Hearing

Sound

If a tree falls in the woods and no one is there to hear it, does it make a sound? This question has been asked many times in various forms, to explore the nature of knowledge and perception. But it can also inform the study of the sense of hearing.

As a tree falls, the crack of the wooden trunk, and the woosh of the air as it falls create ripples in the air, just as a child in a bathtub creates ripples in the water when splashing. These sound waves are really just vibrations in the air surrounding the tree. The particles of air in the atmosphere get pushed into pockets of relatively compressed and relatively decompressed air. If there were no air, for example, if the tree fell completely in a vacuum, there would be no air for the vibrations to move through.

Sound Waves and Limits

Why are some sounds loud and others barely audible? The properties of sound waves are what determine that wave's characteristics. These characteristics include frequency, wavelength, and wave height. Similar to light waves, there are limits to the range of sound waves that humans can perceive. In this interactivity, learn about the characteristics of sound waves and their limits. Click the player to begin.

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The Auditory System: How Hearing Works

Just like the eye does for light, the ear is a magnificent organ that has evolved to convert sound waves into signals that the brain can interpret and understand. However, there is much more to the human ear than merely the outer part that you normally think of. In fact, most parts of the ear are typically unseen, yet their functions are essential to hearing. In this interactivity, learn how the auditory system works. Click the player to begin.

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Sound Levels

Your ability to perceive sound is limited by the range of frequencies that can be detected, and by the intensity of the waves. The average human can perceive an incredible range of volumes, from the faint sounds of air flowing through the heat duct in a quiet house, to the loudest jet engine flying overhead. The faintest detectable sounds are more than a million times less powerful than the sounds that you experience as very loud. For this reason, volume is measured using a logarithmic scale of units called decibels (dB). In this interactivity, learn about the different ranges of sound volumes and how exposure to different decibels can affect human hearing. Click the player to begin.

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Protecting Your Hearing

As you just learned, sounds below 85 dB are considered safe and sounds about 85 dB, with prolonged exposure, can cause hearing loss. Any sound over 125 dB can cause instant hearing damage. Sounds at this level may cause your ears to “ring.” While rare, any sound over 150 dB can cause your eardrum to burst, resulting in deafness. Sounds of 150 dB can occur in war zones from bomb blasts.

For most, the danger to hearing is not from an extremely loud one-time noise like an explosion or a firework, but rather prolonged exposure to sounds over 85 dB. This sometimes occurs when people work in factories or have jobs that require them to use heavy machinery or loud equipment for much of their day. For this reason, laws and regulations exist that require workers on loud factory floors to use protection in the form or ear plugs or ear muffs.

However, increasingly, younger people find that one of the biggest dangers to their hearing comes not from work, but from leisure. Specifically, teens more frequently wear headphones or earbuds, and the music that they listen to, often at louder volumes, can present a significant danger to their hearing. The Occupational Safety and Health Administration (OSHA) recommends no more than eight hours of exposure to sounds of 85 dB a day.

Hearing Loss

If you fail to protect your hearing, the end result is hearing loss. Hearing loss can occur two different ways. Damage to the eardrum or ossicles is considered conductive hearing loss. Damage to the stereocilia causes sensorineural hearing loss. In this interactivity, learn about the two types of hearing loss than can occur. Click the player to begin.

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Sound Location

Your ears’ exquisite ability to detect small variations in sound, combined with the processing power of your auditory cortices, also gives you the ability to detect where a particular sound is coming from. This ability, called sound localization, relies on a property of sound waves. Although sound waves travel quickly, they are quite slow compared to other phenomena like light. This is why you almost always see lightning flash before you hear the thunder that comes with it. The light travels to your eyes incredibly quickly, while the sound moves through the air at a mere 760 miles per hour, or about 1100 feet per second.

While the speed of sound is still very fast, your ears are several inches apart, meaning the sound coming from the left will hit the left ear a fraction of a second before it hits the right ear, which is noticeable to the human brain. The fact that the head blocks some of the sound, making it slightly quieter on one side, also contributes to your ability to locate where sound is coming from. This is why you often see people “cock their ear,” perhaps cupping a hand around one ear and tilting that side in the direction of where they think a sound may be coming from that they are trying to locate. By doing this, they can increase the small differences on which the detection of sound localization rest, and better guess the location from which a sound might be originating.

 

Hearing Review

Now that you have learned about hearing, review your knowledge in this non-graded activity. Click the player to get started.