Why Acoustic Reflections Matter

The early days of recording were all about making live mono recordings of acoustic bands — a bunch of mics wired direct to tape. Next came the multitrack, and recording evolved into click tracks, isobooths, post-processing, and mixdown sessions. Then the digital age showed up. Acoustic recording was out, sampling and DI was in, and everybody’s cousin had a home studio.

Times keep changing, however. Home studios have evolved into more sophisticated studios, and the bands themselves have evolved. These days, there are more mic companies than ever because studios are doing more miking and less going direct. Studios and bands want to do live, ensemble recording. Even rapping is going in this direction. Today’s studios are actually recording more real music, air breathing acoustic sound, than ever before.
But some things never change. Open up a mic, and we get two kinds of sound. The first is the sound we want to get: the direct signal from the talent. The second is the sound we don’t want to get — the sound from the room. We usually end up needing a cleaner signal at the mic and so our goal, as recording engineers, is to figure out ways to boost the direct signal and cut the room signal.
By following in the footsteps of the last few decades of recording, we try to get an acoustically dry signal, as close to an acoustic DI as possible and then perform the familiar post processing on it to get it into the mix. To do this, we have to build acoustically dead spaces and in the process, kill all the reflections.
But in so doing, we are also throwing the baby out with the bath water. Who’d have guessed that some of those hated, hunted, and hammered room reflections actually help make real sound, sound real? Recording used to be full of this type of “real sound” and today, by implementing a few acoustic tricks, recording can once again sound real.


In any room, the mic acts like a two track acoustic premix, as the direct signal is mixed acoustically in with the room signature: reflections, echoes, reverb, and general room noise. And most of the time, the room signature track is too loud. To boost the signal to noise ratio, we need to boost either the direct signal or fade the room noise; usually it’s some combination of both. What we want is an “acoustic fader” but air faders, like air guitars, don’t do much for sound.
We boost the direct by getting the talent to eat the mic and reduce the mic gain back down to zero VU. But now our talent sounds like a radio DJ and that just might not be the sound effect the producer wants. In addition, we lose control on dynamics, plosive, and proximity effects. To regain control we add the wind ball, dial in EQ, compression, and limiters, and hope for the best.
Another way to increase the direct to room signal strength ratio is to change the mic pattern (or position). Start closing it down and narrow the focus pattern of the mic, stopping somewhere between cardiod and shotgun. But the tighter the pattern, the more colored the voice, and we go back to EQ, compression, and limiters to try to doctor the track into a semi-real sound.
The other way to get a better SNR at the mic is to just dump the room and get pure sound flowing into the mic. Forget EQ, compression, and limiters; just set the mic up in a soundproof anechoic chamber, and one would think we have the ultimate recording space — essentially it’s acoustic DI, all direct signal with a –80 dB noise floor. Later, this very dry signal can be revived by post processing, and adding some warmth and depth with a little delay reverb, as well as some sparkle with a spank from an exciter.


When working in dry rooms, any reflection is audible and likely sounds bad. All it takes is one reflection, and the sound we are trying to get picks up a hollow tonality called the Comb Filter effect. This is when the desired signal is combined with a lower level and time delayed signal — in other words, an early reflection. The combination imposes a harmonic set of cancels and adds onto the original signal spectrum, which sounds like the direct signal was recorded at the bottom of a drinking glass. Dry acoustic recording is very sensitive to the presence of early reflections (comb filter effect), late reflections (echo), fast repeating reflections (flutter echo), boundary loading, mode coupling, and finally, reverberation. Still, dry recording seems to be the primary tool for today’s recording engineer.
The rule of thumb in a dry recording studio is “the best room is a dead room.” Engineers are trained in engineering schools and the school of hard knocks to hate reflections. Today, “reflecto-phobia” is rampant. Music is proudly recorded in acoustically sterile environments. Fueled by fears of comb filter coloration, every single reflection, near or far, that might ever hit a mic has been systematically exterminated over the last 30 years. Many of today’s so-called “live rooms” are “reflection-free zones.”


When people, in contrast to microphones, listen to sound, they generally just listen to what they want to hear and pretty much dial out the rest. People can be located pretty far from the talent, compared to a mic, and not even notice the room sound; they just hear the talent. People are able to tune the room out naturally, and focus in on the talent. The engineer with a mic has to work hard to tune the room out and focus in on the talent.
A person is a biological signal processor, not an electronic one. We use a different mechanism to hear than what is built into mics. A by-product of our hearing system is that we automatically mix all early reflections right into the direct signal and end up hearing one composite “direct” sound. Early reflections are those that arrive within about 1/30 second following the direct signal. It doesn’t matter where those early reflections come from, they just add together (correlation signal detection) in a way that makes the perceived sound significantly louder than the direct signal. This sound fusion process creates a composite direct signal, which has easily more than twice the sound power than the direct signal alone.
Although it doesn’t matter to the sound fusion process where the early reflections come from, we aren’t confused by where the direct sound comes from because of something called the precedence effect. We cue in on the direction of where a sound comes from by tracking and locking on where the original sound signal comes from. The process of knowing where a sound comes from is called echolocation.
There is one adjustment to echolocation that we need to mention: the Haas effect. Very early reflections (those arriving within 5ms of the direct signal) will distract us from knowing exactly where the direct signal is coming from. The perceived direction of the direct signal is somewhere between the location of the direct signal and the location of the very early reflection.
People like early reflections. In fact we’re designed to hear direct and early reflections, and to mix them together into one “direct” sound. This process helps us hear more easily what is going on.


