Sunday Morning Hi Fi #5

Room Acoustics Part 2

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Sunday Morning Hi Fi #5

Welcome back to Sunday Morning Hi Fi #5, a blog that talks about music, gear, system tips and tweaks, and the reasons we love our high-end systems.

Last week, we skimmed the surface of room acoustics with a brief introduction to the subject. We talked about how to determine reflection points in your room that might be causing your system to sound bright or your imaging to be slightly imprecise, and we also talked about room modes—the low-frequency waves that tend to build and overlap, which often cause a lot of issues with boomy or bloated bass, or bass that is ill-defined. Unfortunately, most guides that try to explain room acoustics assume that you are using a traditional point-source speaker with cones and woofers in a rectangular room, but the real world is a lot more diverse than that. Rooms have irregular shapes, some people use electrostats, and many of us share our listening spaces with the rest of the family—that’s why they call it a living room.

This week, let’s delve a little deeper into room acoustics. But remember, it would be impossible to cover every aspect and every scenario on the subject in a weekly blog without boring you to death; for those looking for more complete guides to room acoustics, I highly recommend The Master Handbook of Acoustics by F. Alton Everest and Ken C. Pohlmann, or other scientific monographs on the subject of acoustic theory.

First, let’s define some important terms:

Reflection: The direct, linear path mid- and high-frequency sound waves travel after striking a room boundary, such as a wall, a piece of glass, or other non acoustically porous objects.

Diffraction: The bending of (generally) low-frequency sound waves, which tend to travel around and through objects due to the large size of sound waves below 200Hz.

Refraction: The change in the direction of sound due to the velocity of different mediums. Refraction and diffraction are similar, except that the medium through which sound travels is changing the direction of the sound wave, rather than a fixed object.

Diffusion: The scattering of sound waves to multiple points that do not mirror the angle at which the original sound wave struck the corresponding surface. In other words, diffusion is the equivalent of light hitting a disco ball.

Reverberation: The amount of time a sound reflects within a given space before decaying to inaudibility.

Absorption: The conversion of sound energy into minute amounts of heat. Sound energy is never “absorbed,” but rather converted.

Room Boundary: A wall, ceiling, floor, or partition against which mid- and high-frequency sound waves reflect.

Now, let’s talk about non-traditional rooms, and some examples of speakers that perform better with highly reflective room boundaries.

Non-traditional rooms can sometimes sound much better than your standard rectangular or square room; the reason that many guides shy away from using them as an example is because they present many more variables, and it’s more difficult to explain best practices when it comes to resolving acoustic issues.

With non-traditional rooms, it’s more important to determine the exact acoustic problem than with a rectangular room; rectangular rooms allow the listener to assume the problems, rather than measure or listen for each individual problem.

Do the clap or the snap test to determine your type of room, discussed in Sunday Morning Hi Fi #4. With a non-traditional room, you might have one wall open to a foyer or the main living/dining area; your room might be hexagonal with vaulted ceilings; whatever the design, you might be surprised how easy it is to correct acoustic issues.

Most non-traditional rooms have a diverse amount of acoustical surfaces, some of which reflect, some of which absorb, and some that diffuse. Begin thinking of acoustical objects in terms of two things: 1) The object’s physical size in relation to the size of various sound waves (20Hz is almost 55 feet long, whereas 10kHz is about 1.2 inches), and 2) whether the object reflects, absorbs, or diffuses. A typical family room will have a flat-screen TV (reflective), large sofas (absorptive or slightly reflective/diffusive depending on material), bookshelves (diffusive), various furniture (reflective and diffusive), rugs or carpeting (mainly absorptive), and of course walls, floors, and ceilings (typically reflective).

It’s always good to remember that a room is many, many types of surfaces simultaneously. Try walking around your listening room to determine which objects are reflective, absorptive, and diffusive. Some objects can be all three at once!

Strive for a large amount of diffusive surfaces, such as bookshelves with lots of odds and ends, shelves with vinyl records staggered unevenly so that there are no large flat surfaces, and specialty products which diffuse sounds throughout your room. Because you do not have many (if any) parallel surfaces in a non-traditional room, sound can be diffused to the point that most sonic issues found in box rooms are no longer problematic.

Without measuring your room with specialty acoustic software and microphones, you can only assume what your room is doing by careful listening. Listening tests, of course, should not be underrated, because we listen to our system with our ears, not microphones hooked up to our brains. Once you have listening to your system thoroughly and determined the problem areas (too much bass, too little bass, flutter and slap [reflection issues], or lack of image definition), you can then determine which tweaks will work best to make your system shine. There’s no such thing as a perfect room, and there’s no perfect tweak; but we can make our rooms sound much better.

Let’s delve into specific treatments and tweaks in the upcoming weeks.

Some speakers like highly reflective rooms. Yes, not all reflective rooms or “live” rooms are bad. Many dipoles and especially electrostats/planar/ribbon speakers actually perform better when the front wall (the wall behind the speaker) is reflective. Many point-source speakers sound better when sidewalls are slightly reflective, which gives the soundstage more width (though, generally sacrificing some image detail). Omni-directional speakers love a highly reflective room, albeit a room that has lots of diffusive surfaces as well. In order to tame these speakers a bit and make your room sound better, start with a series of diffusive panels or objects, and introduce them to your room slowly. Many times, your room will only need a lot more diffusion, and absorption won’t be necessary.

Next week, let’s get a little more detailed and begin talking about how to resolve certain specific issues using various tweaks.

Until next week, happy listening,

Spencer

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