Hi everyone,
I would like to share my thoughts after trying to modify the worst sounding speakers in my modest collection - a beautiful pair of Blaupunkt Lab 208. They look like a million dollars but have problems with HF (very directional), boxy sound even when not hanging on the wall and almost zero bass under 100 Hz. About 5 l netto volume, closed cabinet with two (Audax?) 13 cm woofers and one paper cone tweeter. They even have a proper crossover with two solid air inductors and two bipolar caps.
The first thing to check when addressing the boxy sound was a textile mesh under a wooden grill (a wooden one with several holes about 11 mm diameter and 3 mm depth) strengthen with some transparent paint. This textile dust protection under wooden grill was so glued with some hard chemicals that under magnifying glass only one per 10-20 micro-holes was open. So first I cut out the textile protection from all the wooden holes (baffle) and replaced with an "acoustic transparent" open cell foam.
To my surprise it brought no effect. No impact on overall SPL or SPL in frequency domain graph. Zero. Null. The boxy sound remained, which was even visible on the measurements (REW, UMIK-1) in the form of a 120 Hz peak before very early bass roll-off. The next solution I came up with was to remove one of the woofers to change the system overall Q value to alter bass roll off characteristic. And yes, that brought some improvement (of course I added an L-pad to the tweeter and parallel resistance to the remaining woofer to match, sealed the baffle hole in place of missing woofer) but the boxy character remained. Replaced not-a-good-looking damping material with polyester wool (3M calls it Dacron) - no effect.
After some research to find the remedy for too small cabinet (these boxes are only 10 cm in depth) I found - that was what I had thought - a solution. Variovent. After reading several sentences on the web telling "oh, I just added a variovent to my subwoofer and it improved bass extension very much" I decided to give resistive loading a chance in my Blaupunkt Lab 208 speakers. As engineering of such s port was not very demanding, the Blaupunkts quickly received a stuffed area under the grille in place of a missing woofer.
And now... I am not sure what to tell 😉
After experimenting with different damping materials, port area, port diameter and damping density, there is inly one thing I can tell: resistive loading does not extend bass. It was obvious after first measurement and proved with the following ones. First thing was a drastic early roll-off on the LF side of graph. And the second was a change in a system's resonant frequency which increased so significantly that it made such a speaker system virtually useless.
As I do not have a specialized gear for impedance measurements, I did it the traditional way with a serious resistor and analog multimeter. Started with the whole "variovent" closed, so the closed cabinet. The woofer has Fs of about 45 Hz. In the closed 5 l cabinet with the other sibling woofer it increased to 100 Hz and with missing second woofer to 85 Hz. That was the starting point. And then I uncovered the holes in the wooden front of the baffle where a resistive loading port was located, one by one. The results destroyed my faith in this "variovent" solution.
Two holes of a port open - 120 Hz. 5 holes - 145 Hz. 15 holes - 185 Hz. And then up to about 220 Hz of a system's resonant frequency with a "variovent" port fully open. "Fully" means an open surface calculated with some equation found on the internet. And now the question comes: does resistive loading make any sense? Is it really a solution for too small a cabinet? Because from my point of view it is only usable in a very low tuned system, to make impedance curve less aggressive, but always at the cost of bass extension.
Could anyone please say or prove that I am wrong? 🙂 I am pretty sure I am because there were several constructions with resistive loading, can be found on the vintage market and they had to be working as some of them are classics. If you wish, I can share some photos of the work with the Blaupunkts Lab 208 as they are too pretty to believe they play so bad 🙂 They are very solid and beautifully made as a furniture, but have so many sonic problems it is hard to believe they made success on the market. Comb filtering of the tweeter signal because of a drilled-holes concept of a baffle? Not sure yet, but that could also be the case 🙂
Vario-vent, or not to vario-vent them?
Regards,
Mike
I would like to share my thoughts after trying to modify the worst sounding speakers in my modest collection - a beautiful pair of Blaupunkt Lab 208. They look like a million dollars but have problems with HF (very directional), boxy sound even when not hanging on the wall and almost zero bass under 100 Hz. About 5 l netto volume, closed cabinet with two (Audax?) 13 cm woofers and one paper cone tweeter. They even have a proper crossover with two solid air inductors and two bipolar caps.
The first thing to check when addressing the boxy sound was a textile mesh under a wooden grill (a wooden one with several holes about 11 mm diameter and 3 mm depth) strengthen with some transparent paint. This textile dust protection under wooden grill was so glued with some hard chemicals that under magnifying glass only one per 10-20 micro-holes was open. So first I cut out the textile protection from all the wooden holes (baffle) and replaced with an "acoustic transparent" open cell foam.
