Report of the meeting of the SC-04-08 Working Group on sound systems in rooms of the SC-04 Subcommittee on Acoustics, held in Rome, Italy, 2013-05-06
The meeting was convened by chair B. Kok.
The chair welcomed those present at this first meeting of SC-04-08, both physically and remotely using the Internet. The meeting included representatives from relevant SMPTE engineering groups.
The agenda were accepted as written.
Because this was the first meeting of this new working group, there was no previous report.
Development Projects
AES-X215 Liaison with SMPTE project to codify current procedures to calibrate the Cinema B-Chain
Scope: "This project will liaise with SMPTE TC-25CSS in their project to 'Codify current procedures as used by skilled theater technicians to calibrate the Cinema B-Chain. The project will consider comments as appropriate for communication to SMPTE' ".
B. Olson pointed out that SC-04-08 is studying large rooms - cinema sound is a subset of this more general case.
M. Yonge presented a summary of SMPTE ST202, pointing out that ISO 2969 and ST202 are essentially compatible. ST202 has grown in size over the years, but still describes the same technique. ST202 uses measurement tools newly available in the mid-to-late 1970s - pink noise and audio real-time analysis. These offered a way to measure the amplitude response of a room quickly enough to be useful in cinemas where technical access is limited to short periods.
Implementation of ST202 has been successful where it been applied within its stated scope, however there has been some confusion in wider circles as people have tried to apply ST202 in situations that are not within its scope.
The X-curve in ST202 is not a roll-off, or frequency restriction, for the audio. Film sound sources in the cinema A-chain have been essentially flat since the introduction of noise reduction processing permitted flat soundtracks, and subsequently. The X-curve is intended to predict flat first-arrival sound in a normally-reverberant normal-sized cinema. The technique is entirely reliant on a "reverberation model".
Yonge sought to clarify the technique. Essentially, the measurement microphone in the cinema receives two audio components: direct sound from the loudspeakers, plus the reverberation within the room. For human listeners, the perceptual content of the audio is carried by the direct sound primarily. By estimating the reverberation contribution based on a normally-reverberant normal-sized cinema, the overall amplitude response of a room can be approximated when the amplitude response of the direct sound is flat. This overall amplitude response is the X-curve.
ST202 does not clearly express the limits of its applicability, although they are mentioned in the standard if you read it very carefully. These were once well understood by the technicians implementing it.
The X-curve predicts flat first-arrival sound, but only when: the loudspeaker & microphone are in a normally-reverberant room; the room is within a specific size range; the microphone is relatively far from the LS, in the reverberant field; the loudspeaker is fed with continuous pink noise, and not any sort of intermittent signal.
The X-curve is not useful for measuring: small rooms (it is hard to place a microphone sufficiently far into the reverberant field for the 'reverberation model' to be effective. This excludes most recording studio control rooms, for example); rooms with unusual acoustic treatment; very large auditoria; and stadia and open-air spaces.
And ST202 does not clearly provide a basis to compare measurements made using this technique with measurements made using alternative techniques.
B. Long agreed that a tutorial approach is necessary and intended.
AES-X216 Liaison with SMPTE project on Calibration Pink-Noise Standard and Test File
Scope: "This project will liaise with SMPTE TC-25CSS in their project to 'Create a pink-noise standard, and a reference pink-noise file available in a digital audio file and DCP formats that conform to the description of the pink noise in ST202:2010 and RP200-2012 and for which the time domain (mean, standard deviation, skew, kurtosis, etc.) and frequency domain (energy per bandwidth, bandwidth, etc.) characteristics are defined. Specify the algorithm used to generate the pink-noise file.' The project will consider comments as appropriate for communication to SMPTE."
Olson pointed out that AES doesn't want to introduce a different pink noise (PN) standard, but to help SMPTE to create a pink noise standard we can all use.
J. Woodgate observed that the pink-noise scope includes time-domain elements - statisticians and electronics engineers use similar terminology to mean different things. Amplitude probability distribution seems important. Woodgate did some work on the subject for BSI - plotting PN distribution on a log-log scale will produce a parabola that is simple to verify. He will distribute a copy of the paper to this group. T. Lago mentioned that there are alternatives to PN with deterministic response that does not need to be averaged.
P. Newell felt that a standard PN source to support repeatable level and response alignment was important.
D. Murphy observed that putting PN through an amplifier with non-linearities changed the peak content which invalidated the crest factor of the PN downstream. Kok agreed, observing that specifying a file for playback may be less useful that defining the characteristics of the PN at a relevant point in the reproduction chain.
P. Soper pointed out that PN has been the standard test signal for a considerable time. In this digital age, we have an opportunity to define a test signal with more rigour. Looking at current PN generators, amplitude spectrum is usually good, but there is no consistency in level or crest factor.
AES-X218 Measurement and calibration of sound systems in rooms
Scope: "to document relevant parameters affecting the audience experience of a sound system in a generalized room, and identify appropriate objective methods of measurements to quantify them. The intent is to identify tools and procedures for a technician to measure the performance of a loudspeaker system in a room, and then support accurate and reliable calibration of this system to a specified performance."
There was a wide-ranging discussion on various aspects of electro-acoustic performance. Kok and Olson felt that we need to start with basic objective measurements, then consider perceptual stuff - going the other way would mean we can never reach a conclusion!
Level calibration is essential but there are many ways to measure sound pressure level that give different results. Woodgate noted that the standard sound-level meter has been established for many years. It has a symmetrical time constant of 125 ms, which is oriented to human hearing, but this doesn't give the whole story regarding equipment limitations. The limiting factor for amplifiers is the programme peaks. This is also true of loudspeakers, although this is more complicated. Thinks this is worth study.
Woodgate felt that A-weighting, originally intended for comparatively low-level acoustic noise measurements is now a discredited measure for maximum output measurements.
The meeting agreed that SPL measurement alone does not provide an indication of quality - there may be other parameters such as distortion that also needs to be measured.
B. McCarty suggested assembling a set of sound system parameters that need to be measured - these may include measurements of non-traditional qualities such as intelligibility.
E. Brixen introduced a Nordic sound system measurement standard "Nordtest Method NT ACOU 108" that listed a set of parameters to be measured. This could be a starting point for further discussion.
Kok proposed that we set some objective measurements, that should include: level, intelligibility, maximum output level, self-noise, frequency response, etc. These measurements won't guarantee that your system will sound better, but will provide a basis for communication. It would be good to use measurement techniques that already exist rather than invent something new without good reason.
Regarding a measurement of intelligibility, P. Mapp suggested that the existing STI measurement, recently revised in IEC 60268-16, will need addition in these applications.
Harmonic distortion should be measured. There was some surprise that this wasn't routinely done already, however it was accepted that distortion was not generally measured for sound systems. The meeting felt that this should change. The issue of non-harmonic distortions, such as intermodulation distortion (smaller amplitude contribution, but greater audibility), was also mentioned.
AES-X219 Method of measurement for frequency and impulse response of sound systems in auditoria
Scope: "to specify a method of measurement for frequency and impulse response of sound systems in auditoria".
A scope for this project awaits refinement in project AES-X218.
Liaisons
No liaison matters arose.
New Projects
No new projects were proposed.
New Business
There was no new business.
The next meeting will be scheduled in conjunction with the AES 135th Convention in New York, NY., US., 2013-10.
