Back before the process was automated, setting up a home theatre receiver required a tape measure. Stephen Dawson explains why your home theatre receiver needs to know where the speaker are.

When you install a new home theatre receiver it has to be calibrated in order to do its job properly. These days that’s easy. You plug in the microphone that came with it, put the microphone on a stand so that it’s where the head of a user would be and start the auto-calibration. The loudspeakers will whoop for a while and perhaps you’ll be asked to move the microphone to a few different places.

And then it will be finished. All set up.

Those auto calibration systems come under various names. Some are specific to brands – Pioneer’s MCACC, Yamaha’s YPAO. At least one – Audyssey – was developed by a third party and is used by several brands. Each has their own wrinkle on some aspects of calibration, but much of what they do is the same.

There are four such commonalities:

• Size.
• Level.
• Distance.
• EQ.

EQ – adjusting the levels of the different frequencies to correct for performance weaknesses from loudspeakers or the room – is something that came with automation. The other three have been there from the start.

Size determination is required to make sure that bass is delivered well. If the speakers are small – especially satellite-sized – then the bass needs to go to the subwoofer.

Direction
The level and distance settings aren’t about tone or frequency performance. They are all about direction. The direction the sound is coming from.

We are talking about surround sound systems, after all. A naïve view might be that simply having loudspeakers behind you and sending some of the sounds to them is enough. But it isn’t if you want to sounds to appear to the listeners to be coming from the directions intended by the movie makers.

The way we tell what direction a sound is coming from is entirely based on two factors: timing and volume.

And unless calibrated, virtually all systems in all rooms will get both of those wrong.

Of the two – timing and volume – timing is the more important. If either is wrong, and especially the timing, then sounds will seem to be coming from a different direction.

The key to understanding this is to realise that while fast, sound takes time to travel. The precise speed varies depending on such things as the temperature of the air, but at room temperature it’s around 344 metres per second (340 at 15 Celsius, 350 at 30C). For practical work you can use the close rule of thumb is that it takes three milliseconds – thousandths of a second – for sound to travel one metre.

Now let’s say a typical home theatre system has the front right speaker 3.0 metres from the listening position, and the right surround speaker is 1.8 metres from where you’re sitting. A movie sound track has a gun shot and this is supposed to sound like it is half way between the front right and surround right speakers, about 120 degrees to the right of dead centre front. Without calibration home theatre system will send the sound in equal levels and at equal times to those two speakers.

But with an uncalibrated system, the sound from the surround speaker may well be louder than that from the front, and it will definitely arrive first at the listening position.

Indeed, the sound from the surround speaker will get to your ears about 3.5 milliseconds before the sound from the front speakers.

So?

Not only will the gun shot not seem to come from halfway between the two speakers, it will actually sound like it is coming entirely from the surround speaker!

Staying Alive
Evolution has equipped us with a way of making sense out of the confusion of noises out there. It is important to know where the sound of danger is coming from. But here’s the thing: there is very little sound that we hear which comes from only one place.

If someone says something you hear their voice transmitted to you directly through the air. But you also hear their voice bouncing off the floor a short time later. And reflected off the ceiling. And the walls. And the furniture.

You hear that person’s words many, many times, in a very short moment.

At least, that’s what the sound waves are doing. What you actually hear in your mind is the person’s voice, just once. The signal processor in your brain picks just one of the sets of sounds coming in and makes that the one that you hear.

But which one does it pick? The loudest?

Well, yes, but only if it is very much louder. Maybe ten decibels louder, or even more. It doesn’t lean heavily on this because loudness is not a very reliable indicator of which sound is the original one. If you are in a highly reverberant, acoustically live space – say somewhere lined with ceramic tiles – then there isn’t a great deal of difference in level between the original and the reflected sound.

The best short cut, or heuristic, is the one evolution gave us: our brains assume that the original sound is the one that gets to us first.

All the reflected sounds are ‘fused’ into the original sound in our brains. We hear only one sound, but our sense of its character varies so that we can hear the reverberant environment. Nonetheless, the sound will seem to us to have come from the direction of the first arriving sound.

This is known as the ‘Precedence Effect’ or the ‘Haas Effect’, from the work done in the late 1940s and early 1950s by several researchers, including Helmut Haas. The same sound delivered from multiple sources, they found, can fuse together into one perceived sound even though they may be several tens of milliseconds apart in timing. How much time is required to make them resolve into discrete sound effects depends a lot on the type of sound. Short sharp clicks may sound like two sounds as little as 5ms apart, while bigger and more complex sounds might fuse as much as 30ms apart.

Home Theatre
All that works well with real sounds in the real world. Real sounds are produced from one source point, so the shortest path for the sound is normally a straight line.

But in our home theatre example, the sound is coming from two places: the front right loudspeaker, and the surround right speaker.

The sound will seem to use to come from whichever speaker delivers it first to our ears. That is, the closer of the two speakers.

And so we calibrate.

Back in early days of home theatre you would have to measure the distance between the listening position and each of the speakers in turn. If you did it in metres, then you’d multiply by three to get milliseconds and enter that into the setup menu. (Then you’d use the test tone and sound pressure level metre to get the levels right as well.) Later receivers did the calculation for you: you just entered the distances.

These days that process is automated, with the test tones and test microphone working it all out for you.

But however it went, the purpose was delay the sound being delivered to any loudspeaker that was closer to you, so that the sound it delivered would arrive at the right time compared to the sound from the other loudspeakers.