Sealed with low Qts

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Dear all,

Lower Qts is often related to smaller boxes for speakers.

From playing with simulations it is clear that low Qts drivers are a requirement for sensible sized bass reflex speakers.

I know sealed boxes are not in fashion but they have big advantages in size over reflex arrangements.

People often say that their is a heuristic for deciding if a Driver "should" be in a bass reflex or sealed box.

What are the penalties in terms of sound quality of not following these heuristics, and say putting a low Qts driver in a sealed box? I am most curious about sound quality and want low distortion bass but super low extension is not super important to me.

Regards

Owen
 
Delay is related to frequency, not box alignment. It is typically high for vented alignments because thay are usually tuned lower. That said, high delay times (~50ms) at very low (<30Hz) frequencies are pretty much inaudible.


Closed box gives a more acurate delay time.

If you don't need extension the BR's then have less extension.
You just need to play with drivers EBP's.:)
 
I currently explore the solution sealed with low Qtc... and some 6 dB bass boost.
If Qtc < 0.5, the system is not oscillatory.
If not having drivers with sufficiently low Qts, low Qtc (more exactly low Qte) can be obtained through velocity feedback or through amplifiers connected such as giving a negative resistance output which lowers the apparent DC resistance of the voice-coil (which however may present a difficulty due its reactive behaviour).
 
Hi,

Another way of looking at delay time is that a sealed enclosure has the characteristic of a 12dB/octave high pass filter, a bass reflex is a 24dB/octave high pass. This means that around the tuning frequency, a sealed enclosure go through a 180 degree phase shift, and a sealed one 360 degrees. I don't believe this difference in group delay is audible, at least not to me.

On distortion: from my own experience I noted that BR typically has lower distortion in the bass region than the same driver in a sealed enclosure. The explanation I have concocted for that is that in a BR at Fb, the driver has very small excursion, and most of the sound is produced by the port. In a sealed enclosure, however, the driver excursion will be much larger to produce the same low notes, and this is a source of distortion.

About Qts: .37 -.4 I find gives best results for BR and sealed; try and sim it. A lower Qts is nice if you want to build smallish enclosures, but the penalty is that it will produce less bass. So, with a low Qts driver, you want to set out with a driver with a low Fs, because in the enclosure the F3 point will go up. With high Qts drivers it is the other way around. In a BR, you will actually be able to go below the Fs of the driver. In a sealed enclosure it is not possible to get much output below the Fs of the driver (unless resorting to electronic compensation a la Linkwitz).

vac
 
Member
Joined 2008
Paid Member
excellent

Hi,

Another way of looking at delay time is that a sealed enclosure has the characteristic of a 12dB/octave high pass filter, a bass reflex is a 24dB/octave high pass. This means that around the tuning frequency, a sealed enclosure go through a 180 degree phase shift, and a sealed one 360 degrees. I don't believe this difference in group delay is audible, at least not to me.

On distortion: from my own experience I noted that BR typically has lower distortion in the bass region than the same driver in a sealed enclosure. The explanation I have concocted for that is that in a BR at Fb, the driver has very small excursion, and most of the sound is produced by the port. In a sealed enclosure, however, the driver excursion will be much larger to produce the same low notes, and this is a source of distortion.

About Qts: .37 -.4 I find gives best results for BR and sealed; try and sim it. A lower Qts is nice if you want to build smallish enclosures, but the penalty is that it will produce less bass. So, with a low Qts driver, you want to set out with a driver with a low Fs, because in the enclosure the F3 point will go up. With high Qts drivers it is the other way around. In a BR, you will actually be able to go below the Fs of the driver. In a sealed enclosure it is not possible to get much output below the Fs of the driver (unless resorting to electronic compensation a la Linkwitz).

vac

Sir: Your written essay above is about the best I have read in these pages.
For some reason I do not understand, the BR enclosure gets included into the list of evil audio folklore.
Thankyou kindly for your post.
p.s. It's been my experience that creating sawdust has much greater merit than running "sims".
 
..............a sealed enclosure has the characteristic of a 12dB/octave high pass filter, a bass reflex is a 24dB/octave high pass. This means that around the tuning frequency, a sealed enclosure go through a 180 degree phase shift, and a sealed one 360 degrees. ..........
Is this true?
What is the difference between tuning frequency and roll off frequency (F-3dB of a Butterworth)?
 
People often say that their is a heuristic for deciding if a Driver "should" be in a bass reflex or sealed box.
Hmmm...really ?
Typically, you will find that low Qts drivers also have a limited xmax because in a vented design you need less excursion capability. If you put such a driver in a CB then xmax might be exhausted very soon leading to increased distortion.

Another variable to look at is the motor itself. There are many underhung designs for vented boxes. But real good bass comes from overhung motors. So a bass driver for a closed box would have both, a decent xmax and an overhung motor.
I'm not sure where heuristics come into play here ;)
 
"Reversal of phase" (+90º) at tuning frequency can be observed on a typical vented enclosure (BR) and phase linearity with the closed enclosure. (Vented in red.)
 

Attachments

  • vented.gif
    vented.gif
    41.1 KB · Views: 925
  • closed.gif
    closed.gif
    23.9 KB · Views: 916
Is this true?
What is the difference between tuning frequency and roll off frequency (F-3dB of a Butterworth)?


