Acoustics & Soundproofing

1 Soundproofing & Isolation Metrics

Soundproofing performance is measured by standardised ratings that quantify how much airborne or impact sound a construction assembly attenuates. The most widely used in the English-speaking world are STC (airborne) and IIC (impact), while ISO countries use Rw and Ln,w respectively. Understanding these ratings is essential for specifying walls, floors, and ceilings in residential, commercial, and studio construction.

Sound Transmission Class (STC) is a single-number rating of an assembly's ability to reduce airborne sound. It is derived from laboratory measurements across 16 one-third octave bands from 125 Hz to 4000 Hz, plotted against a reference contour. The STC number is the value of the reference contour at 500 Hz after fitting. Higher STC means better isolation. STC 25 means normal speech is easily understood through a partition; STC 45 means loud speech is heard but barely intelligible; STC 60 means even a loud shout is barely audible.

Impact Insulation Class (IIC) measures floor/ceiling resistance to structure-borne impact noise - footsteps, dropped objects, chair scraping. A bare concrete slab may achieve IIC 28; add carpet and pad and it rises to IIC 65+. Building codes typically require IIC 50 minimum for multi-family residential floors.

2 Reverberation Time - RT60, EDT & the Sabine/Eyring Equations

Reverberation time (RT60) is the time it takes for a sound to decay 60 dB after the source stops. It is the single most important parameter in room acoustics, governing speech intelligibility, music clarity, and the overall character of a space. A cathedral may have RT60 of 8–10 seconds; a recording studio control room targets 0.2–0.4 seconds; a classroom should sit between 0.4–0.6 seconds for good speech intelligibility.

Sabine's formula (1900) - RT60 = 0.161·V/A - works well for rooms with relatively low average absorption (αm < 0.3). The total absorption A is the sum of each surface area multiplied by its absorption coefficient at the frequency of interest: A = Σ(Si·αi). The 0.161 constant is derived from the speed of sound (343 m/s at 20°C) and assumes a perfectly diffuse field.

Eyring's formula is more accurate at higher absorption levels and avoids predicting negative RT60: RT60 = 0.161·V / [−S·ln(1−αm)]. For αm < 0.2, both formulas give nearly identical results. For αm > 0.5 (heavily treated rooms), Sabine significantly over-predicts RT60 and Eyring should be used.

3 Room Modes - Resonance, Bass Buildup & Control

When sound waves reflect between parallel surfaces, they create standing waves at discrete frequencies called room modes. At these frequencies, the acoustic pressure distribution is uneven - some positions in the room have excessive bass (pressure maxima) while others have very little (pressure nulls). This is why you can walk around a room and hear the bass completely disappear in certain spots.

Room modes are categorised by the number of room dimensions involved. Axial modes involve one dimension only (e.g., 1,0,0 - length only) and are the strongest and most problematic. Tangential modes involve two dimensions and are 3 dB weaker. Oblique modes involve all three dimensions and are 6 dB weaker still. In a typical small room (studio, home theatre), the axial modes dominate below about 300 Hz.

4 Transmission Loss, Mass Law & Partition Design

Transmission Loss (TL) is the ratio, in decibels, of the sound power incident on one face of a partition to the power transmitted through it. Unlike STC (which is a single-number rating), TL is measured and specified at individual frequencies. Designing for TL requires understanding the mass law, the coincidence dip, and the effects of decoupling and air gaps.

The mass law states that for a single solid partition, TL increases by approximately 6 dB for every doubling of surface mass density, and by 6 dB per octave increase in frequency. A 12 mm plasterboard sheet (≈10 kg/m²) achieves roughly TL 30 dB at 500 Hz. Doubling the sheet gives ≈10 kg/m² more, adding ≈6 dB. This is why sheer mass - concrete, brick, dense board - is the simplest path to isolation.

The coincidence effect creates a dip in TL at the coincidence frequency, where the bending wavelength in the partition matches the acoustic wavelength. For standard 12 mm gypsum board, this occurs near 2,500 Hz. Above the coincidence frequency, TL improves at 10 dB/octave (better than mass law). Below it, mass law applies at 6 dB/octave.

5 Acoustic Absorption - Coefficients, NRC & Material Selection

Acoustic absorption converts sound energy into heat through friction in porous materials, or through panel and membrane resonance. The absorption coefficient α represents the fraction of incident sound energy absorbed by a surface at a given frequency. α = 0 means perfect reflection; α = 1 means total absorption (an open window). Real materials fall between these extremes, and α varies significantly with frequency.

The Noise Reduction Coefficient (NRC) is a single-number average of α at 250, 500, 1000, and 2000 Hz. It is the most commonly cited spec for acoustic foam, ceiling tiles, and panels. NRC 0.90 means the material absorbs 90% of sound energy at mid-frequencies. The Sound Absorption Average (SAA) extends this to 12 one-third octave bands from 200–2500 Hz and is preferred in ASTM E1374.

6 Noise Criteria - NC, NR & RC Curves

Background noise in buildings comes from HVAC systems, external traffic, mechanical plant, and occupants. Noise Criteria (NC) curves are the most widely used method for specifying and evaluating HVAC background noise in occupied spaces. Each NC curve represents a permissible spectrum of octave-band sound pressure levels from 63 Hz to 8000 Hz. A space "achieves" a given NC rating if its measured spectrum does not exceed the corresponding curve in any band.

The NC method was developed by Beranek (1957) and is widely referenced in ASHRAE and architectural standards. NR (Noise Rating) curves are the European equivalent, defined by ISO and used in the UK and Europe. Numerically they are similar but not identical. RC (Room Criteria) curves add a quality assessment - N (neutral), L (low-frequency rumble), H (high-frequency hiss) - making them more diagnostic for HVAC noise problems.