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2.2 Analysis and reverberation time

The fundamental quantity used to characterise, define or gain information about the acoustic qualities of a particular space that can be obtained from an impulse response is reverberation time. Reverberation time (or RT60) is formally defined as the time it takes (in seconds) for a steady state signal to attenuate by 60 decibels once the sound source has stopped (Sabine 1922). Perceptually, RT60 is the formal quantity we associate with a space we might consider as being echoey or spacious. Such a space might have an RT60 value of 2 seconds or more. A space considered as being dry or dead sounding might have an RT60 value of less than 0.5 second. An anechoic space is specially designed to absorb all sound and so would ideally have an RT60 of zero seconds. Reverberation time varies with frequency, and is typically quoted in octave bands (the audio spectrum divided into 10 octaves between 20Hz and 20kHz, with an octave defined as a doubling in frequency). Figure 10 shows RT60 values, quoted for each octave band, for a range of spaces found on OpenAIR.

Figure 10
Figure 10: Reverberation time (RT60) measured in seconds across octave bands from 125Hz to 8000Hz, for four varied acoustic spaces available on OpenAIR: Hamilton Mausoleum, Koli National Park (Winter), York Minster, and the R1 Nuclear Reactor Hall

Other acoustic parameters can also be derived from an impulse response measurement, and are commonly used to provide additional detail in the acoustic characterisation of a particular space. These parameters form an important part of the modern architectural design process and have been documented in the relevant international standard (ISO3382-1 2009). They have also been adopted in studies relating to acoustic heritage as they are able to give insight into, for instance, the intelligibility of speech or music in a particular space. However, it is also possible to interrogate this digital data in other meaningful ways. Analysing the frequency content of such time-varying impulse response measurements helps to reveal how low frequency sound behaves, and whether there are specific resonances that might act to influence or colour how sound is perceived. If spatial impulse response measurements are available, as obtained from a Soundfield microphone, it is also possible to conduct a reflection analysis to detect from which directions, and hence from which walls, specific sound reflections come from. Again, such information can help to reveal how humans might interact with such sonic features or the acoustic consequences of how a space is changed architecturally over its lifetime. Figures 11–14 show a frequency and reflection analysis for the main chamber in Maes Howe passage tomb (Murphy 2005), a small but highly reflective and resonant space.

Figure 11
Figure 11: A series of analyses of one impulse response measured in Maes Howe passage tomb, Orkney, UK: time domain waveform plot of the actual impulse response
Figure 12
Figure 12: A series of analyses of one impulse response measured in Maes Howe passage tomb, Orkney, UK: frequency response obtained from time domain waveform plot in Figure 11 – note the large individual peaks below about 300Hz indicating a highly resonant space that would emphasise those particular frequencies of any sound heard within it
Figure 13
Figure 13: A series of analyses of one impulse response measured in Maes Howe passage tomb, Orkney, UK: reverberation time (RT60) obtained from time domain waveform plot in Figure 11, varying with octave band between 250Hz and 8000Hz, noting that the overall decay of sound is very short
Figure 14
Figure 14: A series of analyses of one impulse response measured in Maes Howe passage tomb, Orkney, UK: spectrogram obtained from time domain waveform plot in Figure 11 showing how the frequency content of the impulse response changes over time, with a quiver plot reflection analysis overlaid indicating from which direction parts of the measured impulse response are arriving at the microphone – the direct sound and first reflection are particularly evident in this example

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