these tubes is so thin that sound waves travel as if the tubes weren't separated at all.

The piston action of the stapes moves the fluid in the cochlea. This causes a vibration wave to travel down the basilar membrane.

The stapes moves back and forth, creating pressure waves in the entire cochlea. The round window membrane separating the cochlea from the middle ear gives the fluid somewhere to go. It moves out when the stapes pushes in and moves in when the stapes pulls out.

The middle membrane, the basilar membrane, is a rigid surface that extends across the length of the cochlea. When the stapes moves in and out, it pushes and pulls on the part of the basilar membrane just below the oval window. This force starts a wave moving along the surface of the membrane. The wave travels something like ripples along the surface of a pond, moving from the oval window down to the other end of the cochlea.

The basilar membrane has a peculiar structure. It's made of 20,000 to 30,000 reed-like fibers that extend across the width of the cochlea. Near the oval window, the fibers are short and stiff. As you move toward the other end of the tubes, the fibers get longer and more limber.

This gives the fibers different resonant frequencies. A specific wave frequency will resonate perfectly with the fibers at a certain point, causing them to vibrate rapidly. This is the same principle that makes tuning forks and kazoos work -- a specific pitch will start a tuning fork ringing, and humming in a certain way will cause a kazoo reed to vibrate.

As the wave moves along most of the membrane, it can't release much energy -- the membrane is too tense. But when the wave reaches the fibers with the same resonant frequency, the wave's energy is suddenly released. Because of the increasing length and decreasing rigidity of the fibers, higher-frequency waves vibrate the fibers closer to the oval window, and lower frequency waves vibrate the fibers at the other end of the membrane. In the next section, we'll look at how tiny hairs help us hear sound.

Amplifying Sound

We saw in the last section that the compressions and rarefactions in sound waves move your eardrum back and forth. For the most part, these changes in air pressure are extremely small. They don't apply much force on the eardrum, but the eardrum is so sensitive that this minimal force moves it a good distance.

As we'll see in the next section, the cochlea in the inner ear conducts sound through a fluid, instead of through air. This fluid has a much higher inertia than air -- that is, it is harder to move (think of pushing air versus pushing water). The small force felt at the eardrum is not strong enough to move this fluid. Before the sound passes on to the inner ear, the total pressure (force per unit of area) must be amplified.

This is the job of the ossicles, a group of tiny bones in the middle ear. The ossicles are actually the smallest bones in your body. They include:

• The malleus, commonly called the hammer
• The incus, commonly called the anvil
• The stapes, commonly called the stirrup

Sound waves vibrate the eardrum, which moves the malleus, incus and stapes.

The malleus is connected to the center of the eardrum, on the inner side. When the eardrum vibrates, it moves the malleus from side to side like a lever. The other end of the malleus is connected to the incus, which is attached to the stapes. The other end of the stapes -- its faceplate -- rests against the cochlea, through the oval window.

When air-pressure compression pushes in on the eardrum, the ossicles move so that the faceplate of the stapes pushes in on the cochlear fluid. When air-pressure rarefaction pulls out on the eardrum, the ossicles move so that the faceplate of the stapes pulls in on the fluid. Essentially, the stapes acts as a piston, creating waves in the inner-ear fluid to represent the air-pressure fluctuations of the sound wave.

The ossicles amplify the force from the eardrum in two ways. The main amplification comes from the size difference between the eardrum and the stirrup. The eardrum has a surface area of approximately 55 square millimeters, while the faceplate of the stapes has a surface area of about 3.2 square millimeters. Sound waves apply force to every square inch of the eardrum, and the eardrum transfers all this energy to the stapes. When you concentrate this energy over a smaller surface area, the pressure (force per unit of volume) is much greater. To learn more about this hydraulic multiplication, check out How Hydraulic Machines Work.

The configuration of ossicles provides additional amplification. The malleus is longer than the incus, forming a basic lever between the eardrum and the stapes. The malleus moves a greater distance, and the incus moves with greater force (energy = force x distance).

This amplification system is extremely effective. The pressure applied to the cochlear fluid is about 22 times the pressure felt at the eardrum. This pressure amplification is enough to pass the sound information on to the inner ear, where it is translated into nerve impulses the brain can understand.

