Noise & Vibration Reference · CIH Ch.7
CIH Noise Control and Vibration
The rest of Chapter 7, in the order a noise problem is handled: the physics, the damage, the OSHA program, the controls, then vibration and a self-test. The fact an exam question most often leans on is in red.
How to use this page
BGC’s Noise subject area covers the health effects of noise and vibration, combining sources, octave-band measurement, audiometric programs and controlBGC. Three companion pages go deep on one topic each; this page ties them together and fills the gaps.
- Companion pages: frequency weighting, dosimeter setup, ear anatomy.
- Red text is the recognition cue; yellow cards are memory tricks and traps.
- Original wording, not a copy of any course text or BGC material. Sources are on the CIH references page.
01 Sound Basics You Need
The physics behind every noise formula: what a decibel is, how levels add, and what distance does.
| Quantity | Key fact | CIH study cue |
|---|---|---|
| Sound pressure level Equation card › | Lp = 20 log(p / p0), with p0 = 20 µPa (0 dB, the threshold of hearing) | 20 log because intensity goes with pressure squared |
| Sound power and intensity levels Power level card › Intensity level card › | LW = 10 log(W / W0), W0 = 10−12 W; intensity uses 10 log as well | Power belongs to the source; pressure is what the ear receives at a location |
| Wavelength Equation card › | c = fλ; sound travels about 343 m/s in air at 20 °C | An absorber works best when at least a quarter-wavelength thickOSHA, so low frequencies are the hard ones to absorb |
| Adding sources Total level card › | Doubling the sound power adds 3 dBOSHA; ten identical sources add 10 dB | Two sources more than 10 dB apart: the quieter one barely matters |
| Distance Equation card › | From a point source in free field, the level drops 6 dB each time distance doublesOSHA; indoors, reflections make the drop smaller | Same inverse square law as radiation |
| Directivity (Q) Directivity index card › Point source card › | Q = 1 in free space, 2 on a hard floor, 4 at a wall-floor edge, 8 in a corner; each doubling of Q adds about 3 dB | Moving a machine out of a corner lowers the level |
| Upper limits | Pain threshold around 140 dB (about 200 Pa). OSHA: impulse or impact noise should not exceed 140 dB peakOSHA | Peak limits use unweighted or C-weighted readings, not A |
Frequency weighting and dosimeter settings have their own pages: A-weighting with worked examples and dosimeter setup for OSHA and ACGIH.
02 How Noise Damages Hearing
From a temporary dullness after a loud shift to a permanent notch on the audiogram.
| Effect | What happens | CIH study cue |
|---|---|---|
| Temporary threshold shift (TTS) | Short-term loss of sensitivity after exposure; hair cells are fatigued and measured thresholds recover with quiet, though research links repeated TTS to permanent, sub-clinical nerve damage even after hearing tests return to baseline | The reason a baseline audiogram needs 14 hours without workplace noise |
| Permanent threshold shift (PTS) | Repeated exposure turns temporary loss into permanent loss as hair cells dieOSHA | Noise-induced loss is sensorineural and does not recover |
| The 4 kHz notch | Early noise damage shows as a dip in the audiogram between 3,000 and 6,000 Hz, most often at 4,000 HzOSHA | A 4 kHz notch points to noise, not age |
| Acoustic trauma | A single very intense impulse (blast, gunshot) mechanically damages the ear at once | Different injury and timescale from gradual noise-induced loss |
| Tinnitus | Ringing or buzzing; often an early warning sign | Ask about it in hearing conservation interviews |
| Ototoxicants | Some solvents, asphyxiants (carbon monoxide) and metals (lead) damage the inner ear, alone or combined with noiseOSHA | Combined exposure can harm hearing even below the noise PEL |
Conductive vs sensorineural loss, and why the anatomy drives each rule, is on the ear anatomy page.
03 The OSHA Hearing Conservation Program
Three trigger levels, five program elements, and the numbers an exam asks about.
| Trigger (8-hour TWA, A-weighted, slow) | What it requires |
|---|---|
| 85 dBA (action level; 50% dose) | A continuing hearing conservation program, and hearing protectors available at no costOSHA |
| 90 dBA (PEL; 100% dose) | Feasible administrative or engineering controls first; if they fall short, hearing protectors must be provided and worn |
| 140 dB peak | Ceiling for impulse or impact noise |
| Program element | Requirement |
|---|---|
| Monitoring | When exposure may reach 85 dBA; designed to find who belongs in the program and to choose hearing protectors |
| Baseline audiogram | Within 6 months of first exposure at or above the action level (1 year with a mobile test van, with protectors worn after 6 months), after at least 14 hours without workplace noise |
| Annual audiogram | Compared with the baseline each year |
| Standard threshold shift (STS) | An average shift of 10 dB or more at 2,000, 3,000 and 4,000 Hz in either ear; an age correction is allowed. The employee is told in writing within 21 days |
| Hearing protectors | Must bring exposure to 90 dBA or below, and to 85 dBA or below after an STS |
| Training | Repeated annually |
| Records | Noise exposure measurements: 2 years. Audiometric test records: duration of employment |
85 starts the program, 90 starts the controls
At the 85 dBA action level the program starts; at the 90 dBA PEL, engineering and administrative controls are required. OSHA uses a 5 dB exchange rate; ACGIH uses 85 dBA with a 3 dB exchange rate.
The numbers
6 months to the baseline, 14 quiet hours before it, 10 dB for an STS, 21 days to notify, 2 years for noise records.
