ModalMax is a very innovative means of harnessing the vibration of a piezoelectric actuator to produce an energy efficient low-profile device with high-bandwidth high-fidelity audio response. The piezoelectric audio device outperforms many commercially available speakers made using speaker cones. The piezoelectric device weighs substantially less (4 g) than the speaker cones which use magnets (10 g). ModalMax devices have extreme fabrication simplicity. The entire audio device is fabricated by lamination. The simplicity of the design lends itself to lower cost. The piezoelectric audio device can be used without its acoustic chambers and thereby resulting in a very low thickness of 0.023 in. (0.58 mm). The piezoelectric audio device can be completely encapsulated, which makes it very attractive for use in wet environments. Encapsulation does not significantly alter the audio response. Its small size (see Figure 1) is applicable to many consumer electronic products, such as pagers, portable radios, headphones, laptop computers, computer monitors, toys, and electronic games. The audio device can also be used in automobile or aircraft sound systems.
The result of using the ModalMax techniques is a piezoelectric audio device with the audio response shown in Figure 2. At 1 cm (sufficient distance for headphones use), the response is 93±5 dB for 600–5,000 Hz, (±4 dB for response greater than 1 kHz). The device has extremely good audio response (approximately 92±6 dB) in the range of approximately 2–20 kHz with very low voltage required (±8 V). The device has good sound definition at frequencies less than 1 kHz. The audio device output is very linear with applied voltage. The device impedance decreases with frequency (3,500 ohms at 100 Hz, 83 ohms at 5 kHz and 43 ohms at 10 kHz). ModalMax consists of four methods used to produce high-quality sound from a piezoelectric actuator.
- Mapping Vibration Topography is the first method used to enhance audio output of piezoelectric devices. During vibration, the cyclic surface deformation produces out-of-plane displacements, the reciprocating strokes of which are similar to a piston. Deformation topography that occurs during vibration is measured using a laser vibrometer. The topography is used to identify all out-of-plane displacement lines and points having amplitudes sufficient for driving acoustic devices.
- Tailoring Vibration Response is the second method used to enhance piezoelectric audio output. The center photo in Figure 1 shows a piezoelectric actuator that has been developed to have numerous natural frequencies with high out-of-plane displacement amplitudes. The device has a "T" planform (i.e., throat and crossbar). The throat has a low torsion and bending stiffness; yet, can sustain large-amplitude vibration without breaking. When one piezoceramic layer is used with an applied voltage of ±25 V, the first bending out-of-plane displacement at the edge has been measured to be 0.12 in. (3 mm), with the edge 1.0 in. (2.5 cm) away from the mounting line. The out-of-plane displacements for the 743 Hz and 426 Hz natural frequencies exceed 0.03 in. (0.76 mm). The displacement at the 977 Hz natural frequency can be seen with the naked eye.
- Tailoring Damping Distribution is the third method used and consists of strategically locating damping material on the piezoelectric device. The damping material makes the audio response quickly decay after a stimulus is removed. Eliminating persistent vibration reduces audio distortion. The complete audio response decays in approximately 3.7 ms.
- Applying Acoustic Chambers to one or more out-of-plane displacement lines identified from mapping is the fourth method of enhancing audio output. Locating multiple chambers on the piezoelectric-device surface makes it possible for a single actuator to drive numerous sound sources. Typical audio devices use a single driver (e.g., speaker cone driven by magnet) to produce a single sound source. Each acoustic chamber is formed as a cylinder with its bottom surface removed. The top surface has an orifice. When affixed to the surface of the piezoelectric actuator, a resonating chamber similar to a Helmholtz chamber is formed.
This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to the Patent Counsel, Langley Research Center; (757) 864-3521. Refer to LAR-15959.