Today we’ll be looking at how to measure sound waves and then we’ll discuss how to use those measurements to get the wavelength of those sounds.
We’ll talk about two different tools we can use to measure sound (one is probably free for you). Then I’ll walk you through the very simple calculation you must use after that measurement to calculate wavelength.
What is Wavelength in Sound?
Before I dive into measuring sound, I want to very briefly define wavelength as it relates to sound waves.
Wavelength is the physical distance between repeating parts of a wave.
When a sound is created, it makes high pressure points and low pressure points in the air. The wavelength of a sound wave is the distance between two high pressure points or two low pressure points.
In sound, wavelength determines the pitch of that sound. A shorter wavelength would result in a higher pitched sound and a longer wavelength would result in a lower pitched sound.
For example a tuba plays notes with wavelengths between 8.5 meters and 2 meters (40 Hz to 200 Hz), while a flute plays notes with wavelengths between 1 meter and 0.2 meters.
How to Measure Sound Waves
In order to measure sound waves you need a few things:
- A microphone
- A screen
The microphone will listen to the sound and convert the sound waves into an electrical signal. That electrical signal gets fed to some other hardware that can convert the electrical signal into meaningful data. Once that data has been created you need it to be displayed on a screen for your viewing.
There are two primary tools I know of that can measure sound waves.
1. An oscilloscope
I majored in physics in college and we did a few lab assignments where we measured various types of waves using an oscilloscope.
Our labs were done with generated, clean electrical waves so there were no impurities in the wave shapes. We were able to measure and document these perfect looking waves like the one in the picture above. This tool made it incredibly easy to measure things like wavelength and frequency.
But when you record sounds in the real world you don’t get these perfect sine waves. Natural noise doesn’t behave like that for a few different reasons.
First is just that most natural pitches will fluctuate slightly as time goes on. If you listen to a singer or a musical instrument, you might hear vibrato. This is literally just the wavelength going up and down over time. Natural sounds tend to do this even if we can’t hear it.
Second, and more importantly, in the real world there are always lots of sources of noise. You will have noise from the air conditioning and the traffic outside and your own breathing and dozens of other places. Each one of these noises produces its own unique wavelength. And when all the sounds get added together, the resulting sound wave is going to be quite messy.
The video above shows what real world sound waves tend to look like. Not nearly as neat and tidy as the the waves in my physics lab.
Taking characteristics like wavelength, frequency and amplitude from these real world sound waves is quite the challenge. In order to get a good measurement you would need to either take a snapshot at a point in time, or make the sound sustain itself long enough to get a consistent reading.
2. Your phone
The other way I know to measure sound is with my phone. I’ve written an article entirely about measuring sound frequency on my phone. Basically, you just need to find and download a free app.
In the case of Spectroid, the app I used on my phone, the readings show up as frequency. I was able to sing a note into my phone’s microphone and get a graph like this:
As you can see this phone app displays the sound data differently than the oscilloscope did. It shows the volume of different frequencies.
Remember we talked about how there are dozens of sources of sound in the real world? Well this app does a good job of showing that reality. The top half of the screen shows all the different sound frequencies and how strong/loud they are.
You can see the peak is at 363 Hertz, which was the frequency of the note I sang. But many other frequencies are being registered as well.
Apps like this give us a convenient and cheap way to measure sound waves.
How to Measure Sound Wavelength
Now how would we go about getting the wavelength of a given sound wave?
It’s not as straightforward as it might seem. It’s not easy to directly measure wavelength. In most cases, we can only measure the frequency of a given sound. Then we must use that frequency to calculate the wavelength.
In the case of the oscilloscope, it really just measures signal strength over time. This is a measure of frequency.
Some oscilloscopes will be able to calculate the wavelength of a wave given that it knows the speed of that wave, but that’s not what they measure directly.
So in many, perhaps most cases, we must first measure the sound’s frequency and then perform a calculation to get the wavelength.
Let’s see how that works.
Get Our Sound Frequency
First, we must measure the frequency of the sound. We’ve already seen two ways to measure frequency. First was the oscilloscope and second was the phone app.
And if you want any more detail about measure sound frequency, just read my article on it.
You can use your own measurement to get the frequency of a sound in your environment, but let’s use the example of my 363 Hertz singing note that I measured from the phone app.
How to Calculate Wavelength from Frequency
The basic equation for calculating wavelength from frequency is this:
Wavelength = Speed / Frequency
Well we’ve just gotten a frequency measurement from a phone app or an oscilloscope. That measurement will be in Hertz. But what about the speed?
Well the speed of sound through the air is 343 meters per second.
So if we divide 343 meters per second / 363 Hertz, we get our wavelength in meters.
343 / 363 = 0.94 meters
The wavelength of the note I sang was just under 1 meter. You can do the same calculation to get your wavelength.
343 / Your frequency in Hertz = Your wavelength in meters
OK let’s wrap this thing up. In order to measure sound waves you need a tool with a microphone (to turn sound into an electrical signal), some software/hardware (to turn that signal into data) and a screen (to display the data.
The two tools we can use to accomplish this are an oscilloscope or a phone with a frequency recording app.
When we measure those sound waves we get back a frequency. And we can finally use that frequency to calculate the wavelength of our sound wave.