Simcenter Testing Solutions Microphone Basics

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A microphone sensor measures instantaneous fluctuations (Figure 1) of the sound pressure in its surrounding environment.
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Figure 1: Left - Orange curve – instantaneous sound pressure fluctuations, Right – Microphone sensor

A microphone is typically an analog sensor whose signal is often converted to digital format (for example, using a Simcenter SCADAS hardware) for further analysis.

This article explains the basic operation of microphones, some guidance on how to select them for a given application, and how to use them with Simcenter SCADAS hardware/Simcenter Testlab software.

1. Microphone Components
   1.1 Capsule
   1.2 Preamplifier
    1.3 Connector
2.  Types of Microphones
   2.1 Precision Condenser Microphones
   2.2 Surface Microphones
   2.3 Probe Microphones
   2.4 Intensity Microphones
   2.5 MEMS Microphones
3. Polarized versus Prepolarized Microphones
   3.1 Advantages and Disadvantages
   3.2 Simcenter SCADAS
4. Microphone Frequency Response
   4.1 Engineering Microphone
   4.2 Studio Microphone
   4.3 Comparison of Engineering Microphone versus Studio Microphone
   4.4 Human Hearing and A-Weighting
5. Acoustic Fields and Response Types
   5.1 Free versus Diffuse Acoustic Field
   5.2 Free Field, Diffuse Field, Pressure Types
   5.3 Polar Pattern
6. Microphone Size: Frequency Range and Sensitivity
7. Calibration
8. Care and Handling

1. Microphone Components

There are several styles of microphones based on a moving element that detects pressure changes.  While other styles exist (ribbon, moving coil, etc), most microphones made today are condenser style microphones, which is the focus of this article.  

There are three main components to a condenser microphone set shown in Figure 2:
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Figure 2: Microphone consists of capsule, preamplifier, and connector.

The components of a microphone are the capsule, preamplifier, and connector.  The entire assembly can be referred to as a microphone or a microphone set.

1.1 Capsule

The microphone capsule (Figure 3) converts sound pressure waves into a proportional analog voltage.

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Figure 3: Microphone capsule.

The capsule contains a protective grid which houses the diaphragm and backplate as shown in Figure 4.  
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Figure 4: Cut away view of a microphone capsule showing diaphragm and backplate.

The diaphragm and backplate convert changes in sound pressure into voltage as follows:
  • The diaphragm is at a fixed initial distance from the backplate.
  • The backplate carries an electric charge (Qo).
  • Vibrating air molecules cause the diaphragm to move.
  • The distance between the diaphragm and back plate changes as the diaphragm moves.
  • The capacitance (C) fluctuates proportionally to the distance between the diaphragm and backplate.
The capacitance changes because of the electrostatic force field between the two plates. So as the diaphragm moves closer to the backplate due to the incident sound wave, the capacitance will increase. This electrical change is measured as shown in Figure 5.
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Figure 5: Microphone diaphragm and backplate.

The microphone acts as a capacitor (C) that measures the charge proportional to the change in the distance between the diaphragm and backplate:
  • C: Capacitance in farads
  • Qo: Fixed electrical charge in coulombs
  • Eo: Fixed electromotive force in volts
  • e: Change in volts (proportional to distance between diaphragm and backplate)
These types of microphones are commonly referred to as capacitance microphones. Capacitance microphones are also called condenser microphones because historically capacitors were called condensers.

The diaphragm is typically made of an ultra-thin material such as mylar or aluminum foil. This allows the diaphragm to move easily but makes microphones fragile sensors.

More about the invention of today’s modern microphones in the knowledge article: History of Acoustics.

1.2 Preamplifier

The capacitance signal produced by the backplate and diaphragm is converted by the preamplifier (Figure 6) to an analog voltage.
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Figure 6: Microphone preamplifier.

The preamplifier also amplifies the microphone signal as well.  The electrical signal becomes strong enough for additional processing, while simultaneously preventing noise from contaminating the signal.

1.3 Connectors

The microphone has a connector (Figure 7) which attaches to a Simcenter SCADAS or other recording device via a wire.
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Figure 7: Microphone connectors.

There are several types of connectors that are commonly used with microphones:
  • BNC (Bayonet Neill-Concelman Connector): Used for ICP/IEPE/CCP signal conditioning.
  • SMB  (Sub-Miniature version B): Commonly used on array microphones.
  • LEMO 7-Pin: Used for polarized microphones.
The wires are then connected to devices like a Simcenter SCADAS where the analog signal is digitized and passed to a computer.  The recorded microphone signal can then be viewed and analyzed on a computer.
2. Microphone Types

Even though microphones consist of three distinct components (capsule, preamplifier, connectors), there are several variations in how they are constructed and operate. 

