Home Theater Design In An Immersive Audio World

Some findings from Trinnov's comprehensive Speaker Placement Recommendations design guide

October 2, 2018

on Herron, Trinnov

Movie sound has progressed over the decades from mono, to stereo, to matrixed four channel (Dolby Pro Logic), and finally to fully discrete 5.1 and 7.1 soundtracks. All of these channels were (essentially) confined to the plane of the listener.

A few years ago, the essential third dimension of height was added: Auro-3D, Dolby Atmos, and DTS:X all have different takes on how the height information should be reproduced, but anyone who has heard a good immersive audio system understands that the benefits are huge.

However, there are several obvious problems:

1. All three of these audio formats are defined relative to a single listening position, while we usually watch movies with friends and family. Presumably, the goal is a shared experience that is as similar as possible for everyone involved…not just good for the person in the MLP (main listening position).
2. The environment of a home theater is quite different than that of the commercial theaters. What is the best way to “scale” from one to the other?
3. The three immersive audio formats cannot agree on where the “extra” height/top channels should be placed. Accommodating all three can be challenging, but should be a prerequisite for all your designs. Your clients deserve no less.

At Trinnov, we have been involved in designing and implementing hundreds of high channel-count theaters around the world. Since 2015 and until today, we are the only company that can render Dolby Atmos to as many as 32 unique channels in the home. So, we have a lot of experience designing these larger, more sophisticated theaters.

Back in late 2016, we set out to document what we had learned. It took longer than we had imagined.

Trinnov Audio dealers now have access to a comprehensive Speaker Placement Recommendations design guide. Many of the specific recommendations require more than the standard 12 or even 16 uniquely rendered channels (e.g., 7.1.4 or 9.1.6). But we can share some of the thought processes behind the recommendations with everyone.

Lesson #1: Everyone Benefits from More Spatial Resolution.
One of the most confusing design decisions seems to be how many surround speakers should be used. Everyone seems to understand intuitively that a larger room will need more surrounds. But what is less obvious that you also need more uniquely rendered channels as the size of the listening area approaches the size of the room.

Imagine that you are not the fortunate person at the MLP. Instead, you are sitting right next to the Left Surround (Ls) speaker. Your proximity to an individual surround speaker will cause it to dominate what you hear and largely collapse the surround experience into that single speaker.

Not good.

The only way to minimize this distortion of the soundfield is to have more, uniquely rendered surround channels. Thus, as a sound moves away from the screen and down your wall, you hear it pass through the Lw to Ls1, to Ls beside you, then on to Ls2, and to Lrs behind you. (I am using Dolby’s nomenclature here for clarity.) The amount of time the sound object is in Ls next to you is greatly reduced, and your perception of the sound’s motion is much more like that of the person at the MLP. This is because those sounds were anchored to physical speakers as they moved down the wall.

Note that these benefits are heard only when all those channels are uniquely rendered. If you have the same number of speakers, all playing the same information, both the perceived size and motion of the sound are severely distorted. This last point is true even at the MLP (or “money seat”).

Lesson #2: Place speakers relative to the listening area, not relative to the MLP.
Another Best Practice that sounds obvious but isn’t quite so simple: every sound that is mixed to one side should be perceived as coming from that side by all listeners; likewise, every sound that is mixed to the back should be perceived as coming from behind for all listeners.

Seems simple enough, right?

Yet, how many multi-row theaters have you seen in which, although all the speakers seem to be at proper angles for the MLP, things get weird for someone sitting in one the corners?

This becomes even more common when the room is either a little longer or a little wider than usual, as shown in figure 1. Notice that the speakers are all placed correctly for the normal, recommended angles relative to the MLP. But people sitting away from the MLP perceive sound coming from the wrong directions. (See figure 1.)

Figure 1.

Lesson #3: EQ is NOT the answer to all your problems. Choose speakers wisely.
In practice, this way of understanding the theater and speaker placement leads to exclusion areas. We discuss these in quite some detail in the book. Doing so in a short article would require too much space.

We know, this seems either obvious or impossible: aiming speakers toward the listener is Olde Skool and Audiophile, we’ve been doing it forever; or you can’t “aim” in-wall and in-ceiling speakers. They are what they are.

But here is the problem: if you “shower-mount” all the speakers without aiming them toward the listening area, you are instead choosing to aim that speaker’s worst sound toward the listeners.

Please read that last sentence again.

Yes, some sort of room EQ or room correction is available in most modern surround processors. But if you add a bunch of extra power to certain range of frequencies to make up for poor off-axis response, you are both stressing the speaker precisely where you should not and you are putting a lot of that distorted energy into the room. It will bounce around and sound pretty nasty.

Think about it: speakers become more directional toward the upper limit of each driver’s range. This is especially pronounced in two-way speakers at the top of the woofer range, and again in the top octave of the tweeter. Do you really want to add a lot of extra energy to the room from your Top Front in-ceiling speakers from 1000–2500 Hz, in our ears’ most sensitive range? Do you want to risk a service call for the tweeter that was blown up by a surge in energy above 12 kHz?

It’s far better design to either aim the speakers using appropriate brackets and hiding it all under stretched fabric, or to look to the relatively small number of speakers that can be aimed while remaining flush to the ceiling or the wall. This consideration is especially true for anything that is likely to be far off-axis, for example the Wide speakers, or the Top Front speakers.

This article covers only a tiny fraction of the work we put into our 86-page book on Speaker Placement Guidelines. I hope it has been at least somewhat thought-provoking. The opportunity to improve our designs and methodologies are profound. Your clients should have the benefit of that level of design experience. Their friends will become your next generation of clients.

Read More:
https://www.residentialsystems.com/technology/home-theater-design-in-an-immersive-audio-world