Optimized double bass reflex

[youyoung21147]

Hello,

I'm actually designing a system based on the Supravox 165GMF driver.

I modelled all the enclosures possible for this driver, from horn to TQWT, including bass reflex. (using M.King worksheets)

It seems that the double bass reflex (the one like Fostex's designs, with 1 internal port coupled to another outside port) is the ultimate load for this unit :

I assume the double bass reflex mainly allows a dramatic damping of midrange through the port, letting the driver emit alone the mids and highs (no pollution from the box)

Now I have found the best compromise, I would like to enhance some parameters that are not included in the worksheet :

- I'd like to make non-parallel side panels, both to avoid standing waves and slightly point the driver towards the listener. Will it affect the simulated response, assuming that I keep the same cross-sectional area ?

- I'd like to incline the internal plate to avoid vertical standing waves issues. Is it a good idea ?

- The simulation doesn't precise where to put the internal port. Should it be placed right at the center of the divider plate or rather on a side of the enclosure ? Would it be worth doing some kind of Onken ports on the sides ?

- I plan to simply use the walls thickness to build the ports : will it work like with a conventional tube, or is the thickness too low ?

- I suppose a rear port is naturally the best solution for this enclosure. Am I wrong ?



Thanks in advance

 

[Scottmoose]

Hello,

I'm actually designing a system based on the Supravox 165GMF driver.

I modelled all the enclosures possible for this driver, from horn to TQWT, including bass reflex. (using M.King worksheets)

It seems that the double bass reflex (the one like Fostex's designs, with 1 internal port coupled to another outside port) is the ultimate load for this unit :

I assume the double bass reflex mainly allows a dramatic damping of midrange through the port, letting the driver emit alone the mids and highs (no pollution from the box)

Now I have found the best compromise, I would like to enhance some parameters that are not included in the worksheet :

- I'd like to make non-parallel side panels, both to avoid standing waves and slightly point the driver towards the listener. Will it affect the simulated response, assuming that I keep the same cross-sectional area ?

- I'd like to incline the internal plate to avoid vertical standing waves issues. Is it a good idea ?

- The simulation doesn't precise where to put the internal port. Should it be placed right at the center of the divider plate or rather on a side of the enclosure ? Would it be worth doing some kind of Onken ports on the sides ?

- I plan to simply use the walls thickness to build the ports : will it work like with a conventional tube, or is the thickness too low ?

- I suppose a rear port is naturally the best solution for this enclosure. Am I wrong ?

Thanks in advance

Hmm, DBR cabinet. Quite unusual to see these, not that's there's anything wrong with them! I haven't modelled the Supravox in any of Martin's MathCad sheets, so I'll take your word for it that the DBR is the best compromise for this driver. Dealing with your points in turn, if you want to use non-parallel sides, go ahead -providing that you keep the cross-sectional area the same, it shouldn't make a jot of difference to the simulated response. Ditto your idea for inclining the internal partition.

However, something to think about from Siegfried Linkwitz: "Some people think that by making the room other than rectangular or using curved surfaces, that they can eliminate standing waves. They merely change frequencies, shift their distribution and make their calculation a lot more difficult."

Whilst he was speaking of room acoustics, the same reasoning is equally applicable to the internals of a speaker cabinet, so it's worth keeping in mind that it's not quite the panacea it's often made out to be. I'm not saying it can never work under any circumstances, just that you might get more benefit from careful selection of materials and construction techniques than from altering the angle of a few box baffles.

As for the internal port, I'd keep things simple and use a flared port if possible, placed in the centre of the baffle. Remember it's got another chamber and port to go through, so this one isn't going to be as critical. It might be worth doing something of the nature of the Onken ports, but again, if you want a fancy porting arrangement, I'd save it for the one leading to the outside world -I remeain unconvinced about this sort of port arrangement, though I believe Tannoy use something similar on some of their top models, so there obviously isn't anything fundamentally wrong with it. Perhaps I just don't like slot-type ports much -usually they produce more ripple in my experience than a conventional circular type.

As for where to put the second port -depends where the cabinets are going. Rear ports can give you an extra boost from room gain, and also reduce port-noise (though a properly designed speaker shouldn't suffer from this anyway). But a front-port arrangement can often sound a little more immediate. One other possibility would be to floor-load it by putting the port in the base, and raise the speaker above a plinth on 4 adjustable spacers. Castle do this to good effect with their Conway 3. You can then play with the amount of damping by raising or lowering the height of the cabinet. Could be interesting. Driving a room's vertical mode can often give a fast sounding bass.
Cheers
Scott

 

[youyoung21147]

Thanks for your answer !

I think I will only slightly modify the shape of the base, mainly for aesthetic reasons.

Conerrning ports, I've modelled longer ports than just the thickness of the walls : it leads me to huge diameters.

