There are two basic types of materials based on how they interact with sound waves. Some of them absorb sound, making it lose energy and volume, while others reflect it, causing it to keep bouncing around the room. Both are equally important principles of acoustics — but today, we’re going to focus on sound-reflecting materials and their uses.
You can probably list some materials that can reflect sound off the top of your head. But to make things easier, I’m going to do that for you later on, in addition to explaining their individual sound absorption coefficient ratings. That scale allows us to see the percentage of frequencies that are absorbed by a certain material represented on a scale of 0 to 1. So a coefficient of 0.70 would mean that a material can absorb 70% of frequencies and reflect the remaining 30%.
But before we get into all that, let’s start by explaining the concept of sound reflection.
What Is Sound Reflection?
Sound waves spread in all directions from their source, but their movement isn’t unrestricted. Everyday objects act as obstacles, often causing the sound to bounce against them and travel in the opposite direction. That concept is known as sound reflection, and it explains the existence of echoes and reverberations.
Physicists have observed that different forms of energy move in different ways. Sound waves are most often compared to light energy, even though the two don’t always behave similarly. Still, learning about the way light travels could be a good way to understand the way sound moves as well.
Most of us can explain the concept of energy reflection in the simplest of terms. When you shine a flashlight at a mirror, the angle of its reflection is the same as the angle of the incoming light ray. In other words, according to the Law of Reflection, the angle of incidence is equal to the angle of reflectance when measured from an imaginary line that is perpendicular to the surface at the point of impact. Both of these light rays exist on the same two-dimensional plane.
But does this Law of Reflection apply to sound waves as well? Of course — but it gets more complicated in practice. Instead of a flashlight, imagine a ceiling fixture. Like light coming from a ceiling bulb, sound travels in all directions at the same speed.
Sure, if you were playing music from a speaker, the sound waves would be directed toward one area, but people all around the speaker would be able to hear it. But if sound travels in all directions and bounces off all surfaces — just like light — shouldn’t we hear echoes all the time? Let’s talk about why we don’t often run into that problem.
Why Don’t We Hear Echoing and Reverberation More Often?
Echoes and reverberations are slight delays in our perception of sound caused by reflection. However, in order to hear them, conditions need to be ideal. In addition to being surrounded by sound-reflective materials, we also need to be in relatively large — cavernous — spaces.
The main reason we don’t often hear echoes and reverberations is that our ears aren’t sensitive enough to pick them up. Both phenomena require us to hear the delay between the original sound and the repeated echoes or prolonged reverb that follow. In the case of the echo, we’d hear distinct repetitions of the sound.
Conversely, reverb is an elongation of the original sound accompanied by an increase of volume. It’s caused by the sound waves bouncing off a reflective surface in an acoustic environment at faster intervals. So there’s an overlap of sounds leading to your perception of the volume increase and clarity.
Basically, the human ear needs about a tenth of a second of delay between the original sound and the reflected echo. However, to notice a reverb, we need about 0.05 seconds between the sounds. In practice, we don’t even hear that delay, only its effects.
As we have established, one of the most important conditions we need to be able to perceive these audio delays is a large space. Echoes tend to happen in huge caves or canyons, or even large empty rooms. According to some calculations, the minimum distance between you and the reflective surface needs to be about 19 yards.
Of course, different environments will affect the resulting echo in different ways. Therefore, the material that makes up the surface plays a big part in how echoey a place will be. So let’s see what makes a material a good sound reflector.
What Makes a Material Good at Reflecting Sound?
There are several features that make a material good at reflecting sound. Basically, it needs to be the opposite of absorbent materials — so, hard, dense, and ultimately impenetrable. However, you could also make the point that all materials are sound-reflecting.
To continue our earlier comparison between light and sound waves, let’s think about materials that reflect light. You’re probably envisioning a mirror or another polished surface, right? But the fact of the matter is that every single object reflects light. After all, that’s how we visually perceive everything around us.
The same is true of sound waves — even though most people don’t use sound reflections to orient themselves in space. But just because we can’t hear every instance of sound reflection doesn’t mean it doesn’t happen.