To my surprise it brought no effect. No impact on overall SPL or SPL in frequency domain graph. Zero. Null. The boxy sound remained, which was even visible on the measurements (REW, UMIK-1) in the form of a 120 Hz peak before very early bass roll-off. The next solution I came up with was to remove one of the woofers to change the system overall Q value to alter bass roll off characteristic. And yes, that brought some improvement (of course I added an L-pad to the tweeter and parallel resistance to the remaining woofer to match, sealed the baffle hole in place of missing woofer) but the boxy character remained. Replaced not-a-good-looking damping material with polyester wool (3M calls it Dacron) - no effect.
After some research to find the remedy for too small cabinet (these boxes are only 10 cm in depth) I found - that was what I had thought - a solution. Variovent. After reading several sentences on the web telling "oh, I just added a variovent to my subwoofer and it improved bass extension very much" I decided to give resistive loading a chance in my Blaupunkt Lab 208 speakers. As engineering of such s port was not very demanding, the Blaupunkts quickly received a stuffed area under the grille in place of a missing woofer.
And now... I am not sure what to tell 😉
After experimenting with different damping materials, port area, port diameter and damping density, there is inly one thing I can tell: resistive loading does not extend bass. It was obvious after first measurement and proved with the following ones. First thing was a drastic early roll-off on the LF side of graph. And the second was a change in a system's resonant frequency which increased so significantly that it made such a speaker system virtually useless.
As I do not have a specialized gear for impedance measurements, I did it the traditional way with a serious resistor and analog multimeter. Started with the whole "variovent" closed, so the closed cabinet. The woofer has Fs of about 45 Hz. In the closed 5 l cabinet with the other sibling woofer it increased to 100 Hz and with missing second woofer to 85 Hz. That was the starting point. And then I uncovered the holes in the wooden front of the baffle where a resistive loading port was located, one by one. The results destroyed my faith in this "variovent" solution.
Two holes of a port open - 120 Hz. 5 holes - 145 Hz. 15 holes - 185 Hz. And then up to about 220 Hz of a system's resonant frequency with a "variovent" port fully open. "Fully" means an open surface calculated with some equation found on the internet. And now the question comes: does resistive loading make any sense? Is it really a solution for too small a cabinet? Because from my point of view it is only usable in a very low tuned system, to make impedance curve less aggressive, but always at the cost of bass extension.
Could anyone please say or prove that I am wrong? 🙂 I am pretty sure I am because there were several constructions with resistive loading, can be found on the vintage market and they had to be working as some of them are classics. If you wish, I can share some photos of the work with the Blaupunkts Lab 208 as they are too pretty to believe they play so bad 🙂 They are very solid and beautifully made as a furniture, but have so many sonic problems it is hard to believe they made success on the market. Comb filtering of the tweeter signal because of a drilled-holes concept of a baffle? Not sure yet, but that could also be the case 🙂
Vario-vent, or not to vario-vent them?
Regards,
Mike
I think you may be expecting too much from the approach from a bass extension standpoint. In its basic form, it's not going to fix a seriously undersized cabinet or dramatically lower F3. From The Loudspeaker Design Cookbook: "The Variovent tends to damp the impedance of a closed box in the same way as adding 100% of high density fill material." . . . "The effects of the Variovent are very nearly the same as the 50/50 4 lb/ft3 fiberglass/Acousta-Stuf combination."
I also can't tell from your description if you were using an off-the-shelf Variovent or if you made your own resistive port. If you made your own, there could be issues with how you implemented it. If you got a significantly different system resonance frequency with the resistive port vs plain sealed enclosure, there's likely something wrong with how it was done.
Beyond that, it seems like you are equating a shallow box (10 cm depth) with one that's too small in volume. They are two separate issues. It's not obvious from your descriptions if you are trying to improve limited bass extension or a coloration related to the enclosure shape.
I also can't tell from your description if you were using an off-the-shelf Variovent or if you made your own resistive port. If you made your own, there could be issues with how you implemented it. If you got a significantly different system resonance frequency with the resistive port vs plain sealed enclosure, there's likely something wrong with how it was done.
Beyond that, it seems like you are equating a shallow box (10 cm depth) with one that's too small in volume. They are two separate issues. It's not obvious from your descriptions if you are trying to improve limited bass extension or a coloration related to the enclosure shape.
