Chapter 8.2

Chapter 8.2

Period 2

Sound

This lesson

• Recap
• Paragraph 8.2 — pitch and frequency
•  Working independently

How is sound produced?

• Through a vibration

What is sound made off?

• Sound waves
  

How does sound travel over distance?

• Through a medium
  

Pitch is a word we use to describe how high or low a sound is. For example:

– A whistle has a high pitch.

– A drum has a low pitch.

The pitch of the sound depends on in a string instrument:

1.Thickness: the thinner the string, the higher the pitch

                       the thicker the string, the lower the pitch

2.Length: the shorter the string, the higher the pitch

3.Tension: the tighter the string, the higher the pitch"

We can make sound visible. We do this with an oscilloscope.

With the oscilloscope you can analyze the sound.

The screen of the oscilloscope is divided into small squares. The time base tells you how much time one square represents.

For example: A time base of 500 ms/division means that one square shows 500 milliseconds."

The vibration period and frequency

Each square on the oscilloscope gives an amount of time called

Time base

time base  = 0,5 ms per 

division

And each vibration takes a certain

amount of time to complete

The vibration period and frequency

The vibration period is the amount of time it takes to complete 1 full vibration

In this picture we see multiple vibrations, at which point(s) do we see a completed vibration?

The vibration period and frequency

With the time 1 square takes up, and the amount of completed vibrations we can calculte the vibration period (T)

For example, we have 

3 completed vibrations

What is the time for these 3 vibrations?

The vibration period and frequency

We divide the time it takes to complete a certain amount of vibrations with the number of vibrations and we get the vibration period (T)

T = 2,4/3=0,8ms = 0,0008 s

So one full vibration = 0,0008 s

The vibration period and frequency

If we know the period of a vibration we can calculate its frequency (f) in Hertz (Hz)

We can use this the formula 

f = 1/T

If we know the frequency and want to 

Calculate vibration period: then you can 

Use T= 1/f

The vibration period and frequency

When we calculated T we got the answer of 0,8ms

Now we can calculate the frequency

f = 1/t

f=1/0,0008=1250Hz

Which means 1250 vibrations per second

The vibration period and frequency

We humans are able to hear sounds with low and high frequencies

Our hearing range is between 20hz and 20000hz

See the images below. total time is 8 milliseconds go by.

What is the frequency of this wave? use the formula 

f=​T​​1​​

See the images below. you have 1 ms/div.

What is the frequency of this wave? use the formula 

f=​T​​1​​

Given

T = 2 milliseconds = 0.002 seconds.

Asked

What is the frequency?

Formula

Calculation

f = 1/T = 1/0.002 =500 Hz

f=​T​​1​​