A-Weighted Sound Level
The standard way noise is measured. A-weighting adjusts levels to mimic human hearing, de-emphasizing very low and high frequencies. Reported in dBA — the unit behind nearly every noise regulation.
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Acoustical Glossary
Plain-language definitions of the acoustics terms we use every day — from noise study metrics like Leq and STC to the products that fix them.
A
19 termsAbsorption
When sound energy converts to heat instead of bouncing back — what acoustic panels do. More absorption means less echo.
Absorption coefficient
A 0-to-1 rating of how much sound a material absorbs. 0.90 means 90% absorbed; bare concrete is near 0.05.
Acoustical engineering
The engineering discipline behind quiet buildings and clear-sounding rooms — covering noise control, room acoustics, and vibration.
Acoustical tile
A sound-absorbing ceiling or wall tile, typically mineral fiber or fiberglass. The familiar drop-ceiling tile is the most common type.
Acoustics
The science of sound — how it is produced, travels, and is heard — and the art of shaping it in rooms and buildings.
Active Noise Control (ANC)
Technology that cancels unwanted sound by producing an inverted sound wave — the two waves destructively interfere. Common in headphones; in rooms it's used for low-frequency hum from HVAC or industrial equipment.
Airborne Noise
Sound that travels through the air — voices, music, TV — as opposed to vibration traveling through structure. Blocking it requires mass and airtight construction: walls, windows, and doors with high STC ratings.
Ambience
The background "room tone" you hear in a quiet space — HVAC hum, distant traffic, the room’s own subtle reverberation.
Ambient noise
All the sound normally present in an environment. Measured first in any noise study as the baseline.
Ambient Sound Level
The total background sound at a location from all sources combined, before a new noise source is introduced. Noise studies always measure ambient first — it's the baseline everything else is compared against.
Analog
Sound represented as a continuous electrical wave, as opposed to digital samples. Vinyl records and cassette tapes are analog.
Anechoic
Literally "without echo." An anechoic chamber absorbs around 99% of sound — among the quietest rooms on earth, used for testing.
Area Effect
Spaced-out acoustic panels absorb more than the same material laid flat, because sound diffracts around their exposed edges.
Articulation
How clearly speech can be understood in a space — the practical goal behind STI measurements and classroom acoustics.
ASTM E 84
The "Steiner tunnel test" rating how fast flames spread across a building material’s surface. Required by code for interior finishes.
Attenuate
To reduce the level of sound (or signal). Barriers attenuate; absorbers attenuate reflections.
Audio frequency
Any frequency humans can hear: roughly 20 Hz to 20,000 Hz.
Axial mode
Resonance between one pair of parallel surfaces — the strongest room modes, causing boomy bass at specific spots.
B
2 termsBackground Noise
The residual sound present when the specific source under study is quiet. In practice, the L90 percentile level is the standard proxy for background noise in environmental assessments.
Break-In / Break-Out
Break-out is sound escaping a building to the outside (a noisy factory heard by neighbors); break-in is outside sound entering (traffic heard inside an office). Facade design must control both directions.
C
4 termsC-Weighting
A frequency weighting that stays nearly flat across most of the audible range, unlike A-weighting. Used for high-level sounds like concerts or industrial noise where low frequencies matter.
Class 1 Sound Level Meter
The precision grade of sound level meter required for legal and regulatory noise measurements. Class 1 meters meet tighter tolerances than Class 2 and must be calibrated before and after every survey.
Coincidence Dip
A drop in a wall's sound-blocking performance at a specific frequency where bending waves in the panel match airborne sound waves. Good partition design pushes this dip outside the speech range.
Critical Distance
The distance from a sound source where direct sound and reverberant sound are equal. Beyond it, adding absorption is the only way to make speech clearer — moving closer to the speaker no longer helps.
D
6 termsDamping
Reducing vibration by converting mechanical energy into heat, typically with viscoelastic materials. Damping quiets ringing metal panels, ducts, and machine housings.
Day-Evening-Night Level (Lden)
A 24-hour average noise level with 5 dB added for evening hours and 10 dB for night, reflecting greater sensitivity when people rest. The standard metric for EU noise mapping and many US planning studies.
Day-Night Level (Ldn / DNL)
A 24-hour average with a 10 dB penalty added to nighttime noise (10 PM–7 AM). The primary metric for US airport and highway noise compatibility planning.
Decibel (dB)
The logarithmic unit of sound level. Because hearing spans an enormous range of pressures, decibels compress it: 0 dB is near silence, normal conversation is ~60 dB, and every 10 dB increase sounds roughly twice as loud.