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Allow me to supply an illustrative image:
For reference purposes, I attach information on what a variovent is 'supposed' to do.
Attachments
Thjat was my response upon seeing your title.
VarioVent is a generic, one-size-fits-all ARU (acoustic resistance unit), i figure only moderatly useful.
One of my tech gurus did some of the most advanced research on aperiodic loading in speaker boxes. An ARU is used to decrease the hump in a loudspeaker that is smaller than you might want. Also have benefit in terms of flattening phase, making for an easier load.
My miniOnkens designs push a reflex towards aperiodic loading, and have done a lot of earlier experimentation.
Custom “VarioVents” are easily made, i liked to use plastic gutter mesh and fiberglass or cotton wall insulation. Like the Dyna A25.
dave
If we had T/S parameters for a driver it could help.
Looking at pictures of these driver types. Looks like typical television or radio speakers.
Likely very stiff cone for sensitivity and very small magnet = very high Qts.
Most likely want a very large box.
A large box to support a driver which likely has resonance around 150 Hz or higher.
So even in the " Right" box the bloomy boxy peak would go away. But never make much bass.
Sometimes a speaker just is what it is.
If you like the look and aesthetic of the vintage speaker case.
I would retro fit with modern new speaker.
Something that would work in a small box would actually be low Qts .3 or lower.
Typically very large magnets to make that work.
They are out there.
Keep in mind even modern drivers around 130mm or 5" wont do ground breaking bass.
But will be fast and responsive transients. With low Qts or powerful magnets.
Resonance wont be say 180 Hz like the vintage, maybe closer to 80 to 60 Hz.
Slightly better bass and no boxy boomy slow bass with weak magnets.
Looking at pictures of these driver types. Looks like typical television or radio speakers.
Likely very stiff cone for sensitivity and very small magnet = very high Qts.
Most likely want a very large box.
A large box to support a driver which likely has resonance around 150 Hz or higher.
So even in the " Right" box the bloomy boxy peak would go away. But never make much bass.
Sometimes a speaker just is what it is.
If you like the look and aesthetic of the vintage speaker case.
I would retro fit with modern new speaker.
Something that would work in a small box would actually be low Qts .3 or lower.
Typically very large magnets to make that work.
They are out there.
Keep in mind even modern drivers around 130mm or 5" wont do ground breaking bass.
But will be fast and responsive transients. With low Qts or powerful magnets.
Resonance wont be say 180 Hz like the vintage, maybe closer to 80 to 60 Hz.
Slightly better bass and no boxy boomy slow bass with weak magnets.
Looks like back is held on bt screws and easily removable. The Audax i have used were slightly mire moidern units, 2 x 4.5” woofers in a leaky closed box i would expect to struggle to get to 100 Hz.
dave
Quick rail goes back to Early war production. You can either punch 24 holes at once or drill 24 holes at once.
Actually can be as many holes as you want, Long as the press and die is long enough.
Looks like they went for drilling as many holes as possible in wood.
One way to make a speaker grill.
Distributive port, AKA bunch of holes. Not very useful unless you want high tuning.
Easier to use one vent and add length for lower tuning.
Otherwise velocity is too high, being all a port is = velocity ramp at set frequency.
Makes measurements appear to have more bass. Accuracy is what it is, no wires to a port.
I understand why the aesthetic is enjoyed, was fascinated with punched metal grills as a teen.
Many holes in wood much the same
Aperiodic aka stuff the krap out of a high tuned port so you dont hear it.
The real solution always the same. expect accuracy 2x above resonate frequency.
If you want good clean bass at 40 Hz you need a speaker with resonance down to 20 Hz.
Usually takes a sub to go down to 25 to 30 Hz resonance. They extended " bass" with a port
Is extra velocity with limited accuracy. Ports are not very efficient. For any extended " bass"
beyond 3 dB you use EQ. and boosting 40 Hz with a speaker tuned to 60 Hz is nonsense.
Subs are Subs, Woofers are Woofers, Mids are Mids. Hard to accept, Fs tells all.
Multiply the Fs by 2. That is where the story ends
Actually can be as many holes as you want, Long as the press and die is long enough.
Looks like they went for drilling as many holes as possible in wood.
One way to make a speaker grill.
Distributive port, AKA bunch of holes. Not very useful unless you want high tuning.
Easier to use one vent and add length for lower tuning.
Otherwise velocity is too high, being all a port is = velocity ramp at set frequency.
Makes measurements appear to have more bass. Accuracy is what it is, no wires to a port.
I understand why the aesthetic is enjoyed, was fascinated with punched metal grills as a teen.
