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2008 Winter Symposium: Technical Papers Review All paper titles will link to a PDF file or downloadable presentation of the paper. Note that these are available to ALMA members only. If you have forgotton your username and password to access the members only area of the ALMA site, please email management@almainternational.org to request that your password be emailed to you. To learn more about ALMA membership see the 'membership information' link on the left hand side of the page.
The 25 minute paper was followed by a lively 30 minute discussion of the subject.
Practical Loudspeaker Applications of Time-Frequency Analysis - Steve Temme, Listen, Inc
Thermal Chimney Heat Extraction - Enrique Stiles, STEP Technology Inc.
A Study of Ferrofluid Behavior in a Speaker Gap - Steve Tatarunis
A CD containing all papers and tutorials can be purchased from ALMA by Members Only for $25. Email management@almainternational.org for details.
More from the ALMA 2008 Symposium:
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The current international loudspeaker standards (IEC 60268-5, AES 2) apply entirely passive elements where built-in amplifiers are excluded. However, more and more loudspeaker systems become active using either an analog input with relatively high impedance or a digital interface. Here the electrical input impedance tells nothing about the transducers used and related quantities such as input power, sensitivity, efficiency are meaningless or can not be determined by using the loudspeaker as a black box system. Thus new measurements have to be developed which are based on the sound pressure output independent of the format of the input signal. This paper described the physical problems, suggests technical solutions and opens the discussion about standard projects more dedicated to active systems. 
Time-Frequency analysis has been in use for more than 20 years and many different time-frequency distributions have been developed. Four in particular, Short Time Fourier Transform (STFT), Cumulative Spectral Decay (CSD), Wavelet, and Wigner-Ville have gained popularity and firmly established themselves as useful measurement tools in the loudspeaker industry. This paper compared these four popular transforms, explained their trade-offs, and discussed how to apply them to analyzing loudspeakers. Practical examples given in the paper included loudspeaker impulse responses and a demonstration of the technique for identification and analysis of loose particles, and Rub & Buzz defects.
It is well known that the heating of a loudspeaker and its enclosure produce undesirable effects on the overall system performance. These effects include thermal compression, tuning change, and possible loudspeaker failure. The “Thermal Chimney” offers a significant improvement in reducing the heating of both the loudspeaker and air in its enclosure. A Thermal Chimney in its most simple form consists of a thermally conductive tube running through the enclosure creating heat transfer from the heated air in the enclosure to the cool air outside by the means of natural convection. Mechanically connecting the chimney to the loudspeaker motor, or inserting heat paths in close proximity to the voice coil yield even more effective results. The addition of a fan for forced convection heat transfer, further improves the effectiveness. 
A ferrofluid placed in a speaker air gap is subjected to the magnetic flux distribution of the gap. Depending upon the nature of the distribution, the fluid may experience a change in its microstructure. Such changes may be detected by the frequency response and impedance measurements of the driver.
For this study several high frequency drivers having both concentric and eccentric air gaps were selected, yielding a range of magnetic flux distributions. The majority of the experiments were carried out at a fixed volume of ferrofluid. However, in some cases the fluid amount was varied, exposing the ferrofluid to a different field gradient in the same speaker. Limited data was also acquired with ferrofluids of different colloid stability.
Response curve measurements of drivers under different conditions showed that internal structural changes in ferrofluids induced by the gap flux can be virtually eliminated by a careful speaker design and ferrofluid selection. It was also found that these changes generally are reversible; the coil agitation mixes the fluid to its original condition.