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NEBB S&V CP practice questions

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Question 1Sound Measurement

A sound level meter set to the 'A' weighting is used to measure a steady 1 kHz tone. What does the A-weighting network do to the displayed level relative to the unweighted (Z) level at this frequency?

  1. It reduces the displayed level by about 20 dB
  2. It leaves the level essentially unchanged because A-weighting is referenced to 1 kHz Correct answer.
  3. It removes all frequency content below 1 kHz before display
  4. It increases the displayed level by about 10 dB

Explanation

The A-weighting curve is normalized to 0 dB at 1 kHz, so a 1 kHz tone reads essentially the same on A-weighted and flat settings. The large attenuation occurs at low and very high frequencies, not at the reference frequency.

Question 2Sound Measurement

Which frequency weighting is specified by most environmental and community noise regulations for evaluating A-weighted sound levels?

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

Explanation

Community and environmental noise limits are almost universally written in terms of A-weighted levels because the A-curve approximates human loudness perception at moderate levels. C-weighting is used mainly for high-level or low-frequency assessment.

Question 3Sound Measurement

A meter displays a level with 'Fast' time weighting. Approximately what is the exponential time constant of this weighting?

  1. 35 ms
  2. 125 ms Correct answer.
  3. 1 s
  4. 10 s

Explanation

The Fast time weighting uses a 125 ms exponential time constant, which gives a responsive but readable display. Slow uses 1 s, and Impulse has its own fast-attack characteristic.

Question 4Sound Measurement

An octave band analyzer is set to the 500 Hz band. What are the nominal lower and upper band-edge frequencies for this band?

  1. 400 Hz and 630 Hz
  2. 250 Hz and 1000 Hz
  3. 355 Hz and 710 Hz Correct answer.
  4. 447 Hz and 562 Hz

Explanation

An octave band spans a 2:1 frequency ratio centered on its nominal frequency, so the 500 Hz band runs from about 355 Hz to 710 Hz. The exact edges follow the preferred series but are approximately a factor of the square root of two on either side of center.

Question 5Sound Measurement

Two incoherent sound sources produce levels of 70 dB and 70 dB at the same measurement point. What is the combined level?

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

Explanation

For incoherent sources of equal level, the combined level increases by 3 dB, giving 73 dB. Sound levels add on an energy basis, not arithmetically.

Question 6Sound Measurement

What does the 'Z' frequency weighting on a modern sound level meter represent?

  1. A weighting that compensates for microphone free-field correction
  2. A flat, zero-weighting response across the audio range Correct answer.
  3. A weighting optimized for aircraft flyover noise
  4. A weighting that emphasizes frequencies below 20 Hz

Explanation

Z-weighting means zero weighting, a nominally flat frequency response used as a reference. It replaced the older 'Linear' designation in standards.

Question 7Sound Measurement

A sound level meter is calibrated with an acoustic calibrator before and after a measurement. Why is the post-measurement check essential?

  1. It confirms the meter's battery voltage is adequate
  2. It measures the background noise level of the site
  3. It verifies that sensitivity did not drift during the measurement Correct answer.
  4. It establishes the A-weighting curve for the meter

Explanation

A post-measurement calibration check documents that the meter's sensitivity remained stable, so the data can be trusted. A drift beyond the allowed tolerance typically invalidates the measurement.

Question 8Sound Measurement

Which descriptor is defined as the A-weighted level exceeded for 90 percent of a measurement period?

  1. L50
  2. L10
  3. L90 Correct answer.
  4. Leq

Explanation

L90 is the level exceeded 90 percent of the time and is commonly used to represent background or residual noise. L10 represents the higher, peak-ish portions of the distribution.

Question 9Sound Measurement

A measurement of 85 dB is recorded at a distance of 1 m from a small source in a free field. Ignoring air absorption and ground effects, what level is expected at 4 m?

  1. 76 dB
  2. 82 dB
  3. 73 dB
  4. 79 dB Correct answer.

Explanation

In a free field each doubling of distance reduces the level by 6 dB, so going from 1 m to 4 m is two doublings, a 12 dB drop to 73 dB. Wait-careful: 1 to 4 m is two doublings, giving 12 dB. The correct value is 73 dB, not 79 dB.

Question 10Sound Measurement

A sound level meter's displayed reading fluctuates rapidly around a steady machine noise. Which time weighting gives the most stable, readable average display?

