ASA Lay Language Papers
167th Acoustical Society of America Meeting

Power from Sound; It’s Not Just Noise

Professor Kenneth A. Cunefare –
The Georgia Institute of Technology
School of Mechanical Engineering
Atlanta, GA  30332-0405

Popular version of paper 3pID2
Presented Wednesday afternoon, May 7, 2014
167th ASA Meeting, Providence


Energy harvesting – it’s a term that has been receiving a lot of press over the past decade. As understood by those performing research and developing products that employ energy harvesting, the term refers to the extraction of some level of useful electrical power from “ambient” or wasted energy sources. The energy sources include vibrations, light, heat gradients, and acoustic sound fields. The acoustic noise inside of hydraulic systems and in jet engines has been targeted by researchers at Georgia Tech and the University of Florida for development of energy harvesting technologies. A Georgia Tech team has recently built prototypes capable of harvesting enough energy from the noise in hydraulic systems to power a broad range of sensing or communication devices, while a team from the University of Florida has developed devices to use the noise in jet engines to power devices to actively reduce the noise.

For most energy harvesting applications to date, the extracted power is typically intended for use in some low-power-level sensing and communication device, such as sensors embedded within bridge structures and powered by the vibrations caused by passing vehicles. The benefit of energy harvesting is that it may enable sensing in environments where wires are not practical or are too costly, and where battery replacement would be equally impractical. Imagine a thousand sensors for monitoring structural integrity embedded in the concrete used to build a bridge; the cost and complexity of wiring them all, or of maintaining them all with batteries over the life of the bridge would be prohibitive; but, if the sensors were all powered by vibratory energy harvesters, and communicated via a wireless network, then the cost of installation is just that of the device itself, and there is no long-term cost for wiring or battery maintenance.

Hydraulics systems, which are commonplace on manufacturing and construction equipment, have high levels of noise within them, noise which is commonly viewed as an annoyance or a detriment to system performance. However, this noise also represents a high-intensity power source for energy harvesting. In a hydraulic system, an energy harvesting technology might be integrated with health-monitoring sensors; energy harvesting technology could be used to power sensors embedded within components and devices where it is impractical or impossible to run wires, such as directly within high-speed rotating internal parts.

Researchers at the Georgia Institute of Technology have developed Hydraulic Pressure Energy Harvesters (HPEH) to generate electric power from the noise in hydraulic systems. A complete HPEH-powered wireless sensing node, schematically depicted in Figure 1, would use the energy produced by the HPEH components to power sensors and communications. The HPEH serves as the “battery” within the sensor node.

Multiple generations of prototype devices, depicted in Figure 2 have been developed and tested. Individual prototypes have generated from 3.3 mW to 150 mW of power. These power outputs are sufficient for a broad range of sensing and communication applications, demonstrating that the underlying HPEH concept is flexible and viable. A prototype wireless temperature sensor has demonstrated the viability of the concept, where the sensor and its communication circuit were completely powered by electricity generated from noise in a hydraulic rig.

Figure 1. Schematic of HPEH-powered wireless sensor node.

Figure 2. Prototype HPEH devices. Power generation capabilities from 150 mW to 3.3 mW.

Another successful application of energy harvesting from acoustic noise is the sound field that exists within jet aircraft engines. Such sound fields may be 160 dB or more, with intensities on the order of a kilowatt/m2 as in hydraulic systems. With that level of sound field, it becomes feasible to extract useful power levels using a resonator-based device to further concentrate the acoustic energy. Researchers at the University of Florida have pursued this concept with NASA. The application of the concept would be for powering noise control devices within the jet engine itself, in what the researchers call an active liner, though the harvested power could also be used for sensing.

But what makes these two energy harvesting applications, the HPEH device and the active liner, feasible? In air, typically encountered sound fields will have intensities (power per unit area) that range from microwatt/m2 to milliwatt/m2. In a hydraulic system, as well as in a jet engine, the intensity of the noise can be kilowatts/m2. In other words, it’s the much higher available ambient energy in the noise that makes the application feasible.

The technologies that have been developed to extract energy from acoustic fields, typically use piezoelectric materials, though magnetic induction generators have also been proposed. Piezoelectric materials produce electrical power when a force is applied to them. Piezoelectric films, crystals and nanowires have been used as the key energy conversion component within energy harvesters.

What makes a successful energy harvester application? One where there is enough energy in the target ambient field so that useful power can be obtained, and where the power needed is matched to that harvested. While acoustic sound fields are an ambient energy source that might be considered as an opportunity for energy harvesting, it is one where the available energy, with few exceptions, typically is quite low as compared to other sources. The exceptions to this challenge to date have been exploited through the harvester devices developed for hydraulics and jet engines.

The challenge posed by the available energy in “typical” acoustic field might be appreciated by considering the power conveyed by human speech. Typically, the power in conversational speech is on the order of 0.001 microwatts (μW). If 100% of that energy could be captured, it would take the conversational power of 60 billion people to light a single 60 watt light bulb. That’s not too practical (not to mention it would be a very crowded earth). What about sounds louder than conversational level? What if there was a sound with a noise level 100 dB, which is a level which would prevent speech, and which would lead to hearing damage with long enough exposure; this 100 dB sound field would yield 0.01 watts per square meter; to capture enough power for a 60 watt bulb would require at least 6000 square meters (64,583 ft2) of harvester area; again, not too practical.

But what if you only need microwatts, or milliwatts? Such power levels aren’t useful for lighting, heating, or even for powering cell phones, but they can be used for sensors and short-range wireless communications. But even with this low of a power demand, most acoustic fields that we encounter in our everyday lives have too low of an energy density for any practical application at the present time.

The noise in hydraulic systems and in jet engine nacelles are examples where the available power makes the harvesting of useful electrical power feasible and with immediate technologically relevant applications. While research and development is pursuing extraction of energy from lower-level noise fields, the inherently much lower energy density of those sound fields yields much lower power outputs; not uncommonly on the order of microwatts and nanowatts. Making use of such low levels of power will require development of ultra-low-power sensing and communications devices, as well as identifying applications where the obtained low power levels enable a solution to a technological need.