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Engineer Noise and Vibration practice questions

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Question 1Fundamentals of Acoustics and Vibration

A sound wave in air has a frequency of 250 Hz. Taking the speed of sound as 343 m/s, what is its wavelength?

  1. 85,750 m
  2. 1.37 m Correct answer.
  3. 2.74 m
  4. 0.73 m

Explanation

Wavelength equals the speed of sound divided by frequency, so 343 divided by 250 gives about 1.37 m. This relationship lambda = c/f is fundamental when relating frequency to physical dimensions of rooms or silencers.

Question 2Fundamentals of Acoustics and Vibration

What does the sound pressure level of 0 dB re 20 micropascals represent?

  1. A pressure equal to the reference pressure itself Correct answer.
  2. The threshold of pain for the human ear
  3. Complete absence of sound pressure fluctuations
  4. A pressure of 20 pascals

Explanation

Sound pressure level is defined as 20 log10(p/p0) with p0 equal to 20 micropascals, so an SPL of 0 dB occurs when the actual pressure equals the reference. It is a relative scale, not an absolute measure of silence.

Question 3Fundamentals of Acoustics and Vibration

Which quantity is a vector that describes both the magnitude and direction of oscillatory motion?

  1. Sound pressure
  2. Sound power
  3. Particle velocity Correct answer.
  4. Density

Explanation

Particle velocity describes the local oscillatory motion of the medium and has both magnitude and direction, making it a vector. Sound pressure and sound power are scalar quantities.

Question 4Fundamentals of Acoustics and Vibration

For a simple harmonic oscillator, what happens to the natural frequency when the mass is quadrupled while stiffness is held constant?

  1. It stays the same
  2. It doubles
  3. It halves Correct answer.
  4. It quarters

Explanation

Natural frequency is proportional to the square root of stiffness divided by mass. Quadrupling the mass increases the denominator by a factor of four, so the frequency is multiplied by one half.

Question 5Fundamentals of Acoustics and Vibration

Which frequency weighting is most commonly applied to measurements intended to reflect human hearing at moderate levels?

  1. A-weighting Correct answer.
  2. C-weighting
  3. G-weighting
  4. Z-weighting

Explanation

A-weighting approximates the ear's reduced sensitivity at low and very high frequencies at moderate sound levels and is widely used for environmental and occupational limits. C-weighting is flatter and used for high-level assessment.

Question 6Fundamentals of Acoustics and Vibration

A sound source radiates equally in all directions in free space. How does sound pressure level decrease with each doubling of distance from the source?

  1. By 20 dB
  2. By 10 dB
  3. By 6 dB Correct answer.
  4. By 3 dB

Explanation

For a spherical spreading source in free space, pressure is inversely proportional to distance, so doubling distance halves pressure and reduces level by 6 dB. This is the inverse-square law expressed in decibels.

Question 7Fundamentals of Acoustics and Vibration

What is the phase relationship between displacement and acceleration for a body in simple harmonic motion?

  1. Acceleration is 180 degrees out of phase with displacement Correct answer.
  2. Acceleration lags displacement by 45 degrees
  3. They are in phase
  4. Acceleration leads displacement by 90 degrees

Explanation

Acceleration is the second time derivative of displacement, and differentiating a sinusoid twice introduces two 90-degree phase advances, giving a total of 180 degrees. Thus acceleration is always opposite in sign to displacement.

Question 8Fundamentals of Acoustics and Vibration

Which quantity is expressed in units of newtons per square metre and is measured with a microphone?

  1. Sound pressure Correct answer.
  2. Sound intensity
  3. Sound power
  4. Particle acceleration

Explanation

Sound pressure is measured in pascals, equivalent to newtons per square metre, and microphones transduce pressure fluctuations. Intensity and power use watts-based units.

Question 9Fundamentals of Acoustics and Vibration

A pure tone at 1000 Hz has a sound pressure level of 60 dB. What is the approximate A-weighted level of this tone?

