Most Important Frequently Asked Acoustics Interview Questions
-
Question 1. How Should I Properly Care For And Handle My Microphones?
Answer :
Caution should be taken when handling these sensitive instruments.
-
Question 2. When Should A Surface Microphone Be Used?
Answer :
Surface microphones are used to measure true surface pressure and noise. They are optimized for measurements in confined spaces and to reduce wind induced noise during testing.
Mechanical Interview Questions -
Answer :
Probe microphones are recommended for small hard-to-reach and high temperature (up to 800ยบ C) areas.
-
Answer :
Array microphones offer a lower cost solution.
-
Question 5. Can Noise Floor Calculations Be Used To Discover Sources Of Unwanted Noise?
Answer :
Yes, during product testing noise floor calculations can be used as part of the product design process.
Mechanical Engineering Interview Questions -
Question 6. What Is The Noise Floor Of A Microphone?
Answer :
The noise floor of microphone is defined by the cartridge thermal noise specification. It is important to note that the electrical noise of the preamplifier will impact the noise floor of the microphone and preamplifier combination. Other components in the measurement chain, for example power supplies and data acquisition systems, can also be limiting factors and increase the minimal sound pressure that you can measure.
-
Question 7. Which Microphone Is Recommended For Low Frequency Measurements?
Answer :
Low frequency is a relative term. Standard 1/2" condenser test and measurement grade microphones have a -3 dB point of 1 Hz to 3 Hz, which is sufficient for most applications. Specialty microphones have been developed to exceed that and measure as low as 0.1 Hz.
Vibration analysis and ultra sonic testing Interview Questions -
Answer :
Multiple free-field microphones spaced in a predetermined pattern and combined with the appropriate software; allow spatial transformation of a complex sound pressure field to be projected to effectively map the acoustic energy flow. Array microphones are an excellent choice for large channel count acoustic testing due to their overall value and phase matching specifications. Transducer Electronic Data Sheet (TEDS) are recommended, since they enable the user to quickly and easily identify a particular microphone.
-
Question 9. What Is A Polar Plot And How Is It Used?
Answer :
Polar plots (also referred to as beam patterns) show a radially symmetric view of the angular dependence of the sound pressure detected by the microphone. The data provided in a polar plot will show how the sound pressure level detected by the microphone changes at various angles of incidence. The angle of incidence used is typically a 180 or 360 degree arc with the microphone at the center. The pressure amplitude decreases as you move from the center. Polar plots do not show the frequency dependence of the measured sound pressure and are always acquired at a specific frequency. The measurement frequency should always be specified with polar data because the shape of the beam pattern will change at different frequencies.
Physics Interview Questions -
Question 10. Does The Size Of A Microphone Impact Its Frequency And Dynamic Range?
Answer :
The size of microphone and its sensitivity does influence its capability. Typically, microphones with the smaller diameter and lower sensitivity allow higher amplitudes and frequencies to be measured. Conversely, larger diameter and/or more sensitive microphones provide lower noise floor and lower frequency capability.
-
Answer :
Anechoic chambers allow measurement of low noise levels without reflections. Whether an anechoic chamber is needed depends on the application, what is being measured, and the environment. Anechoic chambers are generally required when you need to make free-field measurements in a location free of objects that could reflect sound. This is particularly true at lower frequencies where a large amount of space is required to obtain a free-field. Anechoic chambers will be limited by the amount of absorption they provide. Typically, at frequencies below 1 kHz, most anechoic chambers provide very little absorption. In most cases where the signal to noise ratio is sufficiently high, an anechoic chamber is not necessary.
Oil and Gas Interview Questions -
Answer :
Yes, a 1/4" microphone can be used with the PCB® or other 1/2" high temperature preamplifiers, but a 079A02 adapter to connect the 1/4" microphone to the 1/2" diameter preamplifier is needed.
Mechanical Interview Questions -
Question 13. What Is The Difference Between Pcb® Model 130e22 And Model 130a23?
