Interpreting and making practical use of the graphical results from measuring programs like Sonarworks’ Reference 4 Measure or AV Nirvana’s Room EQ Wizard (REW) requires a basic understanding of acoustic physics and the way sound behaves in your particular room. Identifying the problems that stand in the way of attaining an accurate music monitoring system is a continuous learning process that is worthwhile to pursue.
The purpose of this article is to teach you to look at a room’s frequency response curve and determine the likely causes of the frequency problems revealed by the analysis. You can then improve the room’s acoustics the room to flatten out the frequency response before refining the sound with software like Reference 4. Read on to learn to interpret this information that is already provided by your room correction software.
Room Acoustics Reviewed
The Physical Constraints Of Small Rooms
Small rooms used for home or project studios are typically a converted spare bedroom or car garage. These rooms usually measure in the range of between 2.5m to 3.5m wide by 1.7m to 3m high by 3.6 to 7.7m long. Finished ceiling height in the US is typically 8-feet (2.5m) and 2.2m in the U.K.
Acousticians define a small room as a space whose acoustics are dominated by the detrimental effects of room modes that are also known as “standing waves” because they seem to stand still in certain locations in the room. In contrast, large rooms and spaces like concert halls or indoor arenas, the reflectivity of the space—the reverberant field dominates the acoustics.
Room modes are caused by sound waves bouncing back and forth between parallel floors, ceilings, front and rear walls, and between the left and right sidewalls and then back at the listening position. These reflections arrive later in time than the direct sound and mix with it. At a particular frequency, when the reflected sound is in phase with the direct sound, a peak in volume called an anti-node is produced. Just by changing the sound’s frequency or physically moving the listening position, the same direct sound and reflected sound could be out-of-phase producing a dip in volume called a node.
The exact frequencies and locations in the room where nodes and anti-nodes exist depend on the room’s physical dimensions—the length, height, and width.
How Room Modes Work
In every room, there are about 80 different room modes present at any given moment produced by reflections coming from all three of the room’s boundaries—the floor, ceiling, and walls. To muddle things further, there are three kinds of modes.
- Axial room modes are the strongest modes and are caused by two waves traveling in opposite directions and reflecting off of parallel walls—front-to-back, side-to-side or floor-to-ceiling.
- Tangential room modes are formed by waves that reflect off two sets of parallel walls (four walls or two walls, floor and ceiling). They have half of the energy of the axial modes.
- Oblique room modes involve eight traveling waves reflecting from all the boundaries in a room. Oblique room modes have only one-fourth of the energy of axial modes.
Acousticians are most concerned with the strongest axial modes because they make up 50% of the total energy of all the modes.
Figures 1a, 1b and 1c are from Dan Siefert’s room mode calculator (provided by Harman) and show the frequencies and relative locations of the nodes and anti-nodes of the first four multiples of the 1st axial room modes in a room. This particular room measures: 22.4-ft (6.82m) long X 9.4-ft (2.86m) wide X 8.25-ft (2.5m) high. Each of the room’s three dimensions are shown starting from the left and going to the right with color-coding for: the 1st room mode (blue), the 2nd room mode (black) whose frequency is two times the first, the 3rd (red) mode at three times the first, and the 4th (yellow) mode that’s four times the first.
All room modes peak at boundaries—the floor, ceiling or walls as shown far left and right in this chart. This is the reason why when you stand near a wall or especially in the corner of a room, the pressure buildup is intense. The 1st room mode (blue) has just one node at 50% of its length dimension with the 2nd room mode (black) peaking at the same location.
Notice on each graph where any nodes or anti-nodes meet at the same location. The exact location(s) in the room where you would hear these phenomena is completely predictable and must be considered when setting up a room for critical listening. Using these room mode tools is a simple way to predict where modes will exist.
Below in Fig. 2 are some 3-D room mode visualizations created by the AMcoustics room mode calculator.
Modal Measurements in the Real World
Now that we have some theory behind us and we know how to use mode calculators to predict our room’s problems, let’s go the other way and electronically measure our room to see what the real-world frequency graph we generate tells us about our acoustic environment. We can use this information to help effectively treat our room.
All the following graphs for this article were done in my Tones 4 $ Studios using a Sonarworks’ calibrated measurement microphone. Some graphs were created using the free Room EQ Wizard (REW) software and some were created using Sonarworks’ Reference 4 Measurement software. The stereo monitors are Kali Audio IN-8 three-way coincident monitors spaced 40-inches (1.02m) apart measured center-to-center.
Before attempting an accurate and realistic measurement of your room using, you should always close all doors and windows to your room. Any doors to other rooms, closets, hallways, and windows should be shut.
Think of your room divided into two frequency ranges at a point called the Schroeder Frequency. For most small rooms, the Schroeder Frequency is somewhere between 200 and 500Hz. Below the Schroeder frequency, the entire room acts as a resonant cavity with low-frequency sound waves bouncing around creating nodes and anti-nodes. This low-frequency sound energy propagates in “waves” and is modal. Above Schroeder, higher frequency sound behaves like “rays” as if lasers are reflecting around the room.
