THX baffle walls - design, build and benefits

by Nyal Mellor March 25, 2013

What is a THX baffle wall and why do I want one in my home theater?! This article explains what a baffle wall is, covers the acoustic and audio benefits and finally provides tips on how to design and build them.

A baffle wall is essentially a false wall into which the screen speakers (i.e. left, center and right) are mounted. It is a core component of a THX certified cinema.

“Unique to every THX Certified Cinema is the THX “baffle wall.” If you were to peel away the screen at any THX Certified Cinema, you will see a massive wall of speakers housed in an acoustic baffle. The baffle wall is approximately the same size as the screen, providing a solid, smooth and uninterrupted surface to distribute sound throughout the auditorium. It produces a large sound image and accurately tracks sound elements with the onscreen action. This makes panning shots and off-screen sounds more believable and natural, helping to pull audiences into the storyline. Without a baffle wall, sound is uncontrolled – producing a weak, uneven image.”

Acoustical and sound quality benefits of baffle walls

Baffle walls have a couple of major acoustical benefits which translate into sound quality benefits:

• No speaker boundary interference from the front wall behind the speakers, since the speakers are mounted flush with the surface.
• Increased low frequency output. The baffle wall essentially removes the ‘baffle step‘ which happens when the speaker radiation transitions from half space to full space as the frequencies exceed those which can be controlled by the baffle. If your speakers are designed for flush mounting in a baffle wall, as Procella speakers are, then you gain 6dB of headroom in terms of the ability of the speaker (or sub) to reproduce reference levels.
• Reduced diffraction. There is little to no diffraction as the front baffle of the speaker is flush with the wall.

From a sound quality perspective these things mean that the sound tracks cleanly from left to right with no jumps, we have more headroom at low frequencies and we have better bass free from boundary interference suckouts.

Note that very few speakers are designed to be baffle wall mounted – most are designed to be used in free space and hence incorporate baffle step compensation circuits or are otherwise designed to counteract baffle step losses. When placed into a baffle wall the frequency at which the baffle step occurs is moved significantly downwards, to 80Hz or lower, which results in a bass boost. It is possible to equalize out this boost using a low shelf filter.

Visual and aesthetic benefits of baffle walls

baffle wall allows you to hide the screen speakers and front wall located subwoofers so that they are no longer visible. This provides for a very neat and visually attractive installation.

If shallow format speakers and subwoofers are selected baffle walls can be made very shallow. The baffle wall in our demo room uses Procella Audio speakers and subwoofers which allow the baffle wall to be only 8″ deep.

 

How we design and build baffle walls

• Speaker’s acoustical center optimally placed. Typically this means 1/2 to 5/8ths of screen height but will vary depending on the vertical off axis response of the speakers in question as we want the relationship between speakers and audience to be within the angular range where the speakers sound good.
• Multi-layer construction using constrained layer damping techniques. A proper baffle wall is at least 1.5″ thick and comprised of multiple layers of material, primarily to stop the baffle wall from resonating and becoming a giant speaker. This might happen if thin material were used as the speakers sit in the wall and hence transfer energy to it.
• Absorbent covering. The front of the baffle wall behind the screen should be covered with a 1″ layer of light and sound to absorb light transmitted through the screen and sound reflected back from the screen to the wall.
• Speakers and subs decoupled from the wall using appropriate isolation pads. These are used to prevent transfer of energy into the wall from the speakers.
• Void between the baffle wall and structural wall filled with absorbent material to prevent resonances developing in the space. In wide or tall rooms cutouts in the baffle wall can be made to allow the area behind the wall to function as an effective bass trap.

We have written an in depth case study of the design and build of our Demo Room baffle wall which explains the steps in the process and how it fits in the overall theater design process.

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