Can i change the surrounds of my woofers to softer ones?
Hello all. I have a pair of 12" woofers currently installed in a 3 way system. Some years ago they went for resoration due to surrounds and voice coils being in bad shape. The surrounds back then were accordian cloth and the speakers could easily go as low as 25hz in 40ltr sealed cabinets. After they were sent back to me, to my surprise i found out the surrounds were changed to some really rough and stiff type which was again accordian. It wasnt the normal cloth type they had before. I contacted the guy who overhauled them and he told me that they were the only suitable ones he could find and that the cabinets i have would no longer work with these woofers. So after sending in the speakers for T/S measurements, with his help i constructed a new pair of speaker boxes which are the current ones. They are vented, around 70-80ltrs volume for the woofer alone. The vent is 10cm in diameter and 3 cm in lenght. The woofers are -3db at 800 hz.
Bass response is very mellow, maybe a even a bit too much (comparing to their previous sealed enclosures where it was more tight and on the punchier side) but what i still cant get over after years of time is the fact that it cuts off at 50 hz. Which just bugs me out as these are big enclosures with big speakers in them. So my question is can i change the surrounds to a softer type without making a mess out of the bass response? T/S paremeters for the woofers are as follow:
Fs: 60 hz
Qms:3.32
Qes:0.60
Qts:0.51
Mms:35.86 g
Cms:0.19 mm/N
Vas:76.27 ltr
Bl: 10.22
Thanks in advance.
Hello all. I have a pair of 12" woofers currently installed in a 3 way system. Some years ago they went for resoration due to surrounds and voice coils being in bad shape. The surrounds back then were accordian cloth and the speakers could easily go as low as 25hz in 40ltr sealed cabinets. After they were sent back to me, to my surprise i found out the surrounds were changed to some really rough and stiff type which was again accordian. It wasnt the normal cloth type they had before. I contacted the guy who overhauled them and he told me that they were the only suitable ones he could find and that the cabinets i have would no longer work with these woofers. So after sending in the speakers for T/S measurements, with his help i constructed a new pair of speaker boxes which are the current ones. They are vented, around 70-80ltrs volume for the woofer alone. The vent is 10cm in diameter and 3 cm in lenght. The woofers are -3db at 800 hz.
Bass response is very mellow, maybe a even a bit too much (comparing to their previous sealed enclosures where it was more tight and on the punchier side) but what i still cant get over after years of time is the fact that it cuts off at 50 hz. Which just bugs me out as these are big enclosures with big speakers in them. So my question is can i change the surrounds to a softer type without making a mess out of the bass response? T/S paremeters for the woofers are as follow:
Fs: 60 hz
Qms:3.32
Qes:0.60
Qts:0.51
Mms:35.86 g
Cms:0.19 mm/N
Vas:76.27 ltr
Bl: 10.22
Thanks in advance.
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It sounds as though you want to go back to the way it was before. You haven't made it clear which type of surround you are considering, the pleated type might still be worth considering for the way it damps the cone at the edge, or you might find variations at higher frequencies.
If they do not go as low, it is almost certainly so that the surround compliance is less/lower.
In general plastic cones do not need a heavy surround because they are self damping, but paper, which I suspect your is needs a heavier damping surround, these days usually rubber to reduce reflections from the perimeter back to the coil. Plastic ones often have light foam surrounds, but they rot relatively quickly.
Minus 3dB at 800 Hz is a high frequency for bass roll-off, and it may just be the natural variation of its response unrelated to the bass alignment.
My own use a 12" woofer in about 75 litres, and that is a fairly reasonable size pairing, but your tube ratio is very different from what Small recommends which is roughly a ratio of 3:1 length to diameter. With your port large area mid range noise, (rear cone energy), coming out of the port will be much higher.
In general plastic cones do not need a heavy surround because they are self damping, but paper, which I suspect your is needs a heavier damping surround, these days usually rubber to reduce reflections from the perimeter back to the coil. Plastic ones often have light foam surrounds, but they rot relatively quickly.
Minus 3dB at 800 Hz is a high frequency for bass roll-off, and it may just be the natural variation of its response unrelated to the bass alignment.
My own use a 12" woofer in about 75 litres, and that is a fairly reasonable size pairing, but your tube ratio is very different from what Small recommends which is roughly a ratio of 3:1 length to diameter. With your port large area mid range noise, (rear cone energy), coming out of the port will be much higher.
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They didn´t "only" replace surround and voice coil, it was fully reconed.
Sorry but I don´t trust your claim of: "flat down to 25Hz", why do you think/say so?
A 12" speaker? In a meager 40 liter closed box?
With only 35 gram moving mass?
No way.
Even with the softest surround *and spider* which you are not considering, air stiffness alone in that smallish cabinet will push resonance way higher than 25Hz.
2 options:
1) Leave and enjoy it as is, which is not a bad response at all.
2) fully recone exactly as original, or get a duplicate.
In any case I doubt the "flat to 25Hz" claim.
Sorry but I don´t trust your claim of: "flat down to 25Hz", why do you think/say so?
A 12" speaker? In a meager 40 liter closed box?
With only 35 gram moving mass?
No way.
Even with the softest surround *and spider* which you are not considering, air stiffness alone in that smallish cabinet will push resonance way higher than 25Hz.
2 options:
1) Leave and enjoy it as is, which is not a bad response at all.
2) fully recone exactly as original, or get a duplicate.
In any case I doubt the "flat to 25Hz" claim.
It´s a typo, I hope, and completely unrelated to current "flat to 50 Hz" claim.
