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"The correct solution for a skewed low frequency response is acoustic treatment, especially bass traps."
"A room treated entirely with thin materials sounds dead and lifeless, yet it is still boomy and muddy due to excessive ringing at low frequencies."
"Rooms where surround mixes are made or listened to are generally less live sounding than regular control rooms and living rooms." |
By
Ethan Winer No matter what type of room you record in, or even if you record all instruments through a direct box, your monitoring must be accurate to make mixes that sound correct outside your own control room. When a mix you create in your control room sounds the same, or at least similar, in other rooms, it is considered to translate or travel well. Indeed, creating portable mixes is one of the biggest problems facing most home and project studio owners. In a previous article (Acoustics: Good or Bad Vibes?), I showed the terrible low frequency response that's typical in all small rooms - numerous variations up to 30 dB, or even more, are common throughout the entire low end. Most small rooms have a deep null somewhere between 70 and 120 Hz right at the mix position, so many people compensate when mixing by adding too much bass. This leads to the common complaint that mixes that sound good in your room sound too bassy and boomy when played in other rooms or in a car. As I explained in that article, the correct solution for a skewed low frequency response is acoustic treatment, especially bass traps. When bass traps are installed in a room, the reflections that cause low frequency peaks and dips are reduced and the response is made flatter. But bass traps are also useful for solving another common acoustic problem - excessive low frequency reverb and ringing that obscures detail and makes it difficult to distinguish notes played by bass instruments. Top THE IMPORTANCE OF A UNIFORM REVERB TIME We all know what reverberation is - both the natural reverb that occurs in rooms and the electronic kind used as an effect - and we all know that too much of it harms the clarity of music and speech. The general specification for reverb is Reverb Time, or RT60, which is the number of seconds it takes for the reverberant sound to decay by 60 dB. In practice, it's difficult to measure RT60 directly because that requires a very quiet room whose residual noise is more than 60 dB below the test signal. Therefore, reverb time is more often measured until the sound decays by only 15 dB. Since reverb decay rate is constant, it's easy to convert RT15 to RT60. However, these reverb times are an average of the decay rates at all frequencies. Just as important is how the reverb time varies in different frequency ranges. I often see people attempt to treat their rooms by placing moving blankets, egg crates, acoustic foam, or other thin materials over all their walls. Although these materials do reduce reflections, they absorb only higher frequencies. So while they do eliminate the most obvious echoes and ambience - clap your hands and the room sounds clearer - thin materials do nothing to stop low frequency reflections, which are just as damaging. A room treated entirely with thin materials sounds dead and lifeless, yet it is still boomy and muddy due to excessive ringing at low frequencies. Unfortunately, you can't assess low frequency muddiness by clapping your hands. Rather, the main symptom is the difficulty in discerning which notes the bass instrument is playing. Top Small room reverb is far more complex than many people realize. In particular, at low frequencies the room's natural resonances sustain some frequencies more than others, as opposed to true reverberation, which sustains a broad range of frequencies. Further, the acceptable range of reverb time for a given room varies with its volume. According to the Master Handbook of Acoustics by F. Alton Everest, the ideal reverb time for a typical control room is about 400-500 milliseconds, and a studio live room is usually about 100-200 milliseconds longer. Rooms where surround mixes are made or listened to are generally less live sounding than regular control rooms and living rooms. One reason is that movie soundtracks often contain more embedded ambience. Another is that the additional loudspeakers require absorption in more places to trap early reflections coming from more source locations. Therefore, surround playback rooms often have more diffusion than regular control rooms and living rooms, in addition to having more absorption. To be sure, there is no one "correct" reverb time for any room, and the personal taste of the recording engineer or listener must also be considered. Lately professional control rooms tend to use less and less mid and high frequency absorption, relying instead on angling the walls and ceilings sufficiently to deflect damaging reflections away from the mix position. This lets the room retain a big, bright, live sound, while still avoiding early reflections. In fact, modern thinking places at least as much importance on eliminating early reflections as it does on controlling overall reverb time. Most of us do not enjoy the luxury of angling the walls and ceiling, but it's still possible to obtain stellar monitoring in a normal rectangular room. Let's take a closer look. Top |
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"To maintain proper stereo imaging, a control room must be perfectly symmetrical and must also avoid all early reflections."
