Hi Everyone,
I'm in the process of modding my speakers - some old Mission 752's with the original metal tweeter - with the goal of getting less cabinet resonance and colouration in the sound.
So far I've done the usual mods - removed the tweeter grills and replaced with a 1mm O ring, strengthened and damped the woofer basket and used felt to reduce reflections back through the cone and changed the speaker stuffing. Originally the bottom third of the cabinet was stuffed with 4 thicknesses of egg crate foam, I've reduced this to 2 sheets front and back and some wool stuffing in the middle. I've also used bitumastic damping sheets and felt on the side walls and top of the cabinet to reduce resonance and reflections. All of these changes had positive results (at least to my ears) but the cabinet sound was still there so stronger measures were needed.
These speakers have two side to side braces as standard but no bracing to the baffle. Yesterday I made two braces for the baffle, both from 1' by 1/2' ply, one placed between the woofer and the tweeter running all the way to the rear cabinet wall, and one placed between the woofer and the reflex ports which runs back to one of the side to side braces. Both of these braces are away from the side walls and offset from the centre line. The result? the speaker timing was greatly improved, leading edges of notes much more sharply defined, the imaging is more stable and the acoustic more realistic so I can hear the size and reflectivity of the space that the recording was made in. The problem is that I've also greatly reduced the lower bass output so now I have to crank the volume up to be able to hear the bottom end of the music.
I'd like to keep the greater precision these baffle braces give but not at the cost of losing the lower bass. Is there a way I can adjust the stuffing of the cabinet to restore the lost bass? I'm not sure whether to add more wool stuffing or remove some of it😕
Thanks,
Mark.
I'm in the process of modding my speakers - some old Mission 752's with the original metal tweeter - with the goal of getting less cabinet resonance and colouration in the sound.
So far I've done the usual mods - removed the tweeter grills and replaced with a 1mm O ring, strengthened and damped the woofer basket and used felt to reduce reflections back through the cone and changed the speaker stuffing. Originally the bottom third of the cabinet was stuffed with 4 thicknesses of egg crate foam, I've reduced this to 2 sheets front and back and some wool stuffing in the middle. I've also used bitumastic damping sheets and felt on the side walls and top of the cabinet to reduce resonance and reflections. All of these changes had positive results (at least to my ears) but the cabinet sound was still there so stronger measures were needed.
These speakers have two side to side braces as standard but no bracing to the baffle. Yesterday I made two braces for the baffle, both from 1' by 1/2' ply, one placed between the woofer and the tweeter running all the way to the rear cabinet wall, and one placed between the woofer and the reflex ports which runs back to one of the side to side braces. Both of these braces are away from the side walls and offset from the centre line. The result? the speaker timing was greatly improved, leading edges of notes much more sharply defined, the imaging is more stable and the acoustic more realistic so I can hear the size and reflectivity of the space that the recording was made in. The problem is that I've also greatly reduced the lower bass output so now I have to crank the volume up to be able to hear the bottom end of the music.
I'd like to keep the greater precision these baffle braces give but not at the cost of losing the lower bass. Is there a way I can adjust the stuffing of the cabinet to restore the lost bass? I'm not sure whether to add more wool stuffing or remove some of it😕
Thanks,
Mark.
Hi Brakspear75,
the biggest danger in reviewing (own) audio gear is ... self-deception.
You write - implicitly - that tonal balance of the speaker has changed noteably, right ?
If so, how can you be shure, that audible effects you mention as being "positive" are due to those mods and not just due to changes in frequency response ("tonality") e.g. ?
I think some Pics of your mods (damping, bracing, etc.) would be helpful.
That could help to estimate, wether you were detuning the cabinet in the bass to such extent, that the lower bass is reduced by now.
When doing such mods it is very helpful, to make a documentation by measurements, just to keep track of what you are doing and be more safe against fooling yourself.
Cheers
Brakspear75 said:
the biggest danger in reviewing (own) audio gear is ... self-deception.
You write - implicitly - that tonal balance of the speaker has changed noteably, right ?
If so, how can you be shure, that audible effects you mention as being "positive" are due to those mods and not just due to changes in frequency response ("tonality") e.g. ?
I think some Pics of your mods (damping, bracing, etc.) would be helpful.
