• Demonstration Highlight: Flame Tube

    The Flame Tube, or Rubens Tube, is a classic device for showing the wave structure of sound. It was named for German physicist Heinrich Rubens, who developed it around the turn of the last century.

     You can see our own flame tube demonstration in action in this video starring Prof. Norbert Linke



    The device is, in essence, and oscilloscope without the electronics – it transforms a sound wave into a visible trace by allowing the changes in pressure in the gas in the tube to drive flames to different heights along the top. In this way, you can see that sound does form a wave, and can show a standing wave and measure its wavelength.

     To learn more about the Rubens’ Tube, explore these links:




  • Demonstration Highlight: Fourier Analysis

    Joseph Fourier and the Fourier Transform

    Joseph Fourier was a French scientist in the late 18th and early 19th centuries. He made important contributions to subjects ranging from algebra to thermodynamics, including early studies on the greenhouse effect on Earth’s climate, but today is best remembered for his discovery that many mathematical functions can be approximated more simply as a sum of basic trigonometric functions (sines and cosines).

     This process is particularly useful to us because of the realization that you can analyze the structure of any waveform by breaking it down into a series of sine waves. By doing this, we can represent the wave as a list of simple sines and cosines, and their relative amplitudes and phases. We can build up a complex waveform by taking a single sine wave, then adding harmonics of it (sine waves whose frequency is an integral multiple of the fundamental sine wave) in different amplitudes and different phases.

     We can then work with these sine and cosine waves mathematically in order to manipulate the original waveform. This is used in modern technology for many things, from audio equalizers on music players, to cleaning up errors in digital photographs, to analyzing the complex interference patterns from spectroscopy and crystallography used to identify substances in the laboratory.

     This all sounds very complex; but the fundamentals of it are quite simple, and you can try it for yourself!

     Each of these pairs of images represents a single waveform. In the first picture, we see the full wave. In the second, we see the Fourier Transform of that wave – the spread of sine waves of different frequencies that can be assembled to build that waveform. Each spike in the Fourier Transform graph represents a sine wave; the height of each spike is how large the amplitude of that sine wave should be to make the full wave.

    A sine wave, and Fourier analysis of a sine wave 

    When the waveform we put in is just a sine wave itself, of course the Fourier Transform of it is a single line – it’s just that same sine wave again!

     A sawtooth wave, and Fourier analysis of a sawtooth wave.

    This more complicated sawtooth wave is made up of many Fourier components – multiple sine waves. As the frequency goes up, the amplitude goes down.

    Each of these sine waves is a harmonic of the first one; the frequency of each is two, or three, or four, etc times the frequency of the first, or fundamental, sine wave. That fundamentalhas the same frequency as the original sawtooth wave.

    These graphs were all created with an oscilloscope and waveform generator in our facility; check one out here!

    Fourier Analysis setup: oscilloscope, oscillator, amplifier, speaker

    Match the Wave!

    Now try it for yourself! Here are some more waveforms:

     Three waves: 1. Triangle wave, 2. Square wave, 3. Pulse Train 

    and some Fourier transforms. Can you guess which Fourier transform came from which wave?

    Three Fourier analyses of waves, A B and C.  


    Make Your Own Waves

    Even without a complex electronic synthesizer, you can try this at home with a simulator.

    This interactive simulatorin the PhET collection lets you build up waveforms by adding Fourier components:

    And the Falstad collection has another interactive simulator to discover the Fourier components of many different wave forms, and see how the breakdown of components changes when the wave does. You can also turn on the sound generator and compare how different waveforms sound to your ear. Try it out, and see what you can change in a wave to change what you hear – and what you can change and have the wave still sound the same. Can you hear a chance in frequency? A change in phase?

    Try out both, and see what waves you can build and explore!



  • Demonstration Highlight: Fourier Synthesizer

    Welcome back! In this entry in our Demonstration Highlights series, we’re taking a look at Fourier Synthesis. You may recall that we addressed Fourier Analysis in a previous entry, the process of analyzing a waveform by breaking it down into harmonic components. This time, we’re taking the process in reverse. In Fourier Synthesis, we assemble a wave form by adding sine waves together.

     Demo H4-01: The Fourier Synthesizer, with speaker and monitor

    Our Fourier Synthesizer demonstration, H4-01 in the demonstration index, lets you generate a sine wave of any frequency between 100Hz and 1,000Hz. The synthesizer then generates harmonics of this frequency, waves with integer multiple frequencies – e.g. 120Hz, 240Hz, 480Hz, etc. You can then choose to add any or all of these harmonics to the output of the synthesizer. For each of these harmonics, you can then adjust two variables: the amplitude of the harmonic, and its phase (whether it is in synch or out of synch with the original waveform).

