Any benefits to stacking ported bass cabinets? - 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 20th January 2008, 01:07 PM   #1
diyAudio Member
 
Andy Westcott's Avatar
 
Join Date: May 2006
Location: Exeter, England, UK
Default Any benefits to stacking ported bass cabinets?

Hi guys.
A pretty simple question really, but this whole area seems shrouded in mystery & magic!

What are the real gains to be had by stacking ported bass bins, particularly in a PA system? Please note - this refers to ported cabinets, not horns.

I have always understoood that the low frequency resonse is extended somewhat, with perhaps a 3dB gain below fall-off for each doubling of cabinet numbers, but no effect higher up.

However, several sources seem to think the efficiency of the system, across the whole of its operating frequency range is increased by 3dB for each doubling up of cabinets - I'm not conviced that this is so, and feel this may be a fallacy brought about by the fact that an ampifier may be able to supply (almost) double the power to half the impedance, giving the 3dB increase in output that way.

So - what's the bare naked truth about stacking ported cabs?
  Reply With Quote
Old 20th January 2008, 01:30 PM   #2
diyAudio Member
 
Join Date: Jul 2005
Location: North Derbyshire
Stacking them in multiples extends the bass response, it's a standard PA technique, obviously it also makes them louder because you've now got twice the power and can move twice as much air.
__________________
Nigel Goodwin
  Reply With Quote
Old 20th January 2008, 03:01 PM   #3
diyAudio Member
 
Andy Westcott's Avatar
 
Join Date: May 2006
Location: Exeter, England, UK
Thanks Nigel for your reply;

But am I correct in suggesting that the overall efficiency of the stack is unaffected? This is what I currently considered to be the case - ignoring the additional SPL available due to lower impedance, higher power handling etc.

Also just what kind of improvement to the bass extension could be expected with, say, a simple pair of bins over a single one?
  Reply With Quote
Old 20th January 2008, 05:22 PM   #4
Speakerholic
diyAudio Moderator
 
Cal Weldon's Avatar
 
Join Date: Jan 2004
Location: Near Vancouver
Hi Andy,

What Nigel said plus:

There is an increase in sensitivity of 3dB every time you double the driver count. Another 3 can be attributed to it if you leave the amp setting the same as you are now delivering twice the overall power - lower impedance/ higher voltage. The best way understand it (for me anyway) is if you have 4 drivers and you series/parallel them you will gain 6dB for the same power in.

As far as the extension goes, I will have to let others with more knowledge handle that.
__________________
Next stop: Margaritaville
Some of Cal's stuff | Cal Weldon Consulting
  Reply With Quote
Old 20th January 2008, 05:34 PM   #5
diyAudio Member
 
Andy Westcott's Avatar
 
Join Date: May 2006
Location: Exeter, England, UK
Thankyou - this is exactly the bit I'm having trouble believing!

Using that 6dB/4 drivers model, you would be achieving unbelievable efficiencies with a largish wall of speakers, and I can't see that as being the case, although I can understand the set having a more extended bass response as compared to a single driver - that's my feeling and may be incorrect, hence the question.

You see - you'd be looking at almost 10dB up using 8 drivers compared to the same power through a single unit - that is extreme, and is the rumour I want clarified as I just don't know what to believe anymore!

Anyhow, thanks for your input.
  Reply With Quote
Old 20th January 2008, 05:51 PM   #6
kstrain is offline kstrain  Scotland
diyAudio Member
 
Join Date: Apr 2006
Location: Glasgow
Quote:
Originally posted by Andy Westcott

You see - you'd be looking at almost 10dB up using 8 drivers compared to the same power through a single unit - that is extreme, and is the rumour I want clarified as I just don't know what to believe anymore!

Anyhow, thanks for your input.
Possibly part of the problem is that the question is not specific enough. Are you asking about far field or near to the speakers (it is different), in a room or in free space? Are the speakers against a wall or in a corner? In each case the result for a given wavelength is different (you already noted that wavelength matters).

Take two well-sealed speakers in a well-sealed room, if they play below the frequency of the lowest room mode you can think of them presurising the room. In that case it is not to hard to accept that you get twice the pressure (6dB pressure ratio) from two speakers.

In free space very far (many many wavelengths) from the speakers how could it be anything but the same as twice the power into one speaker (i.e 3dB increase).

In real situations the "enhancement" can range from complete cancellation (at a particular frequency) to strong buildup (if the second speaker exites a room mode).

Why is there this difference?

In free space you only have travelling waves, there the relationship between sound pressure and the power carried by the waves is simple (and your intuition of conservation of energy works easily). In a room (or within a wavelength of the speakers) the situation is quite different. A standing wave has very much higher antinode pressure than the travelling wave power suggests.

Ken

(ps. I've avoided equations and skipped over much of the physics.)
  Reply With Quote
Old 20th January 2008, 06:08 PM   #7
diyAudio Member
 
Join Date: Jul 2005
Location: North Derbyshire
Quote:
Originally posted by Andy Westcott

You see - you'd be looking at almost 10dB up using 8 drivers compared to the same power through a single unit - that is extreme, and is the rumour I want clarified as I just don't know what to believe anymore!

10dB is only twice as loud - and you've spend a HUGE amount to achieve it. But you don't just pile speakers on, you pile amplifiers to feed them - more power, more volume, and lower bass response from the stacked bass units.

Try it with a guitar amp, a single 12 inch unit fed from an amp, is considerably quieter than a 4x12 (using the same drivers) fed from the same amp.
__________________
Nigel Goodwin
  Reply With Quote
Old 20th January 2008, 06:36 PM   #8
Eva is offline Eva  Spain
diyAudio Member
 
Eva's Avatar
 
Join Date: Oct 2003
Location: Near the sea
Send a message via MSN to Eva
Placing two direct radiators in close proximity results in 3dB/W of efficiency improvement, but only at low frequencies as the wavelenghts start to become longer than the distance between the drivers. This effect (mutual coupling and loading) is only limited by distance and by the fact that "heavier" acoustical loading does not result in improved efficiency past a certain point.

Another advantage of mutual loading is that cone excursion for a given SPL is reduced resulting in increased LF output capability.

In my experience stacked ported speakers tend to exhibit a higher Q and thus more port output than expected. Some EQ is required sometimes (another alternative is to modify the ports).
__________________
I use to feel like the small child in The Emperor's New Clothes tale
  Reply With Quote
Old 22nd January 2008, 04:36 PM   #9
djk is offline djk
diyAudio Member
 
djk's Avatar
 
Join Date: Feb 2001
Location: USA
A method of reducing distortion and allowing for closer driver spacing:

Click the image to open in full size.

You can stack these quite high and have a line, or like this and have a 3'x3' source (conventional mounting would give about twice this size, more distortion, and limit how high they would couple).

Click the image to open in full size.

Each box was loaded with two 15s about 98dB/W/1M, thus about 101dB. Two boxes would then be 104dB, four 107dB, etc. Each box would handle 800W (29dBW), so four would play 142dB/3.2KW/1M (less power compression).

I actually built 24 of these with a B6 design -3dB at 31hz, although I have never run more than four per side.

If you can pack the drivers close enough to couple, the theoretical limit to efficiency is %25 (about 107dB).
__________________
Candidates for the Darwin Award should not read this author.
  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
reducing bass output in a ported bookshelf speaker Sara Multi-Way 1 19th October 2007 04:25 AM
Dbl ported bass reflx enclosure question: GrahamnDodder Multi-Way 33 8th October 2003 03:31 PM


New To Site? Need Help?

All times are GMT. The time now is 09:10 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