Once upon a time on these very pages, we discussed the procedure and benefits of using Real Time Analysis (RTA) as a tool for revealing problems in a live sound system. RTA or spectrum analysis can help expose room anomalies, problems with a P.A. or to help ring out monitor mixes (“Interpreting RTA Measurement Techniques,” March 2013, and “Ten Tips for Better Monitor Mixes,” Jan. 2014).
After such problems have been identified, we can address them by modifying the P.A. system or possibly through use of graphic or parametric EQ’s. This is all very scientific, but as we’ve mentioned before, achieving flat response via spectrum analysis doesn’t necessarily equate to good sound. So why bother with RTA? Among its many other benefits, running RTA while mixing a show can provide an objective window into your mix, revealing sonic characteristics of which you may not be aware.
Let’s say that you want to run RTA at the front-of-house mix position during your sound check and show. It’s probably a good idea to use some sort of measurement microphone, as opposed to using the microphone built into your phone, iPad or laptop, or a standard vocal or instrument mic. You may be surprised to learn that some very good measurement mics can be had for under a hundred bucks. The PreSonus PRM1 and Dayton Audio EMM-6 come to mind. Those of you tethered to your iOS device can check out the tools available from Studio Six Digital (studiosixdigital.com). These mics, partnered with RTA software, can raise interesting questions regarding your mix. However, it’s up to you to provide the answers.
A basic issue that we all encounter at some point or another is feedback. One of the things I like to do at sound check (particularly in small venues) is to crank up the level of the lead vocal microphone to ensure there won’t be feedback between the house P.A. system and the vocal mic. Some P.A. systems can radiate a fair amount of energy from 300 to 500 Hz from the back of the cabinets toward the stage. An RTA will help you identify specifically which frequencies are the prime offenders, at which point you can decide what type of EQ (graphic, parametric, notch) would most effectively alleviate the problem.
Running pink noise through the P.A. system while viewing the results on an analyzer helps you ascertain that all regions of the P.A. system are active (subs, low-mids, etc), and helps identify acoustic issues at your mix position that may or may not be fix-able. Notice that I did not say we should EQ the room until we see flat response on the spectrum analyzer — and many engineers will tell you that if you do EQ for flat response, the high end of the P.A will take your head off faster than Oddjob’s hat (if you have no idea what I am referring to, visit the classic Bond flick Goldfinger). Observing an RTA while you are mixing or even while playing a recording (.WAV files please, no MP3s or similarly compressed, crappy file formats) can help indicate if the energy in your mix is distributed across the frequency spectrum, but keep in mind that high-frequency content typically rolls off gently even when a mix sounds balanced (see Fig. 1).
Fighting Frequencies
Sometimes an RTA can help you fight the Battle of the Bulge. By this I am referring to the abundance of energy in the range between 200 and 400 Hz (we’ll get to the low end in a minute). Many instruments have their fundamentals — and therefore their strongest output in this region. As a result, they compete with each other for space in the mix. Listening to different channels in the P.A. (kick, bass, synths) while watching an RTA can help identify the instruments that are clashing. Using EQ to cut some of the overlapping bands may make the pieces of your musical puzzle fit together more coherently. Ditto for the kick drum and bass, which always seem to be fighting for space. If you see that both the kick drum and the bass have huge peaks at 120 Hz, you might think about cutting one of them at that frequency to make room for the other.
It’s important to correlate this information with your previous view of the RTA when the pink noise was played through the system (most RTAs will allow you to store a screen shot). An inconsistent average curve in the low-end often means that there are problems related to venue acoustics. For example, suppose you have excessive energy in your mix at 160 Hz. Does this excess appear when pink noise is played through the system? If yes, then you’re hearing either a room anomaly or a misalignment of the P.A. If it’s a room anomaly, you’re probably screwed, but at the very least, try moving the measurement mic (as well as your ears) a few feet away from the mix position to see if the response of the analysis changes.
This might be the result of the mic being located at a node or antinode. A node is an area in a room where a standing wave combines with its reflection destructively, reducing the level of a particular frequency. An antinode is an area in a room where a standing wave combines with its reflection constructively, increasing the level of a particular frequency. Nodes and antinodes are a product of room geometry, so there’s little you can do about them (even with EQ), but if you’re aware of them, you can avoid the rookie mistake of applying huge boosts or cuts in EQ that you can’t hear at the mix position anyway, and that might make people in the first few rows sick to their stomachs.
The Inside View
Another useful bit of information that an RTA can reveal is related to your hearing. If you notice your mixes consistently spike in a particular region, it may be indicative of a notch in your hearing. Sad to say, I have seen this far too many times: engineers who have abused their ears and have experienced hearing damage crank the EQ at certain bands to compensate for sounds that their ears can no longer capture. It doesn’t work.
Steve “Woody” La Cerra is the tour manager and front of house engineer for Blue Öyster Cult.
