XL2 Audio and Acoustic Analyzer

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.

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 "Dodec" 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.


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.

RT60 Measurement with XL2 Analyzer

Measurement Principle

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


Test Procedure

In a first step, it is recommended you determine the ambient noise level in the room (silent test signal source) by selecting the "Set Level Markers" function of the XL2. Depending on this noise floor, you may now choose between the T30 (sound source must be at least 45 dB above the noise floor) or T20 (sound source must be at least 35 dB above the noise floor) acquisition modes.

Second, select the test signal type. If you opt for Pink Noise, you have to allow the XL2 to measure for a minimum of 3 test cycles due to the random nature of this signal. The XL2 then calculates the overall result per frequency band, based on the averaged data of all acquired test cycles.

Now press the start button on the XL2.

Using hearing protection, repeatedly play the test signal with sufficient level; the XL2 will measure and calculate the RT60 each time the sound source is switched off.

Note the following:

  • There should be a maximum of two people in the room during the measurements, neither of whom positioned within 1 meter of the XL2.
  • Execute several measurements at different microphone positions and heights within the same room.
  • The sound signal source should also be placed at different locations and heights within the room.
  • In very large rooms (e.g. concert halls) you may operate several Dodec loudspeakers in parallel.
  • The measuring microphone must be located far away enough from the sound source (> critical distance, see below) and from reflecting surfaces (> 1 m).

Result Display

The XL2 display continuously shows the progress of the measurement. At the end of each acquisition cycle, it indicates via a tick symbol, whether the decay measurements have been completed successfully in the individual bands.

XL2 RT60 Reverberation Time

The result page displays the averaged or the individual results of the acquired cycles, together with the related uncertainty and correlation data.


Correlation and Uncertainty

The XL2 automatically calculates two auxiliary results that provide a feedback on the precision of the acquired data.

  • "Correlation" indicates how good the calculated linear fit matches to the actual decay curve. In other words, a high correlation value represents a very linear, non-distorted decay curve.
  • "Uncertainty" is calculated from the measurement bandwidth (1/3rd or 1/1 octave), the reverberation time (T20 or T30 result) and the number of test cycles. For instance, the RT60 results that were acquired with 5 cycles, T30 data and 1/1 octave resolution have a lower uncertainty (i.e. a better certainty) than if they had been acquired with 3 cycles, T20 data and with 1/3rd octave resolution.

Recommendations

Critical Distance

The formula defines the critical distance between the omnidirectional sound source and the measurement microphone. In other words, for a valid RT60 measurement, the microphone must be placed at least as far away from the sound source as the formula determines.


RT60 Measurement Critical Distance
V = Volume of the room [m3]
c = Speed of sound [m/s]
T = Reverberation time of the room [s]

For example, in a small hall, 10 meters by 10 meters with a height of 5 meters, and an expected RT60 of 3 seconds, the microphone must be at least 1.4 meters away from the sound source.

V = 10*10*5 = 500 m3
c = 342 m/s (@ 20℃)
T = 3 seconds

Critical Distance Dc = 2*√ 500/(342*3) = 1.4 meters



RT60 Reverberation Time

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.

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.



RT60 Reporting Tool

STIPA Reporting Tool(Click to enlarge)

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.

The RT60 Reporting Tool is free to download from the XL2 Support website.

Order Information

Exel Set for RT60 Testing

Exel Kit for Basic RT60 Measurements

including
  • XL2 Audio and Acoustic Analyzer
  • M4260 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

Get more Information

Buy at the web shop or contact NTI Americas .
Reverb
Time RT60

Contact NTi Audio

NTi Audio Contact Americas  
Brian MacMillan
+1 503 684 7050
Email

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