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-   -   Vented box alignment: gradual droop a good idea? (http://www.diyaudio.com/forums/multi-way/65992-vented-box-alignment-gradual-droop-good-idea.html)

tcpip 14th October 2005 03:37 PM

Vented box alignment: gradual droop a good idea?
 
This is a newbie question. This is my first attempt to design a pair speakers. I've used SW to measure the T/S parameters, including Vas measurement using delta-compliance method. I'm now modelling the box using Unibox, and I have a question about what final low-end response to go for.

I want this box for music, not HT, and I don't want "flat till 20Hz". Even then, my drivers have an Fs of 42Hz, which is higher than what I'd like to work with. In a vented enclosure, if I tried getting a maximally flat alignment, I was getting the knee of the curve at a fairly high frequency. So I tried lowering the Vb, and I am getting a very shallow drooping curve, followed by a knee at a much lower frequency.

To my (totally inexperienced) eye, this gradual droop followed by the knee seems like a better alignment than a flat graph, because I expect that room reinforcement will neutralise the gradual droop very well. I'm also going to have a downward-firing port about two inches from the floor, so the low freqs will get amplified even more because of the floor, I believe. Here is the graph Unibox is giving me. See the high F3 but the low knee point:
http://www.dhandanought.org/tcpip/au...d-response.gif

The impedance graph looks like this:
http://www.dhandanought.org/tcpip/au...-impedance.gif

Is this gradual droop a good idea? It's not possible that I'm the first person who's tried this, though I couldn't find any mention of this thing in Dickason 5/ed. (In fact, Dickason's entire approach to vented box design seems anachronistic after using unified box modelling software.)

Can I simply keep lowering the Vb arbitrarily like this in Unibox? Other than cone excursion, is there any limit to how low I can set Vb?

Any comments and feedback will be really useful.

lufbramatt 14th October 2005 06:03 PM

i have used a similar approach when i made my enclosures for a vifa pl18- used a slightly larger box tuned a few hz lower than the "ideal" max flat response, as they were going to be used in a fairly small room. They sound great, but you have to keep an eye on the group delay and phase plots, as by lowering the tuning frequency the group delay will increase, making the bass sound slow and muddled. Im not sure what figures unibox gives, but winisd can be used to give an idea of the group delay figures. Basically, you want the curve to be as flat and low as possible for the tightest bass.

I think- if im wrong correct me guys :)

Variac 14th October 2005 06:11 PM

Dear Mr Pip,

I designed my Basszilla bass box to have this "droopiness" I think they call it "an overdamped alignment" Its supposed to minimize some of the supposed negatives of a bass reflex, and also match room bass lift as you mentioned.

The bass is tight tight tight - whatever that means. I like it a lot, but my bass driver- an obscure JBL 15" is already highly damped already electro- magnetically.

Also, I realized that if I add up all the spaces directly connected to my listening room, it is quite a bit bigger than I thought.

Also, if I turn up the bass (its bi-amped) the upper bass gets excessive before the lower.

Sooooo lately I have been thinking of shortening the port tube to try to lessen the droopinesss a bit to see if I like it with a bit more lower bass. Tragically, due to a construction disaster I was forced to glue the port tubes in place, which is going to make the experiment difficult and non-reversable.

So:

I recommend that you make the port removable and/or adjustable so you can then tune it by ear to the room. Remember. the port doesn't even have to be in the box, it can stick out into the air at first if that's easier for initial experiments. In this case the volume might change a bit if you later put the port back into the box, but boxes realy aren't THAT touchy and you could easily adjust for that after getting close.

Perhaps its best to use PVC tube for the port because its cheap, and a consistant diameter and have it friction fit into a hole in the box, so it is easy to remove. After a few experiments, you will have a collection of lengths to play with! It wouldn't be a bad feature to retain some adjustability to tune them to your new bigger living area in Goa when you move...

thoriated 14th October 2005 06:14 PM

You are correct wrt group delay. However, there is a qualifier to that. For minimum phase high pass filters, once you get, say, an octave or more above the highest non-dc pole or zero, the group delay decreases by 40db per decade regardless of filter order, so lowering the tuning frequency can actually *decrease* group delay an octave or more above the tuning frequency depending on the tuning Q, even though it will be greater at and below the tuning frequency. Once you start tuning below 40hz, the group delay tradeoff seems to me to become increasingly satisfactory with most music.

richie00boy 14th October 2005 06:17 PM

Quote:

Originally posted by lufbramatt
i have used a similar approach when i made my enclosures for a vifa pl18- used a slightly larger box tuned a few hz lower than the "ideal" max flat response, as they were going to be used in a fairly small room. They sound great, but you have to keep an eye on the group delay and phase plots, as by lowering the tuning frequency the group delay will increase, making the bass sound slow and muddled. Im not sure what figures unibox gives, but winisd can be used to give an idea of the group delay figures. Basically, you want the curve to be as flat and low as possible for the tightest bass.

