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No port? But bass reflex, how do we work out enclosure alignment?

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  • No port? But bass reflex, how do we work out enclosure alignment?

    I have written previously about the Visaton 890 MK3 and need to understand more about its enclosure alignment. For DIY purposes, so I can tweak it to my liking. But more importantly to be confident that the alignment will suit my preference. Now unfortunately, Visaton is not being really helpful other than boasting about its tight bass which I have come to never ever believe as alignment designs of SBB4 or QTC's of under .5 is very uncommon these days.

    In general I would love to understand more about the enclosure type, usually you just drop in the box size in Winisd and its as simple as that. But the Visaton has no port its just an open vent. Plus to add to the confusion it has 2 woofers, how do I account for the two woofers in Winisd?

    And if its a bass reflex than how is the size of the vent measured according to alignment or in general in Winisd?
    Attached Files

  • #2
    Any opening in a cabinet qualifies as a port and this is easily modeled in WinISD, you can change the port shape from circular to square and then alter the dimensions to achieve a slot port as in the picture you posted. If the port has no depth beyond the thickness of the baffle then it will be tuned relatively high but a slot placed along one of the cabinet walls gains some length from that boundary so the actual tuning will be a little lower than the software suggests The software also allows you to select the number of drivers you want to use when setting up a simulation.
    Paul O


    • #3
      To expand on what Paul said it is a port, with the depth being the thickness of the baffle. As for the two woofers, WinISD accounts for that in the very first data entry screen, where it asks 'Number of drivers and placement'.


      • #4
        Working off the cab drawings, it looks like the reflex box is < 6.5 cu.ft. w/a rather short (4" ?) slot equivalent to a round 6"id x 4" long port, yielding an Fb in the high 30s w/an F3 in the mid 30s. THOSE 12" woofers in there model w/about a +1dB "lift" between 40-50Hz (right where a lot of living rooms are kinda boomy).


        • #5

          Now, the hardest bit is to identify how the transfer function correlates to the alignment.

          I know that in Winisd once you change the port settings the chosen alignment changes but i can get an idea of what the original alignment looked like(when choosing SBB4 etc) and compare it to what I got. But I am still unfamiliar in general to hazard a guess what the transfer function is telling me in regards to the bass reflex alignment and how tout the bass will be. (putting aside all other factors that contribute to tight bass)

          I also am familiar that a longer port usually gives you the more taut bass and aware how it effects the transfer function, so I can play with that. What is also cool is that I can block the vent and the closed box QTC ends us under .5 which is great.

          Chris the port dimensions are 79mm in height 340 mm in length and the depth is about 90.

          Regarding boominess, I have audiolense to flatten out the frequency curve. My concern is not so much the fb I will have 2 subs I just would like transient bass or alignment that has better transients.

          Below I added what i got in Winisd with 2 drivers and the square port with its dimensions. I understand that visaton has designed the speaker the way it is but understanding how and why they designed it will at least give me the DIY ability to make good judgement calls one to either purchase it or not and second to tweak the things we love doing with DIY.

          Can anyone guide me with the the transfer function and how it correlates to the bass reflex alignments?


          • #6
            I'm pretty sure that slot is 60mm high (you're looking at its OD).

            "Bass reflex alignments" are old school. Even if they weren't, this one is "custom". The alignment is just what you see, once you get the port right.


            • #7
              +1. Back when cabs were designed using slide rules and scientific calculators alignments were created, to reduce the hundreds of possible permutations to a manageable few. They were rendered obsolete with the advent of speaker modeling software that allows one to see how as many of those possible permutations function as one wishes. WinISD still uses alignments as a starting place, as you have to start somewhere, but once you have an initial result you're free to change the box volume and tuning frequency as much as you want, with each alteration giving a new 'alignment'.


              • #8
                ^ I agree. Alignments were important when you had to pick an 'alpha' to plug into an equation to arrive at box tuning. Computer modelling pretty much wiped that out.

                edit: Looks like Bill beat me to the post. I still agree.


                • #9
                  That is very helpful thank you. I had no idea that the alignments weren't so important in bass reflex, well I suppose they are but not what alignment it is per se.

                  But am I under the correct impression that the roll off of the transfer function is related to how the transients in the bass reacts? I am sure I have been through posts which explain this and spoken to some experts about it.

                  I am aware that the group delay gives us a kind of an indication of transients but I thought the roll off what is what we look at.

                  Now, you say the box doesn't necessarily have an alignment, so how to go about in guessing how tight the bass is? Or is there any other response I can look at in free software? And if I am right in saying the roll off is what we look at is there any information, articles or examples or a brief explanation on how to understand which roll off correlates to bass transients.

                  And lastly is increasing the port length going to be the best option in providing more taut or transient bass?