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EARLY
REFLECTIONS AND IMAGING Early reflections - also called first reflections - are echoes that arrive at your ears within about 20 milliseconds of the direct sound from the loudspeakers. When a direct sound is followed immediately by an echo, the ear does not distinguish the echo as a separate sound source. Rather than perceiving the reflections as echoes or ambience, multiple sounds arriving quickly appear to fuse together, which obscures clarity and confuses the stereo image. Just as damaging are reflections from the left speaker that arrive in the right ear, and vice versa. You can tell when an instrument is panned hard left or right, but the in-between positions are poorly defined. To maintain proper stereo imaging, a control room must be perfectly symmetrical and must also avoid all early reflections. For perfect symmetry, the left and right loudspeakers need to be the same distance from their adjacent side wall, and both side walls should have the same type of surface and acoustic treatment. Early reflections are avoided either by applying absorption to key places on the side walls and ceiling or by deflecting them away from the mix position. When you eliminate all early reflections, the effect is quite striking and is not unlike listening through headphones with their almost magical sense of added width and spaciousness. Top Figure 1, viewed from above, shows the layout of a symmetrical control room. The black lines represent the path of direct sound from the loudspeakers to your ears, and the red lines show the undesired early reflections. Reflections off the rear wall, shown in blue, are not a problem as long as the rear wall is at least ten feet behind the mix position. Sound travels at a speed of roughly 1 foot per millisecond, so the 20-foot round trip delays those reflections enough so they're perceived as ambience instead of combining with the direct sound. If your rear wall is closer than 10 feet, it should be treated with absorption. It's easy to determine the correct placement of 2x4-foot absorber panels to avoid early reflections. As long as the walls and ceiling are not angled, simply place the panels along the side wall so they're halfway between your ears and the front of the speaker cabinet. Then place each panel vertically so its center aligns with the tweeter. Since early reflections also arrive from the ceiling, you need to apply absorption there as well. Again, the panel should be centered left to right and placed halfway between you and the loudspeakers. By the way, another source of early reflections is the top surface of the mixing console or desk. These are difficult to avoid unless you're willing to get rid of your console! Sometimes you'll see engineers place a large sheet of acoustic foam over the entire console, leaving only the faders exposed, while making final mix decisions. Top |
| "ETF runs on any Windows computer and works well even with ordinary consumer grade sound cards."
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ROOM
MEASUREMENTS Professional acousticians use a variety of hardware and software to help them evaluate the frequency response, reverb time, and early reflections in rooms. Most of these tools are far too expensive for project studio owners to consider. However, one terrific solution is the popular ETF program from Acoustisoft. ETF performs all of the important acoustic measurements yet sells for a very reasonable price. You can read more about ETF at the Acoustisoft web site, so I'll just cover some of the highlights. ETF runs on any Windows computer and works well even with ordinary consumer grade sound cards. The program plays test signals through the computer's sound card, which are routed to your loudspeakers. The result is then recorded through your omnidirectional microphone for analysis. ETF offers two types of test signals - swept sine waves and MLS (Maximum Length Sequence, sounds similar to pink noise). Further, each signal type can be either broadband or band-limited for increased resolution at low frequencies. Figure 2 shows the ETF graph of reverb time versus frequency in my home studio, and this is just one of many reports that are available. Top One big problem with standard third octave measurements is that they hide the true extent of a room's response variations. As you can see in Figure 3, the 1/12th octave response (blue line) reveals the room's peaks and nulls far more accurately than the same data plotted at 1/3rd octave resolution (red line). Also note that at higher resolutions ETF imposes a low frequency limit, so the 1/12th octave display stops at about 200 Hz. You can measure with high resolutions at low frequencies, but that requires a separate low frequency test. Besides measuring detailed frequency response, reverb time, and the arrival of early reflections, ETF has many other useful features, such as displaying energy/time curves and overlaying multiple test results onto a single graph. For example, one of the tutorials on the Acoustisoft web site shows the low frequency response for a subwoofer at eight different locations, so you can easily see which placement is the most accurate. There's also a Device Designer that helps you build Helmholtz resonator bass traps and QRD diffusors. Top Finally, ETF allows you to run a test once, then analyze and report on the data in many different ways later. When you run a test and save the result, all of the data is stored in the file. That data can then be manipulated later using all of the ETF options. AL FINE As you have seen, the key to creating excellent mixes is being able to hear clearly and accurately - even the best monitor speakers in the world are of little use if your control room is lying to you. I'll leave you with one final word about acoustic measurements: Worry most about the big deviations and don't sweat the small stuff. If you can get your control room flat to within +/- 10 dB, you're doing really well. And no, I'm not kidding! Top |
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