Reverberation Time RT60 Measurement

Reverberation Time RT60 Measurement

Reverberation is one of the most significant acoustic properties of a room. Knowing the reverberation time is essential in characterizing rooms, be they performance spaces, ordinary rooms or open office spaces.

While the requirements for measuring reverberation are described in detail in parts 1-3 of the ISO3382 standard, on this page we provide some basic common information such as terminology and how an actual reverberation measurement is done.

We are a leading manufacturer of acoustic test and measurement instruments and on this page we will show you what benefits you receive by choosing the NTi Audio RT60 measurement solution.

You will find the essential tools for your job and gain knowledge along the way.

 

What is Reverberation Time?

Sound produced in a room will repeatedly bounce off reflective surfaces such as the floor, walls, ceiling, windows or tables while gradually losing energy. When these reflections mix with each other, the phenomena known as reverberation is created. Reverberation is thus a collection of many reflections of sound.

Reverberation time is a measure of the time required for reflecting sound to "fade away" in an enclosed area after the source of the sound has stopped.

Reverberation time is important in defining how a room will respond to acoustic sound.

Reverberation time reduces when the reflections hit absorbent surfaces such as curtains, padded chairs and even people, or exit the room through the walls, drop ceilings, doors, window glass, etc.

Here we show you how to get a feeling for reverberation times in various rooms, just by clapping your hands.

 

How is Reverberation Time defined?

The Reverberation Time (RT) is the time the sound pressure level takes to decrease by 60 dB, after a sound source is abruptly switched off. RT60 is thus a commonly-used abbreviation for Reverberation Time.

RT60 values vary in different positions within a room. Therefore, an average reading is most often taken across the space being measured.

 

RT60 Measurement Principle
Visualization of the basic principle of an RT60 measurement.

 

Rooms with an RT60 of < 0.3 seconds are called acoustically "dead".

Rooms with an RT60 of > 2 seconds are considered to be "echoic".

Why is reverberation important?

Too much reverberation has a negative impact on the intelligibility of speech. This can, for example, make it hard to hear what a class teacher is saying.

Reverberation is also particularly noticeable in a place of worship where the sound may be heard for several seconds while it fades away. The main reason religious leaders pronounce their words clearly and talk slowly, leaving small gaps between sentences, is to overcome this reverberation and make their speech clear (such a manner of speaking also has a beneficial side-effect of sounding reverent).

Conference rooms are an especially challenging acoustic environment. Collaborative white boards, stylish glass walls and the obligatory large table are all highly-reflective surfaces for sound. This tends to increase the reverberation time of the room which impacts speech intelligibility.

On the other hand, too little reverberation will reduce the rich, warm acoustic sound from an orchestra in a concert hall.

 

VIDEO: How to measure RT60

In this Video we explain how to measure RT60 with the XL2 Audio Analyzer, DS3 Dodecahedron Speaker and the PA3 Power Amplifier.

 

7 Steps to RT60 Measurement

  1. 1.
    On the XL2, select RT60 from the main menu.
    XL2 RT60 Main Menu
  2. 2.
    When the room is quiet, click SET.
    XL2 RT60 SET
  3. 3.
    On the PA3, play "EQ Pink" noise and adjust the level.
    (wear hearing protection)
    PA3 EQ Pink and Gain
  4. 4.
    Press Start button on the XL2 to measure the RT60.
    Press Start
  5. 5.
    Toggle sound source 3 times on and off.
    PA3 Remote Control
  6. 6.
    Press Stop button on the XL2.
    Press Stop
  7. 7.
    Print your Report.
    RT60 Report with your logo

Download Quick Guide to RT60 Measurement

 

Meter for RT60 Measurement

We recommend the XL2 Acoustic Analyzer for RT60 measurements. The automatic features simplify the operation of the XL2 and minimize the time and effort spent measuring.

 

XL2 Acoustic Analyzer for RT60 Measurement
XL2 Acoustic Analyzer for RT60 Measurement

XL2 Features

  • automatic triggering on both impulse* and gated noise** sound sources
  • automatic averaging of multiple measurements
  • spectral results
  • completely integrated documentation
  • compliant to international standards

* An impulsive sound is defined as an almost instantaneous (thus impulse-like) sharp sound such as a clap, pop or a gunshot.

