Need help with underperforming sub - diyAudio
Go Back   Home > Forums > Loudspeakers > Subwoofers

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 30th September 2007, 04:27 PM   #1
Orbus is offline Orbus  United States
diyAudio Member
 
Join Date: Sep 2007
Unhappy Need help with underperforming sub

Hi all,

The other day, after many, many months of work, I finally completed my first sub project. It's a single driver Dayton DVC 15" (295-190) using the 240 Watt PE Plate Amp (the one without bass boost). Enclosure is a straightforward box, about 190 - 200 liters with bracing, the driver, and amp taken into account. Inside of the box is lined with foam rubber (carpet pad). I have two 4" flared ports, 18" long each, which according to Unibox ought to give me a tuning of about 21 Hz and an F3 at 23 Hz. WinISD Pro gives slightly different numbers, but it's in the same ballpark.

Unfortunately, when I got it set up and in the room, the low frequency response is definitely less than I had hoped for. I've got a radio shack analog SPL meter and a computer that's hooked up to the stereo, so I did a run through with sine wave test signals from 100Hz down to 15Hz and recorded the SPL at each frequency. This is with C-Weighting, slow response, and no added adjustment factored in. This is the graph I came back with:

LINK

As you can see, the response is decidedly less than flat. I'm not expecting perfection here. My room isn't anywhere close to ideal. But a 15 dB dropoff in the low 30s? That just seems wrong to me.

This is my first project, so I'm hoping I'm missing something really obvious here. Because of the shape of the graph, I'm not even really sure how to attack this. I can't decide if I should be regarding it as a huge hump in the middle of the range, or an early dropoff and a room null around 75 Hz.

At the moment, the only thing I can think to do is try elongating the ports - maybe add another 5 inches - and see what that gets me. I know the PE amp also has a highpass filter, but I'm not real sure on the details. Maybe it's cutting off too high? I believe it is moddable...

Any ideas or suggestions are greatly appreciated. Thanks in advance.
  Reply With Quote
Old 30th September 2007, 05:32 PM   #2
Orbus is offline Orbus  United States
diyAudio Member
 
Join Date: Sep 2007
A few things I forgot to mention:

The receiver is set to cross over from the mains at 80 Hz. Lowpass dial on the plate amp is set to 180Hz (highest setting, basically bypassed). The mains are probably coloring the output some in the upper end. Indeed, I probably ought to flip the phase switch on the plate amp and see if it makes any difference in the null area around 75Hz. Will try it and post back.

Last thing is that the ports are up-firing, and the driver is front firing.

I'm running some 20-40Hz sine wave sweeps right now at low volume to break in the driver some. They're running at 10 seconds per sweep and I pretty much hear/feel nothing until it gets to the 4th or 5th second. Definitely not reaching the low end like I want.
  Reply With Quote
Old 30th September 2007, 05:59 PM   #3
GM is offline GM  United States
diyAudio Member
 
GM's Avatar
 
Join Date: Jun 2003
Location: Chamblee, Ga.
Greets!

Nothing in the specs/alignment accounts for a > ~5 dB roll-off except possibly the foam blocking the ports, so just to cover the obvious, the amount of foam is minimal and attached in such a manner that it doesn't hinder driver/vent action, the VCs are wired in parallel and all joints are airtight, right?

Hmm, your second post makes me wonder about how close the vents are to the rear of the driver's diaphragm, too close can increase Fb considerably > predicted.

The 75 Hz notch is probably a floor/ceiling room mode, if so, then there's nothing practical you can do about it.

GM
__________________
Loud is Beautiful if it's Clean! As always though, the usual disclaimers apply to this post's contents.
  Reply With Quote
Old 30th September 2007, 06:08 PM   #4
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi,
less than 6kB in your link. Just post .png as an attachment.
Why did you use image shack with all it's stupid downsides?

What does c weighting look like?
__________________
regards Andrew T.
  Reply With Quote
Old 1st October 2007, 05:21 AM   #5
Orbus is offline Orbus  United States
diyAudio Member
 
Join Date: Sep 2007
Sorry, already had the image posted on imageshack from a post on another board, so I just posted the link. Will go inline next time. Let's see - in answer to questions:

-The box is sealed up tight. I put a bead of caulk along all the inside edges, the amp and driver both have gaskets, and the port flares are epoxied. I haven't seen any indication of leaks.

