Report of the meeting of the SC-04-08 Working Group on Measurement and Equalization of sound systems in rooms, of the SC-04 Subcommittee on Acoustics, held in New York, NY, 2019-10-17
The meeting was convened by chair Peter Mapp.
In attendance were P Mapp, R Cabot, B Olson, D Murphy, E Brixen, C Hughes, S Moulin, G Grazioli, P Martignion, M Ramakrishnan, D Weinberg, K Graffy, N Shaw, D Blore, J Britto*, R Kessler*, B Long* (* denotes electronic link).
The formal notice on patent policy was read.
The chair, Peter Mapp [PM] introduced himself as did the other attendees
Minutes of previous meeting in New York, 2018-10-19 were approved.
AES-X218: Measurement and calibration of sound systems in rooms.
PM summarised the 3 main points of the minutes
(1) Max SPL – it was agreed that Steve Hutt would lead a working group looking into how to characterise/measure max SPL. Subsequently this task was transferred to SC04-03-A and was currently concentrating on ‘M’ noise. Whether this would be the best approach to characterise the maximum SPL capability of an installed sound system remains to be seen. (PM has concerns over the signal dynamics which crest factor alone does not suitably deal with). This topic was therefore on hold until SC04-03-A reached consensus and reported back.
(2) Framework Document – It was agreed at the last meeting in NY, that the document (not clear yet whether this will be an information document or a standard) in order to progress be limited to initially cover purely room acoustic aspects i.e. the measurement of Reverberation Time and Background Noise Level, together with information on associated measurement microphones and reporting of the data. PM had offered to produce the initial text for RT and BGNL measurement. Unfortunately, due to a combination of circumstances, whilst a lot of background research had been carried out, to date it had not been possible to write the text. PM now planning this for early 2020.
(3) Input from other committee members – this had been singularly lacking but hopefully when the initial text was provided, this would move things on.
PM noted that he wanted to produce a stand-alone document that would not require constant referral to other documents or standards. Whilst there were a number of useful documents already available, none dealt specifically with measuring sound systems in rooms or the associated acoustic measurements required to either design or to document a system’s performance or acoustic environment. This was agreed.
Discussion took place concerning whether the sound system should be used to excite the room or space for RT measurements, or whether traditional methods such as using a Dodec speaker or balloon pops etc. should be used. The feeling was that the sound system should be used as this would (a) better excite the space and (b) would actually be the source exciting the space under operational conditions. PM noted that his research and experience suggested that in normal spaces, e.g. meeting rooms and auditoria up to 2000 seats, either method produced similar result. It was noted however, that guidance would need to be added when dealing with larger / irregular spaces such as arenas, stadiums, malls and airports etc. (It was later proposed that these spaces might require their own dedicated sections, although the general principles set out in the main text should cover many of the issues).
PM pointed out, that when it came to noise level measurements, there was a difference between European and American standards. In Europe, a free field microphone is used and the microphone is nominally pointed towards the source, the ANSI standard however, required a pressure microphone to be used and not pointed towards the source. As this is to be an international document, this issue needs to be addressed. In practice, in the type of rooms / spaces envisaged, PM felt that up to approximately 6-8 kHz there would probably little difference when using a ½ inch microphone but some further research was required. One way around the problem is to use a ¼ inch measurement microphone – but the vast majority of sound level meters employ ½ inch mics. Neil Shaw pointed out that there was a useful annex on microphone performance/characteristics in the SMPTE cinema system calibration documents. PM to review during his initial text writing.
This lead to a discussion of using SMPTE documents. It was agreed that they may contain useful information but cinema systems were very different to PA and sound reinforcement systems. The progress of the InfoComm standard on Spectral Balance was also raised. David Weinberg (DW) volunteered to find out, as he was a member of both committees.
It was agreed that then AES document whilst not aiming to re-invent the wheel and tackling the same ground as other documents, needed to be a complete document in its own right and it was precisely because these other documents did not cover the required aspects that the AES was producing its own document. It was again re-iterated that the AES document would be limited (at least initially) to the size of rooms ranging from large meeting rooms to Auditoriums with a capacity of up to approximately 2000. Stadiums were specially excluded due to their unique (abnormal) acoustic properties. Equally, in such large spaces and exciting the space with the sound system effectively becomes essential. It was agreed that in such cases it became essential to look at the decay slope when determining the RT and that most automatic algorithms, as found in typical sound level meters, would produce an erroneous result. It was noted that this needs to be highlighted in the document.
Discussion then took place as to how prescriptive the document should be and who it was aimed at, noting that it was not to be a tutorial. However, some guidance might be needed on the interpretation of the results. It is to be assumed that the reader has some basic knowledge of acoustics, sound systems and measurements.
It was agreed that a decision tree was required to assist the user of the document as to what measurements needed to be carried out, how they are to be made and if any follow up measurements / diagnostics need to be undertaken. Bruce Olson (BO) volunteered to produce this as he felt that had a clear view and opinion as to what was required.
It was agreed that additional parts (chapters/annexes) could be produced to deal with special cases (1) Stadiums, (2) Airports / Malls and (3) Cinemas, as required. It was also noted that recording studios and control rooms were specifically excluded from the applicable spaces, but again the general principles set out in the document could probably be applied.
Discussion also took place as to whether the document should cover Emergency Communication, Evacuation and Voice Alarm systems. It was agreed that it would not specifically do this, as requirements varied considerably around the world but again many of the measurement procedures would be applicable. This raised the question as to whether STI / STIPA should be included. PM concluded that this would be a major piece of work to undertake and to an extent also depended on local codes and standards – which the AES could not address. It was therefore agreed, to leave this for the foreseeable future and concentrate on the agreed areas of Background Noise Level and RT measurement and reporting.
Actions
PM to produce initial framework document and placeholder text
SH to report back on Max SPL as and when SC04-03A reached a useful conclusion.
BO to produce decision tree as early as possible as this might affect draft text/framework
DW to liaise with InfoComm as to the status of their spectral balance document & report back
AES-X219: Method of measurement for frequency and impulse response of sound systems in auditoria
Project remains postponed, as per the 2015 NY meeting, until there is some output of X218
Liaisons
Nothing to report.
New Projects
No new projects were proposed.
New Business
No new business was proposed.
The next meeting will be scheduled in conjunction with the AES 146th Convention, to be held March 2020 in Vienna, Austria.