2.1 Precision Condenser Microphones

For engineering measurements, precision condenser microphones are commonly used (Figure 8). 
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Figure 8: Precision condenser half inch diameter microphone.

These microphones typically come in 1/4", 1/2", and 1" diameters. 

They can be either prepolarized (IEPE/ICP/CCP) or externally polarized (200V required).  See upcoming sections for more information on polarization.

2.2 Surface Microphones

Surface microphones are flush mount microphones (Figure 9) designed with a low profile.  
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Figure 9: Flush-mount microphone examples.
These microphones typically have a height of 10 mm or less. They are ideal for confined spaces or for applications where it is desirable to have no interference between the microphone and the surrounding air (for example an aero-acoustic wind tunnel). 

2.3 Probe microphones

Probe microphones are used in special measurement conditions where the microphone has the chance of being damaged due to contamination from dust or oil, or in extremely high temperature environments (up to 800°C).

Their design is made up a microphone and preamplifier that is encased in a special housing with a needle-like probe tip as shown in Figure 10.  
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Figure 10: A Probe microphone with an extended needle tip to protect the internal microphone

The probe tip can be inserted into the test environment while keeping the microphone electronics safe. Since the probe tip is small, it is also ideal for measuring in small cavities or tubes and can be useful for automotive applications including exhaust noise.

2.4 Intensity Microphones

To measure sound intensity, specialized pairs of phase-matched microphones are often used as shown in Figure 11:
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Figure 11: Left – Microphone pair with a spacer (black) between them, Right – Sound intensity probe used to position the microphone pair.
A sound intensity measurement not only considers amplitude, but also includes the direction of the sound.  To determine the direction, the difference in the sound pressure is used between the two microphones.  This requires an accurate measurement of the phase.

More about measuring sound intensity in the knowledge article: Simcenter Testlab: Measuring Sound Intensity

2.5 MEMS Microphone

MEMS (Micro-ElectroMechanical Systems) are miniature solid-state devices used to measure sound.  MEMS microphones (Figure 12) are used in devices like cell phones.
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Figure 12: Right – MEMS microphones, Left – Simcenter Sound Camera uses MEMS microphones.

MEMS microphones function in a similar fashion to a condenser microphone; a diaphragm is suspended in a small housing with a silicone backplate. As sound moves through the sound port inside the housing, the diaphragm fluctuates proportional to the pressure wave, and the integrated circuit converts the signal to a voltage. 

In addition to being much smaller, MEMS microphones consume far less power than their full analog counterparts. However, they cannot be calibrated like traditional microphones.  As a result, MEMS microphones are typically used in more qualitative applications like acoustic source localization, rather than for quantitative measurements like ISO sound power.

More information on the Simcenter Sound Camera: Simcenter Sound Camera - Everything You Need to Know
3.  Externally Polarized versus Prepolarized Microphones

In a typical condenser style microphone, for the backplate to pick up the movement of the microphone diaphragm and turn it into voltage, it needs to be charged. There are two charging schemes as shown in Figure 13:
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Figure 13: Left – Externally polarized microphone, Right – Prepolarized backplate.

The two schemes are:
  • Externally Polarized: A 200 Volt (DC) charge is applied to the backplate from an external power source.
  • Prepolarized: The backplate is coated with an electret which has a permanent electrical charge.  This makes external polarization unnecessary, because everything is handled within the microphone capsule. 
An electret (formed as a portmanteau of electr- from "electricity" and -et from "magnet") is a dielectric material that has a quasi-permanent electric charge or dipole polarization. An electret generates internal and external electric fields, and is the electrostatic equivalent of a permanent magnet.

Prepolarized microphones are more popular than the externally polarized microphone today. Because of the prepolarized electret, less wiring is required to use the microphone.   An prepolarized type microphone can be powered via a BNC cable (two wires), whereas an externally polarized microphone requires a 7-pin LEMO connector (7 wires).