Hi,

The tuning frequency with a BR is where the port and driver are completely in anti-phase. This is also where the driver is at its minimum excursion in the bass region, and this is how you can measure it: where the driver excursion is minimum, back EMF is minimum, and driver impedance also shows a minimum. Another way to measure it with only a signal generator, is to put the loudspeaker on it's back, throw a couple of grains of rice on the cone, and look for the bass frequency where they dance the least. The point where the loudspeaker FR is down -3dB can be at, below, or above the tuning frequency.

With a sealed enclosure, the frequency is the point where the impedance reaches a maximum, quite the reverse situation from a BR. This is where the spring formed by the driver suspension and the air cushion below the cone coupled to the moving mass have their maximum resonance. This point is always above the resonance frequency of the bare driver. At this point, the response can be expected to be 3dB down.

Butterworth is just EEspeak for a filter with a Q of .7, and both BR or sealed enclosures can be designed to have such a Q, or a higher one (by making the enclosure smaller), or a smaller Q (by making it larger). A higher Q gives you a bump in the response before it starts to go down, whereas a lower Q provides a more gradual decline.

Where exactly the -3dB point in the FR is can be approximated by calculations or sims, but there are so many variables that you really need to measure to find out what it is in a specific implementation.

vac
 
Andrew, I reread my post
Originally Posted by vacuphile
..............a sealed enclosure has the characteristic of a 12dB/octave high pass filter, a bass reflex is a 24dB/octave high pass. This means that around the tuning frequency, a sealed enclosure go through a 180 degree phase shift, and a sealed one 360 degrees.

Sorry for the sloppy editing and the resulting confusion. Obviously a 24dB/oct filter (4th order) has a 360 degree phase shift and the 12dB/oct (2nd order) a 180 degree shift, see context. Please replace the last 'sealed' in this phrase with 'BR'.

vac
 
Member
Joined 2008
Paid Member
Subs and Mid-bass both

The phase shift introduced by a vented alignment will be less audible at the bottom of the speaker's bandwith. For example when the sub is vented and the mid is sealed.

The phase shift will be more audible and hard to work around when the mid woofer is vented and has to cross over to a sub.

Boris,
This has been my finding as well. I use a huge infinite baffle bass system, which is mono, then crossing to mid-bass units @ 50 Hz. Originally I tried running vented mid-bass units, but when switching over to sealed, everything just sounded "right".
I might add here that there's nothing evil about vented bass systems, but every application has a better fit. Of course, there are a great number of variables in every situation. I have heard both excellent and poor implementations of both methods.
 
Member
Joined 2008
Paid Member
obvious

Andrew, I reread my post
Originally Posted by vacuphile
..............a sealed enclosure has the characteristic of a 12dB/octave high pass filter, a bass reflex is a 24dB/octave high pass. This means that around the tuning frequency, a sealed enclosure go through a 180 degree phase shift, and a sealed one 360 degrees.

Sorry for the sloppy editing and the resulting confusion. Obviously a 24dB/oct filter (4th order) has a 360 degree phase shift and the 12dB/oct (2nd order) a 180 degree shift, see context. Please replace the last 'sealed' in this phrase with 'BR'.

vac

Hey vac,
I knew what you meant the first time I read that, but yes, I can see where it could be mis-leading. Still, all-in-all, a great explanation !!
 
Boris,
This has been my finding as well. I use a huge infinite baffle bass system, which is mono, then crossing to mid-bass units @ 50 Hz. Originally I tried running vented mid-bass units, but when switching over to sealed, everything just sounded "right".
I might add here that there's nothing evil about vented bass systems, but every application has a better fit. Of course, there are a great number of variables in every situation. I have heard both excellent and poor implementations of both methods.

Scott, thanks for the kind words.

Interested in knowing how you implemented your infinite baffle bass system.

Right now I am working over a loudspeaker that a friend of mine designed (great looking), meant for use with a subwoofer. However, in sealed form they would only go down to 180 Hz or so, way too high. So, the only solution is to turn them into BR, which will bring them down to about 80 Hz. Still too high in my opinion, but about as good as it gets with this driver. However, I already know now that the aditional 180 degrees of phase shift in the cross over region will cause trouble, certainly at a highish 80 Hz. If you use a second order high pass on the satelites, the total phase shift will be 540 degrees! To properly align this with the subwoofer will require quite some measurements, plus most likely a DSP to do the filtering. It's a pretty hard to get right, and it remains to be seen how my friends speakers will turn out.

This also by means of explanation why your setup with BR midwoofers may have been disappointing; it is quite a puzzle to get the group delays of satelites and subwoofer properly aligned and timed.

vac
 
Last edited:
Hmmm...really ?
Typically, you will find that low Qts drivers also have a limited xmax because in a vented design you need less excursion capability. If you put such a driver in a CB then xmax might be exhausted very soon leading to increased distortion.

Another variable to look at is the motor itself. There are many underhung designs for vented boxes. But real good bass comes from overhung motors. So a bass driver for a closed box would have both, a decent xmax and an overhung motor.
I'm not sure where heuristics come into play here ;)

IME the vast majority of woofers available are of the overhung motor (long coil, short gap) design.
The only underhung (short coil, long gap) ones that spring to mind are ATC and one series of Tang Band drivers.
Underhung designs promise lower distortion but reduced efficiency and they require huge amounts of magnet material, leading to higher costs.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.