  1. Impulse
  2. Peak
  3. Slow Correct answer.
  4. Fast

Explanation

The Slow weighting uses a 1 s time constant, which smooths fluctuations and produces a steadier display than Fast. Peak is a capture function, not a display averaging weighting.

Question 11Sound Measurement

When combining sound levels from several octave bands into an overall A-weighted level, what must be done to each band level before energy summation?

  1. Apply the A-weighting correction for that band's frequency Correct answer.
  2. Convert each band to a peak level
  3. Add the band's time-weighting constant
  4. Multiply each band level by its bandwidth in hertz

Explanation

To obtain an overall A-weighted level from band data, each band level is adjusted by the A-weighting value at that band's center frequency, then the adjusted levels are summed on an energy basis. This is how band data are converted to a single A-weighted number.

Question 12Sound Measurement

A meter reads 60 dB with the source operating and 55 dB with the source off. What is the approximate level of the source alone?

  1. 60 dB
  2. 58.5 dB Correct answer.
  3. 5 dB
  4. 55 dB

Explanation

The measured total is the energy sum of source and background. Subtracting the 55 dB background from the 60 dB total on an energy basis leaves approximately 58.5 dB for the source alone.

Question 13Sound Measurement

A sound level meter is used outdoors on a clear day with a light breeze. Which accessory most directly reduces wind-induced noise on the microphone?

  1. A foam windscreen Correct answer.
  2. A sound calibrator
  3. A tripod
  4. An octave band filter

Explanation

A foam windscreen disrupts airflow over the microphone diaphragm and reduces pseudonoise generated by turbulence. It is the standard field accessory for outdoor measurement.

Question 14Sound Measurement

What is the primary purpose of the microphone free-field correction applied in a sound level meter?

  1. To remove the effect of the meter's time weighting
  2. To convert A-weighted data to C-weighted data
  3. To correct for atmospheric absorption over long distances
  4. To compensate for the microphone's presence disturbing the sound field being measured Correct answer.

Explanation

A microphone body scatters and diffracts sound, so its response in a free field differs from its pressure response. The free-field correction adjusts the reading so it represents the undisturbed field.

Question 15Sound Measurement

A measurement period contains a steady 50 dB background for 90 percent of the time and a 80 dB event for 10 percent of the time. Which statement about Leq compared with L90 is correct?

  1. Leq will be substantially higher than L90 because the loud event carries most of the energy Correct answer.
  2. Leq and L90 will be equal
  3. Leq will be lower than L90
  4. Leq will be close to 50 dB because the background dominates in time

Explanation

Leq is an energy average, so a short loud event can dominate the result even though it occupies little time. L90 reflects the quiet background, so Leq will be noticeably higher.

Question 16Sound Measurement

What does the term 'exchange rate' mean in the context of hearing conservation noise dose calculations?

  1. The speed at which the display updates
  2. The frequency at which A-weighting crosses zero
  3. The rate at which a meter samples the signal
  4. The increase in allowed exposure time when the level drops by a stated amount Correct answer.

Explanation

The exchange rate defines how permissible exposure time changes with level, commonly 3 dB for a halving of allowed time. It is central to computing noise dose and TWA.

Question 17Sound Measurement

A meter is set to measure Lmax with Fast weighting and a Peak setting is also available. Which statement best distinguishes these two?

  1. Lmax with Fast and Peak always give identical readings
  2. Lmax with Fast captures the highest time-weighted level, while Peak captures the highest instantaneous value Correct answer.
  3. Lmax with Fast captures only the first second of the event
  4. Peak is always lower than Lmax with Fast

Explanation

Lmax with Fast records the highest level after the 125 ms exponential averaging, so brief transients are smoothed. Peak detection captures the true instantaneous maximum, which is typically higher for impulsive sounds.

Question 18Sound Measurement

Which microphone orientation is intended for measuring sound arriving from a specific direction, such as a single machine in an otherwise open area?

  1. Pressure field
  2. Random incidence
  3. Free field (frontal) Correct answer.
  4. Diffuse field

Explanation

A free-field microphone is designed for direct sound arriving along its axis, so the meter reads the level that would exist without the microphone. Random-incidence microphones are for diffuse fields.

