Best vented transient response

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I've searched this forum (and dickason): for a given driver - how to best model boxes to predict and optimise vented box transient response . . some learnings:
"more gradual rolloffs have better transient response than steeper rolloffs"

(If I were doing sealed, I'd aim for Qt of about 0.7, but I have some big drivers suited for vented)
"wisely chosen vented alignments can produce bass as refined as a moderate Qtc sealed alignment".

~ Unibox simulates step response/ tone burst
~ Win ISD simulates group delay very interactively.

I believe that settling time (rise time + the time of any "ringing") step response is generally the better compromise of fairly fast rise time with a slight overshoot, that best predicts transient response.

. . ie rather than Win ISD simulated group delay - use Unibox's step response

if I am right, the next step along this line of optimisation is looking at step response graphs - at what point is settling time achieved, and how much time difference is signifcant . .

The multiple wiggles of a vented box's step response make comparisons hard - who could comment on my suggestion of a good yardstick -

the mimimum time by which (Unibox's) step response is no more than ~ 0.1(?)

Thanks
 
The alignment called SBB4 in Dickason is the best vented enclosure for transients, and all I use. Current project uses Scanspeak 25W (the lower Q version) in 6 cu ft, with the box tuned to ~18 Hz. Bass is very clean, no hangover or "one-note-ness".

Several advantages;
Earlier mild rolloff helps compensate for room gain.
Driver is loaded to <20Hz.
Driver excursion is reduced as you approach resonance.
And in my experience, tuning is not particularly critical.

The principal disadvantage is the box size. I also used this once in a commercial design, the 18W tuned to 28?? Hz or so, also with good results.

As a general bias, I believe that keeping the driver loaded for all program content is a good idea, so for smaller drivers I prefer sealed.
 
Beyond SBB4 . . but not Onken

Of the normal vented alignments, yes the SBB4 with those attributes is very appealing. (And the Scanspeak *W's with the SD1 motors are great drivers).

If a driver is suited to the Onken alignment - very high Vas I think is most important - and you can live with a giant box, reports suggest the Onken is best vented enclosure for transients. the Onken seems to be the final step along the spectrum of larger box, tuned low, yielding slow roll off and better transients.

I'm looking at the best way to use a 15" JBL pro driver (the 2035, with a big but not massive Vas of 192 litres). Onken_calc.xls suggests (depending on Rg) a 285 litre box, but I don't want it that big :smash:
The priority is transient resonse rather than depth.

planet10 said in a similar thread a while back
"The classic vented box alignments are an artifact of a time when we didn't have personal computer programs - and not even any calculators. back then it was a long tedious process to calculate out even one alignment. So they picked a few spots on the curve and generated tables & graphs so that anyone could easily generate those boxes.
A vented alignment is a continuum of possibilities. Now with a computer program you can explore all the possibilities."


Has anyone simulated step response in Unibox, and found they could employ the results to fine tune their design, ie go beyond SBB4, knowing in building a bigger box that they gained something tangible ?

Thanks
 
I certainly agree with the continuum comment. I don't use classic alignments as such, I mentioned SBB4 as a way to connect to the Cookbook. I use LEAP for modeling and do final tuning with the Sheffield drum record for minimizing overhang. Frequency response falloff is chosen to more or less match room gain.
 
Yes it does, but I don't use it; I used the delay curves instead. Nowadays, I tend to use the freq response more, and the delay curves a bit less, as I choose the driver and box size to put me in the SBB4 (or Extended Bass Shelf) territory to begin with. I did spend a lot of time the first time through the loop (10 years ago?) to decide that that was the "alignment" I wanted to use.

Oh, and to emphasize the Sheffield drum record; the notes say the kick drum should be totally dead. With a significant alignment error, I've heard it Baarooommm. When it is truly dead, the transient response (which is actually more lack of hangover I think) is very good. I don't think the disk is still available, and I don't know a substitute unfortunately.
 
Hi rick57. greetings from the Sunshine State, I don't want to confuse things but you might reconsider going for a Qt of 0.7. I am aware that this number is frequnetly seen as the optimun and gives "Butterworth" or maximally flat response. On the other hand as the bass end there is a good case for a lower figure. My source is Martin Collom's book "High Performance Loudspeakers" (various editions since 1978. I think the 5th is the latest). He maintains that a figure of 0.6 gives a better subjective response. The maximally flat response tended to sound too bassy and loose. Its a v.good text and might repay checking out at a local library if you can get it. Enjoying the weather down there this week? I lived there during the 70's and can recall leaving a building in sunshine having a coffee in Lygon St Carlton and returning 75 minutes later. During that time it rained and snowed but the sun was shinning by the time I headed back. You've got to love it!
 
