Just Finished My First Build - Of Course I Have Few Questions

Originally Posted by fleek1116 

Let me start by saying thanks to everyone in the forum for all the incredible information. Reading through everyone else's threads has given almost every answer I could've asked for. All the contributors here are incredibly knowledgeable and seem to love sharing their info with any who might ask. That being said, I still managed to come up with a few specific questions. If anyone has read any of my other posts, they'll know I get incredibly wordy, so sorry about that in advance.

For background, I just finished I just finished my first build, a ported build with a 8.15 cu ft gross volume (about 7.8 with driver, bracing and port taken into account). I have a 4" dia, 12" long port which should give me about a 18Hz tune according to WinISD (I know, not enough port area, too much port noise). I'm using the Dayton 15" HF and the Bash 300W plate amp from PE (17.7Hz HPF with 1dB boost from 25-30Hz by default according to PE). I built the box out of 3/4" MDF with PL premium on the joints. I used roughly 2" wide planks of the same MDF for bracing; two front-to-back, two side-to-side and 2 top-to-bottom. I don't believe there are any leaks at any of the joints or around the driver but I'm not sure how to tell definitively. My media room is 15x19x8 in a shape generally lake the state of Utah. It's a 2nd floor room of a new construction house with carpet on the floor and no other treatments.

I will say that first impressions were s**t-eating grin incredible. I popped in "Live Free Or Die Hard" and watched the apartment shootout and the tunnel scenes and was blown away by all the sound I've been missing. So much fun to watch that. Now on to my nit-picking...

1) First, a very specific and unimportant question: During the tunnel scene when the car is tumbling (think barrel roll) through the air and lands on the two cars around the protagonists, there is an amazing LFE slide as it falls. In my system, there seems to be a slight blip where the LFE disappears right before the car makes impact. I was just wondering if anyone has the frequency charts to know if that's just below my sub's capability, or if that is an intended effect.

2) Based on advice from I got from other threads, I used 5 lbs of polyfill to stuff the cabinet (slightly less than the 1lb/1ft^3 often cited). I tried to keep it away from the port entrance but didn't know to leave a clear path from the rear of the driver to the port. Is it possible I really screwed up the tuning of my box, and if so, how would it have changed? I'll think about removing the polyfill when I take off the driver to finish the box

3) I've run some test tones though my system and output seems to disappear when I get sub 25Hz. I realize I won't be able to 'hear' frequencies that low, but I can 'feel' the output at 30 Hz. At 25 and below, this seems to go away (even though various things around the room start to rattle). Did I just do some calculations wrong or is there something wrong with my box? I suppose I really won't know unless I get a measurement mic and work with REW.

4) I tried Audyssey 2EQ (on my Onkyo TX-NR609) and it gave me some unexpected results. It had my fronts and centers crossed over at 50Hz and the subwoofer level set to -13dB (although the fronts and center were around -5dB). Do the levels just indicate my subwoofer is much stronger than my mains (which I expect is the case)? I'm at a loss as to why it set the crossover on the mains so low. Everything I read says to cross over the mains, even if they are towers, pretty high (around 100Hz) and let the sub deal with anything lower since that's what it's there for. Is there any reason why I should xover that low (50Hz)?

5) I feel like there's a little extra 'boominess' to my bass in general. I ignored the myth that ported builds are automatically 'loose' but I'm worried I did something wrong in my design. It seems that I've got a lot of rattling from stuff in my room and this may be the contributing factor to the dirtiness. Short of taking everything off the walls is there anything I can due to reduce these rattles/resonance? I pondered the Subdude/isolation route but if it's simply the acoustic noise exciting the stuff, that may not help. I don't really feel the floor vibrates that much so I'm not sure if isolating will help. Some of the speaker spikes claim to 'clean up' bass but I believe that will couple my cabinet with the floor and transfer more of the energy to the structure/wall hangings and maybe exacerbate the problem. Any easy fixes I might be able to make?

Whew. Again, sorry to the length of this post. I'm sure that will scare away a few answers so shame on me. I may be getting to picky for what I can expect out of a single sub in an untreated room but I'm hoping for some tips or reassurance. Thanks again.

-Adam[/quote

]Well, I will say that the scenes you are using to demo are some of my favorites. The tunnel scene when it flips through the air actually feels like it hits you when we view it in my room.You can feel the pressure build against your body and our clothes shudder as it flips and crashes. I have never heard it anywhere else, so that is what I have to compare it with.

http://www.avsforum.com/t/1333462/the-new-master-list-of-bass-in-movies-with-frequency-charts/6000#post_22583196




I might be inclined to say that the sub might be running out of juice at the levels you want to recreate. What kind of SPL are you getting? What SPL levels did WinISD predict?