The Eardrum

Once the sound waves travel into the ear canal, they vibrate the tympanic membrane, commonly called the eardrum. The eardrum is a thin, cone-shaped piece of skin, about 10 millimeters (0.4 inches) wide. It is positioned between the ear canal and the middle ear. The middle ear is connected to the throat via the eustachian tube. Since air from the atmosphere flows in from your outer ear as well as your mouth, the air pressure on both sides of the eardrum remains equal. This pressure balance lets your eardrum move freely back and forth

The eardrum is rigid, and very sensitive. Even the slightest air-pressure fluctuations will move it back and forth. It is attached to the tensor tympani muscle, which constantly pulls it inward. This keeps the entire membrane taut so it will vibrate no matter which part of it is hit by a sound wave.

Ear illustration courtesy NIDCD Normal ear anatomy

This tiny flap of skin acts just like the diaphragm in a microphone. The compressions and rarefactions of sound waves push the drum back and forth. Higher-pitch sound waves move the drum more rapidly, and louder sound moves the drum a greater distance.

The eardrum can also serve to protect the inner ear from prolonged exposure to loud, low-pitch noises. When the brain receives a signal that indicates this sort of noise, a reflex occurs at the eardrum. The tensor tympani muscle and the stapedius muscle suddenly contract. This pulls the eardrum and the connected bones in two different directions, so the drum becomes more rigid. When this happens, the ear does not pick up as much noise at the low end of the audible spectrum, so the loud noise is dampened.

In addition to protecting the ear, this reflex helps you concentrate your hearing. It masks loud, low-pitch background noise so you can focus on higher-pitch sounds. Among other things, this helps you carry on a conversation when you're in a very noisy environment, like a rock concert. The reflex also kicks in whenever you start talking -- otherwise, the sound of your own voice would drown out a lot of the other sounds around you.

The eardrum is the entire sensory element in your ear. As we'll see in the coming sections, the rest of the ear serves only to pass along the information gathered at the eardrum.

Catching Sound Waves

We saw in the last section that sound travels through the air as vibrations in air pressure. To hear sound, your ear has to do three basic things:

• Direct the sound waves into the hearing part of the ear
• Sense the fluctuations in air pressure
• Translate these fluctuations into an electrical signal that your brain can understand

The pinna, the outer part of the ear, serves to "catch" the sound waves. Your outer ear is pointed forward and it has a number of curves. This structure helps you determine the direction of a sound. If a sound is coming from behind you or above you, it will bounce off the pinna in a different way than if it is coming from in front of you or below you. This sound reflection alters the pattern of the sound wave. Your brain recognizes distinctive patterns and determines whether the sound is in front of you, behind you, above you or below you.

Ear diagram courtesy NASA

Your brain determines the horizontal position of a sound by comparing the information coming from your two ears. If the sound is to your left, it will arrive at your left ear a little bit sooner than it arrives at your right ear. It will also be a little bit louder in your left ear than your right ear.

Since the pinnae face forward, you can hear sounds in front of you better than you can hear sounds behind you. Many mammals, such as dogs, have large, movable pinnae that let them focus on sounds from a particular direction. Human pinnae are not so adept at focusing on sound. They lay fairly flat against the head and don't have the necessary muscles for significant movement. But you can easily supplement your natural pinnae by cupping your hands behind your ears. By doing this, you create a larger surface area that can capture sound waves better. In the next section, we'll see what happens as a sound wave travels down the ear canal and interacts with the eardrum.

How Hearing Works

Your ears are extraordinary organs. They pick up all the sounds around you and then translate this information into a form your brain can understand. One of the most remarkable things about this process is

Ear diagram courtesy NASA
Your ear is a delicate and detailed sensory organ.

that it is completely mechanical. Your sense of smell, taste and vision all involve chemical reactions, but your hearing system is based solely on physical movement.

In this article, we'll look at the mechanical systems that make hearing possible. We'll trace the path of a sound, from its original source all the way to your brain, to see how all the parts of the ear work together. When you understand everything they do, it's clear that your ears are one of the most incredible parts of your body!

