Homeward Bound—The Move to the Small Studio

Jul 1, 2004 12:00 PM, By Bob Hodas

PART TWO: REFLECTIONS, SURFACE TREATMENT AND EQ

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[During the past 25 years, acoustical consultant Bob Hodas has worked in studios all over the world, from Sony Music Entertainment in Tokyo and Abbey Road in London, to Avatar and Electric Lady in New York City, Skywalker Sound in Northern California, and O'Henry, NRG Recording and The Mix Room in Los Angeles. These days, as many top producers and engineers are building their own facilities, Hodas finds himself increasingly dealing with the challenges posed by building professional studios at home. In our two-part series, he shares his tips for building — and tuning — the project studio. Last month, he discussed the unique isolation challenges in existing rooms and detailed the finer points of speaker placement. This month, he moves on to room reflections and EQ.
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As I mentioned last month, you can't just put up a pair of speakers in a room and expect everything to sound good. There are many variables affecting room response (and hence, the quality of your mix) if you're building a studio within the limitations of an existing space, so I'm suggesting options that are reasonable for the project studio designer. Last month, I went over the most costly issue: achieving ideal isolation within an exisiting structure. I also explained the significance of room dimensions, especially orientation and symmetry issues, and ways to determine ideal speaker placement.

This month, I'll explain room reflections, talking about easy ways to control those reflections with surface treatment, and tackle the difficult task of bass management.

DEEP REFLECTIONS

First-order reflections are the initial signal reflections that bounce off the walls, floor and ceiling and mix in with the direct speaker signal. In most home spaces, the reflections are short enough that your brain cannot differentiate them from the direct signal. These destructive reflections cause holes in the frequency response so that you miss parts of the music. They will also affect the phase response, creating problems in the soundstage and imaging, both side-to-side and front-to-back. This theory also applies to reflections off of the console if your speakers are sitting on the meter bridge or too close to the back of the desk. Your high end will be much smoother and more coherent if you put the speakers on stands and move them back from the console to a distance where the tweeter won't interact.

Perhaps you're working at a desk with a controller or keyboard rather than an actual console. The flat desk surface can be even worse for reflections than the raked surface of a console, so you should be especially aware of speaker placement. Also consider the positions of your computer monitors and how they interact with the speakers. If the monitors are right in between the speakers, for example, then you could get low-frequency loading into the surface of the monitor. You will also experience a loss of front-to-back depth imaging. Place the monitors below the speaker level and, if possible, build them into the desk and place them at an angle so that they are easy to view but don't interact with the speakers.

Because sound and light waves behave very similarly above 400 Hz, you can use a mirror and simple geometry to find unwanted reflections. Invest about $30 in a 2×2-foot plastic mirror without the frame. Have someone sit in the listening position while you hold the mirror absolutely flat against the sidewalls and ceiling. Slide the mirror all around to see if the listener can see the speaker components (not the side or top of the speakers) in the mirror. When the speaker is seen in the mirror, that is a first-order reflection point, so you will want to treat in that area. You will probably be able to outline one large area on the sidewalls that shows the reflection of both speakers. Be judicious with the absorptive treatment — there is nothing worse (to my ears) than an overly damped room. Just treat the areas that need it and leave the rest alone.

I like to absorb rather than diffuse the ceiling and side reflections, because absorbing them increases the coherence of the system. Coherence is very important for imaging. (See Figs. 1a and 1b.) Diffusing the rear wall reflections adds space to the room. The diffusion gets rid of the discrete reflections, but leaves most of the energy intact; the energy spreads out over time. Don't try to equalize first-order reflections because they are completely dependent on your position and will change at different seating positions throughout the room. EQ will not fix a high-frequency reflection problem.

BIG BOTTOM

Bass control is a tough subject. I see the same mistakes repeated over and over in small rooms, mostly because people make assumptions but don't take measurements. I really can't tell you how to trap a room without measuring it first. To do so would be irresponsible of me. So I'll start with a few concepts.

Just because you have corners doesn't mean that they are causing a problem. If you're working with a particular room dimension (an 8-foot ceiling height, for example), that doesn't mean you have a problem based on the relative wavelength. I have been in rooms where corners were trapped and the traps were causing more problems than they solved. Or else, clients bought traps that were sold as broadband but turned out to work on fairly specific frequencies with a high Q. Take it from me: In a small room, you'll need to find the problem with measurement tools and treat that problem surgically. It's a process that may require some degree of experimentation, so measure the room after you have treated it to ensure that you have gone about it all correctly.

I'm a real fan of membrane absorbers for smaller rooms because there is generally not enough room in a home studio to do large-scale trapping. These rooms also seem to have specific problems that require more targeted trapping. I would not install membrane absorbers without experimenting to see exactly where in the room they need to be installed and how many are needed. If you put a membrane absorber in the wrong place, it won't do anything because it will not be stimulated by sound waves — and that's a big waste of money.

