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 Berlin, 2014-04-27
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 New York, 2013-10-18, 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".
The meeting understood that a report from SMPTE is expected soon.
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 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.
There was a substantial discussion of the content of a pink-noise signal suitable for these calibration purposes.
A pseudo-random sequence of about 10s seemed reasonable. Shorter sequences will be audibly repetitive. Longer sequences prevent predictably-rapid settling of the meter.
A crest factor of 12 dB was felt to relate closely to practical conditions and should also be straightforward to implement.
G. Hill pointed out that for a given sequence length there are multiple MLS sequences. These sequences may sound different, but will have the same spectral properties over the overall length. However, if the analysis does not consider the sequence length, then the inherent predictability of MLS cannot be guaranteed.
This led to an extensive discussion on the desired sequence length. It was recognized that a longer sequence is beneficial with regards to the audibility of the repetition rate and the frequency resolution. At the same time, it might show that sub-sets of (long) pseudo-random number (PRN) sequences will exhibit a variation of the spectral content and thus result in poor repeatability. Thus, with regards to repeatability, it is beneficial to use a sequence length which is short with regards to the integration time of the applied meter.
J. Woodgate argued for a near-Gaussian probability density function to make the audible content predictable.
It was proposed that a bandwidth of 20 - 20kHz is good for a full-range system. Lower frequencies could be reserved separately for subwoofer measurements. (Infrasonic content in main-channel pink noise may prejudice performance measurements of the full-range systems).
The chairman proposed: repeated iterations of a pseudo-random sequence, sequence length to be further defined, near-Gaussian probability density function, pinked, 20 - 20kHz band limited, 12dB crest factor, will give a good result. Could be generated by an MLS, but other PRN certainly need to be investigated.
With regards to the sequence length it has to be noted that a 10 s sequence seems acceptable with regards to audibility of the repetition rate. However, it has to be investigated what the influence of this sequence length is on the stability of the reading over time on a SLM or RTA at position "slow" (running integration time of 1 s), particularly at low frequencies.
The meeting understood that the SMPTE group had identified a pink-noise signal. The chairman would like to see the proposed SMPTE test signal to confirm stability of the reading over time.
Propose to generate a liaison letter to SMPTE following this meeting.
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."
The meeting was urged to comment on the Nordtest document, "NT ACOU 108" in preparation for developing this project.
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 137th Convention in Los Angeles, 2014-10
