A number of famous orators, vocalists, and writers have earned the distinction of having a powerful voice. However, technically speaking, having a "powerful voice" is not really a distinction at all as sound waves from a human source transport energy, which can be used to generate power.
Energy from mechanical vibrations that result from sound waves is converted into sound energy. The rate at which this energy conversion occurs is the sound power. Given a threshold sound power (W0) of 10-12 W and observed sound power (W), Eqn. (1) can be used to calculate the sound power level, Lw, in decibels (dB) from a source. 
The intensity of sound from a source follows the inverse square law.  Sound intensity (I) is described by the sound power per unit area in units of watts per square meters (W/m2) and can be calculated using Eq. (2).  Assuming sound waves are sent out uniformly in all directions from a source, one can understand sound to be propagated in a spherical shapes; therefore, the area is taken to be the surface area of a sphere.
Sound intensity level (LI) describes how strongly an observer perceives a sound and can be calculated using Eqn. (3), assuming a reference intensity (I0) of 10-12 W/m2. 
Sound power and intensity are used to describe the "loudness" of sound produced from a source. Next, we will discuss an application that uses the principles of sound energy to produce usable power for applications in self-powered electronics.
Liu et al. set a precedent for sound energy harvesting by fabricating zinc-oxide (ZnO) nanowire-based nanogenerators that were excited with ultrasonic waves to produce a current.  ZnO nanowires, approximately 5 microns in length, were grown on gallium nitride/aluminum nitride (GaN/AlN) substrate.  A thin ZnO film was grown on the bottom of nanowires to serve as an electrode for the nanogenerator.  A platinum-silicon electrode was aligned at the top of the nanowires.  The device was packaged with water-proof polymer for testing in an aqueous environment. The nanogenerator was tested in an isolated water bath using a 41 kHz ultrasonic wave generator to excite the nanogenerator. The ultrasonic wave displaces the electrode with respect to the nanowires, building up strain in the nanowires, allowing for the conversion of mechanical energy into electricity.  They report a current of 500 nA and voltage of 10 mV with a surface area of 6 square millimeters.  They found that the current output originated from the nanogenerator as a result of the ultrasonic waves. 
Cha et al. report a similar approach in which arrays of piezoelectric ZnO nanowires were grown on a GaN thin film-deposited sapphire substrate, then topped with a palladium gold (PdAu)-coated polyethersulfone (PES) substrate.  This PES substrate served as an electrode and vibration plate. The vibration plate detected sound waves as they hit the top electrode of the device, causing the nanowires to compress and release. Upon compression, a negative piezoelectric potential is created, causing electrons to flow from the top electrode to the bottom electrode.  This results in the generation of a positive potential at the top electrode. This device produced an output voltage of 52 mV from 100 dB of noise. 
Assuming a source-receiver distance of 3 feet, the average human voice is 60 dB.  The aims of Cha et al. and similar work are reasonably feasible, but require advanced technology that can efficiently harvest as much of that human voice sound energy as possible in order to generate an appreciable amount of energy that can be used to power electronic devices and other applications.
© Ashley Seni. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 D. Davis and E. Patronis, Sound System Engineering, 3rd Ed. (Focal Press, 2006).
 J. Liu et al., "Toward High Output-Power Nanogenerator," Appl. Phys. Lett. 92, 173105 (2008).
 S. N. Cha et al., "Sound-Driven Piezoelectric Nanowire-Based Nanogenerators," Adv. Mater. 22, 4726 (2010).
 A. Karpf, The Human Voice: How This Extraordinary Instrument Reveals Essential Clues About Who We Are (Bloomsbury, 2006).