Quick summary to start this thread.

IB is Infinite Baffle
AP is Aperiodic
BR is Bass Reflex

different ways (other than sealed) of loading a woofer in an enclosure.....

The discussion began like this....

This is an IB install in a car:
http://www.talkaudio.co.uk/vbb/showthread.php?t=172183

and this is a discussion of IB/AP on another forum:
http://www.diyaudio.com/forums/showthread....mp;pagenumber=1

The idea (of AP/IB,etc.) is to control impedance rise at Fb....



OK.. as Impedance rises, amplifier output decreases... we know this.

by controlling impedance rise, we ideally improve 2 things:
1. amplitude linearity - the speaker 'system' no longer 'drops off' below Fb, as the box is not part of the equation (I did say ideally)
2. phase linearity - when a speaker's output is affected by increased and decreased amplitude, this introduces phase anomalies around those frequencies

How does this impact frequency response? anyone?

In basaholic's quote;


This is incorrect, lower Qtc results in a lower impedance peak, not a higher one, this is very easy to test.

There's been some discussion on the DIY audio forums involving what is essentially a very large ported box with the vent either being stuffed or replaced with AP membranes, the goal is similar, to flatten the impedance curve as much as possible.

Also don't discount the damping properties of fill, a correctly filled enclosure can halve the impedance peak straight off the bat, infact you can end up with a lower Qtc than the driver has even in infinite baffle.



The thing with this is you are relying on the damping of the amplifier to (at least in part) control the resonance of the driver at Fb, this no doubt is where people are hearing the difference between different amps (quick call Richard Clarke), if you mechanically/acoustically damp the driver's resonance you have less back emf going to the amp, which is always a good thing.

In regards to the questions

1, the system will still roll off below fb, it'll just be a more shallow roll off, you can't get away from the physics of how the driver behaves.

2, [edit] after some more thought on this, I would think that phase anomalies would only be a concern in relation to other drivers operating in the same frequency range, ie; absolute phase isn't important, only relitive phase at the listening position.

The best way to flatten the impedance curve is to load the driver in a 1/2 wave front horn...

An "AP" enclosure will not necessarily help you with either of those things. Your ears will not tend to detect the phase anomalies, but rather the frequency response anomalies. Also, it is impossible to have amplitude linearity in the real world - and if the frequency response is extended to a lower frequency, then you are in fact lowering the Fc (and the efficiency/sensitivity of the system).
The theory behind "AP" enclosures was that it was meant to increase the mechanical damping for drivers which have poor mechanical damping - the effect is lowering the Qms and thus lowering the Qts slightly. The second factor they use a large enclosure (in this case, the whole boot) which also results in a lower Qtc.

So while it can have an effect on the impedance (the effect of lowering the Qms), the real aim is to control the frequency response.
If a sealed box (or using the boot as the enclosure) already enables you to get the frequency response that you want, then a membrane damped enclosure is a waste of time. If you don't have the luxury of a boot, then you can also consider using a Linkwitz transform filter with a small enclosure.

Despite popular belief, this type of enclosure will have the most effect on HIGH Qts drivers with high Qms (poor mechanical damping). It tended to be used in back in the days when home audio drivers were used which were not really suited for car audio. If you use a subwoofer with reasonable mechanical damping and high electrical damping (low Qes) then this type of enclosure is not suitable. If the final Qtc of the system is below 0.566 then the system is overdamped and the "AP" membrane will make things worse.

But I don't recommend aiming for a particular Qtc, I recommend aiming for a smooth in-car frequency response.

The effect of lowering the impedance at resonance would be a dip in the frequency response around resonance, unless you are using a bass reflex type enclosure.
As Richard Clark has always said, you don't want to lower the impedance so that it is almost flat.



It depends on whether we are talking about mechnical damping or electrical damping. This can be shown in any loudspeaker modelling program too.




But the output impedance is sufficiently low on virtually all car audio amplifiers. The amplifiers are actually designed to be able to cope with the load.
In the case of RCs when comparing a normal amplifier to one with a very high output impedance (normally tube amplifiers), he simply adds a resistor in series with the normal amplifier.
The amplifier doesn't really add damping to the driver - in the case of a high output impedance, this is in series with the driver, so it decreases the damping on the driver.

That was my understanding also.



Your answer 2 - aboslute phase, as it changes with frequency, is audible and certainly affects realism/accuracy - especially where a transient, or wide bandwidth signal appers in both the midbass and subbass regions - the effect is phase (and therefore time) mis-alignment between the two - very noticable to me, as I hear time mis-alignment much more acutely than amplitude anomalies.

Here are Winisd plots to illustrate what I was saying.

Yellow is the reference and has the following specifications. (Qts - 0.5, Qms - 5.778, Qes - 0.547 Fs - 25hz, VAS - 80L, Re - 3.5, Sd 484cm^2)
The Qtc is approximately 0.7.

Blue is basically the same as yellow, except it has a much lower Qms - 2.0 (very low). (Qts - 0.43)
The Qtc is approximately 0.61.

