15. 10. 2024

What are sound reflections and reverberations and why do you need to care?

When we talk about sound reflections think echo.

If you are in mountains, a cave, large hall or an empty room, say something or just clap your hands – that sound gets back to you as an echo (reflections) and repeats itself over and over (reverberations) until it loses its energy and dies (reverberant decay). This is because the sound reflects from one hard surface goes to the other and reflects over and over again until it loses its energy.

The same exact process happens in your studio, except that you might not hear it as loud as in a cave, but sensitive microphone will pick it up and your recording will be “inadequate”.

How the sound behaves in a voice over studio

Sound energy can be reflected by the surfaces in the room and bounce around or it can be partially absorbed, lose some of that energy and bounce around a bit less, or it can hit an odd, shaped object, break apart and go in different directions with diminished energy (diffusion).

These behaviors can be problematic in a recording studio. Sound waves bouncing inside the room can interfere with each other, cancel, or reinforce the energy at certain points, which makes a recording in the room difficult.

How your recording is affected by the way sound waves behave in a room depends on a number of factors – the frequency of the sound wave, the shape and dimensions of the room, the materials that the walls and ceiling and floor are made of and covered with, how many doors and windows are there, and where they are placed, location of the microphone and voice actor in relation to the nearest walls, etc., and the contents of the room (furniture, tapestry, equipment, etc.).

One of the biggest factors that determine what happens with sound in a room is the frequency of the sound wave. As we discussed in another article, lower frequencies wavelengths are long, and sound waves are more powerful. Low-frequency sound wave cannot be reflected by a small obstacle, so it bends around objects in the room and passes through lighter materials.

At mid and high frequencies, say 100Hz or so and above – sound waves are more straightforward and easier to control. Their behavior in the room can be compared to that of a tennis ball: they bounce from hard surfaces and are slowed down (absorbed) by soft surfaces.

1. First reflection:

The harder and flatter the surface – the stronger the reflected sound and the longer it will be bouncing around before it dies.

2. Then it will bounce from a corner:

This is just a simplified example, in reality, a surface has to have a dimension equal to a wavelength to reflect sound in this manner. So, our 100Hz wave will bounce off a roughly 3.5m wide wall. But will diffract (bend around) around objects that have dimensions smaller than 3.5m wide. Also, sound waves are omnidirectional, meaning that they go in all directions at once, so when they reflect, they also interact and affect each other.

However, throw a ball against a pillow and it will stop because the direct energy will be absorbed by the pillow. That’s the basic idea.

3. Then from the other wall:

Why sound reflections are bad for your recording

In reality, the voice over actor is positioned right in front of a microphone, so the microphone will pick up the sound coming directly from you, the speaker, but it will also record the sound waves reflected off by nearby walls and other surfaces.

The problem is that the microphone will record the sound from the voice over actor a very short time before the sound reflected off the walls gets to it too. How long the delay depends on how far away the nearest reflective surface is.

This delay between the two sound waves (direct and reflected) can result in phase differences in the waves.

As the direct sound waves of your voice and the reflected sound combine, the time/phase delay can cause cancellations and/or reinforcements in the sound waves, changing the tonality of what you are recording.

This effect is known as “comb filtering”. And if the tonality is changed, you are not recording what you are really saying, or rather not the way you say it.

Are there bad and good sound reflections?

Well, ideally you do not want any reflections in your recording. You want only direct sound to be recorded. But some reflections are indeed worse than others.

First reflections

The worst and most powerful are the so-called first reflections. These are the sound waves that bounced the first time, less than 20 milliseconds or so after the direct sound. Sound travels at a rate of about one foot per millisecond, so that means that any reflective surface within about 3m or so of the recording position will cause problems and there are also Secondary (second bounce) and tertiary (third bounce) reflections.

Flutter Echo

Flutter echo happens when the sound waves are reflected directly between two parallel surfaces – say opposing walls in a small room – back and forth. Try it in a smallish room with hard parallel bare walls: clap your hands and you’ll hear a rattling sound – that is flutter echo.

