The phenomenon of Standing Waves is a subject that can be challenging for many people just learning about acoustics. Having taught the subject for over a decade now it seems that it can be both confusing and revelatory. Read on.
WHAT ARE STANDING WAVES?
Every room has some sort of standing wave behavior–it is really a matter of the degree of obtrusiveness. Standing waves are types of resonant modes in a space that are produced when sound waves of a particular frequency overlap with themselves causing constructive or destructive effects–dips and peaks in amplitude. These variations will be perceived at particular locations in a room, called nodes or anti-nodes. Nodes are points of zero amplitude and Anit-nodes are maximum amplitude points
Below is a visualization of a standing wave in black (source).
Note that as initial wave and reflection propagate, the standing wave remains stationary, as do the zero amplitude points or nodes seen here as red dots. A listener standing in a room at these locations would experience significant drops in the amplitude of that particular frequency.
Where and at what frequencies nodes occur is a function of wavelength and the dimensions of the room. When the wavelength of a frequency corresponds to a particular room dimension (length, width, height) a standing wave is produced. It is called a “standing wave” because the amplitude dip or peak of the sound wave remains fixed in a particular location as the waves propagate.
Perhaps the most obtrusive standing waves are created from waves reflecting between parallel walls or the floor and ceiling. These are called Axial modes. To determine what frequencies will be problematic requires a very simple formula:
Standing Wave Frequency = Speed of Sound / Room Dimension
(where the speed of sound is considered to be 1132 feet per second on average)
So if we take one dimension of a room, let’s say the width, to be 10 feet, we just plug that into the equation:
Standing Wave Frequency = 1132 / 10 = 113.2Hz
Not only will this frequency be an issue, but all related harmonics and subharmonics will also create standing waves because they are integer multiples or divisors of the fundamental frequency in question, so their wavelengths will also align mathematically with the dimensions of a room. For instance in the example above–56.6Hz, 169.8Hz, 226.4Hz, and other harmonics will also produce standing waves. But lower frequencies will typically be the most obvious and obtrusive.
Of course, we have only discussed one dimension of the room, so also consider that other standing waves will be created between the ceiling and floor and the length of the room.
OTHER TYPES OF ROOM MODES
Axial modes, mentioned above, are just one type of resonant mode involving parallel surfaces. There are also Tangential modes and Oblique modes that are created as waves reflect off the walls, ceiling, and floor in various ways.
Room Modes (source)
Calculating these types of modes calls for a Room Mode Calculator. The AMROC online room mode calculator on amcoustics.com is a great one. Not only will it calculate and list all the room modes created by a particular rectangular or square room, but there is a feature that accounts for non-rectangular rooms as well.
You can scan the bar graph with your mouse to hear the offending frequencies and related pitch and see a 3D visualization of where the nodes will occur so you can adjust your listening position if possible.
The calculator also accounts for the RT60 (a measure of reverberation) of the room. RT60 is the length of time it takes for the reverberation level to drop 60dB from the original level. There’s a nice little app that will approximate the RT60 of your room using your phone and a series of hand claps. Check out ClapReverb on the Apple Store.
The AMROC program will also recommend the total square footage of absorption material and the absorption coefficient required given the size and RT60 of the space.
Clearly, square or rectangular rooms present a greater challenge for mitigating axial mode issues with acoustic treatment like absorbers, diffusers, cloud panels, bass traps, etc. But tangential and oblique modes can also be problematic, so even in rooms lacking parallel surfaces, standing waves can be a problem unless proper acoustic devices are used. Acoustic treatment is all about the control of reflections by lessening their impact with absorption, diffusing or spreading out reflections evenly around the room, or reducing bass buildup in corners and where one surface meets another (E.g. wall and ceiling).
HOW TO TEST FOR STANDING WAVES IN YOUR ROOM?
