Compendium

Acoustics & Noise Control — Family Index

21 min read

Acoustics & Noise Control — Family Index

A consolidated Tier-3 reference covering wave physics, sound level quantification, absorbers, barriers, vibration isolation, room acoustics, HVAC noise, active noise control (ANC), NVH, transducers, and the standards ecosystem that wraps it all. Sister files cover structural dynamics, signal processing, HVAC, and polymers — see cross-references at the bottom.

1. At a glance

Sound is a mechanical wave — pressure perturbations propagating through a fluid or solid medium. Acoustics covers everything from infrasonic seismology to ultrasonic medical imaging, but practising engineers usually live in two overlapping fields:

  • Room / environmental acoustics — how energy radiates, reflects, decays in built or open spaces (concert halls, offices, highways, communities).
  • Noise + vibration control (NVH) — how to keep unwanted acoustic energy out of people (workers, residents) or sensitive equipment (microscopes, centrifuges, lithography).

Speed of sound c depends on medium and temperature:

  • Air ~343 m/s (1125 ft/s) at 20 °C (68 °F); c ≈ 331.3·√(1 + T_C/273.15).
  • Water (fresh, 20 °C) ~1480 m/s (4856 ft/s).
  • Steel (longitudinal) ~5100 m/s (16 700 ft/s); shear ~3200 m/s.
  • Concrete ~3500 m/s; glass ~5300 m/s; rubber ~60 m/s (very slow, why it makes a good vibration isolator).

Applications span: HVAC noise control, traffic + community noise, factory machinery, automotive NVH, audio (recording, hi-fi, hearing aids), sonar, ultrasound (NDT, medical imaging, cleaning, welding), MRI gradient noise, and pedestrian-alert systems for quiet EVs.

2. Wave fundamentals

Wave equation

For small-amplitude (linear) acoustics, pressure p satisfies:

∂²p/∂t² = c²·∇²p

Solutions are travelling waves; for a plane wave moving in +x: p(x,t) = P·cos(ωt − kx) with angular frequency ω = 2πf and wavenumber k = ω/c = 2π/λ.

Wave types

  • Plane wave — constant amplitude across phase fronts (far from source, or inside ducts/tubes well below cutoff).
  • Spherical wave — radiates from a point source; intensity ∝ 1/r², pressure ∝ 1/r → 6 dB drop per doubling of distance (free field).
  • Cylindrical wave — radiates from a line source (highway, train); pressure ∝ 1/√r → 3 dB drop per doubling.

Wavelength

λ = c/f. Practical values in air at 20 °C:

FrequencyWavelengthNotes
20 Hz17.2 m (56 ft)Subwoofer; large rooms only
100 Hz3.43 m (11.3 ft)Below most absorber bands
1 kHz0.343 m (13.5 in)Reference; STC band centre region
10 kHz34.3 mm (1.35 in)Air absorption dominant outdoors
20 kHz17.2 mm (0.68 in)Upper hearing limit
40 kHz8.6 mmUltrasonic parking sensor
5 MHz0.07 mmMedical ultrasound

Acoustic impedance

Z = ρ·c (rayls = Pa·s/m). Air ~415 rayls; water ~1.5 Mrayls; steel ~46 Mrayls. At an interface the reflection coefficient R = (Z₂−Z₁)/(Z₂+Z₁); power reflected = R². Air-to-water mismatch is so large that ~99.9 % of energy reflects — why submarine sonar has problems coupling to surface microphones, and why ultrasound gel matches transducer to skin.

3. Sound levels (decibels)

Acoustic quantities span 12+ orders of magnitude → logarithmic scale.

Three level definitions

LevelSymbolFormulaReference
Sound Pressure LevelL_p / SPL20·log₁₀(p/p_ref) dBp_ref = 20 µPa (threshold of hearing)
Sound Intensity LevelL_I / SIL10·log₁₀(I/I_ref) dBI_ref = 1 pW/m²
Sound Power LevelL_W / SWL10·log₁₀(P/P_ref) dBP_ref = 1 pW

In a free field at distance r from a point source: L_p ≈ L_W − 20·log₁₀(r) − 11.

Frequency weighting

  • dB(A) — A-weighting curve attenuates lows + very-highs to approximate the equal-loudness contours at moderate levels. Used for hearing-damage and community-noise regulation.
  • dB(C) — flatter, used for peak/impulse noise.
  • dB(Z) — unweighted (Z = zero).