Answer :
Both are free-field array microphones with an SMB connector. The 130A23 uses a different design to allow for less variability in frequency response up to 20 kHz and increased dynamic range. The 130E22 allows more variability at high frequencies, and is priced more cost effectively.
-
Answer :
Phase represents the delay between an applied force (electrical or mechanical) and the response to the applied force (electrical or mechanical). This is characterized as absolute phase or relative phase. Absolute phase is the intrinsic phase delay of the microphone and is independent of any external references. It represents the delay between the incident pressure and the output electrical response. Relative phase is the delay between the output electrical response and the response of another microphone. Any relative phase measurement must be established against a standard.
-
Question 15. Is It Safe To Use A Microphone At Its Maximum Rated Temperature?
Answer :
Yes, you can measure to the maximum temperature rating of the PCB® microphone as long as the mated preamplifier is rated to at least that temperature. PCB® microphones are tested to beyond their maximum temperature. It is important to note that every microphone requires a preamplifier and in many cases the preamplifier can limit the microphone and preamplifier mated system temperature specifications.
Nuclear physics Interview Questions -
Answer :
The contribution of the preamplifier to temperature coefficient is negligible. It recommended to always use the microphone and preamplifier system specifications.
-
Answer :
Yes, the temperature coefficient is linear beyond these limits and has been tested to be consistent between -40 °C and 150 °C
Natural Gas Interview Questions -
Answer :
All of PCB’s 1/2'' and 1/4'' microphones have a flat frequency response from 4 Hz to 10 kHz. Model 377B11 is a 1/2" pressure microphone. This is the most cost effective option in the pressure response category for test and measurement quality microphones.
Mechanical Engineering Interview Questions -
Answer :
Yes, the corrections for a PCB® calibration certificate can be provided in Excel format.
-
Answer :
The working standard Model 378B02 is more stable in extreme environments than the low cost Model 130 series. Model 130 microphones will experience larger shifts in sensitivity caused by temperature, humidity and atmospheric pressure. Both microphones should be calibrated in the test environment after they have stabilized. The accuracy of the measured data may be affected by the placement position and/or the geometry of the microphone and any reflections that may be present. The effect of the microphone itself on the sound field at the point of measurement (1/4’’ vs. 1/2’’ microphone) can alter the measurement. If this occurred, then change the position of the microphone and perform another measurement. If the error is only at higher frequencies, it is important to note that the specification of the 130E20 at 10 kHz is ± 2dB. The model 377B02 is flatter to 20 kHz. Variations within tolerance may occur in specific instances.
Ultrasonic Transducer Engineer Interview Questions -
Question 21. Does A Windscreen Affect The Sound Field And Output Of The Microphone?
Answer :
Windscreens can attenuate the signal at higher frequencies.
-
Question 22. Are Drop Tests Performed On Pcb® 377 Series Microphones?
Answer :
Yes, all PCB® model 377 microphone designs are subjected to a drop test to ensure stability.
-
Answer :
Yes, PCB® is the first to market a low noise prepolarized microphone, model 378A04. This matches the noise floor specification (6.5 dBA) of a 1/2" externally polarized microphone.
-
Answer :
These microphones were designed with different diameters to be drop in replacements to other manufactures’ microphones. This eliminates the need to change holders or change set-up.
Vibration analysis and ultra sonic testing Interview Questions -
Question 25. Will Improper Grounding Negatively Impact The Noise Floor Of A Microphone?
Answer :
Yes, noise can be introduced from the line power or from improper grounding.
-
Question 26. Are The Pcb® Model 377 Series Of Microphones 100% Tested?
Answer :
The Model 377 series of microphones are 100% tested and each goes through a series of environmental stress relief tests and are they are calibrated multiple times to characterize stability.
-
Answer :
When the incident pressure is positive on a prepolarized microphone, the voltage output should be positive. Externally polarized microphones are 180 degrees out of phase with prepolarized microphones due to where the charge is stored. On an externally polarized microphone the voltage output will be negative when the incident pressure is positive.