In the 1950s, they had one, maybe two takes and then the session was over. The idea was to use a number of mics distributed throughout the group, adjust their position and gain and get a live, hard-wired mix down direct to tape on a mono track. Their goal was to capture enough signal to recreate the sound that was heard when sitting in the room. Those days are far from the idea of recording separate tracks in isobooths at various times and in various parts of the country, then mixing them together a few months later.
A good example of the tail end of the early days recording technique was in the RCA Victor Studio B in Nashville back in the ’50s and early ’60s. This topic came up during an AES Sectional presentation on the Quick Sound Field (QSF) recording technique, held there in 2003 (for more information on QSF, visit www.asc-studio-acoustics.com/qsf.htm). QSF is a modern way to acoustically capture sound fusion at a mic; see Figure 1. Studio B had finally been renovated but it wasn’t open to the public yet; the room was full of engineers — a lot of new ones who hadn’t even been in the studio since it closed, and some old-timers who worked there when they were young. After the QSF presentation was over, the question and discussion time quickly led back to the recording techniques that used to go on in that room. Studio B is a shrine. It’s enough to just stand there, inside that room and wonder upon all those hallowed vibrations — the ones that hit the floor tiles and bounced off, and those that lie buried still in the wall and ceiling tiles. So many early greats worked and played there: Elvis and the Jordanairs, Roy Orbison, Everly Brothers, Chet Atkins, and many more recorded in this old RCA Studio B.
The QSF lecture reminded the old-timers about recording in this room. They talked about the mic setups and how the band played all together, at one time, one song from start to finish, direct to tape. And that was how they made records.
This was all well before multitracking and mixing capability became available in recording. When multitracking came, in the ’70s, Studio B accommodated the growing interest in this “new sound” of music. The room was deadened and hosted a small village of iso sound shacks lining the walls. Eventually Nashville was overrun with recording studios, and Studio B closed. Now it has been renovated back to the glory of its former years. All the sound shacks are gone now, and the room has been returned to its original, one big recording room, configuration.
Back in the early days, the room had a 3-mic, gain and mix to Ampex tape approach. Later, more mics were added. There were no isobooths. At best, there were gobos. In this environment, each mic got signal from every instrument. For example, if there were 12 mics and six talent sources, there would be at least one direct signal from each talent source arriving at each mic. That means that there were at least 12 different signal path versions of each talent source after mixdown. And then the early reflections have to be added in; floor bounce, glass bounce, other instruments and what not.
The net result after mono mixdown would be that each talent source would have at least 12 direct signals, with time delays ranging from 4ms out to 25ms, and levels ranging from 0VU down to –16dB on the track. And then there would be the reflections, filling in the mix with even more random time offset signals. In a 12-mic setup there would actually be captured up to 30 or 40 distinct time delayed signal paths for each talent source. That qualifies as a “Sound Fusion” effect recording.


During this early period, the ASC TubeTrap factory got a few calls from engineers who heard about the QSF sound (Figure 2). One had been doing a radio for many years. He said he developed a magic black box that was his trade secret: It consisted of a whole bunch of amplitude adjusted time delays inside the box. He fed his mic into one end, and got a synthetic QSF sound (direct + a whole lot of random time offset signals) out the other end. The time delays matched exactly the QSF window of about 25ms.
Another engineer contacted the factory and told his story how he had hooked 30 some mics up over the top of a classic opera singer. Each mic was located at a different distance and angle from the talent. He just added them all together and ran it out to the house sound system. He said the sound was fantastic and used the technique many times. He effectively collected some 30 random-time off set signals, all within the 25ms time window. Each signal was basically the same signal except for the acoustic EQ due to the off-axis coloration of the voice. And, as the talent moved around, the sound package didn’t change; the total sound remained the same, even though the signal fed into the different mics did change. The listener’s brain can’t tell which reflection is where inside the Sound Fusion effect time window.
At that time, digital reverb was starting to become affordable. The reverb plate was being replaced with a four adjustable delay/reverb returns. When ambience was set tight (300 to 500ms) and the delays set shorter (30 to 100ms), it produced a synthetic ambience, much like a room. By setting it even tighter and shorter, the Sound Fusion effect could be generated. But the big advantage with the acoustic version, the QSF, is that it controls the presence of natural ambience in the room at the mic while adding close and natural flush of early reflections into the acoustic mix at the mic position.


Although there are many ways to create early reflections electronically, probably the easiest approach is an acoustical one built around ASC’s StudioTraps. These are tubular structures where the front half is reflective in the treble range, and the rear half is absorptive in the treble range. The entire surface of the trap is bass range absorptive. Originally, they were set up to absorb treble and create a sort of “instant vocal booth,” where room noise was reduced, so it was possible to raise the mic gain. But as engineers experimented with it, they found that pointing the reflective surface of at least eight StudioTraps toward the mic could create the effect of early reflections that decay rapidly.
In a smaller home studio, a typical setup is to set a tight QSF pattern. The smaller the room, the tighter the pattern, and the more intense the Haas reflections, which boost the live effect and at the same time, blocks the room even more. Typical small room recording does very well with only eight StudioTraps in a semicircle setup, 4 to 5 feet in diameter.
While working in QSF, the talent is free to groove to the music without causing a shift in the sound at the mic (Figure 3). As the talent moves, all that’s changing is the arrival time of the various Haas reflections. However, the ensemble package of direct + early reflections remains at the same level and sounds the same.
In any event, regardless of how you add early reflections to your sound, I hope this article has gotten you thinking about why they’re important and useful. Make them work for you, instead of against you, and you’ll be rewarded with a more live, animated sound.