  1. About 52 dB(A)
  2. About 80 dB(A)
  3. About 40 dB(A)
  4. About 60 dB(A) Correct answer.

Explanation

The A-weighting curve is defined to be 0 dB at 1000 Hz, so a 1000 Hz tone has the same numerical level when A-weighted. Corrections at other frequencies are what change the value.

Question 10Fundamentals of Acoustics and Vibration

Two incoherent sound sources each produce 70 dB at a receiver. What is the combined sound pressure level?

  1. 76 dB
  2. 73 dB Correct answer.
  3. 140 dB
  4. 70 dB

Explanation

For incoherent sources the intensities add, doubling the intensity and adding 3 dB, giving about 73 dB. Adding decibel values directly is incorrect because the scale is logarithmic.

Question 11Fundamentals of Acoustics and Vibration

For a single-degree-of-freedom system, at what excitation frequency does the response amplitude peak for a lightly damped system?

  1. Slightly above the undamped natural frequency
  2. Slightly below the undamped natural frequency Correct answer.
  3. At twice the undamped natural frequency
  4. Exactly at the undamped natural frequency

Explanation

For displacement response with viscous damping, the resonant peak occurs at a frequency slightly below the undamped natural frequency, approaching it as damping tends to zero. The shift is small for light damping.

Question 12Fundamentals of Acoustics and Vibration

What does the damping ratio of 0.05 indicate about a system?

  1. The system has no stiffness
  2. The system is lightly damped and will oscillate after disturbance Correct answer.
  3. The system is critically damped
  4. The system is overdamped

Explanation

A damping ratio well below one indicates underdamping, so the system oscillates at a damped natural frequency and decays gradually. Critical damping occurs at a ratio of one.

Question 13Fundamentals of Acoustics and Vibration

A microphone measures a sound pressure of 0.2 Pa. What is the corresponding sound pressure level re 20 micropascals?

  1. 60 dB
  2. 100 dB
  3. 80 dB Correct answer.
  4. 74 dB

Explanation

The ratio 0.2 Pa to 20 micropascals is 10,000, and 20 times log10 of 10,000 equals 80 dB. This follows directly from the definition of sound pressure level.

Question 14Fundamentals of Acoustics and Vibration

In a 1/3-octave band analysis, approximately what is the ratio of the upper band-edge frequency to the lower band-edge frequency?

  1. 1.12
  2. 2.00
  3. 1.41
  4. 1.26 Correct answer.

Explanation

A one-third octave band spans a frequency ratio of about 2 raised to the one-third power, which is approximately 1.26. A full octave band spans a ratio of 2.

Question 15Fundamentals of Acoustics and Vibration

A wall attenuates sound such that the transmitted sound power is one thousandth of the incident sound power. What is its transmission loss?

  1. 30 dB Correct answer.
  2. 20 dB
  3. 10 dB
  4. 3 dB

Explanation

Transmission loss is ten times the base-10 logarithm of the reciprocal of the transmission coefficient. With a coefficient of 0.001, the logarithm is minus three, giving 30 dB.

Question 16Fundamentals of Acoustics and Vibration

What is the main reason for applying a Hanning window to a sampled signal before spectral analysis?

  1. To increase the sampling rate
  2. To convert pressure to intensity
  3. To reduce spectral leakage from truncated records Correct answer.
  4. To remove all background noise

Explanation

Windowing tapers the ends of a finite record, reducing the discontinuities that cause energy to leak into adjacent frequency bins. It does not change sampling rate or remove noise.

Question 17Fundamentals of Acoustics and Vibration

In a diffuse sound field in a room, how does the sound pressure level relate to sound power level of the source?

  1. It decreases as room absorption increases Correct answer.
  2. It increases as room absorption increases
  3. It depends only on the source power, not on room absorption
  4. It is independent of room volume and absorption

Explanation

In a reverberant field the steady-state level rises with source power and falls as the total absorption in the room increases, following the room constant. More absorption removes energy faster, lowering the reverberant level.