Typical SPL values

SourcedB(A)Notes
Threshold of hearing020 µPa
Rustling leaves10
Library (quiet)40
Normal conversation @ 1 m60
Vacuum cleaner @ 1 m75
Lawnmower90OSHA action level region
Subway platform100
Rock concert110
Threshold of pain130–140
Jet engine @ 30 m140Hearing damage in seconds

Combining sources

Incoherent (uncorrelated) sources add on a power basis:

L_total = 10·log₁₀(Σ 10^(L_i/10))

Two equal sources → +3 dB; ten equal sources → +10 dB. Two sources 10 dB apart → ~+0.4 dB above the louder (the quiet one is negligible).

4. Frequency analysis

  • Octave bands — adjacent bands have 2× frequency ratio. Standard ISO centres: 31.5, 63, 125, 250, 500, 1000, 2000, 4000, 8000 Hz.
  • 1/3-octave bands — ratio 2^(1/3); preferred for architectural and product spec work.
  • STC (Sound Transmission Class) — ASTM E413 single-number partition rating; curve-fit to 16 third-octave TL values 125 Hz – 4 kHz.
  • R_w — ISO 717-1 European equivalent (uses 100 Hz – 3.15 kHz).
  • NRC (Noise Reduction Coefficient) — ASTM C423 average of α at 250, 500, 1000, 2000 Hz, rounded to nearest 0.05. NRC 0.85 is “very absorptive”.
  • SAA (Sound Absorption Average) — newer ASTM replacement averaging twelve 1/3-octave bands 200 Hz – 2.5 kHz.
  • NC / RC / NCB curves — ANSI/ASHRAE noise criteria for HVAC; an octave-band spectrum is matched to a curve, lowest enclosing curve = rating.

5. Hearing physiology

  • Nominal range 20 Hz – 20 kHz; upper bound drops with age (presbycusis) to ~12 kHz by 60.
  • Sensitivity peak 2–5 kHz (ear canal resonance ~3 kHz).
  • Equal-loudness contours — ISO 226:2003 (revised from Fletcher–Munson 1933 and Robinson–Dadson 1956). Phon = SPL of an equally loud 1 kHz tone.
  • Loudness (sones) — perceptual scale; doubling sones ≈ +10 phon.
  • Masking — loud band masks adjacent quieter bands; basis for MP3/AAC perceptual audio codecs.
  • Hearing damage mechanisms: NIHL (noise-induced hearing loss) from prolonged > 85 dBA; acoustic trauma from impulse > 140 dB peak (gunshots).

6. Noise standards + regulations

Occupational

  • OSHA 29 CFR 1910.95 (US) — PEL 90 dBA TWA 8 h, action level 85 dBA (hearing conservation program required). 5 dB exchange rate (halve allowed duration each +5 dB) — looser than international.
  • NIOSH REL — 85 dBA TWA with 3 dB exchange rate (more protective).
  • EU Directive 2003/10/EC — action 80 dBA, upper 85 dBA, limit 87 dBA; 3 dB exchange.
  • MSHA (US mining) — similar to OSHA but 90 dBA PEL with 5 dB exchange.

Community + environmental

  • HUD site acceptability — 65 dBA Ldn (day-night) acceptable, > 75 unacceptable.
  • EPA Levels Document (1974) — 55 dBA Ldn outdoor, 45 indoor for “no effect” on public health/welfare.
  • IEC 61672 — Sound Level Meter (SLM) specs; Class 1 lab grade, Class 2 general purpose.

Vehicle pass-by

  • ISO 362-1 / -3 — Phase 2 test procedure. EU Regulation 540/2014 phases in 72–77 dB(A) limits (M1 passenger car: 68 dB(A) by 2024 phase 3).
  • SAE J1470 + J1492 — US in-use motorcycle and truck.
  • EV pedestrian alert — UNECE R138 + US FMVSS 141 (minimum sound for EVs
    • hybrids below ~30 km/h; ~56 dB at 10 mph).

Aircraft

  • ICAO Annex 16 Volume I — Chapters 3, 4, 14; Stage 5 (US FAA Part 36) is the current civil noise certification. Measured in EPNdB (Effective Perceived Noise Level) at takeoff, sideline, approach.

