Considering The T60 Slope Ratio

T60SR6 Scoring & Grading

Much like the subjective MOS (mean opinion score) scoring that is often applied to Alcon or STI grading scales for speech intelligibility, the proposed T60SR6 scoring and grading scale is defined by six numeric scoring tiers, and five grading adjectives, which most people can understand.

Until more exhaustive testing is done to update this table, these values are proposed to describe the measured or predicted SR scoring.

Critics may say the goals are too aggressive. The author suggests they are not. Consider outdoor concerts and festivals. When outdoor rock, pop, blues, rap and country events operate in free field conditions, no one seems to have much problem with the flat, near zero T60s. In many cases, these are the best sound conditions.

Why not strive for an ideal blend between outdoor, free field acoustics and tight indoor acoustics?

VLF Acoustic Treatment

Retrofitting VLF absorption treatment can be quite difficult. It is usually more cost effective if such treatment is factored into a structure during the initial design and construction phases.

Treatment techniques such as perforated panels with deep mineral wool filled cavities, limp mass diaphragmatic absorbers, and Helmholtz resonators can be built into the architectural plan, but these will almost always be custom solutions.

Commercially-available VLF treatment methods and materials are not easily found at this time. The folks at Flex Acoustics [5] in Denmark are perhaps the leaders in today’s market, with no other companies (known to the author) engaging in mid- and large-venue solutions.

There appears to be at least three reasons for the scarcity of VLF acoustic products. First is the lack of testing criteria and standards for materials below 125 Hz. Second – there are very few testing facilities large enough to do T60 testing in the 63 Hz range and below. Third – until the late 20th century there was little need or demand for such products.

This author suggests those hurdles are no longer acceptable reasons to ignore the obvious problems that exist in venues with fair, poor or bad slope ratios.

Electro-Acoustic Enhancement

It is well known and understood there are no electronic systems capable of removing reverberation. It can only be added. Modern electro-acoustical systems such as Meyer’s Constellation, the Lexicon LARES, Yamaha’s AFC, Muller-BBM’s Vivace and others, rely on implementation in a building with a very well-controlled acoustic signature.

A mid-band T60 of one second is generally considered a good starting point. When this technology is properly integrated, adding digital reverberation at various frequencies, and in varying amounts, becomes relatively easy.

However, targeting a mid-band T60 of one second does not reveal the whole story. This thesis suggests that the T60SR6 score must be evaluated, and if necessary, the room be properly treated to achieve an “optimal” grade, or as close as possible. If any other T60s are much longer than the mid-band target, an electro-acoustic system can do nothing to compensate.

Summary

In his paper, Reverberation and the Art of Architectural Acoustics, [6] Robert Sekuler notes that the father of architectural acoustics, Wallace Clement Sabine, had this to say about concert hall acoustics. “Sabine realized that the total acoustic disturbance produced by any ensemble’s instruments must be intense enough to overcome the concert hall’s tendency to absorb sound. Otherwise, the hall will “soak up” much of the available sound, leaving little for the audience to hear, and causing the audience to complain that the music lacks “body.”

Sabine’s comment was made more than one hundred years ago. Musical styles, venues, technology, and consumer tastes and expectations have advanced far beyond anything relevant to Sabine’s concern.

Sabine’s thinking may still be valid in some limited number of settings, but the exact opposite is often the case in many venues today. It is offered here that reverberation decay times, in the six octaves of sound outlined above, are fundamentally critical when evaluating a room that has or may require a high-power, extended-range sound reinforcement system.

Also, consider this. In some situations, a venue with a good reverberation profile may now be viewed to be just as limiting as one with too much or too little reverb. Ideal, classical music T60s only compliment a narrow range of 21st century music, entertainment and production. Like it or not, modern economics requires that venues be built to support the widest possible range of offerings.

The author believes the best way to make a building’s acoustics adequately flexible for the widest range of events and activities, is to start with a reasonably short T60, blend in a optimal Slope Ratio, then install an electro-acoustic enhancement system.

This thesis is offered to help all concerned parties advance and define a room’s reverberation goals, and provide a simple numeric scoring scale and grading vocabulary from which acoustical design specifications can be initiated and evaluated.

Acknowledgements

A special thanks goes out to the following people for the comments and suggestions they provided during the development of this thesis:

Steven Liddle – Springwell Audio AB
Bernd Noack – proAV Consulting
Ron Sauro – NWAA Labs
Pat Brown – SynAudCon
Tony Hoover – McKay Conant Hoover
John Storyk – Walters-Storyk Design Group

References
[1] Adelman-Larsen NW, Thompson ER, Gade AC. Suitable Reverberation Times for Halls for Rock and Pop Music. J Acoust Soc Am. 2010 Jan;127(1):247-55. Updated J Acoust Soc Am. 2012 Nov;132(5):3586.
[2] Margriet Lautenbach of Peutz BV, Zoetermeer, the Netherlands and Maaren Luykx of Peutz BV, Mook, the Netherlands. Room Acoustic Aspects of Some Recently Opened Pop Venues.
[3] Niels W. Adelman-Larsen. Concert Hall Acoustics: Paper ICA2016-95 Acoustics for amplified music.
[4] Bernd Noack. http://www.av-info.eu/Bass Ratio
[5] http://flexac.com/en/
[6] Robert Sekuler. Reverberation and the Art of Architectural Acoustics 2https://webct.brandeis.edu/NPSY_12A/senpro2002/topic_6_stuff/sabine.html
[7] https://ledgernote.com/columns/mixing-mastering/fletcher-munson-curve/
[8] https://amcoustics.com/tools/amroc?l=72&w=35&h=20&ft=true&fo=120&fu=40&r60=1.2
[9] http://www.linkwitzlab.com/rooms.htm
[10] Bonello, Oscar J.: “A New Criterion for the Distribution of Normal Room Modes” – AES-Journal(USA) 29 (1981). Bonello, Oscar J.: “Acoustical evaluation and control of normal room modes” – JASA 66 (1979)

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