Decoupling
Breaking the rigid connection between two surfaces so vibration can't pass through — e.g., resilient channels, isolation clips, or staggered studs in walls. One of the most effective soundproofing strategies.
Diffusion
Scattering sound reflections evenly in many directions instead of absorbing them. Diffusers keep a room sounding live and natural while eliminating harsh echoes — essential in studios, theaters, and performance spaces.
E
2 termsEarly Decay Time (EDT)
Reverberation time measured over the first 10 dB of decay, then extrapolated. EDT correlates better than RT60 with how reverberant a room actually feels to listeners.
Equivalent Continuous Sound Level (Leq)
The single average sound level that contains the same total energy as a fluctuating noise over a given period. LAeq is the backbone metric of nearly every environmental noise study.
F
4 termsFast / Slow Time Weighting
How quickly a sound level meter responds to changes: Fast (125 ms) tracks fluctuating noise; Slow (1 s) smooths it. Most environmental measurements use Slow; occupational assessments often use Fast.
Flanking Transmission
Sound sneaking around a wall through connected paths — ceilings, floors, ducts, outlets — rather than through the wall itself. The reason a high-STC wall can still disappoint in the field.
Flutter Echo
A rapid rattling echo bouncing between two parallel hard surfaces, like bare walls in a hallway or gym. Fixed with absorption on at least one of the facing surfaces.
Frequency Analysis
Breaking a noise measurement into octave or third-octave bands to see which frequencies dominate. Essential for diagnosing problems — a 63 Hz hum needs a completely different fix than 4 kHz hiss.
I
5 termsImpact Insulation Class (IIC)
A single-number rating (higher is better) of how well a floor-ceiling assembly blocks impact noise like footsteps. Tested per ASTM E492; building codes typically require IIC 50.
Impact Noise
Sound generated by physical impact — footsteps, dropped objects, furniture — that travels as vibration through structure. Controlled with underlayments, floating floors, and resilient mounts rather than wall mass.
Impulsive Noise
Brief, high-intensity bursts like hammering, pile driving, or gunfire. Regulations penalize impulsive noise (typically +5 dB) because it annoys far more than steady sound at the same level.
Insertion Loss
How many decibels quieter things get after you install a noise control product — measured with and without the barrier, enclosure, or silencer in place. The honest way to rate product performance.
Inverse Square Law
Sound level drops about 6 dB every time distance from a point source doubles outdoors. The quick math behind setback distances and barrier placement.
L
2 termsLAeq,T
The A-weighted equivalent continuous level over measurement period T — e.g., LAeq,1h for one hour. The workhorse descriptor in noise studies worldwide.
Lmax / Lmin
The highest and lowest A-weighted sound levels recorded during a measurement period. Lmax captures the loudest event (a truck pass-by); Lmin shows the quietest background moment.
M
2 termsMass Law
The rule that doubling a wall's mass (or doubling frequency) improves sound blocking by about 6 dB. It's why concrete and double drywall outperform lightweight partitions — and why mass alone has practical limits.
Maximum Sound Level
See Lmax. The peak level during a survey period — critical for sleep disturbance and single-event assessments like aircraft flyovers.
N
7 termsNoise Barrier
An outdoor wall or berm that blocks the direct path of sound between source and receiver — along highways, around rooftop equipment, or at construction sites. Performance depends on height, length, mass, and placement.
Noise Criteria (NC)
Curves rating background noise in rooms, focused on HVAC and mechanical systems. An NC-35 office is typical; concert halls target NC-20 or lower. Derived from octave-band measurements.
Noise Dose
A worker's total noise exposure over a shift, expressed as a percentage of the allowed daily limit. Measured with a personal dosimeter; 100% means the legal limit is reached.
Noise Isolation Class (NIC)
A field-measured rating of how much sound isolation exists between two real rooms, including flanking paths. Unlike lab STC, NIC tells you what occupants actually experience.
Noise Monitoring
Continuous or repeated measurement of noise levels over days or weeks — for construction compliance, industrial permits, or community complaints. Modern systems log data remotely with alerts when limits are exceeded.
Noise Reduction Rating (NRR)
The EPA-required label on hearing protectors in the US, in decibels. Higher NRR means more attenuation — but real-world protection is typically lower; derating formulas adjust for imperfect fit.
Noise Study
A professional assessment measuring existing noise, predicting future levels, and recommending controls — required for permits, CEQA/NEPA review, and resolving neighbor disputes. Silentical's core consulting service.