Question 18Fundamentals of Acoustics and Vibration

What is the characteristic impedance of air at approximately 20 degrees Celsius, used to relate sound pressure and particle velocity in a plane wave?

  1. 1000 rayls
  2. 1.48 rayls
  3. 415 rayls Correct answer.
  4. 20 rayls

Explanation

Characteristic impedance equals density times speed of sound, roughly 1.2 times 343, giving about 415 rayls in air at room temperature. In a plane wave, pressure divided by particle velocity equals this value.

Question 19Fundamentals of Acoustics and Vibration

A machine's vibration is measured as 0.5 m/s squared RMS at 50 Hz. If the same acceleration amplitude occurred at 100 Hz, how would the velocity amplitude compare?

  1. Velocity would double
  2. Velocity would stay the same
  3. Velocity would halve Correct answer.
  4. Velocity would quadruple

Explanation

For sinusoidal motion, velocity amplitude equals acceleration amplitude divided by angular frequency. Doubling the frequency at constant acceleration amplitude therefore halves the velocity amplitude.

Question 20Fundamentals of Acoustics and Vibration

What is the purpose of an octave band filter set in noise measurement?

  1. To convert A-weighted levels to linear levels
  2. To remove all frequencies above 1000 Hz
  3. To split the spectrum into bands with defined upper-to-lower frequency ratios Correct answer.
  4. To measure only impulsive noise

Explanation

Octave and fractional-octave filters divide the audible spectrum into contiguous bands whose edge frequencies have a fixed ratio, enabling frequency-specific level reporting. They do not selectively delete the high spectrum or convert weightings.

Question 21Fundamentals of Acoustics and Vibration

A vibrating surface has a radiation efficiency much less than one at low frequency. What does this imply?

  1. It radiates more acoustic power than a large baffled piston of the same velocity
  2. It has no vibration at all
  3. It radiates acoustic power inefficiently relative to its vibration velocity Correct answer.
  4. Its sound power equals its mechanical power input

Explanation

Radiation efficiency compares radiated acoustic power with that of an ideal piston of the same area and velocity. A value well below one means weak coupling to the air, so little acoustic power is produced for a given vibration.

Question 22Fundamentals of Acoustics and Vibration

Two coherent pure tones of equal amplitude and slightly different frequencies are summed. What acoustic phenomenon results?

  1. Complete cancellation at all times
  2. A single tone at the average frequency with constant amplitude
  3. Beats, with amplitude modulation at the difference frequency Correct answer.
  4. A doubling of the frequency

Explanation

Summing two equal-amplitude sinusoids of close frequencies produces a signal whose envelope varies at the difference frequency, perceived as beats. The carrier sits near the average of the two frequencies.

Question 23Fundamentals of Acoustics and Vibration

For a mass-spring-damper system, what does the quality factor Q of 20 imply about the damping ratio?

  1. The damping ratio is about 0.05
  2. The damping ratio is about 0.5
  3. The damping ratio is about 2
  4. The damping ratio is about 0.025 Correct answer.

Explanation

For light damping the quality factor is approximately the reciprocal of twice the damping ratio, so a Q of 20 gives a damping ratio near 0.025. Higher Q means sharper resonance and less damping.

Question 24Fundamentals of Acoustics and Vibration

A point source in a room has a sound power level of 90 dB. In the reverberant field the room constant is 200 square metres. Approximately what reverberant sound pressure level results at a distance well beyond the critical distance?

  1. About 96 dB
  2. About 90 dB
  3. About 83 dB Correct answer.
  4. About 77 dB

Explanation

The reverberant level is approximately the sound power level plus 10 log10(4/R), where R is the room constant. With R equal to 200, 4/R is 0.02, whose logarithm is about minus 17, giving roughly 83 dB after accounting for the standard reference constants.

Question 25Fundamentals of Acoustics and Vibration

In a plane progressive wave, what is the phase relationship between sound pressure and particle velocity?

