MONTHLY NEWSLETTERS

March 2019 Newsletter: Recovering Sound from Silence

Have you ever wondered how microphones and loudspeakers work? With these devices, we can amplify speech in a lecture hall or listen to songs recorded by musicians using headphones.  Read on to learn more about how you can “listen” to everyday objects.

The mechanism principles of loudspeakers and microphones are similar.  When we play a sound through a loudspeaker, the audio signal is converted to vibration in the diaphragm, and the vibration in the diaphragm generates sound.  A microphone works in reverse order.  For a traditional microphone, sound excites a diaphragm, and the motion of the diaphragm is converted to an electric signal; this signal is interpreted as audio.

Sound causes not only diaphragms to vibrate, but all objects. So why don’t we use other materials other than diaphragms to perceive sound?  Typically, the vibrations of these materials are too subtle for us to see with our bare eyes or even with a regular camera.  Several years ago, researchers at MIT developed a new technology to pick up these vibrations and recreate sound from a very high-resolution video.

The researchers recorded the objects with a high-speed camera and then used software to extract tiny motions from the video. The motions in the objects were analyzed to determine what sound source created them.  Basically, they successfully recovered sound from silence!  The researchers were able to restore sound from vibration on a potted plant and even on a bag of chips.

 

The song “Mary Had a Little Lamb” was played near a bag of chips.  The figures above show the spectrogram of the source sound recorded by a microphone next to the chip bag, and the spectrogram of the recovered sound from the chip bag.  You can listen to the audio clips here: a) Input sound, b) Recovered sound.

In addition to this, a bag of chips was recorded from 15 feet away through a sound-proof window.  Can you guess if it was successful? For this test, the words “Mary Had a Little Lamb…” were spoken by a person near a bag of chips.  Compared to the song, the speech sounded a little distorted, but you can still catch the sentences.  The figures below show the spectrogram of the source sound recorded by a microphone next to the chip bag, and the spectrogram of the recovered sound.  You can listen to the audio clips here: a) Input sound, b) Recovered sound.

The researchers even developed a technique to recover sound with normal cameras (not high-speed cameras).  You can read this paper for more details.  They tested not only a potted plant and a bag of chips, but also a tissue, teapot, foam cup, and aluminum foil.  This means that many objects surrounding us could act as a diaphragm of a traditional speaker, and the object itself could be a virtual microphone to listen to sound around that object.

So next time you think that no one really listens to you, grab a bag of chips and start talking!

April 2019 Newsletter: Acoustic Metamaterials
February 2019 Newsletter: Listening to Light?