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Be an “Ace of Bass” - Part 5

Wed 05 Jan 2011

Be an “Ace of Bass” - Part 5

In this final part of our “Ace of Bass” series we’re going to look at the rather mysterious subject of audio equalization or “EQ’ as it’s often known.

EQ or Tone control?

For many people their first exposure to audio equalization was a Midi System or some similar “HiFi” system. Most had 3, 5 or more slider controls which offered some simplistic tone shaping to the sound. Results from adjustments were often poor and somewhere downright nasty!

Although fairly crude, these controls are very similar in principle to today’s more complex equalizers. In fact even the basic “Bass” and “Treble” controls found on many amplifiers can be considered basic equalizers.

The principle of an equalizer is to either add gain (make louder) or attenuation (make quieter) a particular part of the audio frequency range. In the recording business EQ can be used creatively to deliberately change sounds for artistic effect. However, in HiFi and home cinema systems it’s there (in theory at least) to correct for inadequacies in the play back system and/or listening room.

Types of EQ

Equalisers come in two basic flavours:

  • Shelving filters – these offer gain or attenuation over a wide range of frequencies. Normally the “Bass” and “Treble” controls found on regular amplifiers are of this type, their effects can often be quite dramatic and rarely achieve accurate reproduction of sound.
  • Parametric Equalizers – rather than affecting a broad range like shelving filters, parametric equalizers can be used for more specific changes. Each “band” of parametric EQ (you could have many per audio channel in sophisticated equipment) has controls for centre frequency, gain/attenuation and width or “Q”.  The width control determines the size of the frequency range either side of the centre frequency where the gain or attenuation will occur. The gain control adding or boosting volume over that range.

Practical uses for EQ

We’ve already mentioned that the regular Bass & Treble tone controls are typically simple shelving filters. Some more advanced equipment uses parametric equalisers at two set frequencies and widths, one each for Treble and Bass. However, these are more rare.

Parametric equalisers are now becoming commonplace in home cinema systems although it may not be immediately obvious where they sit or what type they are. Many modern AV receivers, AV processors and some high-end subwoofers have auto setup routines that offer tonal correction on top of the setting of speaker levels and distances. The Arcam AVR500/600 and AV888 are good examples of this approach. These systems will normally measure the response of a system and add multiple parametric equalisers per channel to try and correct errors caused by room effects. Where the room causes a “hump” in the frequency response (two waves of the same frequency arriving in phase which then add up) the equalizer can be set up to provide a corresponding “dip” in the response which corrects for the hump.

If done accurately this can be a very useful solution, especially for problems in the bass region. While correcting for humps can be achieved this way dips in the response are more difficult to cure and EQ is usually not the answer, particularly in the bass region. Dips are usually caused by cancellation of two waves. EQ will simply make the two waves bigger, which, in turn, still cancel. The result is usually hot amplifiers, hot speaker drivers and no change in sound!

Less is more

You might gather from the above that EQ used incorrectly can cause as many problems as it solves.  Setting up an equalizer requires accurate measurement of the system both before and after changes to make sure all is well. This requires specialist test equipment and a competent operator! Perhaps that’s why the vast majority of equalizers in HiFi and Home Cinema equipment tend to come with automated setup routines to avoid such problems.

We hope you’ve enjoyed this series of articles as much as we’ve enjoyed presenting it. Audio is a fascinating subject. A good understanding of it helps a great deal when setting up systems. The better the system setup the more we can all enjoy our music and movies!

If you have any comments on anything you’ve learned here or have suggestions for future subjects we’ love to hear from you.