Sunday Morning Hi Fi #4

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Sunday Morning Hi Fi #4

Welcome back to Sunday Morning Hi Fi #4, hopefully everyone is having a great weekend of listening and sonic enjoyment from their system! Last week I said that I wanted to talk about room acoustics a bit, and that’s what we will do.

Room Acoustics Tips and Tweaks.

For this brief, introductory installment on acoustics and optimizing your room, I’m going to assume that you have some flexibility with your room setup. Not everyone has a listening room dedicated exclusively to their high-end audio gear; for those of you who use a family room for listening, there are still plenty of things that you can do to optimize your listening experience. But we will talk about those tweaks in future installments. Right now, let’s focus on the “average” listening room, which I will consider as typical North American construction, i.e. a rectangular room constructed on a slab with 16” on-center studs and 5/8” sheetrock. Of course, there are rooms that will be different from this type of construction, but such a room is found in most single-family homes throughout North America. If your listening environment is different—such has solid concrete wall construction, brick and mortar, pier and beam or floor joists, basement construction, et cetera—you can still use some of the following information to improve your listening experience.

Before we get started, I highly recommend purchasing the Master Handbook of Acoustics, Fifth Edition by F. Alton Everest and Ken C. Pohlman. I have the digital version for my iPad, so I like to reference it and search through the book while making adjustments. This book is not a light read by any means, and will be pretty dense for newcomers to room acoustic theory. But the information contained within is invaluable, and will help you get the best out of your listening room and understand sound pressure much more thoroughly. Unless you are an acoustical engineer, this book will have lots of information to digest.

One of the biggest issues I see with my friends and acquaintances when it comes to room acoustics is being overeager. Room acoustics should be the last thing you focus on, well after your speakers and listening position have been set up. Proper speaker placement will make a far larger difference in your listening room than acoustic treatments. Make sure to take your time with speaker setup and finding your ideal listening position. Once you have done everything you can to make things sound “correct,” which is 90% of the equation, then it’s time to employ room acoustic treatments (or remove them, more on this in a bit). Think of room acoustic treatment as the icing on the cake. Again, this will be the most simple of introductions to room acoustics. Those interested in delving deeper should consult scientific monographs on the subject of acoustics.

There are three basic types of rooms with regards to their “acoustic state”: A live room; a dead room; and a neutral room. Generally, we want to strive for a neutral room that leans slightly to the live side. A live room is a room that has a long reverberation decay time. Everyone has experienced an overly live room at some time or another. Think of a large cafeteria or hall where footfalls echo and “ring.” This is the reason why cafeterias tend to be so loud; the acoustic energy in the room has nowhere to go, and therefore reflects off of every surface and builds to the point where one needs to shout in order for one’s neighbor to hear. If you have a room with hardwood, tile, or marble floors with little or no soft material, you will recognize that it is a live room. Stand in the middle of the room and clap your hands and listen. Does the sound of the clap continue to echo and ring for a period after the clap? This is the reverberation, and the decay is how quickly the acoustic energy is dissipated. This type of live room will require materials to absorb the mid- to high-frequency reflections in order to tame the acoustic energy.

A dead room is the exact opposite of a live room. Sounds tend to be muffled, and if you do the same “clap test” you will notice that the sound dies before you have even stopped clapping. Dead rooms are usually filled with carpet, heavy drapes, and have little or no reflective surfaces. With our listening rooms, we want neither a dead room nor a live room. We want a room that is somewhere in the middle, and generally a neutral room that leans to the live side of acoustics.

The reason we want a neutral-to-live room is that the real world is a mixture of absorptive, reflective, and resonant acoustic spaces. We don’t want our rooms to swallow the sound, but we also don’t want it to spit it back in our face.

If your room is too dead, consider removing some of the heavy fabric or carpeting that is swallowing the acoustic energy until you achieve a more natural sounding room. If it is too live, it is generally much easier to pinpoint points of reflection in order to tame reverberation.

I’m assuming that you have a fixed room size and construction. If you are fortunate enough to build your own room, consult an acoustic architect to help design a room to fit your needs.

Determining Reflection Points

Luckily, controlling mid and high frequencies is fairly simple and inexpensive. For $200-$500, you can acoustically treat your room, depending on the size. But you will want to determine reflection points before you purchase acoustic treatment products.

All you will need to do this is a small mirror (at least 12” x 12”), a friend/helper, and some masking tape or a pencil (if you don’t mind marking your walls). Again, do this only after you have tweaked your speakers to perfection.

