Spectra 44, Various, 28 Hz?

Read more on the Acoustat Spectra 44 in our home audio section

John Mastroleo2004-02-23 22:21

Andy, I have owned my Spectra 44 speakers for 12 yrs. I have two questions. Acoustat\'s add literature for the model 2+ 2 rates the speaker with a maximum output of 115db on musical peaks. This output is given in a room with a dimension of 16\'x24\'. The add goes on and gives a listening distance of 20\' feet from the speakers with the speakers positioned four feet from the front wall. It also gives the maximum power capacity for the speaker of 500 watts. The printed frequency response was given at 28 to 20 plus or minus two db. This add is printed in Stererophile vol 7 issue two. John Holt conducted the review.

My two questions are how can the speaker achieve 28 hz.frequency response. I measured the SPECTRA panel ( The older panel not the one built in Europe or China.) It has a resonate frequency of 45 hz. How do you get 28 hz out of that? Was the measurement including room gain and taken at the far field of the listening room where you sit? I would like to know how the measurements were taken.

The other question has to do with the amplitude that is used when measuring any speaker system but specifically the 2 + 2 and model 44. When a speaker manufacturer prints a plus or minus reponse of say 2 db for his speaker at what sound pressure are the speakers measured at. ( 85 ,90 ) Many speaker manufactures take frequency response test with the mike 1 meter from the plane of the speaker but you never see what sound pressure the test is conducted at and you never know how low in amplitude the base range is in relationship to the mid band. If a speaker is tested at say 1 meter and at a 87 db loudness level. To be an acurate rating shouldn\'t the speaker be be able to produce the same amplitude at 28 hz as it is at 1kh? Is there a standard for measuring? How are the Acoustat 2 + 2 and the model 44 measured to achieve the published 28 hz freq response when the panels have a measured resonate frequeny of 45 hz.?

One last thing the new panel that was designed in 1995 for the European Acoustats makes mention of the new wider dynamic range panels. The Audiostatic Company also talks on their web page of a wide dynamic panel. To do this is the mylor placed further away from the charge so it has to stretch further thus moving more air.?

Sincerely, John Mastroleo

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Andy Szabo2004-02-23 22:21

Hello again John! I remember talking to you many times on the phone back in the good ol\' Acoustat days. You\'ve asked a lot of specific questions about speaker specifications and measurements that I cannot answer, but I will make a few general comments.

I have always felt that any loudspeaker\'s acoustic specifications are highly suspect. Due to non-standardized methods of measurement, effects of room acoustics, and a liberal dose of marketing hype, I just don\'t give them much credence.

I was not at Acoustat when those 2+2 measurements were taken and published, so I cannot comment on how they were produced. Personally, I always suspected that the pre-Spectra specifications were … shall we say optimistic?

Attempts were made to produce more meaningful specifications for the Spectra models. I did most of those measurements myself, and I have a little more faith in the Spectra specifications, but still not much. Part of the problem is that measuring a large-area speaker at one meter will not give an accurate picture of its total output - particularly in the bass region. Taking the measurement at three meters (and extrapolating the numbers back to one meter) can give better results for a large speaker. But, unfortunately, this greater distance increases the likelihood of room acoustics affecting the measurement.

If I remember correctly, most published speaker measurements are made at 1-watt input: impedance, frequency response, and sensitivity. Maximum SPL levels are very tricky to measure. Measuring peak music levels is difficult (even with a peak-holding meter), and trying to measure with a continuous high-level tone would quickly destroy most speakers.

I am not familiar with any changes made to the Acoustat panel since the company was sold to the Italians. (I\'d be interested in learning more!) In regards to your question about Mylar spacing, I can make a few comments. If the spacing is increased, dynamic capability may increase slightly, but efficiency would drop dramatically. Electrostatic forces fall off with the square of the distance, so even a slight increase in spacing would significantly reduce efficiency. Since electrostatic speakers have poor efficiency already, a further reduction in efficiency would not be good. When Jim Strickland designed the original panel, I think he arrived at the optimum diaphragm-to-stator spacing, for the materials being used with an air dielectric.

Acoustat (America) did make one change in the panel that increased dynamic capability, without any negative side effects. The conductive coating was increased in resistivity, which reduced charge migration and localization. This means that the charge distribution over the surface of the panel remains more uniform and hence closer to theoretical operation. The result was a speaker that could play louder without the familiar ”crackles” on bass notes. Panels built with the improved coating can be identified by a bias wire that is yellow with a red spiral stripe.

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