1. Noise Fundamentals
Sound is a mechanical wave that propagates through a medium (air, in most industrial applications) as alternating compressions and rarefactions. Noise is unwanted sound, and in oil and gas facilities it originates primarily from rotating equipment, gas flow through restrictions, and combustion processes.
The Decibel Scale
Sound is measured on a logarithmic scale in decibels (dB) because the human ear responds logarithmically to sound intensity. The decibel scale compresses the enormous range of human hearing (a factor of 1012 in intensity from threshold to pain) into a manageable 0 to 140 dB range. A 3 dB increase represents a doubling of sound energy, while a 10 dB increase is perceived as roughly twice as loud.
Key Acoustic Terms
| Term | Symbol | Definition |
|---|---|---|
| Sound Pressure Level | SPL (Lp) | Measured at a point in space; what a microphone detects (dB re 20 μPa) |
| Sound Power Level | SWL (Lw) | Total acoustic energy radiated by a source; independent of distance (dB re 10−12 W) |
| Frequency | f | Number of pressure oscillations per second (Hz); determines pitch |
| A-Weighting | dBA | Frequency weighting that approximates human hearing sensitivity |
| Octave Band | OB | Frequency range where upper limit is twice the lower (e.g., 500–1000 Hz) |
| TWA | TWA | Time-weighted average noise exposure over a work shift |
Reference Sound Pressure Levels
| Source | Typical SPL (dBA) | Subjective Impression |
|---|---|---|
| Threshold of hearing | 0 | Barely perceptible |
| Quiet office | 40 | Quiet |
| Normal conversation (3 ft) | 60 | Moderate |
| Compressor building (interior) | 85–105 | Loud to very loud |
| Gas blowdown valve | 110–130 | Extremely loud |
| Threshold of pain | 130–140 | Painful |
2. Sound Pressure Level & Sound Power Level
Understanding the distinction between sound pressure level (SPL) and sound power level (SWL) is essential for noise calculations. SWL is a property of the source, while SPL depends on distance, direction, and environmental conditions.
Definitions
Sound Pressure Level:
Lp = 20 × log10(p / pref)
Where pref = 20 μPa (threshold of hearing)
Sound Power Level:
Lw = 10 × log10(W / Wref)
Where Wref = 10−12 watts
SPL from SWL at Distance
For a point source radiating uniformly in all directions (free field), the relationship between SWL and SPL at distance r is:
Free Field (spherical spreading):
Lp = Lw − 20 × log10(r) − 11 dB
Hemispherical spreading (source on ground):
Lp = Lw − 20 × log10(r) − 8 dB
Most industrial equipment operates at ground level, making hemispherical spreading the more appropriate model. The 3 dB difference between free-field and hemispherical models accounts for the ground reflection effectively doubling the radiating area.
Directivity
Real sources are not omnidirectional. Equipment like exhaust stacks, air inlet filters, and blowdown valves have distinct directivity patterns that concentrate sound energy in particular directions. Directivity is expressed as a directivity index (DI) in dB, which is added to the calculated SPL in the direction of maximum radiation. Manufacturer data should specify both SWL and directivity characteristics.
3. Distance Attenuation
Sound level decreases with distance from the source due to geometric spreading and absorption by the atmosphere. Both effects must be considered for accurate noise predictions at receptor locations.
Geometric Divergence
As sound radiates from a point source, the energy spreads over an increasing surface area, reducing the intensity at any given point. This follows the inverse square law:
SPL2 = SPL1 − 20 × log10(r2 / r1)
For every doubling of distance: ΔSPL = −6 dB
| Distance from Source (ft) | Relative SPL (dB) | Multiplier from Reference |
|---|---|---|
| 3 (reference) | 0 | 1× |
| 6 | −6 | 2× |
| 12 | −12 | 4× |
| 25 | −18 | 8× |
| 50 | −24 | 16× |
| 100 | −30 | 32× |
| 200 | −36 | 64× |
| 400 | −42 | 128× |
| 1000 | −50 | ~333× |
Atmospheric Absorption
Air absorbs sound energy, particularly at high frequencies. The absorption rate depends on temperature, humidity, and frequency. ISO 9613-1 provides the standard calculation method.
| Octave Band Center Frequency (Hz) | Absorption at 20°C, 50% RH (dB/1000 ft) |
|---|---|
| 63 | 0.1 |
| 125 | 0.3 |
| 250 | 0.7 |
| 500 | 1.2 |
| 1000 | 2.3 |
| 2000 | 5.5 |
| 4000 | 15.5 |
| 8000 | 50.0 |
Ground Effect
Sound propagation over soft ground (grass, soil) produces additional attenuation beyond geometric spreading due to destructive interference between the direct wave and ground-reflected wave. This effect is most significant in the 200–600 Hz range and can provide 3–6 dB of additional attenuation at distances of 200–600 ft. Hard surfaces (concrete, water) may produce constructive interference, increasing SPL by up to 3 dB. ISO 9613-2 provides methods for calculating ground effects.
