RT60 — what it is and why it matters

RT60 — what it is and why it matters

Reverberation time is the most widely used single-number descriptor of a room's acoustic behaviour. It is the foundation of every room acoustic specification, from concert hall design to studio control room treatment. Understanding what it measures, how it is derived, and where it breaks down is essential for anyone working seriously with room acoustics.

Definition

RT60 is defined as the time required for the sound pressure level in a room to decay by 60 dB after a sound source is abruptly stopped. The subscript 60 refers to the 60 dB decay range. It is measured in seconds.

A 60 dB reduction in sound pressure level corresponds to a reduction in sound energy by a factor of one million. In a well-behaved, diffuse sound field, this decay is exponential — it appears as a straight line on a dB-versus-time plot.

RT60 is a frequency-dependent quantity. A room may have an RT60 of 0.4 s at 1 kHz but 0.8 s at 63 Hz. For this reason, RT60 is always measured and specified in octave or third-octave bands across the frequency range of interest.

Derivation — Sabine's formula

The earliest analytical expression for reverberation time was derived by Wallace Clement Sabine at Harvard in the late 1890s. Sabine's formula relates RT60 to room volume and total acoustic absorption:

RT60 = 0.161 × V / A

where V is the room volume in cubic metres and A is the total absorption in sabins (m²). One sabin of absorption is equivalent to one square metre of perfectly absorptive surface.

Total absorption A is calculated as:

A = Σ (αi × Si)

where αi is the absorption coefficient of each surface material (a dimensionless value between 0 and 1) and Si is its area in square metres.

Sabine's formula assumes a diffuse sound field — one in which sound energy is uniformly distributed throughout the room and the direction of propagation is random at every point. This assumption holds reasonably well in large, roughly cubic rooms with moderate absorption. It breaks down in small rooms, highly damped rooms, or rooms with strongly non-uniform absorption distribution.

Eyring's correction

For rooms with higher absorption — where the average absorption coefficient exceeds approximately 0.2 — Eyring's formula gives a more accurate result:

RT60 = -0.161 × V / (S × ln(1 - ᾱ))

where S is the total surface area of the room and ᾱ is the mean absorption coefficient averaged over all surfaces. As ᾱ approaches 1 (a fully anechoic room), RT60 approaches zero, which is physically correct. Sabine's formula incorrectly predicts a non-zero RT60 in a fully absorptive room.

In practice, Eyring's formula is preferred for treated studio spaces and listening rooms where wall panels, bass traps, and other absorbers have significantly reduced the average surface reflectivity.

Measurement — ISO 3382

Measured reverberation time is derived from the room's impulse response, not from interrupted noise. The ISO 3382 standard defines the measurement procedure.

The backward integration method (Schroeder integration) is used to extract decay curves from an impulse response. The squared IR is integrated from the end of the measurement backwards in time, producing a smooth decay curve that is equivalent to the ensemble average of many interrupted-noise measurements. This approach gives a reproducible result from a single measurement.

From the decay curve, several decay time metrics are defined:

T20 — the time for a 20 dB decay, extrapolated to 60 dB. Measured from −5 dB to −25 dB below the initial level. Used in rooms where the full 60 dB dynamic range is not cleanly available, and preferred when background noise is high.

T30 — the time for a 30 dB decay, extrapolated to 60 dB. Measured from −5 dB to −35 dB. More robust than T20 in many situations and the most commonly quoted metric in studio and broadcast specifications.

EDT (Early Decay Time) — the time for a 10 dB decay, extrapolated to 60 dB. Measured from 0 dB to −10 dB. EDT is strongly influenced by early reflections and correlates more closely with the subjective impression of reverberance than T20 or T30. A room can have a long T30 but a short EDT and still sound relatively dry if early energy is well controlled.

In practice, the choice between T20 and T30 depends on the available signal-to-noise ratio of the measurement. A clean impulse response with high SNR allows T30; a noisy measurement may only support T20.

Target values by application

RT60 requirements vary significantly by room type. The following values represent typical targets at mid-frequencies (500 Hz – 1 kHz):

Recording studio live room — 0.2 s to 0.5 s depending on intended use. Rooms intended for classical or acoustic recording may target the longer end; rooms used primarily for close-miked electric instruments can be shorter.

Control room / mixing room — 0.2 s to 0.35 s. A short, consistent RT60 across the frequency range is preferred. The monitoring position should be dominated by direct sound, with controlled early reflections and minimal late reverberation.

Mastering suite — 0.15 s to 0.25 s. Extremely low reverberation, very flat frequency response. Any room character must be inaudible to ensure translation to other playback environments.

Listening room (hi-fi) — 0.3 s to 0.45 s, with preference for consistency across frequency. Variations greater than 2:1 between low and high frequencies are audible and undesirable.

Broadcast voice-over booth — 0.1 s to 0.2 s. Very dry. Dialogue recorded in a highly reverberant space cannot be convincingly treated in post-production.

Frequency dependence and the bass problem

In virtually all small rooms, RT60 rises sharply at low frequencies. This is a consequence of two related factors: most practical absorbers are less effective below 200 Hz, and room modes at low frequencies store energy that decays slowly and independently of the diffuse field.

A room with an RT60 of 0.3 s at 1 kHz may have an RT60 of 0.8 s or more at 63 Hz. This frequency-dependent decay is one of the primary causes of poor low-frequency reproduction in small studios and listening rooms — bass notes sound blurred, pitch is harder to distinguish, and mixing decisions made on such a system will not translate to other playback environments.

Controlling low-frequency reverberation time requires bass absorption with effective depth. Porous absorbers (mineral wool, acoustic foam) are effective above approximately 200–300 Hz but require impractical thicknesses to absorb below 100 Hz. Corner-placed bass traps — exploiting the pressure maxima that all room modes share at room boundaries — are the most efficient placement strategy for broadband low-frequency absorption.

Limitations of RT60

RT60 is a useful summary statistic but it does not fully characterise a room's acoustic behaviour. Several important aspects are not captured:

Modal behaviour — individual room modes may decay at very different rates. A single RT60 value in the low-frequency octave bands is an average that may mask severe individual modal problems.

Spatial variation — RT60 varies significantly with measurement position, particularly at low frequencies. A single-position measurement is not representative of the whole room.

Non-exponential decay — in rooms with strong modal clustering or highly non-uniform absorption, the decay curve departs from the straight line assumed by the RT60 metric. A double-slope decay — a steep initial drop followed by a shallower long tail — may indicate a connected space or a room with strongly coupled modal behaviour.

Perceptual correlates — RT60 correlates with subjective reverberance, but other parameters, particularly EDT and the clarity index C80, are often better predictors of perceived acoustic quality in critical listening applications.

Despite these limitations, RT60 remains the most widely specified and most easily communicated acoustic descriptor. It provides a useful first-order characterisation of a room and a basis for treatment design — provided its limitations are understood.