MECH7029 Chap.3 Hearing, Measurement and Noise Descriptors
Hearing, Measurement and Noise Descriptors
The descriptor belongs to a question
This chapter follows sound through the ear and through a measurement chain. Microphone type, orientation, calibration and field geometry define the evidence before a digital value appears. Equivalent and percentile levels then describe different aspects of a varying record.
A legal or community threshold is kept tied to its descriptor, duration and measurement position rather than treated as a universal boundary.
Measurement choices define the measurand
A microphone converts pressure to an electrical signal, but its design and orientation affect the pressure it estimates. Free-field, pressure and random-incidence responses suit different arrival conditions.
A calibrator checks sensitivity at a known pressure and frequency before and after a survey; agreement does not prove that wind, reflections, mounting or position were controlled.
A defensible record therefore carries geometry, environmental conditions, response settings and calibration results alongside the displayed number.
The ear is frequency-selective because its mechanical pathway and cochlear response vary across frequency. That motivates weighting, but a weighted environmental value is not automatically a hearing-risk measure.
Audibility, long-term exposure, annoyance and compliance are distinct questions that can require different descriptors and procedures.
Time variation requires a declared statistic
Equivalent continuous level converts energy accumulated during an interval into one constant level. A percentile instead reports the threshold exceeded for a stated proportion of that interval.
L10 can reveal upper events while L90 can characterise a quieter background, yet neither is meaningful without duration and ranking convention. Two sites can share an equivalent level while having very different distributions. The conclusion must link the statistic to the response or rule it was selected to represent.
What this chapter covers
- 01
Outer, middle and inner ear roles
- 02
Frequency-dependent hearing response
- 03
Sound-level-meter signal chain
- 04
Microphone type and directional response
- 05
Calibration before and after measurement
- 06
Equivalent and percentile noise levels
Worked example · free
Find a percentile level from ranked samples
- 2Convert ten percent of the duration into a sample count.
- 2Select the ranked boundary value.
- 2Interpret the complete descriptor.
Key terms
- Sound-Level Meter
- An instrument chain combining microphone, processing and display.
- Free-Field Response
- A microphone response corrected for directed sound in open space.
- Pressure Response
- A microphone response suited to flush or pressure-field measurement.
- Calibration Check
- A known-level test used to verify measurement sensitivity and stability.
- Equivalent Level
- The constant level containing the same acoustic energy over a duration.
- Percentile Level
- The level exceeded for a stated portion of measurement time.
Hearing, Measurement and Noise Descriptors FAQ
Why can a calibrated microphone still give the wrong field result?
Calibration verifies sensitivity under the calibrator condition. It does not correct an unsuitable directional response, reflection from the microphone body, poor position, wind or changing background. Those conditions must be controlled and documented separately.
What is the difference between Leq and L10?
Leq is an energy-equivalent average across a stated duration. L10 is an order statistic exceeded for ten percent of that duration. Intermittent peaks can change them differently, so one cannot be substituted for the other without changing the question.
Does an environmental threshold prove a sound is safe for hearing?
Not by itself. Environmental rules use descriptors chosen for a defined community or compliance context. Hearing risk depends on exposure, duration and frequency content, so the purpose and measurement procedure attached to the threshold must remain visible.
Exam move
Revise this chapter as a chain from decision to evidence rather than as a list of instrument settings. Choose a scenario such as a façade survey, a duct measurement or a reverberant room. State the decision first, then select the descriptor, averaging duration, microphone response, orientation, position and calibration sequence that fit it. For each choice, write one causal sentence explaining what bias it controls.
Next, take twelve sample levels, sort them manually and mark two exceedance boundaries. Keep the subscript and total duration attached to every answer, and contrast each percentile with the equivalent continuous level from the same record. Alter the record by replacing one sample with a brief high event; predict which statistic moves most before recomputing.
Create a second checklist for field validity: pre-check, geometry, weather, background, post-check, corrections and rounding. If a before-and-after calibration difference is unacceptable, state that plausible readings do not rescue the survey.
End every practice response with two sentences: what the chosen descriptor supports, and what it cannot establish about hearing injury, annoyance or compliance without the relevant procedure. Review mistakes by locating the first broken link in the chain. Repeating the final calculation is less useful than repairing a wrong descriptor, microphone choice or measurement geometry.
Before closing the notes, write a miniature report header containing location, duration, response, weighting and calibration status so that every numerical result remains attached to the conditions that created it.
