I just finished my first diy sub, a Dayton hifi 15" in a sealed enclosure. I definately don't know what I"m doing because I tried modeling my next sub and the response is falling from 50hz downward quickly.
I asked this on another forum but haven't gotten the info I need and I need to get this sub done or at least mostly done before I have to go back to work in a couple weeks.
I want to use two RL-P 15's in a ported box 40 in wide/ 20 in deep/ 24 in high. I'd like to know what size ports to use and which way would be the best way to point the drivers, down or toward a wall 7' away?
This sub will be in an integrated music/HT system with the priority being HT. I'll use MDF to build it. It looks like the interior will be about 314 liters. If there's a program besides Winisd or Unibox that works with cu ft instead of metrics I'd appreciate it as I keep having to refer to conversion tables. I still must be making a mistake somewhere because the result I am getting in Winisd isn't nearly as good as what I got doing my sealed music sub with the Dayton HIFI 15".
If someone would be kind enough to just tell me about ports a bit and what size and quantity would be best for a box tuned to say 18hz I'd sure appreciate it. If you think I should tune it any where else please just say so as I am guessing. Thank You
You will get increased output if you face your driver to the floor verses forwards. Be careful though, too close and you'll end up getting reflections back against the driver and that's a no-no...
I don't know about getting increased output by facing downwards; do you have some further information on that? It might more easily create vertical standing waves, but those are generally to be avoided.
Someone mentioned reflections pushing back on the driver, but I'm also not sure that's too much of a problem. Many down-firing subs are about 2" away from their base plate or floor. So that happens to be how far I placed my base plates from my own subs.
The increased output is the result of the sub exciting the floor resonances, not so much the output of the sub itself. For results you can go to the Mach 5 TestBed site where I performed measurements on a MJ-18 Sonosub siting horozontal and vertical. You will notice increased output when the driver is mounted vertically.
The closer you have your driver to the floor the higher the chances of waves bouncing back against the driver cone itself and causing cancellation effects.
Make sure you're using WinISD Pro as it will give you a graph of port airspeed rather than just a Mach no.
If you're not going for a slot port, you'll be using multiple 4 inch ports or a 6 incher. The maximum velocity you should design for depends on the port size and whether you are using port flares. With the amount of air those drivers will move, I would recommend the PSP flares.
Four inch flared ports are good for somewhere between 14 m/sec and 20 m/sec.
Six inch flared ports are good for somewhere between 21 m/sec and 26 m/sec.
Choose a port diameter and then increase the number of ports until the airspeed is within this range.
The problem you'll come up against is fitting the ports in. As you increase the number of ports, their lengths increase. This also eats into the effective volume of the box.
For a given tuning frequency, increasing the box size actually shortens the port length, so you may have to go to a larger box or accept a higher tune.
If you have a look at the "LLT Explained" sticky thread at the top of this forum there is an extensive article by Steve Callas on large subs. One of Steve's pet designs is a 320 litre sonotube with a 6 or 8 inch port tuned below 20hz using one of those drivers. For two drivers, you would build a pair of sono's.
Right angled joints, or elbows, (and some other angles 15, 30 & 45 degrees) are all readily available for 4, 6 and 8" PVC drainage pipes.
Joints come in solvent glued and rubber sealed types. The solvent glued type have to be pushed home quickly or they stick partially out of the joint. Once the joint is made they stay together.
The rubber sealed joints will need a suitable lubricant if you ever want to take them apart again! The removal friction is enormous if you don't use a proper lubricant. Pipe wall brackets can be used to anchor your plumbing out of sight inside the box.
My understanding of reflex ports is that only the physical volume of the materials (the PVC itself) is calculated when discussing effective volumes of ported enclosures. Not the swept volume of the entire pipe.
I'd value a more knowledgable confirmation of which volume is correct.
I also understand that the effective port length is calculated by the center length of a folded pipe. Rather than the longest internal edge or the shortest internal edge of a folded pipe.
Perhaps the positive pressure pulses from the front of the downfiring cone do not have time to dissipate fully causing a local overpressure which reduces cone movement? Though in theory the driver's resonant frequency should rise to follow the pressure build up. The effect would be frequency-dependent I imagine.
Lifting the driver (and its enclosure) on progressively longer feet would easily confirm this.