                  • #10
                    written by Josh Ricci of Data Bass:

                    Myth: Bass Speed

                    Subwoofers are Fast / Slow

                    It is a common misconception that sound quality is related to the woofers speed or quickness, but in fact the woofer's speed is related to the frequency it is producing and the SPL. There are two ways to change a woofer's speed. 1. Change the frequency of the signal it is reproducing or 2. increase the output / excursion. It may sound counter-intuitive, but it is true. There are many other factors that go into making a subwoofer sound fast, or slow, or boomy, tight, airy, or whatever subjective terminology is applied, but that divulges into system design and room acoustics primarily, rather the driver itself. Bass wavelengths by definition are slow to develop. Try listening to a subwoofer operating by itself sometime.

                    Another important concept along this line of thinking is that a subwoofer with deeper extension will tend to sound slower than a unit with less extension. This should not be surprising. The deepest frequencies are the longest and slowest to develop. If you are not used to very deep frequencies being reproduced it may sound less accurate or slow, but in reality the sub with deeper extension is reproducing content that the less extended unit is simply not producing well or at all. The fastest sounding bass is when there is no bass.

                    The concepts of overshoot, difficulties overcoming inertia, or ringing after the note ends also fall into this category. These terms indicate smearing in time of the signal. Fortunately it is relatively simple to look at a few things to determine if there is a problem. The driver or system "Q" or damping is one way. There is also group delay, energy decay, etc. Most studies show that the people are insensitive to even moderate amounts of energy delay in the bass range. Generally what causes these types of subjective terms to be used are differences in frequency response, rapid changes in response vs frequency and issues due to room or vehicle acoustics. In most cases the effect that the room has on the subwoofer response and energy decay rates dwarf those inherent to the design, unless the sub exhibits some truly bad issues. What's most important about this myth is that speed is an inappropriate concept of sound quality.

                    Smaller woofers sound faster than larger woofers

                    One of the biggest myths about woofers is that smaller units like 8's and 10's sound "tighter" and "faster" than 15's or 18's. Even bigger drivers such as 21's and 24's are supposedly worse than 15's or 18's. This can be true in some specific cases, but as a blanket statement of all drivers this is factually incorrect.

                    This subjective impression is often caused by factors other than the simple size of the driver. What tends to happen is that the smaller drivers have a lower Qts because manufacturers tend to share one motor design across many different sizes of drivers in the series. They may have the same motor on the 10" driver that they have on the 15 or 18". Unless the motor can compensate for the extra mass it has to push, then the Qts will not be the same as the smaller drivers. Ultimately the larger driver may not be suited for the same kinds of alignments as the smaller driver and ends up requiring a much larger air volume for the same system "Q" and response shape. Typically the larger driver will end up in a higher "Q" undersized system with a less linear response shape that may cause it to sound more uneven or "one note". It may also end up being tuned much deeper in frequency and the increased low frequency output may also cause it to sound "slower" or "heavier". Smaller subs and those using smaller driver sizes are often not asked to go as low and will not have the same low frequency extension in their design. When comparing subs the one with more low frequency extension and output will sound quite different. Having said that, high Qts drivers are not inherently inferior sounding when compared to more efficient, highly damped drivers, but they require larger boxes and less internal pressure to prevent response peaking and increased group delay.

                    Large sub drivers can be made to have the same motor to cone ratio as smaller drivers. The larger woofers also have more room for bigger suspension components and voice coils which can allow higher power handling, more excursion capability and larger mechanical clearances that simply cannot fit on an 8 or 10" driver frame. It is more expensive to make larger driver sizes in this way, but not impossible. There are a number of good examples on the market. It's just a matter of picking the right driver for the job.

                    Impulse Response Indicates Subwoofer Speed or Tightness

                    This goes back to the myth that some subs are fast / slow. An impulse response represents a ton of condensed information. Too much to compare them directly against each other at a glance and make determinations about performance. The quickest way to make a very short, abrupt, impulse response graph, is to remove all of the bass from the speaker being measured. Bass frequencies require a long time to develop and will extend out the impulse response by their very presence. Simply changing the frequency response balance of the system, or changing the frequency range covered by the measurement will change the way the impulse response looks. For these reasons it is almost impossible to eyeball a pair of impulse responses from units having dissimilar frequency response shapes, or measurement bandwidths and make a direct comparison.

                    There is more the Bass Myths article here:
                    Paul O


                    • #11
                      Originally posted by Trdat View Post
                      But am I under the correct impression that the roll off of the transfer function is related to how the transients in the bass reacts?
                      No. Audible transient response occurs in the frequencies well above where bass reflex operates differently than sealed. For that matter in blind listening tests when sealed and ported cabs are EQ'd for identical response one can't tell which is which. The only significant difference between the two is that ported can go louder in the low frequencies via lower excursion demands.
                      I am aware that the group delay gives us a kind of an indication of transients
                      That's not true either. Group delay upsets audiophiles to no end, because it looks like a big deal when charted. If a speaker has so much group delay that it can be heard it would be so flawed in all respects as to be unlistenable.
                      so how to go about in guessing how tight the bass is?
                      Define tight bass in engineering terms. You can't. It doesn't exist. Neither does fast bass or slow bass.
                      And lastly is increasing the port length going to be the best option in providing more taut or transient bass?
                      Increasing the port length lowers Fb. That's it.