Focus on the Lows
Take a look at Fig. 3 and notice the gigantic dip at about 76Hz. This REW screenshot depicts the first axial room mode dip caused by reflections back and forth from the floor to the ceiling at the listening position. At about 152Hz, the second room mode frequency, there is an anti-node or peak. Unless you plan to move to another room or add bass trapping, you will have to live with that ceiling room mode. Having a 10 or 12-foot ceiling would be a practical luxury for a home studio!
Fig. 4 shows the Sonarworks Reference 4 measurement of the same room and we see the same two aforementioned modal issues. Somewhat smoother than the REW measurement, both measurements indicate my Speaker Boundary Interference Responses.
Where do the Dips in Level Come From?
Low frequencies emanate from your monitor in an omnidirectional and spherical pattern and strike all boundaries and reflect back to the speaker and of course, the rest of the room.
Called Speaker Boundary Interference Response, if your monitors are 1/4 of a wavelength of a given frequency away from a boundary—a wall, floor, or ceiling, the total round trip path, to and from that boundary, is a half wavelength. So just like the 50% location in the previously shown 1st room mode graph, there will be a cancellation at the front of the speaker cabinet—a dip at that frequency. At integer multiples of the frequency, you will have alternating reinforcements and cancellations that affect the frequency response of your very expensive studio monitors. See fig. 5.
Practical Considerations for Low-Frequency Modes
Home studios have domestic and practical considerations for sure. If you have space and can move the monitors further away from the side and front walls, the frequencies of reinforcement and cancellation will go lower and/or possibly be less intrusive. For instance, a distance of 85cm from the front wall to the front of your monitors will cause a node at 100Hz, while moving the speakers to 170cm drops the node down to 50Hz. Keep in mind that this cancellation can not be corrected with room EQ. No matter how much we boost that frequency, the room mode will always subtract an equal amount.
Looking back at Fig. 3, the range between 300 and 800Hz is very “choppy” with peaks and dips that are caused by the reflections from the sidewalls 90cm away and the front wall just behind the speakers about 75cm away. I can change the location of the monitors; maybe further away from the sidewalls and closer to the front wall but there are practical limitations.
As an example: my studio room is only 2.87m wide, but I cannot move the speakers too much closer together or I’ll lose stereo width and imaging. For now, I’m at 40-inches or 1.02m apart (measured center-to-center of the woofers) and I’m already not too happy with the lack of stereo imaging and width!
I’ll change it soon but this points out that some issues we have to live with. Knowing these acoustic issues exist is extremely important for making careful judgments during mixing and recording.
Experiment and measuring is the best way to determine the best compromise. I would recommend developing a routine: measure first with Sonarworks then make ONE single change such as moving the monitors further away from the sidewalls. Measure again and assess the sonic changes. Never make more than one change because you might not be able to know which changes were for the better or worst.
Above the Schroeder Frequency
Fig.6 is called a waterfall graph and shows amplitude (y-axis), frequency (x-axis) and decay time in milliseconds (z-axis). Above 1kHz my small room is fairly smooth and Fig. 4 from Sonarworks confirms this. This is because my room is heavily damped with broadband absorption panels—some as thick as 10-inches.
Reflections off my hard desktop and computer monitor can cause reflections and comb filtering that interfere with good stereo imaging. For that reason my monitors are positioned high enough on Sound Anchor stands to look over my computer monitor and small desktop speakers. I also have my 29-inch wide LG computer monitor resting directly on the desktop and tilted back at a steep angle to get out of the way of the sound coming from the monitors behind it. I’ve covered the back of the monitor with absorptive material to reduce reflections off of it plus my desktop surface is covered with thin rubber shelf liner to reduce high-frequency reflections off of it to my ears at the listening position.
Why Setup Is Super Important
Your monitor’s height from the floor, the distance between them, how far away they are from the front wall, the sidewalls, and the listening position have everything to do with maximizing your monitors’ sound in your room. An optimized setup of both the listening position and the location of your monitors is the important first step to minimize the impact of room modes. This should be done before using Sonarworks room correction plus this knowledge will foster more effective acoustic treatment deployment, (bass traps, absorption panels, and diffusers) in your room.
After room treatments are installed and working, then run Sonarworks 4 to get that last 10% of acoustic perfection. When your room acoustics are dialed in, SW doesn’t have to work as hard to get your room to sound super!
While a program like Room EQ Wizard (REW) measures how your speakers sound in your room from a single listening position, Sonarworks Reference 4 directs you to move the mic and measure from many points around the listening position to obtain more information about the entire listening space between the monitors and the listening position.
The goal with Sonarworks is to measure and correct your monitors as they reproduce sound in your own room. The raw measurement results WILL NEVER BE the same as your monitors measured at the factory. Factory speaker specifications are measured in anechoic chambers where there are no detrimental room acoustics. Buying very expensive monitors with near-perfect specifications is no guarantee they will sound their best in your room.
By knowing the limitations of both your room’s size and your monitors, you’ll better understand the reason why your mixes sound a certain way inside of your room and then different outside of your room.