Building

  • IBC (International Building Code) references ASTM E90 (lab TL) + ASTM E336 (field NIC/NNIC) for STC.
  • IECC energy code adjacent; doesn’t drive acoustics directly.
  • ASHRAE Handbook HVAC Applications Chapter 49 — noise + vibration design guidance; defines NC criteria by room type (NC25 for recording, NC30 for patient room, NC40 for open office, NC55 for kitchens).
  • ANSI S12.60 — classroom acoustics; T60 limits 0.6–0.7 s, background ≤ 35 dBA.

Hearing protection

  • NRR (Noise Reduction Rating) — US EPA single-number, on label of all HPDs.
  • SNR (Single Number Rating) — ISO 4869-2 EU equivalent.
  • Foam earplugs typically NRR 29–33; muffs 20–30; doubled (plug+muff) adds ~5 dB. Real-world derating: subtract ~7 dB or 50 % from NRR.

7. Absorbers

Absorbers convert acoustic energy to heat. Characterised by absorption coefficient α (0 = perfect reflector, 1 = perfect absorber) per frequency band.

Porous absorbers

Friction in tortuous pore network. Brands + products:

  • Fibreglass — Owens Corning 703 (3 lb/ft³) and 705 (6 lb/ft³); Knauf Insulation Acoustical Smooth; PinkBoard.
  • Mineral wool — Rockwool Safe’n’Sound, Rockboard 60/80; Roxul AFB.
  • Polyester — Autex Quietspace 3D Tiles, EchoPanel.
  • Open-cell PU foam — Auralex Studiofoam, primacoustic Broadway.
  • Recycled cotton — Bonded Logic UltraTouch.

Performance rule of thumb: porous absorbers work above ~250 Hz; thicker = lower (a 4 in/100 mm panel absorbs down to ~125 Hz; a 1 in panel only to ~500 Hz). Air gap behind panel boosts low-frequency α. NRC 0.85–1.05 for 4 in fibreglass.

Resonant (Helmholtz) absorbers

Perforated/slatted panel over air cavity; tuned to a band. Resonant frequency:

f_r = (c / 2π) · √(σ / (L_eff · d))

where σ = open area fraction, L_eff = effective neck length (hole depth + end correction), d = cavity depth. Good for mid-low frequency (100–500 Hz) where porous absorbers run out of band. Commercial: RPG Modffuser, Acoustical Surfaces Soundblox CMU.

Membrane / panel absorbers

A limp panel (plywood, gypsum) over sealed air cavity acts as mass-spring oscillator. f_r = 60/√(m·d) (m in kg/m², d in m). Targets ~50–200 Hz. Used in studios for bass control.

Diffusors

Don’t absorb — they scatter energy spatially + temporally so the room sounds spacious without going dead. Preferred over heavy absorption in performance rooms.

  • Schroeder QRD (Quadratic-Residue Diffusor) — Manfred Schroeder 1975; well depths follow n² mod p sequence.
  • PRD (Primitive-Root) — alternative number-theoretic sequence.
  • MLS (Maximum Length Sequence) — 1D binary scatter.
  • Geometric — pyramids, polyhedra, BAD panels.
  • Commercial: RPG Diffusor Systems, GIK Acoustics Q7d, Vicoustic.

Bass traps

Targeted low-frequency absorption — corners (where modal pressure accumulates) using thick porous (Owens 703 stacked) or membrane traps (GIK Scopus, ASC TubeTrap, Real Traps Mondo).

8. Sound transmission + barriers

Mass law

For a non-resonant, limp partition normal incidence:

TL ≈ 20·log₁₀(m·f) − 47   (dB)

m = surface mass kg/m², f = Hz. +6 dB per doubling of mass or frequency. Random-incidence diffuse field is ~5 dB less.

Critical / coincidence frequency

At f_c the bending wave in panel matches grazing acoustic wave → TL dips significantly. f_c ≈ c²/(1.8·t·c_L) for thin panels (t = thickness, c_L = longitudinal velocity in panel). For 1/2 in (12.7 mm) gypsum f_c ≈ 2500 Hz; for 1/4 in glass ≈ 5000 Hz.

Double-leaf walls

Two leaves separated by air gap (and ideally absorptive infill) outperform single leaf of equal mass — but suffer mass-air-mass resonance f_0 at low freq; below f_0 you only get mass law of the total. Typical f_0 ~60–100 Hz for standard 2x4 stud walls. Fixes: deeper cavity, lighter leaves, decoupled studs.