Most of apartment and house rooms have parallel walls, so flutter echo can be a real problem when you are recording with a microphone.

Reverberant decay

Reverberation is the sound left ringing in the room after the direct sound from the sound source stops. It is those secondary and tertiary reflections, it is that remaining reflected sound that tends to wash together.

For most recording, too much reverberation that lasts too long is a problem – recording clarity will be compromised, and there may be phase problems.

Equally important is making sure that the reverberant decay across the frequency range is even. If the reverberant decay for high frequencies is different than for low frequencies, the room will have a characteristic sound that probably won’t be desirable – a bright ring, boom low end, or uneven midrange. However, if the reverberant decay is too short, the room will have a dry, dead feeling and sound that will be uncomfortable.

How to manage sound reflection and reverberations.

If you are thinking of setting up your voice over or instruments recording studio you might be choosing a spot in a bedroom, a walk-in closet, or maybe a spare room.

If you start with an empty room with hard surfaces on the walls, ceiling, and floors, what do you do to manage reflections and reverberant decay? There are two different methods: absorption and diffusion, each of which is effective for solving certain problems. But in small rooms such as a home recording studio, diffusion is not as effective as absorption. We will stop on diffusion briefly because they are not exclusive, and a combination of the two methods can be used successfully.

AKUSTIK DIFFUSION

"Diffusion" is breaking a single sound wave into smaller reflections and dispersing them in different directions. Because there are no big reflections going back to the microphone, first reflection problems are reduced, and because diffusive materials are irregular in shape, flatter echo is eliminated. Although reverberation won’t be eliminated, scattering and breaking up the reflections tends to result in a smoother reverberation at a lower level.

While any irregular surface can break down reflections at some frequencies to a degree, to figure out exactly how to make a diffuser that evenly scatters a broad frequency range of sound waves requires specialized diffusion materials. Such specialized diffusers are hard to build and can be expensive.

SOUND ABSORPTION

Remember the example of echo in the mountains? Now, why is it so quiet when the snow is falling? That is because the snowflakes absorb the sound and speed up the reverberant decay.

For voice-over recording studios, absorption is the main method for managing first reflections in mid- and high-frequency ranges, flutter echo, and reverberant decay.

The idea is to use sound absorption materials placed at the main reflection points to reduce – absorb – sound energy so that the level of the reflected sound waves is significantly quieter than the direct sound.

Absorption works best with high- and midrange frequencies. Lower frequencies require large quantities of absorptive material for effective energy reduction.

Very high frequencies have very little energy compared to low frequencies, so thin, soft materials can be used to absorb them. As the frequency gets lower, thicker and heavier absorptive materials are required. Those thicker sound absorption materials are even more effective at controlling high frequencies.

There is an array of sound absorption materials that can be used, and we will talk in detail about them later. Most fall into the category of porous absorbers. These materials consist of mineral wool, acoustic blankets, acoustic panels, clothing, acoustic curtains, carpets, acoustic foam, and other sound-absorbing materials.

The sound-absorbing effect comes from the fact that the sound energy can penetrate the material on hitting the surface. Here, the sound energy is converted into heat energy, so that only a small part is reflected in the form of sound energy. In other words, the material has absorbed some of the sound.

Acoustic foam is a popular material for lining up recording studios. They are often shaped in a conical or egg crate shape. The make of acoustic foam is different from the make of packing foam, even though they might look the same. Packing foam is made out of closed-channel foam, so the air trapped in the channels provides more cushioning. But for the sound to be absorbed, it has to have a way to enter the material. This is why acoustic foam is made with open channels. So-called "Open Cell Foam" allows sound to enter and then travel through the twisted channels, bouncing off the internal walls and losing its energy. The thicker the foam, the more sound it absorbs. This is why anechoic chambers are lined with acoustic foam wedges up to 60cm deep.

Acoustic foam is also rated as fire retardant – an important factor because some types of foam (like packing foam) are highly flammable and therefore not safe for residences and businesses.