** Gated noise has high spectral density and normal amplitude distribution, e.g. pink or white noise, and is gated with an on/off cycle every 1 - 9 seconds.

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For consideration regarding the measurement meter

Correlation and Uncertainty


RT60 decay linear fit

RT60 is calculated using a linear least-squares regression of the actual measured decay curve. In simple terms, the calculation finds the straight line (linear fit) that best fits as a representation of all the measured data.

The XL2 automatically calculates two auxiliary results, correlation and uncertainty. These are both required by the standards, and indicate the precision of the results.

  • Correlation indicates how well the calculated linear fit matches to the actual decay curve. A high correlation value indicates a linear, non-distorted decay curve.
    The correlation factor is expressed as a percentage; 100% represents perfectly linear sound pressure level decay after the sound source has ceased. The natural deviation from this linearity results in lower correlation values. Actual correlation factors are typically between 80 and 100%.
  • Uncertainty is introduced because pink noise is not a consistent signal, rather a random signal. It depends on the reverberation time (longer times produce lower uncertainty) and the bandwidth of the individual frequency band (broader bandwidth produces lower uncertainty). Also, lower bands show a higher uncertainty factor.

    Uncertainty is influenced by the number of test cycles, the measurement method (T20 or T30), and the measurement filter (1/3rd or 1/1 octave resolution).


    So, for a lower uncertainty (i.e. a better measurement accuracy),

    • T30 is better than T20
    • 1/1 octave measurements are better than 1/3rd measurements
    • 5 cycles is better than 3
      (Note: a minimum of 3 cycles is required)
Where should I place the measurement microphone? Critical Distance Dc

It is recommended to place the sound source and the microphone in multiple positions, and average all the readings, to compensate, for example, for any room modes (resonances brought about by the dimensions of the room).

The microphone should always be placed at least 1 meter from reflecting surfaces (walls, doors, windows, floors, tables).

Further, there is a formula that helps us determine where to place the microphone relative to the sound source. It gives us the minimum distance required between any source of sound and the measurement microphone for a valid RT60 measurement.

This is known as the critical distance. The formula may seem complex at first glance, so we've added an example to make it clearer (hopefully).

RT60 Critical Distance formula

V = Volume of the room [m3]
c = Speed of sound [m/s]
T = Expected RT60 for the room [s]

Example: in a small hall, at a room temperature of 20℃, with dimensions of 10 meters by 9 meters and a height of 5 meters, and an expected RT60 of 2 seconds, the microphone must be at least 1.6 meters away from the sound source.

V = 10 * 9 * 5 = 450 m3
c = 342 m/s (the speed of sound @ 20℃)
T = 2 seconds

Critical Distance Dc = 2 *√ (450 / (342 * 2)) = 1.6 meters

Shall I use 1/3rd or 1/1 octave frequency resolution?

The XL2 Acoustic Analyzer measures RT60 with 1/1 octave resolution, or, with the addition of the Extended Acoustic Pack Option, with 1/3rd octave resolution.

For many applications, using a 1/1 octave resolution is sufficient, unless the specification documentation with which you are working requires a 1/3rd octave resolution.

Shall I choose T20 or T30?

RT60 decay measurement

With a sound source creating a sound pressure level
of 100 dB, a room with a noise floor of up to 55 dB
can be measured using the T30 method

Typically the ambient noise in a room (e.g. an apartment or office) would create a noise floor of 40-50 dB. To measure a decay of 60 dB from a sound source, we have to inject the sound at 75 dB (with 5 dB for the auto trigger and 10 dB headroom to the noise floor) above this noise floor. Creating such sound at 125 dB across the whole spectrum, and particularly at low frequencies, requires awfully high sound pressure and is often practically or even technically not feasible.

In practice, therefore, we measure the time taken for the reflections to decay by 20 dB or 30 dB only. If the decay is acceptably linear, these readings can then be linearly extrapolated to a decay time of 60 dB.

 

  • RT60(T20) = 3 * (time to decay by 20 dB) while
  • RT60(T30) = 2 * (time to decay by 30 dB)

 

Generally it is better to choose T30 over T20, as the measurement uncertainty will be lower. However, if the background noise is too high and/or the sound source is not loud enough to create an extra 45 dB, T20 may be your best option.

How do I get a single RT60 result?

A single RT60 result may be calculated by averaging measured values from a selection of frequency bands.

