Sound Waves and Hearing

Learning Target: By the end of this lesson you should be able to describe what sound is, identify its properties, and explain how we hear it.

UP!  What is Sound?

WHAT IS SOUND?

Sound is caused by vibrations.  When you beat a drum, the drum skin vibrates and causes the air to vibrate.

A Vibration is the complete back-and-forth motion of an object.

No matter how different sounds may be, they are always created by vibrations


WHAT ARE SOUND WAVES?

Sound Wave is a longitudinal waves that is cased by vibrations

          Longitudinal Wave has particles that travel in the same direction as the wave, and are also made of compressions and rarefactions

Compression-

Rarefaction-

HOW DO SOUND WAVES TRAVEL?

Sound Waves travel in all directions away from their source.  Think about someone playing a trumpet.  The sound can be heard in all directions.  Sound can only travel if it has a medium.

Remember that sound can only travel in the form of vibrations, and that they travel by disturbing the particles in a medium.  The particles of the medium do not travel with the sound waves themselves.  They simply vibrate back and forth along the path that the waves travel.

Sound can obviously travel through air, but it can also travel through other materials like, water, glass, and metals.  When people talk you can hear them through walls or other objects, but their sound may be muffled.  When you swim underwater, you can hear the people around you moving and splashing around.  

SOUND IN A VACCUM

In a vacuum there are no particles to vibrate, therefore, no sound can be made in a vacuum.  Sound must travel through air or some other medium to reach your ears and be detected.

NOW! What determines Pitch?

WHAT DETERMINES PITCH

Pitch is how high or low you think a sound is.  Pitch depends on the sensitivity of a person's ear.  Pitch also depends on frequency and wavelength of a sound wave.  

FREQUENCY AND WAVELENGTH

Frequency is obviously expressed in hertz (Hz.)  One hertz is one complete wavelength or cycle per second.  In a medium, the higher the frequency, the shorter the wavelength and the higher the pitch.  High-frequency waves have shorter wavelengths and produce high-pitched sounds.  A low-frequency wave has a longer wavelength and makes a lower-pitched sound.  

WHAT MAKES A SOUND LOUD?

Think about a drum.  If you barely raise the drum stick and gently tap the drum, the sound will be quiet.  If you put more force into driving the stick into the drum surface, and the resulting sound will increase.  Loudness is a measure of how well a sound can be heard.

AMPLITUDE

Amplitude is the measure of how much energy a sound wave carries.  As you increase the force used when you strike a drum, you increase the amplitude of the sound waves.  The greater the amplitude the louder the sound.  The smaller the amplitude the softer the sound.  One way to increase sound is to use an amplifier.

DOWN! How is Loudness Measured!

Loudness is a characteristic of sound that can be calculated from the intensity of a sound wave.  We use Decibel to express loudness.  One decibel (dB) is one-tenth of a bel, which is the base unit.  Kinda like meter and centi-meter.  Human ears can hear sounds as quiet at 0 dB, and sounds that are 120 dB or louder can be painful for us.  

HOW LOUD IS TOO LOUD?

When we hear sounds that are loud for even a short period of time, it can result in hearing loss.  Even loud sounds that are not painful to your ear can cause hearing loss if you are exposed to them for long periods of time.  Loud sounds can damage the hairs on the nerve cells in your cochlea.  Once these hairs are damaged, their do not grow back, which results in hearing loss.  How loud have you had your music lately?  Remember that if your teacher can hear the words to your highly appropriate song, you should probably turn it down!

WHAT IS THE DOPPLER EFFECT?

Think back to the last time that you drove by a train.  You probably noticed the sudden change in pitch in the whistle as the train came closer, and when it passed your car.  This is caused by the doppler effect, which happens when the sound source and the observer are both moving.  Look at the diagram below.

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