Ever since I was doing my PhD I had a sort of quest to measure the thresholds of modal decay for low frequency room modes. During that time, the guys at Genelec Oy were doing a lot of work on digital equalisation of room/loudspeaker responses. They realised that the cost of digital filters would be greatly reduced if the offending modes could be reduced by a given amount rather than total equalisation. I managed to measure modal thresholds but due to the experimental methodology they were defined in terms of Q-factor rather than decay times for specific frequencies. This was later published in the Journal of the Audio Eng. Society in 2007.
Andrew Goldberg, who was with Genelec at that time was also keen to measure the thresholds and, as is often the case in research, was getting similar results to mine but through different routes and methods. We published some papers at the reproduced sound conference and I clearly remember Andrew whispering to me as we went through the typical rigmarole of swapping speakers between presentations: ‘This is all so close!”.
A few years later, Matthew Stephenson started his PhD with me at the University of Huddersfield and one of the first avenues of research we discussed was the measurement of these thresholds. During Matt’s PhD we approached Andrew and set out to finally measure the thresholds.
The work finally got published in the Journal of the Acoustical Society of America this year (March 2015). The thresholds are defined for critical listening modes, i.e. measured with tones and in isolation of masking, and for more general music listening, using music samples auralised within virtual room responses which include effects of temporal and frequency masking. The measured thresholds were then used to define a map which indicates time-frequency regions where modal decay can be detected by listeners.
The thresholds published allow a more precise target for modal control techniques; passive absorption treatment or loudspeaker equalisation filters that attempt to ameliorate the problems of modes in the room. Depending on budget and user requirements, the different thresholds can be chosen. For rooms where audio programme is listened to in a distracted mode (see Truax’s definition of listening modes), the music thresholds can be used to ensure no odd artefacts from the room modes are perceived. For critical listening spaces such as recording, mastering and post-production studios, the thresholds measured with artificial stimuli are more appropriate but will also require more expensive treatment. Finally, for those who want to completely eradicate low frequency room effects from their listening experience, thresholds that lie at the 2.5% confidence interval of the measured data will ensure that no modal effect will ever disturb it.
A link to the publication: