The Power of One

Dolby Digital and DTS offer 5.1 surround systems: six channels of sound. But the most important channel when you want to deliver a “Boooom!” to believe in is in the point one of 5.1. It is the power of one, so to speak.

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By  Justin Etheridge Published  August 30, 2001

Introduction|~||~||~|“Craaaszsh!!!”, “Kpow!!!”, “Booooom!” and “Kersplatz!” Good comic-book words to give effect to that jaw-rattling right hook that Superman delivers to knock out a villain. Words that work … in a comic book and when you are under 13! When you have grown up (speaking physically, if not mentally), you want your comic books to turn into movies and that “Kersplatz!” to sound like Superman meant it!

Movies are about delivering entertainment and, today, sound has become a very important part of the magic of cinema, taking you to places where otherwise you could not go (even if that place is in somebody’s imagination). Quite a tall order when you are trying to reproduce those magical cinema sounds from a home theatre system or your hi fi.

Dolby Digital or DTS provides the first giant leap forward because both systems offer 5.1 surround systems: six channels of sound. But the most important channel when you want to deliver a “Boooom!” to believe in is in the point one of 5.1. It is the power of one, so to speak.

This is the output that is fed to your subwoofer and is used to reproduce all the bass sounds that a cinema sound track has. It will make your light fittings rattle and give you explosions that happen right in your front room.

The first rule is: you should be able to feel good bass! The second rule: too much bass is bad for music. It colours and distorts the other frequencies, making your room boom and muddying the lower frequencies.

This then is our first problem. The dictates of cinema and the needs of music hi fi are different. Most audiophiles won’t have a subwoofer in their music system at all. They cite all the problems of too much bass and prefer to go for quality rather than quantity.

However, good bass down to 20Hz and below is said to be essential to the overall sound of the music, because any note is made up of a series of components that range both up and down in frequency from the fundamental (the main note frequency). So, if you are not reproducing the low frequencies, you’re not reproducing the note as it was originally recorded.

||**||Active vs. Passive|~||~||~|Cinema buffs on the other hand have no such problems. They want the house to shake! You don’t get that from a pair of bookshelves. Most standard hi fi speakers work down to a frequency of about 50Hz and then tail off very rapidly.

Most bookshelf speakers don’t even get down that far. Depending on the speakers, you could be losing sound from 100 Hz downwards. Just as a double bass is a big instrument, you need a big speaker to reproduce sounds below 100Hz.

Hoffman’s Iron Law says that a woofer’s efficiency is proportional to the volume of its cabinet and the cube of the lowest frequency it can produce before its cut-off point. If a woofer’s response is flat down to 40Hz in a two cubic foot enclosure, to get it down to 20Hz you must either increase the cabinet volume eight times or use eight times the power.

So it’s easy to see why it’s so difficult to get decent low-frequency response from small “full range” speakers. Since most of the power goes into producing the low notes, which ordinary speakers are not even that good at, the high end suffers. Ironic, since these are the very sounds that ordinary speakers are good at.

Using a powered subwoofer frees up your amplifiers. They are then able to provide much more power to the other speakers in the system and so improve performance. Dolby Digital and DTS take care of this by funnelling all the sounds below 100 Hz to the subwoofer, which then can be specifically designed to take care of those sounds and leave the mid and upper range frequencies to the other audio channels.

You will not be losing any sound-stage information, because frequencies below 100Hz carry very little directional information: that comes from the mid and upper range.

There are two types of subwoofers: active and passive. Active subs have their own mains powered amp built in, whilst passive speakers are little more than a large drive unit in a big box, which will produce a nasty flat sound unless they are specifically designed to complement your speakers.

A powered sub will take much of the strain away from your amp, freeing up a lot of power for your speakers. Anybody can set up a sub to shake the windows and set off their neighbours’ car alarms by plugging it in and turning all the dials to maximum, but to do it properly needs some consideration.

||**||Chassis design|~||~||~|There are three main types of subwoofer, all of which have their own strengths and weaknesses. The bass-reflex enclosure subwoofer has the driver mounted on the side, facing outwards. It also features a port that allows air to flow in and out of the unit. Pressure waves from the back of the woofer cone excite an air mass in the port (or ports), producing low-bass output that augments the waves generated by the front of the cone.

The result is an additional 3dB of output in the octave of the system’s cut off frequency. Although 3dB may not seem like much, to gain it through amplification would take twice the amount of power. Of course, the drawback of a bass-reflex design is that the response rolls off rapidly below the cut off frequency. Another drawback is the potential for port noise.

The closed box subwoofer is very similar, but, as the name suggests, it is a completely sealed unit. Typically, the enclosure is much smaller than a ported application using the same size woofer, but more amplifier power is required to achieve the same level of output.

In a band-pass subwoofer, the driver is mounted in the middle of a dual chambered enclosure and faces internally. The chamber behind the woofer is the normal enclosure and can be either sealed or ported.

A second chamber in front of the woofer, also contained within the enclosure, functions as an acoustic filter, allowing only low frequencies to emerge through a tuned port. This effectively eliminates the need for a crossover but doesn’t allow for variable frequency attenuation.

Put on a CD with known bass content (Prodigy’s Breathe). Set the subwoofer to the lowest cut off frequency possible and mid volume. Gradually increase the cut off frequency until you can hear the room boom and the rest of the music become muddy. Come back a frequency step.

Then increase the volume control until you get the same effect. Cut the volume back to just before the setting that gave you boom. Easy!

You should also ensure that there are no gaps in the frequency range. A subwoofer that works up to 80 Hz and main speakers that roll off at 100 Hz will leave you with a 20 Hz frequency gap! But gaps are not fatal. Most audio experts say that a low frequency gap is better than an overlap!

This magazine (T2) can recommend the REL range. They do everything claimed for them. But if it is big bucks that you want to spend, try the Revel Sub-15/LE1, which we featured last issue. They’re sure to be the envy of your friends and the bane of your neighbours!

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