The Center channel is the most important channel in an immersive audio system because it is right in the middle of all the picture action, and film directors and sound designers naturally put the majority of the sound elements there. Look at Figure 1, which is a distribution chart of averaged levels I measured over ten action movies. The Left and Right have 3 dB less energy than the Center (that’s half the power); the Sides are another 3 dB lower; the Backs, another 3 dB lower…
So, it makes sense to be particularly careful with the quality, clarity, bandwidth, and dynamic range of the Center speaker!
Traditionally, the Left, Center, and Right speakers are concealed behind the screen (see Figure 2). Yet, I have noticed a very disturbing trend. A growing number of speaker manufacturers now offer hybrid speakers that are meant to be placed to the sides of a large, solid display screen. They include both a Left or Right speaker and half of a Center speaker (see Figure 3).
You are supposed to place these like a pair of Left/Right speakers and magically end up with a clear and powerful rendition of that all-important Center speaker content. I don’t think so…
My hesitation comes from the fact that the Center portions of these speakers are simply producing a phantom center image — along with the typical loss of clarity and vague positional accuracy. These problems result from inter-aural crosstalk created by the shadowing of your head when hearing the same signal from two speakers subtending a horizontal angle of about 45 degrees. The crosstalk causes a loss of midrange energy, as well as positional errors, if you aren’t exactly in the middle between the speakers.
In fact, this extra set of Center drivers doesn’t help one bit; you could have just told the surround decoder that you had no Center speaker and it would have placed the Center signals in the Left/Right speakers with less complications and better sound quality. It would have been better to stick with a physical Center speaker placed below or above the display (but not both). See Figures 4 and 5.
Oh, I can see you coming with, “The interior designer didn’t want the added speaker under the display” or, “The manufacturer said the Center section would work better than the Left/Right speakers.” None of that is relevant. Once the customer gets used to a few extra inches of equipment below the TV — and actually enjoys listening to a movie without straining to understand the dialog — the issue will be forgotten.
To make things worse, I find that many of these combined lateral speakers sacrifice performance for the sake of gimmickry. It is visible in their poor bass response. By the time you put enough woofers for the Left or Right channel and the dual-phantom Center channel, there is not enough cabinet volume left to have any bass at all. Look at Figure 6 — which shows the bass response of this rather expensive combined speaker I tested just last week.
This is a nearfield response only 2 feet from the speaker. The response looks exaggerated in the mid-bass, but should be much flatter than this “ski hill” with way too much mid-bass. Let’s apply some equalization and look again.
In Figure 7, we can now see that this speaker barely makes it down to 120 Hz, requiring a lot of help from the subwoofers. 120 Hz is the frequency at which the average listener can start to detect that the speaker and subwoofer are not in the same place — better keep all the subwoofers near the screen!
If the speaker designer had just used one or two better woofers instead of four and not tried to fool you into creating a split Center speaker, you would have gotten better bass.
It’s true that the separate Center section allows you to adjust the ratio of Center-to-Left/Right content, but there is a better way to do this. Tell the surround decoder that you have a Center speaker and set it to “Small.” Then, connect the L/C/R outputs to a DSP equalizer with a mixing function. Equalize the Center feed to compensate for the inherent loss of midrange energy that comes from producing a phantom center image. Add about 6 dB, one octave wide, centered at 1500 Hz. Finally, blend the Center into the Left and Right at –3 dB. We call this process “Phantom+™.” See Figures 8, 9, and 10.
When that’s all set up, you simply adjust the balance of Center dialog in the overall playback and make your customer happy to be understanding all the spoken word without needing subtitles.
P.S. There are also new technologies that bounce the sound waves off the screen, and we’ll talk about those later.
Anthony Grimani is co-founder of Grimani Systems loudspeakers and president of PMI Engineering and MSR Acoustics.