Room reflections add different sonic effects depending on their volume and how long they are delayed: 1) Some blend in seamlessly with the direct sound. 2) Some add spaciousness and increase image size. 3) The most damaging are heard as distinct echoes.

Reflection

First, the good news
One of the reasons that the effects of room reflections are so noticeable is that our ears (actually, our entire auditory system, which also includes the brain) are amazingly sensitive at locating the source of a sound. Even with your eyes closed, you can usually locate the position of someone speaking to you in a room. Your brain uses timing differences between the original and the reflected sound to locate the source. It would be much more difficult in a highly reflective room with uncontrolled echoes. (Or outdoors in an open field, where the only reflective surface is the ground.)

But our ears aren't perfect. Sounds that arrive at our ears soon enough after the direct sound are perceived as being part of the original sound. As the graph at the right shows, early reflections that are not too loud or delayed too long will not only increase the loudness of the sound, but can actually add a pleasant spaciousness.

This effect is similar to the way our eyes fuse together the series of still pictures used to create TV or movies into an impression of continuous fluid movement. How quickly each image follows is the key: there must be at least 16 frames a second to avoid noticeable flicker. When it comes to sound, there are two factors: loudness and length of delay. If the reflection is too loud, or if the delay between the original sound and the reflection lasts too long, you'll generally hear a distinct echo.

Part of the reason that the surround speakers in a Dolby® Digital system can create such a believable impression of spaciousness is that the signal fed to the surround speakers includes a 15-20 millisecond delay.

Now, the bad news
There are several different ways that room reflections can interfere with your enjoyment of music and movie sound. Some can be treated easily and inexpensively, while others are trickier to deal with. Let's start by talking about the unique set of reflections that develop based on the size, shape and dimensions of your room.

Standing waves and room resonance modes
Any time you have a pair of parallel reflective surfaces (like room walls, or the floor and ceiling), you're going to experience some degree of a phenomenon known as standing waves. Standing waves distort the bass and lower midrange frequencies from 300 Hz on down.

Standing waves are created when sound is reflected back and forth between any two parallel surfaces in your room. They affect frequencies below 300 Hz. A room's primary or "axial" resonance modes are based on the room's three main axes: length, width, and height. These resonance modes create bass peaks and dips of up to 10 dB throughout the room.

One way to understand the effects of standing waves in a room is to think of how a microwave oven works. The high-frequency microwaves generated to heat the food on your plate are reflected over and over inside the oven compartment. As these reflections collide, some are reinforced while others are cancelled, creating areas of varying microwave intensity. This translates into definite hot spots and cold spots in your plate of food, from steaming to lukewarm to cool.

The sound from your speakers acts in much the same way. It is reflected back and forth, over and over between the parallel surfaces in your room: the side walls, the front and rear walls, and the floor and ceiling. This creates areas of differing sound pressure or loudness: the "hot" and "cold" spots.

You can easily hear these standing waves if you play some music with a lot of bass, like pipe organ music or reggae, and take a walk around your room, listening at different spots: the middle of the room, near the walls, and in the corners. You'll probably notice that the bass sounds stronger near the walls and especially in the corners, where standing waves tend to collect. These are specific types of standing waves which are called room resonance modes.

Sizing up your room
It's actually pretty easy to calculate the axial resonance modes for your room. Knowing the frequencies of these axial modes will provide valuable information about how your system and room are interacting, specifically on bass notes in the under-300 Hz range.

First, get a tape measure and measure the length, width and height of your room. As an example, we'll use these typical room dimensions: 21 feet long x 12 feet wide x 8 feet high.

The formula for finding axial room resonance modes:

In the example above, we've calculated our sample room's main resonance mode for length. The room's length is 21 feet, so plugging in 21 for our distance variable in the equation, we get a resonance frequency of 27 Hz.

Our sample room has a length of 21 feet, so plugging 21 into the formula gives us our axial resonance mode for length.

Resonance modes occur when the distance between the room's walls equals half the wavelength of the sound, and at multiples of half a wavelength. Notice that there are always sound pressure (volume level) peaks at the walls. The circled frequencies will be reinforced by the room. Frequencies appearing in more than one column will receive added emphasis, causing even more sound coloration. In this example, you can see trouble spots at 141 Hz, 188 Hz, and 282 Hz.

So, the main mode for the length axis of the room falls at 27 Hz (it's actually 26.9, but we're rounding to the nearest whole number). This means that although you'll still be able to hear deep bass sounds from your speakers below 27 Hz, your room cannot provide any reinforcement of frequencies much below 27 Hz.