Whenever you state a single RT60 result, do not forget to report on the frequencies that were averaged, otherwise the result is meaningless.

For example, a single figure reverberation time may be calculated from a 1/1 octave measurement by averaging the times in the 500 Hz and 1 000 Hz octave bands.

Frequency f [Hz] RT60 Octave Band [s]
63 0.90
125 0.87
250 0.76
500 0.67
1000 0.59
2000 0.56
4000 0.56
8000 0.51

(0.67 + 0.59) / 2 = 0.63

This result may be represented thus: RT60[500Hz, 1000Hz] = 0.63

Alternatively, for 1/3rd octave measurements, you may take averages over the six bands from 400 Hz to 1250 Hz.

Frequency F [Hz] RT60 1/3rd Octave Band [s]
50 0.29
63 0.25
80 0.31
100 0.20
125 0.22
160 0.21
200 0.27
250 0.22
315 0.41
400 0.34
500 0.36
630 0.25
800 0.22
1000 0.23
1250 0.22
1600 0.22
2000 0.25
2500 0.21
3150 0.20
4000 0.22
5000 0.22
6300 0.21
8000 0.23
10000 0.22

(0.34 + 0.36 + 0.25 + 0.22 + 0.23 + 0.22) / 6 = 0.27

This result may be represented thus: RT60[400Hz-1.25kHz] = 0.27

According to the ISO 3382-1 standard, either of the above two results may be labelled T30,mid or T20,mid depending on which dynamic measurement range was considered in obtaining the RT60 measurement result (20 dB or 30 dB respectively). Therefore, for compliance in reporting, and in order to be able to reproduce the same testing conditions at a later date, it is recommended that the results should always state the full range of values in every measured frequency in the form of a table (as above) or a graph in your report.

Should I measure alone?

The process and the XL2 Acoustic Analyzer are designed to be operated by one person.

However, although it is loud and therefore possibly uncomfortable, there can be other people in the room during the measurement. It may, for example, be useful for you to have help moving the dodecahedron around.

Everyone in the room must remain still and quiet during measurements. They should all wear hearing protection. Avoid anyone standing near the microphone.

People who are present in the room during the measurement will absorb sound energy and possibly reduce the RT60 value. You should document how many people were present during measurements.

XL2 RT60 Tech Specs
The XL2 Acoustic Analyzer,
  • Conforms with ISO 3382 and ASTM E2235
  • 1/1 octave bands results from 63 Hz - 8 kHz, based on T20 and T30
  • Optional: 1/3 octave bands results from 50 Hz - 10 kHz, based on T20 and T30
  • Range: 10 ms - 30 seconds
  • Minimum RT60 (typical)
    • < 100 Hz: 0.3 second
    • 100 - 200 Hz: 0.2 second
    • > 200 Hz: 0.1 second
  • Measurement based Schroeder-method
  • Test signal: Impulse source or interrupted pink noise generated by the MR-PRO, MR2 or the included NTi Audio Test CD
How the XL2 measures RT60


RT60 decay measurement

The XL2 Acoustic Analyzer autonomously measures the RT60 reverberation time by:

  • detecting that the sound level has decayed by 5 dB; this indicates that the sound source has been switched off and triggers the XL2 to start the time and level decay measurement,
  • determining the time to reduce by 20 or 30 dB, depending on user's choice,
  • applying a linear fit to the acquired decay curve,
  • calculating the RT60 result:
    RT60(T20) = 3 * (time to decay by 20 dB) while
    RT60(T30) = 2 * (time to decay by 30 dB)

 

Sound Sources for RT60 Measurement

Always wear hearing protection, as the sound sources for RT60 measurements can get loud.

Depending on the type and purpose of the room you are measuring, various sound sources are suitable.

In accordance with the ISO 3382 and ASTM E2235 standards, many common RT60 measurements require an omnidirectional sound source, which means that the sound energy has to be distributed uniformly. For precise measurements, the sound source must have an omnidirectional radiation characteristic.

An impulsive sound is defined as an almost instantaneous (thus impulse-like) sharp sound such as a clap, pop or a gunshot. According to ISO 3382-1, impulsive signal sources have the disadvantage of not being exactly repeatable. Further, the ASTM E2235 standard does not permit impulsive sound sources.

