RealTraps - Acoustics: Good or Bad Vibes?

From EQ Magazine, February 2004

..EQ ACOUSTICS SERIES..

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"All acoustic problems are caused by reflections off the walls, floor, and ceiling."

 

 

 

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Figure 1: The horrendous frequency response shown here is typical in the small rooms often used for home studios. Click the image for a larger version.

 

 

 

 

 

 

"If your room lacks deep bass, your mixes will contain too much bass as you wrongly compensate."

By Ethan Winer

Every EQ reader will agree that recording and mixing music is a lot of fun. And if you produce your own music, it's even more fun to hear the ideas take shape and begin to sound like a "real" recording. But most of us encounter frustration somewhere along the way. Perhaps you have an impossible kick drum that sounds like cardboard no matter how you EQ or compress it. Or maybe you just can't get the bass to sit nicely in the mix no matter what you do. When you finally manage to get what you think is a good mix, as soon as you play it in the car or on a friend's stereo, your heart sinks as you realize how poor it sounds compared to your favorite commercial recordings.

The good news is most of these vibe-killers can be solved by understanding and fixing the acoustic problems that exist in all rooms. I'll begin by describing the most common troubles caused by poor acoustics, and then explain some ways to correct them.

All acoustic problems are caused by reflections off the walls, floor, and ceiling. At mid and high frequencies, reflections can cause echoes and excessive ambience that confuse stereo imaging and make it difficult to tell how much reverb and echo you're adding electronically. In severe cases, midrange reflections also cause ringing - new tones that resonate when excited by similar frequencies in the music. Low frequency reflections create standing waves that skew the frequency response and make bass instruments sound muddy. You might have the finest loudspeakers in the world, but they're of little value if the room itself creates numerous peaks and dips throughout the entire low end. Nulls as large as 25 dB are not only common but also typical, as shown in Figure 1 at left.

This graph shows the low frequency response measured in a typical 16x10x8 foot untreated control room. A pair of Mackie HR-824 loudspeakers were against the front wall with the tweeters at ear level, and the measuring microphone was placed precisely at the mix position. Note the peak/dip pair at 110 and 122 Hz where the response varies a staggering 32 dB across a range smaller than one musical whole step. No wonder you can't distinguish bass notes!

In my experience, room acoustic problems are the single biggest cause of dissatisfaction among home recordists. Unfortunately, too many people consider everything but acoustics when they have trouble making a mix sound the way they'd like. There's no denying that audio gear with rows of lights, knobs, and switches is a lot sexier than boring acoustic panels and bass traps. In truth, acoustic treatment will improve the quality of everything you produce far more than your choice of microphone preamp, sound card, or outboard compressor.

Some may question the need for acoustic treatment. After all, how many people who hear your music will be in a treated room? The answer is simple - all rooms have a unique frequency response. So if your room lacks deep bass, your mixes will contain too much bass as you wrongly compensate. If someone plays your mix in a room having too much bass, the error will be exaggerated, and they'll hear way too much bass. The only viable solution is to make your room as accurate as possible so any errors listeners hear are caused by their own rooms, not yours.

Besides improving the accuracy of your entire system, proper acoustic treatment makes recording and mixing a lot more fun. It's a real head slapper the first time you hear every note articulated clearly by an electric bass, and are able to discern even tiny changes in EQ and effects settings. In one brief moment, it becomes clear what you've been missing all along!

Click to see a larger version
Figure 2: For the flattest low frequency response you should face the loudspeakers the long way into the room, and orient your mix position to form an equilateral triangle between the back of your head and the loudspeakers. Click the image for a larger version.

 

 

 

 

 

 

"Panels made of acoustic foam and rigid fiberglass are much better than blankets because they absorb to a lower frequency."

TOP TEN ACOUSTIC IMPROVEMENTS

Anton, start the drum roll please as we examine the ten most common ways to improve a room, including several that are free, and a warning about some popular approaches that don't work.

Orientation: Assuming you're in a normal rectangular room, orient your mix position so the speakers are facing the long way into the room, as shown in Figure 2. The two most important goals are maintaining an equilateral triangle having left/right symmetry within the room, and placing yourself as far as possible from the surrounding walls. Having the mix position slightly forward of the halfway point yields the flattest low end response, and the ideal speaker height puts the tweeters level with your ears.

Speaker placement: Once the basic placement of your loudspeakers and chair are correct, you can try the speakers at different distances from the front wall to see which yields the smoothest sounding bass response. If your room is small, you may have no choice but to put the speakers flat against the wall. That's perfectly acceptable, and many active monitors include switches to compensate for this placement. But if you have a few feet to work with, spacing the speakers a few feet in front of the wall often improves the response.

