残響時間 RT60

Reverberation Time RT60

This page provides you with all the information you need for measuring RT60. We explain everything, from the terminology used to how an actual measurement is done.

We are instrument manufacturers and experts in the field of acoustic measurements.

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.

 

What is RT60?

When it comes to precise measurement of reverberation time with a meter, we introduce the concept of RT60.

RT60 is an abbreviation for Reverberation Time 60dB.

RT60 provides an objective reverberation time measurement. It is defined as the time it takes for the sound pressure level to reduce by 60 dB, measured after the sound source is abruptly switched off.

 

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.

 

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

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For consideration regarding the measurement meter

How many times must I measure? 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. 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). Uncertainty depends on the measured reverberation time and the bandwidth of the individual frequency band; lower bands show a higher uncertainty factor.


    For instance, RT60 results that were acquired with 5 cycles, T30 data and 1/1 octave resolution have a lower uncertainty (i.e. a higher certainty) than if they had been acquired with 3 cycles, T20 data and 1/3rd octave resolution because

    • 5 cycles give a lower uncertainty (i.e. a better measurement accuracy) than 3.
      A minimum of 3 sequential cycles is required
    • T30 gives a lower uncertainty than T20
    • Octave-band measurements give a lower uncertainty than one-third-octave measurements
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 is measures RT60 with 1/1 octave resolution, or, with the addition of the Extended Acoustic Pack Option, with 1/3rd octave resolution.

If you are looking for the longest RT60 time, the 1/1 octave will give you better results than the 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.

For example, a standard may require that a particular resolution be used, such as the ISO 3382 standard specifying the maximum deviation of directivity of source in octave bands of pink noise.

Another example may be that the manufacturer of the material you intend to use for sound absorption specifies by how much a certain amount of their material reduces the 1/3rd octave RT60 value. So to determine how much material you need to buy, you need to measure RT60 with 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.

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,
  • measures RT60 in the range from 63 Hz to 8 kHz
  • produces results in 1/1 octave band resolution. The optional Extended Acoustic Pack extends the RT60 measurement to a 1/3rd octave resolution.
  • provides a choice between T20 or T30 methods
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 = 3 * T20 or RT60 = 2 * T30

 

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 a diffuse 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 120 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 at least 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 need 12 speakers?


A great deal of thought, discussion and research has gone into the decision for NTi Audio to produce a 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)

It has been demonstrated that, where appropriate, an omnidirectional rather than a directional sound source should be used when measuring RT60. This is defined in the ISO 3382 and ASTM E2235 standards.

Weight and power are key considerations when choosing a dodecahedron speaker.

The speaker cabinet should be neither heavy nor bulky. It needs to be portable, to be transported, for example, to those rooms that need measuring on the 5th floor of that newly-constructed building; a building whose confounded lift is not working.

Omnidirectional means sound energy distributed evenly in all directions at the same time. The DS3 radiates an omnidirectional pattern that complies with the standards.

Further, the DS3 loudspeakers deliver a suitably significant amount of sound power.

In weight, the NTi Audio DS3 dodecahedron is surprisingly light, and capable of delivering 120 dB of omnidirectional power.

 

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 the 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.

 

International Standards

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 ISO 3382-3 for open plan offices, as well as 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.

Typically reverberation times can be reduced by damping i.e. by absorption or redirection (reflection or diffusion). Absorbing materials include 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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