That could help to estimate, wether you were detuning the cabinet in the bass to such extent, that the lower bass is reduced by now.
When doing such mods it is very helpful, to make a documentation by measurements, just to keep track of what you are doing and be more safe against fooling yourself.
Cheers
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So you made one change at a time and listened to the results so you could undo what caused the major change?
I have made these changes one at a time and listened to the results using the same selection of test tracks (music not sine waves) after each change. The bass only disappeared after the addition of the baffle braces, not as a result of the earlier mods. I don't have a proper measurement microphone, so can only take crude measurements, but here's the result of the changes before the braces were added, un-modded trace in yellow, modded in blue:
I haven't yet done the measurement after adding the braces - I'll post it as soon as I've got round to doing it.
Cheers
I haven't yet done the measurement after adding the braces - I'll post it as soon as I've got round to doing it.
Cheers
Attachments
Ok, I've now repeated the measurements with the braces in place - frequency response shown below. As before yellow is un-modded, blue modded before bracing, pink with bracing to baffle. Apologies for the quality of the graph, one of these days I'll learn how to use my image software properly! I seem to have lost 3-4dB in the 50-70Hz range and the response is lumpier.
The braces take up 0.45% of the total cabinet volume - I deliberately kept them slim to avoid volume changes. Maybe I've blocked a standing wave that was reinforcing the output of the reflex ports in this area.🙁
The braces take up 0.45% of the total cabinet volume - I deliberately kept them slim to avoid volume changes. Maybe I've blocked a standing wave that was reinforcing the output of the reflex ports in this area.🙁
Attachments
What you did is dramatically reduce 2nd harmonic distortion which is from the modal response of the panel resonances. 2nd HD in the LF region yields "fatter" bass, and speakers with low 2nd HD tend to sound lean, although correct. Older large box speakers have extraordinary levels of said distortion, while modern speakers do not due to the cabinet not contributing so much to the sound. Many people prefer that thicker, heavier sound. it is, however, still distortion.
I guess to answer your actual question, no, you can't. You took an underdamped enclosure that would ring when excited, and now have stiffened it so that it no longer does. The bass you are hearing was always there; there just used to be grunge on top of is (it is not lower bass that you've lost, it's the upper "punchier" bass).
I guess to answer your actual question, no, you can't. You took an underdamped enclosure that would ring when excited, and now have stiffened it so that it no longer does. The bass you are hearing was always there; there just used to be grunge on top of is (it is not lower bass that you've lost, it's the upper "punchier" bass).
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Without knowledge - just by the FR graphs - i would have assumed the cabinet's Qb has been reduced noteably ...
Is that brace horizontally between driver and inner mouth of port ?
How much (in %) is the cross section of the cabinet (in horizontal plane) reduced by that brace ?
How close is it to the inner mouth of the port ?
I agree that what has been taken out is probably box resonances. Having grown accustomed to them, you may now be missing them.
I would suggest you listen to them for some time and see if you like them or not.
I would suggest you listen to them for some time and see if you like them or not.
Don't want to sound silly but is there any chance you accidentally reversed the polarity of one of the woofers during the modifications? I.e. now out of phase. It wouldn't be the first time this has happened.
But I agree with the idea that you now have better bass if not as much of it subjectively.......
But I agree with the idea that you now have better bass if not as much of it subjectively.......
Thanks for the comments/suggestions, they've led me to re-think the bracing strategy. Pictures are a bit of a problem right now as a lot of my stuff is in boxes (I'm sort of between houses).I've removed the lower brace that was positioned between the woofer and the reflex ports (braces were not glued as I wanted to assess the sound first, just held in by being a very tight fit) and centralize the upper brace between the woofer and tweeter. The braces occupy ~ 1/6 of the horizontal plane and due to the 752's design with 3 reflex ports spaced across the baffle 1' below the woofer the brace here was close to the mouth of one of these ports. I thought this wouldn't make too much difference as it only affected one port out of three; I was wrong!
With just the one brace in place the bass is back, even if it is 2nd harmonic distortion😛
I haven't had time to properly listen to the speakers yet or do any measurements - I'll post again when I have. First impressions are that bass response is pretty much where it used to be, but tighter. There's probably not quite the same accuracy as with both braces but I think it's better than without any, so this may well be the config I run with. I'll assess it properly and measure the FR, then I have plans for the tweeters to see if I can get rid of the treble peak...