Choices of Sound Sources 

Dodecahedron Omnidirectional Speaker


DS3 and PA3
Dodecahedron Speaker Set

The professional solution.

This powerful omnidirectional sound source is suitable for most applications, from small to relatively large rooms.

Advantages

  • lightweight for the 121 dB it delivers
  • wireless remote control for mute/unmute
  • the equalized pink noise covers the acoustic frequency spectrum from 100 Hz to 8 kHz
  • low power compression ensures stable sound level over long time period
  • re-useable at no expense
  • meets the ASTM E2235 standard as it is not an impulsive sound source

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Disadvantages

  • a couple of extra things to carry

Download Dodecahedron Speaker Set Product Data

Existing installed PA system


Minirator MR-PRO
MR-PRO Audio Generator

When the venue is very large, injecting pink noise into the existing installed PA system may be your only reasonable option.

Try to get enough power from the PA system, especially in the low frequencies.

Advantages

  • the pink noise source covers the whole frequency range of measurements
  • re-useable at no expense
  • meets the ASTM E2235 standard as it is not an impulsive sound source

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Disadvantages

  • the PA speakers may not be distributed uniformly around the measured space
Powered portable speaker


Minirator MR-PRO
MR-PRO Audio Generator

Make sure you can get enough power from your loudspeaker, especially in the low frequencies.

To compensate for the measurement uncertainty introduced by the directivity of the speaker, you should perform a greater number of measurements at various positions in the room.

Advantages

  • cost-effective if you already own a powered loudspeaker
  • the pink noise source covers the whole frequency range for measurements
  • the size of the room is limited, within reason, only by the size of your loudspeaker.
  • re-useable at no expense
  • meets the ASTM E2235 standard as it is not an impulsive sound source

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Disadvantages

  • this may produce a less-than-optimal result as the high directivity of a single loudspeaker does not have an omnidirectional radiation characteristic
  • you have to lug the powered loudspeaker around – let’s hope it’s not too heavy
Fire a starter pistol

IT IS NOT ADVISABLE TO GO THROUGH AIRPORT CUSTOMS OR INTO SCHOOL BUILDINGS ETC. WITH A STARTER PISTOL IN YOUR HAND / LUGGAGE.

The bigger the caliber of the pistol, the more deeper frequencies it will cover, and the more sound energy it can produce. Thus larger rooms can be measured.

Exploding caps may leave a burnt gunpowder residue – make sure you have access to a vacuum cleaner to clean up if the location is sensitive to mess e.g a restaurant.

Advantages

  • easy to carry around
  • quick setup
  • re-useable at relatively little expense – cost of the exploding caps
  • has an omnidirectional radiation characteristic

Disadvantages

  • may make people in your immediate vicinity nervous to see you brandishing a gun
  • a starter pistol may not create sufficient energy in large rooms
  • a starter pistol may not cover the whole frequency range of measurements
  • initial financial investment in purchasing the starter pistol
  • ongoing financial expense of firing caps
  • does not meet the ASTM E2235 standard as it is an impulsive sound source
Pop a balloon

The larger the balloon, the more deeper frequencies it will cover, and the more sound energy it can produce. Thus larger rooms can be measured.

Make sure you use higher-quality balloons that are fit for purpose. Cheap children party balloons can be difficult to blow up, and may burst prematurely in front of your client.

Advantages

  • easy to carry around
  • has an omnidirectional radiation characteristic

Disadvantages

  • may be time-consuming – a balloon of 1 meter diameter could take up to 5 minutes to inflate with an electric balloon inflator
  • a bursting balloon may not cover the whole frequency range of measurements
  • a balloon may not create sufficient energy in larger rooms
  • does not meet the ASTM E2235 standard as it is an impulsive sound source
Clapper board


Clapper Board

Advantages

  • easy to carry around
  • no setup
  • re-useable at no expense
  • has an omnidirectional radiation characteristic

Disadvantages

  • the RT60 result may not be valid as a clapper board does not cover the whole measurement frequency range
  • the RT60 result may not be valid as a clapper board will not create sufficient energy in larger rooms
  • does not meet the ASTM E2235 standard as it is an impulsive sound source
Clap your hands

A hand clap can give you an estimation of the RT60 value.

