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Friday, January 17, 2014



Technique : Effects / Processing


PAUL WHITE looks at the various types of equaliser, and gives some tips on how EQ should best be applied to your recordings.

Equalisation can come in many forms, from simple treble/bass controls to multi-band, parametric equalisers, but, however you dress it up, equaliser is just another word for 'tone control'. The term equalisation came about because the very first equalisers were developed to help counteract or 'equalise' shortcomings in telephone systems, but today equalisation is used creatively as well as to fix problems. Equaliser circuits are based on electronic filters -- hence the term filter, which crops up quite a lot when you're talking about EQ. Strictly speaking, a filter is a device that removes something, but in the context of active equaliser circuits filters can boost frequencies as well as cut them.

Check any textbook on audio, and you'll see the limits of human hearing quoted as around 50Hz to 20kHz, though those very same books will also point out that very few individuals, other than young children, can hear pitches anything like as high as 20kHz. A more realistic figure might be around 15kHz for an adult, decreasing further as the years pass. What is really puzzling, though -- and this is scope enough for a feature in its own right -- is that even if your measured hearing response starts to fall off well below 20kHz, it is still possible to hear the effect of equalisation applied at the top end of the spectrum, where you wouldn't expect to be capable of hearing any change. The plot thickens further when reputable studio engineers claim to be able to differentiate between two otherwise identical circuits if one has been modified to handle frequencies up to 50kHz and one handles frequencies only up to 30kHz. In theory, both limits are well above the limit of human perception, so it seems that what goes on outside the audible spectrum has a way of influencing what we perceive within the range of our own hearing systems.




Though equalisers all do essentially the same job, there's a great deal of difference between a simple 2-band treble/bass tone control and a multi-band studio equaliser. The simplest is the shelving equaliser -- a device that applies cut or boost, rather like a volume control, but only to the frequencies above or below the cutoff point of the equaliser, depending on whether the equaliser is based around a high-pass or a low-pass filter.

A low-pass shelving filter, as its name suggests, passes all frequencies below its cutoff frequency, but attenuates all frequencies above its cutoff frequency. Similarly, a high-pass filter passes all frequencies above its cutoff frequency, but affects all frequencies below its cutoff frequency. Figure 1 shows the frequency response graphs of a typical treble/bass EQ using high- and low-pass filters. Note that the filter graph shows a slope at the cutoff point -- it isn't possible, or desirable, to have a filter that does nothing at one frequency, and then comes in with full effect when you move up by just 1Hz. Simple shelving filters typically have a 6dB-per-octave slope, so that their influence is felt more progressively --though it is possible to make much steeper slopes if required. The gentler the slope of the filter, the more frequencies outside the range of the filter will be affected.




A filter that passes frequencies between two limits is known as a band-pass filter, and on a mixer with a mid-range control, the Mid knob controls a band-pass filter. On a typical mixer, the band-pass filter will have variable cut and boost, and on more flexible mixers it will also be tuneable, so that its centre frequency can be varied. This is known as a sweep equaliser, because although the filter frequency can be changed, the width of the filter cannot be adjusted. Figure 2 shows a typical band-pass filter response, including a sweep control function. In a typical mixer, the high and low shelving equalisers are used to control the high and low end, while a band-pass filter controls the mid range. However, some mixers use band-pass filters for EQing the low end too. The argument for doing this is that a shelving high-pass filter will also boost all frequencies below the ones you want to work on, whereas a band-pass filter attenuates frequencies both below and above its cutoff points.




A parametric EQ is very similar to a sweep band-pass EQ, except that a third control is added to allow the width of the filter response to be adjusted. The width of a filter response is sometimes described as its 'Q' value, where Q is the filter frequency divided by the number of Hz the filter affects -- in other words, its bandwidth. The formula for Q is:

Q=centre frequency in Hz

Because the filter response is curved, the actual frequency width is measured between the points on the graph where the signal level has fallen by 3dB. A high value for Q corresponds to a very narrow filter; a low value of Q corresponds to a wide filter. High Q values are useful for picking out sounds that occupy a very narrow part of the audio spectrum, whereas lower Qs produce a smoother, more musical sound.

A studio parametric EQ may have several filter sections, enabling three or four parts of the frequency spectrum to be treated simultaneously. Parametric EQs can be time-consuming to set up properly, but they are the most powerful and flexible of the conventional EQ types. Figure 3 (overleaf) shows a typical parametric equaliser response.




A graphic equaliser can be recognised by the row of faders across the front panel, each fader controlling its own narrow section of the audio spectrum. For example, a 30-band graphic equaliser provides independent control over 30 different bands spaced one third of an octave apart.

Other than the highest and lowest faders, which control shelving filters, each of the filters in a graphic equaliser is a fixed-frequency band-pass filter, where boost is applied by moving the fader up from its centre position, and where cut is achieved by moving the fader down. Graphic equalisers have the advantage of being very easy to set up, but must be used sparingly, as if they're not very well designed, they can have an adverse effect on the sound. They are also less flexible than the parametric EQ, which can be tuned exactly to specific frequencies. On the graphic equaliser, the range covered by each fader is fixed, and the width of each individual band of a third-octave equaliser is actually rather wider than a third of an octave, to allow a smooth overlap between bands. Figure 4 shows the response of a typical graphic equaliser.