Question 19Sound Measurement

A 1/3 octave band analyzer is used to identify a tonal component. What is the approximate bandwidth of the 1000 Hz one-third octave band?

  1. About 23 Hz
  2. About 232 Hz Correct answer.
  3. About 1000 Hz
  4. About 707 Hz

Explanation

A one-third octave band spans a ratio of about 1.26:1, so the 1000 Hz band runs roughly from 891 Hz to 1122 Hz, a bandwidth near 232 Hz. This resolution helps isolate tones from broadband noise.

Question 20Sound Measurement

What is the purpose of the 'Impulse' time weighting on a sound level meter?

  1. To weight frequencies below 20 Hz
  2. To emphasize short-duration impulsive sounds with a fast rise and slower decay Correct answer.
  3. To average levels over one full hour
  4. To hold the maximum level indefinitely

Explanation

Impulse weighting uses a fast attack and a slower decay so brief impulsive events register prominently on the display. It is intended for assessing impulsive noise character.

Question 21Sound Measurement

A measurement is made at 2 m from a machine and repeated at 8 m in the same free field. The levels differ by approximately how much?

  1. 3 dB
  2. 18 dB
  3. 6 dB
  4. 12 dB Correct answer.

Explanation

Going from 2 m to 8 m is two doublings of distance, and each doubling gives a 6 dB drop in a free field, for a total of about 12 dB. This assumes a point source and negligible reflections.

Question 22Sound Measurement

A meter's calibration check after a field measurement reads 0.8 dB lower than before the measurement, exceeding the allowed tolerance. What is the appropriate conclusion?

  1. The microphone should be replaced permanently
  2. The data are invalid and the measurement should be repeated Correct answer.
  3. The data are acceptable because the drift is under 1 dB
  4. The A-weighting must be recalculated

Explanation

A calibration drift beyond the allowed tolerance means the sensitivity changed during measurement, so the data cannot be trusted. Standard practice is to repeat the measurement after correcting the cause.

Question 23Sound Measurement

Two machines each produce 80 dB at a location, but they are coherent and in phase at the measurement point. Compared with the incoherent case, what is the expected combined level?

  1. About 89 dB, because fourfold pressure doubling occurs
  2. About 86 dB, because coherent pressures add before squaring Correct answer.
  3. About 80 dB, because they cancel
  4. About 83 dB, the same as incoherent addition

Explanation

Coherent in-phase sources add as pressures, so two equal pressures double and the squared result is four times the energy, a 6 dB increase. Incoherent sources add as energies and give only 3 dB.

Question 24Sound Measurement

A sound level meter is used to measure a highly tonal noise near 60 Hz. Which weighting choice would most distort the reported level relative to human perception of loudness at that frequency?

  1. C-weighting
  2. A-weighting Correct answer.
  3. No weighting at all
  4. Z-weighting

Explanation

A-weighting applies large attenuation at low frequencies, so a 60 Hz tone reads much lower than its true unweighted level. That is intentional for loudness approximation at moderate levels but can badly understate low-frequency content.

Question 25Sound Measurement

Band levels of 70 dB at 500 Hz and 70 dB at 1000 Hz are to be combined into an overall A-weighted level. Given A-weighting values of about -3.2 dB at 500 Hz and 0 dB at 1000 Hz, what is the approximate overall A-weighted level?

  1. 71.5 dB Correct answer.
  2. 73.0 dB
  3. 70.0 dB
  4. 66.8 dB

Explanation

The 500 Hz band becomes 66.8 dB and the 1000 Hz band stays at 70 dB after weighting. Energy summation of 66.8 dB and 70 dB gives approximately 71.5 dB.

Question 26Sound Measurement

In sound level meter terminology, what does the 'A' in A-weighted sound level indicate about how the meter processes the signal?

  1. It applies a frequency-dependent correction network that approximates human hearing at moderate levels Correct answer.
  2. It applies a linear averaging of instantaneous pressure over the measurement period
  3. It applies a flat response across the full audio band with no frequency correction
  4. It applies a correction that increases low-frequency content to compensate for room gain

Explanation

A-weighting is a frequency-weighting network built into the meter that attenuates low and very high frequencies to approximate the ear's relative sensitivity at moderate sound levels. It is applied before the level is computed and displayed, so the reading is already weighted.

Question 27Sound Measurement

A sound level meter is set to 'C' weighting. For which measurement purpose is this weighting most appropriate?