04 Controlling Noise
Source first, then path, then the worker; hearing protectors last, and derated.
| Where | Controls | CIH study cue |
|---|---|---|
| Source | Buy quiet; reduce speed, impact and flow noise; mufflers; vibration isolation; damping panelsOSHA | Free-layer damping suits thin panels (under about ¼ inch); thicker casings need constrained-layer damping |
| Path | Enclosures, barriers and absorptive linings; distance | A barrier blocks sound (dense, airtight); an absorber soaks it up (porous, soft). Concrete reflects almost all the sound that hits it |
| Worker | A quiet control booth, less time in the area, job rotation, then hearing protectors | A purpose-built booth is engineering; simply designating a quiet room is administrative |
TL is not NR
Transmission loss (TL) is a property of the wall material. Noise reduction (NR) is what the enclosure actually delivers: NR = TL − 10 log(S/a), where S is the wall area and a the absorption inside. Sound builds up inside an enclosure, so NR is less than TL unless the inside is lined with absorber. Transmission loss equation card ›
Hearing protectors and the NRR
| Situation | Estimated exposure under the protector |
|---|---|
| C-weighted TWA available | TWA (dBC) − NRROSHA |
| Only A-weighted data | TWA (dBA) − (NRR − 7) |
| OSHA deciding whether engineering controls are needed | TWA (dBA) − (NRR − 7) × 50%OSHA |
| Dual protection (plugs + muffs) | Take the higher NRR, subtract 7, add 5 dB for the second protector |
Worked example
98 dBA TWA, NRR 25. PPE adequacy: 98 − (25 − 7) = 80 dBA, under 90. Engineering-control check: 98 − (18 × 0.5) = 89 dBA. Same protector, two questions, two answers.
Why derate at all
The NRR comes from ideal laboratory fitting. Real-world fit is often far worse, so the label overstates protection.
05 Hand-Arm and Whole-Body Vibration
The mechanical cousin of noise: same idea of frequency weighting, different target organs.
| Feature | Hand-arm vibration (HAV) | Whole-body vibration (WBV) |
|---|---|---|
| Typical sources | Grinders, chipping hammers, chain saws, impact wrenches, jackhammers | Seated or standing on vehicles and heavy equipment: trucks, loaders, forklifts, tractors |
| Health effects | Hand-arm vibration syndrome: vibration white finger (a secondary Raynaud’s phenomenon), numbness and tingling, loss of grip and dexterity | Low back pain and spinal disc damage; fatigue and discomfort; motion sickness at very low frequencies |
| Most sensitive frequencies | About 8–16 HzOSHA | About 4–8 Hz along the head-to-foot (z) axis |
| Measurement | Accelerometer at the point where vibration enters the hand, three axes; frequency-weighted acceleration in m/s² (ISO 5349-1) | Accelerometer in a seat pad where the body meets the surface, three axes (ISO 2631-1); the pad should weigh no more than about 10% of the person |
| Limits | OSHA has no vibration standard. ACGIH TLVs, based on the ISO methods, are the usual reference | |
| Controls | Low-vibration tools, anti-vibration mounts and handles, tool maintenance, limiting daily exposure time, keeping hands warm and dry | Suspension seats, smoother surfaces and speeds, cab isolation, limiting time |
Resonance and isolation
When the forcing frequency matches a system’s natural frequency, motion is amplified. An isolator only reduces transmission once the forcing frequency is more than about √2 (1.4) times the natural frequency; designers aim for 2–3 times or more. Below that, the mount can make vibration worse.
Mount the sensor properly
Fix the accelerometer rigidly along the axis being measured, and keep it light relative to what it sits on, or its own mass changes the reading.
06 Self-Test
Original scenarios. Cover the answer, decide, then open it.
1A worker’s 8-hour TWA is 87 dBA. Which OSHA requirements apply?
The hearing conservation program (at or above the 85 dBA action level), including hearing protectors made available at no cost. Engineering controls are not required until 90 dBA.
2Two identical machines each produce 88 dB at the operator. What is the combined level?
About 91 dB: doubling identical sources adds 3 dB.
3A reading is 94 dB at 2 m from a small source outdoors. What would you expect at 8 m?
About 82 dB: two doublings of distance at 6 dB each.
4An annual audiogram shows an average 12 dB shift at 2, 3 and 4 kHz in the left ear. What must happen?
It is a standard threshold shift. Notify the employee in writing within 21 days, refit or retrain on protectors, and protectors must now bring exposure to 85 dBA or below.
5A worker’s TWA is 100 dBA with earplugs rated NRR 29. Using only A-weighted data, is the protection adequate under OSHA’s method?
100 − (29 − 7) = 78 dBA, below 90, so yes. Using OSHA’s 50% factor for the engineering-control question: 100 − 11 = 89 dBA.
6A fan on rubber mounts runs at 1,200 rpm (20 Hz). The mounts have a natural frequency of 18 Hz. Will they isolate the vibration?
No. The frequency ratio is about 1.1, below √2, so the mounts are near resonance and may amplify vibration. Softer mounts with a much lower natural frequency are needed.
These are original practice scenarios written for this page. They are not BGC questions and do not predict exam content.
Scope from BGC’s subject-area definitions; requirements from OSHA 29 CFR 1910.95 and its Appendix B; control and vibration guidance from the OSHA Technical Manual, all listed on the references page (checked September 24, 2026). SPEP is not affiliated with BGC.