Field Story
Standing Waves Haunt the Control Room
A TV studio monitoring room had an 80 Hz standing wave causing a 16 dB peak at the mix position and a 12 dB null 0.8 meters behind it. Engineers were making incorrect low-frequency mixing decisions. RTA and room mode analysis confirmed an axial mode between the front and rear walls. Bass traps in the rear corners and a slight EQ cut at 80 Hz flattened the response to within 4 dB.
TV StudioRT60 Measurement per ANSI S1.4
TL;DR
ANSI S1.4 specifies sound level meter performance, and its accuracy requirements directly affect RT60 measurement quality. When using the interrupted noise method, your meter detector characteristics, time weighting, and frequency weighting accuracy all influence the captured decay curve. A Class 2 meter introduces more uncertainty in the decay slope than a Class 1 instrument, which propagates into your RT60 calculation. SonaVyx implements digital signal processing that minimizes these analog-domain limitations, but understanding ANSI S1.4 tolerances helps you interpret the confidence interval of your RT60 results.
ANSI S1.4 and Decay Measurement
RT60 measurement via the interrupted noise method relies on the sound level meter accurately tracking a decaying sound field. The detector response, time constants, and frequency weighting accuracy specified in ANSI S1.4 all affect the quality of the captured decay curve.
Detector Characteristics
ANSI S1.4 specifies two primary time weightings relevant to RT60:
- Fast (125 ms time constant): preferred for decay recording, provides adequate temporal resolution
- Slow (1 s time constant): too sluggish for accurate decay tracking, not recommended for RT60
- The detector must accurately follow a 60 dB decay in the time domain
- Digital implementations can use true RMS with shorter integration windows
Dynamic Range Requirements
A valid T30 measurement requires at least 45 dB of usable dynamic range from your measurement system:
- Class 1 meters typically provide 80+ dB dynamic range
- Class 2 meters provide 60+ dB, sufficient for T20 but potentially limiting for T30
- Digital systems like SonaVyx can achieve 90+ dB with 24-bit audio input
- The noise floor of the microphone and preamp sets the practical lower limit
Frequency Weighting for RT60
RT60 is typically measured in octave or 1/3-octave bands using bandpass filtering rather than A/C/Z weighting. However, ANSI S1.4 weighting accuracy is relevant when measuring broadband RT60 with A-weighting, as sometimes required for simplified assessments.
Measurement Uncertainty
The ANSI S1.4 Class 2 tolerance of plus or minus 1.4 dB at 1 kHz translates to approximately plus or minus 5% uncertainty in RT60 at that frequency. Class 1 reduces this to approximately plus or minus 3%.
Common Mistakes
- Using Slow time weighting for decay recording
- Not accounting for meter dynamic range when interpreting T30 results
- Assuming digital meters have no accuracy limitations
- Applying A-weighting to octave-band RT60 measurements
SonaVyx Approach
The SonaVyx RT60 tool uses digital bandpass filtering with tight tolerances exceeding ANSI S1.4 Class 2. Verify your measurement setup with the SPL meter to confirm adequate dynamic range. Use the impulse response method for highest accuracy. Check the RTA analyzer for spectral context. Predict target values with AcousPlan. See our learning hub for measurement techniques.
Standard Reference
ANSI S1.4:
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Last updated: March 19, 2026