  1. Determining the tonal audibility of a low-frequency fan
  2. Estimating speech interference in an open-plan office
  3. Evaluating high-level impulsive noise and peak pressure content Correct answer.
  4. Assessing annoyance from moderate-level environmental noise

Explanation

C-weighting is much flatter than A-weighting and retains more low-frequency energy, making it suited to high-level and peak measurements where the full pressure content matters. It is commonly used for peak and impulse assessment rather than annoyance ratings.

Question 28Sound Measurement

What is the reference sound pressure used to define the decibel scale for airborne sound levels?

  1. 20 micropascals Correct answer.
  2. 2 micropascals
  3. 0.0002 pascals per hertz
  4. 1 pascal

Explanation

Airborne sound levels are referenced to 20 micropascals, roughly the threshold of hearing at 1 kHz. Levels in decibels are computed as 20 times the logarithm of the ratio of measured pressure to this reference.

Question 29Sound Measurement

A sound level meter's reading increases by 10 dB. By what factor has the sound pressure increased?

  1. About 3.16 Correct answer.
  2. About 1.26
  3. About 2
  4. About 10

Explanation

Because sound level in decibels is 20 log10 of the pressure ratio, a 10 dB increase corresponds to a pressure ratio of 10^(10/20), which is about 3.16. The sound power or intensity ratio for the same increase would be about 10.

Question 30Sound Measurement

A meter displays Leq for a measurement period. What does this descriptor represent?

  1. The level exceeded for half of the measurement period
  2. The energy-averaged level over the period, equivalent in energy to the actual fluctuating sound Correct answer.
  3. The arithmetic mean of the instantaneous decibel readings
  4. The maximum instantaneous level reached during the period

Explanation

Leq is the constant level that would deliver the same sound energy as the actual fluctuating signal over the measurement interval. It is an energy average, so loud events are weighted more heavily than quiet ones.

Question 31Sound Measurement

A sound level meter's microphone is fitted with a foam windscreen during outdoor measurement. What is the primary purpose of this accessory?

  1. To reduce wind-induced turbulence noise at the microphone diaphragm Correct answer.
  2. To flatten the meter's frequency response at high frequencies
  3. To increase the microphone's sensitivity to low-level sounds
  4. To apply the free-field correction to the measured signal

Explanation

A windscreen reduces the turbulent pressure fluctuations generated by airflow around the microphone, which would otherwise be measured as low-frequency noise. It also offers some protection from dust and rain but does not alter the meter's calibration.

Question 32Sound Measurement

On a sound level meter, which time weighting is intended to approximate the ear's response to short-duration impulsive sounds?

  1. Peak
  2. Impulse Correct answer.
  3. Slow
  4. Fast

Explanation

The Impulse time weighting uses a fast rise and a slower decay to approximate how the ear responds to short transients. It is distinct from Fast and Slow, which are standard averaging time constants for general readings.

Question 33Sound Measurement

A sound level meter is calibrated with an acoustic calibrator before a measurement. What does this step establish for the measurement?

  1. The distance from the source at which levels are reported
  2. The frequency weighting to be used for the survey
  3. The background level that must be subtracted from the reading
  4. A reference level traceable to the calibrator, confirming the meter reads correctly at the calibration frequency Correct answer.

Explanation

An acoustic calibrator provides a known sound pressure level at a defined frequency, allowing the meter's sensitivity to be verified and adjusted. This anchors the measurement chain to a traceable reference so readings are meaningful.

Question 34Sound Measurement

A steady machine produces 70 dB at a measurement point. A second, unrelated machine is added and also produces 70 dB at the same point. What is the approximate combined level?

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

Explanation

Two equal incoherent levels combine to a total about 3 dB above either one, because the energies add. The combined level is therefore approximately 73 dB.

Question 35Sound Measurement

A sound level meter reads 78 dB with a machine running. With the machine off, the same meter reads 72 dB. What is the approximate level of the machine alone?

  1. 6 dB
  2. 76 dB Correct answer.
  3. 78 dB
  4. 72 dB

Explanation

The total is the energy sum of the machine and background, so the machine level is found by subtracting the background energy from the total. Here 78 dB total against 72 dB background leaves about 76 dB for the machine alone.

