Boom-Boom Baby

The 1960's were probably the heyday for the audiophile movement. Things were simple - you had a good turntable and stereo amplifier and topped things off with a good pair of speakers or headphones. There was little processing of the sound that went onto the vinyl LPs so the aim was to reproduce the sound as closely as possible to the original. Protecting your records and using a good stylus was about all you had to do to keep things "sweet".

Over the intervening four decades a lot of innovations have come (and many have gone), adding variously to the quality, the variety, or the ease with which we listen to our music. So in general is music reproduction better or worse than it was? That's a very subjective thing. Most of us are happy with our types of music reproduction. Notice I say "types" because we accept different quality levels depending on what we're listening to and where we are.

The purists will tell you that a CD simply can't quite reproduce the same frequency range that the humble LP could. Likewise the typical MP3 file is lower quality than the CD. But would most of us go back to the LP days or even the Discman days? No, we wouldn't. That's because the quality of a typical MP3 player is quite suitable for most applications. The amount of music we can store on a tiny device that draws very little power more than makes up for the lack of audiophile purity.

At the high end we expect our cinema experience to include top-quality audio with lots of gut-rumbling bass and surround sound effects. Sound quality is a solid part of enjoying a movie and those surprise whispers over your shoulder certainly add to the effect.

Why then do we often accept a rubbishy middle ground? It's not that hard or expensive to achieve a good quality of sound reproduction in a typical home theatre or music set-up, yet so many of them fall down in a number of aspects. In particular, bass reproduction.

Until someone comes up with another method, true bass reproduction in a room usually requires a large speaker and a larger speaker box. (I say "true bass" because much of what we're being sold is pseudo-bass. But more on that later.) As the bass frequency becomes lower it starts to become infra-sound. That is, it interacts with more than just our ears. The CRUMP of an explosion or rumble of an earthquake adds greatly to the movie.

Bass reproduction in the home has one important characteristic - it doesn't have a directional quality. Although you might think you can determine where a bass sound is coming from, that's only because there are higher frequencies that give our ears the clue. For example, the thump of hitting a bass drum will be accompanied by a subtle click or rustle and our ears will use that to tell us where the drum is.

It's this property of bass that enables the "point one" part of the "5.1" we see in surround sound specifications. That .1 is the sub- bass component of the sound. That is, the very lowest bass notes and even lower such as rumbles. And because we can't tell where the sound is coming from in the room there's only one channel for it and only one speaker - the "subwoofer".

You can put the subwoofer anywhere you like in the listening area. It doesn't have to be in between the main speakers or even on the same side of the room for that matter. That can be an advantage when SMWBO (She Who Must Be Obeyed) is already complaining about too many speakers and electronics devices in the lounge room. It can be placed in a corner, under a table or behind a curtain. Just as long as it isn't going to rattle things off nearby shelves.

Boom-box

Unfortunately the typical subwoofer supplied with surround sound systems is little more than a boom-box. To have meaning, every sound we hear has a frequency component. Take a musical note of 27Hz, for example, a very low 'A' note played on a pipe organ. That's quite a low frequency, below the ability of a typical hifi speaker but within the limits of a good subwoofer. (It's also within the limits of a reasonable pair of over-the-ear headphones.)

A clever piece of sound manipulation, either in the studio or in the sound system can trick your ear into hearing this bass note by emphasising certain higher harmonics of that note (multiples). That's not perfect but it does deliver much of the effect without really cheapening it. An example of this is SRSTruBass which is used in a number of surround sound systems. Some Bose brand systems use a similar auditory trick.

Unfortunately the "boom-box" speakers use no such sophistication. They tend to have a very dominant, single, resonant frequency and any attempt to reproduce a low bass note simply makes the box "boom" at that constant, resonant frequency. To add insult to injury they usually have rattles and non-linear distortion as well. Some are so bad that they seem to "honk". True bass has a musical attribute. It needs to be as smooth and uncoloured as all other frequencies. It isn't distortion and it certainly isn't tub-thumping. If you don't believe me, take your favourite action movie DVD to a reputable hifi store and ask them to demonstrate a few scenes on a good surround sound system.

One interesting point to ponder is bass reproduction in compressed files such as MP3. The first thing to disappear in a digitally sampled audio signal is the higher frequency components. (Read up on the Nyquist Frequency for background information).

Luckily the bass signal doesn't require much data so it usually comes through the analoguedigital-analogue conversion path unscathed. So if you like smooth, low bass in your portable music, check the low frequency specs of your phones and player. And watch for the "3dB" in the figures - without that the specs are meaningless.

Note: Paul Zucker© Permission to copy or quote extracts from this article may only be done with the written permission of the author.

Reprinted from the August 2008 issue of PC Update, the magazine of Melbourne PC User Group, Australia

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