Total Attenuation
Atotal = Adiv + Aatm + Aground + Abarrier + Amisc
Where: Adiv = geometric divergence, Aatm = atmospheric absorption, Aground = ground effect, Abarrier = barrier insertion loss, Amisc = vegetation, buildings, etc.
4. Barriers & Enclosures
Noise barriers and acoustic enclosures are common control measures at compressor stations and gas processing facilities to reduce noise levels at property boundaries and nearby residences.
Barrier Insertion Loss
A barrier works by blocking the direct sound path from source to receiver, forcing sound to diffract over the top (and around the ends) of the barrier. The insertion loss depends on the path length difference (Fresnel number):
Fresnel Number: N = 2δ / λ
Where δ = path difference (A + B − d), A = source to barrier top, B = barrier top to receiver, d = direct source-to-receiver distance, λ = wavelength
Barrier IL ≈ 10 × log10(3 + 20N) dB (for N > 0)
Practical Barrier Performance
| Barrier Type | Typical IL (dBA) | Application |
|---|---|---|
| Earth berm (10–15 ft) | 8–15 | Property line noise; economical for rural compressor stations |
| Concrete/masonry wall (8–12 ft) | 10–18 | Compact sites; permanent installations |
| Metal panel barrier with absorber | 12–20 | Close proximity; limited space |
| Full acoustic enclosure | 20–40 | Compressor buildings; engine enclosures |
Barrier Design Rules
For effective barrier performance: (1) the barrier must break the line of sight from source to receiver, (2) the barrier should extend at least two wavelengths beyond the source on each side to minimize flanking, (3) the barrier surface density should be at least 4 lb/ft² (20 kg/m²) to prevent significant transmission through the barrier, and (4) absorptive treatment on the source side improves performance by 2–3 dB by reducing reflections.
Acoustic Enclosures
Compressor buildings and engine enclosures provide the highest noise reduction but require careful design of ventilation openings, exhaust paths, and access doors to maintain acoustic integrity.
| Component | Design Consideration |
|---|---|
| Walls and roof | STC 35–50; insulated metal panels or CMU with interior absorption |
| Ventilation openings | Silenced intake and exhaust plenums; duct silencers rated for required IL |
| Access doors | Acoustically rated doors with seals; STC matching wall rating |
| Pipe penetrations | Vibration isolation boots; sealed penetrations with resilient material |
| Exhaust stacks | Hospital-grade mufflers; reactive or absorptive silencers for engine exhaust |
5. Combining Multiple Sources
Midstream facilities typically have multiple noise sources operating simultaneously. Because the decibel scale is logarithmic, sound levels from multiple sources cannot be simply added; they must be combined logarithmically.
Logarithmic Addition
Ltotal = 10 × log10(10L1/10 + 10L2/10 + ... + 10Ln/10)
Quick Addition Rules
| Difference Between Two Sources (dB) | Add to Higher Level (dB) |
|---|---|
| 0 (equal levels) | +3.0 |
| 1 | +2.5 |
| 2 | +2.1 |
| 3 | +1.8 |
| 5 | +1.2 |
| 7 | +0.8 |
| 10 | +0.4 |
| 15 | +0.1 |
| 20 or more | +0.0 (negligible) |
Practical Implications
When one source is 10 dB or more louder than another, the quieter source adds negligibly to the total. This means that at most facilities, the loudest one or two sources dominate the overall noise level, and reducing noise from minor sources has minimal impact. Effective noise control must target the dominant source(s) first.
6. OSHA Requirements
OSHA's Occupational Noise Exposure standard (29 CFR 1910.95) establishes permissible exposure limits and requires hearing conservation programs for workers exposed to noise above the action level.
Permissible Exposure Limits
| Duration per Day (hours) | Sound Level (dBA) |
|---|---|
| 8 | 90 |
| 6 | 92 |
| 4 | 95 |
| 3 | 97 |
| 2 | 100 |
| 1 | 105 |
| 0.5 | 110 |
| 0.25 | 115 |
OSHA uses a 5 dB exchange rate (doubling rate): for every 5 dB increase in noise level, the permissible exposure time is halved. This is more lenient than the 3 dB exchange rate recommended by NIOSH and used internationally.
