IB Subwoofer (aka IB subs)- FAQ Pages
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There are two FAQ Sections. The introductory FAQ is below on this page. A multi-page illustrated guide to designing, building, and installing an IB subwoofer is located HERE

To ask questions about IB subs or your IB sub project please join our IB sub forum
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IB - General consideration FAQs

1) What is an IB subwoofer?

For ease of explanation it's convenient to think of an IB sub as a massively large sealed box alignment. This is overly simplistic, but it serves the purpose of an introduction until such time as greater technical explanation benefits the conversation. 

Think of the baffle as an infinitely large boundary that provides isolation/separation between the front-wave and the rear-wave coming off the driver/drivers. For practical purposes this means the drivers are mounted so they share a common boundary between the listening room and an adjacent space. This adjacent space can be the attic, basement, crawl space, garage, or any room that shares a common wall with the listening area. 

However the IB is created, there must be adequate isolation between the front wave and rear waves. Complete isolation isn't mandatory. This means that small cracks and openings, louvered doors, etc., aren't a significant problem.

2) But don't drivers need the 'air-spring' created by pressure in the box to control their behavior?

To answer this let's review how a loudspeaker operates. The incoming AC signal moves the cone in a direction 1/2 the cycle of the frequency. The polarity then reverses, and the AC signal moves the cone in the opposite direction to complete the cycle.  (In lay terms the cone is pushed in and pulled out by the motor reacting to the incoming signal from the amp.)  The suspension (surround and spider) keep the driver 'centered' while the motor is cycling the cone in and out. This operation occurs whether the driver is mounted in a box, in free-air, or in an infinite baffle.

In years past there were drivers designed to utilize the air-spring of the box to support the cone. Today those drivers are rare and certainly not ones we want to use for IB subs.

3) Can I model the operation of an IB sub?

Those inclined to run sims software can approximate some the performance aspects of an IB sub. Use a design program like Unibox, or WinISD Pro, enter the largest box volume the program will allow (for Unibox this is 99,999L). Use 'heavy' fill as the damping amount. One benefit of running a simulation is that it will provide basic information as to the amount of  power and Xmax needed for an IB subwoofer.

As with any simulation modeling the IB doesn't provide accurate information as to the effects of room-gain, or tell us anything about the sound quality of the sub.

4) How does an IB compare to a sealed or ported sub?

Sealed and ported subs are enclosed systems where the enclosure and rear-wave coming off the cone impacts it's operation.

In the case of a ported alignment; the rear-wave coming off the cone interacts with the air-mass inside the enclosure to create a Helmholtz resonator (like blowing over the top of a bottle). The vibrating mass of air colors the sound. 

In the case of smaller sealed box systems; the box interaction of the rear-wave and the compressing air-mass inside the box changes the 'Q' of the system. This too colors the sound.

In poorly designed or inadequately built ported or sealed box subs, the walls of the enclosure itself can be excited by the rear-wave coming off the driver. If the walls of the box are vibrating there's a detrimental effect on the sound quality.  

With an IB sub there is no Helmholtz radiator effect, no compression of the air-mass and in most cases no box walls to negatively impact the output of the driver. 

5) Where should I put my IB?

Due to the design most people don't have the luxury of choosing between multiple placement options.  If possible, placement should be on the front wall, floor, or ceiling, centered between the main speakers. This allows the greatest flexibility of crossover points, and that guarantees the sub won't smear the stereo image. Frontal side locations work well too. Locations behind or beside the listener are not recommended (see #19 for more info about IBs behind the listening position). The more off axis compared to the main speaker, the lower the crossover point should be.

Those with the flexibility to do mount multiple drivers on the front/rear walls might consider doing a the IB version of a  Single or Double Bass Array 

6) How big a space should there be for rear wave?