Typical STC ratings

PartitionSTC
Single 1/2” gypsum on 2×4 studs33
Double 5/8” gypsum each side, 2×4 studs39
Same, with R-13 batt in cavity45
Staggered-stud, 2× 5/8” each side, batt50
Double-stud (decoupled), batt55–60
Double-stud + resilient channel + 2× 5/8”60–65
Mass-loaded vinyl + double-stud65+

Doors, windows, penetrations

  • Doors — hollow-core 17 STC, solid-core 25, acoustic-rated assemblies (Krieger, Overly, IAC) 40–55 with gasket + drop seal.
  • Windows — single-glaze 27, IGU 32, laminated dual-pane 42+, triple-glazed laminated 50+ (Soundproof Windows, CitiQuiet).
  • Penetrations — any unsealed gap dominates. Seal with acoustic caulk (OSI SC-175, USG Sheetrock), back-to-back outlet boxes offset by one stud bay, putty pads on box rears (Hilti CFS-CT).

Outdoor barriers

Diffraction over the top is the limit. Maekawa chart relates path-length difference (δ) to attenuation. Rule of thumb: a barrier breaking line-of-sight gives 5 dB, breaking + adding height for diffraction gives 10–15 dB; ~25 dB is the practical max regardless of height. Highway sound walls (Caltrans Type 1) typically 4–5 m tall, 10–12 dB at first row of homes.

9. Vibration + structure-borne noise

Structure-borne paths often dominate over airborne, because high-impedance solids transmit with little loss. Control = break the path with low-Z elements.

Mass-spring-damper basics

  • Natural freq f_n = (1/2π)·√(k/m).

  • Damping ratio ζ; quality factor Q = 1/(2ζ).

  • Transmissibility for forced base motion:

    T = √((1+(2ζr)²) / ((1−r²)²+(2ζr)²)),   r = f/f_n

  • For isolation you want r ≥ √2; isolation begins above r ≈ 1.4, gets better as r grows. Practical target: pick f_n ≤ 1/3 of disturbance.

Isolator types

TypeRangeUse
Steel coil springf_n 2–8 HzHVAC, generator, machinery (Mason Industries Series SLF, Kinetics FDS)
Elastomer / rubber padf_n 10–25 HzCompressor, motor (Sorbothane, Sylomer, Bilz HBM-V)
Air springf_n 0.5–3 HzLab tables, lithography (Newport S-2000, TMC)
Wire ropef_n 5–15 Hz, broadbandShock + vibe (Aeroflex, Enidine, ITT Enidine WR)
Pneumatic / active< 1 HzMicroscopes, optical (TMC PEPS, JRS BM)
Viscoelastic / frictionbroadband dampingAerospace, civil (3M ISD 110/112, LANXESS)

TMD (Tuned Mass Damper)

Den Hartog 1947 — attached mass + spring + damper tuned to a problematic mode, splits one peak into two smaller ones. Used on Taipei 101 (730 t sphere), Citicorp Center, Millennium Bridge London. Robot-arm + machine-tool versions covered in [[Robotics/vibration-damping-arms]] and [[Engineering/structural-dynamics]].

Constrained-layer damping (CLD)

Viscoelastic film (3M ISD-112) between two stiff layers; shear losses on bend dissipate energy. Automotive door panels, floor pans, brake shims (anti- squeal). Also Roush Quiet Steel, Material Sciences Quiet Steel laminate.

Floating floors + isolated rooms

  • Concrete slab on rubber/neoprene pucks (Mason Industries Mason WP, Wilson WGP, Kinetics RIM) or jack-up springs.
  • Box-in-box room construction — independent inner shell with isolated walls + floor + ceiling. Recording studios, MRI suites, anechoic chambers (Eckel, IAC, Acoustic Systems).

Resilient mounts

HVAC compressors on spring isolators with seismic snubbers, gensets on spring + neoprene combos (Vibration Mountings & Controls), piping suspended on resilient hangers (Mason 30N).