Dose Calculation
D = ∑ (Ci / Ti) × 100%
Where Ci = actual duration at level i, Ti = permitted duration at level i
TWA = 16.61 × log10(D/100) + 90 dBA
Hearing Conservation Program Requirements
When worker exposure equals or exceeds 85 dBA TWA (the action level), OSHA requires: (1) noise monitoring, (2) audiometric testing (baseline and annual), (3) hearing protection made available, (4) training on noise hazards and protection, and (5) recordkeeping. When exposure equals or exceeds 90 dBA TWA (the PEL), feasible engineering and administrative controls must be implemented to reduce exposure.
Community Noise Standards
In addition to OSHA workplace requirements, many jurisdictions impose community noise limits at property boundaries or the nearest receptor:
| Standard / Jurisdiction | Daytime Limit | Nighttime Limit |
|---|---|---|
| Typical rural residential | 55 dBA Leq | 45–50 dBA Leq |
| Typical suburban residential | 60 dBA Leq | 50–55 dBA Leq |
| Industrial zone | 70 dBA Leq | 65 dBA Leq |
| FERC Compressor Stations | 55 dBA Ldn at nearest NSA (day-night average) | |
7. Equipment Noise Levels
Accurate noise predictions require reliable source data. Equipment manufacturers should provide sound power level (SWL) data in octave bands as part of equipment specifications. When manufacturer data is unavailable, the following typical values can be used for preliminary estimates.
Typical Equipment Noise Levels
| Equipment | Typical SWL (dBA) | SPL at 3 ft (dBA) | Primary Frequency Range |
|---|---|---|---|
| Reciprocating compressor (1000 HP) | 115–125 | 95–105 | Low-mid (63–500 Hz) |
| Centrifugal compressor (5000 HP) | 110–120 | 90–100 | Mid-high (500–4000 Hz) |
| Gas turbine (unenclosed) | 120–130 | 100–110 | Broadband |
| Gas engine exhaust (unmuffled) | 125–140 | 105–120 | Low (31–250 Hz) |
| Air-cooled heat exchanger (per bay) | 95–105 | 75–85 | Low-mid (125–500 Hz) |
| Control valve (gas service, high ΔP) | 100–125 | 80–105 | Mid-high (1000–8000 Hz) |
| Blowdown valve | 130–145 | 110–125 | Broadband (high energy) |
| Electric motor (100 HP) | 85–95 | 65–75 | Mid (500–2000 Hz) |
Piping Noise
Piping downstream of pressure-reducing valves, relief valves, and blowdown valves can radiate significant noise. The pipe wall acts as a speaker, transmitting aerodynamic noise generated inside the pipe to the surrounding air. Pipe lagging (acoustic insulation) can reduce pipe-radiated noise by 10–25 dB depending on the insulation system and frequency. IEC 60534-8-3 provides methods for predicting valve noise, including pipe radiation.
8. Noise Mitigation Strategies
The noise control hierarchy prioritizes reducing noise at the source, then along the path, and finally at the receiver. The most effective and economical noise control is achieved during the design phase before equipment is purchased and installed.
Source Controls
| Measure | Typical Reduction | Application |
|---|---|---|
| Low-noise equipment specification | 5–15 dB | Specify noise limits in purchase orders |
| Hospital-grade exhaust mufflers | 25–35 dB | Engine and turbine exhaust |
| Multi-stage pressure reduction | 10–20 dB | Control valves with high pressure drop |
| Low-noise valve trim | 10–25 dB | Cage-guided, multi-hole, or diffuser trim |
| Inlet silencers | 15–25 dB | Compressor and turbine air inlets |
Path Controls
| Measure | Typical Reduction | Application |
|---|---|---|
| Acoustic enclosure | 20–40 dB | Compressor buildings, engine packages |
| Noise barrier wall | 8–18 dB | Property boundary; residential proximity |
| Earth berm | 8–15 dB | Rural sites with available space |
| Pipe lagging | 10–25 dB | High-energy piping downstream of letdown valves |
| Increased setback distance | 6 dB per doubling | Site layout optimization |
Cost-Effective Noise Control
Specifying low-noise equipment at the design stage is almost always more cost-effective than retrofitting noise control after installation. A 5 dB noise reduction specification on a compressor package may add 2–5% to the equipment cost, while a post-installation acoustic enclosure achieving similar reduction can cost 20–40% of the equipment price. Early engagement of an acoustical consultant during facility design is strongly recommended for sites near noise-sensitive areas.