Abstract:
Microphone Array Processing technologies are playing a vital role in every field of daily life by enhancing voice quality from the smart phone, inside the modern vehicles, conference audio systems and also in the concert halls. The visualization of sound field and identification of noise sources is also carried out in the various industrial manufacturing processes and building architectural design. microphone sensor array that is comprised of multiple microphones shows significant advantages as compared to the use of a single microphone. This article focuses on the essentials fundamentals of sensor array theory, microphone array signal processing and presents an innovative design idea of acoustic measurement system.
1. Sensor Array System
A Sensor array system is composed of multiple sensors, arranged in a spatial distribution manner and it translates the physical quantities (i.e., sound, vibration) in a combined electrical output that can be represented by a sequence of numbers in the digital form.
FIG1 .Microphone Array for Architecture Acoustics
A sensor array system utilizes either Frequency or Temporal filtering with an additional domain, i.e. Space, to perform spatial filtering of signals. In other words, a sensor array system samples propagating wave fields of physical variable in both domains i.e. time and space.
The response of the sensor array system is subject to post processing and analysis by involving the digital signal processing algorithms related
- Source separation and localization
- Direction finding
- Estimation of informative parameters
- Suppression of Noise, interference, reverberation, and echo
These Sensor Arrays are used in much different area like;
- Radar and Sonar for detection, target localization, anti-jamming
- Communications
- NVH instrumentation
FIG. 2. Microphone Array for Automotive Acoustics
2. Sensor Array Signal Processing
Array Signal Processing includes such statistical and adaptive signal processing techniques; those are applied to extract the information of interest by processing those signals which propagate in space through a medium and their wave-fronts are acquired by a sensor array. The most popular application areas are
- Estimation of the Direction of Arrival (DOA) of Source Signal at Array
- Beamforming, i.e. spatial filtering: A process to estimate input data by combining the individual signals of all the sensors with multiplying factor, i.e. weighted coefficients, in a linear manner.
2. 1. Basic Concepts and Classification
The received signals are processed by the Sensor Arrays in the space-time domain. A brief review related to signal is useful to understand the Array Signal Processing
- Signals:
Signal:
A signal is a set of information, which is comprised of variations of physical variable at any instant or position.
Time-Domain Signal:
A signal which comprised of temporal variations at a given position, or signal strength i.e., Amplitude is a function of time. It is written as s (t)
Spatial-Domain Signal:
A signal which comprised of spatial variations at a given time, or signal strength i.e., Amplitude is a function of position. It is written as s 
.
Space-Time Signal:
A signal that is comprised of both position and time based information is called Space-Time or Spatio-Temporal signal. It is written as s (x,t) or in case of 3D as s (P,t).
Sensors Arrays are classified in many ways to analyze and synthesize according to the application.
- Array Geometry
The arrangement of Sensors in the Array is also lead another classification as
- Linear (1D): Sensors are fixed on a straight line in one dimension
- Planer (2D): Sensors are fixed on a plane in two dimensions (Rectangular, Square or Circular aperture)
- Volumetric (3D): Sensors are place in a volumetric body in three dimensions
The linear array is further characterized according to the intra-spacing of sensors
- Uniform spacing
- Non-uniform spacing
- Random spacing
- Sensing
The sensing elements of any kind of Array capture the signal either in Passive or in the Active way
- Passive Array: sensors only collect data to extract the information from the incoming signals.
- Active Array: sensors emit signals to gather information from the returned echoes.
- Placement
The Sensor Arrays are termed on basis of their placement w.r.t to direction of input signal as
- Broadside Arrays: All sensors of Array are placed perpendicular to the direction of input.
- End-fire Arrays: All sensors of Array are placed in line with the direction of input.
- Type and Location of Signal
The Sensor Arrays are characterized according to the distance between Array and source.
- Far-Field Array
A Sensor Array placed such distance from a simple big source, which is non-estimable, large and also much bigger than the dimensions of Array, is called Far-Field Array.
The input signal arrives at the Far-Field Array in form of
- Spherical wave-fronts (mid-range distance)
- Planer wave-fronts ( far enough distance)
- Near-Field Array
A Sensor Array placed such distance from a distributed small source, which is estimable, small and also comparable to the dimensions of Array, is called Near-Field Array.
The input signal arrives at the Array in form of complex wave-front.
- Important Definition
- Uniform Linear Array:
An Array of sensors that is spatially arranged in a straight line and with equal spacing between the sensors element
- Aperture of Array:
The spatial region, i.e. Length, Area or Volume, which can collect the incoming signal is called Aperture of Array.
- Spatial Filter:
A mathematical tool that used to enhance the signals associated with certain locations; by using a sensor array signal processing techniques.
- Element Pattern:
The spatial response of the individual array element is called the element pattern.
- Beam Pattern:
The total response of an array is called Beam Pattern that is the function of
- Array geometry
- Radiation pattern of individual array element