To understand how your ears hear sound, you first need to understand just what sound is.

An object produces sound when it vibrates in matter. This could be a solid, such as earth; a liquid, such as water; or a gas, such as air. Most of the time, we hear sounds traveling through the air in our atmosphere.

When something vibrates in the atmosphere, it moves the air particles around it. Those air particles in turn move the air particles around them, carrying the pulse of the vibration through the air.

To see how this works, let's look at a simple vibrating object: a bell. When you hit a bell, the metal vibrates -- flexes in and out. When it flexes out on one side, it pushes on the surrounding air particles on that side. These air particles then collide with the particles in front of them, which collide with the particles in front of them, and so on. This is called compression.

When the bell flexes away, it pulls in on the surrounding air particles. This creates a drop in pressure, which pulls in more surrounding air particles, creating another drop in pressure, which pulls in particles even farther out. This pressure decrease is called rarefaction.

In this way, a vibrating object sends a wave of pressure fluctuation through the atmosphere. We hear different sounds from different vibrating objects because of variations in the sound wave frequency. A higher wave frequency simply means that the air pressure fluctuation switches back and forth more quickly. We hear this as a higher pitch. When there are fewer fluctuations in a period of time, the pitch is lower. The level of air pressure in each fluctuation, the wave's amplitude, determines how loud the sound is. In the next section, we'll look at how the ear is able to capture sound waves.

How Amplifiers Work

When people refer to "amplifiers," they're usually talking about stereo components or musical equipment. But this is only a small representation of the spectrum of audio amplifiers. There are actually amplifiers all around us. You'll find them in televisions, computers, portable CD players and most other devices that use a speaker to produce sound.

Sound is a fascinating phenomenon. When something vibrates in the atmosphere, it moves the air particles around it. Those air particles in turn move the air particles around them, carrying the pulse of the vibration through the air. Our ears pick up these fluctuations in air pressure and translate them into electrical signals the brain can process.

Electronic sound equipment works the same basic way. It represents sound as a varying electric current. Broadly speaking, there are three steps in this sort of sound reproduction:

• Sound waves move a microphone diaphragm back and forth, and the microphone translates this movement into an electrical signal. The electrical signal fluctuates to represent the compressions and rarefactions of the sound wave.

• A recorder encodes this electrical signal as a pattern in some sort of medium -- as magnetic impulses on tape, for example, or as grooves in a record.

• A player (such as a tape deck) re-interprets this pattern as an electrical signal and uses this electricity to move a speaker cone back and forth. This re-creates the air-pressure fluctuations originally recorded by the microphone.

As you can see, all the major components in this system are essentially translators: They take the signal in one form and put it into another. In the end, the sound signal is translated back into its original form, a physical sound wave.

HowStuffWorks
A home stereo amplifier and receiver in one unit.
See more amplifier pictures.

­ In order to register all of the minute pressure fluctuations in a sound wave, the microphone diaphragm has to be extremely sensitive. This means it is very thin and moves only a short distance. Consequently, the microphone produces a fairly small electrical current.

This is fine for most of the stages in the process -- it's strong enough for use in the recorder, for example, and it is easily transmitted through wires. But the final step in the process -- pushing the speaker cone back and forth -- is more difficult. To do this, you need to boost the audio signal so it has a larger current while preserving the same pattern of charge fluctuation.

This is the job of the amplifier. It simply produces a more powerful version of the audio signal. In this article, we'll see what amplifiers do and how they do it. Amplifiers can be very complex devices, with hundreds of tiny pieces, but you can get a clear picture of how an amplifier works by examining the most basic components. In this next section, we'll look at the basic elements of amplifiers.­

Alternative Speaker Designs

Most loudspeakers produce sound with traditional drivers. But there are a few other technologies on the market. These designs have some advantages over traditional dynamic speakers, but they fall short in other areas. For this reason, they are often used in conjunction with driver units.