  1. They are in phase Correct answer.
  2. Pressure lags velocity by 90 degrees
  3. They are 180 degrees out of phase
  4. Pressure leads velocity by 90 degrees

Explanation

In a plane travelling wave, pressure and particle velocity reach their maxima together, so they are in phase. A 90-degree difference appears in reactive near fields or standing waves.

Question 26Fundamentals of Acoustics and Vibration

A sound wave propagates through air at 20 °C. If the frequency of the wave is doubled while the medium remains unchanged, what happens to the wavelength?

  1. It is quartered.
  2. It is doubled.
  3. It remains unchanged.
  4. It is halved. Correct answer.

Explanation

The wavelength λ is related to the speed of sound c and frequency f by λ = c/f. Since c is constant for a given medium and temperature, doubling f halves λ. Thus the wavelength is inversely proportional to frequency.

Question 27Fundamentals of Acoustics and Vibration

Which of the following best describes the relationship between sound pressure level (SPL) and sound intensity level (SIL) for a plane progressive wave in air under standard conditions?

  1. SPL and SIL differ by a factor that depends on frequency.
  2. SPL and SIL are numerically equal in decibels. Correct answer.
  3. SIL is always 10 dB higher than SPL.
  4. SPL is always 20 dB higher than SIL.

Explanation

For a plane progressive wave in a free field, the sound intensity I is related to the mean square sound pressure p_rms^2 by I = p_rms^2/(ρc), where ρc is the characteristic impedance. Under standard conditions, the reference values (20 µPa and 10^-12 W/m²) are chosen such that the decibel levels coincide numerically. Hence SPL and SIL are approximately equal for a plane wave in air.

Question 28Fundamentals of Acoustics and Vibration

A simple harmonic oscillator has a displacement amplitude of 2 mm at a frequency of 50 Hz. What is the approximate acceleration amplitude?

  1. 197 m/s² Correct answer.
  2. 39.5 m/s²
  3. 9.87 m/s²
  4. 0.63 m/s²

Explanation

For simple harmonic motion, acceleration amplitude a = ω²X, where ω = 2πf and X is displacement amplitude. Here ω = 2π×50 ≈ 314.16 rad/s, so a = (314.16)² × 0.002 ≈ 197 m/s². This illustrates the strong frequency dependence of acceleration.

Question 29Fundamentals of Acoustics and Vibration

In a single-degree-of-freedom system, what is the phase difference between the excitation force and the displacement response at resonance for a lightly damped system?

  1. 90 degrees Correct answer.
  2. 0 degrees
  3. 180 degrees
  4. 45 degrees

Explanation

At resonance, the excitation frequency equals the natural frequency, and the displacement response lags the force by 90 degrees for a system with light damping. This is a standard result from the frequency response function of a mass-spring-damper system.

Question 30Fundamentals of Acoustics and Vibration

A sound source emits a sound power level of 80 dB. What is the sound power in watts? (Reference power = 10^-12 W.)

  1. 0.0002 W
  2. 0.0004 W
  3. 0.001 W
  4. 0.0001 W Correct answer.

Explanation

Sound power level L_W = 10 log10(W/W0). Rearranging gives W = W0 × 10^(L_W/10). With L_W = 80 dB and W0 = 10^-12 W, W = 10^-12 × 10^8 = 10^-4 W = 0.0001 W.

Question 31Fundamentals of Acoustics and Vibration

Which of the following statements about the addition of two sound pressure levels from incoherent sources is correct?

  1. The combined level is always 3 dB higher than the higher level.
  2. The combined level is the logarithmic sum of the two levels in dB.
  3. The combined level is the arithmetic sum of the two levels in dB.
  4. The combined level is found by adding the squares of the sound pressures and then converting back to dB. Correct answer.

Explanation

For incoherent sources, sound intensities (or mean square pressures) add linearly. Therefore, the combined sound pressure level is calculated by summing the mean square pressures and converting the total back to a level. This is not a simple arithmetic addition of decibel values.

Question 32Fundamentals of Acoustics and Vibration

A microphone measures a sound pressure of 0.1 Pa. What is the corresponding sound pressure level in dB re 20 µPa?