10. HVAC noise

Primary noise source in most office and lab buildings. Sources + fixes:

Fan noise

  • Broadband (turbulent flow) + tonal BPF (Blade Passing Frequency): f_BPF = blade count × shaft RPM / 60.
  • Backward-curved centrifugal fans quieter than forward-curved; plenum fans cleaner than housed; mixed flow quiet for given pressure.
  • Variable speed drive reduces noise non-linearly (P_acoustic ∝ tip-speed⁵ for many fans).
  • Acoustic enclosure for big AHUs (Greenheck, Loren Cook).

Duct silencers (attenuators)

Lined absorptive baffles, often perforated metal over fibreglass. Brands: Vibro-Acoustics, Industrial Acoustics Co. (IAC), Aeroacoustic, Ruskin, Greenheck SAFD. Performance specced as insertion loss (IL) dB per octave band for given face velocity. Packless / film-lined versions for clean-room + hospital (avoid fibre shed).

Diffuser + grille noise

Terminal noise from VAV box, dampers, grilles — function of pressure drop and outlet velocity. Stay below catalog NC rating. Oversize and use balanced-flow designs (Titus, Krueger, Price).

Air-side velocity guidelines (ASHRAE)

  • Main supply duct < 17 m/s (3300 fpm) industrial, < 7.5 m/s (1500 fpm) office, < 4 m/s (800 fpm) hospital patient.
  • Return < 2.5 m/s near grille for quiet.

Duct break-out / break-in

Low-frequency rumble radiates through duct wall. Wrap with mass-loaded vinyl (Tecsound, Acoustiblok) or external lagging (Owens Corning Quiet R + jacketing). Round + oval ducts much better than rectangular for break- out.

11. Room acoustics

Reverberation time T60

Sabine 1900: T60 = 0.161·V/A_total (SI; V in m³, A in m²·sabin); US units T60 = 0.049·V/A. A_total = Σ S_i · α_i over surfaces. Better-fitting variants for highly absorptive rooms: Eyring, Millington-Sette.

T60 targets (Long; Beranek)

UseT60 (s) at 500 Hz
Speech/conference room0.5–0.7
Classroom (ANSI S12.60)0.6–0.7
Recording control room0.2–0.4
Recording live room0.4–0.8
Drama theatre0.9–1.2
Symphony hall1.8–2.2
Pipe-organ church / cathedral3–6

Below the Schroeder frequency:

f_S ≈ 2000·√(T60 / V)   (Hz; T in s, V in m³)

individual room modes dominate; above it, a diffuse field assumption is reasonable. Small rooms (≤ 100 m³) have f_S of several hundred Hz, meaning bass is mode-driven. Strategies: bass traps, non-parallel walls, room ratios (Bolt area, “Bonello criterion”).

Early reflections + late reverberation

  • Direct sound (< ~5 ms) sets localisation.
  • Early reflections (5–80 ms) fuse with direct (Haas effect) → spatial impression.
  • Late reverberation sets envelopment + clarity.

C50 (clarity for speech), C80 (music), D50 (definition), STI (Speech Transmission Index) — quantitative metrics from impulse response. Measured with sweep + Farina deconvolution (ESS — exponential sine sweep).

12. Outdoor sound propagation

  • Geometric spreading: point 6 dB/dd, line 3 dB/dd, plane 0 dB.
  • Air absorption — increases with frequency, sensitive to humidity (ISO 9613-1). At 1 kHz ~5 dB/km, at 8 kHz ~40 dB/km. High-frequency content of distant sounds is gone (thunder rolls vs. close lightning crack).
  • Ground effect — soft ground (grass) absorbs more than hard (asphalt, water); destructive interference dip 250–500 Hz typical.
  • Refraction — wind and temperature gradients bend rays. Downwind / inversion → focused shadow-free; upwind / lapse → shadow zone with sharp attenuation cliff.
  • Atmospheric stability (Pasquill A–F) influences turbulence + ray scattering.
  • ISO 9613-2 — standard for outdoor propagation calculation (used by CadnaA, SoundPLAN, Predictor-LimA).

13. Active Noise Control (ANC)

ANC superposes an anti-noise wave 180° out of phase with the disturbance. Effective for low frequency (long wavelength, large coherent zone) and periodic signals. Above ~1 kHz spatial coherence breaks down outside the ear canal.

FXLMS algorithm

Burgess 1981 — Filtered-x Least Mean Squares. The reference x is filtered through a model of the secondary path (speaker → error mic) before update, because the actual error contains delay/colour from that path.

w(n+1) = w(n) + μ · x̂(n) · e(n)

Variants: leaky FXLMS, normalised NLMS, frequency-domain partitioned-block, RLS for fast convergence.