To hear the effects of standing waves you’ll need a way to generate a sine wave and a measuring tape. Most DAWs have a signal generator plugin (typically found under the Utilities category) that will provide a sine wave. If you don’t have a measuring tape just walk off the distance and use your steps to get an approximate dimension. Using the example above for a room with a width of 10 feet, you should expect problematic frequencies to be: 56.6Hz, 113.2Hz, 169.8Hz, and 226.4Hz (in terms of axial modes).
Test for standing waves throughout the room
- Measure one dimension of your room (let’s say width)
- Use the formula below to calculate one of the axial modes
Standing Wave Frequency = 1132 ft per second / 10 ft = 113.2Hz - Divide that number in half to get a subharmonic (56.6Hz)
- Multiply the subharmonic 3x and 4x to get two additional problematic frequencies.
- Standing waves should occur at: 56.6Hz, 113.2Hz, 169.8,Hz, 226.4Hz, etc.
- Now dial in one of these frequencies on your sine wave generator and play it back through your reference monitors.
- With the sine wave playing, walk around the room and notice the peaks and dips in amplitude that occur. You are now hearing the standing waves in action. Try it again with the other frequencies calculated above. You should hear similar effects in different areas of the room.
I’ve done this in classrooms where the sound all but disappeared in particular locations. It’s pretty magical to me that just pushing around some invisible air molecules can result in such drastic effects.
Test for standing waves at the listening position
Another way you can check for problematic frequencies at your listening position is to slowly sweep a sine wave and observe which frequencies dip in amplitude. If you experience a strong dip at a particular frequency you will undoubtedly find that its wavelength (= speed of sound/frequency) relates to one of the dimensions of your room.
WHY STANDING WAVES MATTER
Obtrusive standing waves in a room are problematic because they create a situation where the room itself is contributing to the sound you hear from your speakers. That is not to say you can’t make a perfect-sounding mix in the room, you can. But it will only sound perfect in that room at that particular location in the room. This is sort of useless unless you invite every potential listener to sit in your chair. (although that might be possible for experimental music – but I digress)
The goal of acoustics regarding mixing and mastering spaces is to remove the room from the equation so that your mixes translate well to all sorts of listening scenarios. Ideally, you want a neutral-sounding space that doesn’t inordinately accentuate and attenuate particular frequencies.
Using sine waves is a rather extreme case since most sounds are more complex than the purity of a single sine tone. So you may not experience complete dropouts of a sound, but it may drastically thin or rob a sound of its natural timbre because of standing wave issues. Without the knowledge of the effects of standing waves, you may try to solve these issues by boosting or attenuating frequencies to compensate for the effects of the room. That would be a big mistake.
WHAT TO DO ABOUT PROBLEMATIC STANDING WAVES
Okay, so you’ve followed the steps above, done the calculations and tests and now you know there are standing waves in your room. Here are your options:
- Move to a different room. I say this partly in jest, but if you have several rooms in your house that can be studio spaces, you may find some are more appropriate for music production.
- Change the location of your mixing desk. In a typical rectangular room, the desk should be centered on one of the short walls so that there is symmetry at the listening position and the distance behind the listener is maximized to an extent. Ideally, you’d want the listening position to be located at a distance from the front wall which is about 38% of the total length of the room. More to come on this in future articles.
- Treat your room with appropriate acoustic devices such as absorption panels, diffusion panels, bass traps, floor and ceiling treatment, acoustic curtains, etc. Stay tuned for future articles about what acoustic treatments to use and where to place them.
- Use corrective EQ software such as Sound Reference ID by Sonarworks. While not a substitute for a well-treated room, software like this can help tweak and correct problematic frequencies to some extent.
CONCLUSIONS
Knowing that standing waves exist, what they are, and how they are produced is the first step to dealing with related problems. Without a neutral sounding space you cannot accurately mix music because levels and stereo or multichannel imaging will not be accurate. Acoustics is a game of inches, so if you think your room sounds great at your listening position, try moving your head 12” to the right or left and see I mean. It’s both magical and tyrannical depending on your state of mind.
EXTRAS
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