Newbie Questions

[Mjr7531]

I've got a few questions, I've proceded to make speaker for my sister for christmas, I've got a workshop and limited carpentry skills at my disposal.
I'm using two NSBs per speaker in order to get 4 ohms, each boxs is using to two ports, and I've to make them a slim tower, (inner dimensions) w 4" x h 24" x d 11.5" Should I have any problems with the overly tallness and narrowness of the enclosure? I'm going for imaging if that helps.
Also, I read somewhere that you don't stuff a ported box, then I looked in a book on speaker building and show you how to arrange stuffing to accomadate for a stuffed box.
I would like to stuff the box, if that is not frowned upon, so I can build the box smaller. If the stuffing is used, what is the aproximate change? I saw somewhere a 70% percent box is 20% "larger"
Lastly, WinISD doesn't give a 10% oversize compensation, do they?
Thanks guys,
Matt

 

[planet10]

these may well be getting long enuff to support a quarter wave resonance, which will affect the required port tuning.

dave

 

[BillFitzmaurice]

Stuffing or lining is an absolute requirement for smooth response. With a simple box of the sort you're making I prefer to use 1" thick high density polyester batting (furniture padding) fully lining the box. The 24" inner dimension will get problematic up around 140 Hz, and that frequency is down where damping materials aren't so effective, so I'd put a double layer of lining on each end to be sure it's adequate. Do not make the box smaller even if you fully stuff, as most box programs are too conservative with regards to box size and also don't consider the volume occupied by the duct and the drivers. A bit too big is preferable to a bit too small.

 

[Mjr7531]

Ah, what would I do to stop this? Make it wider and deeper then?

 

[planet10]

Ah, what would I do to stop this? Make it wider and deeper then?

Or take advantage of it and model it as an ML-TL

dave

 

[Mjr7531]

I'm ready to drill holes in the baffle to place the NSBs, I remembered reading somewhere that putting the speakers 1/4 (or was that 1/5?) down from the top of the tower would cancel problems with harmonics. Is it true, and to what extent? Is speaker placement that important on the baffle as long as they are alligned? Lastly, ports in front are not an issue as long as they are 3 in. from the woofers, right? Thanks,
Matt

 

[wintermute]

I think what you are talking about I read in one of Weems' books. I think it says something along the lines of "Placing the dirver one fifth of the distance down the box (from the top) helps suppress the 5th harmonic."

The technique is to help stop pipe resonance in long narrow cabinets, I think...... I'm not at home so can't place my hands on the book right now.

Tony.

 

[wintermute]

OK quote from [bold] Designing, Building, and Testing your own Speaker System[/bold] by David B. Weems. pp 27

Placing the driver at a point that is pne-fifth the distance from the end of the pipe suppresses the fifth harmonic, a major cause of resonance in closed pipes

now I guess the critical thing is that it says the fifth harmonic is a major cause, not the only cause, so while this will help, I'm guessing it isn't a magic cure all

Tony.

 

[planet10]

It is fairly standard procedure now-a-days to move the driver from the end of a quarterwave line to take advantage of its ability to suppress a harmonic of the primamary resonance (i don't know if 1/5 and 1/3 are actually the best...

A snapshot of the MJK offset table is attached -- this offset kills the 1st/lowest undesired harmonic. To get the whole paper "Classic Transmission Line Alignment Tables" go to Quarter-Wave.

dave