SC-04-08 meeting report, New York, 2013-10

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., US., 2013-10-18

The meeting was convened by chair B. Kok.

The chair welcomed those present at this meeting of SC-04-08, both physically and remotely using the Internet. The meeting included representatives from relevant SMPTE engineering groups.

The agenda and the report of the previous meeting, held in Rome 2013-05-06, were accepted as written.

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. Vessa presented on SMPTE activity and cinema context. This included recording requirements, the influence of screens and acoustics, current SMPTE standards, X-curve, SPL settings, variations in EQ, etc. The main goal is to achieve consistent results, taking into account ear/brain compensations and headroom requirements.

From the SMPTE perspective it is essentially a 3-step plan:

  1. Optimize current methods;
  2. Move forward, using contempary measurement techniques and insights;
  3. Anticipate, or initiate, future developments

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 SMPTE ST 202:2010 and RP 200-2012 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.

P. Soper discussed the SMPTE need for a pink noise test signal. Already in wide use to support existing measurement methods, but poorly specified so that variations in different pink-noise sources were a source of error in practical measurements. Future measurement methods are not constrained to use pink noise, but for optimization of current methods it seems essential to have a well-defined signal.

A replacement was proposed by using a cyclic signal based on a repeated pseudo-random noise sequence containing all frequencies, for example MLS with pink shaping. This will be easier to specify as a more stable and reproducible source. If the sequence length is chosen to be similar to the settling time of the analyser (typical 1 s for setting 'slow'), this would ensure a stable reading as temporal irregularities in the measured response will repeat with the sequence length. This would substitute for less-well-defined PN generators in existing measurements using RTAs.

An intensive discussion was on the required length of the repeated sequence, aspects to consider are:

• Frequency resolution (typical 1/(sequence length), thus 1 Hz for a 1 s length) The bandwidth of the 20 Hz 1/3-octave band is app 4,5 Hz is 4.5 point in this octave band sufficient for a good measurement?
• Longer sequences offer better frequency resolution, but that would require the use of Leq measurements, it is not clear of the equipment used for the current methods support this.
• The (sequence length)/(frequency resolution) is mainly a concern at lower frequencies and when narrower band analysis than 1/3-octave is used; maybe it is an option to define a dedicated low-frequency test signal with a longer sequence, to be used only during tuning the system and not for regular maintenance.
• There was much discussion about an acceptable settling time. 10 s felt to be high; 30 s very difficult. 3 to 4 s would be acceptable to a technician for quick checks in a cinema.
• The repetitive character might be (subjectively) disturbing for some technicians, as they have always learned that pink noise should not be repetitive.

Some further research will be needed to evaluate this concept.

J. Woodgate preferred a pseudo-random generator using a shift register as a simpler implementation. A practical pinking filter will need a sensible specification. Needs to produce a fairly rapid settling time - it isn't practical to spend minutes assembling a spectrum.

The test signal needs some bandwidth limits to avoid effects caused by out-of-band components. Proposed is to develop a tolerance window with minimum and maximum level per frequency, similar to frequency response tolerances for microphones, filters, etc.; this then also defines the allowable ripple in the pass-band.

Amplitude stability - the rms level of pink noise should be referenced to -20 dB FS

Vessa would like versions of pink noise at 48 kHz and 96 kHz sampling frequencies.

The strawman crest-factor specification was constrained between 11.5 and 12 dB. No real significance to this number - just seems to be close to current generators. Soper was aware that crest factor will be modified by downstream elements (amplifiers, transformers, filters), but feels this should not change the PN spec - the distortion is all part of the B-chain. Others felt that a lower crest factor would be better. The group decided to consider the 12dB to be a maximum CF and not the upper limit of a 0.5 dB window.

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."

E. Brixen's Nordic document (NT ACOU 108, available for download from the group's document site) is interesting as a starting point for this project.

Formed a pilot group to create a list of topics:

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 136th Convention in Berlin, Germany, 2014-04.

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