Article by Jeff Hedback 

A great mix communicates a song directly to the listener's emotions.  The building blocks of a great mix (apart from great material, performances and capture) can be identified as separation, clarity, punch, image and depth.  These blocks are significantly affected by the time domain.  The time domain is one of the most important and least understood factors in the mix environment.  This article is going to show how your decisions when using compressors, limiters, reverbs, etc. are directly related to acoustical reflections in the time domain.

Let's begin in the comfort zone, the electronic/digital manipulation of the time domain. We know a flanger combines two identical signals with the second signal delayed no more than 10 milliseconds (ms).  A chorus is the same concept with the delay time ranging from about 10 ms to about 50 ms.  We also know that discrete echoes can be created by extending the delay time of the second signal by 50 s or greater.  You are also certainly familiar with variable attack and release times on compressors and limiters, yet another example of manipulating the time domain.

But, do you know what the time domain is acoustically?  The time domain simply refers to how sound (frequency) reacts over time (or distance).  I just snuck in the term distance.  Here is a rule of thumb to keep handy: it takes about 1 millisecond for sound to travel 1 foot.  This becomes more important as we look at your mix environment in the time domain.

Early reflections are sound reflections that arrive at the listener within about 30 ms.  Imagine a 34 foot section of string.  Stretch this string, in your mind, from a speaker out in any direction to a surface and return to your ears.  I bet you are surprised at how many of your room surfaces could promote early reflections.  The extra four feet accounts for a typical distance the direct sound travels from speaker to your ears.

Figure 1 arrival of reflection is approx 7 ms after the direct sound: (4.6'+6.6') – 4.4'= 6.8'

Assuming I have your attention…so what?  Early reflections cause comb filtering.  Comb filtering results in your mixes translating too wet, too dry, bad imaging, lack of separation, no depth and all kinds of similarly undesired things.

Now that we have identified the bad thing, how do we correct the bad thing?  We can design a better space to mix in, we can locate speakers and ears for optimal results in a space, and we can install acoustical treatments. But we cannot use EQ to address early reflections*.  The reason is that EQ happens before the speaker but the reflections are after the speaker.  The reflections are still there, they simply have a different frequency character than before the EQ.

*Note: there are significant advancements and offerings in active room correction DSP technology.  Many speaker manufacturers are now offering such devices with their products.  These can indeed be helpful, but do not negate room issues.  These systems can greatly improve the frequency response at a single mix position.  But, think about how many times your mix surroundings change with gear moving in and out, etc.  I suggest you consider these systems as another tool to be used along with acoustical materials and room design, Q-Tips, etc...back to the time domain.

There is a transition frequency range where sound reacts in a directional manner above and increasingly omnidirectional below.  The transition range is around 300 Hz.  Because music is dynamic and complex, this transition point can shift from between 200 Hz to 500 Hz.  Below this point, room modes (resonances) are the primary result of room issues.  Let's focus above the transition point and save the low stuff for a later day.

Deeper into the time domain:

• There is a quantifiable arrival time of the direct sound to your ears.
• There is a quantifiable arrival time of the first significant reflection to your ears.
• Then there is the reverberant field that covers up to approximately 250 ms in small rooms (after this the background noise level comes into play).

What is good and bad?

• We want to extend the arrival of the first reflection as long as possible.  At least 10 ms, but preferably greater than 20 ms.  We achieve this by working at The Hit Factory.  Right! Huh? Apart from it no longer being in existence, I doubt you have that budget.   However you can locate speakers properly (part of good room symmetry), locate the mix position properly, and acoustically treat problematic reflection points.
• There is a trick with a mirror that can help you identify the problematic points: you sit in your mix chair.  Next, have a friend hold a mirror and move this along the wall surfaces.  Wherever you see a reflection of a speaker, you have identified a likely reflection spot. 
• The ceiling is another area to treat.  You almost always need to treat the ceiling area halfway between your speakers and ears.  Beyond that, each room has its own factors.  The floor is rarely an issue.  However, the desk can certainly be a problem.  Simply placing your speakers on stands rather than the desk itself can reduce these reflections.
• Now that we have nudged the arrival of the first reflection point, we want to look at its amplitude.  Meaning: how loud is the first reflection in relation to the direct sound?  If they are too close in amplitude you can get a "smeary" flanged or chorused response.  This is rarely heard as dramatic as just described, but accuracy is none the less out of question.
• We want the amplitude of the first reflection to be at least 10 dB down from the direct signal.  The best rooms are greater than 15 dB down.   Example: you are mixing at 85 dB and the first reflection is coming back to you at 70 dB.
• Absorption panels are a primary way to reduce the amplitude of reflected energy.  The main concerns are first and second order reflections.  By the time a sound hits off the third boundary, its amplitude is typically not a problem.

Figure 2 shows absorption panels on right sidewall

• The off-axis polar pattern and frequency response of your speakers have impact here as well.  So get out your manuals.
• Now is a good time to mention the term NRC (Noise Reduction Coefficient) which describes the sound absorption properties of a material.  It is not a percentage but rather a helpful tool used to compare one material verses another.  In simple terms: the higher the NRC, the better the performance below 500 Hz will be.  Remembering that the transition range can vary from 200 Hz to 500 Hz, you should select higher NRC acoustical panels (above .9 NRC) if your room is exceptionally narrow or shallow.
• Diffusion…yes, diffusion can be used to tame early reflection issues.  It is a much more complicated task than using absorption materials.  The use of diffusion in the near field is wrought with danger and controversy.  For the purposes of this article, we will leave diffusion alone at this point.

Starting to piece this together???

The time domain is a battle of milliseconds.  Gaining control of early reflections even a few milliseconds and a few dB will produce more accurate mixes.  You can make confident decisions with attack and release times, set up "perfect" reverbs and nest a snare to speak through even the most complex tracks.  Look at your space using this information.  There is help available.  There are books on the topic, a great deal of information on-line, there are several manufactures who offer product application support and there are professional consultants who can assist.  Master your time domain and Happy Mixing!

Jeff Hedback is Chief Designer at Hedback Designed Acoustics an acoustical design and consulting firm specializing in small rooms.  www.HedbackDesignedAcoustics.com