Reverberation Time: Spilling the "T" on RT60

Created by Jake Bedard, Modified on Mon, 7 Oct at 10:17 AM by Jake Bedard

Reverberation Time: Spilling the "T" on RT60

RT60 (aka T60) is a measurement of Reverberation Time, specifically the duration required for the sound energy in a room to decay by 60 dB after a sound has stopped. It is plotted through time-domain processing. Typically a short, loud impulse is used to test this, such as a balloon popping or a hand clap, either live in the room or played as a dry sample through an omnidirectional speaker. A frequency sweep can be used in some scenarios, as well.

Reverberation time is calculated from the reverse integration of an impulse response that has been filtered into octave bands (typically from 125 Hz to 4 kHz). Reverse integration, also called Schroeder integration, refers to the fact that it is measured backwards in time from the saddle point, or the point in time where the decay of the original sound meets the noise floor of the room. The amount of time (in seconds) from this saddle point back to the arrival of direct sound is the reverberation time.

Here are some examples of common reverberation times:

Church: 2-10 sec
Concert Hall: 1-2 sec
Office: 0.5-1.1 sec
Classroom: 0.4-0.6 sec

If the reverberation time is too long in a space, speech and music become less intelligible, as the reverberations within the space create additional background noise. The metric for measuring the speech intelligibility of a system and/or space is STI, which is explained in this article. Conversely, shorter reverberation times reduce background noise, but can also muffle voice and music. Overly short reverberation can also exacerbate comb filtering, while making speakers more localizable.

T10, T20, T30 and EDT

It is not always possible to measure a full 60dB of reverberant decay, so smaller ranges can be used instead. This is where the evaluation ranges T10, T20, T30, and EDT come in handy. T10-T30 are sometimes called "late reverberation times," as they measure the later part of the RT60 reverberation curve (often called a Schroeder curve). These ranges are measured between two points on the Schroeder curve. The first point lies where the direct sound has decreased by 5dB, and the endpoint is then 10dB, 20dB, or 30dB down the curve (this value correlating to T10/T20/T30). This is only possible, however, if this endpoint is also at least 10dB above the noise floor. Since these ranges are fractions of the total decay time, they are then extrapolated to the equivalent of a 60 dB decay time (multiplied by 6 for a T10 measurement, by 3 for a T20 measurement, and doubled for a T30 measurement).

Early Decay Time (or EDT), on the other hand, is measured from the arrival of direct sound to a point 10dB down the integration curve. Like a T10 measurement, this value is multiplied by 6 to find its 60dB equivalence.

Measuring Reverberation Time in Smaart

You can calculate the reverberation time of a captured Impulse Response measurement in Smaart by clicking the RT60 button at the bottom of the Control Bar. You can also click the Schroeder button to make the Schroeder curve appear on the plot. Impulse Response recordings captured in .wav format can also be dragged into Smaart, if captured using other software.

Our support portal hosts a full guide on capturing Impulse Response measurements in Smaart, available here:

The Schroeder Curve

Smaart uses a series of points to denote the anatomy of the Schroeder/reverse integration curve, pictured here.

These markers are as follows:

Ld, or Level Direct: This marker corresponds to the arrival time of direct sound.
Le, or Level Early (Decay): This marker is automatically positioned 10 dB down from the Ld marker on the reverse integration curve. The slope between Ld and Le is used to calculate EDT.
Lr1, or Level Reverberant 1: This marker designates the top of the reverberant decay range, 5 dB down the integration curve from the Ld marker. All level markers are user-adjustable, but positioning these three is fairly straightforward and shouldn't require much tweaking.
Lr2, or Level Reverberant 2: This marker designates the end point for the reverberant decay slope. If there is sufficient dynamic range, it should be 30 dB down the reverse integration curve from Lr1. If not, 20 dB works instead. Lr2 is one of two markers you may sometimes want to adjust by hand (the other being Ln).
Ln, or Level of Noise: The most subjective of the five, this marker serves as the basis for positioning all of the other markers. Ln ideally corresponds to the saddle point, the starting point for the reverse time integration curve. The magnitude coordinate is used to estimate the level of the measurement's noise floor and the Lr2 marker needs to be at least 10 dB above that.

"Optimize Graph" is checked by default, causing the Noise Floor (Ln)

to appear lower than what's shown in the IR trace.

The Ln marker's magnitude is fixed, as it is based on an estimation of the measurement's ambient noise floor. You may notice that its magnitude level is typically below the apparent level of noise in the displayed Log IR or RTC trace. This is because by default, Optimize Graph is active in the Impulse Response Graph Options dialog. IR mode's noise floor estimate is based on RMS levels, but the Optimize Graph feature uses Peak levels, hence the visual discrepancy. You can, however, adjust the time coordinate of the Ln marker by clicking and dragging it with your mouse.

When these level marker widgets are visible, a block of statistics appear in the top right corner of the plot. These include the 60 dB Reverberation Time (RT60), Early Decay Time (EDT), and the differences of time and level between each pairs of markers.

