June 09, 2015
Spectrograms help us see sound. Okay, we can’t really see sound. Sound is invisible but this hasn’t worked in its favour. As far as our five senses are concerned, vision will always be the favorite. For every word we have to describe the way something sounds, there are hundreds of words to describe the way it looks. We are creatures of vision.
This is why people get so excited about tools that can show us sound. For a long time scientists went to oscilloscopes to describe sound waves, and they learned a ton!
The sounds of synthesizers were largely created with the visual aid of waveforms in the oscilloscope. Credit: Omegatron.
But, there is a big problem with the wavy line presented by oscilloscopes: sound waves don’t look like that (or work like that) so a confusing image is being presented.
Spectrograms take a different approach. Spectrograms take sound from a microphone or recording and lay them out in a type of real time musical notation. Here’s what they look like:
When you look at a spectrogram along with a recording, time scrolls across the screen from right to left. Pitches show up as these bright bar, with low pitch sounds at the bottom and high pitch sounds up top.
A whistled scale looks like this. The bars line up with the numbers on the side, telling us the exact frequency of the pitch.
But here’s the reality, most sounds don’t vibrate at just one speed.
So most sounds show up on the spectrogram like this:
At first this might seem disappointing. It looked so simple before, but all those lines just look confusing!
This is actually a blessing in disguise because it helps us explain one of the most interesting, exciting, misunderstood and mispronounced things about sound: timbre (rhymes with amber).
Timbre is often described as the quality of a sound or the tone colour of a pitched note. When a trumpet and a flute play the same pitch at the same volume, we can tell the difference. This is because of timbre. When we pick out the sound of a loved one calling our name from across a loud and crowded space, it’s because of timbre. When we shape the bright sound of our vocal cords (slapping together hundreds of times a second) into vowels and words to communicate our thoughts… Timbre. Timbre is really interesting and yet most people know little to nothing about it.
This is why the spectrogram is so handy: the lines you see are directly connected to the timbre of a sound.
Let’s dive in to some specifics:
The speed of a vibration determines the pitch of a sound. Humans hear vibrations between 20 and 20,000 Hertz (Hertz = vibrations per second). Spectrograms display these numbers along the side.
Most sounds vibrate at more than one speed. When we see the stacked lines on the spectrogram, all of these different pitches are vibrating at the same time, but they blend together to create the timbre. Musicians talk about these sounds as overtones.
Some sounds contain lots of overtones, and we call those timbres bright. Sounds with few overtones we call dark.
One pitch is louder than the rest (often the lowest line) and this is the “note” that we hear, or the fundamental.
What this all means is a sound’s timbre will leave a sort of “fingerprint”, and with not too much time spent, you can start seeing the sound’s details. To avoid the pitfall of the oscilloscope, I want to say that the images in the spectrogram do not show us what sound waves look like, but the images give us a hook, a path to deeper understanding.
These tools are freely available to you if you have a connected computer or a smart phone. The images in this blog were captured with a downloadable program called Sonic Visualiser, which has a steep learning curve, but is easily the most complete tool for scientific study (you can check out the documentation on their website here). If you want to jump right into looking at sound through a spectrogram, there is a free app called Spectrum View or for a few dollars you can get Signal Spy, which has a beautiful looking spectrogram (with inaccurate frequency numbers) and it has an oscilloscope, decibel reader and spectrum analyzer.
Spectrograms are also a main feature in our Air Powered program, which is available for Calgary area schools to book.
So go out and explore the world of sound through this amazing virtual visual aid. It brings a lot of things into focus.
This is easier to understand if you can see and hear it, so I have started a series of videos looking at how spectrograms work and what they can teach us about sound, and there are three so far. The first is a general introduction and quick look at how we control the timbre of our voice to create vowel sounds:
The second video looks at the “fingerprint” (“soundprint”?) of several different instruments:
The third video is a closer look at overtones, and how they are controlled on the ever-popular Hammond Organ: