Did you know you can use a laser as a microphone? By pointing a laser on an object and carefully observing vibrations of that object it is possible to gather information about sound waves in that area. The rumor goes that laser microphones were used to spy on conversations inside the Ecuadorian embassy, the hideout of Julian Assange for several years.
If a sound wave hits an object, said object starts to vibrate. By carefully measuring the distance towards the vibrating object it is possible to recover the soundwave that introduced the vibration. In the case of Julian Assange, some people assume this technique was used to record vibrations on windows, making sound recordings by security agencies possible.
Disregarding the question whether this happened or not, the technique is using high precision optical measuring techniques, making the topic difficult to explore at home.
However, in some cases it is even possible to gather acoustic information analyzing a video. The youtuber “Veritasium” visited Dr. Abe Davis, a Stanford PostDoc, to create a small presentation of this technique. The special part is that in this video no high quality camera or optical equipment is used. Instead all information is gathered by filming a cheap bluetooth speaker with a standard camera.
The basic principle behind all these techniques is sound waves vibrating objects. Luckily for all of us who want to try this and don’t want to spend too much time with reading and building, Steve Mould, another youtuber, created a very neat video in which he uses a speaker and a balloon with a small piece of mirror on it to show these vibrations. By pointing a laser on the piece of mirror and vibrating the ballon via the speaker, the laser gets deflected creating beautiful patterns on the wall.
Homepage of Abe Davis:
original video of Brian Mackenwells, inspiring Steve Mould:
Project of building a laser microphone by Mark Chounlakone, Julian Alverio in collaboration with Justin Tunis:
wikipedia; Laser microphones