A classic question that comes up whenever people are first learning about EQ is whether it’s better to use Additive or Subtractive EQ. Naturally, Additive refers to boosting and Subtractive to cutting, and while it’s obvious that you can do both as you choose, the Additive vs Subtractive argument is really more of a “philosophy of EQ” question - is it generally better to approach a task by boosting or cutting. Let’s take a look at that.
From teaching recording and studio techniques for years, it’s always seemed that the instinct of most newbies when it comes to applying EQ - especially when mixing - is to initially gravitate to boosting. I guess it’s a natural tendency to want to add something to a sound that presumably has been deemed wanting in tone, and boosting - Additive EQ - has a much more obvious effect on tonal color than cutting - not only is the tonal change more pronounced, but the overall level is increased, and subconsciously we all tend to equate louder with better. But many budding mixers are told, at some point, that it’s better to EQ by cutting - Subtractive EQ. The conventional wisdom holds that this will provide not only better tone but that it also avoid a number of potential problems.
Well, like most conventional wisdom, there’s a grain of truth to those assertions, but the advice often leads people to believe that Subtractive EQ is somehow inherently superior to Additive EQ. In fact, the suggestion to look to Subtraction rather than Addition is intended as more of a hedge against novice EQ practitioners developing bad habits, and a way of nudging them to listen more carefully and develop an ear for more subtle EQ changes.
There are a few assertions commonly made about Additive vs Subtractive EQ practices that, while they may have a germ of truth, are not really valid, and a few that are, at least to some degree.
We all know that conventional analog EQs (and standard digital EQs) introduce a small amount of phase shift; it’s sometimes claimed that Subtractive EQ introduces less phase shift than Additive. From a technical standpoint, this is not true - the amount of phase shift is a function of the degree of boost or cut, and is the same for both. This notion may have sprung up from the idea that experienced EQ'ers may be able to accomplish a desired tonal adjustment with less aggressive EQ by cutting, and that may be so, as far as it goes, but there’s nothing inherently less “phasey” about Subtractive.
It’s also suggested that since Subtractive EQ doesn’t utilize gain boosts it’ll be cleaner - obviously this assumes analog EQ, since with digital EQ the issue would be moot. Now, if an EQ was noisy, then there might be some benefit to not bringing up a high inherent noise floor by aggressive boosting, but most EQs are clean enough that shouldn’t be an issue. I’ve heard some people suggest that passive EQs (like the classic Pultec and Lang models) are better (cleaner) because they cut only, but even though technically a passive EQ only cuts, those designs do offer both boost and cut, by incorporating a constant gain boost and then setting the zero positions of the EQ gain controls back down to actual unity gain. In fact, this gain boost (often achieved with colorful tubes and transformers) may be a big part of the characteristic analog warmth of those designs, even when run flat.
Another point of conventional wisdom is that Subtractive EQ is less likely to cause overloads, especially when aggressive boosts are dialed up that may push the track or the overall level of the mix into the red. This can be true, but only if the mixer fails to maintain proper gain-staging when EQing. Gain-staging, of course, is the practice of maintaining unity gain - the same level going into and coming out of each stage in the signal path, ensuring a clean, appropriate level at all times - and this bit of EQ advice is most likely intended for newbies who haven’t yet had that good-practice technique ingrained in their workflow.
But better than avoiding EQ boosts for the risk of overloads down the line is to make gain-staging an integral part of the EQ process - just about all EQs I can think of provide at least an overall output level control, and some have both input and output gain, along with meters to help the user maintain unity gain.
Besides promoting the safe use of both boosts and cuts, there’s another benefit to proper gain-staging within an EQ: As I mentioned before, we all tend to subconsciously equate louder with better, and when boosts have been applied to a signal - even subtle ones - we may find ourselves preferring that version to the original because it’s very slightly louder, rather than because the tone is actually an improvement over the original. If unity gain is carefully maintained as the EQ is being applied, then any tonal changes - either from boosts or cuts - will stand on their own merits, giving better feedback as to the effectiveness of the EQ - this will be especially obvious when A/Bing the original and EQ’d versions.
The final bit of Additive vs Subtractive conventional wisdom I want to mention is the idea that, in general, Subtractive EQ is smoother than Additive EQ. There can be a grain of truth to this as well - not because there’s something technically better, but because of how the application of EQ is approached. An important aspect of EQ, in general, is the bandwidth of the boosts and cuts being applied. Many EQs are celebrated for being especially “musical”, and this term is often applied to EQs that have broader curves - wider bandwidths. Each EQ band’s bandwidth, or “Q” - “quality factor” - will have a significant effect on the character of the tonal variations dialed up.