                      • #12
                        Your telling me when I switched from my closed box SVS sub at around .9 QTC to a .7 QTC DIY sub then improved that same DIY sub to a .58 QTC I was imagining things?

                        There was clearly an improvement in the overhang, sounded so much better that when putting the SVS sub against the my DIY sub I threw the SVS sub out to the trash. Tight bass is no overhang, I can't define it i am not an expert but its there when you lower QTC, I know it doesn't have as much to do with the higher frequencies hence it might be something I dont need to worry about on the Visatons especially if I am crossing over with a sub.The Visaton should be a well designed enclosure but I am designing the Visaton in accordance to my taste and want to make sure the trade offs are in my favor.

                        How can transient bass not exist when clearly its obvious. What could I be hearing if it doesn't exist? I thought it was one of the most important parameters of designing woofer enclosures....

                        I totally understand bass cant be fast but there has to be a way to choose how transient the bass is with box volume and port length or by other measures?

                        Are you suggesting that this is something I don't need to worry about when designing an enclosure for woofers?


                        • #13
                          Have you ever heard of "Q" (more of a closed box function than a reflex box) described as a "quality" factor? MOST people are okay w/a closed box Q above 0.7, up to about 1.0 before it starts sounding "boomy". (You've described changing the Q on other subs you've had by making the box larger.) You can SEE the diff. between a (closed box) Q of 0.50 vs 1.0 by modeling in your box program. While the rollofff curve shape (the "Q") of a closed box gets less "steep" as the box gets larger, it's ALSO (probably) limiting the system's extension. If you NEED 40Hz in a room, and you've got a box w/a Q of .80 but it's also got an F3 of 20Hz, you can make the box larger (lowering the Q) but still have adequate extension (maybe still down to 30). OTOH if you've got a box w/a Q of 0.80 (but its F3 is BARELY 40Hz) and you make the box larger, your Q WILL drop, but you'll probably also lose your 40Hz extension. (NOTE: MANY air-suspension designs of the 60s, 70s, 80s (think AR, KLH, and ADVE\T - all Henry Kloss designs) did NOT have Qs near 0.50, or EVEN 0.70 ... Qs of .80, .90 and even 1.0 were pretty typical. They kinda needed Qs that high to hit that 40Hz F3.)

                          You can't mess around w/ANY closed box rolloff curve and duplicate the tuning you're looking at from your MK3s. THAT design is trying to achieve pretty high SPLs, and one thing they've done is (apparently) created a "hump" in the 40-50 Hz range w/their tuning. Yes, you CAN push that hump down (SIMILAR to lowering the Q of a closed box system) by making the port longer. You'll also lose extension.

                          If you feel you can hear that "hump" (and don't like it), an easier way to TRY lowering the hump (rather than lengthening the slot vent) would be to throttle down your port cross-section, like by inserting a wood block the length of the port. Instead of doubling (or tripling) the slot length (sorta harder to try), just block off 1/4, or 1/3, or even 1/2 the width of your slot (but it's got to be the entire length(depth) of the slot, not just the opening at the front). Sure, at high output levels you'd expect chuffing, but by keeping the SPLs lower (to avoid chuffing) you should still be able to assess the "Quality" of your rolloff situation. If you find a rolloff curve you like better, monkey around in WinISD to dupe THAT curve (permanently) by THEN lengthening your port.


                          • #14
                            Lower QTC = less low bass = better transient response just like the article says. And as stated in the article and from what I have seen countless times drivers with weak motors don't have as much control over the cone as those with stronger motors.
                            Paul O


                            • #15
                              Originally posted by Chris Roemer View Post
                              You can't mess around w/ANY closed box rolloff curve and duplicate the tuning you're looking at from your MK3s. THAT design is trying to achieve pretty high SPLs, and one thing they've done is (apparently) created a "hump" in the 40-50 Hz range w/their tuning. Yes, you CAN push that hump down (SIMILAR to lowering the Q of a closed box system) by making the port longer. You'll also lose extension..

                              I am happy to loose extension. But what I would like to understand is if I am crossing over the Visaton's to subwoofers around 150hz would the cutting out the lower frequencies from the Visaton's have the same effect on 150hz and above if I was to make the port longer? Or would I still need to extend the port to get rid of that hump even though its crossed over?

                              Essentially its just that hump that is not making it as transient as it could?

                              I can stuff the port and essentially get a lower QTC and make it more like a sealed box, I will loose sensitivity and SPL but its an option. But the same question applies here, if I am crossing over with subs at around 150hz would what is above 150hz be effected by the stuffing of the port if the crossover will ultimately cut of the long wavelength frequencies?