The rule of thumb is that for each driver used, there should be a space no smaller 10 times larger than the Vas of the individual driver. That is then multiplied times the number of drivers. 10 times the total Vas or larger is considered optimal. For a more in depth explanation regarding the 'why' of the of the 10 times Vas recommendation click HERE

I want to add a bit of an addendum to the answer to question #6. For those that can't do a 10 times Vas IB, but can do at least 4 times Vas, go ahead and build it. Now this won't be a true IB (Qts =Qtc) . But anytime you can get a space that's 4 times Vas, you'll have a sub with better sound quality than a standard portable box sub. Note, one will need to use standard sealed box construction techniques with a 4 times Vas box. Be sure to choose low Vas drivers if you take this approach. 

7) How many drivers should I use?

This is a difficult question to answer accurately given the variables involved. To error on the side of safety, we recommend using at least double or more the Sd of that needed by a box sub used in the same room. In a small room 2-15"s should be adequate. For larger spaces or for high output, multiple 12"s, 15"s or even 18"s  are desirable. People should understand the IB bass is much different than the bass from a standard 'box/tube' sub. Because of that, we recommend use of more drivers than one would normally associate with a 'normal' box sub. Please note, the primary function of using significantly more drivers is to maintain extremely low distortion and protect the drivers, not to create insanely high SPLs.

7a) Why do I need more drivers in an IB than a standard box sub?

The enclosure (for a small sealed box) functions as an acoustic high-pass filter. That acoustic filter acts to protect the driver from over excursion at lower frequencies because it rolls of the output as the frequencies go lower. Since an IB has no 'box' per say, it doesn't benefit from the protection from the acoustic filter. This lack of protection from the acoustic filter also means that an IB easier to drive at lower frequencies. As a result it's easier to hit the limits of the suspension because IB subs play so low.

8) Won't the drivers be damaged since they're not mounted in a box?

The output of IB subs is limited by the suspension travel of the driver/drivers. And they will reach the excursion limits of the suspension before reaching the thermal limits of voice coil. As a result it's a good idea to use larger drivers, or more drivers than are normally used for a standard 'box' sub.

One example of how this effects a driver is the Parts Express IB woofer. It has a 'normal' VC thermal rating of 350 watts. PE however 'de-rates' the power handling to 140 watts, when the driver is mounted in an IB. This is to protect the suspension and keep the VC from bottoming. 

It is a good idea to keep this 'de-rating' guide line in mind, when choosing the drivers. When possible check with the manufacturer if you have questions about using one of their drivers in an IB. 

9) How do I choose my drivers?

Chose a driver preferably with a 1/2 roll rubber or foam surround. Most accordion surrounds are limited in their usefulness and are usually noisy when driven hard. (Note; as of the summer of 2006, new drivers with updated accordion surrounds have been developed for IB use. See the forum for specifics regarding which drivers are recommended)

Look for woofers with a Qts that is in keeping with the sonic characteristics you want. Generally a low Fs is desirable, however drivers with a higher Fs will work, they simply require slightly more EQ. (Note; contrary to popular belief driver and and do play well below their Fs) 

We do not recommend using pro-sound drivers. Most have too small an Xmax for use in an IB. Also we do not recommend a majority of the drivers designed by car audio manufacturers. There are a few car audio woofers that are suitable for our needs, but by and large most aren't appropriate for IB use. 

10) Which Qts/Qtc should I chose?

For an IB, the Qtc of the system is approximately the Qts of the driver. If you like very tight bass, chose a lower Qts driver. If you like the "HT" sound, then chose a driver with a higher Qts. Dual voice coil woofers can be used with a variable resistor in series with the inputs of an unused VC. This allows the "Q" to be adjusted to the listener's wants/needs. One drawback of this is that the power handling is cut by at least 25%. Here's a LINK to the Adire Audio article on RDO (resistively damped operation).

NOTE 1: using RDO or leaving one VC open will decrease the output of each driver -3dB.

NOTE 2: using narrow bandwidth EQ will raise the Qtc of any system. Wideband or shelving filters have significantly less impact on the Qtc

I wrote a post on the forum going into detail as to my personal recommendations for the choice of driver 'Q'.  Here's a LINK to that post. There are those rather emphatic that the Qts for a driver operating in free-air must be ~0.7. If that's what you want by all means use driver with that Qts.

