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? :)
 
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?
 
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.
 
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.
 
Andy Westcott said:

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.)
 
Andy Westcott said:

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.
 
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).
 

djk

R.I.P
2001-02-04 4:23 am
USA
A method of reducing distortion and allowing for closer driver spacing:

PPSL2.gif


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).

PPSL3.gif


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).