There are a handful of ways to measure sound frequency, but none are easier or cheaper than doing it with your phone. Today I’ll show you how I measured the frequency of my own voice using a free app called Spectroid (there are other free apps that you can use as well).
Then we’ll briefly talk about another method for measuring sound frequency intended for laboratory use.
Some Remarks on the Nature of Sound
Measuring sound frequency is actually quite easy if you have the right tools. However, before I go into that I want to talk a little bit about the nature of sound. I want to make sure you know what to expect, and know more or less what you’re looking at when you start measuring sound.
Frequency, wavelength and pitch
First, let’s talk about the relationship between frequency, wavelength and pitch. Frequency and wavelength are two ways of measuring or describing a sound wave. And pitch is just the way our ears interpret sounds of different frequency or wavelength.
So let’s image you’ve just hit the middle C key on a piano. This creates a sound wave that hits our ears and is interpreted as a note. That’s pitch.
If you were to measure the frequency of that sound (we’ll talk about how to do this later), you see a value somewhere between 255-265 Hertz. This roughly means that the sound’s wave repeats itself 255-265 times each second.
If you were to measure the wavelength of that same sound, you would find that it is about one and a half meters (about 4.5 feet). That’s because the speed of sound is about 343 meters per second and the sound wave repeats itself about 260 per second. So 343 meters divided by 260 waves equals about 1.3 meters for each wave form.
Frequency and wavelength inverse relationship
I wanted to mention one more thing before moving on. Sound frequency and wavelength have an inverse relationship. This just means that as one goes up the other goes down.
We said middle C was 260 Hz and a 1.3 meter wavelength.
The note A above middle C has a frequency of 440 Hz. That’s an increase in frequency compared to middle C. That means that the wavelength will decrease (it ends up being 0.8 meters).
As one goes up the other goes down.
Natural sound (sound with no apparent pitch)
OK moving on. Great we can mostly understand what’s going on with musical notes that have pitch. They have a specific frequency and wavelength and our ears are able to interpret that sound as having pitch.
But for most sounds we hear in the world, our ears don’t hear a pitch.
When you’re having a conversation it certainly doesn’t sound like anyone is singing. If you’re walking downtown the traffic just sounds like noise. And what about the sound of rain? Not very musical either.
This happens because there are many sounds each with a different frequency/pitch all happening simultaneously. Your ears can’t differentiate all the different frequencies because there could be dozens or hundreds of them.
When we measure the frequency of natural sounds like these, we can’t assign it a single frequency. We must describe natural sounds with many frequencies, some of which will be stronger or weaker than others.
What you need to measure sound frequency
In order to record sound you need roughly three tools.
- You need a microphone to actually record the sound and convert it into an electrical signal.
- You need some hardware/software that can convert that electrical signal into meaningful data
- You need a way to display the data, usually on a screen
That’s an oversimplification, but I don’t think you’re reading this article to build your own oscilloscope. Like most things of this nature you can do a rough measurement with cheaper tools and you can do a much more accurate measurement with expensive, precision tools. We’ll be spending most of our time looking at the cheap ways to measure frequency.
Measure Frequency the Easy/Free Way (Phone App)
OK we’ve gotten the prelude out of the way, let’s talk about actually measuring sound waves. The first way is to just download a free phone application.
I’ve downloaded an app called Spectroid from the Google Play Store.
When you open the app it immediately starts recording the sound around you. If you used the same app you would see a screen like this:
On the top part of the screen you can see all the frequencies the app is measuring right this moment. It shows the current amplitude/strength/loudness of a given frequency.
In the middle of the screen is the frequency axis. It’s labelled 10, 100, 1000, 10000 Hz, which is a logarithmic scale. It basically just means there are more frequencies being measured on the right side of the screen than on the left. This axis applies to both the upper graph and the bottom half of the screen.
Then you’ve got the bottom half of the screen. This shows how the frequencies have changed over time.
Interpreting the graphs in the app
When I use the app I definitely find it most helpful to look at the top half of the screen. The top half will show you when a particular frequency is very strong. It will even show you the frequency of the strongest sound it records.
Here’s another screenshot from the app while I hummed a random note. You can see on the top graph that the peak is labelled 363 Hz. That’s the frequency of the note I was humming.
You can still see that the phone is picking up other frequencies from around the room. There’s a baby monitor near me and a fan going on the hood above my stove. There are also just ambient sounds from the house, the outdoors and the phone itself.
That’s why you see so many other frequencies being recorded.
Still not too bad for a free phone app.
One thing I notice when I sing a note is that there will be a primary peak (the 363 Hz), but there are also other, smaller peaks that show up above the primary peak.
I’m not completely sure why that is, but my hypothesis is that the phone, or maybe objects near me are resonating some overtones of the note I sing. Singing one note can sometimes bring new pitches out of the air or objects around you, usually in octaves from the primary note.
I’m not sure, but that’s my guess as to what’s happening with those secondary, tertiary, etc. peaks in the app.
Measuring Frequency with an Oscilloscope
Now there are, of course, other instruments used to measure frequency of sound. Phones just happen to be very convenient for most people.
Keep in mind though, that every way to record frequency requires those same three things: A microphone to transfer sound to electrical waves, hardware/software to convert the electrical waves into data, and a way to display the data.
Oscilloscopes are tools specifically made to measure all different characteristics of waves. This includes sound waves.
An oscilloscope like this one can take in electrical waves and put them up on the screen. You can view the actual waveform, and you can see other characteristics of the wave like its frequency.
In order to measure a real sound you would also need a microphone that can plug into the oscilloscope.
Tools like this are generally meant for laboratory use, which means they aren’t ideal for complex sounds. To my knowledge, oscilloscopes don’t attempt to break out all the different frequencies in a natural sound. they count the wave’s repetitions and just give you a single frequency for the electrical input signal.
This makes them excellent tools for measuring sound waves in controlled laboratory environments, but not so great for measuring sound waves in natural, uncontrolled environments.
Not an easy setup
I majored in Physics in college and I remember a couple lab days where we used an oscilloscope to measure some generated waves (not real sound waves). I can tell you it was super confusing.
There were dozens of buttons and several dials on the device and we had to hook them up through a power supply and make sure the input signal was going in correctly.
Basically, it took guidance from our lab tech to get everything working correctly. I wouldn’t recommend going this route unless you’re willing to put in some serious study time to get your measurements.
We’ve reviewed some characteristics of sound and looked at how to measure sound frequency using a phone application (I used Spectroid, which is free). You can easily see the frequency of a single pitch on the app, and you can also see the many frequencies of the ambient sounds that surround us.
If you need to measure frequency in a controlled environment, then you’ll likely want a specialized tool, like an oscilloscope. But if you go that route prepare to spend some money and do your research. If you’d like to see a detailed post about measuring frequency with an oscilloscope, let me know in the comments below.