Too often, centre channels are downplayed or ignored to the detriment of multi-channel systems. Anthony Grimani explains.

Last week, I walked by a well-meaning salesperson at an AV shop who was spreading misinformation about the purpose of the centre channel in a multi-channel audio system.

He, like many people out there, was espousing that the centre channel exclusively contained dialogueue and should be optimised for that sound range. While it is true that the centre channel contains a lot of dialogueue and must resolve the spoken word with clarity and ease, it is also true that the centre channel has just about everything else in it, too – including music and sound effects.

In fact, sound designers use the centre channel for all sounds that correspond to action near the centre of the picture. So, screeching tyres, first violins and even dinosaur foot stomps are funnelled into the all-important centre speaker. Don’t believe me? Play your favourite movie, disconnect everything but the centre and listen.

To better understand the centre channel, you need to start thinking like a filmmaker. Everything is about the action in the middle of the screen, from lighting to photography and yes, sound.

A soundtrack expands out from the centre to left, right, surrounds and now even the height dimension. Stereophonic sound – which was pioneered for cinemas – always employed three screen speakers and an array of surround speakers, expanding on original monophonic. Everything was rooted in the centre and grew out into the wider spans of the screen and around the room. It was the hi-fi business (which desperately wanted to get on the bandwagon) that dumbed down ‘stereo’ to only two channels. That’s all they could fit on the grooves of an LP record!

So, for film guys, the main channel is centre, not left/right as is commonly thought by people from the music side. In fact, the centre channel on average has more than twice the power requirement of the L/R channels, and four times that of surround.

So how do you optimise a centre speaker for home cinema sound? Well, there are a few important rules to follow:

1. Choose a centre speaker with plenty of sound pressure capability. It only needs to play down to about 80Hz, the typical subwoofer crossover frequency. I have seen many centre speakers with blown tweeters and the occasional blown woofer. This usually results from underestimating the peak and average sound pressure requirements of action movies. You will need to play up to 105dB peaks, and 100dB sustained levels. As a quick rule of thumb, a centre speaker with 90dB sensitivity will need about 200W for a 30m² room and 400W for 60m². Very few speakers with typical 25mm tweeters will take this abuse. Thus the common failures. It is best to rely on speakers with higher sensitivity for bigger rooms.
2. Make sure that the centre speaker has a smooth frequency response. Many speaker designers take liberties in order to ‘enhance’ the musical character of their product. One such trick is to dip down midrange frequencies to make the speaker sound smoother. That is just wrong; it reduces dialogue clarity and action impact.
3. Choose a speaker with wide horizontal dispersion. Avoid the typical horizontal woofer-tweeter-woofer configuration. The pair of woofers usually lobe, which means holes in the midrange frequencies (dialogue not intelligible) if you sit only one or two seats off-centre. Instead, use a three-way speaker with dual woofers, a midrange and a tweeter. Better yet, stand the speaker vertically to avoid the horizontal dispersion issues.
4. Place the centre speaker behind an acoustically transparent screen. The sound and picture perfectly match, yielding a more satisfying and convincing experience. You’ll need to switch from big TVs to projectors. You’ll also need to control ambient light and room colours to avoid contrast-smashing light bounces. It’s not necessarily easy, but if it were, they wouldn’t need you!
5. Aim the centre speaker up or down towards the listening area to get the best coverage at all the seats.
6. Avoid mid-bass resonances by refraining from placing centre speakers inside cabinets. If you must use a cabinet, treat the area around the speaker with sound absorption. Also, build a small baffle around the speaker to prevent cavity resonance. This takes some creativity, but will really improve dialogue clarity.
7. Tune the frequency response of the centre speaker to smooth out strong peaks or dips. You will need to specify separate surround decoder and amplifiers, and interconnect an equaliser between them, or use an upscale receiver with manual EQ built-in. Look for models that provide at least 2/3 octave resolution. Note that I remain unconvinced that auto-EQ systems work well…
8. Listen to the results of your work. Dialogue should be clear, powerful and dynamic. I like to listen to worst case scenarios like the amazing movie My Left Foot, where Daniel Day Lewis plays a writer with cerebral palsy. If you can understand him, you will understand any dialogue out there! Also, make sure that the peak dynamics of action movies come through undistorted and crisp, without sounding too aggressive.

Think of the centre channel as the foundation of a 5.1, 7.1 or Atmos speaker system. Once you get it right, it will be easier to work through the other elements of setup and tuning. Your clients will benefit from better intelligibility, clarity and dynamic range.