     As Joseph Fourier showed us last time, you can create approximations of any other wave by assembling harmonics in this way.

     animation of a Fourier Series approximation of a sawtooth wave, public domain gif by Jacopo Bertolotti

    You can try this at home with the updated Making Waves simulation in the PhET Collection at the University of Colorado. This simulator works much the same way as our demonstration, allowing you to select the amplitude of each harmonic, and display them both individually and in sum. Try it here:

    In the top third of the screen, you set the amplitude of each harmonic. The middle third shows graphs of each harmonic, and the bottom third shows the sum of all of them. Try building a square wave, or a sawtooth wave, and see how close you can get!




  • Demonstration Highlight: The Theremin

    Demonstration J4-51: The Theremin is a fun and exciting way to illustrate electrical capacitance. You can see it in action in this new video with Angel Torres.

    The theremin is an electronic musical instrument invented in the early 20th century by Russian scientist, engineer, and cellist Leon Theremin. As well as his musical work, Leon Theremin developed many other electronic devices in his career as an engineer, including an early motion detector and listening devices for espionage.

    The two metal “antennas” on the sides of the theremin are not antennas in the usual sense. Each one functions as one plate of a capacitor. When you move your hand near the antenna, your hand serves as the other plate of that capacitor. Thus, each functions as a variable capacitor, where the capacitance, the ability of this air-filled capacitor to store electrical charge, varies as you move your hand and body near the antenna.

    Each of these capacitors is part of a variable RLC (resistor-inductor-capacitor) oscillator circuit. One of these variable oscillators controls a second internal oscillator circuit; these together create the output frequency (or pitch) of the sound from the theremin.

    The other variable oscillator, meanwhile controls the output amplitude. The resulting signal is fed through an amplifier circuit to a speaker. Together they form an electronic system that can create music, controlled by the motions of your body – without the player ever actually touching the device.

    The theremin has been used in a wide range of music. Much of the early technique of playing it was developed by classical violinist and thereminist Clara Rockmore. The theremin can be heard in the work of orchestral composers like Dmitri Shostakovich and Percy Grainger, and in rock bands including the Rolling Stones and Led Zeppelin. And you can hear it on the sound tracks of movies ranging from Cecil B. deMille’s The TenCommandments, to the science fiction classic The Day the Earth Stood Still, to the 2006 animated film Monster House.  It’s a beautiful way to see and hear the fusion of art and science.

     J4-51 Theremin; with stand, amplifier, and speaker

    Our theremin seen here was built by the Moog Corporation, best known for their electronic synthesizers.


    Read More:

  • Seeing Sound: Vibrations on a Plate

    This week, a tweet by Simone Schnall of Cambridge University went viral on Twitter, sharing a video by British science presenter Steve Mould

    Coincidentally, this week also marks the birthday of Ernst Chladni, for whom this phenomenon is named- Chladni Figures. 

    sand forms patterns on a square steel plate as it is stroked by a violin bow

    Sand is sprinkled on top of a plate. As it vibrates, the sand traces out the pattern of node and antinodes, accumulating along the lines where the plate is at rest, and being driven away from the areas where the plate is moving up and down with the sound wave. In its simplest form, the plate would be clamped at the center and driven by the bow at one edge; as you change the bow position, you can excite different vibrational modes of the plates and form different patterns. But pressure elsewhere on the plate, even something as simple as pressing your thumb against the edge, can form a node and thus change the pattern of standing waves.

    Of course, this can be made far more complex, and the formation of these patterns can be used to staudy how different objects vibrate in different conditions. This is still used today to help in the design of musical instruments.

    If you are teaching this topic at UMD, consider using our own set of Chladni plates, and invite your students to try it for themselves. Or, for greater complexity (and volume), try the oscillator driven version. By using an audio oscillator to drive the plate from the center, a wide range of modes can be observed by carefully varying the driving frequency. For this version, we can also offer a wider variety of plate shapes, including a model of a violin back. 

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    To learn more about Ernst Chladni, Chladni Plates, and the history of acoustics, visit these pages at the Smithsonian and at Cambridge's Whipple Museum.


  • STEM News Tip: Sonic Tourism

    Perhaps you can't get out and see the world right now, but here's a way to hear it! Prof. Trevor Cox of the University of Salford in the UK has created a website of remarkable sounds from taround the world. Explore the Sonic Wonders map at .

    In addition to being a lot of fun, there are some exciting bits of acoustical physics to explore here! Check out whispering galleries in Massachusetts and New York; resonant sound sculpture in Icelandbooming and whistling sands around the world. The site is well worth checking out if you're curious about sound - or teaching about it! Be sure to also check out Prof. Cox's acoustic and audio engineering blog at . 

  • STEM News Tip: The Physics of Music Recital Rooms

     A new article in Physics Today shares recent research on the physics underlying how we practice and hear music.

     We’ve all seen how the resonant modes of an object or space can affect and be affected by sound; and, of course, our collection has many demonstrations exploring this. But what makes for fascinating physics experiments can also make for a fabulous concert or a cacophony when performing music. The shape and size of a room can have an important effect on what parts of complex sounds are reinforced or damped – and those effects do not scale linearly. New studies have found some interesting results that can shape the future shape of small recital rooms, particularly important in this era of digital recording and streaming.