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Synthesizers and the Doppler Effect

Question: How can you recreate THX’s “Deep Note” with a simple setup?

Introduction | The Physics | The Simple Solution | More Advanced


Whether or not you know it, you’ve probably heard THX’s trademark sound “Deep Note.” If you need a refresher you can read a bit about the sound and listen to some samples at THX’s website, or just play the sample below:

When people are asked to describe the sound, their initial response is usually either “loud” or “big.” However, the loudness aspect is a bit of an illusion. Sound Designer Gary Rydstrom pointed out, “from a technical standpoint, 'Deep Note' just feels loud because it has a spectrum of frequencies that grows from small to large.” With that in mind, today we’ll try to reproduce that “big sound” with some basic physics and some cool electronics.

The Physics behind "Deep Note"

When we deal with sound in physics we’re dealing with waves. From that perspective it is easy to come up with a way to get a growing spectrum of frequencies. We start with one low frequency, and some how we need to accelerate it to produce a ramping effect of low to high frequency. That’s where the Doppler Effect comes into play.

A traditional example of the Doppler Effect would be a police car passing a stationary observer. As the police car is approaching the observer the siren sounds as if its frequency is increasing, and the opposite happens as the car passes the observer and continues away. To the person inside the car the frequency remains the same. So what’s the cause of this effect? In this scenario we are dealing with a moving sound source. Therefore, every time a new wave is emitted, its starting point is further up than the previous wave. So, in this pattern, every time a new wave is released it ‘pushes’ the wave ahead of it. Our observer hears this: as the car is approaching the waves are being pushed towards him (therefore the frequency is increasing), as it moves away, the waves are being pushed away from the observer (so the frequency decreases).

Using this principle we can make a growing sound by starting with a low frequency wave, and pushing it towards progressively higher frequencies. This is all good and fine on paper, but how are we physically going to make this happen?

A Basic Experiment

So after going through the physics of the process, I’ve designed a hypothetical model for an experiment. I’d like to generate one deep note, and bombard it with progressively higher frequencies. This will yield the same results as the observer listening to the approaching car, and quick increase in frequency. This will also make a large and loud sounding sound.

To make this effect happen physically, we need a system that can only produce one note at a time, a monophonic layout. This will let us do something that I like to call “stacking.” We generate one deep note, but then continue to add higher and higher notes. These notes are “blocked” by the first note, so they begin to push it.

The solution to this problem is rather archaic. We need a monophonic analog synthesizer! Today’s synthesizers are basically computers. They produce digital signal which can be modified with built in software and electronics. But back in the good old days, synthesizers were circuits complete with potentiometers, oscillators, and other components. These components would modify the sound physically—hence the difference between analog and digital!

So, now that we have this monophonic synth setup, we begin by playing our first deep note:

Then we increase the frequency:

And reach our peak:

When all of this is put together this is the result:

While the sound produced here is not as complex and sexy and THX’s sound, it is comparable, and much simpler to reproduce. Music Thing has a great background article that highlight’s the birth of THX’s “Deep Note.” It turns out that it was produced on a supercomputer and took thousands of lines of code to create. The price must have been in the hundreds of thousands of dollars when parts and labor are considered. This solution, however, was a quick and cheap physical trick that costs less that $200. Not bad for a first try!

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Cite Our Experiments & Research

If you have used any of this information or any of these images please go ahead and cite them in your bibliography. For your convenience, this is what the citation would look like in MLA format:

Family, Afrooz. “Synthesizers and the Doppler Effect.” September 29, 2006 Mad Physics. dd mmm. yyyy


We are glad to share our knowledge with you as long as you cite all of our info, and contact us before you use anything for non-educational purposes (commercial, etc.).

† In the bibliography you must insert the day you visited the site (this is relevant because the site could change at some point), therefore, in the bibliography above replace dd with the day you visited, mmm with the abreviated month, and yyyy with the year (ex: dd mmm. yyyy becomes 23 Dec. 2004).

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