Understanding Acoustics in a Room

Today, room acoustics play an important role in traditional performance and assembly venues as well as in everyday spaces. In addition to schools, restaurants, open-plan offices and shopping malls, more and more people are looking for the best possible hearing and sound experience in their own homes. Whether you’re setting up an elaborate stereo system or simply trying to hold a conversation, some rooms seem to have a better sound than others. When either of these effects happens, it’s rarely due to the sound source. More often than not, it’s caused by the environment - specifically, the structure of the room and the materials in it.

Understanding room acoustics starts with knowing what acoustics entails. Acoustics are the way sound waves interact with the space around them. When something emits a sound, whether it be your voice or a stereo speaker, it projects sound waves outward. How the waves interact with those surfaces depends on the nature of the surface. Sometimes the sound waves will bounce off, while other times they’ll be absorbed.

It may not seem like a big deal if a room echoes a bit or if the sound falls flat. Bad acoustics may be bearable, but good acoustics can bring some worthwhile benefits to everything you do in a room.

Room acoustics refers to the acoustic properties of a room. Sound does not only propagate directly within a room. The sound waves emitted by a loudspeaker reach the listener partly as direct sound, but also via reflections from walls, ceilings or furniture. Each surface absorbs some of the incident sound. The reflections therefore gradually lose sound energy as the surface ‘swallows’ some of the energy by converting it into heat. The sound fades. The less absorption that takes place on the surfaces of the room, the longer this decay process lasts: the room sounds ‘reverberant’, it has a long reverberation time.

Reverberant rooms have poor acoustic quality and are subjectively unpleasant for users. Room acoustics are strongly influenced by the size of the room, the geometry and the sound absorption of the room surfaces. In large rooms, to minimise reflections, it is advantageous to have absorption in more than one plane. For example, it is advisable to provide absorption on the ceiling and on another level. Where very high demands are placed on the room acoustics, it is possible to use surface design in a targeted manner.

Measures against sound generation and sound transmission are also distinguished according to whether the sound source and the listener are in the same room or in different rooms. In the former case, sound control is achieved by sound absorption. A distinction is made between airborne and structure-borne sound insulation.

An important area of structure-borne sound insulation is the reduction of impact noise or the reduction of sanitary noise caused by fittings or piping. DIN 18041 is the internationally recognised standard for the design of room acoustics and is entitled: Acoustics in rooms - Requirements, recommendations and guidance for design. The standard specifies desirable reverberation times for rooms, depending on the volume of the room and the purpose of use.

Why Good Acoustics Matter

Bad acoustics may be bearable, but good acoustics can bring some worthwhile benefits to everything you do in a room:

Improved Concentration: One way bad acoustics can manifest is by magnifying small sounds. This type of environment is far from ideal when it comes to concentrating. If people are trying to study or work on something, they probably won’t have much luck amid that many distractions. Improving the acoustics of a room can make a world of difference when it comes to concentration.
Increased Productivity: Once you’ve adjusted the acoustics of your room to allow for greater concentration, it follows that you’ll be able to work more productively. Without the distractions of all the small movements within a 20-foot radius, people can stay on task and get more done in a given amount of time.
Enhanced Sound Quality: Sometimes, you’re focused on the sound itself in addition to its effects.

Factors Influencing Room Acoustics

Since acoustics are driven by interactions between sound waves and different surfaces, it makes sense that many different factors are at play in a specific room’s acoustics.

Surface Hardness: Much of what drives how a sound wave interacts with a surface is whether the surface is hard or soft. That’s why it’s essential to consider the arrangement of hard surfaces in your room. While soft surfaces tend to absorb sound waves, hard ones reflect them, causing reverberation. The more hard surfaces there are in a room, the more reverberation there will be.
Multiple Voices: Sometimes the surfaces aren’t the only thing that could be causing problems - the sound waves themselves are just as significant. If you’ve ever been to a party in someone’s home, you may have experienced the effect of multiple voices mixing together at once, preventing you from understanding anyone.
Ceiling Height: Even if your sound source is facing a horizontal direction, sound waves will still go upward and come into contact with your ceiling. How that happens depends on the ceiling’s height. The farther the sound waves have to travel, the more reverberation there will be. If you don’t want reverberation in your room, you’ll want to make sure you have a low ceiling.