Red is essentially the same as yellow, except it has a much greater BL and so the Qes is much lower (so it has greater electrical damping). (Qts - 0.272, Qes - 0.286)
The Qtc is approximately 0.4 (overdamped).

Edit - Note - the images are in PNG format, so they look fine, but the compressed jpg thumbnails do not look so great.
Also, these values (Qms for Blue, Qes for red) which were chosen to show a clear difference in damping - it is unlikely that the Qms could be reduced from 5.778 to 2.0 with a normal "AP" membrane install.

Further notes - the enclosure was kept at a constant 87L.
Secondly, the same Qts/Qtc/frequency response as blue can also be achieved with greater electrical damping (eg higher BL), rather than mechanical damping - and the impedance would peak at about 33 ohms.

Those are very linear drivers you have there... are they pehaps mathematical models??

I'd be more interested to see the effect of different acoustic damping on an actual woofer in a car.


I'm not saying the results would be different... but the idea is to compare various damping methods, rather than different woofer's specs.

The purpose was to show the effects of increasing the electrical or mechanical damping of a driver, while controlling the other variables.
(The other specifications are quite possible for a real world driver.)
It is not that hard to see how the graphs relate to a subwoofer inside a car - if the other factors are considered, but held constant, then the major difference is still the difference in frequency response. (other factors such as the transfer function of the vehicle held constant, the voice coil inductance, linear xmax, amount of power etc)

I agree that all those things make a difference, but I want to know why would someone go to great trouble to make their sub IB or AP in a car if there was no benefit??

You can essentially eliminate the rear wave in an AP design, venting to outside the car (in the rear qtr panel), and controlloing the cone via the AP mat over the woofer...

Where's Dean when we need our brains exploded?

would be nice to see a phase graph too

if i didnt read the information on the graphs i would have assumed that they were just differently aligned sealed boxes.

BTW Pulse i read your links that you gave us on the AP...and it seems that they were just adding a AP mat to the BR port.

just think to gain 6db you have to make the VC travel 2 times further!!!!!!

Not exactly jas, they are drilling a series of ports - to make a grille for the AP membrane to vent through.
There's also a link somewhere to a US car audio site - where they actually weld in a steel louvre to vent the sub out of the car...

I'll add both the links into the first post, for reference.

Normall an AP membrane would be 1/2 to 2/3 the size of the woofer, and consist of a 'filler' material, such as dacron, or insulator Batt... with the idea of being able to adjust the 'rate' of air allowed to flow through - so a tightly controlled leaky box - like a port, but smoother.

this is where it got confusing with those links simon

some people were adding material to their BR ports

and other people were drilling more holes in thier boxes and changing the material and density to each hole to adjust the alignment of the box sub combination.


i remember when UDS had pre-made AP mats used for certain subs (i can not remember the range of specs they needed). The box was meant to be as small as posible to just fit the woofer and AP mat in side. This is different from these AP mat designs since they tend to use quite large "leaky" sealed boxes. If you have the space then why not just use a sealed box with the alignement you want (Qtc) since the sealed box will be more efficient why use the AP?? The sealed for the same DB level will have less THD thats for sure due to lower cone movement.

From what ive read about AP design is that they are typically less efficient than a sealed box. If this is the case to keep up with a ported design in terms of output they will require a larger distance to travel. SO basically what im getting at is the benefit of an AP setup vs BR (ported) would be lost at the same DB level in terms of THD. We know that phase should be better with AP (due to only one impedence peak) and IMD....not sure about that one.

The reason for IB (Infinite Baffle) or AP is much the same - It is a 1st order 'enclosure', and so allows the sub to play 'naturally'.
As we know sealed is 2nd order, BR is 3rd order, Bandpass 4th, etc...

The part I was confused - as you seem also to be - was the AP they talk about is controlling the leakiness of a larger box - effectively making a controlled damped BR. Boyd Kraigher of Total Recoil is doing the same thing with his full-range drivers - a medium sized box using a combination of AP and BR to smooth the low-end response of the woofer.

When we talk of AP - as per the USD AP mats you mentioned - we are meaning an AP damped Infinite Baffle.

So AP-IB versus AP-BR.

Infinite baffle is still 1st order, using the AP to dampen the low end resonance of the driver (Zrise at Fs).

BR is still 2nd order, using the AP to dampen the box resonance (Zrise at Fb).

Now what is the advantage or disadvantage of each technique???

i have an AP mat here and Blanky has some for his USD 15's at home.
i did a fair bit of reserch on them a while ago. most people seem to not know much about them, esp on this forum.


what about TL box's? i looked into these quite some time ago, seem good but no one seems to have any way to design a box properly, it's all hit and miss.

TL is like a damped BR, I guess.
I have heard a TL system, and it wasn't as nice as sealed - you could still hear the "ripples" in the bass created by the TL. But then, I didn't like the crossovers they used either... The drivers themselves sounded fantastic.

have a quick look at this link

http://www.elitecaraudio.com/article.php?sid=18