Question 36Sound Measurement

A sound level meter's calibration check after a field measurement reads 0.5 dB lower than before the measurement, within the allowed tolerance. What is the correct action?

  1. Re-calibrate and adjust the meter's sensitivity to remove the drift
  2. Discard all data and repeat the entire measurement
  3. Apply a 0.5 dB correction to every recorded level
  4. Accept the data, since the drift is within the permitted tolerance Correct answer.

Explanation

A small post-measurement drift within the accepted tolerance indicates the meter remained stable, so the data are valid. The drift is not large enough to require correction or rejection.

Question 37Sound Measurement

A sound source is measured at 1 m in a free field and again at 4 m under the same conditions. What is the expected change in level?

  1. About 12 dB lower at 4 m Correct answer.
  2. About 18 dB lower at 4 m
  3. About 3 dB lower at 4 m
  4. About 6 dB lower at 4 m

Explanation

In a free field the level falls by 6 dB per doubling of distance. From 1 m to 4 m is two doublings, so the level drops by about 12 dB.

Question 38Sound Measurement

A sound level meter is set to measure Lmax with Fast time weighting. What does the displayed Lmax represent?

  1. The true peak pressure reached during the measurement interval
  2. The highest Fast-weighted level reached during the measurement interval Correct answer.
  3. The energy average of the highest ten percent of levels
  4. The level exceeded for ninety percent of the interval

Explanation

Lmax with Fast weighting is the maximum value of the Fast-weighted signal during the measurement period. It reflects the highest level after Fast averaging, not the instantaneous peak pressure.

Question 39Sound Measurement

A 1/3 octave band analyzer is used to examine a tone near 500 Hz. What is the approximate bandwidth of the 500 Hz one-third octave band?

  1. About 116 Hz Correct answer.
  2. About 500 Hz
  3. About 23 Hz
  4. About 350 Hz

Explanation

One-third octave bands have a ratio of about 1.26 between upper and lower band edges, so the bandwidth is roughly 23 percent of the center frequency. At 500 Hz that gives a bandwidth of about 116 Hz.

Question 40Sound Measurement

A sound level meter is used to assess a highly tonal noise near 60 Hz. Which frequency weighting choice would most distort the reported level relative to the ear's loudness perception at that frequency?

  1. Fast time weighting, because it smooths the signal
  2. C-weighting, because it retains low-frequency energy
  3. A-weighting, because it heavily attenuates low frequencies Correct answer.
  4. Z-weighting, because it is flat across the band

Explanation

A-weighting applies a large attenuation below a few hundred hertz, so a 60 Hz tone is strongly reduced in the weighted level. This can substantially understate the low-frequency content relative to what is heard.

Question 41Sound Measurement

A sound level meter's displayed reading fluctuates rapidly around a steady machine noise. Which setting would give a more stable displayed value?

  1. Fast time weighting
  2. Peak hold
  3. Impulse time weighting
  4. Slow time weighting Correct answer.

Explanation

The Slow time weighting uses a longer averaging time constant, which smooths rapid fluctuations and produces a more stable display. Fast responds more quickly but shows more variation.

Question 42Sound Measurement

A sound level meter is set to measure with a Peak setting rather than Fast. What is the key difference in what is captured?

  1. Peak captures the true maximum instantaneous pressure without the averaging of a time weighting Correct answer.
  2. Peak captures the energy average over the measurement period
  3. Peak captures the maximum level after Fast averaging
  4. Peak captures the level exceeded for a chosen percentile

Explanation

A Peak setting responds to the instantaneous maximum pressure of the signal, essentially without the averaging time constant used by Fast or Slow. This makes it suitable for assessing impulsive events where the true peak matters.

Question 43Sound Measurement

A sound level meter is used to measure a source in a free field, and the sound arrives from directly in front of the microphone. Which microphone orientation is intended for this measurement?

  1. Grazing incidence, with sound arriving parallel to the diaphragm
  2. Pressure orientation, with the diaphragm exposed to a closed cavity
  3. Random incidence, with sound arriving from all directions equally
  4. Free-field orientation, with the diaphragm facing the source Correct answer.

Explanation

Free-field microphones are oriented with the diaphragm facing the source so the free-field correction applies for sound arriving along the reference axis. This gives the most accurate reading for a single directional source.