With just the one brace in place the bass is back, even if it is 2nd harmonic distortion😛
I haven't had time to properly listen to the speakers yet or do any measurements - I'll post again when I have. First impressions are that bass response is pretty much where it used to be, but tighter. There's probably not quite the same accuracy as with both braces but I think it's better than without any, so this may well be the config I run with. I'll assess it properly and measure the FR, then I have plans for the tweeters to see if I can get rid of the treble peak...
It's a frequent occurrence in industry. A product goes out for review against another, and the one with the more faithful reproduction has less "slam" or is "drier", but has a more "audiophile" sound. Take said verbage as you please. I have encountered this many a time and always will go for the "drier" sounding, well damped bass. Reason being, when the speaker gets turned up, that "audiophile" one will have the same tonal balance. The heavier bass starts to spiral out of control as the resonances become more prominent. Even if this is just due to poor driver design and not from the cabinet, it's still much the same effect. True story: many of the lowest LF distortion speakers are reviewed poorly for their bass even though they do exactly what the record indicates. It just comes down to people liking bass. 🙂
it is possible for vibrating cabinet wall to generate and so radiate more acoustical output than the driver in the cabinet. So If you effectively brace and stiffen you ought to expect a reduction in this radiated noise. Cabinet wall resonance is how an LS35A is able to seem to make bass. You did not lose bass output you just lost some distortion. Best regards Moray James.
I normally cld brace drivers to the corners eg side wall back wall junction. Not the true corners. This reduces the distortion while not inducing torsion related coupling. Enclosure walls are cld braced amoungst each other.
Funny how your measurements closely mimic the improvements and flatter response I also saw in the 500 to 2k Hz region after mods on my bookshelf speakers. Only mod I did was taking out tiny amount of poly fill and stuffed with fiberglass insulation. I did see very minimal improvements by removing plastic grille and putting tape over poorly placed screw holes in the tweeter waveguide.
Also, after taking apart my old Cerwin Vega D7's, I realized that the woofers, tweeters, crossover controls and connection terminal in the rear all leaked air and chuffed under heavy bass test at low frequency sine waves. After putting gaskets on everything, I noticed the bass was a little deeper, cleaner and lacking just a tiny bit as you described. But it sounded way better. I have not braced the cabinet, yet. Needs bracing bad.
Thanks for your mods and measurements.
Also, after taking apart my old Cerwin Vega D7's, I realized that the woofers, tweeters, crossover controls and connection terminal in the rear all leaked air and chuffed under heavy bass test at low frequency sine waves. After putting gaskets on everything, I noticed the bass was a little deeper, cleaner and lacking just a tiny bit as you described. But it sounded way better. I have not braced the cabinet, yet. Needs bracing bad.
Thanks for your mods and measurements.
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The funny thing is; if you place fingertips on the walls of LS3/5A they are solid and little to zero panel vibration can be felt.... They sound rough from over powering them before it is noticeable. So I personally give that theory for LS3/5A 'bass' very little credence at all. They have an underdamped Qb, but very little cabinet contribution, and the Q hump is the cause of the apparent bass response.
What has happened here with the 752 in the first attempt is due to modification of the BR alignmend.
Cabinet vibration may not play a significant role at the "very lower" end, especially as the "tower" shaped cabinet is expected to be rather stiff at lowest frequencies.
Maybe somewhere above 100Hz, there might be influence of cabinet vibration.
Explaining lower bass performance with regard to cabinet vibration with this enclosure is "beside the point" IMO, as it is with nearly all cabinets, especially the small ones.
Cabinet vibration may not play a significant role at the "very lower" end, especially as the "tower" shaped cabinet is expected to be rather stiff at lowest frequencies.
Maybe somewhere above 100Hz, there might be influence of cabinet vibration.
Explaining lower bass performance with regard to cabinet vibration with this enclosure is "beside the point" IMO, as it is with nearly all cabinets, especially the small ones.
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With all due respect, this is completely incorrect. There is a reason why subwoofer cabinets which operate solely below 100Hz are designed to be as stiff and as massive as possible, even though their individual panel resonances occur above the passband. It is the driver motion itself causing excitations in the enclosure that said bracing eliminates.
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