Acoustic Forensic Analysis: RT60 and the ETC
Once you move beyond loudspeaker tuning and start analysing the room itself, you enter the more forensic side of acoustics. In Smaart v9 Suite, this work happens in Impulse Response mode. Two measurements dominate this domain: the Energy-Time Curve (ETC) and RT60 (Reverberation Time). Together, they describe how a room behaves once sound leaves the loudspeaker and fate takes over.
1. The ETC: Identifying the Enemy
The Energy-Time Curve is a top-down view of sound energy over time. It clearly separates the direct sound from early reflections and, eventually, the diffuse reverberant field.
For the system engineer, the ETC is a diagnostic weapon. A discrete reflection arriving within roughly 20–30 ms of the direct sound will interfere with it, producing tonal coloration through comb filtering. Reflections arriving later — typically beyond 50 ms — are perceived as discrete echoes, the classic “slap-back” effect.
Smaart allows you to place a cursor directly on any reflection spike in the ETC and read its delay as distance. In practice, this means you can identify the offending surface — rear wall, balcony front, side wall — without wandering the venue like a lost tourist with a tape measure.
2. RT60: The Room’s Decay
RT60 is defined as the time it takes for sound energy to decay by 60 dB. In Smaart, this is derived from the impulse response using Schroeder integration, which converts the impulse into a cumulative decay curve.
T20 and T30
In real rooms (with HVAC noise, traffic, or the occasional dropped fork), measuring a full 60 dB decay is often unrealistic. Smaart therefore calculates RT60 from the first 20 dB (T20) or 30 dB (T30) of decay and extrapolates the remainder. This is standard practice and generally more reliable in non-laboratory conditions.
Frequency dependency
A room does not have one RT60; it has many. Low frequencies almost always decay more slowly than high frequencies. Smaart displays this clearly using frequency-dependent decay views, allowing you to see exactly which bands are overstaying their welcome.
3. Practical Application: Choosing Speaker Coverage
Measuring RT60 before committing to loudspeaker deployment tells you how aggressive you can be with acoustic energy. If the room shows a long decay in the low-mid region (around 250 Hz), flooding it with wide-dispersion coverage is a reliable way to produce a warm, indistinct fog of sound.
In such cases, tighter pattern control — often via a directional line array — is not a luxury but a necessity. The goal is simple: put energy on the audience and keep it off reflective boundaries.
The professional insight
Use the spectrograph view in Impulse Response mode. If you see bright horizontal streaks that persist at specific frequencies, you are looking at room modes. Equalisation will not fix this. Not now, not later, not ever. The only solutions are directivity control or acoustic treatment — preferably both.
Smaart is excellent on its own, but most users find they get much more out of it after some structured training. That’s where our seminars come in. At TZ Audio we run practical seminars, both online and in-venue. We offer seminar-only or full “all you need packages” including software & hardware. It’s simply the fastest way to become comfortable and confident with the measuring a sound system.
If you’re in Norway, Sweden, Denmark or Iceland – or elsewhere – we offer is online seminars and traveling to Norway is a valid option too of course. We’re here if you have any questions about the software or upcoming seminars.
Thanks for reading!