4th BP experts? - 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 19th July 2009, 05:37 PM   #1
danielm is offline danielm  United States
diyAudio Member
 
Join Date: Dec 2006
Default 4th BP experts?

Cleaning out our storage building and found four of these beast. They are two 15" drivers in a 4th order BP box (JBL MP255 style) I used in a fixed install many yrs ago.

Pulled one out of storage yesterday and checked it outdoors in an open field. Test mic is a rat shack SPL meter placed on ground. The box is 8 ohms so, with 2.83 volts here's what I get at 1m:

30hz 95db
40hz 103db (sounded like I reach xmax here at around 129db, may have been the amp clipping, I failed to check )
50hz 101db
60hz 99.5db
70hz 100db
80hz 102db
90hz 104db
100hz 105db
110hz 105db
120hz 102db
130hz 99.5
140hz 96db
150hz 93db

I'd like to seperate these beast into a single driver per box. Also, it would be nice to get rid of the dip if possible.
And, I don't really plan on operating them above 110hz.

I don't know the driver specs (yet) and the box is about 3 cf sealed and 2 cf vented (per driver). Vent is a 4" slot 6" deep.

I'm wondering if I have any hope at loosing the dip at the cost of upper BW?

Hoping each new box is not larger than what it is now... 4cf would be nice. I can live with the dip if it requires a larger box to smooth it out.

Ideas?
  Reply With Quote
Old 19th July 2009, 07:52 PM   #2
diyAudio Member
 
chris661's Avatar
 
Join Date: Feb 2008
Location: Sheffield
Blog Entries: 8
Those figures don't look right.

4th order band pass boxes tend to have a frequency response which (from 20Hz up), rises in amplitude, stays flat for a while, then drops at the same rate as it came in.

Having a dip in the response just seems weird to me...

Still, I'm no expert, just expressing my interest

Chris
__________________
"Throwing parts at a failure is like throwing sponges at a rainstorm." - Enzo
My setup: http://www.diyaudio.com/forums/multi...tang-band.html
  Reply With Quote
Old 19th July 2009, 08:25 PM   #3
Ron E is offline Ron E  United States
diyAudio Member
 
Ron E's Avatar
 
Join Date: Jun 2002
Location: USA, MN
A dip typically means the Q of the sealed chamber is high - that it is too small, or the driver has a high Qts or both. To design this properly you need T/S parameters (preferably real, not published).

Here's a link, with a design spreadsheet (works in open office)
http://www.diysubwoofers.org/bnd/4thorder.htm
__________________
Our species needs, and deserves, a citizenry with minds wide awake and a basic understanding of how the world works. --Carl Sagan
Armaments, universal debt, and planned obsolescence--those are the three pillars of Western prosperity. Aldous Huxley
  Reply With Quote
Old 19th July 2009, 11:35 PM   #4
danielm is offline danielm  United States
diyAudio Member
 
Join Date: Dec 2006
I was afraid of that.

My old back just can't deal with anything bigger and was hoping to get by with a smaller box.

What about isobaric loading? Although, doubling up on the drivers would yeild me half as many boxes and I had plans to make a steerable array from many small boxes. Might just have to live with the dip.

I suspect the driver Qts is high. I don't belive they were very expensive. I'll see if I can make some measurements when I pull them apart.

Thanks.
  Reply With Quote
Old 20th July 2009, 04:47 PM   #5
GM is offline GM  United States
diyAudio Member
 
GM's Avatar
 
Join Date: Jun 2003
Location: Chamblee, Ga.
My take on it is that the dip is centered somewhere around 66 Hz, so increasing the front chamber's Vb is required to flatten it out, but if used in a high average power app, then it should have an even larger front chamber (higher Q) to account for the response flattening due to VC heating.

Isobaric loading reduces chamber Vbs, but then the vents are much bigger to have a low vent mach, so may have pipe harmonics in the speaker's pass-band that may need damping which in turn affects output, ergo chamber Vb adjustments in a 'chasing your tail' scenario as you try to find an acceptable trade-off. Going isobaric in a BP requires a PR solution IMO if some form of pipe/horn alignment isn't used.

GM
__________________
Loud is Beautiful if it's Clean! As always though, the usual disclaimers apply to this post's contents.
  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


Similar Threads
Thread Thread Starter Forum Replies Last Post
DHT experts, i need your help Cassiel Tubes / Valves 10 25th February 2009 01:42 PM
any kicker zr experts out there? marko Car Audio 2 30th September 2006 09:10 PM
Any TL experts out there? jmiyake Subwoofers 8 8th September 2006 03:17 AM
How low can shiva go??EXPERTS!!!!!!!!!! Andre Baptiste Subwoofers 5 5th March 2004 02:30 AM
Need help from the experts bateman Multi-Way 1 22nd June 2003 02:29 AM


New To Site? Need Help?

All times are GMT. The time now is 11:17 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