How to Maximize Dialog Intelligibility

Maximize Intelligibility With Improved Center-Channel Performance October 12, 2017

Dialog clarity is perhaps the most important performance aspect for audio systems–not just home theaters, but pretty much everything: public address, commercial cinema, or sound reinforcement. Unfortunately, I still get regular calls to fix systems with compromised dialog clarity. If clients are complaining, that’s a bad thing.

But how does it happen? Most of the time, the culprit is dialog that is being reproduced by two or more speakers. Here are some examples: (A) the master bedroom TV has a soundbar with drivers on both ends producing dialog, or (B) stereo speakers flank the screen with no center speaker, or (C) the sound mix or routing in the system causes dialog to come from the L/R speakers even when there’s a physical center speaker.

Why is it bad to have the dialog coming from two or more different speakers? The short answer is that our ears/brains add the sound sources together. If multiple speakers produce the same sound of a person talking, then our brains don’t add it correctly, and we end up hearing mushy, hollow sound with no clarity or crispness.

Leon’s Horizon SEVEN

Let’s talk about some practical solutions, starting with the simple bedroom stereo soundbar. The core problem is that the soundbar is two channels, but dialog is mono. Drivers in both the left and right part of the soundbar add together in different destructive ways depending on where you are in the room. There are a couple of solutions. The first is to use a mono sound bar. It may not have the great “separation” or “envelopment” of a stereo sound bar, but clients are going to be using it for plain vanilla dialog on TV news/sports the vast majority of the time anyway.

Another solution is to use a soundbar with separate drivers for the left, center, and right. Dialog is placed in the center drivers, while the left and right drivers add envelopment and separation (as much as is possible from speakers so close together). Make sure the center drivers are not the horizontal mid-tweeter-mid (M-T-M) array that is so common. Can you guess why this is undesirable? Look back to the discussion about the same sound arriving at your ears from different sources. Those two woofers separated by that tweeter represent two different sources! There are things that can be done, such as using a tweeter that crosses over lower than the typical 2-3kHz, or placing the drivers very close together to mitigate the interference patterns between the two horizontally spaced drivers, but this level of engineering is fairly unusual in soundbars.

Last but not least, take advantage of the companies like Triad and Leon that will custom build your soundbar. Tell them that you want an M-M-T (orientation frequently used for left/right drivers in soundbars). It’s not perfect, but it’s better than M-T-M.

In situations where stereo speakers flank the screen where a center speaker is not available, then you must choose the speakers you’re going to use for phantom center very carefully. Look for flat frequency response, especially around the middle frequencies. A lot of designers intentionally introduce a mid-range dip to make a speaker sound more “laid back.” All this really means is that the speaker is missing critical midrange detail for dialog clarity. Second, make sure the speaker has really wide, uniform, broadband horizontal dispersion. To make this selection, you need detailed axial and polar frequency response plots. Not many manufacturers provide them, so you are going to have to press them or maybe even switch to a brand that actually has that data.

Of course, don’t ignore the room. Echoes/sound reflections really tend to kill clarity. If the room is right on the verge of being too “live,” then switching from a physical to phantom center can tip the scales into mushy-land. Go with a little more absorption in the room than normal. The response of the L/R speakers must match precisely, so, if all else fails, you need an EQ.

Finally, what can you possibly do if the mix is bad (like dialog mixed into the L/R of a 5.1 mix), or your source device outputs 2-channel mixes from the L/R speakers without giving you the option to engage matrix decoding? This happens a lot with cable/satellite boxes (particularly on local channels) or gaming console boxes with internal audio mix engines that want a 5.1 or 7.1 pipe open all the time. You have to create an alternate audio path for the compromised content. Usually, this takes the form of downmixing multi-channel to a two-channel analog output using a little device called an HDMI audio de-embedder. This device doesn’t remove the audio from HDMI, so that pipe stays open. But, with access to both a multi-channel/bitstream feed from HDMI and a two-channel downmix through analog, you can program a button for the client that switches to the analog backup when the mix is wrong. This takes a little creative programming and client education, but it’s nice to have a fallback, instead of fielding complaints that the client can’t understand the news anchor or play-by-play guy.

Chase Walton contributed to this article.

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