Acoustic Modems for Ubiquitous Computing

S ound has been largely ignored or dismissed in digital communications. Given that computer systems have been traditionally aimed at long-distance and high-bit-rate communications, this dismissal is understandable. The data rates for sound are relatively low compared to media such as radio, and the sounds can be annoying. Designers of most current digital communication systems assume applications need comminicating devices to exchange large amounts of information, and so they focus on maximizing the transmission bit rate. This is the case, for example, with transferring files over a local area network (LAN) or video over the Internet. As small, mobile computational devices become more pervasive, however, the need for short-range, localized communications is as urgent as the need for longdistance communications. Ubiquitous computing1 refines the very notion of computers and computation, envisioning a world in which small devices with embedded computational and communication capabilities interact with other devices and with humans for purposes other than traditional desktop work. Efforts such as IrDA, Bluetooth, and, more recently, impulse radio, or ultrawideband signaling2,3 target the short-range communications critical in ubicomp. Ubicomp also breaks the assumption that computing applications need high bandwidth. In fact, interconnected devices’ communication needs are diverse, ranging from very low to very high bit rates, and from short-range, localized messaging to long-range connections. Motivated by the specific characteristics of the aerial acoustic communication paradigm used by humans and other animals, we examine the use of device-to-device aerial acoustic communications in ubicomp applications. Unlike radio, sound is contained within walls, which can help solve interference problems at the physical layer. And, unlike infrared, sound doesn’t need line of sight and so can go around corners, which can be useful for moving or hidden devices. Unlike both media, sound can also expose the communication to humans when necessary, as well as some location aspects of the communication. Another reason to revisit acoustic modems 40 years after their invention is that CPUs are several orders of magnitude faster than they were in the 1960s. Thus, we can now implement, entirely in software, cheap acoustic modems that use ordinary microphones and speakers. The Digital Voices project explores device communications using audible sound.4 (The “Sound in Device Communications” sidebar gives an overview of the use of sound in device communications.) Within the project, we investigate bridges between human and computer commuSound offers features not available with other short-range, low bandwidth communication technologies, such as radio and infrared, enabling communication among small computing devices and humans in a ubiquitous computing environment.