Introduction: Harvesting Sound Energy From Passing Cars

About: I am a University of South Carolina Mechanical Engineering Graduate Student doing ultrasonic wave propagation research in the field of structural heath monitoring. I happen to enjoy making things in my spare t…

There is energy everywhere around us and in many different forms. Many devices have been developed to harvest light, wind, waves, and more. One unusual place of energy harvesting is from passing cars. As cars pass by some of their energy is released in form of sound. Even though the overall energy maybe small it can be harvested. In this Instructable I will show how to apply the solution of Euler–Bernoulli beam theory to design a cantilever beam to oscillate at such a frequency to adsorb sound waves as well as converting its mechanical motion into electricity.

Step 1: Materials and Prices

The materials to make one or a similar device can vary based on what someone wants. The most important part is that the oscillator (described in next step) is cut to proper dimensions and firmly clamped to an object so that its vibrations will have little dampening (Less dampening = more power). A few of my components were salvaged from used metal plates and old circuit boards. I encourage testing out your own designs with varying parts. Anyway, this is what I used totaling $39.37:

  1. 20 pcs piezoelectric wafers..............................7.07 (need for number of oscillators made)
  2. 20 pcs MB6S full wave rectifiers......................6.43
  3. 10 pcs Printed circuit board (PCB)...................5.99
  4. Tripod................................................................Already owned (suggested but not needed)
  5. 4 pcs 1/4 x 2 inch bolt.......................................4 x 0.25 (estimated)
  6. 4 pcs 1/4 inch washer.......................................4 x 0.10 (estimated)
  7. 3 x 0.75 x 0.75 inch square rod.......................Already owned (Not needed)
  8. 2 pcs 3 x 2 x 0.25 inch plates..........................Already owned (Not necessarily needed)
  9. Hookup wires...................................................Already owned
  10. 2 pcs 10K ohm resistors..................................Already owned (Could use others)
  11. 2 pcs 4.7 micro F capacitors...........................Already owned (Could use others)
  12. 24 x 12 x 0.062 inch Nylon 6/6 sheet...............18.48
  13. 6061-T6 1mm thick Aluminum sheet................Already owned

One will also need a device to cut, drill, solder, glue and more. I suggest one watch/read this and recognize what tools are needed for what project that it may be applied to. For example, changing the 0.25 inch metal plates to 0.5 inch wood could potential work just as fine if metal working tools are not available.

Step 2: Creating the Oscillator

The oscillator is the fundamental piece on this device. However, it is nothing more than a cantilever beam (a protruding beam with one fixed end) with designed dimensions. This step explains the calculation of the dimensions.

To begin, sound not only can be seen within a time domain but also a frequency domain. When one records sound it is in a time domain, meaning it can be plotted on a time vs amplitude graph. To transform the audio sound from its time domain to the frequency domain a Fast Fourier Transform (FFT) was preformed. This was done in MATLAB software, but other free software is available. After obtaining the sound of passing cars using an audio recorder the signal (sound waveform) was transformed into the frequency domain (seen in FFT from passing vehicles). From that image we see that the the highest amplitudes lay within 0 to 75 Hz and 100 to 250 Hz. From this a beam can be designed where it's natural frequency would correspond with the peak power frequencies from passing cars.

For experimentation I choose to make 3 oscillators: an aluminum 6061-T6 dual 8 inch that operates at 20.55 Hz, a Nylon 6/6 dual 8 inch at 8.12 Hz, and a Nylon 6/6 2 inch and 3 inch at operates at 129.99 and 57.77 Hz respectively. Note: these are just the first mode resonance peaks. This was calculated using the equation in the "1mm thick natural vibrations of various materials" image. The equation is the derived expression for a rectangular cross-sectional beam from Euler–Bernoulli beam theory. C is the vibration mode constant (multiple modes exist) for the first vibration mode. Also in the image are length vs frequency plots of 1 mm thick Nylon 6/6, 6061-T6 aluminum, and 304 stainless steel. The two metals (aluminum and stainless steel) behave similarly. Nylon, because of its lower elastic modulus has a much lower natural frequency trend.

These 3 oscillators will be tested to see how each will perform in laboratory and field testing.

Step 3: Device Hardware and Assembly

Each hardware component with it's dimensions are shown if the first picture. Based on your desired operating frequencies will determine the length and material. For my oscillators I choose 6061-T6 aluminum and Nylon 6/6. For one of my oscillators (a second nylon) the dimensions of the lengths are 3.5 inches and 2.5 inches instead of the 8.5 inches in the dimensions figure. The oscillator in the pictures was not used for testing. Regardless, it is assembled in the same way.

Assembly is straight forward and easy. The series of pictures shows how the components are assembled. Before the piezoelectric sensors are bonded on the oscillator, a test fit should first done. Then a pencil will be used to mark the point where the top of the base plate meets the oscillator. Next they device will be dissembled. Bond the piezoelectric sensors just above this line to avoid them from being crushed. Once they are glued reassemble the device. Wires now can be soldered to the wafers. Insure a tight clamp on the oscillator by securely tightening the nuts.

Step 4: Electric Components (Power Generating)

The electrical system consists of the oscillator, resistor, and capacitor (in image). The oscillator will act as the voltage supply, however the voltage produced is alternating. To convert the voltage to DC a MB6S full wave rectifier will be used. The effects of the rectifier can be seen in the second image where the signal was measured after a flick was applied to the oscillator.

The resistor and capacitor were added to measure the system. The system follows the equation above (where τ = R*C) assuming the voltage in (Vin) is a constant power supply. However the voltage in will be sporadic but there can be a somewhat close estimation of the actual voltage produced. Using the equation above and inputting Vin = 1 v, C = 4.7 μF, R = 10 kΩ results in a near full charge within a half of a second. Realistically, since the voltage in varies, the time it takes to reach that value will take longer, however the voltage gain will eventually level out as it reaches the input voltage.

