Making Music
2. Perception

(see below for terms of use)


Introduction
When it comes to detecting sound, dogs really have us beat. Not only are their ears much larger than ours, but the inner structure of their ears allows them to detect sounds that human ears never could.

That's probably a good thing, though. If you could hear sounds from four times further away and hear frequencies that are higher than even the highest-pitched squeals, it could probably get rather irritating. 

In this lesson, we will learn how ears detect the traveling disturbances of sound waves. We will also learn about the modern marvel of science called a "cochlear implant" that acts as an electronic ear helping some deaf hear again - or for the first time.

 

Lesson

  • We will begin this lesson by considering how the human ear perceives sound waves. Watch this YouTube video for an explanatory animation (http://youtu.be/ahCbGjasm_E)

  • The hearing process depends on the intricate and delicate machinery of the ear. Perhaps no part of the ear is more delicate than the tiny hairs inside the cochlea. Watch this amazing video for views of what the ear looks like when detecting sound (http://bit.ly/M3sJeW).

  • There is an entire branch of science known as "psychoacoustics," which explores how humans perceive sound in different situations. The previous video introduced one interesting topic known as the "cocktail party effect". Our brain has an impressive ability to sort out sounds in a crowded and loud environment. Simply by choosing which sound you want to focus on, your brain can ignore all the others. Well, most of the time that is. It turns out that having two ears is an important part of your brain's ability to sort out the sounds.

  • Listen to the following audio file of two women speaking at the same time. The audio will play twice - once with both voices coming out of both speakers and the second time with one voice coming out of each speaker separately. In each case, you should try to listen to what just one of the voices is saying. (For this to work, you will need to have stereo output either in the form of two speakers or a pair of headphones). (Cocktail_Party_Effect.wav)

  • You probably noticed that it was easier to hone in on just one of the voices when it was coming from its own speaker. Having two ears allows us to locate the source of a sound and once we pin down the location, our brain can focus in on it. 

  • Our ears detect the very slight changes in timing and volume between each of our ears, which allow us to estimate (quite accurately) the direction from which a sound is coming. Explore these real stereo recordings, which make it seem like you an right in the middle of the action. (These really only work well when you are wearing headphones: http://www.virtualbarber.org/page.php?3)

  • Other animals, such as dogs, have significantly improved hearing over humans. Watch this brief video for a description of the ways in which dog hearing differs from our own. (http://bit.ly/OJCHly)

  • As we age, we loose the tiny hairs in our cochlea, which makes it more difficult for us to detect very low or very high frequencies as well as very quiet sounds. Explore the range of your hearing by using this online test. Click on the orange rectangles to play a sound. The vertical scale represents the volume (don't go too loud or it could hurt) and the horizontal scale represents pitch. See if you can hear the sounds at the highest frequencies - up to 16 kHZ (that's 16,000 Hz). (http://www.phys.unsw.edu.au/jw/hearing.html) Note that the lower frequencies may not play well on standard speakers. You can also try this hearing test to figure out exactly how high of a pitch you can still hear (http://bit.ly/NuLZF4)

  • The cochlea is the part of our ear that actually detects the sound (it's where all the tiny hairs are). It can be damaged or fail to develop properly in small children, resulting in deafness. Cochlear implants are electronic replacements for damage cochlea which detect sound and pass the signals on to the brain with artificial hairs (electrodes). Depending on the number of electrodes used, a recepient of the transplant can go from being almost completely deaf to hearing muffled voices that can understood. Explore this simulation which shows what it would be like to hear with a cochlear implant - starting with just "1 hair" (or "channel") the simulation goes all the way through 22 channels of hearing. (http://to.pbs.org/Q2FCfm)

  • Lastly, watch this incredible story of a young girl, who hears for the first time after receiving a cochlear implant. Watch from the beginning up to 13:30. (http://video.pbs.org/video/1335053789)


Assignment
Your assignemnt in this lesson is to write a 50-word summary of how the human ear normally detects sound. Return to the main course site and submit your summary under "MM-2 Summary." 

Return to Sample Curriculum
Continue on to Lesson 3


Terms of Use: These materials are subject to copyright, "All Rights Reserved." However, if you are interested in using this curriculum, please contact us and we will almost certainly grant you permission to use it under a Creative Commons (attribution-noncommercial) license. Please note that this curriculum is composed largely of free and open educational resources. Our copyright claim extends only to the text and instructional sequence outlined in this curriculum.