Hello again. Perhaps i didnt make myself completely clear for which i am sorry, English is not my native language. The speakers in question are the Gamma LA1231. An old model with an alluminum chasis, alnico magnet, light paper cone and accordian cloth surround. It is what we call a guitar bass speaker around were i live. It is by no means a subwoofer. The speaker's frequency response is 25-6000 hz. There is a plate on the back of the magnet stating it. But perhaps though it was because of the room gain that they managed to play so low in the sealed boxes, as the room was no more than 20 square meters. It has been years since they played in that configuration and i admit that the 25hz statement may have been exaggerated. Still though in their current boxes they cant reach the low frequencies they used to and by alot. The new ones definately wont go lower than 50 and the guy who designed the boxes told me so. The previous sealed ones are what the manufacturer offered the speakers with - a 2 way way system with an crossover frequency of 2500 hz, 2nd order for the bass, 4rth for the horn tweeter. Eventhough it doesnt look like the best setup, in reality the system played really nice. Here is a picture from the internet. The horn has those replacable alluminum type foils.
The new system is a 3 way with a crossover frequency of 800 hz for the bass speaker. I dont see why it would be a typo. I am currently out of town and i cant post a picture of the actual drivers i have but i can tell you that:
1. voice coils are changed
2. surround is changed to a hard, crude one with definate resonating capabilities and it has been impregnated
3. the cone hasnt been changed but the dust cap and the base of the cone (radius of 2-3 centimeters) have been impregnated as well
4. im posting the frequency responce of the new driver regarding the concerns about the port dimensions and crossover frequency. I have other measurements called MLS analysis and cumulative spectral decay. If needed, i can post them.
As for the choice of surround, i havent given it a thought yet as i first wanted to make sure that it's a worthwhile project to begin with. If anything, id like to reduce the vocal spectrum by 2ish decibels as my midrange drivers seem to be quite "tallented" at it by themselves. Again, im sorry if i've confused anyone. I dont really know much about audio and thought i'd ask the guys that do if anything could be done with these speakers.
An externally hosted image should be here but it was not working when we last tested it.
The new system is a 3 way with a crossover frequency of 800 hz for the bass speaker. I dont see why it would be a typo. I am currently out of town and i cant post a picture of the actual drivers i have but i can tell you that:
1. voice coils are changed
2. surround is changed to a hard, crude one with definate resonating capabilities and it has been impregnated
3. the cone hasnt been changed but the dust cap and the base of the cone (radius of 2-3 centimeters) have been impregnated as well
4. im posting the frequency responce of the new driver regarding the concerns about the port dimensions and crossover frequency. I have other measurements called MLS analysis and cumulative spectral decay. If needed, i can post them.
An externally hosted image should be here but it was not working when we last tested it.
As for the choice of surround, i havent given it a thought yet as i first wanted to make sure that it's a worthwhile project to begin with. If anything, id like to reduce the vocal spectrum by 2ish decibels as my midrange drivers seem to be quite "tallented" at it by themselves. Again, im sorry if i've confused anyone. I dont really know much about audio and thought i'd ask the guys that do if anything could be done with these speakers.
Do you think it is why my mids have a peak at the "A letter" of the vocal spectrum? I believe that's around 1k hz? Maybe there was a reason the port was designed the way it is?
Silvermax,
Your BR (bass reflex) enclosure's tuning frequency (Fb) appears to be around 60Hz from the volume and port dimensions given. Below Fb, response drops at 24 dB per octave, 50 Hz being 1/3 octave below Fb will be about 8dB down in level, sounding less than half as loud. At 25 Hz, the output would be basically nothing.
A smaller sealed cabinet will have less output, but will only drop at 12 dB per octave- it may have been 6-10 dB louder at 25Hz than the bigger BR cabinet, while having less output around the BR's Fb of 60Hz and above.
Extending the port another 10CM (130CM total) would lower Fb to around 40Hz, which should extend your bass around 2/3 octave, and reduce "boom" in the 60Hz range. You can use cardboard or PVC tubing of the same diameter externally mounted as your present port to experiment with.
Since you preferred the sound of the sealed cabinet before, you may even prefer dropping the Fb even lower, using a longer port, though if a very long port is put inside the cabinet, the reduction of cabinet volume may end up with little port output.
Art
Art, thank you for that. I suppose that the person who designed the enclosures had something else in mind for them than me. I see now that its worth experimenting with the lenght of the port, but what about the diameter? As Pharos stated earlier above, simulatons recommend a ratio roughly of 3:1 lenght to diameter?
Silvermax,
The 3:1 length to diameter guideline helps people avoid making tiny ports to tune low using a hole drilled in typical plywood depths, which results in such small port volume that it can be "blown out" at moderate drive levels.
You did not mention the Xmax of your driver, but assuming they are not "pushing a lot of air" the 10cm diameter port is large enough to avoid audible turbulence, "chuffing noise".
Since you will be extending the port to reduce Fb, you will also be increasing it's aspect ratio, and will have a bigger slug of air than you had before, less likely to chuff.
Art
The 3:1 length to diameter guideline helps people avoid making tiny ports to tune low using a hole drilled in typical plywood depths, which results in such small port volume that it can be "blown out" at moderate drive levels.
You did not mention the Xmax of your driver, but assuming they are not "pushing a lot of air" the 10cm diameter port is large enough to avoid audible turbulence, "chuffing noise".
Since you will be extending the port to reduce Fb, you will also be increasing it's aspect ratio, and will have a bigger slug of air than you had before, less likely to chuff.
Art
