Posts Tagged ‘modes’

Sight of sound

Your eardrum converts the motion of the air into something you can hear, but what if everything around you could work the same way? In a recent TED Talk, MIT researcher Abe Davis demonstrates cutting edge research into extracting audio from silent video of everyday objects exposed to sound. Using high-speed video equipment and even a consumer-level camera, he extracts intelligible music and speech just by watching a nearby houseplant or a snack bag—the proverbial “fly on the wall”. Acknowledging the surveillance possibilities (which were already feasible using lasers), the research pushes beyond audio to expose the natural modal movement of an object by simply ensonifying it and recording what happens, allowing one to push, pull, and shake something virtually without ever touching it.

Drop-less droplets

In a setup that’s equal parts science and Harry Potter, scientists at Argonne National Laboratory acoustically levitate liquids in midair to further critical pharmaceutical reasearch. Using a technology originally developed by NASA to simulate microgravity conditions, the pharmaceutical droplets are suspended in midair using standing waves of inaudible ultrasound generated by small speakers above and below.

By suspending a drug this way—free from any container or other physical contact—scientists can study its various forms and the ways it might be absorbed by the body. Not to mention putting on a pretty cool show in the process!

See the unseen

To promote their new vibration analyzer, measurement instrumentation company Fluke commissioned some amazing high-speed video of—you guessed it—things vibrating!  Putting the fascinating physics of vibrating plates and cylinders aside, you will have to admit that it’s interesting how a cymbal deforms at 1,000 frames per second.  (And if that doesn’t do it for you, the shaking basset hound at the end is pretty nice too!)

Catch a wave

You may know that the sounds you hear travel through the air as waves, but the invisibility of air makes this concept a tricky one to visualize.  For those who like physics demonstrations (and who doesn’t), we recently came across this video of a series of pendulums—and the pendulum is perhaps the most accessible form of wave motion we witness in everyday life.

A pendulum’s length determines its frequency, just as sound waves in air have a frequency that corresponds to pitch.  The demonstration superimposes different frequencies to illustrate traveling waves, standing waves, beats, and “random” noise, which are all phenomena that come from mixing different frequencies together.

Visualizing modes

The Graves on SOHO VoIP blog tipped us off to this cool video showing the vibration of a square plate at different frequencies. By covering the plate with salt, we can see the areas where the plate vibrates a lot (the salt rolls away) and the areas where it doesn’t vibrate at all (the salt collects). These spots and lines of little or no movement are called nodes. As the driving frequency (and sound) gets higher and higher, the patterns (called mode shapes) get more and more complex (and cool looking!)

This behavior is a great example of why we don’t use big speakers (woofers) to generate high-frequency (treble) sound. Instead of moving in unison like a piston, the speaker cone resonates internally at high frequencies, and different parts of the cone are moving in different ways, like a group of uncoordinated smaller speakers. This causes a poor frequency response, with lots of peaks and dips — often referred to as “breakup”. Smaller drivers (tweeters) are more at home with high-frequency sound, since their resonant “breakup” occurs at frequencies near or beyond the limit of hearing.

This is also a great way to visualize the analogous effect of room modes. Instead of a vibrating plate, a room is full of vibrating air, and at certain frequencies there will be points or areas in a room’s volume — nodes — with very little sound. This is most pronounced in hard, reverberant rooms, and at low frequencies. Still, even though the effect is more subtle in normally absorptive rooms, it can wreak havoc with the reproduction and recording of low-frequency sound, so identifying and managing room modes is a common task in the design of recording studios and listening rooms.