Physics Interview Questions -
Answer :
The 1/4" model 378A12 has the lowest sensitivity at 0.25 mV/Pa. This allows it to go to extremely high amplitudes.
-
Answer :
Changes in temperature, humidity and atmospheric pressure may change microphone sensitivity. This is why environmental conditions are stated on PCB® calibration certificates. The sound field can also have a significant effect on the measured sound pressure at frequencies above 5 kHz. Be sure to use the correction curves that are specified for the sound field where the measurement is being performed. It is also noteworthy that these sound fields are often idealistic. In practice, a "free" field may not be complete free of reflections or a random incidence field may not provide reflections from all directions. The mounting configuration of the microphone and objects near the microphone can affect the measured response even when located behind the microphone. Every configuration is different. Signal conditioning and data acquisition equipment can affect the sound measurement by limiting the dynamic range or attenuating the signal at different frequencies.
-
Question 30. What Is The Anticipated Impact Of Using A Microphone At 6290 Ft. Above Sea Level?
Answer :
As static pressure increases, sensitivity goes down. Higher pressure means higher air density, which increases the acoustic impedance on surface of the diaphragm and in the gap between the diaphragm and the backplate. This has an effect on displacement. At 6290 ft. the barometric pressure is 47 kPa, which is a change of -54 kPa. If we apply that to the pressure correction, the result is -54 kPa x -0.013 dB/kPa = 0.70 dB re 1V/Pa.
-
Answer :
The seal may accumulate debris or residue over time. The intention is to remove the grid cap and replace the seal on model 130A24. If a change in frequency response (at high frequencies) after repeated use in hostile environments is seen, then it may be time to replace the seal. The grid cap should be installed immediately after changing the pad. Instructions on replacing the pad are provided in the product manual.
-
Question 32. What Is The Rise Time Of Pcb® Model 377a12 Microphone?
Answer :
PCB® model 377A12 microphone is an under-damped system (z ≈ 0.4) and has a rise time of roughly 10 ms. This estimate needs to be verified by direct measurement on individual microphones, variability could be high because resonance frequency is not controlled in manufacturing.
-
Question 33. What Is The Rise Time Of A Microphone?
Answer :
The rise time is determined by the resonance frequency and damping factor of a second order system.
Oil and Gas Interview Questions -
Question 34. How Does A Preamplifier Impact The Dynamic Range Of Microphone?
Answer :
The preamplifier can impact both the noise floor of the microphone and preamplifier system, due to the electrical noise it adds. In some cases it can limit the upper end (3% distortion). This depends upon the microphone specifications and the level of voltage provided by the preamplifier.
-
Question 35. Can A Microphone Be Flush Mounted With The Grid Cap Removed?
Answer :
Yes, you can flush mount a microphone without a grid cap on. The delicate diaphragm will be exposed and precautions should be taken to prevent anything from coming in contact with it. The best approach (for accuracy) is to thread the microphone into the hole with grid cap removed so that diaphragm is flush mounted. You must be careful not to damage diaphragm.
-
Answer :
The two sensitivity specifications represent the same thing. The first specification is open circuit sensitivity in reference to 1 V/Pa and the second is the converted result in mV/Pa. For example the PCB® 377B02 is -26 dB re 1 V/Pa which equals 50 mV/Pa.
Nuclear physics Interview Questions -
Question 37. What Is The Operating Temperature Of Pcb® Model 377b02?
Answer :
The operating temperature for the microphone is listed at 150◦ C. It is important to note that the microphone requires a preamplifier and the preamplifier will be the limiting factor. Standard 1/2" preamplifiers range from 60◦ C to 80◦ C but PCB® was the first manufacturer to introduce a high temperature preamplifier that when mated with 377B02 can be used to 125◦ C.
-
Question 38. How Does Distance Impact The Results For My Measurement Microphone?