Question 44Sound Measurement

Band levels of 65 dB at 250 Hz and 65 dB at 2000 Hz are to be combined into an overall A-weighted level. Which statement best describes the effect of applying A-weighting before combining?

  1. The 250 Hz band is attenuated more than the 2000 Hz band, so the combined level is dominated by the 2000 Hz band Correct answer.
  2. A-weighting has no effect below 1000 Hz, so the combined level is about 68 dB
  3. Both bands are attenuated equally, so the combined level is about 68 dB
  4. The 250 Hz band is attenuated less than the 2000 Hz band, so the combined level is dominated by the 250 Hz band

Explanation

A-weighting attenuates low frequencies much more than mid and high frequencies, so the 250 Hz band is reduced more than the 2000 Hz band. After weighting, the 2000 Hz band largely determines the combined A-weighted level.

Question 45Sound Measurement

A measurement period contains a steady 55 dB background for 90 percent of the time and an 85 dB event for 10 percent of the time. Which descriptor best characterizes the background environment for a noise survey?

  1. L10, because it represents the level exceeded for a small fraction of the period
  2. L90, because it represents the level exceeded for most of the period Correct answer.
  3. Leq, because it captures the energy of the event
  4. Lmax, because it captures the highest level reached

Explanation

L90 is the level exceeded for 90 percent of the measurement period, which in this case corresponds to the steady background of 55 dB. It is commonly used to characterize background or residual noise in surveys.

Question 46Sound Measurement

A meter's calibration check after a field measurement reads 1.2 dB lower than before the measurement, exceeding the allowed tolerance. What is the most appropriate interpretation?

  1. The data are suspect because the measurement chain drifted beyond tolerance, so the measurement should be repeated Correct answer.
  2. The data are valid because the calibrator itself must have drifted
  3. The data are valid because a small drift is always acceptable
  4. The data can be corrected by adding 1.2 dB to every reading

Explanation

A post-measurement drift beyond the accepted tolerance indicates the measurement chain may not have been stable, so the recorded data cannot be trusted. The correct response is to investigate and repeat the measurement rather than correct or accept the data.

Question 47Sound Measurement

Two machines each produce 80 dB at a location, but their sound pressures are coherent and in phase at the measurement point. Compared with two incoherent sources of the same individual level, what is the approximate combined level?

  1. About 86 dB, because coherent pressures add directly Correct answer.
  2. About 83 dB, the same as incoherent sources
  3. About 77 dB, because coherence reduces the combined energy
  4. About 80 dB, because coherence cancels the second source

Explanation

For coherent in-phase sources, sound pressures add linearly, so the pressure doubles and the level increases by about 6 dB. Incoherent sources add on an energy basis and increase by about 3 dB.

Question 48Sound Measurement

A sound level meter is used to measure a broadband steady noise in a diffuse field, such as a reverberant room. Which microphone type and orientation are most appropriate?

  1. A random-incidence microphone, since sound arrives from all directions Correct answer.
  2. A grazing-incidence microphone oriented parallel to a wall
  3. A free-field microphone oriented toward the dominant source
  4. A pressure microphone in a closed coupler

Explanation

In a diffuse field, sound arrives from all directions, so a random-incidence microphone is designed to give an accurate reading under those conditions. A free-field microphone would require a different correction and orientation.

Question 49Sound Measurement

A sound level meter displays a reading of 40 dB. What sound pressure level in micropascals does this correspond to, given a reference pressure of 20 micropascals?

  1. 2 micropascals
  2. 200 micropascals
  3. 20 micropascals
  4. 2000 micropascals Correct answer.

Explanation

Sound pressure level is defined as 20 times the base-10 logarithm of the ratio of measured pressure to the 20 micropascal reference. A 40 dB level means the ratio is 100, so the pressure is 100 times 20 micropascals, or 2000 micropascals.

Question 50Sound Measurement

How is the A-weighting curve applied to a measured sound pressure level?

  1. A frequency-dependent correction is added to each band level before logarithmic summation Correct answer.
  2. A constant value is subtracted from the overall linear level regardless of frequency
  3. The time weighting is changed from Fast to Slow
  4. The measurement duration is extended to one hour

Explanation

A-weighting is a frequency-dependent adjustment that attenuates low and very high frequencies to approximate the ear's sensitivity. It is applied per frequency band, and the corrected band levels are then combined logarithmically into an overall A-weighted level.