I think- if im wrong correct me guys :)

What you have is more of an extended bass shelf (EBS) alignment. Keeping Fb the same and reducing box size is not the same thing. Also, Keeping box size the same and lowering Fb will lower group delay, not increase it.

lufbramatt 14th October 2005 06:37 PM

I see, ive re-read that bit of the loudspeaker design cookbook and understand it a bit better now. :cool:

As a side note, I also found that the simulations on loudspeaker cad programs gave me a port length that was too long to give the desired tuning frequency. Although i was meant to use a 19cm port for a tuning of 45hz, after making an impedance curve the port was reduced to 15cm to give the 45hz- as given, it was around 40hz and the bass was seriously lacking in umph, sounding too rolled off.

Variac 14th October 2005 06:43 PM

Quote:

lacking in umph, sounding too rolled off.
manifestation of "excessive droopiness"
My problem too.

We are certainly technical here ;)


I mean my SPEAKER has "excessive droopiness"
I'm fine , really:eek:

tcpip 14th October 2005 09:42 PM

Quote:

Originally posted by Variac
Dear Mr Pip,
Dear Mr Ack,

As usual, it's a pleasure hearing from you.

Quote:

I designed my Basszilla bass box to have this "droopiness" I think they call it "an overdamped alignment"
Great. I'm glad to hear that you and others have already trod the path.

Quote:

The bass is tight tight tight - whatever that means. I like it a lot, but my bass driver- an obscure JBL 15" is already highly damped already electro- magnetically.
If it's thy Basszilla that thou dost refer to, sooth, was it not built to play through the large 15" mouth of yon Selenium driver? Or hast Age caught up with my mental faculties and clouded the visions of my memory, vile demon that it is? If Age dost not play cruel tricks with me and if Qtc rightfully be a measure of damping of the driver, then yon curvy Selenium and my puny Kevlar upper bass from local shores both have damping in similar measure; between 0.3 and 0.4 fall their Qtc values, so the Great Oracle "TS Param" says.

Quote:

Sooooo lately I have been thinking of shortening the port tube to try to lessen the droopinesss a bit to see if I like it with a bit more lower bass.
Here I am confused. Why will reducing the droopiness reduce your upper bass boom? Won't a parametric bass equaliser for the bass frequencies help to cut room peaks, short of actual DRC? Will reducing this droopiness actually cut your upper bass boom? Are you sure the boom is not due to a mis-tuned port?

Quote:

I recommend that you make the port removable and/or adjustable so you can then tune it by ear to the room.
Yes, this is something I have already decided to do, having no confidence in my ability to build a vented enclosure and having the luxury of a downward firing port. My box's bottom surface is made of three sheets, for thickness and solidity. I have decided to cut out a 6"x6" square piece from the lowermost of these sheets, and fit the tube to it. The square piece thus acts as a wide flange to my duct. I fit it to the bottom surface and plug the duct into the box, and attach it with four screws and seal it with a piece of closed-cell foam. It's like attaching a speaker driver, only it is at the bottom of the box. So my tube is actually removable.

I will use this method and prepare a few identical square pieces, each with a piece of tube of a specific length. This will let me switch tubes and try different lengths. I find it hard to conceive of a port tuning approach without this sort of well-sealed but removable tube. I can apply this approach to the rear or the bottom of the box... it's the front-firing port which is hard to do this way, for cosmetic reasons.

Quote:

It wouldn't be a bad feature to retain some adjustability to tune them to your new bigger living area in Goa when you move...
Oh cruel taunt, fly back to thy master from whose keyboard thou hast come to haunt me across the seven seas. Fly back and torture his dreams, let him toss and turn in his bed of feathers in sunny San Francisco, and devour his sweet succour. Oh, the cruelty of reminding a poor man in Bombay of dreams of owning a house with a large living room in Goa.... why, Paradise appears more in my mortal reach now than Goa.

:D

Who says we audio geeks can't create winged words of beauty, even if it's all ****? :D

Variac, your Basszilla is doomed to complete obsolescence. You are not using a G2Si, which, in another thread the Gooroos have just told me is about the only tweeter worth using for high-xo uses. Suffer. Regret and suffer. :D

tcpip 14th October 2005 09:47 PM

Quote:

Originally posted by richie00boy
What you have is more of an extended bass shelf (EBS) alignment.
Yes, I've read this EBS term and I kept wondering what it was.

Quote:

Keeping Fb the same and reducing box size is not the same thing. Also, Keeping box size the same and lowering Fb will lower group delay, not increase it.
So, it seems my lowering of Fb has a good side-effect. Let me see if Unibox gives me any group delay curve... I'll try posting it.

Also, let me clarify that I screwed up in my opening post. I have written in that post:
Quote:

Other than cone excursion, is there any limit to how low I can set Vb?
I meant "Fb", not "Vb". I hate to screw up like this, because it almost defeats the very purpose of my posting and asking the question.

So, to get the question right, is there no limit to how low I can set the Fb?

PeteMcK 14th October 2005 09:52 PM

Tannoy low q
 
This article might be relevant:

http://users.bigpond.com/tunnelgap/Tannoy/lowq.html


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