11) Is there a formula for calculating the amount of displacement (Vd) needed for a given room size?

This is the same as question #4, but people keep asking so obviously the info in #4 isn't sufficient.

It isn't possible to create a formula calculating the Vd per a specific cu ft of room, given all the large number of variables involved with these types of installations. Room size alone isn't that important. The primary considerations are the distance from the listening position to the IB, and the desired output level at the listening position

12) Will any driver work?

Most 'normal' woofers satisfactory for sub use in sealed or ported designs will be satisfactory. Access the forum for guidance in choosing an appropriate driver. Avoid drivers smaller than 12" due to the problems associated with trying to move large volumes of air with small cones (aka radiating resistance).

13) Won't the heat of an attic/humidity of a crawl space harm the driver?

If a driver is suitable for use in a car environment, it is usually acceptable for use in an attic. Many drivers designed for home use, are approved for car use by their manufacturers. Check with the manufacturer to be sure. 

14) If I need/want multiple drivers, must I cut a hole to mount each of  them?

There are two basic mounting systems. One is simply a line of drivers attached to a baffle board. The other is called a "manifold array", "transition box" or "plenum". This system allows multiple drivers to share a common opening, and be located in a more compact space.  Check the generic design/build/install FAQ and the gallery pages for examples of each type installation. 

15) How big an opening should there be for my manifold ?

In theory the best choice for a manifold would be an opening equal to the summed swept volume of the diameters  of the drivers being used. In the real-world an opening this large isn't practical nor is it necessary. The recommendation is make the opening as large as practical given the limits of stud/joist spacing or other physical considerations of the install.  That said.....

As a basis for determining a practical outlet size we can reference Collo's gap calculations for the distance between the driver and the bottom plate of a sono-sub. Below is the quick reference guide as seen on Collo's website.

As an example we see a pair of high excursion 18"s would have a recommended opening 18" dia X 12" wide. With an IB the height of the opening is often larger than the diameter of the driver, and the width is usually that of a stud-bay or the joist spacing. So we're within the recommendations seen in the graphic

Invariably someone asks "how small can the opening be?" It's best to avoid having an opening smaller than 50% of that from the calculations.   

16) Is there an optimal design for the transition box (manifold) ?

The box should be as short and compact in length/height as possible. Designs like Tom Nousaine's TSTSTW or 12 Shiva's Dancing have cancellation issues due to the arrival times between the drivers nearest the listener vs those farther away. In certain circumstances these cancellations are beneficial, under normal circumstances they should be avoided. The ideal box is a single driver in height/length. An example of this would be a 18"-20" cube with drivers mounted facing in from the sides. This design allows 2 15" drivers to be used initially, and 2 more drivers could be add later if additional output is needed/wanted. For more 'hands-on' information about manifolds please read the Generic Design/Build/Install pages

17) Do I need a large power amp to drive an IB?

The IB will be as efficient as the woofer's rating, with expected increases if additional drivers are utilized. So chose your amp accordingly. The number of drivers being used, the efficiency of the drivers, size of the room and the desired sound pressure levels, are all factors to consider when choosing the correct amplifier. As noted in FAQ #1 many aspects of the performance can be approximated using a box modeling program.

18) Will an IB vibrate the entire house?

Yes, but so can a portable 'box' sub if played loud enough. An IB however will have the same amount of sound coming off the rear of the drivers as comes off the front. This should be taken into account especially if placed in an attic or basement and played at high SPLs. Using an even number of drivers with them mounted firing toward each other (opposite wall mounting in a manifold/plenum) will result in the cancellation of some of the mechanical forces on the structure. Note that high output/high excursion standard 'box' subs will cause an entire house to vibrate too. So an IB isn't alone in this situation.

19) Where should the IB  be placed?

