March 2003 meeting of SC-04-03

Report of the SC-04-03 Working Group on Loudspeaker Modeling and Measurement of the SC-04 Subcommittee on Acoustics meeting, held in conjunction with the AES 113th Convention in Amsterdam, Netherlands on 2003-03-23

The meeting was convened by Vice Chair N. Harris.

The agenda and the report of the previous meeting in Los Angeles were accepted as written.

Open Projects

AES-1id-R: Review of AES-1id-1991 (r2003) AES Information Document - Plane-Wave Tubes: Design and Practice
M. Dodd enquired about optimum placement of the microphone in plane wave tubes (PWT). J. Panzer suggested that near the wall avoids pressure nodes; a short technical discussion ensued.

D. Gunness and Dodd suggested that the PWT is now not commonly used for compression driver design - it is only good for low frequency measurements. The main area of concern for this type of driver is its high frequency performance. Gunness said that he tests drivers through a reference horn.

J. Woodgate reminded the committee that the Call For Comment (CFC) for reaffirmation period closed on 2003-03-14.

AES-5id-R: Review of AES-5id-1997 (r2003): AES information document for Room acoustics and sound-reinforcement systems -- Loudspeaker modeling and measurement -- Frequency and angular resolution for measuring, presenting and predicting loudspeaker polar data
No action was proposed or required

AES2-R: Revision of AES2-1984 (r2003): AES Recommended Practice -- Specification of loudspeaker components used in professional audio and sound reinforcement
This project is awaiting contributions. Woodgate gave a verbal report from Task Group SC-04-03-A. He noted that, for example, graph scales and some words were inconsistent with the IEC 60268-5 specification.

D. Clark recommended one mandatory sheet, and one optional sheet. The group recommended using a graph per IEC-60263 with 25dB/decade ratio for scale. Woodgate to clarify. The reference should point to the IEC document.

A number of questions need to be considered. 'Point of rotation' for polar measurements should be defined. 'AES Music program' should be clarified. Measurement of power rating should be compared with the 'long term maximum power' specification of IEC 60268-5.

It was felt that the simulated program signal should be tailored to driver bandwidth. Bandwidth-limiting filter slopes need to be clarified: IEC 60268-5 uses 24dB per octave. Sound pressure levels should be specified to a precision of 0.1 dB.

W. Klippel proposed using DC resistance of the voice coil to derive figures for power compression. In discussion, those present were not in favor of abandoning "Xmax", a parameter describing maximum driver excursion. It should instead be clarified.

AES19-R: Review of AES19-1992 (r1998): AES-ALMA Standard Test Method for Audio Engineering -- Measurement of the lowest resonance frequency of loudspeaker cones
Noted that a Call for Comment for Withdrawal of this standard was in progress. The document will subsequently be maintained by ALMA International.

Development projects

AES-X72: Acoustic Center of Loudspeakers
In discussion it was felt that the term "Acoustic Center" was unhelpful. Instead a number of alternative terms was explored: "Point of Rotation", "Point of Reference", "Reference Point"(IEC), "Wavefront shape center", "Apparent Source Location", "Temporal Center", or "Time Center".

Woodgate expressed concerns over the use of the term "Point of Reference" as he felt it could be confused with the IEC "Reference Point". Gunness felt that the the two terms were equivalent. Woodgate pointed out that such a Reference Point did not "appear to have the properties of an acoustic centre - it is purely mechanical".

It was noted that for purely practical purposes, that the point of rotation is typically the centre of gravity of the loudspeaker. It usefully provides an unambiguous reference point.

It was noted that temporal centre is not a fixed point at any frequency. Centre of wavefront is also frequency dependent. These are not good references, but it should be stated why this is so in the document, together with the reasons why we cannot define "acoustic centre".

The literature suggests that knowledge of a driver's physical location and rotation axis should allow proper reconstruction of polar data. Woodgate suggests that a more clearly defined name convention is needed.

AES-X103: Large Signal Parameters of Low-Frequency Loudspeaker Drivers
The meeting felt that the 'Xmax' name should remain because people would continue to use the term in any case. However, it needed a proper definition in AES2. Klippel gave a verbal report on his progress on this project.

Various approached to measurement of Xmax were discussed. Physical displacement calculation could use the Keele method or it could be measured using a laser displacement meter.

Klippel suggests clarification for Xmax, that it could be specified as 10% THD or M%THD, or M% stiffness; in other words it could be specified in terms of any nonlinear model parameter. In practice there were not many measurement system options.

For two-tone methods some defined symbol or nomenclature is needed. For example, "[email protected]" indicating that the upper frequency, F2, was 8.5 times the frequency of F1.

The IEC method could be used for measurement of percentage Total Harmonic Distortion (THD) or Intermodulation Distortion (IMD), whichever is higher. There was some discussion of the IEC method where F1 was at Resonance and F2 was set to give a level 12 dB below the F1 level. An alternative suggests that the Maximum SPL measurement is made using a two-tone test where F1 is set to give a level 3dB below that at resonance.

It was felt that a standard format is needed for use with these nonlinear model parameter approaches. Gunness observed that most designers were using the physical measurement approach of M. Gander.

Klippel will prepare a draft document for the group.

AES-X129 Loudspeaker Distortion Perception and Measurement
R. Heinecke-Schmitt presented a synopsis of her research work from 1996 into the audibility of non-linear distortions as typically generated by loudspeakers. correlation of distortion to listener perception using different source materials with the distortion electrically generated using Klippel's mirror filter method. Testing was made to determine perception thresholds of distortion in different source materials. The importance of signal type was emphasised. Two AES preprints exist - 4016 and 4131, and this re-presentation of her 1996 paper was felt to be useful.

R. Cabot requested that her presentation be posted to the group document site. Heinecke-Schmitt agreed that, with some modification, it would be possible. Heinecke-Schmitt is also in the process of completing her doctorate on an extension of the same work and indicated that upon completion, and with an invitation, she would present the results to the Working Group.

J. Stewart asked whether there was a good measurement - even for a specific class of signal - that would correlate with listener perception.

Research indicated which model parameter was most critical, but not how you would measure the objective effects of that parameter.

Steve Temme had made contact with representatives from Opticom who could not attend our meeting, but agreed to run PEAQ evaluations on some recordings of loudspeakers.

Should a standard specify the parameters or the distortions? Following a brief discussion of the values that might appear on a spec sheet - direct measures, or model parameter values - the meeting was closed with the discussion to continue.

New projects

No new projects were received or introduced

New business

There was no new business.

The next meeting will be held in conjunction with the AES 115th Convention in New York, NY, US, 2003-10.

AES - Audio Engineering Society