Here's an example: A client's room had a 29dB hole at 71 Hz. So how could absorption possibly fix this problem? In this case, the hole was caused by excessive out-of-phase information arriving at the mix position. So if we can remove that information, the hole will fill in. The big question for this client was where to place the traps for the most effective results. Because the cancellation frequency corresponded to the room width, we tried placing the traps on the sidewalls centered at the mix position. This didn't give us results at all. Then we tried placing the traps in the front corners of the room behind the speakers. This was quite effective when we placed them as high in the corners as possible, and less effective when down by the floor. We also tried trapping the front wall/ceiling juncture with little result, but had good response in the rear corners when the traps were placed up high. Any wall/ceiling juncture we tried didn't work; in fact, noncorner placement generally yielded poor results. Trap placement needed to be very specific to be effective. We ended up with five RPG Corner Modex traps in each front corner and three in the rear, reducing the notch by about 20 dB. While we did not solve the entire problem acoustically (as EQ needed to be applied), we made significant improvement given the allowed budget. With more money and time, we could have filled in the hole completely by just using bass traps.

I cannot say that this trap will always be the perfect solution. I measured a room that had a large hole at 70 Hz and an 8-foot ceiling, which indicated an ideal situation. We found little or no change in this room utilizing the traps. The reason for this was that one wall was sucking out the problem frequency, so the wall needed to be reinforced to fix the problem. Test each individual situation and base your placement decisions on science — not rules of thumb. That's a good philosophy no matter what type of treatment you plan on using. As this is a complex acoustical problem, I can't give you a single solution. Remember that some bass issues can be solved just by placing the speakers in the proper spot.

SUBWOOFERS

A number of my clients have asked me if they should add subwoofers to their main system. There is often the need to hear deeper bass when working on small close- or mid-field monitors with limited low-end response. Also, we cannot always place the monitors in the best position or properly trap to get a smooth low-end response. In these cases, adding a sub allows you to place the main monitors without a great deal of concern for their low end. In small rooms, the proper mains placement may be bad for imaging or simply be unachievable due to room dimensions or lack of symmetry. A subwoofer, on the other hand, can be placed just where it needs to go.

I am a firm believer in stereo subwoofers. There's a common misconception that bass is omnidirectional and subwoofer room position is not important. Place your subwoofer off to one side and listen for its location. I'm sure you will be able to find it. If you use a mono subwoofer, then it's very important to place the subwoofer symmetrically between your speakers. Placing the subwoofer off to one side will cause an asymmetrical response in the left and right speakers at the crossover point based on the uneven distance of the left/right speakers to subwoofer. This will require equalization to balance the system.

If the room is quite small, however, you may get away with an asymmetrical sub placement due to the long wavelength at the crossover point.

If you are buying a sub to make up for poor low-end response in your mains, then there are several features to consider. Phase adjustment will allow you to dial in the phase crossing at the crossover point and help make up for placement problems. The downside is that this is really difficult to do by ear, so you will want to measure phase with an analyzer. The sub should have a lowpass frequency adjustment if you really want to dial it in to the maximum — although many subs are fixed at about 80 Hz and can still get the job done. Make sure the unit has both high- and lowpass frequency adjustments; in other words, true crossovers. It would be great if you could passively integrate the mains without putting them through a crossover, but most rooms have mains with a very erratic low-end response and you don't want to add that in with the sub.

Placing any subwoofer system requires a good analyzer and someone who knows how to use it. Subwoofer manufacturers often suggest that you use test tones to set up subs, a valid option but with crude results; personally, I have not seen a studio that set up subs properly without analysis. To get the best frequency response, you'll want to achieve a linear phase response at the crossover point. An analyzer with phase display is a must for this process. The process can be time-consuming and requires experimenting with multiple sub placements and phase switch adjustments. The best results can sometimes be achieved by raising the sub off the ground, turning it upside-down or even backward. Moving a subwoofer a mere six inches to a foot can make a significant difference. Unfortunately, if there is a rule of thumb, I haven't found it yet. (To see the results of improper and proper sub placement, see Figs. 2a, 2b, 3a and 3b.)

EQUALIZATION

The whole point of the above instruction is to get your studio into shape so that the music you make translates properly to the outside world. If you take your mixes to mastering and there is more than a dB of EQ being applied here or there, then you need to tune your studio. With the right room dimensions and proper speaker placement, there is no need for EQ. Ideally, you should do everything possible to fix your room acoustically because that will create a nice large sweet spot. But EQ can be a very cost-effective means of solving low-frequency room problems.

The proper tool to tune a room is a parametric equalizer because it allows you to dial in the exact center frequency to address your problem. Then it allows you to shape the curve to give a proper fit solution. A ⅓-octave equalizer is simply hit or miss on the center frequencies and employing brute force with its fixed Q, so that you'll wind up EQ'ing more than necessary or often won't fix the problem at all. Parametric solutions just make sense.

Figures 4a and 4b demonstrate this point: Figure 4a shows the low end of a room curve with a ⅓-octave solution applied to it. Notice that you can't quite get to the problem and wind up affecting more frequencies than necessary, especially around 150 Hz. Figure 4b shows the same room with a parametric solution. (Note: The EQ curve is an inverse of the EQ that is being applied to show how it fits into the room curve.) The parametric solution allows a better match. I also encourage you to tune your room with an analyzer that displays frequency in at least 1/12-octave resolution. Phase display is also very important.

I hope the above information has been helpful in transitioning your acoustics in a smaller room. Remember, simple solutions apply to these complex acoustics issues, so experiment!


For more of the author's practical tips for optimizing your personal studio listening environment, visit www.bobhodas.com.






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