RT60 Quality Indicators Reduce Measurement Uncertainty

Quality indicators and ratings (Poor, Fair, Good) for Room Acoustic measurements are adapted from measurement procedures described in ISO 3382-1(2009), ISO 3382-2(2008) and application engineering recommendations for the SoundAdvisor™ Sound Level Meter Model 831C. They are designed to help determine the quality of a given measurement and guide measurement improvements.

Bandwidth-Time Product (BT)

BT is the product of the 1/1 or 1/3 octave filter bandwidth and the T20 or T30 decay time for that frequency band. Good verifies that the measured decay is acoustic and not of the instrumentation. A poor rating may indicate the reverberation time was influenced by the decay time of the bandpass filter (ISO 3382-2:2008, 7.3, equation 4).

Poor	BT ≤ 16
Fair	N/A
Good	BT > 16

Background Noise Level (BK)

BK is the difference between the signal used to energize the room and the background noise. (ISO 3382-2:2008, Sections 5.2.1 and 5.3.2). It offers an indication of how sufficiently the room was energized above the background noise level.

Poor	T20 Measurement: BK < 35 dB T30 Measurement: BK < 45 dB
Fair	N/A
Good	T20 Measurement: BK ≥ 35 dB T30 Measurement: BK ≥ 45 dB

Non-Linearity (NL)

NL is the measurement of how well the decay follows a straight line. Non-linearity is reported as per mille ‰ (parts per thousand) deviation from zero or perfect linearity for T20 and T30 (ISO 3382-2:2008, Annex B.2).

Non-linear results are not always an indication of a poor measurement, but may be the result of reverberations off of acoustic ceiling tiles, coupled spaces, etc. Follow best practices recommended in your chosen standard. Ensemble averages typically reduce non-linear responses from abnormal causes.

Decay with non-linearity – further examination is required 

Poor	> 10% deviation between the Decay Curve and the Regression Line
Fair	5% < deviation ≤ 10%
Good	≤ 5%deviation

Curvature (Cu)

Cu is the ratio of T30 to T20 decay times expressed as a percentage (ISO 3382-2:2008, Annex B.2). When the T20 and T30 decays times are the same, the Curvature is 0%.

Poor	Cu >10% The graph of this type of decay may show the T20 or T30 lines intercepting the x-axis (time domain) at very different times. Rather than indicating a poor measurement, it may instead be the result of reverberations off acoustic ceiling tiles, coupled spaces, etc. Follow best practices recommended in your chosen standard.
Fair	5% < Cu ≤ 10%
Good	0% ≤ Cu ≤ 5% A negative value may indicate a measurement error.

Standard Deviation (SD)

SD is an estimate of uncertainty due to the number of averages used to calculate T60. The standard deviation typically decreases as the number of decays and positions increases. The 831C reports SD/RT as a percentage. See ISO 3382-2:2008, section 7.2 for calculation details.

Poor	SD > 10%
Fair	5% < SD ≤ 10%
Good	SD ≤ 5%