The most popular alternative is the electrostatic speaker. These speakers vibrate air with a large, thin, conductive diaphragm panel. This diaphragm panel is suspended between two stationary conductive panels that are charged with electrical current from a wall outlet. These panels create an electrical field with a positive end and a negative end. The audio signal runs a current through the suspended panel, rapidly switching between a positive charge and a negative charge. When the charge is positive, the panel is drawn toward the negative end of the field, and when the charge is negative, it moves toward the positive end in the field.

The diaphragm is alternately charged with a positive current and a negative current, based on the varying electrical audio signal. When the diaphragm is positively charged, it fluctuates toward the front plate, and when it is negatively charged it fluctuates toward the rear plate. In this way, it precisely reproduces the recorded pattern of air fluctuations.

In this way, the diaphragm rapidly vibrates the air in front of it. Because the panel has such a low mass, it responds very quickly and precisely to changes in the audio signal. This makes for clear, extremely accurate sound reproduction. The panel doesn't move a great distance, however, so it is not very effective at producing lower frequency sounds. For this reason, electrostatic speakers are often paired with a woofer that boosts the low frequency range. The other problem with electrostatic speakers is that they must be plugged into the wall and so are more difficult to place in a room.

Another alternative is the planar magnetic speaker. These units use a long, metal ribbon suspended between two magnetic panels. They basically work the same way as electrostatic speakers, except that the alternating positive and negative current moves the diaphragm in a magnetic field rather than an electric field. Like electrostatic speakers, they produce high-frequency sound with extraordinary precision, but low frequency sounds are less defined. For this reason, the planar magnetic speaker is usually used only as a tweeter.

Both of these designs are becoming more popular with audio enthusiasts, but traditional dynamic drivers are still the most prevalent technology, far and away. You'll find them everywhere you go -- not only in stereo setups, but in alarm clocks, public address systems, televisions, computers, headphones and tons of other devices. It's amazing how such a simple concept has revolutionized the modern world!

Other Speaker Enclosures

Other enclosure designs redirect the inward pressure outward, using it to supplement the forward sound wave. The most common way to do this is to build a small port into the speaker. In these bass reflex speakers, the backward motion of the diaphragm pushes sound waves out of the port, boosting the overall sound level. The main advantage of bass reflex enclosures is efficiency. The power moving the driver is used to emit two sound waves rather than one. The disadvantage is that there is no air pressure difference to spring the driver back into place, so the sound production is not as precise.

A bass reflex speaker produces two sound waves by moving one driver. When the driver compresses air forward, it rarefies it backward, and vice versa. The second sound wave is emitted from a port at the base of the speaker enclosure.

Passive radiator enclosures are very similar to bass reflex units, but in passive radiator enclosures, the backward wave moves an additional, passive driver, instead of escaping out of the port. The passive driver is just like the main, active drivers except it doesn't have an electromagnet voice coil, and it isn't connected to the amplifier. It is moved only by the sound waves coming from the active drivers. This type of enclosure is more efficient than sealed designs and more precise than bass reflex models.

Some enclosure designs have an active driver facing one way and a passive driver facing the other way. This dipole design diffuses the sound in all directions, making it a good choice for the rear channels in a home theater system.

The backward air compression and rarefaction caused by the active driver push and pull on the passive driver. A speaker with a dipole design emits sound waves in both directions.

These are just a few of the many enclosure types available. There are a huge range of speaker units on the market, with a variety of unique structures and driver arrangements. Check out this page to learn about some of these designs.

Sealed Speaker Enclosures

In most loudspeaker systems, the drivers and the crossover are housed in some sort of speaker enclosure. These enclosures serve a number of functions. On their most basic level, they make it much easier to set up the speakers. Everything's in one unit and the drivers are kept in the right position, so they work together to produce the best sound. Enclosures are usually built with heavy wood or another solid material that will effectively absorb the driver's vibration. If you simply placed a driver on a table, the table would vibrate so much it would drown out a lot of the speaker's sound.

Additionally, the speaker enclosure affects how sound is produced. When we looked at speaker drivers, we focused on how the vibrating diaphragm emitted sound waves in front of the cone. But, since the diaphragm is moving back and forth, it's actually producing sound waves behind the cone as well. Different enclosure types have different ways of handling these "backward" waves.

A typical sealed speaker enclosure that holds a tweeter, a woofer and a midrange driver.

