T60 Slope Ratio: Symbolically – T60SR6.
A proposed standard for condensing six octaves (63 Hz – 2 kHz) of reverberant decay data into a singular-quotient, qualitative score for medium and large performance, worship and entertainment facilities. Specifically, a defining metric for scoring and grading the ratio between the longest and shortest of the six T60 values, measured or predicted, and applied to fully-enclosed venues employing sound reinforcement systems.
Even before Sabine, and the subsequent developments in the field of architectural acoustics, it was believed a concert hall’s ability to support and sustain low-frequency reverberation was desirable, yet at times difficult to achieve. In addition, low- and very low-frequency (VLF) sounds from acoustic instruments and voices are weak and difficult to propagate into a large room.
In the context of this backdrop much of the existing body of concert hall design literature focuses almost exclusively on classical music, and the need to properly support acoustic instruments and voices. Yet, today there are many more performances of popular music, including rock and pop. Classical music, opera and ballet are no longer the mass-market entertainment draw they once were.
Fast forward to the 21st-century. Like it or not, live music supported by powerful, full-bandwidth sound reinforcement systems sells tickets. The quantitative expectations of today’s audiences are higher than ever, and easily met with the right equipment. However, one key reason the qualitative expectations aren’t easily accommodated is poor room acoustics; specifically, excessive low- and very low-frequency reverberation.
In place of traditional concert halls, we now have multi-purpose performing arts theaters, strip mall and warehouse sanctuaries, casino showrooms, sports arenas, and gymnatoria. Introduce thousands, if not tens of thousands of watts of subwoofer amplification, and you can easily find yourself in a thunderous cacophony.
If we’re lucky, such venues have variable acoustics or some form of installed, mid- and high-frequency sound absorption. The obvious examples are carpet, padded chairs or pews, fiberglass wall or ceiling panels, and maybe a little architectural shape for diffusion and/or modal complexity. Still, it’s rare that any attention is paid, or treatment specifically assigned, to absorb reverberant energy below 125 Hz.
Consequently, the problem is often compounded because mid-high absorption, whether it comes in the form of soft finish materials and/or people, causes a dramatic imbalance in the reverberant character of a room. It’s not uncommon to encounter an “acoustically treated” facility that has a T60 that’s two or three times longer at 63 Hz than at 2 kHz.
To further complicate things, ISO 11654, which defines absorption classes, actually does not reference the 125Hz octave band. This is possibly part of the original problem, as many manufacturers haven’t felt the need to test for or show absorption coefficients at or below this band.
In a recent study, Margriet Lautenbach et al., of Peutz BV concluded, “At least as important as the actual reverberation time is a flat reverberation spectrum, from at least 63 up to 4,000 Hz. Special attention has to be paid to absorbing efficiency at low frequencies: the 63 and 125 Hz octave bands. If the reverberation level at low frequencies is high, a soup of sound ‘swallows’ all higher frequencies and kills the definition. In order to achieve a clear and perceivable bass rhythm, in balance with the mid- and higher-frequencies, the reverberation time in the 63 and 125 Hz may not exceed the average RT by more than 10%.” 
While the proposed T60 Slope Ratio thesis was developed independently, and without any prior knowledge of the works of Peutz, Niels W. Adelman-Larsen and others, the T60SR6 guidelines correlate nicely with their findings.