Hearing Enhancer Student Materials

STUDENT

WHY IS THIS PROJECT IMPORTANT?

Most people know that sound takes time to travel because it is easy to observe --like when we see lightning and wait for the thunder or watch fireworks on the 4th of July. But do you know how sound travels? Engineers and architects study acoustics and apply their knowledge of sound waves and sound propagation to design. They may be designing a sound proof room or a surround sound stereo system. In either case, the knowledge of how sound travels helps them make predictions about the behavior of their designs, and therefore guides the development of a final design.

Biomedical and electrical engineers use their knowledge of sound to design devices that help people hear better. In this project, you and your design team will learn about sound waves by watching and discussing some demonstrations, designing and testing Hearing Enhancer prototypes, and redesigning to get a good final design.

WHAT ARE WE SUPPOSED TO DO?

Your design team is going to design, construct, and test a device that will enhance your hearing. The first goal of your design is to maximize the distance from which a series of sounds can be heard. In other words, your design team will construct a device that will let the user hear test sounds from a distance beyond the spot they can be heard from without the device. The second goal of your team's device is to improve the users ability to locate the direction a sound is coming from. Because there are two major design goals, there will be two different testing phases. The only requirements are that your team uses only the materials listed below and that no part of your team's Hearing Enhancer goes in any body's ear.

WHAT MATERIALS CAN WE USE?

You may use any of the following materials to construct your Hearing Enhancer.

• Tin foil

• paper

• glue

• card board

• tape

• milk jug

• two liter bottle

• rubber band

• saran wrap

• glass jar

• soup can

• string

HOW WILL WE KNOW HOW WELL WE HAVE SUCCEEDED?

First, there is the Line-up phase of testing (Fig 1) in which one person from each design team stands in a line and tests their enhanced ability to hear a constant volume sound from increasing distances. In the Line-up phase, testers will be asked to give a knee bend to signal they heard the test sound. Your team will score points each time the person testing your device correctly signals hearing a sound. If the tester signals when there is no sound being played, points will be deducted.

Figure 1. In the line-up phase of testing, the number of times the tester correctly signals hearing a constant volume test sound along with the distance will be used to calculate the design's performance.

Next, there is the Stand-in-the-Middle phase (Fig 2) which involves one person from each team standing in the middle of an imaginary circle and determining, with the help of their Hearing Enhancer, the location of the sound source on the perimeter of the circle.

Figure 2. In the Stand-in-the-middle phase of testing, the tester uses their team's Hearing Enhancer to determine what direction the test sound is coming from. The tester for your team will stand in the middle of an imaginary circle and while three, 10sec test sounds are played, they must locate the source the of the sound on the perimeter of the circle. In the Stand-in-the-Middle phase, testers will be asked to point to the sound source.

During both stages, the people testing Hearing Enhancers will be blind folded. To measure the accuracy of your tester's prediction about the location of the sound source, two people will stand an arm distance away from the person holding the sound source. Two more people will stand an arm distance from them. When the tester points to the direction they think the sound is coming from, the four people standing by the sound source will help decide how much error was in the tester's prediction. If the tester points directly to the person holding the sound source, their team receives 1000 points. If the tester points to either of the persons next to the sound source, their team receives 750 points. And finally, if the tester points to either of the persons standing the furthest away from the sound source, their team receives 500 points. Remember, each tester will get three attempts at locating the sound source.

Two performance indexes may be calculated to evaluate the performance of your team's Hearing Enhancer. Your teams Basic Performance Index (BPI) can be calculated using results from the Line-up phase of testing.

Your team's Extra Performance Index (EPI) can be calculated using the results form both phases of testing.

WHAT ELSE MIGHT BE USEFUL TO KNOW?

Sound travels from its origin to our ears in waves. The waves only exist in matter; sound does not travel in a vacuum. Just like water waves, even though the water itself doesn't move very far as the wave passes, the material a sound wave travels through doesn't move very far even though the wave travels with high velocity (344m sec-1 in air and 1484m sec-1 in water). Waves have a frequency (the number of cycles per second called Hertz, Hz) and amplitude (the height of the wave). Humans hear sounds with frequencies between 20 and 20,000Hz. These three simple equations let you relate the frequency of the sound wave, f, to the wave length (lambda).

Water waves are different from sound waves because they rely on gravity to exist and involve an interface of two different fluids. Sound waves are actually waves of changing pressure. But, like water waves, two different sound waves will interfere (add to make a larger wave, or cancel to make a smaller one).

Figure 3. The law of reflection says that the angle both rays make with the reflecting surface are equal.

Figure 4. For sound waves going from 1 to 2, the amplitude of the sound wave decreases due to "geometrical attenuation."

Acoustic reflectors are used in many applications. The first thing you need to know before you can use an acoustic reflector is that if the wavelengths of the sound wave are very small (equivalent to saying the frequency is very large) in comparison with the reflecting surfaces you are interested in, you can use simple geometry to analyze them. In other words, you may treat the problem as a geometrical one in which sound rays spread out from the source in straight lines and are reflected from the surfaces according to the law of reflection. Sound rays are simply imaginary lines drawn from the sound origin in all possible directions of propagation. The law of reflection simply says that the angle a ray is reflected at will be equal to the angle of the incident ray (Fig. 3). This rule also applies to curved surfaces like cones and parabolas.

Horn shapes have been used for a very long time to improve the radiation or reception of sound waves. Keep in mind that for very high frequencies (>105), a wave generated in the throat (point 1) of the horn is propagated as a free sound beam near the axis without being influenced significantly by the walls of the horn. But, for lower frequencies, the resistance to the sound radiation is influenced by the shape of the horn walls (Fig 4.). One basic reasons why the horn works is that the area at opening 2 is greater than at opening 1 which makes a sound wave entering at 2 more concentrated (more energy per cross sectional area) at 1. Effective horns have very hard walls that absorb as little sound energy as possible.

A simple horn is not an extremely effective way to locate sounds because it is open to capture sounds from so many directions. Devices like the one shown in figure 5 have been used as directional sources and receivers of sound. Only the sound waves entering from a direction parallel to the tubes do not cancel each other out at the other end. When two waves from a source take two paths, one could get ahead of the other and would be called out of phase. In a tube, the phase velocity of a sound wave is practically equal to free sound speed, therefore all components of the sound wave through the tubes arrive in phase.

Figure 5. A tubular microphone works because the phase velocity of sound in a tube is practically equal to the free sound speed. Sound waves that do not enter the microphone in a direction parallel to the tubes will not arrive in phase at the other end. Therefore, these waves interfere and the result is they cancel each other out.

SAFETY CONSIDERATIONS

This project is very safe if you do not put anything in your ears. Just remember to think ahead and avoid dangerous situations.

This project was developed by Eddie Richert.