While early studio equipment and, to a greater extent, telephone lines needed a lot of corrective EQ to make them sound natural, modern recording equipment is capable of storing and reproducing sound that is virtually identical to the original. Nevertheless, the original sound isn't always what we want to hear, so EQ has evolved to take more of a creative role. What's more, the simple bass and treble 'tone' controls found on early equipment have been replaced by sophisticated multi-band equalisers which place far more precise control in the hands of the user.

At heart, no matter how complicated an EQ is, it is still really just a frequency-selective volume control, but its subjective effect on the sound is often more profound than this description might leave you to expect. In February 1996's SOS I wrote an article that explored some of the psycho-acoustic aspects of EQ, and concluded that the most logical place to start searching for reasons why EQ has the effect on us that it does was in nature -- specifically, the Earth's atmosphere. Here, low frequencies travel slightly faster than high frequencies, so the further away a sound source is from the listener, the more delay there'll be between the low frequencies (which arrive first), and the upper harmonics, which follow. The higher up the spectrum you move, the greater the delay will be, and, in effect, the harmonic structure of the sound becomes progressively more smeared as it travels. Air also absorbs high frequencies more readily than low ones, so the further away a sound is, the less bright it is.

Obviously, we're used to sound behaving like this, so we accept the effect as natural, which, by definition, it is. Our brains don't recognise the phase distortion -- they just recognise the sound as being distant. By the same token, louder sounds that have little or no phase smearing are perceived as less distant. In recorded sound, however, we're faced with the problem that all recorded sounds travel the same distance -- if the hi-fi speakers are three metres from the listener, then that's how far the sound travels. To fake the illusion of distance in a mix, therefore, we need to recreate the smeared harmonics and reduced HF response that occur in nature.




We've already established that EQ acts as a selective volume control, targeting only certain parts of the audio spectrum. However, it's well known to circuit designers that EQ doesn't only change the level of specific parts of the spectrum, it also changes the phase of the affected frequencies relative to those that aren't being cut or boosted. In other words, the equaliser is creating a similar effect to changing the distance of the sound source -- it's affecting both the frequency response and the phase relationships of the signal. As I suggested in last year's psycho- acoustics article, this may be one of the reasons why brightening up a sound makes it seem closer, and why taking off some high end might make a sound seem more distant. Of course, every design of EQ affects the audio spectrum and phase response in a different way, and -- leaving aside technical criteria such as noise and distortion -- this might explain why some EQs have a more natural, musical sound than others.




It's a well-documented fact that the human hearing curve isn't flat, but instead is more sensitive to mid-range sounds than to frequencies at the extreme high and low ends of the spectrum. Of course, we don't notice this, because we've heard sounds this way all our lives. However, as the level of sound we're listening to increases, the mid boost of the hearing system becomes less, and the result is that high- and low-frequency sounds seem proportionally louder. This is yet another of those interesting physiological facts that can be exploited to fool the ear into believing something that isn't entirely true. For example, if we know that extreme high and low frequencies stand out more when we listen to loud music, we can create the impression of loudness at lower listening levels by attenuating the mid range and boosting the HF and LF ends of the spectrum. The loudness button on a stereo system does exactly this, and if you look at the graphic EQs used in a club or PA system, you'll often see them set up with a smile-shaped EQ curve to promote the illusion of loudness and power. Of course, this works just as well in the studio, although it's always most effective just to treat some of the sounds in a mix, to maintain a contrast between the different sounds.




As a general rule, equalisation should be employed only after all efforts have been made to obtain the best sound at source. What's more, there's a huge subjective difference in sound between a budget equaliser and a top-quality studio equaliser: if you have to work with a budget EQ, or the EQ section built into your desk, you'll probably have to use it sparingly if the overall sound isn't to suffer. Though the character of a really nice equaliser is difficult to quantify, the best equalisers allow you to make more drastic changes without the sound appearing unnatural.

Most often, a combination of cut and boost is required -- but always use the Bypass switch to flip back and forth between the equalised and unequalised sounds, to make sure you really have improved matters. Equally, if you feel the need to EQ an instrument in isolation, check again with all tracks playing to make sure that the settings you're using work in context with the rest of the mix. More often than not, you'll have to make further adjustments, but it really is worth striving to get your sounds right at the outset. EQ is an invaluable ally in shaping well-recorded sounds, but even the best equalisers have their limits when faced with difficult material.




In general, the less EQ boost you use, the more natural the final sound will be. The human ear is far more tolerant of EQ cut than it is of boost, so, rather than adding lots of top to vulnerable sounds such as vocals in order to get them to sit at the front of the mix, try applying high-end cut to other sounds in the mix that are conflicting with the vocal.

Some classical purists might say that you don't need EQ at all, but in the world of pop recording, where the emphasis is on appropriate rather than accurate sounds, equalisation has become a way of life. The close miking of drums was originally devised to cut down on spill from other instruments, but now it's become the normal pop drum sound. EQ plays a very large part in creating a modern drum sound, but because no-one is trying to emulate a natural acoustic drum sound, the EQ is used in a creative context rather than a corrective one.




EQ can be used creatively in many ways, but one of the most popular applications is to separate two similar sounds within a mix where the degree of overlap is causing the sound to become confused or muddled. For example, if two sounds are fighting for the same part of the spectrum, a peaking equaliser can be used to add a degree of bite to one sound at one frequency, while the other sound can be peaked up at a different frequency. Similarly, the top or bottom end of a sound can be 'trimmed' to avoid conflict, a typical example being the acoustic rhythm guitar in a pop mix. 

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