Basically, I was looking for a slot port directly cut in the walls, but maybe a circle also directly cut in the wall is better. Anyway, I don't think port noise will be an issue here : the external port is basically 27cm * 5cm ! ( 2.5" radius if a circle)

Flaring the port will be easy, directly in the wood. I'm designing this for a friend who is carpenter, so he knows about woodworking

I'll keep the port on the back to get room boost (the speakers will be near corners.


Thanks for your help !

 

[vmac011]

Perhaps something like this: http://www.diyaudio.com/forums/showthread.php?threadid=61017
Different driver though.

 

[youyoung21147]

Looks quite like I'm going to build !

It conforts me in the idea that DBR is a good kind of enclosure to preserve the sound from being polluted by midrange output through the port. I will be using a driver that has a great Xmax (6mm p-p ) and I assume it will have good and fast bass.


Talking about ports, some people say it isn't precise just to keep the surface and length of a round port, and convert it in a rectangle shape. Is it true ?

Concerning flare, I did a lot of maths to precisely calculate the average surface of the port when flared. (in fact the average virtual width of the port)
Is it a good method or shall I use a different one ?

 

[Scottmoose]

Talking about ports, some people say it isn't precise just to keep the surface and length of a round port, and convert it in a rectangle shape. Is it true?

I doubt it. If the basic 'surface' and length of the port remain the same, then why is it 'less precise'? Beats me. The only warning to raise is that square and rectangular ports do tend to generate more ripple than a circular or ovoid shape, so look out for that. For example, Bob Brines moved away from the slot port originally used in his FT1600 MK1 to a circular type a few inches up in his MK2 version, which reduced ripple between I believe, 500 & 1000Hz. They way you are calculating flared ports sounds fine to me. Most commercial manufactures don't bother doing this -they just use the standard diameter without making any calculations at all for the flare, so you're already streets ahead of them. I'll be interested to hear your reflections once these cabinets are built.

Cheers
Scott

 

[youyoung21147]

I think it is mainly for me to sleep quiet if I calculated the flare (I like exact things lol) : in fact, after 1h of maths, it appears that a a double flare (at the beginning and the end of the port) of 6.5mm radius on a 22mm long port only increased the average width of ... 1mm !


Concerning the port shape, I choose it rectangular to avoid using big pipes (< 130mm in diameter) and because it makes the construction easier. Anyway, I doubt there might be any ripple issues : the double bass reflex gives me - 35dB @ 1kHz ! And -22dB above 200Hz. Following your advice, and other's, I will put the port on the back to furthermore decrease midrange pollution and get room boost.

Maybe I will look for a 9.5mm radius flare, so there will be hardly no parallel sides in a 22mm long port ! Means even less ripple I assume.



I'm still waiting for the speakers unit, and the speakers won't be finished before 1 month 1/2. I designed this for a friend, but he will build them himself (he's a carpenter lol ). So yes, I will post my opinions about it. I'm very impatient myself to hear these !

 

[jsb]

Just feel like a grave digger, to unearth this thread, but did you eventually build this enclosure with that driver. There seems to be a lot of talk about the "official" supravox DBR enclosure ("Carla"). Quite unusual with that brand of drivers.

Anybody else had a chance to listen to supravoxes in DBR??

 

[youyoung21147]

Yes, these speakers have been built.

The tweeter is a FT17H

A symmetrical crossover circuit has been implemented (the "just one cap on the tweeter" solution really didn't work well) :

12dB Q=0,5 Fc=4500Hz

The port has been placed on the back but is circular and flared.

However, the enclosures will be modified to put the port on the front : the back port causes problems because of its proximity to the rear wall and need the speakers to be very far from the wall.


Now, talking about sound :

First of all, I must say the 165GMF are very fast and precise drivers. As far as you don't lead them to Xmax, they deliver excellent dynamics that are not usual for this kind of driver.
Along with the Fostex tweeter, they reproduce quite many details and imaging remains very precise despite the 2-way config.
The midrange zone really impressed me : I could hear details that my FX120 and F120A drivers didn't show.
Concerning bass, there is much output down to 42Hz. Bass is quite precise and clean considering it is a huge ported box. Measurements show a zero group delay down to 50Hz but I find it very hard to believe.

However, I must say these speakers are quite hard tu tune. First of all, they need to be far from the rear wall. Then, I strongly recommend to install some reinforcements inside to avoid vibrations.
And finally, careful stuffing should be installed because the sound without it is awful.


I'm sorry I didn't add this project on my website, which could be done soon. Studies first !!

 

[jsb]

Thanks! that's interesting...
Sorry they were so hard to tune.

 

[youyoung21147]

Lol, now I know how to make them sound good, it shouldn't be hard for me or anyone else to get good results from the start !

I will update my website so everybody can see them.


Salut !