Have your helper hold the mirror and move about your listening room while you sit in the listening position. Your sidewalls to your left and right will have a primary reflection point and a secondary reflection point, two on each side. While you sit in your listening position, have your helper hold the mirror against the left wall about ear height, and then slide the mirror until you see the tweeter of your left-channel speaker in the mirror. This is your first reflection point. Have your helper mark that spot with the pencil or masking tape. Now have them slide the mirror along the left wall until you see the tweeter of your right-channel speaker, and mark that spot. Do the same thing for the right wall, everything in reverse. You should now have two spots marked on your left wall and your right wall.

The same thing applies to your floor and your ceiling. Most people will have carpet or a rug over hard, reflective surfaces, so reflection points on the floor can be generally disregarded. If you have hard flooring and no rug, consider placing a rug in front of your speakers to combat early reflections from the floor. Most people will have a bare ceiling, though, and will need to mark two spots for acoustic treatment. Have your helper stand on a stepladder or stool (safety first!) and hold the mirror on the ceiling. Move the mirror along the ceiling until you see the left-channel tweeter, mark that spot, and do the same thing for the right channel. You don’t need to worry about the rear wall or the front wall right now.

Once you have the reflection points marked, it’s easy to apply mid- to high-frequency absorptive material to these spots. Remember, acoustic panels do not absorb frequencies below 300Hz (generally, there are exceptions), so these reflection points are to control brightness, “slap,” and reverberation.

Apply acoustic treatment sparingly. Add acoustic treatments a little at a time, until you achieve the desired room control and type. You don’t want to add so much that you create a dead room, which will suck the life and energy out of your system. If you have a dead room, remove heavy drapes, rugs, et cetera until you have a room that is neither dead nor live.

Determining Room Modes

The best way to determine room modes and standing waves (lower frequencies that tend to propagate, build, and overlap) is with room acoustic software. Some of this software is rather expensive, requires setting up microphones and using technical graphs (generally waterfall) to analyze your room. If you have this software, or are really dedicated to your room acoustics, I highly recommend using it. But if you just want a general idea of how your room interacts with your system, check out http://amroc.andymel.eu

This room mode calculator allows you to input your room dimensions, and then graphs your room’s theoretical room modes. What you want is a smooth parabolic graph of room modes in order to achieve the best sound. What you will find is that room modes begin to bunch together and overlap from 60Hz to about 250Hz. When multiple room modes around the same frequency occur, “standing waves” can form, which create objectionable bass bloat, muddy and ill-defined bass, and that buzzing bass sound. When you use the amroc calculator, the website produces a graph of your room’s modes, and as you move your cursor along the graph it will play the graphed frequency through your speakers so that you can actually hear the modes. I inputted a theoretical room size of 20’ x 25’ x 10’ and found that modes overlap and build at 56, 72, 87, 106, 112, 114, 116, 121, 125, and 140-150Hz. As the frequency moved higher, the more overlap and the more standing waves there will be in this room. Those frequencies are problem areas that will need to be corrected. Because lower frequencies are much larger waves (20Hz is roughly 52 feet long!), they are much harder to control, and you cannot absorb them like you can higher frequencies with shorter wavelengths.

Because controlling bass modes and standing waves is a much more in-depth project, I’m going to save this portion for the coming weeks. In the next few weeks, I will show you how to build your own acoustic panels and to “tune” them for the desired frequency range that needs to be controlled, and also how to build a Helmholtz Resonator to control the low end a bit. Unfortunately, some of these techniques require lots of space to implement properly, and small rooms won’t benefit as much because of lack of space.

Ideally, if you can have a listening room with more than 1500 cubic feet of air volume, you will have a much easier time with room acoustics. You can calculate this by multiplying the length, width, and height of your room to determine your room volume.

If you want to play along and work on your room with me week by week, that would be great! Calculate your room volume, and your room modes, determine your reflection points and whether your room is live, dead, or neutral, and in the coming weeks we will build Helmholtz Resonators and DIY acoustic panels to help maximize your listening experience. I will also include links to pre-fabricated panels and “bass traps” for those who don’t have the time to build their own. There are a lot of really great products out there, many of which are actually cheaper than building your own. But if you like to DIY like I do, then we can do this together.

I’m looking forward to maximizing our listening experience in the coming weeks and months!

Happy listening,

Spencer

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