I wonder if this is a cheap upgrade for SVS subs? I could sell "special" risers for very silly "audiophile" prices! :devil:
Though I imagine Tom V spent some time working out an optimum height for his sub's feet under exhaustive testing. :nerd:
I'm sure the person who designed SVS subs knows the best height. However watching people by things like Magic Pebbles and magic wire for alot of money has stopped them. The wire makers have done well, at least the big names have. Me I hear so little difference I consider it inconsequential.
So yes selling some "tuning" stands may be a worthwhile effort. Actually some rooms may benefit from a different height so you better make them adjustable:bigsmile:
This is something that can be scientifically verified too. You don't need to kill virgins for their blood, although that may make them worth more.
Having the woofer farther away from the floor, and particularly with some sort of absorption material, will improve decay. Measurements on Secrets and/or Stereophile for a "subwoofer stand" prove this. I can't remember where. Maybe both.
Having the woofer farther away from its base plate is a separate question, and one that I'm a bit skeptical of right now.
The bottom line is the designer knows what they're doing with their own design better than anybody else. I suppose it's possible for the height to vary a bit and help but I'd try everything else first.
I did some work on downfiring clearance when developing sonosub.exe
The principle I adopted, was that the air leaving the front of the driver should not encounter a cross-sectional area smaller than the area of the cone itself.
An approximation using nominal diameter gives:
An 8 inch driver needs 1-9/16 inch gap
A 10 inch driver needs 1-15/16 inch gap
A 12 inch driver needs 2-3/8 inch gap
A 15 inch driver needs 3 inch gap
An 18 inch driver needs 3-9/16 gap
Downloads of that software are past 700 with no negative feedback about this issue.
If you go downfiring with twin 15inchers, there is no need to double the gap - just add a little because they're in close proximity.
The principle I adopted, was that the air leaving the front of the driver should not encounter a cross-sectional area smaller than the area of the cone itself.
Could you elaborate on this? I'm not sure what you mean by cross-sectional area. What would be the result of failing to meet this guideline? And what information was this conclusion based on?
This drawing is for a flared port instead of a driver, and is upside down, but illustrates the principle:
Replace the blue port with a driver.
Flip over and imagine the light yellow circle to be a baseplate or the floor.
The area of the buff-coloured cylinder = driver circumference * clearance
For the air being moved by the driver not to encounter any restriction, this area must not be less than the area of the driver ( represented by the orange area )
This just seemed common sense to me.
Having a smaller clearance would increase air velocity somewhat, but not as much as what you would get from a port, so this shouldn't be a problem ( unless clearance is very small )
The other thing that could happen is that the driver could experience some loading which may change the tuning. I couldn't say how much of a problem this would be.
The combined cone area n x Sd is the desirable opening for an IB manifold.
This is achieved automatically in an array since the front and back of the drivers "see" the full area of the cones without any constriction or obstruction.
Anything less than n x Sd must reduce sound quality by causing some pressurisation or constriction in front of the drivers.
It follows that if a bottom plate (or the floor) are too close to a downfiring driver then the skirt area will be less than Sd and some pressurisation will occur.
This should be very easy to prove in practice since one merely changes leg length and retests.
Physically lifting a sub from the floor is another matter unless the bottom plate is removed. Otherwise it would only affect the coupling of the entire subwoofer to the floor boundary.
I find it interesting that SVS do not downfire their large and efficient ports. While some compact subwoofers do. This can't be a simple case of cramming long ports into the box. Almost any port configuration could be achieved by using elbow joints. Whether this is desirable is another matter.
I find it hard to believe that the SVS ports have been arranged purely as a cosmetic feature. The proximity to the floor of downfiring ports must be disadvantageous from a performance point of view. Could it be a simple pant's leg flapping issue? :devil:
The SVS subs use PSP flared ports. That would imply that they're using as high a velocity as the flares allow. Firing them into the floor would run the risk of audible turbulence.
Thanks for the explanation, collo. I guess I got a little confused since you called things area instead of volume. But I understand your reasoning now.
My TC-2000 15" drivers are 2" from the base plate on the bottom, and on top covered by a grille (from Parts Express, the same one on top of the SVS cylinder ports). So I'm probably restricting the airflow as you indicate.
I suspect that the reason the SVS ports are facing up, in their cylinder subs, is that this way they can put the driver on the bottom and the ports on top, which is less "dangerous" from a consumer use point of view. Maybe technical reasons too.
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