These level markers are relative, so moving the Ln marker will automatically recalculate the four others, updating their displayed values.

When the Ld, Le, Lr1, or Lr2 markers are auto-positioned, their early decay and reverberation times are based on a linear regression (or "least squares-fit line") of the reverse-time integration slope within each range. The calculated slopes are not displayed. These markers are manually adjustable as well, and moving them will instead recalculate the associated decay slope (EDT or RT60) based on the slope of the connecting line. Clicking or moving the Lc marker will restore the other four to their auto-calculated positions, reverting back to regression analysis.

When moved manually, the associated level difference value (Ld-Le or Lr1-Lr2) is updated in real-time. The Ld-Ln delta provides an estimate of overall dynamic range. D/R is the Direct/Reverberant ratio, an early-to-late energy ratio that comes from the time coordinate of the Le marker.

Saving Your Work

If you manually adjust any level markers by hand, you can save the adjusted positions to the IR .wav file header by clicking the Save button on the Data Bar. Smaart 8.5 and newer no longer require companion .csv files to store this information, but if you wish to import marker positions stored in a legacy .csv file, you can click the Info button on the Data Bar and then click the Import button in the Trace Info dialog to access the file.

The All Bands Table

Ideally, reverberation time should be measured from several locations throughout the room and averaged together, octave-band by octave-band to get an average decay time for each octave. To view reverberation time data by band, click the All Bands button on the Control Bar. This will open the All Bands table, which contains nearly every acoustical quantity that Smaart can calculate from an IR, for both octave bands and 1/3-octave bands (where applicable).

The "All Bands" table allows you to view reverberation data for each individual

octave band. You can also export this table to a .txt file by clicking the

Export button at the bottom of the window.

Reverberation times for the 125 and 250 Hz bands may be averaged together to get a T(Low) figure, as shown on the All Bands table. The average of the 500 Hz and 1kHz band is called T(Mid). When a single number figure is given for reverberation time, it is assumed to be T(Mid) unless otherwise stated.

Dividing T(Low) by T(Mid) gives you the Bass Ratio. Bass Ratio quantifies the "warmth" of sound in a venue and is a particularly important parameter for concert halls. The word "Bass" in this case refers to the vocal or instrument bass registers and should not be confused with PA-type sub-bass frequencies. Acceptable values are dependent on expectations. For instance, a Bass Ratio of 1.1-1.25 is considered good for fairly reverberant concert halls (RT60 greater than 1.8 seconds) but the upper figure could be increased to 1.45 for less reverberant spaces.

Reporting Your Findings

The standard evaluation range for reverberation time consists of 6 one-octave bands from 125 Hz to 4 kHz. Average times for each octave band can be presented in a table or on a graph. When using a graph, the frequency axis of the graph should be labeled with the IEC standard nominal octave band center frequencies. The y-axis of the graph should have an origin of 0 and be labeled in seconds. In either scenario, it should also be noted whether T20 or T30 was used. ISO 3382-1 specifies that if a graph is presented, it must be a line graph with a standardized aspect ratio of 2.5 cm per second and 1.5 cm per octave. ISO 3382-2 is less specific, simply stating "a graph" should be used.

The Histogram Display

Selecting Histogram as your display type for an IR mode graph will plot a chart of all reverberation times or (early-to-late energy ratios) by 1/3-octave bands. The type of data the Histogram displays and the resolution are selected by means of the list control in the upper-right of the graph. You can change the Histogram to a line chart by opening the Impulse Response options page
(Options > Impulse Response) and selecting Plot as Line under Histogram Settings.

Frequency Domain Analysis

Selecting Frequency as your graph type in the main graph area automatically transforms the IR into the frequency domain to show you its spectrum. The Frequency graph has a frequency in Hertz on the (horizontal) x axis and magnitude in decibels on (vertical) y axis. The Smoothing control in the upper right corner of the Frequency graph works exactly the same way as smoothing on the real-time transfer function display.

What is Considered a "Good" Reverberation Time?

Preferred reverberation times vary according to room size and purpose and the type of program material presented. Shorter reverberation times such as 0.4-0.5 seconds for smaller rooms and 0.8-1.2 seconds for larger rooms are preferred for auditoriums, classrooms, theaters, and cinemas, where speech intelligibility is a primary concern.

Opera houses and mixed-use performance spaces, where both speech intelligibility and musical appreciation are equally important, typically aim for the 1.2-1.8 second range. Large halls intended for symphonic performances and organ music can range from about 1.8-3 seconds or more.

It is generally preferable to see reverberation times that are roughly equal across all frequencies. An exception is environments meant for the performance of choral, organ and romantic classical music. In these scenarios, it may be preferred to see a reverberation time curve toward lower frequencies. Since higher frequencies tend to decay faster than lows, but you don't want to see wildly different reverberation times in neighboring octaves. Acoustical treatments and/or physical changes to the sound system are typically required to effectively address any problems you may find, such as the installation of soundproof foam, sound defusing paneling, or hanging baffles.