RealTraps - Recording Spaces

..EQ ACOUSTICS SERIES..

Also see this Comb Filtering VIDEO.

"The main difficulty when recording in a small room is that the floor, ceiling, and walls are all close to both the performer and the microphones."

By Ethan Winer

For many project studio owners, calling their small recording space "a live room" is a bit of a cruel joke. Many people are lucky to have two rooms so they can record in one room while monitoring in the other. Big-ticket professional studios have very large recording spaces for a good reason - several of them, actually - but you can definitely improve whatever space you have to get the most out of it. In this article I'll explain why large rooms are generally better for recording than small rooms, then show how to minimize the problems inherent in smaller rooms to help make them sound like larger spaces.

COMB YOUR HEAR

The main difficulty when recording in a small room is that the floor, ceiling, and walls are all close to both the performer and the microphones. These nearby surfaces create two fundamental problems: One problem is comb filtering that creates a series of many peaks and deep nulls in the frequency response. This gives a hollow sound similar to a phaser or flanger effect. The other problem with small rooms is they foster many short echoes that can thicken the sound in a bad way. When a wall is only a few feet away from either the performer or the microphone, the inevitable echoes arrive too quickly to be perceived as echoes. Rather, they often make instruments or voices sound muddy and poorly defined. Top

Click the images below to see them full size.

Figure 1: A microphone was placed 20 inches from the wall to measure the comb filter effects of the reflections. The loudspeaker was placed directly behind the microphone, about six feet away.

Figure 2: This graph shows the frequency response measured with the microphone placement shown in Figure 1. All of the peak and dip frequencies are related to the 20-inch distance.

"Unlike absorption, diffusion retains the pleasing qualities of an ambient space while avoiding the direct reflections that cause comb filtering and short echoes."

The photo in Figure 1 shows a measuring microphone placed 20 inches away from a sheet rock wall, and Figure 2 shows the resulting frequency response as measured by ETF, the room analysis software I use. Reflections from the nearby wall create a continuous series of peak and null frequencies at uniform intervals, and the frequencies are all related directly to the 20-inch distance. The cause of this skewed frequency response is called a comb filter because the graph looks like the teeth of a hair comb. Note that comb filtering can occur when a room boundary is near a microphone and also when it's near the performer.

Comb filtering occurs near reflective boundaries because the reflections collide in the air with the original sound after a slight delay. The general term for this collision is acoustic interference, and it can be either constructive or destructive. At frequencies where the waves are more or less in phase, they combine to increase the level. At other frequencies that are out of phase by some amount, they will instead cancel. To get a deep null the two waves must be exactly 180 degrees out of phase and also very similar in level, or another combination of phase shift and volume that results in two waves being exactly equal but opposite. See the sidebar What Causes Comb Filtering for a more detailed explanation. Top

SOLUTIONS

Both of these problems - the comb filtering and the short echoes - are responsible for creating the typical boxy, "off-mike" sound that makes instruments and voices sound like they were recorded in a small room. The solution for echoes and comb filtering caused by reflections off nearby surfaces is adding an appropriate amount of absorption, and optionally diffusion, on the walls and ceiling. The most common type of absorption is a panel one to four inches thick made of foam or rigid fiberglass covered with fabric. The reflections I've been addressing cause problems mainly at mid and high frequencies, so even fairly thin absorbing panels are sufficient to reduce the reflections enough to avoid a boxy sound.

In most cases you should cover only part of the walls with absorbent panels, rather than covering the entire walls. This spreads the absorption around the room more uniformly, yet still provides sufficient coverage to avoid the damaging reflections. If a room is so tiny that even a small amount of its "room sound" is detrimental, you'll have to cover nearly every surface and accept that any desired ambience must be added electronically during mixdown. Otherwise, I prefer to see absorption applied in stripes or a checkerboard pattern, for a total coverage of about 30 to 40 percent. Top

One exception is when you need to create an area that's more dead sounding to record vocals or spoken narration. In that case you can place absorbing panels that cover both walls completely in one corner, extending about three or four feet in each direction. Then you'll put the microphone in the corner facing out toward the room, and have the performer face into the corner.