Narrower Qs can sound peaky if large boosts are applied, and this may be the reason that newbies are encouraged to gravitate to the Subtractive approach - at least until they get more familiar with the effects of different Q settings. Broader Qs will be less likely to emphasize a specific range of frequencies, and more likely to maintain the natural tonality with a more subtle overall variation in tone rather than a more obvious, more “EQ’d” character. But once a novice EQ user’s ears have become a bit more attuned to the subtleties of EQ, there’s no reason to avoid boosting - it’s just a matter of applying it effectively, with a more subtle hand.
So while it can be good advice for novices to suggest taking the Subtractive EQ approach at the outset, to avoid the potential pitfalls of the careless use of Additive EQ, ultimately best practice will normally involve both boosts and cuts. So when and how should an intrepid practitioner of the mysterious art of EQ use each?
Well, another bit of conventional wisdom (there it is again!) suggests boosting with broad curves for tone (creative EQ), and cutting with narrower bandwidth for problem solving (corrective EQ) but once again, while there’s some good advice in that, it’s too broad an overall suggestion to really stand on its own. As I said earlier, broad EQ boosts do tend to sound smoother and more natural - more “musical” - than narrower, peaky ones, but once a good ear for tonal variation has been developed (practice, practice, practice), it can be effective to dial up a narrower, more targeted boost for a deliberate and somewhat more obvious tonal tweak - the key is to apply it in such a way as to not call attention to the mechanics of the EQ, making sure the resulting tone still sounds like it could have come from the instrument (or voice) that way.
Likewise, narrow cuts are a good way to approach corrective EQ. Often, a track will suffer from a narrow resonant peak (thanks to room artifacts or mic placement) that imparts an unpleasant ringing on certain notes, but the overall tonal balance may not require EQ in that range. A very narrow cut (high-Q) can zero in on the problem frequency and tame it, with minimal (or no) impact on the overall tonal balance. To locate the precise problem frequency, a large narrow boost can be dialed up, and the frequency swept slowly; when the problem resonance is encountered it’ll jump out - then switch to a deep, narrow notch, and Bob’s Your Uncle - problem solved.
But of course Subtractive EQ is not just for problem solving - broader cuts can be an excellent way to make subtle, smooth overall tonal changes. That’s part of the argument for it, and it’s definitely a good way to approach EQ - it doesn’t have to be used in place of Additive boosts, but ideally in conjunction with them, for a combination of overall tonal re-balancing and more targeted tweaks. Here’s a simple example...
Let’s say you have a snare drum track that - like most close mics on the top of the snare - needs some EQ to alleviate an overly thick, midrangy tone and a lack of fullness.
A strictly Additive EQ approach might be to add some boost at around 3kHz or so, for presence, another at 100-150 Hz, for some fatness and thump, and possibly a bit of extra boost up above 8-10 kHz , for overall air and to bring out the sound of the snares themselves. This could sound fine - once unity gain is restored - though the resulting balance will be heavily dependent on not just the gain in each band but the bandwidths (Q) of the various boosts employed.
A purely Subtractive approach might dial up a broad dip in the lower midrange, around 500-800 Hz. Again, once unity gain is restored, the resulting curve may look quite similar to the one dialed up with the Additive approach - the upper mids and treble emphasized, and a small boost on the low end. The main difference may be in how smooth the lower midrange is, and again, that will have a lot to do with the bandwidth and the resulting shape of the EQ curves - the latter can also be a function of the design of that particular EQ, though with full-featured parametrics, it’s usually possible to dial up just about any shape with a little bit of experimentation.
Combining both Additive and Subtractive approaches, since the overall impression of the original snare sound is too midrangy, I might start with the same broad midrange cut, but then instead of fiddling with the exact frequency and bandwidth, to fine tune the effect on the adjacent upper midrange and bass regions, I might turn to Additive EQ boosts to target the most appropriate areas in the upper midrange and treble, as above, and fine-tune the amount of thump more directly with a small extra bass boost in the 80 Hz range. This combined approach could make it easier and quicker to dial up the desired tone - just don’t forget to maintain unity gain throughout the process.
So both Additive and Subtractive EQ have their place. While it’s not a bad idea to start off with an emphasis on Subtractive EQ, as your ears becomes more attuned to the tonal nuances, a mix of both boosts and cuts is typically the most efficient and effective path to tonal nirvana.