Answer :
As the distance increases between a sound source and the sensing element, the sound pressure level will decrease. In a perfect environment, the sound pressure level will decrease approximately 6 dB for every doubling of distance.
-
Answer :
The static pressure limit is the positive pressure limit on the diaphragm. At 40,000 ft. the ambient pressure is 21 kPa (vs. 99 kPa in Western NY). This is a pressure differential of 13.5 dB, but the microphone will only see this pressure if the altitude is changing faster than the microphone is venting. It is important to know what the rate of change in pressure the microphone will experience. PCB® recommends no more than 0.5 psi/s (3.5 kPa/s).
-
Question 40. What Is The Recommended Microphone For Wind Turbine Testing?
Answer :
The PCB® model 378A07 is the preferred model for very low frequency measurements (to 0.1 Hz) that are needed for wind turbine measurements.
Natural Gas Interview Questions -
Question 41. Which Prepolarized Microphone Is Recommended For Qualifying An Anechoic Chamber?
Answer :
PCB's model 378A04 has a noise floor rating of 6.5 dBA. This is suitable for most anechoic chambers.
-
Answer :
The adapter should not affect performance at temperatures up to 120° C.
Ultrasonic Transducer Engineer Interview Questions -
Answer :
Although the diameters are slightly different, the PCB® model 079B10 is designed to accommodate both diameters.
-
Question 44. What Is The Importance Of The Vent Hole Position In A Preamplifier?
Answer :
The vent hole should be in the sound field whenever possible and operators must be careful not to cover the vent hole during mounting. This only affects low frequency response. But large changes in static pressure may be problematic if a microphone is not properly vented. A side vented microphone is recommended for measuring low frequencies within a cavity or tube where there is a large pressure differential between the pressure inside and outside the cavity.
-
Answer :
An adapter is not required. Using a CAL250 (or a pistonphone with a 1" orifice) is recommended. Before performing a calibration, ensure that the microphone has stabilized in the environment where it will be used. Then, install the black rubber fairing pad onto the model 130B40 surface microphone to help ensure a proper seal. Place the microphone and faring pad on a flat, level surface. Turn the handheld calibrator upside down, and place the 1” (25mm) opening onto the faring, centered around the microphone sensing element. Press down slightly to ensure a seal and to minimize outside noise. Then perform the calibration.
-
Question 46. Can I Calibrate Pcb® Model 378a04 Low Noise Microphone With Any Handheld Calibrator?
Answer :
PCB® model 378A04 is a specialty microphone used to measure extreme low noise. To accomplish this, it has a very high sensitivity. A result of this high sensitivity is that the 3% distortion level is lower than some handheld calibrators and pistonphones output levels and will overload the microphone. A calibrated reference source needs to be used that cannot exceed 100 dB and must be below 5kHz. The CAL200 calibrator with a reference signal of 94 dB at 1 kHz is recommended for model 378A04 calibration.
-
Answer :
Vent holes allow for equalization of the external pressure. This also impacts the low frequency specification. If the preamplifier vent hole gets covered or sealed, the microphone will not work properly. A microphone holder can block this vent. A short preamplifier has very little surface area to clamp onto, so there is a higher probability of blocking the vent. This is why a second vent hole on a different axis was added.
-
Question 48. Is It Better To Calibrate A Microphone And Preamplifier Separately?
Answer :
It depends upon whether the microphones will be used with different preamplifiers. If the microphones are stored separately from the preamps and are intended to be used with different preamps for each test, then the recommendation is to calibrate the units separately. When calibrated separately, each component is tested with a ‘reference’ component which typically is more stable and has tighter tolerances than stock ‘off the shelf’ components. This will provide a more accurate calibration for an individual microphone or preamplifier. If the microphone and preamplifier will always be used together as a mated pair, it is recommended that they be calibrated together as a ‘mated system.’ The preamplifier can affect the system sensitivity slightly and by calibrating them together, you get an accurate system calibration, for when the same pair (microphone and preamplifier) are used together.