  1. 80 dB
  2. 68 dB
  3. 86 dB
  4. 74 dB Correct answer.

Explanation

Sound pressure level L_p = 20 log10(p/p0), where p0 = 20 µPa = 2×10^-5 Pa. For p = 0.1 Pa, the ratio is 0.1/(2×10^-5) = 5000, and 20 log10(5000) ≈ 20 × 3.699 = 73.98 dB, approximately 74 dB.

Question 33Fundamentals of Acoustics and Vibration

In a 1/3-octave band filter, what is the approximate ratio of the upper band-edge frequency to the lower band-edge frequency?

  1. 1.41
  2. 2.00
  3. 1.26 Correct answer.
  4. 1.12

Explanation

A 1/3-octave band has a bandwidth where the ratio of upper to lower band-edge frequencies is 2^(1/3) ≈ 1.26. This is a standard definition in octave band analysis.

Question 34Fundamentals of Acoustics and Vibration

A wall has a sound reduction index (transmission loss) of 40 dB at 500 Hz. What fraction of the incident sound power is transmitted through the wall?

  1. 0.0001 Correct answer.
  2. 0.01
  3. 0.001
  4. 0.1

Explanation

The sound reduction index R = 10 log10(1/τ), where τ is the transmission coefficient. For R = 40 dB, 1/τ = 10^4, so τ = 10^-4 = 0.0001. Thus one ten-thousandth of the incident power is transmitted.

Question 35Fundamentals of Acoustics and Vibration

What is the primary purpose of applying a Hanning window to a sampled signal before performing a Fast Fourier Transform (FFT)?

  1. To remove high-frequency noise from the signal.
  2. To compensate for the microphone's frequency response.
  3. To reduce spectral leakage caused by non-integer number of cycles in the sample. Correct answer.
  4. To increase the frequency resolution of the FFT.

Explanation

A Hanning window tapers the signal at the beginning and end of the sample to reduce discontinuities, thereby minimizing spectral leakage that occurs when the signal does not contain an integer number of cycles within the sample window. It does not improve frequency resolution or remove noise.

Question 36Fundamentals of Acoustics and Vibration

In a diffuse sound field in a room, how does the sound pressure level relate to the sound power level of the source?

  1. It depends on the room constant and the distance from the source. Correct answer.
  2. It is directly proportional to the sound power level in dB.
  3. It is independent of the room's absorption.
  4. It decreases by 6 dB per doubling of distance from the source.

Explanation

In a diffuse field, the sound pressure level is determined by the sound power level and the room constant R (which depends on total absorption). The relationship is L_p = L_W + 10 log10(4/R) for the reverberant field, so it depends on room absorption and not on distance in the diffuse field.

Question 37Fundamentals of Acoustics and Vibration

What is the characteristic impedance of air at 20 °C and standard atmospheric pressure, commonly used to relate sound pressure and particle velocity?

  1. 20 µPa·s/m
  2. 1.21 Pa·s/m
  3. 415 Pa·s/m Correct answer.
  4. 340 Pa·s/m

Explanation

The characteristic impedance of air is ρc, where ρ is the density (about 1.21 kg/m³ at 20 °C) and c is the speed of sound (about 343 m/s). This gives approximately 415 Pa·s/m (or N·s/m³).

Question 38Fundamentals of Acoustics and Vibration

A machine's vibration is measured as 0.5 m/s² RMS at 50 Hz. If the same acceleration amplitude occurred at 100 Hz, what would be the approximate velocity amplitude?

  1. 0.0008 m/s Correct answer.
  2. 0.0016 m/s
  3. 0.0004 m/s
  4. 0.0032 m/s

Explanation

For sinusoidal vibration, acceleration amplitude a = ωv, so velocity amplitude v = a/ω. At 100 Hz, ω = 2π×100 ≈ 628.3 rad/s. Using the RMS acceleration 0.5 m/s², v_rms = 0.5/628.3 ≈ 0.000796 m/s, approximately 0.0008 m/s.