Like light, sound bounces off every physical object, even our own bodies. But even though all surfaces are potential reflectors or sound, some are better at it than others. Certain properties can make sound reflection more successful. The material in question would need to be:
- Hard — which is why acoustic foam is a sound absorber, rather than a reflector
- Dense — because porous materials can trap air and, therefore, sound as well
- Flat — even though irregular surfaces are reflective, flat ones are better at bouncing the sound evenly in all directions
If a material lacks one of those properties, it loses some of its reflective ability. For example, MLV is dense, but it’s malleable, so it is reflective but also somewhat diffusive. When sound waves come into contact with the material, they usually bounce off, but not before vibrating the material and losing some of their energy. Therefore, all the traits I’ve listed must be present for a material to be truly reflective.
How Do Sound-Absorbing Materials Work?
Unlike sound-reflecting materials, products like acoustic foam are soft and porous. Those are the main properties that make it ideal for trapping sound. When sound waves come into contact with these kinds of materials, they lose all their energy trying to bounce around the foam.
Before I start listing sound-reflecting materials, it might be useful to consider the properties of absorbent materials. That should help you understand where each of the materials on my list fits on the spectrum between completely absorbent and completely reflective materials.
To get back to our example, acoustic tiles have an excellent rate of absorption across all frequencies. They perform best at the 1,000 Hz mark, but they exhibit only a slight dip in higher and lower frequencies. Low-frequency sounds are the most difficult to fend off, as evidenced by the fact that they tend to move structurally, rather than through the air.
Still, acoustic foam can trap even those frequencies — provided that you use the correct shape and placement. Overall, foam products successfully absorb upwards of 80% of frequencies between 125 and 2,000 Hz. Meanwhile, the best sound-reflecting materials can do is block sound from passing through a surface.
Examples of Materials That Reflect Sound
There are many materials that reflect sound all around us — I can hardly list all of them. Still, this list should provide you with some more and less obvious examples. So let’s start with some of the most reflective surfaces out there.
Marble is among the most reflective materials on this list because of its density and strength. It only absorbs about 1% of all frequencies in the 125–2,000 Hz range.
Still, with a hardness rating of 3 on the Mohs scale, it’s not even the strongest mineral there is. Therefore, it probably isn’t the most sound-reflective material on the planet. Believe it or not, marble is also somewhat porous, which allows it to absorb water, if not sound.
Granite is a composite material with a hardness of 6.5 on the Mohs scale. It also has a density of 168 pounds per cubic foot, which makes it denser and harder than marble. Like most rocks, granite is naturally porous. However, by the time it gets into our homes, it’s usually completely sealed, making it a perfectly reflective surface.
There are other strong rocks I’m choosing not to include on this list. For example, while diamonds are probably the strongest mineral on Earth, no one has a diamond surface large enough to reflect sound waves.
Clay bricks basically emulate stone in the way they interact with sound waves. Even though they’re significantly less dense than granite or marble at about 120 pounds per cubic foot, they can successfully reflect most sound frequencies. An unpainted brick wall might only absorb between 3 and 5% of all frequencies. If you add a layer or two of paint (maybe even soundproof paint), you might bump that up by a few percentiles.
Ceramic tiles have an incredibly low absorption rate even though they’re not as hard or dense as the previous materials I’ve listed. They can absorb up to 2% of all frequencies, so they’re mostly reflective. That’s why so many people enjoy singing in the shower. Tiles can make even the worst singers sound like the next Adele thanks to the power of reverb!
Concrete is an excellent construction material — when it cures, it’s as hard and dense as rock. Still, depending on the finish we’re talking about, it can be more or less absorbent. For example, rough concrete can absorb up to 7% of high-frequency sounds in the 4,000 Hertz range and 4% in the 2,000 Hertz range. But smooth concrete is even more reflective, and a layer of paint or glaze can bring those numbers down to 1–2%.
Clinker concrete walls, with their rocky texture, can actually absorb between 10 and 60% of frequencies across the spectrum. Those numbers are also reflected (no pun intended) in concrete floors, which have to deal with more impact noise as well.
If you have concrete walls, chances are, they’re probably covered with plaster. Once again, the texture of the finish would have a small part in determining how reflective the surface is. But generally, plaster over concrete can absorb between 5 and 10% of frequencies, performing better on the lower end of the spectrum. Plaster has also been shown to improve the absorbency of masonry walls, but not by much.
Metals tend to have a flat shape and a polished surface, which usually makes them highly reflective. Aluminum, copper, and steel can all amplify and enhance sound waves. In fact, steel has a sound absorption coefficient of only 0.03, which means that it can only absorb about 3% of all sound waves that hit it. Therefore, it reflects the other 97%!