Room Acoustics Explained

Key Parameters in Room Acoustics

A number of terms and parameters are important for understanding room acoustics. The sound absorption of surfaces varies greatly depending on their texture, material, and sound frequency. The harder and smoother the surface, the lower the sound absorption. Soft surfaces absorb a large portion of sound. The sound absorption coefficient ? is used as a measure of sound absorption, which represents the ratio of absorbed to incident sound energy. A value of ? = 0 means complete reflection, while ? = 1 represents complete absorption. The sound absorption coefficient also depends on the frequency. Therefore, it is common to represent ? in a diagram for precise representations. Many materials exhibit very different absorption behavior at certain frequencies.

The use of the equivalent absorption area (A) is common in order to make a specific room acoustic statement about the absorption of individual wall surfaces. A in square meters indicates how small a wall surface could be if it were to absorb ideally. Therefore, the term “absorption capacity” is also common.

A = ? A simplified example that does not consider frequency-dependent differences illustrates the relationship. An absorber with 12 m² absorber area S and ? The same absorption capacity is achieved with an absorber area of 7.5 m² and ? Similarly, three ceiling panels with S = 2 m² each and ? In practice, the classification of acoustic elements according to DIN EN 11654 into absorber classes A, B, C, D, and E is established.

The most important parameter in room acoustics is the reverberation time (T) of a room. The reverberation time is the time in seconds it takes for the sound pressure in a room to decay to 1/1000 of its initial value after the sound source has been abruptly silenced. This corresponds to a decrease in sound pressure level of 60 dB. In the early 20th century, the American physicist Wallace Clement Sabine discovered that the reverberation time, room volume, and sound absorption of the room’s surfaces are interrelated: the larger the room volume (V) and the lower the absorption (A), the longer the reverberation time. In practice, rooms with low absorption sound “echoey” and are perceived acoustically as uncomfortable. Desired reverberation times depend on the intended use and the size of the room. Good average reverberation times for speech are around 1 second, while for symphonic music they are about 2 seconds. By varying the absorption surface, the desired reverberation time can be adjusted depending on the frequency range. It is important to consider all other surfaces and materials, as well as the number of people in the room, as they also affect sound absorption.

For rooms with conventional use such as classrooms, open-plan offices, waiting rooms or multi-functional areas, there is a widely used rule of thumb that good reverberation times can be achieved by equipping an area corresponding to the room footprint with an effective sound absorber.

Reverberation describes the perception of an auditory event in which direct sound and reflected room sound are present. The reflected sound, however, is not perceived as a repetition of the sound signal. Reverberation depends on the ratio of the late arriving sound level to the early parts. A reverberant listening experience in practice reduces speech intelligibility.

Airborne sound is familiar to us all. Structure-borne sound is a term used in building physics to describe the propagation of sound waves in solid objects.

Direct structure-borne sound: Mechanical vibrations on a body lead to structure-borne vibrations.
Converted structure-borne sound: Airborne sound is converted into structure-borne sound. At the transition between air and solid body, for example, at the wall of a room, several physical phenomena occur. Part of the airborne sound waves are reflected, creating room sound. Another part is absorbed by the wall surface, producing heat energy. A third component of the airborne sound waves causes the wall itself to vibrate. Structure-borne sound is created when the sound source is strong enough. For example, in a loud music system, a portion of the airborne sound is converted into structure-borne sound. Another form of direct structure-borne sound is impact noise.

We humans cannot hear structure-borne sound, but we can feel it.

Evaluating Room Acoustics

To some extent, you can tell if a room has good acoustics simply by walking in and creating sound. But that won’t tell you everything, especially if you don’t know what to look for.

Reverberation Time (RT60): Reverberation time, or RT60, refers to the time it takes for a sound to fade away in a room - or more accurately, to decrease by 60 decibels. Reverberation is caused by sound waves bouncing off various surfaces, and the more reverberation occurs, the longer the sound will remain audible. When you optimize a room acoustically, consider reverberation time. For the best results, you’ll want to take precise decibel measurements, but you can gauge by ear if the reverberation is exceptionally high or low. Good reverberation time depends on the size and purpose of the room.
Sound Distribution: When you’re judging a room’s acoustics, make sure you don’t do it all from one spot. A room may have excellent acoustics in one spot and worse acoustics in another. Sound distribution is particularly important for large rooms, like auditoriums. In those settings, you want to make sure the sound from a speaker, musical group or any other source reaches everyone in the room.
Sound Levels: Improper sound distribution is frequently the problem when people are not able to hear sounds in a room. Other times, the problem is that the sound source isn’t loud enough. This factor has less to do with the room and more to do with the people or sound system it’s intended to host. You need to be sure your sound levels are high enough for you to hear them over any other noises.
Background Noise: The focus of acoustics is usually on the sounds you want to hear, but you can’t ignore the fact that there will be unwanted sounds as well. Whether it’s cars driving past outside or the air conditioning unit across the room, background noise is inevitable. That’s not to say you can’t limit it. For the best acoustics, try to find a room that isn’t near any intrusive external sounds.
Echo: In many cases, you probably don’t want your room to have much echo, if any. Most echoes occur due to concentrated reverberation, where a single surface reflects the sound waves toward you. In both cases, the echo can quickly become a problem. When you hear the same thing more than once in rapid succession, especially if it overlaps, it can lower intelligibility. It’s advantageous to find a room that doesn’t cause echoes or work to reduce existing echoes in your current room.