In addition to this fundamental mode at 27 Hz, there will be other weaker modes at multiples of the fundamental mode (2x27, 3x27, 4x27, etc...). So, along with the first mode at 27 Hz, there will be other resonance modes at 54 Hz, 81 Hz, 108 Hz, etc....

Now we can use the same formula for the room's width and height. Plugging the 12-foot width into the formula gives us a fundamental mode at 47 Hz, with multiples at 94 Hz, 141 Hz, 188 Hz, etc.

Using the formula again, our fundamental 8-foot height mode is at 71 Hz, plus multiples at 141 Hz, 212 Hz, etc.

It's a little easier to see what's going on if we arrange our room modes into a table (see right).

There's actually more to the story than just the axial modes involving two walls, described above. There are also tangential resonance modes involving four room surfaces, and oblique modes involving all six surfaces. These other room modes don't affect the sound as strongly, but as we've mentioned before, all reflections affect the overall sound.

How to deal with room resonance modes
So now that you know what room resonance modes are and how they can distort your system's sound, what can you do about them? In many cases, not much. These room modes are based on your room's dimensions, which are difficult to change. (Even bass-loving audiophiles will hesitate to move a wall just to hear more accurate low frequencies.) And room treatment products that are great for controlling treble reflections with short wavelengths don't work at all on long-wavelength bass reflections.

Here are some tips and things to keep in mind concerning room resonance modes:

• Certain room shapes are fundamentally bad from a room-mode standpoint. A cube is one of the worst shapes for a room (each resonance mode gets triple emphasis). You'll also hear more standing wave distortion in rooms with two equal dimensions, or rooms with dimensions that are multiples, ie. 8' x 16' x 24'.
• If you're building a house or finishing a room, here are some room dimension ratios that are superior soundwise:

Applying the 1 : 1.4 : 1.9 room dimension ratio (see table) to a room with an 8-ft. ceiling yields dimensions of 8'H x 11.2'W x 15.2'L.

• In general, the smaller the room, the more its resonance modes will color bass response.
• A high, sloped ceiling tends to scatter ceiling mode effects.
• Common types of wall construction such as drywall or wood paneling on 2x4s will absorb a significant amount of added bass reflections in the under-125 Hz range (see table below).
• Try moving the position of your chair or sofa closer to or farther from your speakers to get out of a standing wave hot spot.
• Standing waves are always stronger next to walls. If your chair or sofa has its back against a wall, moving it out away from the wall should reduce standing wave boominess.
• Room corners are notorious collection points for standing waves. If your room has an 8-foot ceiling, professionally designed bass traps can help reduce or eliminate these standing waves. This is accomplished by soaking up the bass reflections created by the 71 Hz fundamental resonance mode of the 8-foot ceiling.

Flutter echo
Probably the most common and immediately noticeable room problem results from having parallel surfaces (walls, floor and ceiling) with a hard finish that reflects sound. The resulting effect is called flutter echo, a ringing reverberation that remains after the direct sound has stopped.

The sound-absorbing effectiveness of some common room surfaces. Fibrous materials like carpet and drapes provide significant absorption above 500 Hz, but have little effect on lower frequencies. Conversely, window glass and drywall can absorb bass frequencies, but are very reflective above 500 Hz. The most successful approaches combine materials like these with professionally-designed room treatment products.

If you've ever stood in an empty uncarpeted room or hallway, and clapped your hands, you've heard flutter echo. The original clap sound is reflected back and forth between two surfaces. Because the wavelengths of mid- and high-frequency sounds are so much shorter than those of bass notes, the reflections bounce around very directionally, like reflected light. The resulting sound is this ringing flutter echo rather than the boomy standing waves described previously.

Flutter echo affects music by blurring transients (fast musical attacks) and adding an unpleasant harshness to the midrange and treble. Flutter echo and other primarily side wall reflections affect sounds above 500 Hz, and are a major reason why the same pair of speakers will sound different in different rooms.

To treat flutter echo you need to control the reflections on one or both of the parallel surfaces. This usually means applying some sort of sound-absorbing or sound-diffusing material to the side walls between the speakers and your listening position. Carpeting or acoustic ceiling tile will reduce floor/ceiling flutter echo. We'll go into detail about locating and treating your room's points of reflectivity later.

Reflection effects on movie dialogue
The movie industry certainly understands how sonically damaging reflections can be. Think about all the reflection-absorbing surfaces in your neighborhood movie theater: heavy drapes all around, upholstered chairs, and a human audience (that's right, our bodies act as sound absorbers too).

Studies have shown that dialogue is more easily understood in rooms using one or more types of reflection control. Reflections can be controlled in listening rooms and home theater rooms by sound absorption, sound diffusion, or some combination of both.