Advantages

  • good for a quick indication
  • no financial investment
  • has an omnidirectional radiation characteristic

Disadvantages

  • the RT60 result may not be valid as it is difficult to clap hard enough to trigger the measurement
  • the RT60 result may not be valid as a hand clap will not cover the whole measurement frequency range
  • does not meet the ASTM E2235 standard as it is an impulsive sound source


For consideration regarding sound sources

For how long must I play the sound source signal?

Before being switched off to trigger a measurement, the sound source should be played for a long enough time period to ensure that a balance between injected and absorbed acoustic energy has been reached. In other words, the sound reflections should be given enough time to fill the whole room.

As a rule of thumb, ensure that the pink noise is played for a few seconds and at least half the time period of the estimated RT60 test result.

If in doubt, play the sound source for at least 5 seconds before each measurement.

Thinking outside the box

If you find yourself in a very large or long room with no installed PA system, you may have to think of innovative ways to create a loud, deep bang. To encourage you to think outside the box, we can share the following experience with you. A loud and low-frequency bang can be produced using a telephone book hit against a stable and robust table.

Do you have any innovative ways of creating noise for RT60 measurements?
Tell us

Why do we use 12 drivers in the DS3 Dodecahedron Speaker?


Dodecahedron
noun
a three-dimensional shape having twelve plane faces, in particular a regular solid figure with twelve equal pentagonal faces. from Greek dodekaedros meaning ‘twelve-faced’ (thanks to the Greeks for this tricky word, and democracy, philosophy, art, architecture, science, and sport, to name but a few others things)

Sound insulation and precise reverberation time measurements require the use of an omnidirectional sound source. Omnidirectional sources radiate sound equally in all directions. Loudspeakers mounted on the surfaces of a polyhedron will give such a uniform, omnidirectional radiation.

These are only five possible regular polyhedron shapes for creating an omnidirectional source

tetrahedron with 4 triangular faces
hexahedron or cube with 6 square faces
octahedron with 8 triangular faces
dodecahedron with 12 pentagonal faces
icosahedron with 20 triangular faces

The international standards ISO 3382-1:2009 and ISO 16283-1:2014 specify the directivity response of omnidirectional speakers. To give an adequate approximation of uniform omnidirectional radiation, it is stated that the dodecahedron (12 faces) is the preferred polyhedron.

In the design of the NTi Audio Dodecahedron Loudspeaker enclosure, consideration was given to the practical advantage of transporting a smaller, lightweight enclosure, and providing sufficient sound power output, while ensuring a flat frequency response, and optimizing the spectral uniformity - and all this at an affordable price

 

Producing Compliant Reports

We have done most of the work for you already by building a free RT60 Reporting Tool for producing measurement reports based on the acquired XL2 data.

STIPA Reporting Tool

Get your free RT60 Reporting Tool here.
Download now

The XL2 Acoustic Analyzer saves the RT60 measurement results on its SD card. The data contains all relevant information formatted as a text file so, as an alternative for reporting, the data can be easily imported into any spreadsheet program such as MS Excel.

 

Recommended RT60 Values

The RT60 reverberation time measurement is defined in the ISO 3382-1 standard for performance spaces, the ISO 3382-2 standard for ordinary rooms, and the ASTM E2235 standard.

Here is a list of location types with suggested RT60 values:

Location Volume Critical Distance Dc Recommended RT60
 Recording Studio < 50 m3 1.5 m 0.3 s
 Classroom < 200 m3 2 m 0.4 - 0.6 s
 Office < 1'000 m3 3.5 m 0.5 - 1.1 s
 Lecture Hall < 5'000 m3 6 m 1.0 - 1.5 s
 Concert Hall, Opera < 20'000 m3 11 m 1.4 - 2.0 s
 Church     2 - 10 s

 

Recommended Reading

Here you can get a feeling for RT60 values just by clapping your hands.

The RT60 reverberation time measurement is defined in the ISO 3382-1 standard for performance spaces, the ISO 3382-2 standard for ordinary rooms, and the ASTM E2235 standard.
- ISO 3382 Part 1: Performance spaces
- ISO 3382 Part 2: Reverberation time in ordinary rooms
- ASTM E2235

Typically reverberation times can be reduced by damping using absorbing materials such as thick carpets, curtains, upholstered furniture or dedicated sound-absorbing panels. Furthermore, the presence of people in a room reduces the reverberation, and therefore produces a lower RT60 value compared to the unoccupied room. Read more about the soundproofing options.

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