October 3, 2022

If you don’t have a treatment plan for your recording space and have the money, acoustic treatments will be a better option than new preamps, interfaces, plugins… or any other options.

Vocals are especially important because they are the most important element of any mix and also the one that we, the listeners, most connect with. We all listen to voices every day so we are pretty adept at identifying bad or unusual vocal sounds. A comb-filtered vocal isn’t very appealing to most people.

However, life is what it is and there will be times when you have to treat your room with respect.

Perhaps you are renting. You might be renting the space because it has multiple uses. Maybe your girlfriend/wife/partner/cat doesn’t like frames covered with black speaker cloth (hint: change the color!). You might be terrible at DIY or you don’t have the budget to buy ready-made absorbers.

Maybe that new shiny Best microphone isolation shield was too tempting to resist and you spent your entire budget (Rockwool, wood, and Cara fabric are much less fun).

No matter what reason, manufacturers are willing to help by selling reflection filters.

These devices are claimed to increase the isolation of your from other rooms, remove reflections (hence their name), and improve your recordings. Although they are usually used for vocal recording, there is no reason to stop you from using one with the mic on your 4×12 guitar amp cabinet.

There are many types of filters, but they often resemble a parabolic surface that supports the mic. The filter’s walls surround the mic, which is supposed to prevent reflections and comb filtering. Google is your friend if you don’t understand what I mean.

It begs the obvious question: can they eliminate acoustic treatment? (I joke: you won’t).

They do not add enough sound quality.

There are many reasons to do this.

One reason is that filters, due to their usual thickness and materials, are only partially absorbent. They tend to absorb better at higher frequencies and middle-high frequencies and gradually absorb less low down.

Most filters are used with cardioid microphones. We know that cardioid mics are often “cardioid” from the middle. Mice behave more omnidirectionally at lower frequencies. A reflection filter won’t do much in the areas where a cardioid microphone would need the most help, as it is not omnidirectional at lower frequencies.

Bass trapping can still be useful unless you are whispering.


Another reason is that reflection filters, while not perfectly absorbent, act as reflectors and create new reflection patterns around the mic (different from if the filter was absent).

A reflection filter is defined as one that is very close to the mic. Because of the less dissipation, any reflections it creates are higher than if they were coming from walls.

The distance a sound wave travels to dissipate is measured in meters. For example, doubling the distance between a source and a receiving point will result in a 6 dB drop in level. This means that the closer a reflective surface to a microphone is, the louder the reflected sound is when captured by your microphone. It will also lead to greater cancellation when it meets the original sound waves.

This means that the filter causes some comb filtering, which alters a bit the sound picked up from the microphone. Lows and low-mids are particularly affected by this, as the reflection filter acts more like an absorber (as opposed to a mirror).

Reflection filters for vocals cause coloration to occur primarily in the lower register of the voice (the stuff between 70-150Hz and more in male voices). This effect is more noticeable the louder and busier the voice.

This is, in principle, a negative thing.

It may or not be necessary depending on your voice and what sound you want. Keep this in mind and if the vocal sound isn’t what you want, you can remove or reposition the reflection filter.

Do reflection filters aid with anything? They do. These shiny gadgets can reduce mid to high-frequency reflections from the mic.

The Achilles heel of low-cost mics is their off-axis response. Even if the filter does not reduce side reflections, it can make simpler capsule designs sound better. Side absorption is better than no side absorption.

It is important to note that the reflection pattern generated by the device (the “bad”) and the reduction in off-axis reflection (the “good”) depend on the location of the microphone within the filter. You have some freedom with most devices, so it is worth looking into which position offers the best compromise. The microphone should be placed as close to the user’s ears as possible.

You may not notice the differences until you mix them. Use good headphones to focus on the low end of the sound for ‘the bad’, and the high frequency for ‘the good’. If you have a hi-pass filter, it’s a good idea to use it.

Broadband absorbers can be placed on the sides of your mic position to achieve the same result. This is something most studios do, but most home recordists don’t. A studio will have better mics, which means they are more responsive to off-axis sounds. This is why it’s worth the effort.

You can’t reproduce side absorption at your home unless you have good mics.

The geometry of the filter is another important factor. Except for the Kaotica Eyeball, filters “protect” the sides and back, but leave the top and bottom, and most importantly the front open to reflections. Reflections from the wall behind the singer can cause filtering in cardioid microphones used for vocals.

The back of the capsule rejects mid-high frequency sounds, so any absorption material at the of a cardioid mic doesn’t make much difference. The back of the singer is the best place to absorb, and not the microphone.

It’s possible to have very good vocals wherever you are by placing some high-frequency absorb material behind your singer. Although a heavy duvet placed on a T-shaped microphone stand might not look as good, the results are quite impressive.

The reflection filter won’t do much in this regard so it isn’t enough to produce a killer vocal. It can be combined with good mid-high absorbers to the back of the singer to help reduce off-axis reflections, resulting in a better sound.

You will need to weigh the benefits of slightly less rejection and the extra comb filtering/coloration caused by the filter.

However, some comb filtering (or coloration) is inevitable when recording. This is why diffusers are used in recording rooms. The pros and cons of filters depend on each situation. For example, the color may or not be appropriate for a particular voice, room, microphone, and position.

If I need to record vocals in untreated rooms (typically on-location), I set up a heavy duvet and a stand beside the singer. Two more are usually added to the sides, if possible. If three duvets don’t exist or you have limited time, a reflection filter can still be helpful. I usually bring one.

It is important to consider what frequencies you are recording when recording other material than vocals.

If you are micing a kick and tom microphone or a bass cabinet, it would not make sense to use a reflection filter. It may be beneficial when micing a guitar cabinet or a snare drum – or as overheads. The material’s high-frequency content is higher than the low and can absorb off-axis reflections.

There you have it. It is impossible to expect miracles, and you must still have proper room treatment to avoid heavy comb filtering. However, a reflection filter can be useful in certain situations, particularly if your microphone has a poor off-axis response.