# 2.15: Period and Frequency

**At Grade**Created by: CK-12

**Practice**Period and Frequency

While working on an assignment about sound in your science class, your Instructor informs you that what you know as the "pitch" of a sound is, in fact, the frequency of the sound waves. He then plays a note on a musical instrument, and the pattern of the sound wave on a graph looks like this:

He then tells you to find the frequency of the sound wave from the graph? Can you do it?

Don't worry. By the end of this Concept, you'll understand what frequency is and be able to find it from a plot like this one.

### Watch This

In the second part of this video you'll learn about the period of trigonometric functions.

James Sousa: Amplitude and Period of Sine and Cosine

### Guidance

The **period** of a trigonometric function is the horizontal distance traversed before the

**Frequency** is a measurement that is closely related to period. In science, the frequency of a sound or light wave is the number of complete waves for a given time period (like seconds). In trigonometry, because all of these periodic functions are based on the unit circle, we usually measure frequency as the number of complete waves every

Period and frequency are inversely related. That is, the higher the frequency (more waves over

After observing the transformations that result from multiplying a number *in front of* the sinusoid, it seems natural to look at what happens if we multiply a constant *inside* the argument of the function, or in other words, by the

Notice that the number of waves for **2 waves** over the interval from 0 to *half* that distance—so the graph has been “scrunched” horizontally. The frequency of this graph is therefore 2, or the same as the constant we multiplied by in the argument. The period (the length for each complete wave) is

#### Example A

What is the frequency and period of

**Solution:** If we follow the pattern from the previous example, multiplying the angle by 3 should result in the sine wave completing a cycle **three times** as often as

This number that is multiplied by *decrease* the frequency, or multiply by a number that is less than 1. Remember that this dilation factor is *inversely* related to the period of the graph.

Adding, one last time to our equations from before, we now have:

#### Example B

What is the frequency and period of

**Solution:** Using the generalization above, the frequency must be

Thinking of it as a transformation, the graph is stretched horizontally. We would only see

#### Example C

What is the frequency and period of

**Solution:**

Like the previous two examples, we can see that the frequency is

### Vocabulary

**Period:** The ** period** of a wave is the horizontal distance traveled before the 'y' values begin to repeat.

**Frequency:** The ** frequency** of a wave is number of complete waves every

### Guided Practice

1. Draw a sketch of

2. Draw a sketch of \begin{align*}y = 2.5 \cos \pi x\end{align*} from 0 to \begin{align*}2\pi\end{align*}.

3. Draw a sketch of \begin{align*}y=4 \sin \frac{1}{2} x\end{align*} from 0 to \begin{align*}2\pi\end{align*}.

**Solutions:**

1. The "2" inside the sine function makes the function "squashed" by a factor of 2 in the horizontal direction.

2. The \begin{align*}\pi\end{align*} inside the sine function makes the function "squashed" by a factor of \begin{align*}\pi\end{align*} in the horizontal direction.

3. The \begin{align*}\frac{1}{2}\end{align*} inside the sine function makes the function "stretched" by a factor of \begin{align*}\frac{1}{2}\end{align*} in the horizontal direction.

### Concept Problem Solution

By inspecting the graph

You can see that the wave takes about 6.2 seconds to make one complete cycle. This means that the frequency of the wave is approximately 1 cycle per second (since \begin{align*}2 \pi\end{align*} is approximately 6.28). (You should note that in a real wave of sound, you would need to use the speed of the wave and so the calculation would be different. But if you read the graph the same way you read trigonometric functions to find the frequency, this is the result you would find.)

### Practice

Find the period and frequency of each function below.

- \begin{align*}y=\sin(4x)\end{align*}
- \begin{align*}y=\cos(2x)\end{align*}
- \begin{align*}y=\cos(\frac{1}{2}x)\end{align*}
- \begin{align*}y=\sin(\frac{3}{4}x)\end{align*}
- \begin{align*}y=\sin(3x)\end{align*}

Draw a sketch of each function from 0 to \begin{align*}2\pi\end{align*}.

- \begin{align*}y=\sin(3x)\end{align*}
- \begin{align*}y=\cos(5x)\end{align*}
- \begin{align*}y=3\cos(\frac{2}{5}x)\end{align*}
- \begin{align*}y=\frac{1}{2}\sin(\frac{3}{4}x)\end{align*}
- \begin{align*}y=-\sin(2x)\end{align*}
- \begin{align*}y=\tan(3x)\end{align*}
- \begin{align*}y=\sec(2x)\end{align*}
- \begin{align*}y=\csc(4x)\end{align*}

Find the equation of each function.

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Amplitude

The amplitude of a wave is one-half of the difference between the minimum and maximum values of the wave, it can be related to the radius of a circle.Frequency

The frequency of a trigonometric function is the number of cycles it completes every units.horizontal stretch

Horizontal stretch describes the stretching of a graph from the axis. For a sinusoidal function with equation , the coefficient controls horizontal stretch.Period

The period of a wave is the horizontal distance traveled before the values begin to repeat.sinusoidal function

A sinusoidal function is a sine or cosine wave.sinusoidal functions

A sinusoidal function is a sine or cosine wave.Vertical shift

A vertical shift is the result of adding a constant term to the value of a function. A positive term results in an upward shift, and a negative term in a downward shift.### Image Attributions

Here you'll learn how to find the period and frequency of a trig function from either the graph or the algebraic equation.