Audyssey - History of 5.1

Posted on May 27, 2009 By Chris Kyriakakis on

"Do you have a subwoofer in your system? Great. Then your speakers are small. Before you get all upset, read on. This is one of those audio myths whose time has come to be busted. To understand why, we need to talk about Bass Management.

In the early days of home theater it was thought that in order to reproduce the full movie surround experience at home it was necessary to place 5 large loudspeakers in the room. The reason for the size was the woofers. To play at theatrical reference levels and reproduce the deepest bass available in the content requires each speaker to have 12” or larger woofers. Let’s just say that this theory didn’t get very far in the real world.

A better and more practical approach came after studying human perception. The mechanisms that we use to determine the direction of arrival of sound depend on the frequency. At high frequencies the wavelength of sound is small and so sound coming from the side is shadowed by our head. That creates a level difference between the sound reaching the ear closest to the source and the ear on the other side. Our brain analyzes these level differences and produces an estimate of where the sound is coming from. But at lower frequencies, the wavelength of sound gets longer and our head is not large enough to produce a level difference at the two ears. Instead, we analyze the difference in time of arrival of sound at the two ears. Sound arrives first at the closest ear and we use that to determine the direction. But even that ability fails us below about 80 Hz. The wavelengths get very large and it was found in listening tests that 80 Hz is the frequency below which most people can not localize the direction of sound.

Taking advantage of this apparent “deficiency” in our hearing was what made home theater practical for millions of homes. Five satellite speakers of reasonable size could now be used because they no longer required large woofers. A subwoofer (or two) can reproduce the lower octaves and it can be placed out of sight since its content is not directional.

But there is also a practical advantage: directing the bass to a dedicated subwoofer channel with its own amplifier greatly improves the headroom in the main channels. The idea behind this was proposed in a Society of Motion Picture Engineers (SMPTE) meeting in 1987. The participants could not agree on the minimum number of channels required for surround sound on film. Various numbers were being shouted out until a voice was heard from the back: “We need 5.1”. Everyone’s head turned around to look at Tom Holman. He proceeded to explain what he meant: Take the low frequency content from all 5 channels and redirect it away from the satellite speakers to the subwoofer. If we do the math, then the content below 80 Hz is 0.004 of the audible 20,000 Hz bandwidth. But 5.004 didn’t sound as catchy so Tom rounded up to 5.1. By the way, don’t make the amateur mistake of calling it 5 dot 1. It is a decimal: 5 point 1.

Fast forward to the early 90s when the first DSP powered home theater receivers started to appear. Along with progress came complexity. Some industry forces believed that Bass Management should be an option that could be turned on and off by the consumer. That’s not necessarily a bad idea, but to make an informed decision requires much more knowledge about the system than what was available to the typical consumer. So, the Large and Small rule of thumb was established. The idea was to look at the size of your speakers and decide whether their woofers were “large enough” to reproduce the lowest octaves at the required levels. It was a noble thought, but looking at it 15 years later I believe that it has led to nothing but massive confusion. The poor consumer was led to believe that Large is somehow a good thing and was then left wondering why there was nothing coming out of their subwoofer.

Redirecting the bass to the subwoofer relieves the receiver amplifiers from having to work on reproducing the low frequencies and this greatly improves the headroom. If you happen to be using Audyssey MultEQ for room correction, you will achieve much better low frequency performance because the MultEQ subwoofer filters have 8x higher resolution than the filters in the other channels.

Here is a better rule: All speakers are Small. In today’s complicated AVR lingo that just means: If you have a subwoofer you should always turn bass management on. Always. Even if your receiver clings to the past and automatically sets your speakers to Large."

I particularly like the comment "Here is a better rule, all speakers are SMALL".
Thinking of purchasing large speakers or worse..."powered towers"? Nah. Save your money and maximize the speaker's ability down to 80hz. Then, you can redirect all the money you just saved to your system's bass performance..

Tom V. 
Power Sound Audio