 How many times must I measure? Correlation and Uncertainty

RT60 is calculated using a linear leastsquares 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.
 Where should I place the measurement microphone? Critical Distance D_{c}

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 [m^{3}]
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 m^{3}
c = 342 m/s (the speed of sound @ 20℃)
T = 2 seconds
Critical Distance D_{c} = 2 *√ (450 / (342 * 2)) = 1.6 meters
 Shall I use 1/3rd or 1/1 octave frequency resolution?

The XL2 Acoustic Analyzer measures RT60 with 1/1 octave resolution, or, with the addition of the Extended Acoustic Pack Option, with 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 4050 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)
T20 and T30 are measurement methods used to derive RT60 values. Choosing either the T20 or the T30 method on a measurement device will give an RT60 result. i.e. the time it takes for the reverberation to decay by 60 dB.
T20 and T30 do NOT, in themselves, represent actual measured values. They are not, as the labels may incorrectly imply, the time taken to decay by 20 and 30 dB respectively.
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.
 How do I get a single RT60 result?

A single RT60 result may be calculated by averaging measured values from a selection of frequency bands.
Whenever you state a single RT60 result, do not forget to report on the frequencies that were averaged, otherwise the result is meaningless.
For example, a single figure reverberation time may be calculated from a 1/1 octave measurement by averaging the times in the 500 Hz and 1 000 Hz octave bands.
63 
0.90 
125 
0.87 
250 
0.76 
500 
0.67 
1000 
0.59 
2000 
0.56 
4000 
0.56 
8000 
0.51 
(0.67 + 0.59) / 2 = 0.63
This result may be represented thus: RT60_{[500Hz, 1000Hz]} = 0.63
Alternatively, for 1/3rd octave measurements, you may take averages over the six bands from 400 Hz to 1250 Hz.
50 
0.29 
63 
0.25 
80 
0.31 
100 
0.20 
125 
0.22 
160 
0.21 
200 
0.27 
250 
0.22 
315 
0.41 
400 
0.34 
500 
0.36 
630 
0.25 
800 
0.22 
1000 
0.23 
1250 
0.22 
1600 
0.22 
2000 
0.25 
2500 
0.21 
3150 
0.20 
4000 
0.22 
5000 
0.22 
6300 
0.21 
8000 
0.23 
10000 
0.22 
(0.34 + 0.36 + 0.25 + 0.22 + 0.23 + 0.22) / 6 = 0.27
This result may be represented thus: RT60_{[400Hz, 500 Hz, 630 Hz, 800 Hz, 1000 Hz, 1250Hz]} = 0.27
According to the ISO 33821 standard, either of the above two results may be labelled T_{30,mid} or T_{20,mid} depending on which dynamic measurement range was considered in obtaining the RT60 measurement result (20 dB or 30 dB respectively). Note that these labels do not indicate if 1/1 or 1/3rd octave frequency resolution bands were used during the measurement. Therefore, for compliance in reporting, and in order to be able to reproduce the same testing conditions at a later date, it is recommended that the results should always state the full range of values in every measured frequency in the form of a table (as above) or a graph in your report.
 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