Fulton FMI speakers. Model and parts reference thread.

[Brian Beck]

Josh, it’s hard to describe how stiff or loose they are on an absolute basis because it’s very subjective. But comparing a crusty one to a supple one side-by-side does reveal a quite noticeable difference in stiffness. Yes, the Fulton 10” and 12” woofer cones are easier to move, but that is to be expected.

The gold standard is to measure free-air resonance (“fs”) which is directly affected by surround stiffness. The Rola factory’s measurement from 1974 shows an fs of about 42 Hz. This matches what Bill measured in his good drivers.

The cotton or linen surround provides the basic centering and support function (along with the spider). But without a sealant the surrounds would leak too much air. The sealant would act to slightly raise Fs (to the desired 42Hz). The sealant might also make a better mechanical impedance match with the cone, dampening resonances.

Dayton Audio used to sell the very affordable WTS and DATS devices which measure impedance vs frequency. Now their cheapest DATS is about $200. Still, if you play with old speakers or the design of new ones, buying their latest DATS or finding a used WT3 or DATS might be worth the cost. These are very powerful PC-based tools for not a lot of money.

 

[Brian Beck]

Rola 2203 1974 test

 

[Bill Grandia]

Josh, it’s hard to describe how stiff or loose they are on an absolute basis because it’s very subjective. But comparing a crusty one to a supple one side-by-side does reveal a quite noticeable difference in stiffness. Yes, the Fulton 10” and 12” woofer cones are easier to move, but that is to be expected.

The gold standard is to measure free-air resonance (“fs”) which is directly affected by surround stiffness. The Rola factory’s measurement from 1974 shows an fs of about 42 Hz. This matches what Bill measured in his good drivers.

The cotton or linen surround provides the basic centering and support function (along with the spider). But without a sealant the surrounds would leak too much air. The sealant would act to slightly raise Fs (to the desired 42Hz). The sealant might also make a better mechanical impedance match with the cone, dampening resonances.

Dayton Audio used to sell the very affordable WTS and DATS devices which measure impedance vs frequency. Now their cheapest DATS is about $200. Still, if you play with old speakers or the design of new ones, buying their latest DATS or finding a used WT3 or DATS might be worth the cost. These are very powerful PC-based tools for not a lot of money.

More importantly, without the sealant, the surround would leak sound from the back side of the cone which would tend to cancel sound from the front side of the cone and cause a premature low-frequency acoustic rolloff.

 

[Brian Beck]

Actually what you describe is the same thing. Leaks allow acoustic pressure to influence both sides of the cone at low frequencies. A truly sealed box is a second-order high-pass filter. One with leaks (and they all leak somewhat) behaves as a weak fourth-order high-pass filter at very low frequencies, essentially behaving as a degenerate ported system.

We can see another interaction going on in the factory curves. That dip at about 420Hz followed by a peak at around 1300Hz is probably due to what is called “table bounce” or even “floor bounce”. Placing a loose woofer over a flat surface can create peaks and dips because sound pressure from the back side of the cone is reflected off the surface and passes back through the cone, either cancelling or reinforcing the initial wave depending on distances versus wavelength. I’ve seen smoother curves in the past for a 2203.

 

[Bill Grandia]

Actually what you describe is the same thing. Leaks allow acoustic pressure to influence both sides of the cone at low frequencies. A truly sealed box is a second-order high-pass filter. One with leaks (and they all leak somewhat) behaves as a weak fourth-order high-pass filter at very low frequencies, essentially behaving as a degenerate ported system.

We can see another interaction going on in the factory curves. That dip at about 420Hz followed by a peak at around 1300Hz is probably due to what is called “table bounce” or even “floor bounce”. Placing a loose woofer over a flat surface can create peaks and dips because sound pressure from the back side of the cone is reflected off the surface and passes back through the cone, either cancelling or reinforcing the initial wave depending on distances versus wavelength. I’ve seen smoother curves in the past for a 2203.

Although it is true that a leak through the surround will also affect the pressure inside the enclosure at low frequencies, it's the pressure that is outside the enclosure that really matters. ,

 

[Bill Grandia]

Rola 2203 1974 test

Fortunately, I doubt that anyone who is at all familiar with making acoustic measurements would take seriously the plot of acoustic output of the Rola woofer that you show.

 

[Brian Beck]

As someone very familiar with acoustic measurements, I offered that 1974 B&K plot only because the impedance portion of the plot offers a valuable piece of data about what Fs should be in a new driver, where there is a resonant peak of impedance at low frequencies: 42Hz. We can ignore the rest of the plot for our purposes here. Since the resonant frequency is influenced by the stiffness of the surround (along with other factors) it is a good target to measure when refurbishing the surrounds. This tells us what it was, for a brand new woofer, way back in 1974, right when FMI 80s were being made. We’re fortunate to have that data.