-Pad is 1/2 - 3/4" thick foam rubber carpet pad from home depot. It's stapled in there pretty good.

-How close is too close on the distance from ports to basket? I'd have to take the amp off to check, but the ends of the ports are probably 5 or 6 inches back from the back of the driver. They're hanging down from the top of the box. The ends are probably more or less in line with the magnet though.

-Wiring is indeed parallel.

-I agree on the 75 Hz notch. I walked around the room with the spl meter and it's definitely localized.

-The graph I posted was done with C weighting. I'd have to run another sweep if you want to see A weighting, but I don't think I want to run it this late at night - bass carries through walls, and I live in a townhouse.

Now, here's what I've done today: I cut the front speakers out of the loop (turned their external amp off), so what I'm getting now should be all sub. The high end will be covered by the crossover in the receiver, but no more pesky other speakers to worry about. It's still down quite a bit in the 70s though.

I opened it up, disconnected the plate amp, ran speaker wire down one of the ports, and hooked up a pro amp. Then I put the plate amp back in place to close up the hole. I haven't done a graph, but just from running sweeps and frequency spot checks, performance seems to be largely similar. Amp is an Ashly FET-500, using one channel into the parallel wiring (I wasn't going to rewire the driver just for the experiment.) That pretty much exonerates the high pass filter in the plate amp as the culprit.

Next thing to try is probably experimenting with the port length. If having the ports in line with the driver is a problem, my only option is probably to put some elbow joints in them and point the ends in another direction.
  Reply With Quote
Old 1st October 2007, 07:39 AM   #6
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi,
keeping the port ends about 1.5diameters away from the cabinet walls and from the driver is supposed to be sufficient.
Your 5 to 6inch gap sounds about right for 4inch ports.

What software did you use to help with the cabinet design?
__________________
regards Andrew T.
  Reply With Quote
Old 1st October 2007, 03:10 PM   #7
Orbus is offline Orbus  United States
diyAudio Member
 
Join Date: Sep 2007
Unibox. I've run it through WinISD too. The numbers in WinISD come out a few dB/Hz different, but overall yield pretty similar results. I've attached the graph WinISD Pro spit out. WinISD gives me a bit higher F3 than UniBox predicts, but not by a lot.

I took the amp off this morning, and checked on the position of the ports. The ends are more or less inline with the magnet. Off axis though - so they're maybe 30 degrees to the left and to the right. Port centers are 6" away from the magnet, so the edges of the ports would be 4" away.
Attached Images
File Type: png winisdshot1croppedresized.png (50.5 KB, 300 views)
  Reply With Quote
Old 1st October 2007, 04:02 PM   #8
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi,
I had to set Fs to 22Hz, Vas to 250L and Qts to 0.415 to mimic close to your curve in WinIsdpro. Is that about right for the Dayton?
I am very surprised that you find this lacking.

Could the sensitivity be too low and there is insufficient signal getting to the driver to bring the level up to match the mid/treble?

What is the Frequency response of the two amps you have tried?

4inches from the magnet must achieve at least 6inches (1.5diam) from the cone.
__________________
regards Andrew T.
  Reply With Quote
Old 1st October 2007, 04:08 PM   #9
diyAudio Member
 
RobWells's Avatar
 
Join Date: Aug 2002
Location: UK
Are you using both voice coils ? - Are they connected out of phase ?

Rob.

Edit: just re-read thread and GM beat me to it..
  Reply With Quote
Old 1st October 2007, 04:20 PM   #10
diyAudio Member
 
Join Date: Apr 2001
Location: Leeds, UK
I think most of the problems you're having are room related. When I was setting my sub up I get an almost identical set of 'problems' as you have there except they are all higher in frequency as I have quite a small room.

The two peaks at 33Hz and 50Hz are related to the modes of the length and width of the room, the actual level of the sub at a guess will be around the 67db point on that graph. The suckout at 75Hz I'm not sure, I have the exact same thing in my room at 110Hz, but only from the main speakers, not the sub.

The lack of low end is probably the RS SPL meter not being able to pick up low frequencies very well. Google 'Room EQ Wizard', it's a piece of software that allows you to connect the RS SPL meter to your computer and measure/graph your sub's FR really easily. It's also got an RS meter calibration curve built in so you will get a more accurate measurement
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



New To Site? Need Help?

All times are GMT. The time now is 06:53 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2