If you're making a traditional IB with wall mounted drivers, if possible they should be mounted immediately behind the front main speakers. If you're using an outlet box/manifold array then the opening should be placed as close as possible to an area centered between the mains and in line with their front baffle. Those wanting stereo IB's should place them immediately adjacent to their respective mains. For people that don't have access to the a centered position, there are successful installations many feet to either side, but one should seriously consider using a phase adjustable crossover to obtain maximum performance. Also it's best keep the outlet box/manifold array as short as possible to minimize pipe resonance's. This also avoids cancellation resulting from the differences in arrival times when drivers are mounted at differing distances along in the pipe.

Generally speaking we do not recommend placing the IB behind or immediately beside the listening position (too many potential phase related issues with the mains/smearing the stereo image). If this is the only option, test the location (see FAQ#15)  prior to cutting any holes. For the best performance in this situation consider using sealed boxes in the front of the room, augmented by the rear mounted IB with a very low crossover point. Also see FAQ#16 regarding phase adjustable crossovers. For more info on this topic see page #1 of the Generic Design/Build/Install page.

19a) I have an unused fireplace. Can I use the firebox and chimney for an IB?

No you won't have an IB, you'll have a grossly mistuned transmission line

20) Is there any way to test the placement before I start cutting holes in my house?

Yes there are two methods, both require using an existing portable box/tube sub.  1) Place the sub in the current listening 'sweet spot'. Then go to the proposed spot for the placement of the outlet for the IB. For those contemplating attic mounting this means getting up on  a ladder. If there are going to be major playback issues with the IB, they should be audible from the spot where the outlet will be placed. 2)Take the existing sub and place it where the planned opening is for the IB. Go to the listening position and perform the same tests as listed above. 

If you don't have a portable sub to use for testing, one option is to make a test box using one of the drivers for your IB. A simple sealed enclosure will do the job. 

A lazy man could build up a manifold with a removable panel and use this as his test box. When the placement tests are completed, the side with the removable panel becomes the opening into the room. Make sure the assembled box can be moved to and placed in the intended location prior to prefabricating any manifold.  

21) What about phase? My IB and mains are located at different distances from the listening position.

Use the placement test described in FAQ #15. If phase is an issue it will be apparent with this test. If there is a phase problem, that can usually be fixed with a phase adjustable crossover. FYI, the Paradigm X-30 is an example of a moderately priced outboard active crossover with a variable phase adjustment. Also many of the available plate amps offer adjustable phase control. Note that phase controls effect phase at the crossover point, they aren't a 'magic bullet' solution to all room related issues. For those wanting a phase controller independent of the power amp, Rod Elliot offers a PCB for those into DIY, it's Rod's Project 103.

22) Will I need an equalizer?

Although not mandatory, use of a parametric EQ will allow the best overall performance and is highly recommended. Generally speaking  1/3 octave equalizers lack the flexibility for obtaining the best performance. A good inexpensive parametric EQ for subwoofer use is the Behringer DSP1124P or FBQ2496. Use the EQ to cut the peaks in the frequency response, and create the flattest possible curve. Don't try to boost room induced nulls. This is impossible and a serious waste of amplifier power. Use of small amounts of boost to 'shape' the FR fine. Avoid using large amounts of boost, and understand that any boost puts additional load on the amplifier.

23) Is there a fire danger with attic mounted IBs?

Although theoretically possible, the odds of this occurring are extremely low. When an amplifier dies it can send DC into a speakers, that instantly melts down the voice coil of the driver. This almost always results in an open circuit. The voice coil will smell burnt, that's a function of the overheated voice coil former, not an actual fire. If fire remains a serious concern for someone, install a smoke detector beside the sub.

24) I have a ceiling mounted front projector setup. Will an attic mounted IB cause the picture to vibrate?

Several people have ceiling mounted manifold subs, and are using ceiling mounted CRT or digital projectors. To date, no one has reported problems with the picture vibrating when a manifold was used. Use of a line array might be problematic.

We've had people note waves in large front projection screens associated with high IB output levels. This situation however isn't unique to IB subs. Any sub that sufficiently pressurizes the room can create ripples or waves in a fabric screen.