 

[Bill Grandia]

As someone very familiar with acoustic measurements, I offered that 1974 B&K plot only because the impedance portion of the plot offers a valuable piece of data about what Fs should be in a new driver, where there is a resonant peak of impedance at low frequencies: 42Hz. We can ignore the rest of the plot for our purposes here. Since the resonant frequency is influenced by the stiffness of the surround (along with other factors) it is a good target to measure when refurbishing the surrounds. This tells us what it was, for a brand new woofer, way back in 1974, right when FMI 80s were being made. We’re fortunate to have that data.

It also provides an easily-measured objective reference for how much the surrounds have hardened which probably goes unnoticed especially when the 80s are not used full range, such as in the E modular and J modular systems..

 

[Brian Beck]

I agree; the condition of the surrounds of FMI-80s used in Js or Es should not be as critically important because the subwoofers are handling the bass below about 375Hz or so.

 

[Bill Grandia]

I agree; the condition of the surrounds of FMI-80s used in Js or Es should not be as critically important because the subwoofers are handling the bass below about 375Hz or so.

The situation is worse than I thought. According to the data sheet for the FMI E Modular System, the crossover point between the FMI 80 and the E woofer is 100Hz. I don't have a data sheet for either the J Modular or Premiere System.

 

[Brian Beck]

The situation is worse than I thought. According to the data sheet for the FMI E Modular System, the crossover point between the FMI 80 and the E woofer is 100Hz. I don't have a data sheet for either the J Modular or Premiere System.

That is close. The spec for the J Modular claims 375 Hz.

 

[Bill Grandia]

That is close. The spec for the J Modular claims 375 Hz.

That's because the J Modular has a 10-inch upper bass driver that takes over for the Rolla woofer that is in the 80. With the E System, the Rolla woofer in the 80 needs to carry the low end down to 100Hz.

 

[Bill Grandia]

As someone very familiar with acoustic measurements, I offered that 1974 B&K plot only because the impedance portion of the plot offers a valuable piece of data about what Fs should be in a new driver, where there is a resonant peak of impedance at low frequencies: 42Hz. We can ignore the rest of the plot for our purposes here. Since the resonant frequency is influenced by the stiffness of the surround (along with other factors) it is a good target to measure when refurbishing the surrounds. This tells us what it was, for a brand new woofer, way back in 1974, right when FMI 80s were being made. We’re fortunate to have that data.

We are indeed fortunate to have that data because there is no way of knowing how many FMI 80s that are currently offered for sale and that have hardened surrounds that need to be either refurbished or replaced..

 

[JoshHendi]

Of the four pairs of FMI-80s I have owned, three pairs have had harden surrounds. So at least in my experience, it is more common than not

 

[Bill Grandia]

Of the four pairs of FMI-80s I have owned, three pairs have had harden surrounds. So at least in my experience, it is more common than not

Which is why the first question that anyone who is purchasing a used pair of FMI 80s should ask about is the condition of the Rola woofer surrounds.

 

[Bill Grandia]

Those are not the correct tweeters, unfortunately. I don’t know what they are. The original tweeters are Peerless MT-225-HFC Alnico tweeters. Despite having simple-looking paper cones they are actually very smooth and articulate.

View attachment 3704230


View attachment 3704228

I removed a mechanically-damaged tweeter from a pair of FMI-80s that I purchased last year on eBay and was surprised that there is no part number on it. Do you know if it's the general case that the tweeters in FMI-80s do not have a part number on them?

 

[JoshHendi]

I removed a mechanically-damaged tweeter from a pair of FMI-80s that I purchased last year on eBay and was surprised that there is no part number on it. Do you know if it's the general case that the tweeters in FMI-80s do not have a part number on them?

Most I have seen do not have any sort of part number printed... I have seen some with a Peerless brand sticker, but never a stamped model number

 

[Bill Grandia]

Most I have seen do not have any sort of part number printed... I have seen some with a Peerless brand sticker, but never a stamped model number

So, then, you are assuming that the tweeters that are used in the FMI 60, 80, 100, 120 as well as in the tweeter array for the FMI E-Modular system are all MT225.. Do I have that right?

 

[JoshHendi]

So, then, you are assuming that the tweeters that are used in the FMI 60, 80, 100, 120 as well as in the tweeter array for the FMI E-Modular system are all MT225.. Do I have that right?

Yes as far as I'm aware of, the MT-225s were the tweeter of choice for all the regular 60-120 models (at least during that production phase, I know some later Fulton speakers switched up driver selection)

 

[Brian Beck]

The FMI-60*used a CTS tweeter, and the FMI-120 used the slightly larger MTS-25-HFC. The first version of the 80 also used a single MTS-25-HFC, and this was the speaker that JGH first raved about in Stereophile. Shortly thereafter Fulton switched to using pairs of the MTS-225-HFC in each 80, the version we know best.

*The original FMI-60 used a 6” or 7” full-range driver with a whizzer cone. No separate tweeter. I have both versions of the 60. My full-range version has the usual foam surround rot and I need to fix that, someday.

The attached Image is of the original (and rare) single tweeter version of the 80