Written by 5:00 am Room Acoustics

What Are Standing Waves and How Do They Affect Your Listening Experience?

Dennis Foley explains the biggest issue in room acoustics: standing waves.

Let’s start this article with the definition of standing waves: what they are and how they impact your listening experience. Standing waves are areas of high and low pressure within your room. A room is a box. It can only hold so much pressure. Up to a certain level it is fine. After that point, based on its size and volume, it will tell you how much and at what frequency it’s not happy. It expresses this discontent through room modes: areas of high and low pressure for frequencies below 300 Hz.

Think o3. modes pressure graphic 225.jpgf a weather map on television and you’ll see that you have areas of low and high pressure. This is atmospheric pressure but it’s pretty much the same concept when it comes to your room. In your room, it’s sound pressure. If you mapped the areas of pressure in your room, it would look much like that weather map, with areas of high and low pressure are distributed throughout the room. They do not move clockwise and counterclockwise like atmospheric pressure, but they do exist between two or four room boundary surfaces and can be 3 to 4 feet in width. Their amplitude, their strength, depends on the degree to which low frequencies do not fit into the room.

So if we have areas of high and low pressure in our room, and we’re producing music, the music contains all of the frequencies that the human ear can hear. We want to make sure that we are able to hear all of that information. Speakers and amplifiers today produce a lot of information and its good information, it’s clean and accurate. They have a good balance in representation of all the frequencies that we can hear. We have to get our room in a position, so we’re able to hear all of that. In short, we need to get our room out of the way.

If we have areas of high and low pressure in our room, the energy from our speakers has to go through those areas of high and low pressure. Think of the energy in your room that’s coming out of your speakers as an airplane. It’s flying through the air. If there are areas of high and low pressure turbulence, the plane is not going to fly straight. It’s going to zigzag and go up and down. We want to make sure that we equalize the pressure in our room, so there is no “soup” to fly through.

Manage the Pressure

Can we eliminate it? No, but we can manage it. We can get it to a point where it doesn’t interfere with our music. That’s the best we can do because most of our rooms today are too small to allow for the size of the room without extensive surface area treatment to equalize the pressure. We have to manually install technology to treat the unwanted pressure areas so equalization upon wanted room pressures is the key component. We equalize in the passive domain, not the electronic or active. This is the domain that does not have to manipulate the signal in the recording to try and fit into the room. Just let the true signal from the source go into the room and then have the room set up to handle all source frequencies equally. It is harder to do especially in the low-frequency range, but well worth it when it comes to low-frequency attack and decay rates.

How do we do that? Well, we do that thorough a series of technologies that deal with pressure. Here’s where many people make a big mistake: They use technologies that don’t work with pressure. They use foam, they use boxes filled with building insulation. These are not pressure reactive devices. Foam is not a bass trap. There’s nothing that traps bass. You can’t trap bass with two 8-inch poured slabs of concrete with a 6-inch air space in between. Tons and tons of mass, and you still can’t stop 30-, 40- or 50-cycle energy completely. So how are you going to expect to do it in your room? You’re not going to, so management is the key.

Getting the pressure issues under control, so that they don’t interfere with our airplane’s flight path, so to speak, or the music from our speakers. We want a smooth flight with no turbulence. We want to have a cocktail and not have it fall in our lap. We want to relax. We don’t want the room interfering with our music. It’s just not acceptable. The music’s too important in our lives and it’s too important to our spirits, so we must equalize all of that pressure within the room.

How do we start? Low frequencies are our first concern. All pressure below 125 Hz needs to be managed. That’s the problems in all rooms, it’s the problem people face all the time, it’s the main reason for you not connecting with your music because you have something between you and your music: namely, the room. So you have to treat these areas of pressure and, as explained, low pressure must be treated with pressure reactive technologies. Foam, boxes filled with building insulation are not pressure reactive technologies. They can be if they’re 10 to 14 feet thick. That’s unrealistic and far from cost-effective, and no room today can afford to give up that amount of space to treat low frequencies.