Question 39Fundamentals of Acoustics and Vibration

What is the main function of an octave band filter set in noise measurement?

  1. To amplify the overall sound level for better audibility.
  2. To convert sound pressure level to sound power level.
  3. To apply A-weighting to the signal.
  4. To separate sound into frequency bands for analysis of its spectral content. Correct answer.

Explanation

Octave band filters divide the audible frequency range into contiguous bands (octaves or fractions thereof) so that the sound energy in each band can be measured. This allows identification of dominant frequency components in noise.

Question 40Fundamentals of Acoustics and Vibration

A vibrating surface has a radiation efficiency much less than one at low frequency. What does this imply about its sound radiation?

  1. It radiates less sound power than a theoretically perfect radiator of the same area and vibration velocity. Correct answer.
  2. It radiates sound only at high frequencies.
  3. Its radiation is independent of frequency.
  4. It radiates more sound power due to increased directivity.

Explanation

Radiation efficiency (σ) is the ratio of actual radiated sound power to the power that would be radiated by a piston of the same area and RMS velocity in an infinite baffle. A value much less than one indicates inefficient radiation, often due to acoustic short-circuiting at low frequencies.

Question 41Measurement, Instrumentation and Signal Analysis

A sound level meter is set to 'A' frequency weighting. Which statement describes what the meter's weighting network does to the signal before it computes the level?

  1. It applies a constant gain across the entire audio band so the reading equals the unweighted level.
  2. It attenuates low and very high frequencies relative to the mid-band, approximating the ear's loudness response at moderate levels. Correct answer.
  3. It boosts all frequencies below 1 kHz to compensate for the reduced sensitivity of the ear at low frequencies.
  4. It removes frequency content above 20 kHz to suppress ultrasonic noise that could bias the level reading.

Explanation

A-weighting is a fixed filter whose gain is roughly 0 dB in the mid-band and falls off at low and high frequencies, approximating equal-loudness contours at moderate sound levels. It is defined by standardised pole-zero characteristics, so the meter's displayed level is the weighted, time-averaged result rather than a flat-band measurement.

Question 42Measurement, Instrumentation and Signal Analysis

An integrating sound level meter reports an equivalent continuous level over a 10-minute measurement. If the sound pressure level were a steady 70 dB for the entire period, what value would the meter display?

  1. It cannot be determined without knowing the crest factor.
  2. 60 dB
  3. 70 dB Correct answer.
  4. 80 dB

Explanation

The equivalent continuous level is the constant level that would deliver the same mean-square pressure over the measurement interval. For a truly steady signal the mean square is constant, so the energy average equals the instantaneous level. Crest factor affects peak measurements, not the energy average of a stationary signal.

Question 43Measurement, Instrumentation and Signal Analysis

Which quantity is obtained directly from a single-sided amplitude spectrum of a stationary tonal signal?

  1. The cumulative energy above a chosen frequency band edge
  2. The root-mean-square amplitude at each discrete frequency component Correct answer.
  3. The phase lag of each component relative to the trigger instant
  4. The probability density of the instantaneous amplitude

Explanation

A single-sided amplitude spectrum folds the negative-frequency half onto the positive axis and scales it so each line represents the rms amplitude of that sinusoidal component. Phase information is discarded in an amplitude-only spectrum, and neither amplitude distributions nor band energies are read directly from it.

Question 44Measurement, Instrumentation and Signal Analysis

A vibration accelerometer is mounted with a thin layer of soft adhesive tape rather than a stud. What is the most likely effect on the measured vibration?

  1. A raised mounted resonance, causing underestimation of low-frequency vibration
  2. No measurable effect, because the accelerometer crystal determines the response
  3. A lowered mounted resonance, causing overestimation of high-frequency vibration Correct answer.
  4. An increase in transverse sensitivity only, with no change in the axial response

Explanation

A compliant mounting layer acts as a series spring that lowers the mounted resonance frequency of the accelerometer. Once the measurement band approaches that resonance the response is no longer flat and high-frequency amplitudes are overestimated, so compliant mounts must be checked against the required frequency range.

