Spectra 11, Various, Thielle Small parameters

Read more on the Acoustat Spectra 11 in our home audio section

Ralf H. Hetzel2004-02-23 22:21

Hello, I found my speakers inside your web page and could get some good information from this. Thank you very much for this support. I would have one more question and maybe you also do know the answer. Would you know the Thiele Small parameters of the bass speaker, which is used in the Acoustat spectra 11? And also, would you know the cross over frequency between the bass and the electrostatic panel? I tried to calculate it based on the crossover design, but I am not sure about the result. Thanks for your answer, regards, Ralf H. Hetzel


Andy Szabo2004-02-23 22:21

I do not have the Thiele-Small parameters for any of the Acoustat woofers. This would certainly be valuable information, to allow easier replacement of what were custom-designed woofers, with no exact replacements available.

The crossover for any of the Acoustat woofers can be a bit confusing. All of them use what is basically a -12 dB/octave filter, but with the two poles aligned at different frequencies. This gives the effect of initially rolling off the woofer at -6 dB/oct, but then increasing to -12 dB/oct at a higher frequency. This technique seems to match well with the roll-off of the esl.

For the Spectra 11, the esl rolls off at -6 dB/oct, starting at about 160-Hz. The woofer rolls off at -6 dB/oct starting at 187-Hz, but then increases to -12 dB/oct at about 240-Hz.

Note: awhile back, I answered a similar question from Don Layman. There, I said the crossover point was about 250-Hz. I\'m not sure if I mistyped, or was merely mistaken at the time. In any case, the above is a more complete (and hopefully correct) answer to the question.

Andy Szabo2004-02-23 22:21

Please note that there is an owner’s manual for the Spectra 1100 available on this site. While not exactly the same speaker as the Spectra 11, much of the information is the same.

There are three components that make up the Spectra 11 (or 1100). They are the woofer box, the interface electronics, and the electrostatic panel. Of these three, only the panels are “left”or “right”. The other two may be used interchangeably.

Acoustat marked “left”or “right”on these panels in several ways. There may be a colored dot on the top or bottom of the speaker: red for right, and green for left. Or, there may be a label on the rear surface of the panel, with the word “left”or “right”. If these label(s) are no longer present, you can still tell by listening.

While playing music, place your ear close to the panel and move from side to side. You will note that the high frequencies seem to come more from one side than the other. The panel with the high frequencies coming from the left side is the left speaker, and vice-versa for the right panel. (Note that this is opposite to the test for full-range Spectra’s.)

The Theile-Small parameters for the woofer were available at one time, but I did not keep a record of them. (Little did I know that ten years later, I would be helping Acoustat owners via the Internet.) I have measured these parameters before, and still have the necessary equipment, but my instructions for the procedure is currently misplaced (not lost!).

The woofer on the Spectra 11 is clearly its weak point, and replacing it with a better unit could improve the bass considerably. Perhaps it might be better to approach such a project by determining the desired parameters of the new woofer, rather than being concerned about the specs of the old one. You’ll want the new woofer to have as low a resonant frequency as possible, and a compliance that is suitable for the given closed-box volume. Efficiency is not an issue, since the woofer crossover coil contains extra resistance to reduce the woofer’s output to match the panel. This resistance (and other crossover values) may need some modification to suit a new woofer. If you find a new woofer that works well, please post the information on this site. Many Spectra 11 owners would be very interested!

Ralf H. Hetzel2004-02-23 22:21

Hello again, thank you for answering my first questions. Actually regarding my last question about the Thiele Small parameters; if the supplier did not provide you with this parameters, it is also possible to measure and calculate this numbers, and I hoped that somebody else did this already. It is not too much of an effort but still demands some equipment and skills, both of which a speaker designer should have.

Well my next question is much simpler. Which speaker (Acoustat Spectra 11) shall be positioned to the right and which to the left side. I bought them used and have no manual, and on top of each frame is on marker. Is this a kind of indication? Or can each speaker be the right or the left one? Thanks for your answer, best regards, Ralf H. Hetzel

Post a reply

Your name will appear on the website next to your contribution. Your email address will only be used to contact you if something is wrong with your contribution. It will not be shared with others.