-
Answer :
It is recommended that a single point calibration be performed in the field, both before and after your test. Testing in the actual environment helps account for changes to the sensitivity stemming from fluctuations in temperature, humidity and atmospheric pressure. If a calibration is done with a speakerphone (Models CAL200 or CAL250) or a pistonphone, both before and after the tests, there will be a much higher degree of confidence that the test results are valid.
-
Question 50. Can I Use A Pcb© Microphone Beyond Its Specified Frequency Range?
Answer :
Microphones are commonly specified to be within +/- 2 dB across the entire frequency range, stated by the manufacturer. You can use it beyond this frequency range, but may have less accurate readings, for example +/- 3 dB tolerance.
-
Answer :
For most applications both will provide the same test results. An externally polarized (200V) microphone is better for high temperatures, up to 150o C but at a higher cost due to the required 200V power supply and the 7-pin cabling and may be limited to the temperature rating of the preamplifier required. A prepolarized system uses an electret, where the charge is embedded. When operating at temperatures above 120o C, the charge may escape the electret material causing a loss of sensitivity.
Prepolarized microphones are better for humid applications, which can short the 200V designs. Prepolarized designs use low cost constant current supplies, which makes them convenient for portable applications and for use with Sound Level Meters. They are interchangeable with most accelerometer set-ups and other sensors using 2-20 mA constant current power supplies. This provides savings in set-up costs. This design has become increasingly popular by using standard coaxial cables and low cost power supplies.
-
Question 52. What Is A-weighting And What Is The Best Way To Implement It?
Answer :
Different frequencies affect the human ear in different ways. Equal loudness tests determined how each frequency affects the human ear and showed that the average human ear is most sensitive around 4 kHz. A 100 dB signal will only feel like 70 dB at 50 Hz. This is why a guitar may sound louder than a bass drum, or a brake squeal is more annoying than an engine knock, when in fact they are the same decibel levels on a linear scale.
An A-weighting scale is basically a filtering system, which provides the relative dB effect on a human ear, at each frequency. This can be accomplished in the software system, within a Sound Level Meter, or with an in-line filter.
-
Question 53. What Is Nah And Its Relationship To ’array’ Microphones?
Answer :
NAH is Near Field Acoustic Holography. A single microphone can measure amplitude and frequency. By using multiple quality microphones with good phase matching characteristics, tests can be performed to show direction, particle velocity and intensity. NAH software systems can utilize an array of microphones to enable engineers to reduce the time and cost to minimize noise and vibration, by performing sound pressure mapping and holography tests for noise sources and transmission paths on 2D (planar) surfaces.
-
Question 54. What Is The Maximum Spl Limit Of A Microphone? How Is It Specified?
Answer :
When the microphone experiences a change in pressure the output voltage of the preamplifier produces a waveform proportional to the amplitude and frequency of the sound. At very high amplitudes mechanical and electrical characteristics of the system may become non-linear resulting in distortion of the waveform.
The maximum sound pressure level that a microphone can accurately detect is characterized by the total harmonic distortion (THD), expressed as a percentage. The dynamic range limit is defined as the peak sound pressure level required to produce 3% to THD in the output of the preamplifier. The actual maximum SPL that can be measured is based upon the exact sensitivity level of the microphone and the peak voltage supplied by the preamplifier, the DC bias voltage in the preamp, and the excitation voltage of the signal conditioner. To maximize the upper limit of dynamic range, select a microphone with low sensitivity, high maximum peak voltage output, and use a signal conditioner with sufficient excitation voltage to allow for the full voltage swing (DC bias + peak output voltage).
-
Answer :
The operating temperature limits are usually supplied in the manufacturers specification sheets. A typical precision prepolarized microphone from PCB® will be guaranteed to operate up to 120o C. A typical externally polarized microphone will operate up to 150o C. The sensitivity may change slightly as the operating temperature changes. A reputable manufacturer will provide a temperature coefficient of sensitivity (example –0.007 dB/ oC.) The operating temperature for preamplifiers is usually the limiting factor.