Improving Room Acoustics

The above criteria are great things to consider when looking for a room with good acoustics. But what if you’re already stuck with a room that’s lacking in those areas?

Diffusion: If the problem with your room is mainly bad sound distribution, diffusion may be the best solution. Diffusion refers to the process of spreading sound waves out evenly across a space. You can accomplish diffusion by using specialized objects or wall panels to deflect sound outward. These objects often have a convex shape to distribute the sound waves widely, though new designs are continuously being developed.
Absorption: In contrast to diffusion, absorption is the best solution for removing sound waves from an environment. Absorbers often come in the form of wall or ceiling panels. Since they’re made of soft material, they absorb the sound waves that hit them instead of reflecting them.
Tuning: Diffusion and absorption technically qualify as tuning in the broadest sense, but there is a more specific meaning of the term tuning. Digital tuning refers to the process of altering a room’s acoustics by projecting sound waves into the air in a certain way. You can accomplish digital tuning with the help of a digital equalizer, or digital EQ. Digital EQs appear in many sound systems to improve the quality of the projected audio, which makes them ideal for rooms intended for sound system use. As a digital EQ projects audio, it adjusts the volume of different frequencies for optimal acoustics.
Soundproofing: Sometimes the issue with your room’s sound stems from outside sources. You can put up as many absorber panels as you want, but it may not stop those outside sounds from getting in. The alternative option is to soundproof. Unlike the other options, soundproofing doesn’t involve manipulating sound waves within the room - it keeps them from getting in to start with.

Room Acoustics in Home Theater

When setting up your dream home theater, understanding room acoustics basics is crucial to enhancing sound quality. In a home theater context, room acoustics can make the difference between a good and a great cinematic experience. Poor room acoustics can lead to sound distortions, reducing the clarity and impact of the audio.

Room Size and Shape: Your room’s dimensions significantly influence how sound waves reflect within it. Rectangular rooms generally provide the best acoustics.
Surface Materials: The materials on your floors, walls, and ceilings can significantly affect sound absorption and reflection.
Room Furnishing: Furniture plays a critical role in controlling sound reflection.

As a home theater enthusiast, applying this knowledge can greatly enhance the clarity, immersion, and overall quality of your audio experience. Room acoustics is how sound behaves in an enclosed space, influenced by the room's dimensions, materials, and design. The key elements include room size and shape, surface materials, and room furnishing.

Improving room acoustics usually requires a combination of measures. Initial reflections are those sound vibrations that hit a wall first and are reflected back to the ear. They can cause cancellation or overemphasis in a particular frequency range, i.e. interference. Measures for optimizing room acoustics can be carried out during the construction project or afterwards.

Acoustic carrier panels with BASWA’s fine marble sand acoustic plaster for seamless acoustic solutions are executed smooth, colorful, curved or thermally activated and are very discreet and seamlessly integrated into the architecture. Ceiling panels are mounted with distance to the ceiling for maximum sound absorption. Holes in the ceiling are required for the installation of the panels. Sound-absorbing wall panels are directly attached to existing walls. For optimal visual integration, a selection of colors and image motifs is available. Acoustic foam panels are usually made of polyurethane foam.

It’s nearly inevitable that when people find out I review audio and home theater equipment they ask me the same question: “Who makes the best speakers?” My reply is always the same: “Describe your room.” My room? What on earth does the room have to do with getting great speakers? As it turns out, it has everything to do with it.

I have a degree in recording engineering, a field that deals with audio engineering and the hardware, software and techniques needed to capture, mix and produce great recordings in a studio. As I learned more and more about audio, and as I practiced my craft throughout the past two decades, I began to notice a consistent trend. Moderately good speakers could sound great in room that had good acoustics, and great speakers could sound downright awful in rooms that had poor acoustical properties.

Have you ever had difficulty understanding the dialogue in a movie you were watching at home? There’s a reason that happens-even when you have a really nice center channel speaker. If you have wood, tile or terrazzo floors, or perhaps there’s simply a lot of glass in the room-this might be windows, or it might be a coffee table in between you and the television-you have a recipe for lots of reflections.