Many holes in wood much the same
Aperiodic aka stuff the krap out of a high tuned port so you dont hear it.
The real solution always the same. expect accuracy 2x above resonate frequency.
If you want good clean bass at 40 Hz you need a speaker with resonance down to 20 Hz.
Usually takes a sub to go down to 25 to 30 Hz resonance. They extended " bass" with a port
Is extra velocity with limited accuracy. Ports are not very efficient. For any extended " bass"
beyond 3 dB you use EQ. and boosting 40 Hz with a speaker tuned to 60 Hz is nonsense.
Subs are Subs, Woofers are Woofers, Mids are Mids. Hard to accept, Fs tells all.
Multiply the Fs by 2. That is where the story ends
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Here are data from impedance measurements showing system Fr vs. "Variovent" area. The measurements were done by starting with all the vent area completely covered and then uncovering step by step. Blaupunkt Lab 208 with one of the woofers removed and a "variovent" made in its place, so firing front. I tried with a few different "variovent" prototypes, from a simple polyester wool under a mesh to a plastic pipe with the wool highly compressed. Always, always, nevermind what I did, any of these vents made overall resonance frequency of a system higher. I sacrificed some serious time for experimenting and now when I read somewhere in the net that resistive loading "extends bass response" it makes me call for exorcist 🙂 Here, the data:
...in a minute I will come back with some photos of these speakers, drivers, ports etc. For now the project is left with only one woofer per system and l-pad for the tweeter, parallel 8 Ohms for the remaining woofer and Dacron inside. It works far better than in its original form Blaupunkt made. Problematic HF radiation and a grille multiple holes comb filtering (that's what I suspect from the SPL graph) remain, part of a boxy nature is lost, but... It's still hard to believe that Blaupunkt made something sounding so bad. Mod to passive drivers is waiting, but that probably won't change the nature of these boxes. PS. Original drivers have huge magnets, almost the diameter of the cones. Very soft suspension, but no way to measure TS parameters because of the lack of proper gear. Still have a hope that someone here had better experience/success with resistive loading.
Best,
Mike
...in a minute I will come back with some photos of these speakers, drivers, ports etc. For now the project is left with only one woofer per system and l-pad for the tweeter, parallel 8 Ohms for the remaining woofer and Dacron inside. It works far better than in its original form Blaupunkt made. Problematic HF radiation and a grille multiple holes comb filtering (that's what I suspect from the SPL graph) remain, part of a boxy nature is lost, but... It's still hard to believe that Blaupunkt made something sounding so bad. Mod to passive drivers is waiting, but that probably won't change the nature of these boxes. PS. Original drivers have huge magnets, almost the diameter of the cones. Very soft suspension, but no way to measure TS parameters because of the lack of proper gear. Still have a hope that someone here had better experience/success with resistive loading.
Best,
Mike
So, here we go with some vintage audio porn 😉
This is how - apart from a piece of polyester wool - it came from Blaupunkt some 50 years ago:
...macro of a mesh under a wooden grille almost completely sealed with some dried-out chemical:
...the same from the front:
...old mesh ready to be cut out:
...new open cell foam is ready:
...one of the "Variovent" prototypes. Open area calculated from some equasion find on the web and then checked with A-25 vent:
...I could change the quantity/compression of the wool as well as a number of open holes:
...and now the "bass extension" after many experimenting 😉
...and finally something for the Blaupunkt Fetishists. Maybe I'll run a separate thread on these Lab 208 boxes, have tons of photos, some measurements, can advise what to do and what to do not with these beauties:
And still, all the resistive loading supermen who succeeded with such a solution, are highly welcome.
All the best,
Mike
This is how - apart from a piece of polyester wool - it came from Blaupunkt some 50 years ago:
...macro of a mesh under a wooden grille almost completely sealed with some dried-out chemical:
...the same from the front:
...old mesh ready to be cut out:
...new open cell foam is ready:
...one of the "Variovent" prototypes. Open area calculated from some equasion find on the web and then checked with A-25 vent:
...I could change the quantity/compression of the wool as well as a number of open holes:
...and now the "bass extension" after many experimenting 😉
...and finally something for the Blaupunkt Fetishists. Maybe I'll run a separate thread on these Lab 208 boxes, have tons of photos, some measurements, can advise what to do and what to do not with these beauties:
And still, all the resistive loading supermen who succeeded with such a solution, are highly welcome.
All the best,
Mike
You say, Mike, that the volume of the enclosure is about 5 litres net (about 0.18 cu ft).