Larger rooms often benefit from diffusion as well as absorption. Unlike absorption that removes reflections and makes the room less live sounding, diffusion retains the pleasing qualities of an ambient space while avoiding the direct reflections that cause comb filtering and short echoes. Instead of reflecting the waves back toward the source, diffusors scatter different frequencies in different directions. However, be aware that some diffusors work better than others, and some products touted as diffusors are really just deflectors. For example, an angled wall or a large curved surface do not scatter waves at different angles based on their frequency - they just redirect all frequencies at the same fixed angle. Also, some diffusor designs are effective over a fairly limited range of frequencies. Top

Figure 3: A reflective floor gives the best results when recording acoustic instruments. You don't need to pay for fancy wood - linoleum and even bare or painted cement work just as well.

"I believe that for studios where the total amount of space is limited, one large room is a better choice than two small rooms because it avoids the nearby walls."

HARD FLOORS RULE

If you've ever seen a live room in a professional recording studio, it almost certainly had a floor made of hardwood or linoleum. Likewise, every stage in every auditorium in the world has a reflective floor, and for good reason. A reflective floor offers a desirable ambience that can enhance a recording to sound more present and lifelike. Figure 3 shows my home studio, which has a hardwood floor chosen specifically because I often record orchestral and other acoustic instruments. Even though a reflective floor is nearby, it's not as detrimental as when many such surfaces are nearby. Avoiding all other nearby reflections lets you place microphones farther from the source without the sound becoming distant and hollow. Another reason a hard floor with absorbent ceiling is better than the other way around is you can use thicker material on a ceiling than on a floor. Two to four inches of rigid fiberglass on a ceiling absorbs to a much lower frequency than even the thickest carpet.

Close-miking acoustic instruments, as opposed to a guitar amp, can be a problem because acoustic instruments radiate different frequencies in different directions. This is especially true for stringed instruments like violins, cellos, and acoustic guitars; but clarinets, saxophones, and other woodwinds are directional too, with different notes coming from different places on the instrument. Therefore, there is no single close-miked position that will give a uniform, balanced sound. Only by pulling the microphones back a few feet can you capture the entire range of an acoustic instrument in the proper balance. Top

To complement a reflective floor in a small room, it's a good idea to have most or all of the ceiling be absorbent. This serves two purposes: First, it avoids flutter echoes that otherwise occur between two parallel reflective surfaces. More important, an absorbent ceiling is equivalent acoustically to a ceiling that's infinitely high - either way, sound that travels up is never reflected back. Having a ceiling that's either very high or very absorbent is especially important when using overhead microphones. If you mike a drum set from above and the mikes are only six inches below a bare sheet rock ceiling, the nearby reflections will make the drums sound thin and hollow no matter how much EQ or compression you apply.

Finally, I believe that for studios where the total amount of space is limited, one large room is a better choice than two small rooms because it avoids the nearby walls. Yes, you'll need headphones when recording and overdubbing, and you'll also have to defer all mixing and processing decisions to mixdown. (Aside from effects integral to the instrument, like playing with a wah pedal, this is a good approach anyway.) But the loss is not as great as you might think. To really hear a miked drum set properly in another room requires serious isolation; otherwise, what you hear is influenced by the low frequencies that inevitably leak through the walls. Since true isolation is very difficult to achieve in a house, not to mention expensive, in my opinion it's not even worth trying for in most home-based studios. Top

Figure 4: Delaying audio is equivalent to shifting its phase by some amount at certain frequencies. When the delay equals 180 degrees of phase shift at a given frequency, combining the original and shifted versions creates a null at that frequency and at all higher related frequencies.

Figure 5: Delay-induced phase shift occurs at predictable distances from every room boundary. When the distance between a listener or microphone and the boundary equals 1/4 wavelength, the total round trip is 1/2 wavelength resulting in a deep notch at that frequency.