Step 5: Electric Components (Voltage Recording)

Early testing using a voltmeter seemed to drain the capacitors' voltage. To decrease the drain and to better record the data an Arduino nano along with a SD card will be used. Before each test the capacitors are drained using the two jumper wires seen in the image above (white wires left of SD modular). The code is attached as well as a simple wiring schematic. This device was only used in field testing.

Step 6: Testing and Results

Before I took the device to the road for experimentation I decided to preform laboratory testing first to see where the resonance frequencies were and the normalized voltage amplitudes between the oscillators. To record the data, I borrowed an oscilloscope after a few failed attempts with the Arduino device (too low of voltage to be detectable at a too low sampling rate).

Design parameters for the test follows; Each oscillator was exposed to a sub woofer as it played a frequency sweep from 1 to 200 Hz over a duration of 20 seconds. The voltage from the piezoelectric wafers was measured at a sampling rate of 40 kHz from the wafers themselves and not the capacitors. This was to see not the stored voltage but the voltage being actively produced.

The website for generating the audio frequency sweep:

Reviewing the data the true resonance peaks can be measured (from frequency plots), seen in the table. The aluminum provided the most accurate results, however all the nylon resonance peaks varied in error from 10.78 to 20.32%. This is likely due to a wrong value of Young's modulus since the raw material did not come with a data sheet and the value was guessed from the lower range of Nylon property values.

The aluminum oscillator's second mode, with a C=4.694^2 and a resulting peak of 126 Hz, had significantly higher amplitude.

Since the output voltage to the sub woofer was unknown but remained constant throughout the testing, the signals can be compared to see which oscillator produced the most power. The aluminum oscillator, even though it was it's second resonance mode, produced significantly more power varying from 60% to 70% more from its nylon counterparts. All the nylon oscillators produced nearly the same power, regardless of their resonance frequencies. The nylon producing lower voltage could be due to their higher damping.

After the data was gathered and analyzed the device was then prepared for field testing.

The device was taken to the same street and location where the audio waveform FFT was taken from. Before each experiment the capacitors were shorted out to discharge their voltage. Voltage was recorded every second for 10 minutes for each oscillator. The time of testing occurred at 10:32 am till 11:11 am under sunny conditions with sporadic wind gusts of 0 - 5 mph. After the data was collected the voltage was then plotted against time.

Analysis of the Energy harvesting field experiment results seem to reveal the mild gusts of wind, that accelerated the voltage increase, especially on the 8 inch nylon oscillator (which was the most prone to such effects). These voltage increases are indicated with the highlighted circles in the images. From the charging capacitor equation, the slope will approach 0 when maximum voltage is obtained. Therefor a power of two polynomial tread lines are added and extrapolated past the recorded data to have a somewhat rough estimate of the input voltage. These numbers and tread lines are shown in another image. The 2 inch and 3 inch nylon oscillators did not have a conclusive tread line. Where the slope turns to 0 are indicated and corresponding voltages recorded. The 8 inch nylon left oscillator preformed the best with an estimated input voltage of 0.275 volts, most likely due to energy gathered from wind. Oddly enough the right 8 inch nylon oscillator performed much worse with an estimated input voltage of 0.16 volts. The aluminum oscillators performed consistently with an estimated input voltage of 0.225 volts.

Step 7: Conclusions

In the laboratory testing the various oscillators were examined to see the accuracy of the calculations and to compare the voltages. The aluminum oscillators produced significant more power on their second vibration mode than any of the nylon oscillators. Also the aluminum oscillators suffered little error while the nylon oscillators error ranged from 10-21% possibly due to a wrong property value for Young's modulus (E).

Once in the field, the 8 inch Nylon oscillator had an maximum input voltage of all of roughly 0.275 volts of power harvested from the sound of passing cars. However the energy produced was certainly effected by a 0 - 5 mph breeze which produced significant jumps in input voltage of the 8 inch Nylon oscillators. The aluminum and 2 inch and 3 inch Nylon oscillators seemed to be less resistant to the effects of wind. The most consistent results was from the aluminum oscillators which both reached an estimated input voltage of roughly 0.225 volts. The 2 and 3 inch Nylon oscillators were inconclusive on the precise input voltage as their tread lines failed to reach an extrapolated peak.

Improvements could be made to optimize the vibration modes to better match the energy available since higher frequencies contain more energy. Also more metals should be explored since they have less damping.

From the experiments it is clear that harvesting wind with this device was more effective. If the oscillators were lengthened, significantly more power could be harvested from a mild breeze. However, the point of this project is to not only create a nifty device but to also open the mind to abstract ideas. Even though sound harvesting of cars might not be a useful way to produce power doesn't mean that the concept is completely useless. The fact that the device is actively removing sound energy means that for high traffic areas such as highways, prone to noise pollution, could benefit from a similar device. If sound is being turned into mechanical motion the volume could potentially be decreased in such an environment.

Step 8: Update: Raw Voltage Produced

Due to questions and concerns regarding the voltage in, I went out and collected the raw signals from the wafers themselves using an oscilloscope. Sure enough the wind produced the highest amplitudes seen in the long nylon figure (8 inch oscillator). However, no passing vehicles could be seen in that oscillator's signal. Passing cars can be seen in the other figures. It appears that the aluminum preformed the best.

Hopefully this resolves a few questions in the comment section.

Explore Science Contest 2017

Second Prize in the
Explore Science Contest 2017

Invention Challenge 2017

Runner Up in the
Invention Challenge 2017