Products

  • Headphones — Bose QC Ultra, QC45; Sony WH-1000XM5; Apple AirPods Pro 2 (H2 chip); Sennheiser Momentum 4. Use feedforward (ext mic) + feedback (in-cup mic) hybrid.
  • In-cabin auto — Bose QuietComfort Road Noise Control (Honda, GM), Mercedes Quiet Cabin, Lotus ANC, Hyundai RANC. Uses accelerometers on body + cabin mics + door speakers.
  • Engine-order cancellation — Active Sound Design (Audi, BMW); plays matched anti-phase through speakers (also synthesises tuned sport sound).
  • Open-window / open-air ANC — research (Nanyang Tech 2020 grating speaker array); not commercial yet.
  • Active vibration control — piezo stack actuators on machine tool spindles, magnetorheological (LORD MR fluid) damper in semi-active vehicle suspension.

14. NVH (auto + motorcycle)

Sources prioritised at different speeds:

  • Low speed (< 50 km/h) — engine, powertrain, accessory whine; for EV the gear whine + inverter coil whine dominate.
  • Mid speed (50–100 km/h) — tyre/road noise (CPB, cavity resonance ~200 Hz).
  • High speed (> 100 km/h) — wind / aero (A-pillar vortex, mirror, seal leak).
  • Transient — brake squeal, suspension clunks, body shake.

Treatments

  • Bitumen / butyl damping pads — Dynamat, HushMat, Second Skin Damplifier on floor pan, doors, roof, trunk; reduce panel resonance + drumming.
  • Closed-cell foam + barrier mat (mass-loaded vinyl) sandwiches for airborne TL.
  • Sylomer / EVA pads on subframe + suspension mounts; engine mounts hydraulic or active (BMW EMA).
  • Hood + dash insulators — moulded fibre, melamine foam, microperforated PET.
  • Tyres — silica compound + foam-in-tyre (Continental ContiSilent, Pirelli Noise Cancelling System) reduce cavity 200 Hz peak.
  • Brake NVH — shim layers (Eagleburgmann, Trelleborg), chamfers + slots, rotor damping clips.
  • Exhaust — reactive (chambered) mufflers tune; absorptive (perforated + fibre) take broadband; resonators tune narrow band; helmholtz volume.
  • EV pedestrian alert — UNECE R138 + FMVSS 141; minimum sound (synthesised tonal complex 56 dB at 10 mph, increasing with speed); discontinued above ~30 km/h where tyre noise takes over.

15. Microphones + speakers

Microphones

  • Condenser — capacitor diaphragm, needs polarising (true 48 V phantom) or permanent-charged electret. Wide-band, sensitive. B&K 4189 (Type 1 measurement), Neumann U87, DPA 4006, Earthworks M30.
  • Dynamic — moving coil; rugged, high SPL handling. Shure SM57/SM58, Sennheiser MD421, Electro-Voice RE20.
  • Ribbon — corrugated metal ribbon; figure-8, smooth high end. Royer R- 121, Coles 4038, AEA R84.
  • MEMS — silicon micro-machined capacitive; in phones, hearing aids, laptops. Knowles SiSonic, Infineon IM69D130, Bosch Sensortec, AAC, Goertek. SNR ~65–70 dB(A) top units.

Speakers / transducers

  • Dynamic cone — voice coil in magnet gap pushes cone (Adams, Scan-Speak, SEAS, Eminence). Workhorse of audio.
  • Compression driver + horn — high efficiency mid/high (JBL, B&C, RCF). Cinema + PA.
  • Planar / ribbon tweeters — Magnepan, Mundorf AMT, RAAL.
  • Electrostatic — Quad ESL, MartinLogan; full-range diaphragm between perforated stators.
  • Balanced Mode Radiator (BMR) — Tectonic, Naim wide-dispersion driver.
  • Piezoelectric — buzzers, ultrasonic, tweeter (Murata, CUI Devices, Pui Audio).

Beamforming

Phased microphone or speaker array, electronically steered. Used in conference systems (Shure MXA920, Sennheiser TeamConnect Ceiling 2), smart speakers (Echo, HomePod), hearing aids (Phonak StereoZoom), automotive in-cabin voice (Cerence). Algorithms: delay-and-sum, MVDR (Capon), generalised sidelobe canceller, neural beamforming.

