Illustration: Jordan Moss
If you just haven’t gotten enough sonic science and music theory knowledge from the Splice blog, you’ve come to the right place.
Of course, if this is your first time joining us, welcome! We’re eager to dig into a broad range of topics to help you become a better music creator, listener, conversationalist, and fan. Today, we’ll discuss the Doppler effect, one of the world’s many phenomena named after the scientists who studied them—this sonic breakthrough observed by 19th-century Austrian thinker Christian Doppler. What he developed helps explain a regular occurrence in everyday human life, backed by fundamental physics for which we should first define a few terms.
Let’s get started!
Defining key terms
|Cycle:||One repetition of a repeating waveform, commonly measured by the distance between the recurring peaks (highest points) of the sound wave (the distance between the lowest points can also constitute a cycle).|
|Period:||The amount of time required to complete one cycle.|
|Hertz:||The unit that measures frequency, indicating cycles per second; a frequency of 1 Hz indicates a period of one second.|
|Velocity:||The speed at which a given sound wave, or the source of that wave, is moving.|
What is the Doppler effect?
The Doppler effect is the experienced alteration in the observed frequency of a sound, due to motion of either the source or the observer.
But what exactly does all of this mean?
The Doppler effect is a phenomenon of wave propagation, which you probably experience to some degree every day. In particular, it’s likely that the more time you spend around motor vehicles, the more you naturally experience the Doppler effect. This is because high-speed movement, particularly involving an object with high noise pollution like cars, places the basis of a physics experiment directly in the middle of everyday human life.
Let’s use the example of a fire truck—it’s loud, impossible to ignore, and is often jetting past you with sirens blaring and rattling you to your core. We often hear emergency sirens of all kinds alternating a small number of notes (such as tritones) or using sliding expressions rather than ones that are purely chromatic. We can analyze these sorts of sirens through a music theorist’s lens—however, it’s important to distinguish that in order to do this with accuracy and consistency across repetitions, both the truck (the “source”) and the listener (the “observer”) need to be at rest.
If both the source and the observer are at rest, the perceived frequency won’t change, and you’ll hear the siren the same way each time it completes its loop. How you experience and engage with that looped sound based on relative location, however, is where the Doppler effect comes into play.
What the Doppler effect sounds like
Let’s put ourselves in the middle of that blaring sound for a moment by joining me on my street. Before pressing play below, please ensure your volume is at a conservative level. The gain of the initial sounds has been lowered, but the source is, after all, meant to be impossible to ignore!
Captured with the Shure MV88 via the Motiv app.
As the fire truck passes, can you define the note (or notes) of the siren? Does it sound the same to you at the start of the recording as it does towards the end? Take out an instrument or open your DAW and try to identify what you hear before continuing, and also consider recording a siren in your own city to do the same or use it as a way to practice your pitch recognition.
In this example, the primary notes of the siren are alternating in a perfect fourth interval, between A and D. In a matter of seconds, the distance traveled has already shifted our perceived frequency nearly a half step down, to just about at G♯ / A♭ and C♯ / D♭. At the tail end of the siren being shut off, we can still hear a whisper of that D note, but it’s certainly not the first frequency the brain picks up.
The root of this change is a differentiation in cycles per second, defined above via a metric producers are very familiar with (Hz). As an object emitting sound moves in your direction, the perception of this sound is not only reflective of the core ‘audio file,’ but also of the velocity with which the source is approaching you that alters the cycles per second. This increase accounts for what we perceive as a higher frequency as an object approaches, and a lower frequency as it moves away from us, like in the sample above.
You may have already guessed that there’s a second way to manifest this experience. Currently, we’ve made the assumption for the sake of a basic example that the listener is standing still. The observer is at rest, experiencing sound in relation to another object that’s moving. If the listener, however, is the one moving and passes by a fire truck in park, blaring its siren anyway, this would also result in an experience of the Doppler effect—as would be the case if both parties were moving.
Curious about lesser-known ways in which this idea is experienced beyond traffic? Have a read here for applications to medicine, space doings, and police meeting their monthly speeding ticket quotas.
The common misidentification of the Doppler effect
The reality is, much like many commonly-known audio effects, this phenomenon is very easily mixed up with others we commonly experience, and leverage, in our musical concoctions. In my case, this arose around a calling card of the live performances by multi-instrumentalist Andrew Bird.
Due to the loyal (obsessive) fandom of my parents, I was exposed to this artist and what I would still consider my favorite approach to live event surround sound. Perhaps it would be my second favorite had I attended the original Animals tour as my father did, as I was regularly assured growing up just how much they built upon their innovative early years.
So, what exactly does Andrew Bird do? Well, it may be helpful to first share this image of one of Chicago-based Specimen Audio’s trademark products with you.
Those who’ve attended an Andrew Bird show likely now see the whimsical, whistling violin looper as synonymous with this striking shape. Now, imagine dozens of these scattered throughout Chicago’s famous Fourth Presbyterian Church, which is what occurs annually at a show called Gezelligheid. Loosely translated to ”cozy” in Dutch, these shows are tucked inside one of the few buildings to survive the Chicago Fire, often with the worst of the city’s icy winters lurking outside. Compounding this setup is the direct effect of these Specimen Janus Horn Speakers, which can rotate 360° at varying speeds to make for a unique, mesmerizing show.
The first time I saw this, I wondered if this was just a visual gimmick with a significant price tag. What is the listener actually experiencing with these rotating tubes reminiscent of some of the oldest electric audio technology in human history?
As spoiled by the headline of this section, the audio behavior being exhibited here by a Janus Horn Speaker is not the Doppler effect. Because this product is stationary, as is most often the observer, there isn’t a significant change in velocity or frequency. This is especially the case because of the dual horn feature (rather than a singular horn rotating); there’s very little time without at least some part of the internal refractive surface of one of the horns facing the listener.
What’s instead being experienced is a sort of analog tremolo effect, with an intensity that differs based on the rate of rotation. It takes nothing away from the wonder of the experience—but no, it isn’t an example of today’s topic.
The Doppler effect is just one of many strange—yet commonplace—experiences in which the physics of sounds and our own bodies creates illusions. If you liked this topic, have a look at our other pieces related to psychoacoustics.
Years after Christian Doppler’s breakthrough, the idea of an inverse Doppler effect would also take center stage in research, which attempts to understand the conditions under which the perceived frequency of a sound increases, or sounds higher, when the object is further and / or moving away. There are a number of studies which relate this phenomenon to factors of electronic fields, specific instruments such as flutes and other wind instruments, and so on.
The expanses of scientific curiosity are ever-growing, so please feel free to reach out to me directly at firstname.lastname@example.org if you’d like to see us feature this or another psychoacoustics topic further. Of course, you’re also welcome to share general feedback, ask questions, or request additions to this post specifically.
Whether you embark on a field recording journey to integrate the Doppler effect into your music or this knowledge simply becomes another piece of your understanding of sound waves, go fourth and create!
Explore more topics on within the vast world of psychoacoustics:
August 21, 2023