So you’ve got yourself one of those FFT measurement systems and a shiny new microphone and you want to know where to put the mic!
This is one of the most asked questions about measurement systems I hear and the stock answer “it depends” isn’t very helpful. Let’s see if we can put people on the right track in obtaining useful, meaningful data. (How you interpret the data we’ll leave for another time).
The “it depends” answer really hinges on what you are trying to measure. Are we measuring the performance of the loudspeaker(s), or are we measuring how our loudspeaker(s) perform in a given space? It can be difficult to separate the two. Speaker designers/manufacturers are usually interested in the former and want the room to “go-away,” whereas end-users have to make the system “fit” or “play-nice” in the performance space.
Let’s assume for now you know how to set your measurement system up and get “wiggly-lines” to appear, now we need to decide where to put the microphone(s). If we accept every position is unique and all are important and the adjustments we make for one position will affect some or all of the other locations. We will never have the time to measure them all, and even if we could, what could we do about it? We will have to prioritize them practicing “audio-triage” to achieve a minimum variance throughout the coverage zone using our time efficiently.
Measurement microphone placement plays a critical part in the alignment process. While it is important to maintain good techniques for mic placement, do not give too much thought to finding the “perfect” position. Our goal is to provide us with global information so the system can be aligned for maximum consistency. Each microphone provides a local view of the system’s response with the global characteristics more or less hidden. The key to mic placement is to use positions that provide a global representation with a minimum of unique local conditions.
The primary position for any system or sub system would be on-axis (+/- 10%), about half way through coverage zone and not too far back the room reflections intrude and not too close you are into cabinet reflection issues. This position represents the “average” seat in the speaker system’s coverage area. This position is the reference point in terms of speaker positioning and will take first priority in terms of delay, level and EQ setting.
The coverage pattern can be verified by comparing side coverage areas to this position. When you have reached the –6 dB point you are at the edge of the pattern. If the pattern edges do not occur as intended, it is time to reposition the speaker.
This position should be average in level for the seating area. As you move closer, expect to see a rise in level. As you move farther expect to see a loss. This is a complex function of the axial attenuation and propagation loss, which hopefully will combine to create a minimal difference in level over depth. (See Fig. 1)
Guidelines for Measurement Mic Placement:
- Avoid positions with obvious unique local conditions, e.g., a pillar a few feet away.
- Avoid placing mics in aisles. They tend to have strong reflections from the open floor in front of them and are not representative of the area at large.
- Always point the mic toward the speaker source. Even “omnidirectional” microphones become directional above 5 kHz at angles of 90° or greater.
- Beware of exact center points in rooms since they will have unique reflection patterns.
- If at all possible avoid the offer to run your mic through the house patch bay to save running the cable. It almost never works.
- Do not place the mic at sitting head height. This position will have a strong local reflection off the next row. This is not representative of the response with audience members seated. Standing head height usually works better. When working in extreme proximity, however, such as frontfills, the sitting height may work better because the standing height will be out of the vertical pattern. (See Fig. 2)
Once the speaker position is determined, the primary mic position serves as the relative level marker for this subsystem with respect to others. If all subsystems are set to create this same relative level, then maximum consistency will be achieved throughout the hall. This will be complicated, however, by the interaction of the subsystems, which tend to cause some additions.
The primary position offers a good representation for delay alignment. For most situations the best result will be achieved when the primary mic position is used as the synchronous point of alignment.
The primary position provides a first look. However, it is highly recommended additional positions be looked at and factored in to the final decision on the EQ setting.
The Mix Position
You might notice that no mention has been made of placing a mic at the mix position. This is because the mix position is, after all, just another seat. It is best if the mix position is a primary location. However, if it is not, it will not help to pretend otherwise. In fact, aligning a system to a poor position will create a worse effect for audience and mixer alike. There are many reasons why a mix position may not be suitable for alignment (or mixing for that matter), such as being off-axis, at the back wall, under the balcony, or all of the above. While it is true that the mixer’s reference point is critical, it is futile and destructive to align the system for a bad mix position.
Secondary Mic Positions
No matter how well placed the primary mic is, it is still only a single point in the room. I cannot say enough about the benefits of analyzing additional mic positions— or about the potential dangers of basing your entire alignment on a single position. Every position has unique local response characteristics in addition to more global ones. Extreme peaks and dips can be found at one position and disappear a few seats later. Moving the mic or using multiple mics is a form of insurance against making decisions that will not create global solutions. I can attest to many instances where a problem appeared to be solved at one position, only to be revealed later that the “solution” had merely repositioned the problem a few seats away.