16. Ultrasound applications

Frequencies above 20 kHz, with industrial + medical + sensor branches.

Industrial

  • NDT (Non-Destructive Testing) — pulse-echo, TOFD, phased-array (Olympus OmniScan, Zetec, Sonatest). See [[Engineering/Tier3/ndt-methods]].
  • Cleaning — 25–80 kHz cavitation in surfactant bath; PCB, medical instrument cleaning (Branson, Crest, Elma).
  • Welding — 20–40 kHz titanium horn, plastic + metal foil welding (Branson, Herrmann, Dukane); battery tab welding for EV.
  • Machining — UAM (Fabrisonic), rotary ultrasonic for ceramics.

Medical

  • Imaging — 1–15 MHz transducer arrays (linear, curved, phased); GE LOGIQ, Philips EPIQ, Siemens Acuson, Mindray. Doppler, elastography, 3D/4D.
  • HIFU (High-Intensity Focused Ultrasound) therapy — non-invasive ablation (prostate, fibroid, essential tremor). InSightec Exablate, EDAP Focal One.
  • Lithotripsy — kidney stone fragmentation.

Sensor

  • Distance / parking assist (40 kHz piezo, Murata, Bosch).
  • Liquid level, anemometry, gas flow (ultrasonic transit-time, Sick, Endress+Hauser).

17. Software + tools

Measurement hardware

  • SLMs (Sound Level Meters) — Brüel & Kjær 2245 / 2250 / 2270, NTi Audio XL2, Larson Davis 831, Svantek SV 971, Pulsar Nova. Class 1 or 2.
  • Mic preamps + DAQ — B&K LAN-XI, GRAS 12HF, Dewesoft DEWE-43A, NI cDAQ + 9234 IEPE module.
  • Reference mics — GRAS 46AE, B&K 4189, PCB 378B02. Calibrated pistonphone (B&K 4231, 94 / 114 dB at 1 kHz).
  • Accelerometers (for vibration) — PCB Piezotronics 352C series, B&K 4514, Wilcoxon. IEPE / ICP standard.
  • Acoustic camera — gfai tech Acoustic Camera, Sorama Sound Camera, Brüel & Kjær Array.

Simulation

  • FEA + BEM: ANSYS Mechanical Acoustics, ANSYS Twin Builder, COMSOL Multiphysics Acoustics Module, Siemens Simcenter 3D Acoustics, Actran (Hexagon/MSC), VA One (ESI).
  • Mid/high frequency SEA (Statistical Energy Analysis) — VA One, Wave6.
  • Aero-acoustics CFD — ANSYS Fluent + CAA, Siemens STAR-CCM+, OpenFOAM (compressible / hybrid LES), FW-H far-field analogy.
  • Auralisation + interior NVH: ANSYS VRXPERIENCE Sound, Siemens LMS Virtual.Lab Sound Design, Müller-BBM PAK.
  • Architectural ray-tracing: ODEON, CATT-Acoustic, EASE (AFMG), RAMSETE.
  • Outdoor propagation: CadnaA (DataKustik), SoundPLAN, IMMI, Predictor-LimA.

DAW + measurement plug-ins

  • DAWs: Avid Pro Tools, Apple Logic Pro, Steinberg Nuendo, Cockos Reaper.
  • Live audio: Smaart (Rational Acoustics), SysTune (AFMG).
  • Free: Room EQ Wizard (REW), ARTA, Audacity, Sonic Visualiser.

