One thing I thought was interesting was that I'd heard in open baffle that the restoring force of the box air helps linearize the driver. While I have not measured that effect or seen measurements of it, another interesting effect I noticed is that in open baffle, every harmonic of distortion is 6db higher sensitivity up to the baffle frequency. So for example if a woofer plays a 20 hz signal with a -20db second harmonic or 10%, it would actually be -14db or 20? due to the higher frequency at 40hz. And a 4th order harmonic for example would be 12db more sensitive and would increase the 4th order distortion percentage by a factor of 4. So every harmonic distortion would be multiplied by its order, up to the baffle frequency.
Just thought this was an interesting thought I had had and this may contribute a lot to why we look for low distortion woofers in open baffles and try to keep excursion down.
Just thought this was an interesting thought I had had and this may contribute a lot to why we look for low distortion woofers in open baffles and try to keep excursion down.
Open baffles are an exercise in futility for low frequencies, yet, amazingly enough, still enjoy a cult following. I hear the distortion as well. The owners/builders are usually in denial. Sorry about that ugly truth. I'm sure I'll get blasted, as usual.
Engineering is often a balance act of compromises.
Linear vs nonlinear distortion in this case..
Where's Earl?
Linear vs nonlinear distortion in this case..
Where's Earl?
Haha join the club! Whilst religion and political debate is prohibited here, the main subject matter (DIY audio) has its own pontiffs and prophets 😀
Well, something's gotta suspend the woofer moving assembly and keep it from leaving the magnetic gap.
If it's not the air inside an enclosure, you'll need a stiffer suspension to compensate, so I'd say your findings are right on.
The fix? Theoretically the same woofer but with higher mechanical Q.
If it's not the air inside an enclosure, you'll need a stiffer suspension to compensate, so I'd say your findings are right on.
The fix? Theoretically the same woofer but with higher mechanical Q.
Oh I wasn't bashing OB at all. I love open baffle and think it's great. As someone said engineering is a balancing act of pros and cons and this is just one I'm willing to deal with. There are ways around it by having a subsonic woofer to take over everything below say 40-50hz where the crossover is unnoticeable or you can do what I did and listen at reasonable levels with super overkill on the woofers so you use like a tenth of the xmax, and I just deal with the distortion at higher volumes since I'm usually not critically listening at loud enough volumes for it to matter.
Also Id like to see some facts about how the air in a box linearizes the woofer. I feel like when pushed to a similar xmax, you would find similar distortion of the cone movement whether it's in a box or not. Using an accelerometer on the cone rather than measuring the distortion to compare ob and sealed would be an interesting experiment.
In a closed box air pressure changes works as a spring. It gives power compression but thus reduces distortion. In an open baffle there is no air spring and we must equalize due to acoustic dipole loss - we need loads of more power which means more travel and more distortion to reach same acoustic power (spl).
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Actually I haven't seen a comparison of same driver in closed box and equalized ob. Perhaps it is irrelevant? One of main benefits of bass reflex is reduced distortion in bass and all simulation programs know how to compare closed vs. BR.
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Actually I haven't seen a comparison of same driver in closed box and equalized ob. Perhaps it is irrelevant? One of main benefits of bass reflex is reduced distortion in bass and all simulation programs know how to compare closed vs. BR.
The dipole need less power than the boxed woofer for a given excursion.
The dipole must move much more than the boxed woofer though so driver nonlinearities will be much worse. Also since the fundamental will be cancelled more than the harmonics or upper range IMD this means that distortion vs bass ouput will be even worse.
It's really a stupid idea.. and I'm guilty of using and liking it as well. 🙂
As mentioned, with decent dimensioning and moderate output things are not so bad. I often check the woofers when I hear good omph in the bass only to notice that the woofers only move a mm or so.
For the effect of the box air compliance vs driver suspension compliance you need to look at the specific case.
Air nonlinearities are what they are (can be calculated for given box volume and driver displacement) and driver suspension nonlinearities differ a lot from design to design.
No dogma though, helmholtz, infinite baffle, acoustic suspension, dipole.. anything goes as long as it sounds good and I've heard all of them do that.
The dipole must move much more than the boxed woofer though so driver nonlinearities will be much worse. Also since the fundamental will be cancelled more than the harmonics or upper range IMD this means that distortion vs bass ouput will be even worse.
It's really a stupid idea.. and I'm guilty of using and liking it as well. 🙂
As mentioned, with decent dimensioning and moderate output things are not so bad. I often check the woofers when I hear good omph in the bass only to notice that the woofers only move a mm or so.
For the effect of the box air compliance vs driver suspension compliance you need to look at the specific case.
Air nonlinearities are what they are (can be calculated for given box volume and driver displacement) and driver suspension nonlinearities differ a lot from design to design.
No dogma though, helmholtz, infinite baffle, acoustic suspension, dipole.. anything goes as long as it sounds good and I've heard all of them do that.
Small box, high pressure means higher distortion.
If you're lucky you can partly cancel the asymmetric part but the symmetric nonlinearities will be worse and cause symmetric compression and distortion.
If you're unlucky the surround will collapse by the stress.
+1 on the above.
I tried an OB fairly recently. Had a couple of spare boards around, so threw a few drivers at them.
1x EV DH1a on HP64 horn
1x Faital 10FH520
1x Beyma 15P1200Nd
Active crossovers around 150Hz and 1.2kHz, brute-force EQ'd everything flat from about 25Hz upwards (around 25dB of boost at the bottom end).