Reflections aren’t all bad. After all, we hear tons of reflections when we speak and listen to others, when we interact with the word around us…reflections are everywhere. You can’t really get away from them to be honest. In a home theater setting, however, you want to control reflectivity so that music and movie soundtracks are reproduced more faithfully to the original. When the same sound comes at you repeatedly from different locations, it does so at different times. One instance of the sound may be bouncing off a glass wall behind you, while another comes up from the floor only to meet yet another that ricocheted off the glass coffee table. When these sounds converge at your ears, the net effect is best described as “muddiness”. And this will happen with a $100 stereo system or with a $10,000 pair of speakers.

If I’ve just described your system (and when I talk to people I often see approving nods and thoughtful gazes) you don’t have to abandon hope. There are solutions. Those solutions can come in the form of doing some minor redecorating, moving your speakers slightly, or even adding a few key pieces of room acoustics. The bottom line is that you can often achieve better sound in ways that aren’t going to: a) break the bank, or b) cause you to have to remove all your furniture and start from scratch.

When I talk about “redecorating”, alarms and bells go off-particularly as my friends consider what their wives may say regarding moving around furniture (most of my discussions are with guys, though I suppose there are more than a few die-hard women who love home theater.) The truth is, the recommendations I make often work right into the leanings of the homeowner. A great example is this one couple who had some large glass sliding doors at the back of their room. I suggested they install some fabric-based shades to break up the reflections occurring there. As it turns out, the wife had wanted those shades for a good long time.

Another time I was speaking with a couple who told me that they had moved, and the same sound system simply didn’t seem to yield the same results for watching movies. They complained of having to turn the volume up a lot just to hear the dialogue. When I asked about their former room and the new one, I found that they used to have a carpeted space, but now enjoyed a renovated terrazzo floor. I suggested a small throw rug for the terrazzo floor right in front of the television. This would cause some of the reflections from the center channel speaker to diminish greatly and improve the clarity of the audio.

I’m always amazed at how little people will do their own adjusting of speakers to achieve a better sound. Most simply set them up and never again touch them. Sound varies greatly with speaker positioning. In one example I can remember, the center channel was pushed way back on a glass AV shelf (which, as far as I’m concerned, should be outlawed). The sound from the tweeter was decidedly bouncing off the shelf and combining with the direct sound at the listening position, creating a less than idea effect. I barely sat down before I saw (and heard) the problem and was able to simply scoot the speaker forward so that it was firing past the edge of the shelf.

In another home there was a combination issue. A subwoofer was placed in the front corner of the room, which created a bit of a bump around 60Hz. That bump was rattling dishes in the kitchen every time they watched a movie. It was a minor annoyance, but it was very real and happened quite a bit. The homeowner had turned down the subwoofer to try and alleviate the problem, but then their movies lacked the punch he wanted. I suggested spreading out the front speakers a bit more, and placing the subwoofer just inside the left speaker position, about 18″ away from the wall.

While some of my more innocuous suggestions may work for many, there are some instances where you simply have to bite the bullet and purchase or build some sound absorbers. The good news is that you can do this fairly inexpensively, and you can also color match it to anything you need. Sound absorption panels don’t need to be ugly. They can also take on just about any shape or size. The traditional panels are either 2 ft x 2ft or 2 ft x 4 ft, but you can really and truly make them any shape you want. Absorption panels are simply fabric coated frames filled with dense fiberglass that “scrub” sound. They remove audio from the room and don’t give it back. That means that they eliminate reflections.

The trouble is, they need to be used sparingly or you’ll end up with a “dead” room that sounds lifeless, or which requires a ton of amplification power and some tweaking to sound right. Movie theaters are “dead” rooms, but they are designed that way, and the sound systems are designed to compensate for how they are built.

So how do you go about getting room acoustics? Well, you can Google how to make your own, or you can purchase pre-made models and get them in the colors you need (I’ve done both). If you have a minor issue with reflective walls, then a few panels decoratively placed throughout the room will do a lot to reduce these reflections.

The true way to do it right, however, is to actually have a room analysis done by a professional. Many companies offer these services for a very inexpensive fee (some even do basic evals for free) because they also offer solutions that you can then purchase. A good rule of thumb is to stick to less than 30% covering of any wall to avoid over-dampening your room.

There really is a correlation between good room acoustics and getting great sound for your home theater or stereo system. If you don’t get the room right, it’s going to be nearly impossible to get good sound out of even the best speakers money can buy.