The rule of thumb for a home made resistive vent according to David B. Weems is that it should have an area of 10 sq in per cu ft of enclosure volume.
In your case that would indicate a vent area of 1.8 sq in (11.6 sq cm) which, according to your graph, would result in an enclosure resonance frequency of just over 150 Hz.
To make the vent any larger in area would be counterproductive.
EDITED due to mistake in unit conversion!
P.S. I love your internal shots - very interesting.
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I do not have my "Loudspeaker Design CookBook" on hand for now, but IIRC, Vance Dickason studied the Variovent, and explained that it is a way to "shape" more or less the impedance curve of a loudspeaker and attenuate some unwanted resonances.
T
T
...and that tells the truth: resistive loading makes any sense only in a very, very low tuned system. Or the one designed to work with subwoofer. But for sure tuning this simple device is not that simple as some could expect. PS. If you like the photos - here we go 😉
New open cell foam waiting for the glue to dry under preassure:
New open cell foam fine-sealed with a special matt neutral silicone:
Now from the front:
The tweeter with a mesh dust cup and a Faraday copper ring underneath:
An L-pad for the tweeter almost ready:
...and now ready:
L-pad for the tweeter and power resistor for the woofer:
Filled out with Dacron:
...and on place with Dual system from the rigth times:
For now the Blaupunkts have only one woofer per box and sound much better than in the factory double-woofer form. Sealed enclosures remained. No variovent. The Dual CV20 "amplifier" is a joke, turntable fantastic, C901 tape deck visionary, tuner has a wooden (playwood) main PCB 😉 but that's exactly as old-school as it should be. When a Variovent-hero appears on this forum and tells how-to, resistive loading may return 😉
Regards,
Mike
I disagree. It is my preferred technique for midband enclosures. Works well.
dave
Eyeballing things from a distance, yours may have been acting like a lightly damped port instead of like a Variovent. That could explain why the Fb went up when the vent got more open (essentially larger diameter). The depth of your cylinder is significant, and the damping material doesn't look that dense. How that interacts with your perforated opening could be modeled, but I'm not sure an accurate "seat of the pants" estimate can be made (unless it works out that it's mostly behaving like an array of 16 ports of x diameter and x length).
The black side of the ScanSpeak model has fibers that are quite tight. The open area overall is significant, and the depth is minimal.
None of this means that a properly functioning aperiodic enclosure is going to do what you originally wanted, but it might point you in some directions about why things behaved the way they did in your test.
https://www.madisoundspeakerstore.com/flares/ports/vents/scanspeak-290001-aperiodic-vent-ssv/
In one of the "prototypes" I just stuffed the original wooden-drilled baffle with polyester wool from the back and pushed it with a plastic mesh as Scan Speak or Dynaudio did. The measurements were pretty the same as with the pipe and drilled cap. Front baffle holes are about 11 mm in diameter and about 3 mm in depth - a bass-reflex port with such dimensions would appear on the FR graph somewhere much higher than 100-200 Hz region if I had done the math properly.
Hi Dave,I disagree. It is my preferred technique for midband enclosures. Works well.
dave
could you axpand/explain the above graph more detailed? Is "TL" a transmission line?
Best,
Mike
Dave,
to Dyna A25 resistive loading solution from your picture: was the filling made of rockwool/fiberglass or some other material? Was it safe to have these speakers e.g. in sleeping room as fiberglass/rockwool may produce dust?
Best,
Mike
Back in the late 1960s, no one seemed to be bothered about such matters!
The filling in the port was simply referred to as "acoustically resistant material".
This is what Dynaco had to say about the A-25:
"The aperiodic design is not a bass-reflex approach, since there is no acoustic output through the port. The characteristics of the "plug" in the port are quite critical, necessitating individual adjustment of each system. This added acoustical impedance damps the woofer, improving its response to transient signals. Examination of the woofer cone motion shows that, with this aperiodic design (on which patents are pending), the cone follows the input signal all the way down to DC with far greater precision than is the case with either bass-reflex or acoustic-suspension designs."
The way that Dynaco adjusted the critical density of the aperiodic vent filling was "by observing the back EMF of a 5 Hz square wave which has been fed into the speaker system. Damping material is added and compressed until an optimum square wave is shown on an oscilloscope."
It is the imedance of the midTweeter in Tysen V2. It is a 10:1 aperiodicaly stuffed transmission line about 27 cm long. With the taper and length about 250 Hz tuning.
Woofers in a PP ML-TL that reachs about 35 Hz.
dave