18. Selection heuristics

NeedRecipe
Office cubicle privacySTC 40 partition + NRC 0.9 ceiling tile (USG Mars, Armstrong Ultima) + sound masking (Cambridge Sound QtPro, Biamp) at 45 dBA
Recording studioBox-in-box, double-stud + RC, decoupled floor, broadband absorbers + diffusors, bass traps in corners, T60 0.3 s flat
Home theatreSTC 60 walls, sealed door, 5.1+ treatment, T60 0.3–0.4 s, mass-loaded vinyl over framing
HVAC equipment roomSpring isolators (Mason SLF), spring-hung pipes, duct silencers, STC 55 wall, acoustic louvre
Auto cabinDamping pads + closed-cell foam barrier + carpet underlay + ANC + tuned tyres + laminated glass
Concert hallShoebox or vineyard shape, diffuser-treated upper walls, variable reverb (banners, rotating elements), Yasuhisa Toyota / Nagata Acoustics design language
HospitalASHRAE 170 + FGI Guidelines, NC30 patient rooms, STC 50 walls, NRC 0.85 ceilings, T60 < 0.6 s
ClassroomANSI S12.60: T60 ≤ 0.6 s, background ≤ 35 dBA, NC25
Hearing aidDSP with directional beamforming + ML noise reduction (Phonak SmartSpeech, Oticon More with on-board DNN, Widex MoMENT, Starkey Genesis AI)
PA systemDistributed line array (L-Acoustics K2, d&b GSL, Meyer Leopard) + acoustic treatment to lower T60 < 1.5 s
Outdoor highway barrier4 m absorptive concrete + 1 m diffraction cap, set ≤ 30 m from receiver

19. Cross-references

  • [[Engineering/vibration-dynamics]] — single-DOF + multi-DOF + modal basics; the math behind isolators and TMDs.
  • [[Engineering/structural-dynamics]] — full-building / aerospace dynamics; Den Hartog, TMD design tables.
  • [[Robotics/vibration-damping-arms]] — TMD + active damping in manipulators; piezo + MR.
  • [[Engineering/Tier3/heat-transfer-correlations]] — paired with HVAC noise design.
  • [[Engineering/hvac-fundamentals]] — system-level HVAC; this file covers the acoustic layer.
  • [[Engineering/Tier3/polymers-taxonomy]] — rubber + foam properties for isolators and absorbers.
  • [[Engineering/Tier3/seals-taxonomy]] — gaskets influence flanking transmission and door + window TL.
  • [[Engineering/signal-processing-dsp]] — FXLMS + adaptive filtering for ANC; FFT + octave-band analysis.
  • [[Engineering/Tier3/ndt-methods]] — ultrasonic flaw detection cross- reference.

20. Citations

  • Kinsler L.E., Frey A.R., Coppens A.B., Sanders J.V. — Fundamentals of Acoustics, 4th ed., Wiley, 2000.
  • Beranek L.L., Mellow T.J. — Acoustics: Sound Fields and Transducers, 2nd ed., Academic Press, 2019.
  • Long M. — Architectural Acoustics, 2nd ed., Academic Press, 2014.
  • Cox T.J., D’Antonio P. — Acoustic Absorbers and Diffusers, 3rd ed., CRC Press, 2017.
  • Pierce A.D. — Acoustics: An Introduction to Its Physical Principles and Applications, 3rd ed., Springer, 2019.
  • Bies D.A., Hansen C.H., Howard C.Q. — Engineering Noise Control, 5th ed., CRC Press, 2018.
  • Den Hartog J.P. — Mechanical Vibrations, 4th ed., McGraw-Hill, 1947 (Dover reprint).
  • Schroeder M.R. — “Diffuse sound reflection by maximum-length sequences”, J. Acoust. Soc. Am., 57(1), 1975.
  • Burgess J.C. — “Active adaptive sound control in a duct: A computer simulation”, J. Acoust. Soc. Am., 70(3), 1981 (FXLMS).
  • Standards: ISO 226:2003 (equal-loudness contours); ISO 362-1/3 (vehicle pass-by); ISO 717-1 (R_w airborne); ISO 3382-1/2/3 (reverberation time); ISO 9613-1/2 (outdoor propagation); ISO 11904 (binaural measurement); IEC 61672-1 (SLM); IEC 61260 (octave-band filters).
  • US standards: ASTM E90 (lab TL); ASTM E413 (STC); ASTM E336/E1110 (field NIC/AIIC); ASTM E1414 (ceiling); ASTM C423 (absorption); ANSI S12.60 (classroom); ANSI S3.29 (community noise); OSHA 29 CFR 1910.95; NIOSH Publication 98-126.
  • Building / HVAC: ASHRAE Handbook — HVAC Applications, Ch. 49 — Noise and Vibration Control (2023); ASHRAE Standard 170 (healthcare); FGI Guidelines for Design and Construction of Hospitals.
  • Transportation: ICAO Annex 16 Vol. I; FAA Part 36; UNECE R51.03 (vehicle noise); UNECE R138 / FMVSS 141 (EV pedestrian alert).

Family index — Engineering Tier 3 — last updated 2026-05-16.

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