Reverberation Time RT60

Reverberation Time RT60

Reverberation time is the time required for the sound to "fade away" in a closed area. Sound in a room will repeatedly bounce off reflective surfaces such as the floor, walls, ceiling, windows or tables. When these reflections mix with each other, a phenomena known as reverberation is created. Reverberation reduces when the reflections hit absorbent surfaces such as curtains, padded chairs and even people. Reverberation is a key parameter when qualifying the acoustic status of a room. Particularly, too much reverberation has a negative impact on the intelligibility of speech.

At a Glance

  • Lightweight test solution
  • Frequency range 50 Hz - 10 kHz
  • Standardized reporting

Overview

If a sound is created in a room and then the sound is abruptly stopped, the reflections will linger in the room for a short period thereafter. This is particularly noticeable in a church, for example, where the sound may be heard for several seconds while it fades away.

A reverberation time measurement is used to calculate the time required for the sound to "fade away". That is, for the sound pressure to reduce by a predefined value. RT60 is the standard reverberation time measurement and is defined as the time it takes for the sound pressure level to reduce by 60 dB, measured from the moment the generated test signal is abruptly ended. The sketch below visualizes the basic principle of an RT60 measurement.

RT60 Measurement Principle 


The RT60 reverberation time measurement is defined in the ISO 3382-1 standard for performance spaces, and the ISO 3382-2 standard for ordinary rooms. RT60 is measured in seconds and often stated as a single value. The actual measurements span the frequency band from 50 Hz to 8 kHz with 1/1 or 1/3rd octave resolution. Rooms have individual absorption capabilities for each frequency, so the RT60 values within each band will vary.

Typically the ambient noise in a room 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 15 dB headroom) above this noise floor. Creating such sound at 125 dB across the whole spectrum, and particularly at low frequencies, is technically often not feasible.

In practice, therefore, we measure the time taken for the reflections to decay by 20 dB or 30 dB only. These readings can then be extrapolated to a decay time of 60 dB. Thus the reverberation time RT60(T20) is calculated as 3 * (time to decay by 20 dB) and RT60(T30) is calculated as 2 * (time to decay by 30 dB).

Test Signal Generation / Sound Sources

RT60 measurements require a diffuse sound field in the room, which means that the sound energy has to be distributed uniformly. For this reason, the sound signal source must have an omnidirectional radiation characteristic for precise measurements in accordance with ISO 3382.

 

NTi Audio Dodec
 

 

Omnidirectional Loudspeaker

The DS3 Dodecahedron Loudspeaker is a powerful sound source with omnidirectional radiation characteristics. It consists of 12 loudspeakers of the same type, mounted to a dodecahedron-shaped chassis. The optimized frequency spectrum delivers a high level of equalized sound energy for precise reverberation time measurements.
 

Power Amplifier PA3

 

Power Amplifier PA3

The dodecahedron speaker is powered by the portable Power Amplifier PA3, a product especially developed for precise acoustic measurements. The PA3 offers a built-in pink noise generator with an equalized output signal. The wireless remote control allows the test signal to be switched on/off from outside of the measurement room.


Minirator MR-PRO
 

Minirator MR-PRO

The NTi Audio Minirator MR-PRO generates a gated Pink Noise with a low crest factor dedicated to RT60 measurements. In addition, the MR-PRO offers a GlideSweep signal with editable cycle time (< 1.5 seconds for RT60 measurements using the XL2 Acoustic Analyzer). Both test signals are replayed via a power amplifier and a dodecahedral loudspeaker.
 

Other Test Signal Sources

Other Test Signal Sources

As an alternative to the omnidirectional loudspeaker it is also possible to use an impulsive noise source for RT60 measurements. Such an impulse may be generated for instance by a starter pistol, a balloon or clapping boards.

Measurement

The XL2 measures the energy decay in the range from 63 Hz to 8 kHz by the Schroeder method in 1/1 octave bands. Automatic triggering and averaging of multiple measurements simplifies the operation and minimizes the measurement time. Use either an impulse source or an interrupted pink noise as the test signal. The optional Extended Acoustic Pack extends the RT60 measurement to a 1/3 octave resolution.
 

XL2 RT60 Reverberation Time


The XL2 Acoustic Analyzer calculates the RT60 reverberation time by measuring T20 or T30; your choice. The XL2 follows this procedure:

  • detects 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,
  • applies a linear fit to the acquired decay curve,
  • determines the time T20 or T30,
  • calculates the RT60 result: RT60 = 3 * T20 or RT60 = 2 * T30

 

RT60 decay measurement

Data Export and Documentation

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

STIPA Reporting Tool

The RT60 Reporting Tool creates measurement reports based on the acquired XL2 data. The tool displays the acquired RT60 results in 1/3rd octave or 1/1 octave frequency resolution.

Download here

Recommendations

The ISO 3382 standards list a number of location types for which the reverberation time should be optimized, and recommends the following 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

 

A room with an RT60 of < 0.3 s is called acoustically "dead" (e.g. anechoic chamber), whereas sounds in rooms with reverberation times > 2 s are "echoic".

Typically reverberation times can be reduced with the introduction of 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.

Configurations

Exel Set for RT60 Testing

Exel Kit for Basic RT60 Measurements

including
  • XL2 Audio and Acoustic Analyzer
  • M4261 Measurement Microphone
  • Minirator MR-PRO
  • ASD Cable for XL2
  • 2x Mains Power Adapter
  • Exel System Case
 

Exel Kit for Extended RT60 Measurements

including
  • XL2 Audio and Acoustic Analyzer
  • Extended Acoustic Pack for XL2
  • M2230 Measurement Microphone
  • Minirator MR-PRO
  • ASD Cable for XL2
  • 2x Mains Power Adapter
  • Exel System Case

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