The idea of this chapter is to give an overview of a relatively new technology – that of using ultrasound to transmit data at short ranges, within a room say. The advances that have made this a useful technology include the ability to utilize a sufficiently wide bandwidth, and the availability of instrumentation that can send and receive ultrasonic signals in air. The chapter describes this instrumentation, and also covers the various aspects of ultrasonic propagation that need to be discussed, such as attenuation, spatial characteristics, and the most suitable forms of modulation. Provided such details are considered carefully, it is demonstrated that ultrasonic systems are a practical possibility for in-room communications.
TopBackground To Ultrasound
Sound is generated by the oscillation of particles. For it to propagate, it needs a medium, and that could be solids, gas or liquids. Sound waves are periodic, which can be described by the term frequency, which can be expressed in the following form:
, (1) where c donates the speed of sound, and λ is the wavelength.
The audible range of sound for various animal species is different. Humans can hear sound within the range 20 Hz – 20 kHz. For this reason, sound waves with frequencies above 20 kHz are classified as ultrasound, whereas bats are capable of detecting much higher frequencies. In gases, and the majority of liquids, ultrasound propagate as longitudinal waves, where the displacement of particle is parallel to the direction of travel. The propagation velocity of ultrasound varies in different media, as it is controlled by the density and the elasticity of the medium. The velocity of longitudinal waves in gases and liquids is given by
(2) where
Ka is the adiabatic bulk modulus,
ρ is the mean density of medium. The sound velocity in air is approximately 331 ms
-1, although this varies with temperature and humidity. An approximate value for variations in temperature can be calculated using
,
(3) where
c0 is the speed of sound in air at atmospheric pressure at 273° K (331.0 ms
-1),
is the temperature coefficient (0.61 for air), and
d is the temperature difference from 273° K.
When ultrasonic waves pass though an interface between two materials at an oblique angle, both reflection and refraction take place, provided that the two materials differ in acoustic impedance Z, where Z = ρc in each medium in question. A large difference in the values of Z in two media A and B results in a large reflection coefficient, which for plane waves at normal incidence can be estimated from
.
(4)