Added February 5, 2016: It turns out this next item is wrong because most speaker cabinets don't vibrate enough to generate much sound:

Decouple your speakers: Sound travels through solid materials faster than through air. So when loudspeakers are sitting on a desk, low frequencies can transmit from the speaker's enclosure through the desk and floor and arrive at your ears before the waves in the air. If the secondary path is strong enough, the phase shift caused by this time delay contributes to low frequency response errors. One solution is to buy speaker isolation pads sold for just this purpose. You can optionally make pads from rigid fiberglass or even kitchen sponges - the kind that become stiff when dry work best for this.

Mid/High frequency absorbers: I distinguish between materials and products that absorb from 300 Hz and up and those that are effective at lower frequencies. One common treatment that helps a little is heavy blankets or thick bedding hung on the walls. Please understand this is at best a partial solution because these materials absorb only the highest frequencies. All rooms need broadband absorption, and treating only the high frequencies is usually worse than having no treatment at all.

Panels made of acoustic foam and rigid fiberglass are much better than blankets because they absorb to a lower frequency. For a given thickness, rigid fiberglass absorbs to about an octave lower than foam. Typical foam that's two inches thick is useful down to 500 Hz. Contrast that with rigid fiberglass of the same thickness that absorbs well to below 250 Hz. Fiberglass is also fireproof. The downside is it must be covered with fabric to prevent the fibers from escaping into the air. Regardless of which product you choose, please don't make the common mistake of covering the entire room with thin material. This makes the room sound creepy and lifeless, yet boomy at the same time.

 

 

 

 

 

"One common cause of poor stereo imaging is early reflections."

 

 

 

 

 

 

 

 

 

"Many people wrongly believe that bass traps should be tuned to specific frequencies based on the room dimensions."

 

 

 

 

 

 

 

 

 

"I am convinced that the most important 'gear' you can own is proper room acoustics."

Reflection free zone: One common cause of poor stereo imaging is early reflections - echoes that arrive within 20 milliseconds of the direct sound from the loudspeakers. Instead of sounding like echoes, early reflections fuse with the direct sound and obscure clarity. Worse, if sound from the left speaker bounces off the right wall into your right rear, or vice versa, imaging suffers. This is easily solved by placing 2x4 foot absorbing panels in selected locations on the side walls and the ceiling. Place one panel vertically on each side wall halfway between the loudspeakers and your ears, and a third panel horizontally on the ceiling, also halfway between you and the speakers.

Bass traps: Although reflections at mid and high frequencies are more easily noticed, low frequency reflections are far more damaging. Any device worthy of the name bass trap will have substantial absorption at 300 Hz and lower - hopefully much lower. The cheapest bass traps you can buy are 4-foot bales of fluffy fiberglass, left in their plastic wrapper, and stacked in the room corners. They're big and ugly for sure, but they perform reasonably well, especially for their low cost. The next step up is rigid fiberglass panels wrapped in cloth and placed straddling the room corners, including the ceiling corners. Commercial bass traps include foam corners, corner mounted rigid fiberglass, and wood or fiberglass-based membrane traps.

Many people wrongly believe that bass traps should be tuned to specific frequencies based on the room dimensions. But this ignores the fact that severe peaks and nulls exist in all rooms at all frequencies. Therefore, the best bass traps for all rooms are those that absorb the entire range of low frequencies.

Diffusion: Diffusion avoids the damaging echoes and comb filter effect caused by reflections off nearby walls, though it's used most often in larger rooms. Diffusors range from small lightweight plastic panels that resemble a city skyline, to large expensive Quadratic Residue Fractal designs. If the room and budget are both small, simply placing absorber panels on troublesome surfaces makes sense. But when money is no object and you want to retain as much liveness as possible, diffusion is very useful.

Room EQ: Trying to use equalization to correct low frequency room problems simply does not work. The peaks and dips shown in Figure 1 are very localized; if you move even two inches away the response changes drastically. So while EQ can help a little to reduce mid and high frequency response deviations, it is inappropriate for the more damaging low frequency errors.

Egg cartons and non-acoustic foam: These do not work either. Egg cartons are far too thin to be useful, and non-acoustic packing foam lacks the porous "open-cell" structure needed to absorb sound waves.

Styrofoam panels: You've probably seen these lightweight inexpensive panels at home supply stores, about an inch thick and often pink. Unfortunately they have no useful acoustic properties.

IN SEARCH OF EXCELLENCE

I visit many audio newsgroups and web forums every day, and every day I see people who are dissatisfied with their mixes ask what gear they should buy next. I am convinced that the most important "gear" you can own is proper room acoustics. After all, who cares about 0.01% versus 0.001% distortion, 192 KHz sample rates, or jitter that's 120 dB below the music, when the frequency response in your mixing room varies by 30 dB or more? I hope this brief overview of acoustic treatment inspires you to treat your room. You will be astonished by how much your mixes improve.

 

Ethan Winer has been keen on acoustic treatment since he built his first bass trap in 1977. He now heads up RealTraps in New Milford, CT - visit him at www.realtraps.com. Special thanks to studio design guru Wes Lachot for generously sharing his expertise and advice.

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