The most common type of enclosure is the sealed enclosure, also called acoustic suspension enclosure. These enclosures are completely sealed, so no air can escape. This means the forward wave travels outward into the room, while the backward wave travels only into the box. Of course, since no air can escape, the internal air pressure is constantly changing -- when the driver moves in, the pressure is increased and when the driver moves out, it is decreased. Both movements create pressure differences between the air inside the box and the air outside the box. The air will always move to equalize pressure levels, so the driver is constantly being pushed toward its "resting" state -- the position at which internal and external air pressure are the same.

In a sealed speaker setup, the driver diaphragm compresses air in the enclosure when it moves in and rarefies air when it moves out.

These enclosures are less efficient than other designs because the amplifier has to boost the electrical signal to overcome the force of air pressure. The force serves a valuable function, however -- it acts like a spring to keep the driver in the right position. This makes for tighter, more precise sound production.

Chunks of the Frequency Range

To produce quality sound over a wide frequency range more effectively, you can break the entire range into smaller chunks that are handled by specialized drivers. Quality loudspeakers will typically have a woofer, a tweeter and sometimes a midrange driver, all included in one enclosure.

Of course, to dedicate each driver to a particular frequency range, the speaker system first needs to break the audio signal into different pieces -- low frequency, high frequency and sometimes mid-range frequencies. This is the job of the speaker crossover.

The most common type of crossover is passive, meaning it doesn't need an external power source because it is activated by the audio signal passing through it. This sort of crossover uses inductors, capacitors and sometimes other circuitry components. Capacitors and inductors only become good conductors under certain conditions. A crossover capacitor will conduct the current very well when the frequency exceeds a certain level, but will conduct poorly when the frequency is below that level. A crossover inductor acts in the reverse manner -- it is only a good conductor when the frequency is below a certain level.

The typical crossover unit from a loudspeaker: The frequency is divided up by inductors and capacitors and then sent on to the woofer, tweeter and mid-range driver.

When the electrical audio signal travels through the speaker wire to the speaker, it passes through the crossover units for each driver. To flow to the tweeter, the current will have to pass through a capacitor. So for the most part, the high frequency part of the signal will flow on to the tweeter voice coil. To flow to the woofer, the current passes through an inductor, so the driver will mainly respond to low frequencies. A crossover for the mid-range driver will conduct the current through a capacitor and an inductor, to set an upper and lower cutoff point.

There are also active crossovers. Active crossovers are electronic devices that pick out the different frequency ranges in an audio signal before it goes on to the amplifier (you use an amplifier circuit for each driver). They have several advantages over passive crossovers, the main one being that you can easily adjust the frequency ranges. Passive crossover ranges are determined by the individual circuitry components -- to change them, you need to install new capacitors and inductors. Active crossovers aren't as widely used as passive crossovers, however, because the equipment is much more expensive and you need multiple amplifier outputs for your speakers.

Crossovers and drivers can be installed as separate components in a sound system, but most people end up buying speaker units that house the crossover and multiple drivers in one box. In the next section, we'll find out what these speaker enclosures do and how they affect the speaker's sound quality.

Driver Types

In the last section, we saw that traditional speakers produce sound by pushing and pulling an electromagnet attached to a flexible cone. Although drivers are all based on the same concept, there is a wide range in driver size and power. The basic driver types are:

• Woofers
• Tweeters
• Midrange

Woofer

Tweeter

Midrange

Woofers are the biggest drivers, and are designed to produce low frequency sounds. Tweeters are much smaller units, designed to produce the highest frequencies. Midrange speakers produce a range of frequencies in the middle of the sound spectrum.

And if you think about it, this makes perfect sense. To create higher frequency waves -- waves in which the points of high pressure and low pressure are closer together -- the driver diaphragm must vibrate more quickly. This is harder to do with a large cone because of the mass of the cone. Conversely, it's harder to get a small driver to vibrate slowly enough to produce very low frequency sounds. It's more suited to rapid movement.

