Sound & Light

Sound: Definition, Types, Characteristics & Frequencies

Sound is all around us. We use our sense of sound to navigate our environment, to communicate and to enjoy music. But what is sound? How is it made and how does it transmit from one location to another?

What Are Sound Waves?

Sound is a type of mechanical wave or an oscillation of matter. A wave is a disturbance that travels from one location to another in a medium. The key here is that the points in the medium oscillate in place while the disturbance itself travels.

For example, consider a wave done by a crowd at a ball game. The fans in their seats serve as the wave medium. Individually, they stand up, raise their arms and then sit back down – they oscillate in place. The disturbance, however, travels all the way around the stadium.

Oscillations in a medium tend to come in one of two varieties: Transverse waves oscillate at right angles to the direction of travel (as with the audience at the stadium, or a wave on a string) and longitudinal waves oscillate parallel to the direction of travel.

Sound waves are longitudinal waves. When a sound wave propagates through a medium, such as air, it does so by causing the air molecules to vibrate, which causes changes in air pressure, resulting in compressions (regions of high pressure) and rarefactions (regions of low pressure) in the air as the wave travels.

Think of a toy spring like a Slinky stretched out across a table with one person holding either end. If one person plucks the Slinky toward themselves, it will send a longitudinal wave down the Slinky. You will see regions of the Slinky coils that are more closely spaced (compressions) and more loosely spaced (rarefactions). Any given point in the Slinky oscillates back and forth in place as the disturbance moves from one end to the other.

Again, this is exactly what happens with sound waves in air, or any other medium, for that matter.

How Are Sound Waves Created?

Just as with any other wave, sound waves are created by an initial disturbance or vibration. A struck tuning fork, for example, vibrates at a specific frequency. As it moves, it bumps into the air molecules around it, periodically compressing them.

The compressed regions transfer this energy to their neighboring air molecules as well and the disturbance moves through the air until it reaches your ear, at which point it transfers energy to your eardrum, which will vibrate at the same frequency – and be interpreted by your brain as sound.

When you speak, you vibrate your larynx (a small hollow tube at the top of your windpipe), which in turn vibrates the air around it, which then propagates the sound energy to the listener. By contracting and expanding the tissue in your larynx as well as manipulating the articulators in your mouth (your lips, tongue and other mouth structures), you can create different sounds.

All objects can be sound sources that create sound in the same way – by vibrating and transferring those vibrations to an adjacent medium, such as the air.

The Speed of Sound

In dry air, sound travels at a speed of

v = 331.4 + 0.6T_c

where ​Tc​ is the temperature in Celsius. On a standard 20 degree Celsius (68 degrees Fahrenheit) day, sound travels at about 343.4 m/s. That’s about 768 miles per hour!

The speed of sound is different in different media. For example, the rate at which a sound wave travels in water can be greater than 1,437 m/s; in wood it is 3,850 m/s; and in aluminum, in excess of 6,320 m/s!

As a general rule, sound travels faster in materials where the molecules are closer together. It travels the fastest in solids, second fastest in liquids and slowest in gases.

Experiment: Measuring the Speed of Sound

You can perform a simple experiment to measure the speed of sound. To do this, you will need a sound-emitting source (which could be a tuning fork, a hand clap or your own voice), and a reflecting surface a known distance away from the source (such as a solid cliff wall several meters in front of you, or the closed end of a simple tube).

Provided you have equipment (and/or reflexes fast enough) that can measure the time-lapse between when sound is emitted and when it returns to the source location via an echo off of the reflecting surface, you will have enough information to determine the speed.

Simply take twice the distance from the source to the reflecting surface (since sound travels from the source to the surface, and then back again) and divide it by the time between sound emission and echo.

As an example, suppose you shout into a 200-m-deep canyon and receive an echo back in 1.14 seconds. The speed of sound would be 2 × 200 / 1.14 = 351 m/s.

Exceeding the Speed of Sound

You may be familiar with the phenomenon of certain aircraft breaking the sound barrier. What this means is that the aircraft flies faster than the speed of sound. At the moment it exceeds this speed, it creates a sonic boom.

An aircraft traveling at ​Mach 1​ is traveling at the speed of sound. Mach 2 is twice the speed of sound and so on. The fastest aircraft in the world was the North American X-15, which reached a speed of Mach 6.7 on October 3, 1967.

