Room Acoustics Principles: A Comprehensive Guide
The acoustical characteristics of any room are determined by its architecture. Room architecture consists of room size, room shape and room finishes. This article is for anyone who is just entering the field of room acoustics. We at Acoustic Bulletin writes for both acousticians and sound enthusiasts of all types and levels of experience.
Understanding Sound Waves
Sound arises when particles in the air are set into motion. This happens, for example, when we speak. The movements of the particles create pressure waves that are transported through air at a speed of approx. 343 meters per second. The amount of energy contained in a sound wave determines the distance from wave crest to trough and is called the sound level and is measured in decibels (dB). The decibel scale is not linear, but logarithmic. This means that people experience 10 dB as a doubling of the sound level. A sound consists of many waves with different wave lengths, which together form the timbre of a given sound. This ratio of waves cycles pr. second is also called frequency. It is measured in Hertz (Hz) and is determined by the length of a given wave. The audible spectrum is roughly between 20 and 16,000 Hz, going from a dark to a bright pitch. For the sake of simplicity, this spectrum is often divided into frequency bands where each band represents an average of a frequency range.
Sound waves are variations in air pressure, alternating plus or minus the static atmospheric pressure of a given space.
Key Factors Influencing Room Acoustics
1. Room Size
The first factor affecting room acoustics, room size, is usually determined by the number of seats needed for the room and the layout of those seats. The aspect ratio (ratio of the ceiling height to room width to the room length) is also a critical factor in the room’s acoustic functionality.
2. Room Shape
The second factor in determining the room acoustics is the room shape. Frequently, rooms are designed in a fan shape with a curved back wall and wrap-around seating. When designing a room, plan a room shape that does not include concave or focusing wall or ceiling shapes, parallel wall surfaces, or extremely large continuous surfaces. With regard to room shape and how sound behaves as it bounces around the room, we want to provide early reflected sound energy to the seating areas and eliminate late reflections that are perceived as an echo. Whether sound arrives early or late in time is relative to how long after the direct sound it arrives. In general, the larger the room, the farther sound has to travel to be reflected, and the later it arrives back at a listener.
3. Room Finishes
The third element that determines the room acoustics is the room finishes. Since we’re looking for early energy, not late energy, we will generally want the surfaces close to listeners to be hard or reflective and the surfaces far from listeners to be soft or absorptive. This is just the opposite of what happens in most rooms. A typical room is designed so absorptive surfaces, such as carpeting and padded pews or chairs, are closest to listeners, while reflective surfaces, such as walls and ceilings, are farthest from listeners. Ideally, we want the surfaces close to the listener to be reflective (for example, hard flooring instead of carpet beneath seats) and the surfaces far away to be more absorptive.
Reverberation Time
The main descriptor in room acoustics is reverberation time. The reverberation time (T) specifies the time period in which sound energy dissipates in a space. It roughly represents how noisy a space is. Reverberation time is measured in seconds within specific frequency bands. In short, reverberation time depends on the ratio between the volume of a room and the absorption area. The larger the room is, the longer the reverberation time will be. This is particularly important if you for example are dealing with rooms of double height and/or open environments. On the other hand, the reverberation will decrease the larger the equivalent absorption area of a room is.
A room’s absorption area (A) is an expression of the total amount of sound absorption that the room contains. The larger the absorption area of a room is, the faster the sound energy will die off. For large rooms such as open learning spaces and office environments, norms in some countries set demands on absorption area per volume rather than reverberation time alone. When it comes to acoustic materials, it is important to keep in mind that their actual area is not necessarily equal to their absorption area. It depends on the quality of the materials. You should therefore always ask or look for the absorption classes of the acoustic products, which are listed from class A and lower.
Room Modes and Standing Waves
Getting your music room to sound good can be a confounding and frustrating experience if you don’t take some basic physics into consideration. There are some fundamental principles of acoustics that apply to all small rooms with parallel walls. Once you are aware of the behavior of sound waves in these spaces, it becomes much easier to understand why music sounds the way it does when played in a certain room.
Two terms, which you will invariably encounter in any discussion of studio acoustics, are Room Modes and Standing Waves. The two terms are very closely related, practically synonymous. A room mode is essentially a resonance, an area of increased amplitude that results when a sound wave reflects off a boundary surface (wall, floor, or ceiling) and combines in phase with the original direct sound wave. Sound waves are variations in air pressure, alternating plus or minus the static atmospheric pressure of a given space. Areas of maximum amplitude, representing the maximum change in air pressure, are called Antinodes. Areas of minimum change in air pressure (essentially the zero-crossings between positive and negative halves of the pressure wave cycle) are called Nodes.
