Moulton Lectures
On
Electro-Acoustics
|
Lecture
30 An Introduction To Microphones |
Presented
by:
Dave
L Moulton
Location:
Racal Acoustics Limited
Harrow UK
Date: 19-May-2004
Content
In this lecture I will give a very simplistic
introduction to microphones, basically describing what they are and the
fundamentals on how they work. In later lectures I will go into more detail on
the operation of specific types of microphone. For the purposes of this lecture
I aim to cover the following:
·
Definition of a Microphone.
·
Bi directional and Omni directional Characteristics.
·
Definition of Sensitivity and Frequency Response.
·
Typical response Profiles.
On With The Lecture
What is a Microphone?
A microphone is an electro-acoustic transducer which
converts acoustic pressure fluctuations into an equivalent correlated
electrical output fluctuation.
A simple block diagram is shown in figure 30.1 below:
Figure
30.1
The acoustic input is generally represented as a pressure wave. In audio acoustics these waves can usually be described as either Planar or Spherical. There are of course other inputs that can be described as derivatives of the spherical wave, such as cylindrical waves. Figure 30.2 shows a planar pressure wave input.
Figure 30.2
Now let us have a look at the transformation process.
We already know from previous lectures that when an acoustic pressure wave is incident on a surface, it will exert a force on that surface. If the surface is not rigidly fixed it will move in sympathy with the pressure fluctuations in the wave. Thus, we have an acoustic to mechanical transformation taking place.
Figure 30.3
If we now consider that our pressure wave is acting on a diaphragm and the diaphragm is attached to a device which converts mechanical movement into an electrical output, then we have the basis for a microphone. A typical example would be the rocking armature transducer described in lecture 28. In this device the diaphragm is attached to a balanced armature which controls the magnetic flux density across an air gap. If the flux density across the gap is able to vary, then the flux density through each of the coils will also vary, resulting in an electrical voltage being generated across the input terminals of the transducer. This is a typical example of a transducer which exhibits the property of reciprocity, thus it can act either as an earphone or a microphone.
Figure 30.4
The same principal applies to the Central Armature
transducer described in lecture 29.
Clearly the process of converting the mechanical
action into an electrical output will require another transformer. Thus the
rocking armature microphone system can be described as:
Figure
30.5
Note: there are two transformers needed to go between
the acoustic domain to the electrical domain. The mechanical domain is the bit
that links the two together.
In the case of the rocking armature transducer I have
shown the output transformation to be ‘G’. I have purposely not put in
the actual transformation ratio because it means changing from an impedance
equivalent system to a mobility equivalent system. I do not wish to get into
this type of detail in this lecture. But to give you some idea of the
Mechanical to electrical conversion for the rocking armature, one must consider
the relations ship:
Where F = Force, B = Magnetic Flux Density, I = Induced
Current and l represents the coil length.
Note: F is mechanical
and represents an ‘across variable’ whereas I is electrical and
represents a through variable. This relationship between a through variable and
an across variable represents the conversion of an equivalent circuit expressed
in an impedance format to one expressed in a mobility format. Still, that’s
enough said about that for now. I will cover this in more detail in a
subsequent lecture.
Microphone Schematic Symbol
The most common schematic representations of a microphone symbol are:
Figure
30.6
Passive Microphones
Microphones that are Non Polarity conscious, tend to be also known as passive microphones. This means that they require no external power in order to operate. Examples of passive microphones are:
Rocking Armature, Central Armature, Moving Coil and
Piezo.
Active Microphones
Other microphones are polarity conscious and require
an external power source in order to work. These microphones generally have
some form of electronic amplification circuitry integrated within the
construction. A typical example of this type of microphone is the electret
(also known as a condenser microphone).
An exception to this rule is the carbon granule microphone.
This microphone has no additional circuitry to provide amplification, and is
not polarity conscious, but does require external power in the form of a DC
current for it to work.
Microphone Classifications
Microphones can generally be described in one or a combination of the following Classes:
Pressure Microphones
A pressure microphone is a microphone in which its electrical output response is a function of the variations in the acoustic pressure of an incident sound wave.
Velocity Microphones
A velocity microphone is a microphone in which its electrical output response is a function of the particle velocity resulting from an incident acoustic wave.
Omni-Directional (Non-Directional Microphones)
Omni-directional microphones exhibit the same sensitivity when subject to the constant acoustic sound pressure from point source located at any spherical coordinate relative to the microphone. The microphone being located at the centre of the coordinate system.
Let me give a simple explanation.
First we locate the microphone at the centre of a room and then map out a perfect circle at a fixed radius from the microphone.
Next we locate anywhere on the circumference of the circle an acoustic sound source, the source must be able to provide a constant sound pressure output at a given frequency.
