What is decay time?
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 frquency 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 four different problems that can exist with decay times – they can be too long, too short, uneven across the frequency spectrum or vary excessively over time.
Our targets, taken from our white paper on acoustic measurement standards, 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 +/- 25% across the same frequency band using one third octave smoothed bands.
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.
- Too short - short decay times result in a lack of spaciousness and envelopment and lead to increased listening fatigue. Overly short decay times are considered those under 0.2s. Music does not come to life but rather sounds dry and sterile.
- 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 spectrum.
How can I find T60 using acoustic measurements?
An experienced acoustic consultant or sound engineer will be able to detect an overly live room by merely speaking or clapping their hands and listening to the decay of the sound field. Most of us, however, will need to use acoustic measurement equipment to get a sense of how live or dead the space is and identify frequency bands with overly long decay times or those that vary significantly across the audio spectrum.
How can I check if my room has decay time issues?
Use XTZ Room Analyzer to measure your room. T60 for 6 different frequency bands is shown. Watch the video below to see how easy it is!
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