i made this recording with a custom laser microphone. i used a very small amount of water in a bottle cap. i got the water to produce tones by sending feedback through it. click through to listen.
over the past few months, i have been developing a laser microphone as part of a larger project. the process has been painstaking, and much care has poured into apparently tiny details such as linearity and signal to noise ratio. that documentation will come in a later post. for now, let this recording serve as an example of why i'm interested in optical acoustic measurements at all. the fact that i can record vibrations in an object without making contact with that object is extremely important for the types of field recordings and instruments i'd like to make.
in this example, i've pointed the mic at the surface of a capful of water. clearly, a piezo-electric hydrophone would behave differently in this context. i'm sending feedback through the bottle cap by using a vidsonix transducer, which basically turns any object into a speaker, to vibrate a little plate that the cap is resting on. i was using this setup for my calibration / systems testing. the 'little plate' in this case is actually a precision mirror.
the apparent distortion is not clipping, nor is it poor linearity in the opto-electrical components, rather, it is the result of the nonlinear filtering performed by the surface of the water. i made a few overdubs, but applied no effects to any track. i didn't use any eq's or compression. the high frequency bands of noise are not present during typical operation of the mic, but i added a substantial amount of gain to a few of the overdubs because i had found some interesting low level signals. i believe most of it is thermal noise from the resistors. the sensor and laser driver circuit were still on breadboards during the time of this recording.
the sensor amplifier has been the most volatile element in the design process, having undergone eight major revisions. i will likely document findings from the specific topologies of the major revisions-- including their strengths and weaknesses-- in a later post. the topology being used in this recording is a differential transimpedance amplifier, followed by a unity-gain voltage amplifier. here's a schematic for the interested. i will likely analyze this circuit in a later post.