3.1 Advantages and Disadvantages

Externally polarized microphones (200V) have some advantages over prepolarized (0 V) microphones:
  • More stable at higher temperatures (150 C and higher).
  • Higher voltage preamplifier increases the maximum dB limit of the microphone by 3-6 dB.
On the other hand, prepolarized microphones (0 V) have some advantages over externally polarized (200V) microphones:
  • Prepolarized microphones are less expensive than externally polarized microphones due to their simplified cabling (based on coaxial cable and BNC connectors).
  • Prepolarized microphones and IEPE/ICP accelerometers use the same signal conditioning. This makes test setup easier when using both microphones and accelerometers.
  • Externally polarized microphones are more likely to short in humid conditions due to the 200V polarization voltage.
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3.2 Simcenter SCADAS

Simcenter SCADAS equipment can condition and power both prepolarized (ICP/CCP/IEPE) and externally polarized (200V) microphones.

The terms ICP/CCP/IEPE all refer to commercial implementations of prepolarized microphones: 
  • IEPE: Integrated Electronic Piezo-Electric, generic name for constant current supply powering of sensors.
  • ICP: Integrated Circuit Piezoelectric, a PCB Piezotronics company trademark.
  • CCP: Constant Current Power, commonly used by GRAS corporation.
All three (ICP/CCP/IEPE) modes use the same type of BNC connector. They all use a 2 to 20 mA constant current supply. 

Simcenter SCADAS input cards, like the V24 and V8 card, work with IEPE/ICP/CCP sensors that use BNC connectors.  In the “Input Mode” field of Simcenter Testlab, “ICP” should be selected.

The Simcenter SCADAS VM8 card works with both IEPE/ICP/CCP sensors and externally polarized microphones (200V supply) as shown in Figure 14:
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Figure 14: The Simcenter SCADAS VM8 card supports both externally polarized microphones as well as ICP/CCP/IEPE microphones.

To power a 200 Volt polarized microphone, the fields “Polarization” and “Power Supply” need to be activated under “Tools -> Channel Setup Visibility”. They should be set to “200” Volts and “28” Volts respectively.

To use the Simcenter SCADAS VM8 cards with an ICP/CCP/IEPE sensors, a 7-pin LEMO to BNC adaptor cable is needed.
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4. Microphone Frequency Response

Frequency response indicates if a microphone measures sound levels differently across a frequency range.

4.1 Engineering Microphone

For engineering applications, most microphones are designed to have a flat frequency response. This means that the microphone has the same sensitivity at all frequencies.

The frequency response of a typical engineering microphone is shown in Figure 15.
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Figure 15: In an engineering microphone, the frequency response (expressed in decibels of gain or attenuation) is as flat possible across most of the frequency range.  

The frequency response (blue curve) represents that the ratio of a measured level to an applied reference level.  The frequency response is expressed in decibels of gain or attenuation from the reference. Ideally the value should be zero across the entire frequency range.

In this example, the deviation is zero at most frequencies. At high frequencies, there is some deviation.  Engineering microphones are designed with the goal to measure all frequencies equally. 

In Simcenter Testlab, there is a "frequency dependent calibration" feature which can be used to compensate for deviations in flatness as shown in Figure 15

4.2 Studio Microphones

On the other hand, “studio” microphones do not have a flat frequency response (Figure 16). 
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Figure 16: Frequency response of a studio microphone.  At low frequencies (100 Hertz or lower) and high frequencies (above 15,000 Hertz) the measured levels are lower than the actual physical levels.  Frequencies important to speech communication (1000 to 6000 Hertz) are measured at higher levels than the actual physical levels.

These microphones often attenuate low frequency noise and high frequency noise to help remove clicks and pops that are undesirable in sound recordings. There is no standard for the frequency response of a studio microphone - the frequency response can vary widely between different models.

Studio microphones are used in non-engineering applications like music recording, radio broadcasts, etc.

4.3 Comparison of Engineering Microphone versus Studio Microphone

Measuring the same sound with an engineering microphone versus a studio microphone will yield different results.  For example, in Figure 17, the same pink noise sound source was measured with both microphones.
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Figure 17: Pink noise 1/3rd octave spectrum from engineering microphone (green) versus studio microphone (blue).  Pink noise should have a similar level at all octave frequencies.

Pink noise is designed to have the same level at all octave bands across the frequency range of human hearing.  The green curve (measured with engineering microphone) is much flatter than the blue curve (measured with studio microphone). 
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4.4 Human Hearing and A-Weighting

The human ear does not have a flat frequency response, it also attenuates low and high frequencies.  The frequency effect of human hearing is captured in the A-weighting curve (Figure 18).
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Figure 18: An A-weighting filter alters the frequency content of a measurement signal on purpose to mimic the effects of human hearing.  In order to be applied properly, the frequency response of the recording microphone should be flat.

