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. In a church, for example, the sound may still be able to 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 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.
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 measurements are known as T20 and T30 respectively. Because the decay is linear, we can then extrapolate the result as RT60 = 3 * T20 or RT60 = 2 * T30.
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.
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.
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.
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:
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.
The result page displays the averaged or the individual results of the acquired cycles, together with the related uncertainty and correlation data.
The XL2 automatically calculates two auxiliary results that provide a feedback on the precision of the acquired data.
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.
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
The ISO 3382 standards list a number of location types for which the reverberation time should be optimized, and recommends the following RT60 values:
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.
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.
(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.