Question 45Measurement, Instrumentation and Signal Analysis

The calibration of a sound level meter is checked before and after a measurement with a pistonphone. What physical quantity does the pistonphone provide to the meter?

  1. A known reverberation time in a closed coupler
  2. A known sound intensity in a defined direction
  3. A known sound pressure level at a specified frequency Correct answer.
  4. A known particle velocity amplitude in free field

Explanation

A pistonphone or acoustic calibrator generates a stable sinusoidal pressure of known level, typically near 94 or 114 dB at 250 Hz or 1 kHz, inside a sealed cavity over the microphone. The meter's sensitivity is adjusted so its indicated level matches that reference, which verifies the whole pressure-measurement chain.

Question 46Measurement, Instrumentation and Signal Analysis

A signal is sampled at 25.6 kHz. A 14 kHz tone is present in the continuous input. At what frequency does it appear in the discrete spectrum?

  1. 14 kHz
  2. 1.2 kHz
  3. 11.6 kHz Correct answer.
  4. 2.4 kHz

Explanation

Frequencies above the Nyquist frequency of 12.8 kHz alias to a mirror image about Nyquist. The 14 kHz tone folds to 25.6 minus 14, which is 11.6 kHz, so it appears in the spectrum as a component that is not physically present at that frequency in the signal.

Question 47Measurement, Instrumentation and Signal Analysis

Which detector characteristic determines how quickly a sound level meter's displayed level rises toward a steady value after the sound starts?

  1. The microphone's open-circuit sensitivity
  2. The time constant selected for the exponential averaging Correct answer.
  3. The range setting of the input attenuator
  4. The frequency weighting network selected

Explanation

Slow, fast and impulse settings change the exponential time constant used to average the squared signal, which governs how fast the indicated level tracks a change. Frequency weighting shapes the spectrum, and sensitivity or range affect scaling rather than the temporal response.

Question 48Measurement, Instrumentation and Signal Analysis

A measurement microphone with a nominal open-circuit sensitivity of 50 mV/Pa is connected to an amplifier of 20 dB voltage gain. What output voltage corresponds to a sound pressure of 1 Pa?

  1. 0.5 V Correct answer.
  2. 5 V
  3. 0.05 V
  4. 0.1 V

Explanation

At 1 Pa the microphone produces 50 mV, and a 20 dB voltage gain multiplies amplitude by ten, giving 0.5 V. Keeping track of whether a decibel figure is a power or voltage ratio is essential, since a 20 dB voltage gain is a factor of ten, not one hundred.

Question 49Measurement, Instrumentation and Signal Analysis

A rotating machine produces strong vibration at exactly the shaft rotation frequency. Which processing step best isolates the amplitude of this component from a broadband spectrum?

  1. Applying synchronous time-domain averaging triggered once per revolution Correct answer.
  2. Increasing the analyser's input range setting
  3. Averaging successive spectra in linear magnitude rather than power
  4. Applying a Hanning window before the transform

Explanation

Synchronous averaging keyed to a once-per-revolution tacho signal reinforces components coherent with rotation and averages away non-synchronous content such as random noise and other machine tones. It therefore extracts the shaft-rate component with a much improved signal-to-noise ratio compared with a single spectrum.

Question 50Measurement, Instrumentation and Signal Analysis

An engineer increases the FFT record length from 4096 to 16384 samples at the same sampling rate. What is the effect on the spectral result?

  1. Frequency resolution worsens and the measurement time per record decreases
  2. Frequency resolution improves and the measurement time per record increases Correct answer.
  3. The Nyquist frequency halves and the resolution stays the same
  4. The amplitude accuracy improves while the frequency resolution is unchanged

Explanation

Frequency resolution equals the sampling rate divided by the number of samples, so quadrupling the record length quarters the bin spacing. The longer record also takes four times as long to acquire, which matters for non-stationary signals.