Blog Categories

Understanding small room reverberation time measurements

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.

What is the T60?

If you need to understand what T60 is and how to measure it please read our introductory article ‘Room Acoustic Measurements 101‘.

 

How do you interpret T60 measurements?

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.

 

Allowable range for T20 and T30 of +/-25%. Measurement from Room EQ Wizard.
Allowable range for T20 and T30 of +/-25%. Measurement from Room EQ Wizard.

 

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.

This room is too live, with a T60 of 0.9s. Vertical scale is 0 to 2 seconds.
This room is too live, with a T60 of 0.9s. Vertical scale is 0 to 2 seconds.

 

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.

Very low T60 (around 0.15s).
Excessively low reverberation time (around 0.15s) – vertical scale 0 to 1 second.

 

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.

Unbalanced T60.
Unbalanced reverberation time – vertical scale 0 to 1 second.

2 thoughts on “Understanding small room reverberation time measurements”

  1. Hello,
    Is it significant to consider RT in a small room? Since RT describes the statistical decay of sound in a room, a question arouse me. Small rooms can be considered as “statistical”? Several aspects need to be considered, irregular response due to eigenmodes, lack of diffusion, small mean free path, etc.
    Sabine, fundamented RT stating that a diffuse space is the basis for its calculation. This means that the field should be homogeneous and isotropic, but in a small room where absorption takes a major role, there are little (or there aren’t) spots where we can consider presence of a diffuse field. However, a series of discrete reflections influence the field, generating strong directional characteristics due to early reflections and dissimilarities over time and space on the field (taking measurements in two different positions may yield extremely different responses!). So my question: Is RT a relevant parameter in small rooms? Does measuring it yield valuable information of the room response?
    Thanks!

    1. Hi Ignacio

      Personally I don’t see why you can’t use decay time measurements, although I don’t think that they are psychacoustically that relevant in determining sound quality in small rooms. They are just straight line fits to the energy time curve showing the time it takes for the SPL to drop from xx dB to xx dB relative to 0dBFS i.e. direct sound (where xx depends on whether the measurement is T20, T30, T60). Note the subtle difference in meaning between decay time and reverberation time. If you were being pedantic / “text book”, then you would not use the word reverberation time to describe anything in a small room. Reflections and resonances (room modes) are much, much more important to focus on from an acoustics perspective. There are many times when I have had to make an acoustical design choice to sub-optimize decay time in order to control reflections appropriately. Any acoustics practitioner who focuses on the reverberation time in a small room doesn’t really understand what is important and not important in a small room. You often see acoustics professionals from large room backgrounds using reverberation time measurements and not much else. That might be fine for a concert hall or auditorium, but definitely not for a home theater, home recording studio or critical listening room.

      Thoughts?

Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>