On land, the speed of sound was broken on October 15, 1997 by Andy Green who went 763.035 miles per hour in a ThrustSSC jet car in the Black Rock Desert in Nevada.

Frequency and Wavelength

The frequency of a wave is the number of oscillations that occur at a given point in the medium per second. It is measured in units of hertz (Hz) where 1 Hz = 1/s. The wavelength of a sound wave is the distance between two consecutive regions of maximum compression. It is typically measured in units of meters (m).

The speed of a sound wave, ​v,​ is directly related to frequency ​f​ wavelength lambda via ​v = λf​.

The speed of sound in a particular medium does not depend on frequency or wavelength, but is instead a constant of that particular medium. The frequency of a sound wave will always match the frequency of the sound source, so it does not depend on the medium or the wave speed.

Hence, in two different media, the frequencies will be the same, while speeds will be specific to the mediums and the wavelengths will vary accordingly. (High frequency corresponds to small wavelengths, and vice versa.)

Frequency ranges that are typically detectable by the human ear run from 64 Hz to 23 kHz, though people tend to lose their ability to hear the higher frequencies as they age. In contrast, dogs can hear all the way up to about 45 kHz (which is why they respond to dog whistles that are inaudible to humans), cats can hear up to 64 kHz and porpoises can hear all the way up to 150 kHz!

“In Space, No One Can Hear You Scream”

You’ve no doubt come across this quote from the 1979 movie ​Alien​, and it’s true: sound does not travel in a vacuum. This is because it needs a medium. There has to be some material between the sound source and you in order for the sound to propagate.

So all of those space battle scenes you see in movies with the loud explosions? Completely false! There would be no sound because there is no medium for it to travel through.

Sound Intensity and Sound Energy

Sound intensity, ​I​, is the sound power per unit area. The SI unit for sound intensity is watts/m2 where ​I0​ = 10-12 W/m2 is considered the threshold for human hearing. Colloquially, sound intensity is what we consider to be the “loudness” of a sound.

A common way of presenting perceived loudness of sound is by using the decibel (dB) scale, where sound intensity is in decibels:

This scale is useful because humans don’t perceive loudness linearly. That is, a sound with twice the intensity can seem like more than twice as loud when it started out quiet, and less than twice as loud if it started out somewhat loud already. The decibel scale provides numbers more consistent with our perceptions.

The sound of light breathing rates at about 10 dB, while conversation in a restaurant is about 60 dB. A jet flyover at 1,000 ft is about 100 dB. A borderline painful thunderclap is 120 dB, and your ear drums rupture at 150 dB.

The energy in a sound wave is directly related to the intensity. The units of intensity, W/m2, are the same as J/(sm2) or energy in joules per second per square meter.

Musical Instruments

Recall that the speed of sound only depended on the medium, and not on the frequency of the wave. This is a good thing because otherwise listening to a concert would be a terrible experience, with different musical notes reaching you out of order.

Different frequencies of sound correspond to different pitches, or musical notes. When a singer sings, they produce different frequencies by changing the size and shape of their larynx. Musical instruments are designed to create sound of pure tones typically by creating standing waves, whether in a tube or pipe, or along a string.

Consider a string instrument such as a guitar. The frequency at which a plucked string vibrates depends on its mass density (how much mass per unit length), the tension in the string (how tight it is held) and its length. If you look at a guitar, you will see that each string has a different thickness. The tuning knobs on the end of the handle allow you to adjust the string tension, and the frets give you places to put your fingers to alter the string lengths as you play, allowing you to create many different notes.

Woodwinds, in contrast, consist of hollow tubes where standing waves can be created in columns of air (just like in your larynx). The different tone holes on such an instrument allow you to change the types of standing waves that can form, and hence change the notes that can be played.

For an instrument like a trombone, you can also adjust the tube length by moving the slide back and forth, allowing for different frequency standing waves, and hence different notes to be played.

Percussive instruments, such as drums, rely on vibrations of a membrane (such as a drum head). Much like plucking the strings of a guitar, when you strike the drum head at different locations, standing waves form on the membrane, creating sound. The frequency and quality of the sound depends on the size of the membrane, its thickness and tension.

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