Metal, along with wood and concrete, is frequently used to construct reflective noise barriers around highways. They’re supposed to block traffic noise from spilling out into surrounding neighborhoods. However, putting these barriers on both sides of the road often only succeeds in amplifying the noise. That’s why many people advocate for absorbent barriers instead.
Glass windows and mirrors are fairly similar to polished metal surfaces, in that they also have a sound absorption coefficient of 0.03. But even that changes based on the kind of glass you’re talking about.
For example, 4 mm glass can absorb up to 30% of low-frequency sound waves and 2% of high-frequency ones. However, thicker glass reflects between 90 and 98% of frequencies.
Even though plastic is generally pretty malleable, it’s also firm and smooth enough to reflect sound. Since it’s pretty dense and non-porous, it can reflect between 95 and 100% of all sound frequencies. But again, for this to have a significant effect, you’d have to be surrounded by plastic walls. You likely won’t hear an echo bouncing off your plastic patio furniture.
In our homes, we are surrounded by wood and its byproducts — it’s what our furniture, doors, and floors are made of. On its own, wood isn’t particularly good at absorbing or reflecting sound. Still, its intrinsic properties make it somewhat better at the latter than the former.
For example, plywood on studs has an absorption coefficient of about 0.30 on the low end of the frequency range, but only .09 on the higher end. So it reflects between 70 and 91% of sound waves. Solid timber products are even more reflective though, reflecting between 86 and 92% of sounds. So it all comes down to the density, hardness, and shape of the surface in question.
Water can also be a sound-reflecting material, if only in certain situations. You see, if you’re standing on the shore and trying to call out to someone on a boat, your success will depend on the water texture.
If the water is choppy, your voice won’t carry far. But if it’s smooth, the sound waves will bounce off the surface and make it to the person you’re trying to reach. The water will cool the air above the surface, which will also help your voice travel farther. However, the distance the sound waves have to travel is another potential obstacle.
Heavy Flat Curtains
Even though curtains are certainly better at absorbing sound waves than walls or windows, they can also be reflective. If you’re using heavy soundproof curtains and you don’t want to amplify the sound waves in the room, don’t pull the curtains taut on the curtain rod. That could lead to the curtain behaving more like a flat wall. So if you want to prevent reflection, keep the fabric falling naturally.
Since talking about curtains in this context is already somewhat of a stretch, let’s end this list here. You get the gist: any hard, impermeable surface you come across is going to be a good sound reflector. Now let’s talk about how we can use these materials in acoustic projects.
How Do We Utilize the Principle of Sound Reflection?
If you’re interested in soundproofing your house, you might be looking to eliminate or deaden any sound-reflective materials you have in there. However, sound reflection isn’t necessarily a bad thing. In fact, there are many positive uses of materials that reflect sound.
Completely sound deadened rooms tend to sound a bit dull, but they’re a necessary part of recording studios. Still, most pop artists nowadays add reverb in post-production simply because it enhances our audio experience. A quick perusal through videos of people singing in tall stairwells or tiled bathrooms should convince you of the benefits of sound reflection in that case.
The principle of sound reflection also influenced the design of many musical instruments, from metal horns and trumpets to wooden guitars. But sound reflection isn’t only useful in the music industry. Nautical sonars are our version of echolocation, which we borrowed from bats and dolphins.
Megaphones and hearing aids both use this technology to amplify sounds. Even stethoscopes are a great example of the way we’ve used sound-reflecting materials to make it easier for doctors to hear irregularities in our heart and lung activities.
Yet despite the many good uses of sound reflection, it can also be incredibly irritating. Let’s imagine a speaker in a large auditorium with bare marble walls — their lecture is probably going to echo like nobody’s business! On top of that, the reflective material will amplify every murmur and cough coming from the audience. So what can you do to keep the room looking presentable while also playing up its grand appearance?
Preventing Sound Reflection
If you want to stave off some of the echoes and reverberations, you should use absorbent materials on the main reflective surfaces of a room. Line the walls with curtains and cover the floor with carpets. You could also use acoustic tiles on the ceiling or hang sound absorbing panels as vertical baffles instead. The goal is to make the room softer and even a bit smaller, without taking away from its total square footage.
Just be sure not to overload the room with absorbent materials. If it starts sounding a bit too dull, opt for diffusers instead of absorbers. That should help you strike a good balance between absorption and reflection.