There are numerous installations with the IB outlet behind an AT screen. To date no one has reported any issues with the output from the IB causing the screen to move.

25) A drop ceiling would be the only thing separating the front and rear waves, can I install an IB?

No, a drop ceiling doesn't provide adequate isolation between the front and rear waves. 

FYI, a drop ceiling functions as a bass-trap. 

People have built IB subs in the attic above a drop ceiling, then extended the outlet of the manifold below the ceiling tiles. In this instance it's a good idea to place 6" or more of fiberglass batts on top of the tiles so they're less likely to bounce from the pressure of the bass output.

26) I have 2 drivers from company "X" and can't buy more. Can I buy 2 drivers made by another company and use them with the drivers I have?

Mixing drivers from different manufacturers isn't recommended, unless the majority T/S parameters of the drivers are very closely matched.

27) I can put my IB so the rear wave exits into the laundry room, it has louvered openings in the bottom of the door. Will they ruin the performance of the IB ?

This is sort of a repeat answer from FAQ #1 but it's frequently ask. Where the front and rear waves come together there will be a null. If the listening area is immediately adjacent to the door, you will probably have some audible cancellation. If you're seated a distance away from the area of cancellation, you'll hear no negative effects.

28) Will your driver work mounted in a horizontal orientation?

 Here are links to the Adire Audio  and the Parts Express 5% sag calculators. Note the effects of gravity are cumulative, this means the lifespan of horizontally mounted drivers will be shorter than those in a vertical orientation. If this is the only choice choose drivers with as stiff a suspension as possible. This usually means using drivers designed for car audio use.

29) I live at or near the North/South Pole, what considerations are there for ceiling mounted IBs in colder climates ?

The drivers themselves are pretty bulletproof. Until the voice coils and suspension warm up the T/S parameters are slightly changed. One enterprising fellow reverse mounted his drivers (fame and magnets in the manifold), thus keeping the metal parts of the speakers in the heated section of the house. He found this improved the performance during the colder months.

Implications regarding heat loss through the manifold are those of convective loss through the cones. If this is problematic or worry some, use a removable insulated door or plug that can be quickly removed when the IB isn't operating.

30) Ok, I completed my IB what happens now?

Sometime after sending in pictures of your installation, you'll get a visit. It will be late at night, people wearing gowns and hoods will show up. They'll have bass demo CD's. They'll teach you the secret "Cult" handshake and salute. In the morning when you awaken, you have a vague memory of this as a dream. But remember........... 
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Misc really important stuff.... ;^) 

Why are we called the "Cult of the Infinitely Baffled''?  

We are a cult because like all irrational institutions, we know our faith and beliefs represent the only true path to low frequency enlightenment (LFE). And because no amount of rational thought, scientific knowledge, law suits or political pressure, will change these beliefs. 

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I located a copy of the first generation Shiva whitepaper. It contains a brief section discussing IB operation. Click HERE  to see the whitepaper, as a big bmp file. Click HERE  to see the whitepaper, as a smaller .pdf file. 

This article is reproduced with permission from Adire Audio
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Here are links to the various pages in the gallery

Manifold Designs are on pages 1 and 2
Wall mounted speakers are on page 3
Unusual and Extreme Designs are on page 4
"Infamous" Designs are on page 5

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Additional links

If you don't know what T/S Thiele/Small parameters are, HERE'S a link to their definitions.

For those wanting info on how to wire multiple drivers in series or parallel use this LINK. People continue to get confused about wiring multiple driver configurations, so here's another LINK to a page with illustrations.

Here's a LINK to a very good PC based tone generator

LINK to the Linkwitz Lab spreadsheet used to compare the max output of drivers in dipole and monopole operation

For those wanting more info on sub placement and use of multiple subs here's a LINK to the Harman International white paper.

For those wanting info on calculation of room resonant modes here's a LINK to the SGHT xls worksheet

For those wanting more info on active and passive room treatments including links to setting up and using the Behringer EQ use this LINK

For those wanting a relatively accurate but somewhat more complicated way to calculate 'room gain', use this LINK

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