- How many times must I measure? Correlation and Uncertainty
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).
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?
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
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