Now we move the sound source 360 degrees around the circle, ensuring that it always points in a direction along the radius towards the microphone. At the same time we plot the microphone electrical output sensitivity for each position of the sound source on the circumference.
The result is a plot of sensitivity versus sound source position angle. This plot is commonly known as a polar plot of the microphone. For an Omni-directional microphone this plot will be a complete circle. The radius of the circle represents the microphone sensitivity for a given angle of incident sound.
Figure 30.7
Bi-Directional Microphone (Directional Microphone)
Bi-Directional Microphones have an output sensitivity that is a function of the location of the sound source. Typical examples of directional microphones are those types that have the front and back of the diaphragm exposed to the incident sound wave. These microphones are commonly known as differential or pressure gradient microphones.
Note: Pressure microphones tend to have only the front face of the diaphragm exposed to the incident sound wave.
An example of a Bi-Directional polar plot is shown in figure 30.8 below:
Figure 30.8
Note: The polar response shows that the microphone is more sensitive to incident sound pressures on its front and back faces than on the side faces. This shows that the Bi-directional microphone exhibits directionality.
The outputs from an Omni and Bi directional microphone can be combined to give what is known as a cardioid Polar Response, thus:
Figure 30.9
From a constructional point of view the difference between a directional and none directional microphone is shown in figure 30,10 below.
Figure 30.10
Microphone Sensitivity and Frequency
Response.
Microphones are usually characterized by the following:
1. Sensitivity.
2. Frequency Shape (20Hz to 20kHz).
3. Polar Response (For a range of Frequencies in the band 20Hz to 20kHz).
4. Total Harmonic Distortion.
5. Acoustic Noise Discrimination (Pressure gradient Microphone).
6. Impedance .
Items 1 and 2 can be combined in a single profile known as the frequency response of the microphone. The frequency response is a plot of sensitivity versus frequency.
Sensitivity
The sensitivity is defined as the decibel ratio of output voltage relative to 1V for a constant acoustic sound pressure level.
Normally the sound pressure level is maintained at 1 Pascal (94dBSPL) at each of the measurement frequencies in the audio band. Thus the microphone sensitivity is expressed in terms of Decibels relative to One Volt per Pascal. For example the sensitivity of a rocking armature microphone at 1kHz could be typically described as:
-56dBre1V/Pa
Which equates to an output voltage of 1.6mVrms per Pascal
Where:
Passive microphones tend to have very low sensitivities ranging from tens of microvolts per Pascal to a couple of milli-volts per Pascal. Thus passive microphones tend to have sensitivities at 1kHz in the range:
-100dBre1V/Pa
(10μVrms) to –50dBre1V/Pa (3mVrms)
Active microphones such as Electret and Carbon types, tend to have much higher sensitivities than the Passive versions. Typical sensitivities at 1kHz are:
-50dBre1V/Pa (3mVrms) to 0dBre1V/Pa (1Vrms)
Frequency Response
The microphone frequency response can be measured by making use of an artificial voice simulator (Typically a B&K 4227 Voice) and an audio analyser (Typically a B&K 2012).
Calibration
Before any measurements are taken it is first necessary to calibrate the analyser using a high precision condenser microphone and a pre calibrated 1 Pascal acoustic sound source. The idea is to use the sound source to set an acoustic reference for the analyser.
Figure 30.11
Voice Correction
After the calibration at 1kHz it is then necessary to obtain a frequency response of the output of the artificial voice. The idea is to sweep the voice output over the audio band of interest, then to create a correction curve that the analyser can use to normalize the voice output to 1 Pascal at all frequencies in the sweep band.
Figure 30.12.
Figure 30.12 shows the Voice set up for a near field measurement. Thus the precision microphone is located 10mm away from the voice lip ring. The analyser can now be set up to sweep the acoustic output from the artificial voice and apply the correction curve to represent a constant 1 Pascal sound pressure at 10mm from the voice lip ring.
For a far field set up, the artificial voice is replaced by a loud speaker and the reference distance would be 1m instead of 10mm.
Microphone Frequency Response Set Up
The microphone unit to be measured can be placed at 10mm from the artificial voice lip ring, ensuring that the centre of the microphone is coaxial with the centre of the artificial voice.
Figure 30.13
Microphone Frequency Response Graph
Figure 30.13 below shows the typical frequency response profile for a Racal Acoustics Rocking Armature Microphone (Type 27160). The microphone has been measured without any load impedance put across its output terminals, we normally call this an open circuit response. Note, the frequency response shape for a Rocking armature transducer used as a microphone is completely different to when it is used as an earphone (Ref Lecture 28). The reason for this will be explained in a later lecture.
Figure 30.14
End of Lecture