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Understanding small room reverberation time measurements
by ct_acoustic_frontierJuly 06, 2022
This article explains how to interpret T60 and other variants of mid and high frequency decay / reverberation time measurements in the context of small rooms like home theaters, listening rooms and recording studios. Note that a lot of things written about T60, both online and in text books, are written in respect to acoustically large rooms such as concert halls.
Single figure T60 measurements cannot do much more in a small room than tell you whether a room is overly live or overly dead. More useful is to look at how sound decays across the critical midrange frequency bands above the transition frequency from 250Hz to 4kHz and to examine whether the speed of this decay is consistent over time.
What are the targets for decay time?
There are three different problems that can exist with decay times – they can be too long, too short and/or uneven across the frequency spectrum.
Our targets, taken from our Acoustic Measurement Standards white paper, are:
Time taken for sound to decay 60dB (T60) should be between 0.2s and 0.5s from 250Hz to 4kHz.
T20 and T30 should be within +/- 25% from 250Hz to 4kHz when using one third octave smoothed bands.
[caption id="attachment_3363" align="aligncenter" width="672"] Allowable range for T20 and T30 of +/-25%. Measurement from Room EQ Wizard.[/caption]
Note that the standards in our white paper are based on other, well accepted standards such as those proposed by the EBU (European Broadcast Union - pdf here) and AES (Audio Engineering Society - pdf here).
How does decay time influence sound quality?
We are looking for decay times that are like Goldilocks oatmeal preferences - 'just right'. Decay times that either too long, too short or are notably different to the decay times in other frequency bands all cause audible sound quality degradations:
Too long - overly long decay times, even when obvious echoes cannot be detected, results in a loss of low level detail in much the same way as a room with a high noise floor. This is because low level sounds within the music are obscured by the slow decay of other louder sounds. This phenomenon is also known as masking and can be employed for good in noise nuisance reduction systems. Slow decay times consequently reduce the ability of a system to reproduce critical acoustic cues such as the decay character of a recording venue or the precise location of a musician within the soundstage. A room with long decay times also tends to sound harsh and brittle and can be an unpleasant place to listen resulting in rapid fatigue. Overly long decay times are considered those over 0.5s.
[caption id="attachment_3368" align="aligncenter" width="600"] This room is too live, with a T60 of 0.9s. Vertical scale is 0 to 2 seconds.[/caption]
Too short - short decay times can result in a lack of spaciousness and envelopment. Overly short decay times are considered those under 0.2s. Like most things in acoustics, however, things are not simple. Two rooms with identical single figure reverberation times can sound quite different depending on where the absorption is located. For example our showroom home theater has very little mid/high frequency absorption on the lateral surfaces but a lot on the ceiling and floor. That room will sound different to a room with a lot of absorption on the side walls and not much on the ceiling or floor.
[caption id="attachment_3367" align="aligncenter" width="600"] Excessively low reverberation time (around 0.15s) - vertical scale 0 to 1 second.[/caption]
Vary significantly across frequency bands - a room that exhibits uneven decay characteristics, where the sound decays much faster at some frequencies than others can at worst sound noticeably unbalanced with a ‘dull’ treble or ‘bloated’ bass. Uneven decay is most often caused by furnishings within the room such as thin drapes and carpets that absorb significantly more energy at treble frequencies (above around 1kHz) than they do at midrange frequencies. Our target here is for T20 and T30 to be within +/-25% across the frequency range from 250Hz to 4kHz.
[caption id="attachment_3366" align="aligncenter" width="600"] Unbalanced reverberation time - vertical scale 0 to 1 second.[/caption]
Much confusion still exists about what a room correction product does, what problems it can (and cannot) solve and therefore its 'place' in a modern high quality sound reproduction system. Part of the challenge of understanding room correction is that it requires a reasonable level of understanding of sound quality, acoustic science, acoustic measurement and psychoacoustics (how humans perceive sound). The majority of the articles I have read online or in print magazines do not cover the fundamentals in enough depth to allow the curious and committed reader a chance to understand room correction on anything more than a cursory level. By the end of this article I hope that you will have learnt enough to judge for yourself what room correction can and cannot do and how best to apply it in the context of a world class music or home theater system.