Recording - October 2013 - 58

Peter D’Antonio
Yes, as long as it’s broadband absorption, so you don’t introduce
the filtering issues we just discussed.
Okay, so we’ve dealt with the lows, the mids and the highs.
What’s next?
You’re done! [laughs]

What About Subwoofers?
Before we wrap up, can we talk a little about subwoofers?
People will say, “I’m not hearing enough lows, so I’ll add a sub,”
and then put a sub anywhere in the room where they won’t trip
over it, and call it done. But it’s a bit more complex than that,
isn’t it? If we’re using a sub, how do we keep from hurting ourselves more than we’re helping?
Most people will put a sub right in the corner because the efficiency is
the highest. And there’s nothing wrong with that. But one thing we noticed
early on, in an analysis that was continued by Todd Welti and Floyd Toole,
was the benefits of using multiple in-phase subs, whether it be two or four.
If you put one sub 1/4 of the room’s length from the front wall and 1/4
of the room’s width from the side wall, and do the same symmetrically on
the other side of the room, and they’re in phase, what happens at those
particular distances is that the second harmonic mode is at a null. So you
will not excite the second harmonic with those subs. And if you look at a
diagram of pressure vs. distance from the boundary, at those positions the
positive and negative parts of the pressure cancel for the first and third
harmonics. So those two subs cancel the first harmonic mode, don’t excite
the second, and cancel the third. There’s not a lot left, aside from perhaps
a ceiling mode you can’t do much about. (See Figure 5.)
So that allows you to move from left to right across the console
and not have to worry about how those modes change your perceived sound depending on where you’re located. It gives you a lot
of freedom at the console, a wider sweet spot for the low end.

Figure 4. Dr. D’Antonio’s early drawings of Reflection-Free Zones.
In the drawings on the left and center (top row) and left (bottom
row), you can see how sound from speakers bounces off walls,
ceiling, and console to interfere with direct sound where the listener sits. In the drawings on the right (top and bottom), the
room is shaped so that indirect sound goes right past the listener to the back of the room, where it’s hopefully handled by diffusors to add a sense of natural ambience.

RPG employs scientists in many disciplines, including mathematics and acoustics, and we relish the opportunity to
design innovative projects for the future—from our initial RPG
Diffusor to modern designs like our Waveform shape-optimized curvilinear diffusors, Topakustik microperforated
absorptive wood panels (with 30,000 holes per square foot),
the Quietstone absorbers made from recycled glass and
stone, and the Lumaphon LED light and sound diffusor that
provides dynamic lighting for classrooms.

So: one sub maybe good maybe bad, two subs good?
And four subs even better! [laughs] The analysis shows that having a second pair situated across from each other across the back
wall improves things as well over the whole room. But that’s often
impractical; Todd Welti’s analysis went on to show that if these 1/4
distances weren’t feasible, you got lesser but still good advantage if
the two or four subs were placed directly in the corners, or against
the walls halfway along the length and width of the room.

Final Thoughts
So that’s our survey of the four problems in room acoustics, and
their solutions, and how some of them can be fixed with treatment products and some cannot...
End users can go out and buy the latest room treatment packages
advertised in the trade magazines, and that’s fine, they serve a very
useful purpose. But there’s been very little innovation beyond what
we introduced decades ago. The current packages all attempt to
provide solutions for these four basic causes of acoustic distortion.
Rooms have come a long way; you can have a project studio now
that can not only compete with the electronics of a major studio, but
it can also compete with the acoustics of a major studio.
When it comes to project studios, remember that placement is
free, and a lot of what you can do, you can do before you buy anything. So when you do buy something, you’re buying it to solve a
specific problem that you have isolated and understood.
Any other resources you’d care to share?
Having identified and solved the acoustical issues of the small project studio many years ago, RPG has decided to focus on a businessto-business recording studio model; we collaborate with professional studio designers and commercial architecture projects rather than
the direct-to-project-studio market.
58

RECORDING October 2013

Figure 5. Multiple subwoofers in a room. All of these arrangements
give good results; the best (theoretically) would be the arrangement on the far right, which is impractical in many real rooms.

Since we no longer offer the Room Sizer and Room Optimizer
programs, there is a website that incorporates my research at
www.acoustic.ua/forms/rr.en.html that your readers might want
to check out. There are also Technical Presentations, which are
heavy on math in some places but have diagrams that illustrate a
lot of these points, available for free download at rpginc.com.
One of the slides from one of the Technical Presentations
described an interesting “thought experiment”, where you
would secretly insert a plug-in that has multiple eq peaks
and short delays, and then expect an engineer to get his
best mix with it. Then you would switch off the plug-in and
surprise the engineer! The point of the exercise is that this is
exactly what your room is doing to your mixes, all the time,
and when you leave the room, you “turn off” the plug-in!
Exactly. If you can’t take the room out of the mix, you
can’t take the mix out of the room!

http://www.acoustic.ua/forms/rr.en.html http://www.rpginc.com

Recording - October 2013

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