Sheffield Drum Record

Curmudgeon said:
~~~~~~~~~~Snip~~~~~~~~~

Oh, and to emphasize the Sheffield drum record; the notes say the kick drum should be totally dead. With a significant alignment error, I've heard it Baarooommm. When it is truly dead, the transient response (which is actually more lack of hangover I think) is very good. I don't think the disk is still available, and I don't know a substitute unfortunately.


Curm and any others,

If you want the Sheffield Drum record, it's available on CD and for sale by Winston Ma of FIM.

Go here: http://www.fimpression.com/detail.aspx?ID=85

HTHs,
TerryO
 
Curmudgeon said:
Well, that's an interesting site! Thank you.


Winston Ma is a member of our audio club, the Pacific NorthWest Audio Society. He has served as a judge in the last three of our diy speaker contests ("The Puget Sound!" DIY Speaker Contest).
A real gentleman, very bright and exceedingly modest, he has given several presentations on music reproduction at our club's meetings over the years.

If you're interested in what he listens to, here's a URL:

http://www.positive-feedback.com/Issue3/maroom2.htm

Best Regards,
TerryO
 
Hi Curmudgeon

Your experience and evolution is very educative
So choosing the delay curves are better guide in some way than step responses?

I had decided one test disc (a Stereophile one) was enough - so the Sheffield drum record is better test than your favorite drum music? Answering my own question - when listening to my fave drum music I tend to forget about the test and just enjoy the music ;)

I saw that the original Sheffield Lab CD, was replaced in 2004 by the XRCD24 CD that TerryO linked to, an XRCD.

. . As the drum is my favorite instrument . . that's a good idea
 
Hi Jonathan

"I don't want to confuse things" . .
can't be as confusing as the weather in Melbourne:
Greetings from the very rare Aussie state that had a White Christmas, even if it was only hail, interspersed with sun and almost tropical downpours. Repeat 5 times in 4 hours . . :confused:

A guy I know with 50+ speakers built, suggseted with pop/ rock tastes I should go for a Qt of 0.8, "maybe as low as 0.7"
Interesting that Colloms (I have a copy his book somewhere here) suggests 0.6.
My current thinking is only experimentation both up front with a model, and later add eg blocks of wood to lower Vb, will yield the answer for me.

Sealed: a heavy fill Qt of say 0.53 requires a Vb of 139 litres, with an f3 of 93 Hz would need to be supported a sub below (was going to build them for HT anyhow), and play with the Qt in the range 0.53 - 0.75.
A 139 litre box with a vent in it yields an f3 of 45 Hz . . going much bigger vented with this driver can only drop f3 a further 3 Hz - a marginal return.

I alternate beween being attracted to the extension of vented and the tuneability (add wood/ bricks/ etc to raise Q) of sealed. I think tuneability is probablity most important.
As the best for my tastes may end up with a Qt of say 0.65, that's only a 57 litre box, so the 139 litre tests of concepts boxes probably should be just eg 12 mm (1/2 inch) MDF.

sorry if it evolved into a "thinking aloud" post

Cheers
 
I don't know if the transient simulation is better or not; I just found that the delay curves seemed more intuitive for me. LEAP is excellent, and the transient simulation would not be included unless it was useful.

I use the Sheffield record because it was recorded as a test record, and the notes indicate what you should listen for. I really only use it for the dead kickdrum, but have found that to be very effective for final box tuning.
 
Hi Curmudgeon

"I just found that the delay curves seemed more intuitive for me. LEAP is excellent, and the transient simulation would not be included unless it was useful"
. . fair enough

"very effective for final box tuning"

What final tuning options are there for a vented box - vent length & stuffing?

Thanks
 
Hi rick, have you got a set of Theile's work? His original papers were published in Oz around 1961 and not reprinted in the JAES until 1970 when he was suddenly "discovered"and seen as having solved the bass-reflex issues. I know RMIT, the State Lib' and Monash Engineering Library (The Hargrave) hold copies of the JAES. You'd enjoy all the theory there. Small from Syd'Uni then did a more extensive series in the early-mid 70's where he applied rigorous maths to sealled boxes, reflexes ABR etc. That work spans many editions of the JAES. All those places will allow photocopying by members of the public. Sorry if I'm telling you stuff you already know!
 
rick57 said:
What final tuning options are there for a vented box - vent length & stuffing?

Thanks

On the current project we didn't use stuffing, but rather 1" wool felt on all surfaces. So we don't have to worry about stuffing density, or settling. The drawback to the felt is cost and moths. (Most DIY members strongly prefer the stuffing.) That was not the result of a lot of testing. We did a couple of comparisons very early, that probably weren't all that valid. Tried the felt, liked the results, and stayed with it. We use water (NOT solvent) based contact cement. When I used stuffing in the past, it was wool; in one case where volume was really too small, Red/Navajo/Rug wool saved me. Screened ports worked out well.

Cabinet sides and front are vertical, back is sloped.

First proto evaluation was fine, and then we moved the cabinet, and everything went sour. We'd put the cabinet equidistant from the side walls; not our smartest move. Fierce noding. We put the cabinet more or less where it would be used, which helped a lot. We expected the deep bass to get too heavy when we turned up the second cabinet, but it did not. Not sure why, but cheerfully accepted the result.

One unexpected issue was driver heights. Because we're using mild slopes, with multislope crossovers, floor bounce was more of an issue than expected. Some heaving and ho-ing of the proto cabinet to adjust the height was needed.

We got the cabinet size about right on the first pass, (LEAP) and were happy enough with our first woofer that we did not look further, so we were a little surprised at the other issues like height and placement making such a big difference.
 
J & J

Thanks for the suggestion
I find Dickason often unclear and RMIT is close, so I’ll have a go with Theile (not Thiele?) and Small.
The early-mid 70's material sounds better – (I’m not on hols so to allow enough time) approx how many articles might that be?


Curmudgeon

> Cabinet sides and front are vertical, back is sloped.

I like the discreet removal of a resonance

> floor bounce was more of an issue than expected

I believe MJK’s latest worksheet can model this

Cheers
 
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