On the other hand, VocalBoothToGo sound blankets work differently. Unlike moving blankets filled with low-quality recycled polyester and cotton fibers, specialized akustikplatte known as "VocalBoothToGo acoustic blankets (acoustic curtains)" are filled with highly absorptive cotton. As the sound gets in, it agitates the cotton fibers, and by making them vibrate, the sound loses its energy.

The thicker the sound blanket, the better its sound absorption efficiency and broader its frequency range. Using VocalBoothToGo acoustic blankets in pleated fashion increases the absorption surface and the total quantity of soundproofing panels in the room.

Fiberglass insulation:

Glass fibers are another common sound-absorbing material. The rigid variant such as: Owens Corning™ 700 fiberglass series provides good absorption in the high and mid frequency range. SAFE'n'SOUND® rock wool is used to fill the space between drywalls during construction for better soundproofing. It can also be used in the corners as a bass trap. Rock wool and glass fibers can of course be very irritating, so cover them and do not disturb them to prevent strands from becoming airborne. Normal household items such as soft furniture with fabric covers, cushions, curtains and drapes, blankets, rugs etc. all absorb sound from mid and high frequencies to a certain extent.

How much sound absorption should you use?

So, you have to get rid of reflections. Do you just line up the walls, floor, and ceiling of your studio with sound-absorbing material and start recording?

But it’s not quite that simple. Many types of absorptive materials only work well on higher frequencies, leaving midrange sound bouncing around without much control.

This can result in an odd, dark-sounding room with no high end. If you have too much absorption in the upper frequencies, the mid- and high-frequency reflections will be reduced, but the bass frequencies will be bouncing around, resulting in a boomy, bassy room that sounds muddy. Too much overall absorption, and the room will sound too dead and uncomfortable to work in.

The idea is to use just enough to control first reflections and to tame reverberation decay. Keep in mind that you can always add more absorption if the room still seems too live.

The right balance is key to finding the right amount of absorption in the proper location so it will be as effective as possible – enough to control the mid and high range, and enough bass traps to end up with a balanced room response and an even reverberant decay across the frequency range.

How does it relate to voice over & instrument recording?

The purest and cleanest sound is direct sound energy. This is the sound that travels in a straight line from the voice actor or instrument to the microphone.

Sound energy reflected from objects in a room confuses the direct sound of your voice and makes recordings sound “muddied.”

You want to capture everything in the recording, including emotions and different intonations, without hearing any sonic additions from your recording room.

Unfortunately, you must record in a "box" or room that has walls, a ceiling, and a floor. When you speak into a microphone, the sound radiates in many directions. The sound of your voice goes through the room until it hits an object or wall, then it reflects back and gets recorded by your microphone over what you are saying at the moment.

Unlike reflections in a control room where the sound comes from a speaker and reaches the engineer at a distance, in voice-over recording, you position yourself right in front of the microphone. So your first reflections would be from the nearest reflective surface: laptop screen, walls to the right or left of you, and a wall in front or behind you if you are sitting close to a wall. Wherever the closest reflective surface is, that will be your first reflection point to treat acoustically.

Do not forget the ceiling and the floor. They are also reflective.

Reflections arrive at your microphone delayed in time and add sonic distortions to your voice narration. To minimize the sound of the room, sound absorption works best for small recording studios such as home voice-over studios.

Untreated reflective surface reflecting the sound back at the microphone.

For voice-over actors who do individual work (one-person commercial or book narration), having a small acoustic vocal booth might be an easier and more effective and practical way to create a great sound recording environment, rather than acoustically treating the whole room.

Portable or small acoustic vocal booths are easy to set up and also easy to take with you if you have to move.

Reflective surface below treated with a thick sound blanket to reduce the sound bouncing back to the microphone.

First reflections of sound waves have to be treated with sound absorption panels or sound blankets to minimize reflections. Thick absorptive material also works well for soundproofing.

Signature:

VocalBoothToGo is specializes in providing effective and inexpensive acoustic room treatment products, such as VocalBoothToGo Sound Blankets, Sound absorbers for sound and film production, mobile sound booth, portable vocal booths and booth rentals.

You can find contact to us here.