Figure 6: The ionosphere is composed of the D, E, and F layers. The D layer is good at absorbing AM radio waves. The D layer disappears at night, and the E and F layers bounce the waves back to earth. Drawing and text are from THIS page at The Lyndon State College of Meteorology.

SIDEBAR: WHAT CAUSES COMB FILTERING

You've heard the effect of comb filtering many times, but perhaps not its name. Phaser and flanger effect units use time delay - or phase shift, which is closely related - to create the characteristic hollow sound caused by a series of peaks and nulls in the frequency response. When used as an electronic effect, the filter frequencies are usually swept up and down slowly to add some animation to, for example, an otherwise static-sounding rhythm guitar part. The peaks and nulls occur when audio is sent through a delay line or phase shift circuit, and then combined with the original signal.

For any given delay time, some frequency will be shifted exactly 180 degrees. So when the original wave is positive, the delayed version is negative, and vice versa. If the two are mixed together equally, the result is complete cancellation at that one frequency. Nearby frequencies that are shifted less, or more, than 180 degrees also cancel, but not as much. Figure 4 shows a single frequency tone that has been delayed in time so its phase has shifted by 90 degrees, and again by a longer delay equal to 180 degrees. If the original tone is combined with the version shifted by 180 degrees, the result is complete silence. Other frequencies present in the audio will not be cancelled unless they are related to the fundamental frequency. That is, a delay time that shifts 100 Hz by 180 degrees will also shift 300 Hz by one full cycle plus 180 degrees. The result is a series of deep nulls at 100 Hz, 300 Hz, 500 Hz, and so forth. This is what creates the hollow swooshy sound associated with phaser effect units, not the phase shift itself. Top

This same hollow sound occurs acoustically in the air when reflections off a wall arrive delayed at your ears or a microphone. Figure 5 shows that for any frequency where the distance between a listener (or microphone) and a reflective wall is equal to 1/4 wavelength, a null occurs. Understand that a 1/4 wavelength distance means the total round trip is 1/2 wavelength, so the reflection arrives after 180 degrees of phase shift, not 90. Nulls also occur at related higher frequencies where the distance is equal to 3/4 wavelengths, 5/4, and so forth. This is why the response has a series of nulls instead of only one. Note that comb filtering also occurs at lower frequencies when the distances are larger, causing peaks and nulls there too. [You can use the RealTraps Frequency-Distance Calculator HERE to determine the relation between frequency and spacing.]

Added February 17 and March 12, 2007: Comb filtering also intrudes in our lives by causing reception dropouts and other disturbances at radio frequencies. If you listen to AM radio in the evening you'll sometimes notice a hollow sound much like a flanger effect. In fact it is a flanger effect! The comb filtering occurs when your AM radio receives both the direct signal from the transmitting antenna and also a delayed version that's been reflected off the ionosphere. This is shown in Figure 6 at left.

Likewise, FM radio suffers from a comb filtering effect called "picket fencing" whereby the signal fades in and out rapidly as the receiving antenna travels through a series of nulls. As with audio nulls, radio nulls are also caused by reflections as the waves bounce off nearby large objects such as a truck next to your car on the highway. The signal fades in and out if either you are moving or if the large object is moving, and this is often noticeable as you slow down to approach a stop light in your car.

Reception dropouts also occur with wireless microphones used by musicians. The solution is a "diversity" system having multiple receivers and multiple spaced antennas. In this case it's the transmitter (performer) that moves, which causes the peak and null locations to change. When one receiving antenna is in a null the other is likely not in a null. Logic within the receiver switches quickly from one antenna to another to ensure reception free of the dropouts that would otherwise occur as the performer moves around.

The same type of comb filtering also occurs in microwave ovens, and this is why these ovens have a rotating base (or the rotor is internal and hidden). Even with rotation hot and cold spots still occur. Wherever comb filter peaks form the food is hot, and at null locations the food remains cold. Top

Ethan Winer eats, drinks, and sleeps acoustic treatment. He now heads up RealTraps in New Milford, CT where you can visit him at www.realtraps.com.