Inside the box.

The term "infinite baffle" really means an infinitely large, rigid panel with a loudspeaker drive unit fixed somewhere in the middle. The great size of the baffle forces the sound waves from the driver to to travel a very long way before they reach the edge of the baffle. When the sound waves reach the edge of the baffle they would be of opposite phase which would cause cancellation at a frequency depending on the length of the wave which matched the distance from the driver. The positive pressure wave from the front of the driver cone cancels the negative partial vacuum of the rear wave. Since frequency is directly related to wavelength the baffle must be made very large indeed so that cancellation can only occur at very low frequencies. Hopefully at a frequency below that required to reproduce music and film effects with full power.

In real-world situations it is far easier to use a large, enclosed rear volume as an IB enclosure. The infinitely large baffle then becomes the wall between the listening room and the rear enclosure volume. The edges of the baffle have been effectively wrapped around to seal off the rear space from the AV room. Since the sound waves from the front and rear of the cones cannot interact with each other there is no out of phase cancellation at any frequency. Because the rear volume is so large there is no back pressure on the drivers as there would be in a small sealed box. This lack of back pressure allows the IB to reach very low frequencies indeed at high output levels while maintaining low distortion and very reasonable efficiency. When the drivers are housed in a box manifold sealed into the dividing wall the IB can be thought of as a local folding of the (infinite) baffle.

The listening room itself must be sealed from the IB enclosure to avoid out-of-phase sound wave cancellation. Open doors, serving hatches or windows between the two spaces would cancel the bass at certain frequencies depending on the wavelength at which the sound waves met each other at the opening in the baffle wall. Airtight sealing is not that important though. The large volumes of air movement required at very low frequencies would struggle to squeeze around the narrow gaps of a closed door for example. A perfectionist might like to draught seal the door so they can sleep soundly at night. If nothing else it will help to stop the door from rattling against its frame. You'd be amazed how a door can flex when driven by cyclic low frequency pressure waves.

A view inside the open manifold from the AV room: Click for a larger image. Backclick to return to the text.

The drivers in my own IB are now fixed in pairs on either side of a manifold (box). The box is completely open on only one side to the listening room. The open area (or vent) of the manifold is calculated to match the total cone area of the 4 x 15" drivers (or 4 x Sd) to avoid any compression effects in the manifold. (In my own case the opening is 40" x 13.5")

The driver cones in an IB must all move in the same direction (towards the open side of the manifold ) when a signal is applied. If any cones moved in opposite directions they would cancel each other's output. A torch battery connected briefly to the speaker cables can be used to check that all the cones jump in the correct direction. The cables can then be inserted into the power amplifier speaker terminals observing the correct polarity.

The unusual two-facing-in, two-facing-out arrangement of the drivers in my manifold in this image is supposed to cancel out some distortion components. The drivers which face into the box will not produce exactly the same audio response as the drivers which face out of the box. This imbalance helps to cancel out any variations between them. It is considered a better arrangement than having all the drivers facing inwards or all facing outwards. (Where unwanted driver effects would be additive)

Opposing the drivers on either side of the box cancels mechanical vibration due to cone movements.

Mechanical vibration was the weakness of my original vertical, line array IB. The downside is that a rather large box has to be housed next to the baffle wall. Whereas an array takes up no space in either room. The box can be hidden in the enclosure as I have done. Or it can be placed in the listening room. In which case it can be made to look like a large box subwoofer. This type is usually referred to as an "outie". My manifold could have been made smaller but I wanted to avoid any chance of compression effects in the box by making the opening into the AV room as large as possible. The opening into the room can be covered in breathable cloth without affecting the IB's performance. Some IB builders use decorative screens to cover the manifold opening. Grills are more practical with underfloor IB manifolds where the manifold opening might be walked on accidentally.