Abstract— This document illustrates the fundamental physics working principle, construction of Microphone Sensors those are extensively used in the different stages of automotive manufacturing industry and also for the in vehicle applications. In this paper the all necessary information regarding Microphone sensor, like signal conditioning, hardware interfacing and Array signal processing will be explained, which are required to complete understanding of its application.

Index Terms— Wave equation, Condensers, MEMS Microphone, Automotive Audio Bus (A2B), Array Signal Processing, Electrical and Acoustic Parameters, Beamforming

I. INTRODUCTION

Sound is a universal, ever-present and wonderful component of our environment [1]. Sound is used as source of situation awareness of what is happening around. Various parameters of the automotive environment can be inferred from sound In recent years, applications in the automotive sector that utilize “sound” as input have become more and more common. Nowadays, Microphone-dependent technologies have expanded to offer more than just convenience and safety for top-of-the-range vehicles. Even in the mid-class, drivers value the use of hands-free microphones and the regulated noise dependent volume controls of car stereos and entertainment units because these technologies actively reduce the chance of accidents. Voice recognition enables users to command and control various functions without taking their hands off the steering wheel [2].

II. PHYSICS OF SOUND

It is necessary to know about relevant physics and acoustic parameter to sound to understand the measurement and information processing techniques of the Microphone sensor. Acoustics is the science of sound and deals with the origin of sound and its propagation [3]. The main areas of acoustics are

 Airborne sound

 Underwater sound

 Structure-borne sound.

A. Airborne sound

Airborne sound is precisely measured by the Microphone sensors and is used as input by the various automotive applications.

Airborne Sound: (A) Sound is a wave phenomenon by which energy is transmitted through a medium via vibration of the medium and pressure (or stress) fluctuations within it [1]. (B) Sound consists of very small, rapid fluctuations in pressure relative to the atmospheric pressure [1].

Fig. 1. Sound Waves (Wavelength-Frequency Relation) [4]

A. Basic Terminology and Parameters of Sound

The characteristics of a sound wave are described by a pressure oscillation of a pure tone. A “pure tone” is a sinusoidal pressure wave of a specific frequency and amplitude, propagating at a velocity determined by the temperature and pressure of the medium (air) [5].

• dB Scale:  Sound levels are quantified by log-based scale, i.e. Decibel (dB) scale. It used to measure the sound power level, sound pressure level, and sound intensity level relative to some 0 dB reference.
• Pressure of sound (p(t)): It is the difference between instantaneous pressure of sound and reference (atmospheric) pressure.

The sound pressure    p(t) is also called acoustic pressure. It is scalar quantity and measured by

Microphone sensor. It has dimension of [Nm^-2] or pascals [6].

I. Microphone Sensor: Introduction and classification

1. A. Brief History and Definitions

The first electrical (carbon) microphone was designed by David Hughes in 1876 for use in the telephone The condenser microphone was invented in 1916 by Christopher Wente and was greatly improved by him in 1922 Wente’s design principle still forms the basis for the majority of microphones used for scientific measurement [1].

Microphone: Microphone is a sensor for capturing the small changes in air pressure we call sound and converting them into an electrical signal [9].

Ideal Microphone: Ideal Microphone that can convert the waveform of sound pressure to an electrical signal having the identical waveform. [8]

Practical Microphone: Practical Microphone will feature Nonlinearities and Self-noise. [8]

Microphones are electro-acoustic devices having a diaphragm or moving surface that is excited by the acoustical wave. It converts acoustic pressure waves into electrical signals that are proportional to one of the sound field quantities, i.e., sound pressure or sound velocity. Analog signal conditioning and digital signal processing techniques are used accordingly to achieve best possible output.

B. Classification

  There are two major approaches to sort the Microphone sensors on technical criteria i.e., Electro-acoustical Conversion principle and Receiver principle

Electro-acoustical Conversion principle refers to the physical nature of the transducing element to convert the sound waves into the electrical signal. The different types of microphones based on transduction are:

1. Carbon Microphones,
2. Condenser Microphones,
3. Electro-dynamic

   iV. Piezoelectric Microphones

Receiver/Pickup principle based classification corresponds to the exposure of microphone diaphragm to the sound Pressure or the direction or angle from which the sound arrives. This property of Microphone is called Directivity or Directionality. Directional properties of the Microphone are described by Polar response patterns/ Directivity plots. The graphically depiction shows the variation in sensitivity 360 degrees around the microphone, assuming that the microphone is in the center and that 0 degrees represents the front of the microphone.  The three basic directional types of microphones are omni-directional, unidirectional, and bidirectional [10].

• Omni-directional Microphone:It has equal output or sensitivity at all angles. Its coverage angle is a full 360 degrees. An omnidirectional microphone will pick up the maximum amount of ambient sound [10].

• Unidirectional Microphone:It is most sensitive to sound arriving from one particular direction and is less sensitive at other directions. It is also called Cardioid Microphone. It has the most sensitivity at 0 degrees (on-axis) and is least sensitive at 180 degrees (off-axis). Its effective coverage or pickup angle about 130 degrees that is up to about 65 degrees off axis at the front of the microphone.  It picks up only about one-third as much ambient sound as an omnidirectional Microphone [10].

Cross Section of Condenser Microphone [12]

[Complete Document is attached as PDF File.

Attachments:

Microphone Sensors for In-Vehicle Applications V.2.pdf