Secondary mic positions provide secondary opinions on the data. The global aspects of the system become readily recognizable when multiple placements are compared. These are the major tendencies of the system, the ones that will be the keys to getting the system under control. Secondary positions are found within the coverage area of the speaker but away from primary position.
• Can be placed over a wide range within the speaker’s intended coverage area.
• Provide additional local information to help ascertain the global parameters of the speaker system response.
The unfortunate side effect of taking secondary measurements is the presence of blatantly contradictory data. The simple technique of overlaying the inverted EQ curve over the room + speaker response becomes complicated in the face of these discrepancies. Typically, the equalizer settings are appropriate in some frequency ranges and contradictory in others, indicating the corrections for one measurement position hinders another.
In most cases the new positions will show normal variations in frequency response due to the interaction of the speakers and the room, they may also turn up unexpected results indicating coverage gaps or excess overlap. The secondary data can be compared to the primary. Major trends will emerge where the responses match. Decisions must be made about those areas where they differ.
At first glance it might appear that taking the two different samples and creating an average would make a suitable “average” response. But unfortunately, it doesn’t work that way (unless you use “coherence-averaging”). A 20 dB peak and a 20 dB dip of a one-tenth octave bandwidth average out to 0 dB but we hear the 20 dB peak as a massive coloration, while the 20 dB dip is only marginally perceived. If you leave the 20 dB peak in the system you will soon be looking for employment.
In some cases a similar peak in the response may appear in both positions but differ in amplitude or bandwidth. In such cases, an average between them is applicable.
Tertiary Microphone Positions
Tertiary positions are a third class of placements used to verify various aspects of the speaker, such as proper wiring, gain structure and position. The data from these positions is not typically used to make level, delay, or equalization decisions.
Tertiary Mic Position Sample Applications
• Coverage angle verification: The mic is placed at the expected axial edge.
• Seam analysis: The mic is placed at the transition zone between systems to verify the coverage has no gaps.
• Sound leakage onto the stage: The mic is placed on stage after the system is aligned to observe the nature of the leakage.
• Analysis of a particular seat your client is very concerned about, such as for a critic.
Common Misconceptions about Multiple Microphones
When multiple measurement microphones are mentioned there are a few misconceptions that tend to arise. The most common is we will sum the microphones electrically to produce an average response. This has no validity whatsoever due to the comb filtering resulting from the summation of the signals with their different propagation delays. Such an idea could only work in a world without phase. Another misconception is we multiplex the mics, switching from one to another in rapid succession. This is a vestige of real-time analysis, and again totally neglects phase.
A third misconception is the idea of “spatial averaging,” where the mic is moved around while measurements are in progress. This may be useful for noise analysis but not for the alignment of a speaker system.
What does work in multiple microphones is “coherence-averaging” where only the good, or high coherence data is summed and poor or bad data is rejected.
A final point regarding multiple mic positions is they are not a random sampling as might be performed to check the chlorine content of a swimming pool. Each mic position is carefully chosen to give data about a particular speaker system, so decisions can be made about that speaker’s position, level, delay and equalization. A random sampling may provide interesting data but this is an alignment process, not a survey.
Ground Plane Measurements
A final tip: Placing a microphone on a stand introduces reflections from a boundary – in this case the floor. This will introduce a comb filter as the late arriving reflection is summed at the microphone. To get a clearer look at the response, you can place the mic on the floor (See Fig. 3)
Another solution is to place a surface on the seats. (See Fig. 4)
About Martyn “Ferritt” Rowe
Industry veteran and OSA’s Director of Engineering Services Martyn “Ferrit” Rowe brings nearly three decades of real-world experience in live event technical services. Ferrit most recently came from Martin-Audio as the technical training manager for MLA, and uses his vast knowledge and expertise of the multi-cellular technology to support client projects as well as support and train engineers and technicians.
Ferrit began his career running cables on a Thin Lizzy “Live and Dangerous Tour,” and then taking on the roles of running monitors, front of house, and system technician for some of the most popular acts in music, such Judas Priest, Ozzy Osborne, Black Sabbath, The Police, KISS, The Who, Elton John, Poison, Bon Jovi and Van Halen.
Ferrit’s training career began in 2000 when the first of the line array’s from V-DOSC emerged and became an instructor on the line array theory and continued that path with various systems over the years before joining the Martin-Audio MLA division, and then bringing his knowledge and expertise to OSA International, Inc.