To mimic the perception of the human ear in the recorded data, A-weighting filters are often applied to measurements made by engineering microphones. A-weighting would not be useful with studio microphones due to their non-flat frequency responses. 

More about A-weighting filters here: What is A-weighting?.
5. Acoustic Fields and Response Types

Sound measurements are often performed in different acoustic fields (free and diffuse).  Depending on the field, different microphone response types can be used to account for the field.

5.1 Free versus Diffuse Acoustic Field

In an acoustic free field, sound radiates freely from a source with no reflections.  In an acoustic diffuse field, sound reaches the microphone from multiple directions due to a reflective environment. These environments are graphically illustrated in Figure 19.
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Figure 19: Left – Acoustic Free Field, Right – Acoustic Diffuse Field

In a free field, the sound source appears like a point source off in the distance from the microphone measurement location.  A diffuse field means sound reaches the microphone from all directions. In artificial environments, a diffuse field can be created that produces equal sound levels that reach the microphone from all directions.

5.2 Free Field, Diffuse Field, Pressure Types

In an ideal world, a microphone should have an equal response at all frequencies due to the same sound level, regardless of the angle with which the sound reaches the microphone.  This is not always possible in practice, so there are different types of microphones that can be used to compensate for unequal behavior as shown in Figure 20:
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Figure 20: Three microphone types include free-field, diffuse field, and pressure field.

When the wavelength of sound and the size of the microphone are similar, interference causes the microphone levels to vary.  In general, this occurs at higher frequencies (10,000 Hertz or higher):
  • Free Field: A free field microphone is designed to compensate for its presence in an acoustic free field.  It must point directly at the sound source (zero-degree angle of incidence) to do so. When measuring in the zero-degree angle of incidence, any distortions in the measurement angle due to the microphone orientation are eliminated. This type of microphone is typically used in anechoic environments or in open areas with no reflecting surfaces.
  • Diffuse Field: Diffuse field microphones, also called random incidence microphones, are designed to respond in a uniform manner to any sound arriving from any angle. These are useful in reverberant chambers, vehicle interiors, etc.
  • Pressure Microphones: Pressure microphones are designed for flush mount applications where the sound source is at a 90-degree angle to the microphone. They are typically used in small enclosures and used in near-field applications.
5.3 Polar Pattern

The amount that a microphone will distort sound levels is dependent on frequency of the sound and the angle of sound incidence. This can be either given as part of the microphone specifications or can be measured.  

This information can be consolidated into a polar plot.  To create a microphone polar plot, a consistent sound level is generated at various incidence angles and at specific frequencies.  The results are shown in Figure 21 below:
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Figure 21: Example polar sound plot showing sound attenuation for the same sound source from different angles (numbers) and various frequencies (colored circles) for a free field microphone.

To read a polar plot, the angles show the sound field around the microphone, with zero degrees being directly in front of the diaphragm. Free-field microphones are designed to be consistently sensitive at zero degrees, so the zero degree position will measure sound at every frequency without losing amplitude.  Therefore a free field microphone should always be pointed directly at a sound source as described previously.

Each frequency circle plotted in different colors shows how much attenuation will occur if the sound comes in from different angles to the microphone. Unsurprisingly, behind the microphone shows the highest levels of attenuation. These polar plots, also sometimes known as “pickup patterns” will determine how the microphone measures sound in a given acoustic field.
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Microphones with specific patterns can be chosen ahead of time to get the frequency response desired during measurement, depending on application.

6. Microphone Size: Frequency Range and Sensitivity

When selecting a microphone, the size of the microphone diaphragm has a direct effect on the frequency and amplitude range (lowest to highest value) of sound that can be measured. Generally, the larger the diameter of the microphone diaphragm, the lower the level that the microphone can measure (and vice versa) as shown in Figure 22.
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Figure 22: Microphones with larger diameter diaphragms can measured lower sound levels.  Microphones with smaller diaphragms can measure very high sound levels.

