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:
What is good and bad?
Figure 2 shows absorption panels on right sidewall
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