How much does the woofer move at 25 Hertz? What frequency does the driver stop moving? The tuning was calculated, was it actually measured yet?

If I was you, I would run my mains as large to start. I would set the LFE to 120 and see what happens. From there, I would make adjustments. Try it and see what happens.

Do you have an SPL meter?


LTD02 might be right about the port. It might be on the small side. Most of the 12's I have seen that really do the lows justice use at least 2- 4 inch ports. A 15 might need at least that much. There was a thread about this a while back and this might just be proof of the port area being too small although it models OK in WinISD. Any thoughts?


Keep cranking,

Robert

Originally Posted by fleek1116 

I have a 4" dia, 12" long port which should give me about a 18Hz tune according to WinISD (I know, not enough port area, too much port noise).

Based on advice from I got from other threads, I used 5 lbs of polyfill to stuff the cabinet (slightly less than the 1lb/1ft^3 often cited). I tried to keep it away from the port entrance but didn't know to leave a clear path from the rear of the driver to the port. Is it possible I really screwed up the tuning of my box

I've run some test tones though my system and output seems to disappear when I get sub 25Hz. I realize I won't be able to 'hear' frequencies that low, but I can 'feel' the output at 30 Hz. At 25 and below, this seems to go away (even though various things around the room start to rattle.

Short of taking everything off the walls is there anything I can due to reduce these rattles/resonance?

Originally Posted by fleek1116 

The woofer is moving visibly at 25Hz. Obviously can't give you an actual deflection number but it is moving a fair amount. I never got down to a frequency where the driver stopped moving. What would that tell me if I got there? I haven't measured tuning in any way; I was going by the WinISD models.

Originally Posted by fleek1116 

So equal loudness charts say at low levels I need an extra 5dB of SPL to perceive the same loudness at 25Hz that I do at 30Hz? Interesting to note. Maybe I don't have as big of an issue as I first thought.

Originally Posted by nograveconcern 

Just get some 4" pvc and 90 degree elbows. At this point it is easier to double up to 2 4" ports than trying to retrofit a slot port.

You really don't need any polyfil at all in a ported sub and definitely shouldn't be stuffing it w/ 5lbs.

I would leave the HPF where it's at. Lowering it is only going to reduce its ability to protect your sub.

Originally Posted by Audiophile1178 

I would have to concur with nograve but do remember that as you add another port you have to double the length of both ports to keep the same tuning.

Originally Posted by nograveconcern 

Yes, but re-calculate the whole thing. Box volume will go down as the ports displace more volume.

Originally Posted by fleek1116 

Contemplated the slot port and discounted it due to the extra construction complexity. When I came across the 6" port issues, I really should have put it back on the list. That box you linked looks quite nice.

Is it feasible to plug my round port and retrofit my box with a slot port on top (think spoiler on a car hood)? I'm imagining cutting a 2in x ?in rectangle out of the top of the box near the back edge. Then I'd build the other three side of the port toward the front of the box to give the proper volume. A slotted box plan below has small piece coming up from the port into the box. Was this likely added to give the port extra length? Or is it a normal part of the slot port to help the air enter less violently?



The first issue I can think of is not being able to roundover the edges entering the slot from inside the box, but hopefully that is a minor turbulence issue and I'll come out ahead of where I was with the 4" port. Any other glaring issues I'm missing with this approach?

Originally Posted by robertcharles 

I would put the extra 4 inch port or 2 six inch ports, but let them hang out the box in ordeer to see if they achieve the desired effect. Then, You will have to modify the box by allowing them to stay external or build a larger encloser to account for the added port volume.



http://www.subwoofer-builder.com/vent-options.htm


Check these out!!!


Keep cranking,

Robert

Originally Posted by nograveconcern 

Experimenting with external ports is a good approach, but his 7.8cu ft net box is big enough to allow proper port area inside without starting over from scratch.

Speaking of proper port area, 26.23 sq in is what you should shoot for with the 15 HF. That means a single port 5.78" or larger will do. 2x4" ports have an area of 25.1 sq in so they are still just under the optimal size, but close enough with some flared ends. They will end up being about 26" long.

Originally Posted by fleek1116 

I was thinking of plugging the front port and putting the other 2 on top since the back of the sub sits rather near a wall. Any reason I should avoid ports on top of the box?