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Acoustic Energy Harvesting

May 5, 2011

We live in a noisy world. This is most apparent if you, or your neighbors, have young children. Have psychologists determined why little girls scream so much? If we could harvest all that acoustical energy , perhaps we could put it to good use. Imagine a wireless baby monitor powered by the child's own screams.

The main problem here is the amount of energy that's actually available in ambient noise . In a previous article (Environmental Energy Harvesting, April 19, 2011), I referenced a report that acoustic noise at 75 dB contains only 3 nanowatts/cm² of harvestable energy.[1] This decibel level is about the sound level of a television played at normal volume. It's a little louder than normal conversation.

If your equipment is in an environment with more noise, you can get more power. In some noise environments, you wouldn't want to be anywhere near your equipment. Here are some noise examples, as listed in Wikipedia :

Noise Source	dB (20 μPa ref.)
Hearing damage for long-term exposure	85 dB
Traffic on a busy roadway at 10 meters	80 - 90 dB
Jackhammer at 1 meter (approx.)	100 dB
Jet engine at 100 meter	110 - 140 dB
Possible hearing damage	120 dB
Threshold of pain	130 dB
Jet engine at 30 meters	150 dB

Sound pressures are given in decibels, relative to a reference pressure of 20 μ pascal . So, how to you relate decibels to how much additional power you can obtain from increased sound level? The definition of a decibel (dB) in power calculations is

dB = 10 log₁₀(P/P_ref)

where P is the power and P_ref is the reference power. Acoustic scientists have set P_ref to a very low level, so the dB numbers encountered are positive, as in the above table. Audio and radio frequency engineers , however, have set it to a middle level of one milliwatt , so their dB values are most frequently negative.

What this equation means is that, independent of whatever reference level is chosen, a 10 dB increase corresponds to ten times more power. The early example, that 3 nanowatts/cm² of harvestable energy can be obtained at 75 dB, means that an increase in sound level to 105 dB (all other things being equal) gives you 3 microwatts/cm². A jump to 135 dB brings you into the milliwatt/cm² range.

A large part of the acoustic energy harvesting problem involves coupling the acoustic energy into your transducer . Resonant systems are always the most effective, and if your transducer has a resonance, it can be placed in a device called a Helmholtz resonator . This device, named after the famous physicist, Hermann von Helmholtz , allows harvesting more energy at that resonant frequency . The figure below shows the structure of a practical Helmholtz cylindrical resonator.

Helmholtz resonator. Figure 3A of US Patent No. 5565847.

Helmholtz resonator.

Figure 3A of
US Patent No. 5,565,847 (simplified).[2]

In this figure, ν₀ is the resonant frequency in Hz , S is the area of the aperture , l_e is the thickness of the aperture, C is the sound velocity , and V is the cylinder volume. The equation does not give precise values for all cylinders, but it's a good starting point.

Helmholtz resonators have been used by Mark Sheplak , a Professor of Mechanical and Aerospace Engineering at the University of Florida , and his research team.[3-5] The Florida team demonstrated the possibility of acoustic energy harvesting of several tens of milliwatts at a sound pressure level of 160 dB.

Where would you find such an extreme sound level? The intended application is an active noise reduction system for turbine engines in aircraft. The active systems would replace the present passive systems that aren't very effective over the wide spectrum of engine operating modes from takeoff, cruising and landing.

Engine of a Jet Airways Boeing 777-300ER.

Now, that's an engine!

Engine of a Jet Airways Boeing 777-300ER.

Photo by Cory Barnes, via Wikimedia Commons.

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