A lot of studio designers are still using old-fashioned low-frequency absorption technologies, and that’s fine if you have the space. But most people don’t have the luxury of that kind of space. So we need a very high-powered, low-frequency absorption technology that works in small space, and that’s why I created our carbon technology. I was sick of using 40 to 60 inches of real estate to solve of what we can do now in 8 inches with our activated carbon diaphragmatic absorbers. And that to me is a huge improvement.

So what does managing our low-frequency pressure issues accomplish? It allows our middle and high frequencies to come through more easily. Low-frequency pressure is like a bull in a china shop, it breaks everything. And often people don’t really understand that. When they come into a room, like our test studio, that is completely balanced between high and low pressure areas, they sense it and hear it immediately. And if you don’t have that as a reference, then you really can’t do any subjective comparisons, and data won’t give you that feeling. You have to have a personal reference point and that’s when people come to our studio, their reference point changes and immediately they want to seek a more balanced low- and middle-frequency approach.

Low-frequency pressure moderates and blurs the middle and high frequencies, so it must be equalized and managed to a point where it doesn’t interfere. Where is that point? It’s different for every room usage. It’s different in a live room than a listening room. It’s different in a control room versus a vocal room. All room usages have to be taken into consideration because it’s the usage in the room size and volume that must be married together.

roomvolumes-1-v2 225.jpgAlso, low-frequency pressure issues cause those transition areas between low, middle and high frequencies to be blurred because low-frequency pressure issues are so powerful that they bleed, if you will, into other areas. When a bass drum note starts with the fundamental and then decays naturally through the harmonic, the fundamental and the harmonic must stop resonating in the room before the next bass note is played. In order to do that you have to get the room out of the way. You must have these areas of transition taken care of properly through low- and middle-frequency pressure management.

Once you have the low-frequency issues balanced in your room, then you’re going to be able to hear what your equipment is capable of producing. Treating the room means that you are on a mission to get the room sound out of your music and that particular goal is really what I’m all about.

Once the low end is under good, sound (sorry) management, then you can focus on the reflected energy from the wall surfaces. You want to pay particular attention to the side walls, for it is that time signature that is critical in two-channel and even multichannel monitoring. Reverberation times must match usages. Time signatures must be absorbed or diffused depending on the room usage. Rates and levels of absorption must be adhered to for all room surfaces. There is not just one rate and level that works on all surfaces.

So the bottom line is, in order to treat a room, in order to minimize the high- and low-pressure areas, and get the effects of low-frequency pressure out of middle and high frequency ranges, we have to make the room smaller. Your initial room size plus treatment has to end up being a size that’s conducive to the usage. I see this a lot in our customer inquiries. They have a room — an extra room in the house, in the office, or somewhere — and they think about making it a music room. Well, just because it’s an extra room doesn’t mean it qualifies to be a music room because each room size and volume has to be married to the usage.

As I explained in a previous article, you have to make sure there’s a marriage between the usage and the room size and volume. Sometimes the room size and volume is just too small for music because the amount of treatment that you’d have to put in the room to minimize the unwanted pressure issues would leave you no place to sit in the room. So you’re best off spending your energy looking at larger room sizes.

In Summary

We need to manage our low-frequency pressure issues in the room because both high- and low-pressure areas have a negative impact on our music. We must equalize through proper room treatment. Here’s another thing to realize: Once you treat a high-pressure area, it may have an impact on a low-pressure area because you’re going to absorb some of the energy but not all. So that energy will go somewhere else. Just remember that you’re in a box and the pressure really doesn’t have any place to go. Those of you who are listening to music in a basement really understand this concept because with earth and concrete surrounding your room, there’s nowhere for the low-frequency energy to escape to. Your problems are even compounded in the basement because there is no bleed of sound through adjacent structures.

Equalize the pressure to the room, get it under control so it doesn’t interfere with your middle and high frequencies, and you will start to hear things in your music you never heard before. Just by managing the low-frequency issues, you will immediately start to hear so much more information. That information’s always been there. You just couldn’t hear it before because your room is “upset” and it won’t let you hear it. First, match the room size and volume to the usage,  then manage the low-frequency pressure issues. If you can devote your time and energy to that, the rest of the process is relatively easy. Let’s put it this way, the process of treating reflections is much less expensive and much less time-consuming.

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