If a sound wave is exactly one-half the length of any single room dimension, its nodes will be at that boundary surface. Double that fundamental frequency and a full wave cycle now fits in the space between boundaries. The nodes will still occur at the room boundaries, causing the reflections to be in phase. However, there is now an additional node in the middle of the room; the antinodes of the harmonics do not line up physically with the location of the fundamental’s antinodes.
These sound waves whose wavelength is equal to one-half (or any whole number multiple of) a room dimension, and hence whose nodes occur at boundary surfaces, are called Standing Waves. Actually, they’re a very specific type of standing wave: Axial Mode is the name given to standing waves that exist between two parallel surfaces (front and back walls, left and right side walls, or floor and ceiling). Since axial modes are the most troublesome standing waves in music studios, they are our primary concern. Incidentally, they are called standing waves because unlike most sound waves in an enclosed space, axial modes do not propagate through the room.
Calculate the fundamental axial modes for each of the three dimensions of your room using the formula 1130 / 2L = f. 1130 is the approximate speed of sound in feet per second, and L represents the length of a room dimension in feet. The result f is the frequency of the axial mode in Hertz. Multiply each of those three frequencies by 2 through 8 to figure out the first eight multiples of each axial mode. You should wind up with a list (or a table, or a graph, if you’re industrious) of 24 frequencies. Yep, there’s a standing wave at each one of those frequencies.
Picture this: There is a standing wave between your 8-foot ceiling and floor at 70.6 Hz (…there will be a standing wave (between floor and ceiling) at 70.6 Hz). This means the antinode of this fundamental will be equidistant between the floor and ceiling. There’s also a standing wave at 141.2 Hz, only because a complete wave cycle fits between those room boundaries, the node (area of least amplitude) of that wave is four feet off the ground. Regardless of what type of monitor speakers you use, or where in the room they are placed, your perception of 70.6 Hz and 141.2 Hz will be colored by the existence of these standing waves. Move your head to another place in the room (e.g., stand up) and the response at 70.6 Hz and 141.2 Hz is different, because now instead of sitting in the antinode and node of those respective waves, you’re in a place where they have a different amplitude relationship. And this is only the effect of the first two axial modes of one dimension we’re talking about! Imagine what happens when you add in a third axial mode. Now imagine what happens when you add in axial modes from the other two dimensions.
But don’t panic yet. If they’re fairly evenly spread out, you got lucky, and you should put down this magazine and go make some music now!
The control room I use as an example is 12′ x 10′ x 8′. Plotting the axial modes indicates potential problems around 141 Hz (the second multiple of the 8′ dimension is 141.3, and the third multiple of the 12′ dimension is 141.2… for all practical purposes those numbers are identical).
Dealing with Standing Waves
So what can you do about standing waves? Here’s what won’t help: Sticking foam, or fiberglass, or heavy drapes on the walls. Remember that standing waves occur as a result of the physical dimensions of the room. To a 141 Hz sound wave (whose wavelength happens to be eight feet long), a few inches of foam on the wall is essentially invisible. If you want to alter the standing waves in your room, you have to change the dimensions of the room. That’s right… knock a wall down, and move it closer or farther away. This is why it’s a good idea to plot the axial modes of a room before you decide to turn it into your recording studio; if you’ve got a choice between several rooms, choose the one whose dimensions yield the more evenly spaced axial modes. (Hint: Try to stay away from rooms where any two dimensions are multiples of one another, and avoid perfectly cubical rooms at all costs. Plot the axial modes and you’ll see why.) And if you’re planning on building any walls (say, to separate your control room from your recording space), calculate the axial modes to determine what the best dimensions for those spaces will be.
The Impact of Room Acoustics on Sound Quality
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.
Early reflection points in a room can cause muddiness and negatively impact sound quality.
Practical Solutions for Improving Room Acoustics
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.
Sound Absorption Panels
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.
Table: Common Acoustic Issues and Solutions
| Issue | Solution |
|---|---|
| Excessive Reverberation | Add sound-absorbing materials (e.g., acoustic panels, carpets) |
| Standing Waves/Room Modes | Adjust speaker placement, use bass traps, modify room dimensions |
| Early Reflections | Position speakers correctly, use diffusers or absorbers at reflection points |
| Lack of Clarity | Reduce reflective surfaces, add absorption near the listening position |