The tradeoffs of microphone size include:
  • Microphones that are larger in size have higher sensitivity and a lower cartridge thermal noise (CTN) floor. The CTN is the quietest sound a microphone can detect above the electrical noise floor. Microphones of 1" in diameter are often used when the sound of electronic devices (for example, a smart phone should be silent (when not playing music), with no electronic buzzing noise.  The sound levels are low enough that a human may not hear them unless they hold the device up to their ear.
  • A 1/2" microphone diameter is often used to measure the interior of a car or airplane. They cover the full human hearing range (20 to 20,000 Hz).
  • Smaller microphone diaphragms (for example 1/4") have a lower sensitivity, but can measure a higher maximum decibel (dB) level. They can also measure higher maximum frequency. 
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The size also affects the sensitivity and range that a microphone can measure.  In general:
  • 1" Microphone: 50 mV/Pa, 0 dB to 140 dB
  • 1/2" Microphone: 50 to 12.5 mV/Pa, 15 dB to 160 dB
  • 1/4" Microphone: 4 to 0.25 mV/Pa, 30 dB to 187 dB
7. Calibration

Making sure that the microphone is reading accurately is one of the most important steps in testing. To do this, engineers will calibrate microphones on a daily or yearly basis depending on the need:
  • Daily: Temperature and atmospheric pressure vary every day. Because microphone diaphragms are so sensitive, calibration checks can be done before and after testing each day.  This check is usually performed at a single frequency, usually 1000 Hz for most hand held sound calibrators. 
  • Yearly: Check entire frequency range. This is to ensure that the microphone is maintaining a flat frequency response. This is usually done by returning the microphone to the manufacturer so it is checked in repeatable, consistent conditions.
There are many different types of calibrators to be used for different applications. The most common is a sound level calibrator (typically Class 1 or better) which uses a calibrated speaker to play a tone as shown in Figure 23.
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Figure 23: Microphone (bottom) being calibration with a 1 kHz and 94 dB RMS tone.
To do simple microphone calibration, a microphone is inserted into a hand calibrator that plays a known tone at a known amplitude level. For most microphone calibrations, this is typically a 1000 Hz sinewave signal at 1 Pascal (94 dB) RMS. The microphone is inserted into a slot that isolates it from other sounds to ensure that the sinewave is the dominate source at full volume. During this time, the time domain signal or frequency signal is measured. Since the calibrator is playing a known tone, the microphone should be reading a 1000 Hz signal at 1 Pascal RMS, so the only portion left to solve for is the voltage sensitivity. This is typically calculated in mV/Pa.

If using the time domain to find the sensitivity, the RMS of the signal is taken and divided by the amplitude of the calibration tone. In the example below, a microphone measures the voltage to be at 63.6 millivolts Peak.  This can be converted to a RMS value by multiplying 63.6 by 0.707 as shown in Figure 24.
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Figure 24. An example of using a calibrator and the time domain measurement window to calculate the microphone sensitivity. 

The RMS level of the 63.6 millivolt signal is 45 millivolts. The sensitivity value is the ratio of the measured mV RMS value divided by 1 Pascal RMS. More on RMS versus Peak in knowledge article: Root Mean Square (RMS) and Overall Level.
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Microphone calibration does not need to be calculated by hand as described here. Siemens Testlab offers calibration for many different types of sensors, including microphones.  See knowledge article: Simcenter Testlab Calibration.

Simcenter Testlab also supports Transducer Electronic Data Sheets (TEDS) for sensors like microphones.  TEDS allows the calibration sensitivity, serial number, model number, calibration date, etc of a transducer to be stored on an electronic chip within the sensor.  While TEDS is useful for retrieving this basic information, it is still best practice to calibrate a microphone as the calibration value may change due to environmental conditions. It is also an end to end check of the microphone, signal conditioning, and other parts of the measurement chain are working properly.  It is possible for a TEDS enabled transducer to still read a sensitivity even if it is not functioning properly.  See the knowledge article: Simcenter Testlab: Using Transducer Electronic Data Sheets (TEDS) for more information.

8. Care and Handling

Microphones are highly sensitive instruments and should be treated as such to prolong the equipment life span.  Example shown in Figure 25.
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Figure 25: Left - Always hand tighten the microphone assembly Right - Never use pliers to connect the microphone capsule and preamplifier.

Here is a general list of handling recommendations:

  • Store microphones in clean and dry place
  • Only hand tighten microphone capsule to preamplifier
  • Refrain from touching diaphragm
  • Always leave grid cap on
  • Do not leave/store in dirty environment (test cell, etc)
  • Do not drop
  • Do not clean with shop air
  • Do not use vice grips to tighten
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Hope these microphone tips were helpful!

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KB Article ID# KB000112200_EN_US



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