Do I have a problem with replaced dustcaps and foam surrounds?

[Kuja]

Hello Everybody!

I have restored a cosmetically beaten up pair of classic vintage Acoustic Research AR-90 loudspeakers.



It is a four way loudspeaker with dual paper 10" woofers, paper 8" lower midrange, textile dome upper midrange and tweeter.







Woofers and lower midrange drivers needed refoaming.



8" lower midrange drivers had smashed in dustcaps:






They were kinda ugly, so I replaced them with new ones.

Original dustcaps were porous and made from textile, so I replaced them with the same type.

New dustcaps are larger and maybe are more porous since they have larger openings in the fabric.

Here they are:








Now, after reading some articles here,
I started worrying that I might have degraded performance of these drivers...


New dustcaps are a little bit larger (and I guess that they are couple of grams heavier too?) and are potentially more porous due to larger holes in the material.

Here are the old and the new dustcaps compared:







Also, the new foam surrounds seem to be made of little bit thicker and stiffer foam.
I couldn't find a closer match in my country.





I have two questions:


1:

I have a new dustcap that is larger (maybe a bit heavier too?) than the original
and has larger holes in its porous material.

Textile porous material of the new dustcaps also seems to be a little bit thicker and rougher.

Is it going to significantly / audibly change the sound of a paper 8" lower midrange driver,
operating in the 200Hz - 1.2kHz range?

If the answer is yes, how exactly?

Am I going to have some new dips or peaks in the frequency response? Where and how big?



2:

What can be the influence of a slightly thicker and stiffer foam surround replacement
in the case of this paper 8" lower midrange driver?





If you need them, here are some more details regarding the crossover:









Thanks in advance!





.

[Kraniet]

very nice work. looks incredible the way you managed to freshen the drivers.

as for the effect on the sound its hard to tell without comparing measurements. The stiff foam might work it self out with time, becomming softer. It might also be that the slightly heavier dustcap will be balanced by the stiffer suspension.
I believe the more porous dustcap will have soem effect from maybe 1000Hz sinze that should be the point where the 8" stop behaving like a piston. So it shouldnt have that big impact. The porous dustcap would on the other hand help to ventilate the coil.

[AllenB]

I would expect the weight, stiffness/compliance and porosity to primarily affect resonance. Since this is a mid (I assume it's crossed at the lower end) it probably wont have a significant difference. Furthermore, some of these differences will work against each other to some degree, to your advantage. You may see a slight reduction in sensitivity but hard to say if it will be significant.

[Arty]

hard to tell, but my bet is that the extra weight might produce a lower Fs.
Actualy i think You have nothing to worry about.

[Kuja]

Thank you all for the answers!




Carl from classicspeakerpages.net AR forum, gave me an idea to measure each driver separately from 0.5 cim distance,
so that I can better see how they behave regarding their crossover points.




So I did:








Here are the results (click on the image to enlarge it):




Green = woofer
Blue = lower midrange driver
Red = upper midrange driver
Violet = tweeter

I used 1/6 octave smoothing on the graphs, in order to visually avoid effects of room reflections.


Vertical thicker blue lines are crossover frequencies found in AR-90 literature:







To me, they look quite different from what I have measured!




The lower midrange driver seems to drop in frequency earlier than it should?

Could the new dustcap and foam surround be the cause for this?



Crossover frequency between the UMR and the tweeter should be 7000Hz, but it measured around 5000Hz?

According to the AR literature, crossover slope on the tweeter should be 18dB/octave at 7000Hz and the graph looks (to my uneducated eyes) far from it.






Crossovers are all original except for the old aging capacitors, that were replaced with new polys of the EXACTLY same values.





What are your thoughts?



PS
Both loudspeakers had the same measured results, difference was less than +/- 1dB.

So I guess crossovers and all drivers are OK in both speakers, and they are functioning properly.


PS 2

Here is the measurement I made from 1m distance, measuring mic pointing at the upper midrange driver (the way that AR measured these speakers):



.

[quikie22]

I replaced the foam surrounds for my JBL woofers. They were also slightly thicker, stiffer and heavier too like yours. Initially, the bass response suffered with hardly any bass.
After about 200hrs of breaking-in, they sound like the originals. :-) I guess you need to allow the surrounds to soften as proposed by Kraniet.
So, I think yours should be OK.

[phase_accurate]

The drop in response below 1 kHz could actually be a measurement artefact. If you measure close-up you have the advantage to minimise room effects. The disadvantage is that the measured response of larger drivers gets very irregular at higher frequencies (even if they have a nice response when measured at a distance) due to the propagation delay of the sound that emerges from different points of the cone.

The in-room response doesn't look too bad. Is there a reference measurement available somewhere that you can compare against ?

Regards

Charles

[limono]

Sorry to hijack the thread a little but what was the source for JBL driver surrounds?

[quikie22]

these were some generic no name surrounds available in my local electronic supply store which happened to fit.

I tried contacting JBL regarding the surrounds and they did not offer any solution apart from asking me to send the drivers back to them for refoaming. The postage/freight from my country to them alone would have cost more than the worth of the speakers.

[AllenB]

Agreeing with phase_accurate, you can't interpret that kind of measurement in quite that way.

You should pull the speaker and mic out into an open area of the room. You will then properly measure the high frequencies and all diffraction. The mic distance needs to be at least a foot, preferably three and if possible more, but in order to avoid the room in the measurement there needs to be a clearance from all obstacles around the mic and the speaker in all directions, which is greater than the distance between the mic and speaker so there are limits there. You should then use gating to exclude all reflections. This will be good from the lower midrange and up.