A typical preamplifier specification limits the operating temperature to 60oC to 80oC. High temperature preamplifier designs have been developed that meet 120 oC. Probe microphone designs allow for measurements to be made up to 800oC by performing the measurement at the end of the probe tip. The probe will allow the sound signals to pass through, while isolating the microphone and preamplifier in a separate housing away from the heat source.
-
Answer :
A coaxial cable is designed to be used with an ICP® type prepolarized microphone or accelerometer. The coaxial cable houses a conductor enclosed within a shield (ground.) The connector ends are generally BNC, SMB or 10-32 (microdot) connectors. A LEMO® cable is designed for externally polarized microphone applications. While the coaxial cables are very cost effective (typically, range ¼ to 1/10th the price of the LEMO® cables) and can be run long distances with minimal loss of signal, the LEMO® cables offer flexibility to carry multiple signals., The is necessary for 200V power supplies, mV output signals, and power for heaters, etc. For externally polarized applications, the most popular is a 7-pin LEMO® connector and cable.
-
Question 57. Will A Preamplifier Change The Sensitivity Of A Microphone?
Answer :
Yes, but the extent of the change depends on both the microphone and the preamplifier. All manufacturers will provide the gain (or attenuation when expressed as a negative value) usually in units of dB (re 1V/Pa). The attenuation is subtracted from (or the gain added to) the microphone sensitivity to give the nominal system sensitivity. For example, a microphone such as the 377B02 with a sensitivity of 50 mV/Pa or –26.02 dB (re 1 V/Pa) when used with a preamplifier such as the 426E01 with a nominal gain of –0.05 dB results in a nominal system sensitivity of–26.07 dB or 49.71 mV/Pa.
For this case, the change in the sensitivity is only -0.6%. Some preamplifiers available on the market can have gains as great as -0.3 dB, which will drop the sensitivity by more than 3%. The best way to account for the preamplifier gain is to calibrate the sensitivity of a microphone with the preamplifier. PCB® offers both individual microphone cartridges as well as combined microphone and preamplifier systems.
-
Question 58. How Does Distance Affect The Sound Pressure Level?
Answer :
This depends on the geometry and relative size of the sound source. A simple source projects sound from a single location and the size of the source is small compared to the wavelength of sound. As a rule of thumb, if the wavelength of sound is greater than 10 times the radius of the source, then this assumption is valid. In these cases sound propagates in the form of spherical waves with the source at the center. These waves are radially symmetric where the pressure is constant at a given radius. The distance from the source corresponds to the increasing radius of spherical waves as they propagate outward.
As distance from the source increases the sound power is spread over greater and greater distances thus reducing the sound pressure level. The decrease in sound pressure, under ideal conditions, is inversely proportional to the distance from the source. This is equivalent to a 6 dB drop each time the distance is doubled. For example, if the sound pressure level at 5 meters is 100 dB (re 20uPa), then at a distance of 10 meters away it will be 94 dB (re 20uPa). If the source is not small compared to the wavelength of sound (either because it is very large or the frequency is very high resulting in shorter wavelengths) the assumption of a simple source is not valid and additional mathematical analysis is required to determine how the pressure changes with distance.
-
Answer :
ICP® is a registered trademark of PCB Piezotronics Inc. It is a PCB® sensor containing built-in electronics which can be powered by 2-20 mA of constant current power. An accelerometer that runs off ICP® sensor power will have electronics in the housing. For a condenser microphone to run off this same ICP® sensor power, you will require a prepolarized microphone and a preamplifier which houses the built-in electronics.
-
Question 60. When Can I Remove The Microphone Grid Cap?
Answer :
It is recommended that the grid cap never be removed. It is in place to protect the delicate diaphragm. The only time it should be removed is by a qualified calibration house to test sensitivity over a wide range of frequencies.
Topic: Acoustics Interview Questions
No comments:
Post a Comment