© 2019 by The Audio Blog

Are reflection filters worth the money?

If you've read a little of this blog, you know that the quality of a recording result depends critically from the acoustic properties of the space where you record.­­­­­

The consequence is simple: if you still haven't treated your recording space, and you have some spare cash, use it on acoustic treatment! It'll give way more bang for your buck than better monitors, microphones, new preamps, interface, plugins... or anything else, really.

This is particularly true for vocals as they're usually the most important element in any mix and the one which we, as listeners, connect with the most. After all, we listen to voices every day, so everybody is pretty good in detecting a strange or bad vocal sound. And a comb filtered vocal doesn't sound that pretty to most people.

Life is what it is however, and that means that there are cases when, for one reason or another, you can't treat your room.

Maybe you're renting. Maybe the room has multiple uses and you can't permanently hang panels. Maybe your girlfriend/wife/partner/cat doesn't like frames covered with black speaker cloth (hint: change the color!). Maybe you suck at DIY and you can't afford ready-made absorbers.

Or maybe that shiny new microphone was soooooo tempting and you've spent all your budget on it (because of course rockwool and wood and Cara fabric are far less fun).

No matter the reason, some manufacturers promise to come to the rescue by selling reflection filters.

These devices claim to improve the isolation of your microphone from the room, removing reflections (hence the name) and make your recordings better. They're usually targeted to vocal recording, but nobody's stopping you from placing one around that mic you're using on that 4x12 guitar amp cabinet.

They come in all kind of shapes and types, but most often they resemble a sort of parabolic surface with a support for the mic inside. The mic is basically surrounded by the filter's walls, which is supposed to stop reflections and comb filtering. If you don't know what I'm talking about, Google is your friend.

The obvious question arises: do they help? Can you really get rid of acoustic treatment if you use a reflection filter? (I'm joking: no, you can't)

The bottom line is that while they do color the sound some and they help more with cheaper mics than more expensive ones, and while they are better suited to male voices, they're absolutely not enough.

There's a bunch of reasons for that.

One is that - given their typical thickness and materials - filters are usually only partially absorbent: they tend to do OK in higher and mid high frequencies, and progressively absorb less lower down.

Filters are mostly used with cardioid mics. And we know that cardioids very often are "cardioid" only from mids up. In other words, microphones tend to behave as omni-directional in lower frequencies. This means that a reflection filter will not really do much exactly where a cardioid mic would need the most help - the more omnidirectional lower frequencies.

Unless you're whispering, some bass trapping is still useful.

Another reason is that, since reflection filters are far from perfectly absorbent, they themselves act as reflectors, creating new reflection patterns around the mic (different than if the filter wasn't there, that is).

By definition, a reflection filter is placed very near to the mic. That means any reflections it creates have more level than if they came from the walls, because less dissipation happens.

Dissipation is a function of the distance traveled by a sound wave (doubling the distance from a source to a receiving point produces about 6 dB drop of level). This means that the nearer a reflective surface is to a microphone, the louder the reflected sound will be when it is captured by your mic - leading to stronger cancellation when it meets the original sound wave.

That means that the filter itself causes some comb filtering (‘coloration’), altering a little the sound picked up by the mic. This especially affects lows and low-mids, since that's where the reflection filter acts more as a mirror (as opposite to an absorber).

When reflection filters are used for vocals, the coloration occurs primarily in the voice's lower registry (the stuff around 70-150Hz, of which there's typically more in males voices). The ‘bassier’ and louder the voice, the more this effect will be pronounced.

That's in principle, a bad thing.

In practice, it may or may not be important, depending on the voice and the sound you're after. Better keep this effect in mind though, and if you don't like the vocal sound, you can try to remove or re-position the reflection filter.

So, do reflection filters help with something? Yes, they do. What these shiny contraptions can do is reduce mid-high frequency reflections coming off-axis to the mic.

Off-axis response is the Achilles heel of lower-cost mics. So even if the filter reduces side reflections just a little, it can make less sophisticated capsule designs sound decidedly better. Some side absorption is better than no side absorption.

It's important to point out that both the reflection pattern created by the device (the ‘bad’) and the reduction of off-axis reflection (‘the good’) depend on where you place the microphone in the filter. Most devices allow you some degree of freedom and it’s worth exploring a little which position gives the best compromise. Usually, the more "inside" the microphone is, the better.

Differences can be subtle until you start mixing, so make sure you use good headphones and concentrate on the lower end for ‘the bad’ and the higher frequency sound for the ‘good’. It's not a bad idea to engage a hi-pass filter on the mic if you have it available.

You can achieve a similar result by placing broadband absorbers on the sides of the mic position - something that most studios do (but most home recordists don't). Of course, a studio also tends to have better mics (with better off-axis response) - which is a reason why it's still worth going there.

Unless you buy good mics and - having read this blog - you know how to reproduce the side absorption at home.

A final factor to consider is the geometry of the filter itself. With a few exceptions (like the Kaotica Eyeball), filters ‘protect’ the back and sides, but leave top and bottom and, most importantly, the front of the mic open to reflections. And in a cardioid mic used for vocals, reflections from the wall at the back of the singer are the ones responsible for most filtering.

Cardiods already reject mid-high frequency sound coming from the back of the capsule, so any absorption material at the back of a cardioid microphone doesn't really make a great deal of difference. In other words, the most important place where to put absorption is at the back of the singer, not the back of the microphone.

That's why it's perfectly possible to get quite good vocals anywhere, simply by temporarily placing some mid-high frequency absorbing material behind the singer (a heavy duvet on a T-shaped mic stand may not look that good, but the result is quite pro).

A reflection filter doesn't help anything in that regard, so by itself is not enough to get that killer vocal. Combined with a good mid-high absorbers on the back of the singer, however, it can help reducing off-axis reflections and therefore contribute to a better sound.

It's up to you to evaluate the tradeoff between slightly better off-axis rejection and the additional comb filtering/coloration occurring due to the presence of the filter itself.

Having said that, a degree of comb filtering (coloring) always happens and when recording, as opposed to mixing, it's not necessarily a bad thing (that's why you have diffusers in recording rooms, for example). So the pros and cons of a filter really depend on the specific situation: the coloring may or may not suit a given voice, room, mic and mic position.

For what it’s worth, if I have to record a vocal in an untreated room (typically on location), I always set up a heavy duvet on a stand behind the singer. If possible, I put two more on the sides. But if three duvets aren't available or time is short, a reflection filter will still help, so I generally pack one with me.

When recording material other than vocals, it's important to think about what frequencies are the core of what you’re recording.

It would make little sense to put a reflection filter if micing a bass cabinet say, or kick and tom mics (well, unless you like the additional comb filtering, that is!). On the other hand, it may help when micing a guitar cab or a snare drum - or even as overheads, as the material has more high frequency content than low, and so can benefit from the absorption of off-axis reflections.

So there you have it. You cannot expect miracles and you still need to have proper treatment in your room to avoid heavy comb filtering... but a reflection filter can be a tool worth having in certain situations, especially if you are using a microphone with less than stellar off-axis response.

Good recordings!

(thanks to Chris Saunders for his invaluable editorial contribution)