O
1 termsOctave Band
A frequency range where the top is twice the bottom (e.g., 500–1000 Hz). Noise is analyzed in octave or third-octave bands because problems and fixes are frequency-specific.
P
1 termsPercentile Level (Ln)
The level exceeded n% of the time — L90 is the quiet background, L50 the median, L10 the intrusive peaks. The statistical fingerprint of any noise environment.
R
5 termsReal-Time Analyzer (RTA)
An instrument displaying all frequency bands simultaneously, so you can watch a noise spectrum live. Invaluable for tracking down the source of a hum, whine, or rumble.
Reflection
Sound bouncing off a hard surface, like light off a mirror. Controlled reflections add pleasant spaciousness; uncontrolled ones cause echo and muddy speech.
Reverberation Time (RT60)
How long it takes sound to decay 60 dB after the source stops — the defining number in room acoustics. Classrooms target ~0.6 s; concert halls 1.8–2.2 s; a bare warehouse can exceed 4 s.
Room Criteria (RC)
A refinement of Noise Criteria curves that also flags rumbling or hissing quality in HVAC noise. Used where occupant comfort is critical — offices, hospitals, schools.
Room Modes
Resonant frequencies where sound waves fit exactly between parallel walls, causing boomy buildup. Small rectangular rooms suffer most; bass traps and non-parallel surfaces are the cures.
S
14 termsSabine Equation
RT60 = 0.161 × volume / absorption. The 1898 formula still used to predict reverberation time and calculate exactly how much absorption a room needs.
Single Number Rating (SNR)
The EU/ISO single-number rating for hearing protectors, analogous to NRR. Often published alongside HML values for low, mid, and high frequencies.
Sound Absorption
See Absorption. In products, rated by NRC (Noise Reduction Coefficient) — the average absorption at four speech frequencies.
Sound Barrier
See Noise Barrier. Also used for indoor partitions designed to block sound transmission between rooms.
Sound Exposure Level (SEL)
A single event's total sound energy compressed into one second — lets you compare a 3-second truck pass-by with a 20-second aircraft flyover fairly.
Sound Insulation
Construction that resists sound passing through — walls, floors, windows, doors. Rated in labs as STC/IIC and in the field as NIC/AIIC.
Sound Level Meter
The calibrated instrument for measuring noise, from handheld survey meters to logging analyzers. Regulatory work requires Class 1 meters with current calibration certificates.
Sound Masking
Adding a controlled background sound (like soft airflow noise) to cover distracting speech in open offices. Done right it's barely noticeable; done wrong it's just more noise.
Sound Power Level (Lw)
The total acoustic energy a source radiates, in decibels relative to 1 picowatt. Unlike sound pressure, it's independent of distance and room — the right way to specify how loud equipment is.
Sound Pressure Level (SPL)
What a microphone actually measures — the fluctuation of air pressure caused by sound, in decibels relative to 20 micropascals. Depends on distance, so always stated with one.
Sound Transmission Class (STC)
The single-number lab rating of how well a wall, door, or window blocks airborne sound — higher is better. STC 50 is typical code minimum between dwellings; recording studios need 60+.
Speech Intelligibility (STI)
A 0-to-1 index predicting how well speech is understood in a space. Offices and classrooms need STI above 0.60; below 0.45, conversation becomes guesswork.
Standing Waves
See Room Modes. Stationary patterns of loud and quiet spots at resonant frequencies — why bass sounds boomy in one chair and thin in another.
Structure-Borne Noise
Vibration traveling through solid structure — footfalls, plumbing, machinery — then radiating as sound elsewhere. Fought with isolation and damping, not with thicker drywall.
T
3 termsTonal Noise
Noise dominated by a distinct pitch — a transformer hum, fan whine, or alarm. Regulations add a penalty (typically +5 dB) because tones annoy far beyond their measured level.
Tone Assessment
The standardized method (e.g., per BS 4142 or ISO 1996) for deciding whether a noise contains a penalizable tone, using narrow-band analysis.
Transmission Loss (TL)
How many decibels a partition removes at each frequency — the raw data behind the STC rating. Product specs should show the full TL curve, not just the single number.
V
1 termsVibration Isolation
Supporting equipment on springs, rubber, or air mounts so vibration doesn't enter the building structure. The first line of defense against mechanical noise complaints.
Z
1 termsZ-Weighting
Flat frequency weighting (10 Hz–20 kHz, no filtering) for uncolored measurements. Replaced the old 'Linear' setting; use it when you want the raw truth before applying A or C curves.