I did try running without the LF drivers - got to about 60Hz at good listening levels before the excursion-related distortion became a problem. With the 15"s, I was seeing a few mm of excursion at reasonable levels, and up to 20mm p/p when turned up. The 15"s ran out of headroom first, unless playing small-scale singer + guitar music.
IMO, using nice drivers and active crossovers is the way to do it (for anything, really). An Alpha-15 on the end of an ohm or two of inductor isn't my idea of fun.
Chris
I tried an OB fairly recently. Had a couple of spare boards around, so threw a few drivers at them.
1x EV DH1a on HP64 horn
1x Faital 10FH520
1x Beyma 15P1200Nd
Active crossovers around 150Hz and 1.2kHz, brute-force EQ'd everything flat from about 25Hz upwards (around 25dB of boost at the bottom end).
I did try running without the LF drivers - got to about 60Hz at good listening levels before the excursion-related distortion became a problem. With the 15"s, I was seeing a few mm of excursion at reasonable levels, and up to 20mm p/p when turned up. The 15"s ran out of headroom first, unless playing small-scale singer + guitar music.
IMO, using nice drivers and active crossovers is the way to do it (for anything, really). An Alpha-15 on the end of an ohm or two of inductor isn't my idea of fun.
Chris
hi I have got to ask
have you tried open baffle
And the claims that you state are they from your own open baffle designs
or not,
your opinions seem to be very subjective, and not objective as to one who has measured or experienced these things.
open baffle is good very good indeed I speak from experience not subjective but from objectivebly
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It's what reaches my ears that I care about and OB interacts with the room in a different way than a box speaker - reflections and room loading are their own source of distortion. I would not want to see a woofer moving more than a mm or so anyhow.
The topic is touched upon on this page:
Woofer measurements
"It is interesting to note that a 830500 driver in an open baffle has somewhat lower distortion at the same excursions and frequencies as above. The enclosed air in the THOR box stiffens and linearizes the driver suspension, but it also causes more roll-off in low frequency response. Thus the distortion generated harmonics are of higher amplitude for the closed box than the open baffle. In addition, linearizing the compliance does not necessarily lead to better overall driver performance, because it might change the degree of cancellation that existed between different distortion mechanisms. The acoustic output at the same excursion, though, is higher for the closed box."
As mentioned, obviously there are trade-offs to open-baffle woofer systems. I still prefer them over the alternative though. 🙂
Cheers,
Dave.
Woofer measurements
"It is interesting to note that a 830500 driver in an open baffle has somewhat lower distortion at the same excursions and frequencies as above. The enclosed air in the THOR box stiffens and linearizes the driver suspension, but it also causes more roll-off in low frequency response. Thus the distortion generated harmonics are of higher amplitude for the closed box than the open baffle. In addition, linearizing the compliance does not necessarily lead to better overall driver performance, because it might change the degree of cancellation that existed between different distortion mechanisms. The acoustic output at the same excursion, though, is higher for the closed box."
As mentioned, obviously there are trade-offs to open-baffle woofer systems. I still prefer them over the alternative though. 🙂
Cheers,
Dave.
front wave / back wave
The sheer fact that the front wave cancels with the back wave, to me, makes them a non viable option for any device that has the task of producing bass.
I'd rather spend more time and energy investigating on what type of enclosure construction yields less and less "box" colouration.
I have had pretty good results with the forced-reaction-cancelling types, such as the ppsl (push-pull, slot load)
That gentlemen, is a device that WILL produce bass.
Peace.
The sheer fact that the front wave cancels with the back wave, to me, makes them a non viable option for any device that has the task of producing bass.
I'd rather spend more time and energy investigating on what type of enclosure construction yields less and less "box" colouration.
I have had pretty good results with the forced-reaction-cancelling types, such as the ppsl (push-pull, slot load)
That gentlemen, is a device that WILL produce bass.
Peace.
I don't post much any more but one thing I have always taken issue with is the idea that the compliance of a sealed box is a linearizing factor. It can be argued that IF the force applied to the driver by the box compliance is linear, then the total force applied to the drive has a greater linear component and therefore the nonlinear components will be less. However, the issue is that the force due to the box compliance is not linear. It goes like
F = X /[Cab *(1 + Sd * X/Vo)^2.4]
where X is the driver displacement, Sd = cone area, Vo is the box volume when the driver is at rest, and Cab is the conventional box compliance. The factor (1 + Sd * X/Vo)^2.4 shows that Cab is not actually constant but varies nonlinearly with driver displacement. If Sd * X is small compared to Vo (small drive in big box, very small displacement, etc) then Cab can be considered constant and there is little nonlinear contribution. But, for a large woofer in a smallish box the nonlinearity of the box compliance can be significant and an increase in nonlinear distortion results.
F = X /[Cab *(1 + Sd * X/Vo)^2.4]
where X is the driver displacement, Sd = cone area, Vo is the box volume when the driver is at rest, and Cab is the conventional box compliance. The factor (1 + Sd * X/Vo)^2.4 shows that Cab is not actually constant but varies nonlinearly with driver displacement. If Sd * X is small compared to Vo (small drive in big box, very small displacement, etc) then Cab can be considered constant and there is little nonlinear contribution. But, for a large woofer in a smallish box the nonlinearity of the box compliance can be significant and an increase in nonlinear distortion results.
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