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Comments on reaffirmation of AES2-1984 (r1997)
Comments to date on reaffirmation of AES2-1984 (r1997), AES Recommended Practice -- Specification of loudspeaker components used in professional audio and sound reinforcement published 2002-11-30 for comment. The comment period has closed without unresolved comment and the standard is consequently reaffirmed.
Comment from Klippel, 2003-02-24
The voice coil peak displacement Xmax is an important driver parameter for assessing the maximal acoustic output at low frequencies. The existing standard AES 2-1984 defines the peak displacement Xmax in 4.3.2 Large-Signal Parameters:
"(2) Voice-coil peak displacement at which the "linearity" of the motor deviates by 10%. Linearity may be measured by percent distortion of the input current or by percent deviation of displacement versus input current. Manufacturer shall state method used. The measurement shall be made in free air at fS."
The critical review of the numerical simulation on a fictitious loudspeaker and practical measurements on real loudspeakers (see AES preprint 5508) show that the method in AES2-1984 does not provide a clear and useful definition of Xmax. This is mainly caused by some ambiguities in the wording and more importantly by using assumptions, which are not valid in theory and practice. Clearly, the measurement of harmonic distortion is not sufficient for assessing all important aspects of the large signal performance. Nonlinearities inherent in transducers such as force factor Bl(x), inductance Le(x) and Doppler effect produce significant modulation distortion. The current IEC standard 60268 provides all of the methods required for assessing these kinds of distortion and for defining Xmax more clearly and reliably:
Suggestion for a new wording:
Xmax is the voice-coil peak displacement at which the maximal value of either the total harmonic distortion dt or the 2nd order modulation distortion d2 or the 3rd-order modulation distortion d3 in the radiated sound pressure is equal to a defined threshold d. The driver is excited by the linear superposition of a first tone at the resonance frequency f1=fs and a second tone f2=8.5 fs with an amplitude ratio of 4:1. The total harmonic distortion dt assesses the harmonics of f1 and the modulation distortion d2 and d3 are measured according to IEC 60268. It is recommended to operate the driver in a baffle (half-space), to measure the sound pressure in the near field and to use the threshold d=10 %. Manufacturer shall state Xmax, the dominant type of distortion (dt, d2 or d3) and the value of the threshold d used.
The new definition is based on a two-tone measurement that can be accomplished with straightforward equipment (microphone, sinusoidal generator, FFT analyzer).
1. Measure the resonance frequency fs of the driver.
2. Excite the driver under voltage drive with a two-tone signal at f1=fs and f2=8.5 fs with an amplitude ratio of 4:1.
3. Perform a series of measurement while increasing the input amplitude and measuring the sound pressure in the near field of the driver. If a displacement sensor is available measure the voice coil displacement.
4. Perform a spectral analysis of the sound pressure signal and determine the total harmonic distortion and intermodulation distortion according IEC 60268. The sound pressure measurement for assessing the distortion does not require a calibrated microphone. This measurement may be performed in the near field of the driver operated in free air without any enclosure because the cancellation from the rear radiation is negligible. However, if the voice coil displacement is calculated from the sound pressure level of the bass tone f1 a calibrated microphone is required and the driver should be operated in a baffle (half space environment).
5. Search for the minimal value of the peak displacement where either dt, d2 or d3 are equal to the threshold d. In lack of better arguments and to preserve consistency with the existing AES 2-1984 it is recommended to use d =10 %. However, the manufacturer may use a different value if stated with the measured Xmax.
6. State the peak displacement Xmax, and the type of distortion limiting the excursion.
For example, a statement
Xmax= 3.8 mm @ d2=10 % (dt, d3 < 10 %)
means that a driver provides a maximal peak displacement of Xmax=3.8 mm where the 2nd-order modulation distortion is dominant and produce the threshold of 10 % distortion. This statement implies that the total harmonic distortion and 3rd-order distortion are less than 10 % which can be added in parenthesis optionally. Thus the suspension and the voice-coil height are most likely not the limiting factors for the excursion of this driver.
Reply from Prince, 2003-03-03
In response to your comment on the reaffirmation of AES-2:
Due to mandatory deadlines, AES-2 required reaffirmation or withdrawal at the scheduled time. Since the document will be revised per the work of SC-04-03A in the near future, scheduling suggested that a revision or retraction of the document would not be quicker than the current revision schedule. Your note that Xmax should be revised is well taken and should be noted. We anticipate the inclusion of this measurement method and a similar definition in the upcoming revision.
Comment from Klippel, 2003-03-05
The existing definition of the peak displacement Xmax in 4.3.2 Large-Signal Parameters in AES 2-1984 (r1997) is obviously wrong and leads to inconsistent results. However, Xmax is one of the most important large signal parameter which have to be published on a serious driver specification sheet. Due to the lack of a clear definition in AES 2 - 1984(r1997) manufacturers use alternative methods producing Xmax values which are not comparable.
In consideration of these common known facts the affirmation of AES2-1984 (r1997) with the erroneous Xmax definition would damage the authority and reputation of this document as a standard. However, I think that an affirmation of AES2-1984 is acceptable if a footnote is added to the document to make the reader aware of the limitations on Xmax (as you suggested). If this is not possible and the document can only be reaffirmed "as it is" then a withdrawal should be preferred and a new document should be issued when the work of SC-04-03A is finished
Best regards, Wolfgang
Note from Secretariat, 2003-03-07
In terms of pragmatic implementation, it may help to point out some procedural issues that relate to this standard and its public consensus process.
- If this Call for Comment on reaffirmation of AES2 fails, a withdrawal of AES2 will require a new Call for Comment and another 3-month comment period.
- It is very difficult to revise a standard that has been withdrawn. While the existing definition of Xmax in AES2 may be in need of revision, withdrawing the existing standard will only serve to slow up the introduction of an alternative, and I believe this is exactly the opposite of what is desired.
- It is very difficult to revise a standard that is already in the formal process of a Call for Comment.
- I can find no alternative standardised definition of Xmax. IEC 60268-5 specifies techniques for measuring various distortion components but does not relate these to cone excursion and also doesn't define parameters for Xmax. This means that the current AES2 definition of Xmax, although maybe not perfect, does not appear to be in conflict with other standards.
- An amendment or revision of AES2 will require Working Group consensus and a formal Call for Comment. However, provided that the current AES2 is reaffirmed, this can happen at any time - there is no need to wait five years. Indeed, if wording can be agreed quickly, this could be offered as a formal revision in little more than a month from now. I believe that this is absolutely the fastest way to improve the definition of Xmax.
Accordingly, I would like to support David Prince's proposal of reaffirmation followed quickly by revision. Please let me know if you still object to this procedure.
Comment from Klippel, 2003-03-07
Many thanks for your comments. I have not been familiar with the practical consequences of a withdrawal. I am absolutely not interested in slowing down this process or producing more work for anybody. If you think reaffirmation ist the fastest, simplest and most pracmatic way than I do not disagree with it.