Making Sound: Voice Coil

If you've read How Electromagnets Work, then you know that an electromagnet is a coil of wire, usually wrapped around a piece of magnetic metal, such as iron. Running electrical current through the wire creates a magnetic field around the coil, magnetizing the metal it is wrapped around. The field acts just like the magnetic field around a permanent magnet: It has a polar orientation -- a "north" end and and a "south" end -- and it is attracted to iron objects. But unlike a permanent magnet, in an electromagnet you can alter the orientation of the poles. If you reverse the flow of the current, the north and south ends of the electromagnet switch.

This is exactly what a stereo signal does -- it constantly reverses the flow of electricity. If you've ever hooked up a stereo system, then you know that there are two output wires for each speaker -- typically a black one and a red one.

The wire that runs through the speaker system connects to two hook-up jacks on the driver.

Essentially, the amplifier is constantly switching the electrical signal, fluctuating between a positive charge and a negative charge on the red wire. Since electrons always flow in the same direction between positively charged particles and negatively charged particles, the current going through the speaker moves one way and then reverses and flows the other way. This alternating current causes the polar orientation of the electromagnet to reverse itself many times a second.

Making Sound: Diaphragm

A driver produces sound waves by rapidly vibrating a flexible cone, or diaphragm.

• The cone, usually made of paper, plastic or metal, is attached on the wide end to the suspension.
• The suspension, or surround, is a rim of flexible material that allows the cone to move, and is attached to the driver's metal frame, called the basket.
• The narrow end of the cone is connected to the voice coil.
• The coil is attached to the basket by the spider, a ring of flexible material. The spider holds the coil in position, but allows it to move freely back and forth.

Some drivers have a dome instead of a cone. A dome is just a diaphragm that extends out instead of tapering in.

Making Sound

In the last section, we saw that sound travels in waves of air pressure fluctuation, and that we hear sounds differently depending on the frequency and amplitude of these waves. We also learned that microphones translate sound waves into electrical signals, which can be encoded onto CDs, tapes, LPs, etc. Players convert this stored information back into an electric current for use in the stereo system.

A speaker is essentially the final translation machine -- the reverse of the microphone. It takes the electrical signal and translates it back into physical vibrations to create sound waves. When everything is working as it should, the speaker produces nearly the same vibrations that the microphone originally recorded and encoded on a tape, CD, LP, etc.

Differentiating Sound

We hear different sounds from different vibrating objects because of variations in:

• Sound-wave frequency - A higher wave frequency simply means that the air pressure fluctuates faster. We hear this as a higher pitch. When there are fewer fluctuations in a period of time, the pitch is lower.

• Air-pressure level - This is the wave's amplitude, which determines how loud the sound is. Sound waves with greater amplitudes move our ear drums more, and we register this sensation as a higher volume.

A microphone works something like our ears. It has a diaphragm that is vibrated by sound waves in an area. The signal from a microphone gets encoded on a tape or CD as an electrical signal. When you play this signal back on your stereo, the amplifier sends it to the speaker, which re-interprets it into physical vibrations. Good speakers are optimized to produce extremely accurate fluctuations in air pressure, just like the ones originally picked up by the microphone. In the next section, we'll see how the speaker accomplishes this.

Sound Basics

To understand how speakers work, you first need to understand how sound works.

Inside your ear is a very thin piece of skin called the eardrum. When your eardrum vibrates, your brain interprets the vibrations as sound -- that's how you hear. Rapid changes in air pressure are the most common thing to vibrate your eardrum.

An object produces sound when it vibrates in air (sound can also travel through liquids and solids, but air is the transmission medium when we listen to speakers). When something vibrates, it moves the air particles around it. Those air particles in turn move the air particles around them, carrying the pulse of the vibration through the air as a traveling disturbance.

To see how this works, let's look at a simple vibrating object -- a bell. When you ring a bell, the metal vibrates -- flexes in and out -- rapidly. When it flexes out on one side, it pushes out on the surrounding air particles on that side. These air particles then collide with the particles in front of them, which collide with the particles in front of them and so on. When the bell flexes away, it pulls in on these surrounding air particles, creating a drop in pressure that pulls in on more surrounding air particles, which creates another drop in pressure that pulls in particles that are even farther out and so on. This decreasing of pressure is called rarefaction.