🎚️ Issue #7: The Glossary You Need to Survive (and Thrive) -- Part 3

THIS WEEK’S SETLIST

What You’ll Learn Today

• Stop Feedback at the Source: How to use smart microphone placement ('Null Point') and disciplined muting ('NOM') to prevent squeals before you even have to reach for an EQ knob.

• Fix Feedback with Precision: Learn how to create a surgical 'Notch Filter' to eliminate a stubborn feedback ring without thinning out the rest of the voice or instrument.

• Unlock a Clearer, Fuller Sound: Understand the secret 'why' (things like 'Path Length Differential') that can cause a hollow or weak sound, helping you make better decisions on stage to improve your mix.

Just for today… (more coming next week)

31. NOM (Number of Open Microphones)

32. Notch Filter

33. Null Point (of a Microphone)

34. PA Summation (Mains, Subs, Front-fills)

35. Path Length Differential (Δx)

👋 WELCOME

BUILT FOR VOLUNTEERS IN THE BOOTH

Every week in SundayMix, you’ll get no-fluff FOH tactics to help you run tighter, cleaner, more confident worship mixes — even when service starts in five and chaos is in the air.

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31. NOM (Number of Open Microphones)

• In Plain English and How to Use It: This simply means how many microphones are currently turned on (unmuted) and active in your sound system.
  

  • How to Use It: The more mics you have open, the more likely you are to have feedback and a messy, unclear sound because each open mic picks up all the room sound and bleed from other instruments. Try to only have mics open for the people or instruments that are actively making sound. Mute mics that aren't being used (e.g., the guest speaker's mic during worship, or a guitar mic when it's not being played).

• Advanced but Still Usable: The Number of Open Microphones (NOM) directly impacts the overall system gain and its susceptibility to feedback. For every doubling of NOM, the potential acoustic gain (PAG) of the system decreases by approximately 3 dB, assuming all mics contribute equally to picking up ambient sound and bleed. This means the overall system level must be reduced by 3 dB to maintain the same feedback stability margin. Minimizing NOM by muting unused microphones is a fundamental practice for maximizing gain-before-feedback, reducing overall stage noise/bleed in the mix, and improving clarity. Automated mic mixers are sometimes used in speech applications to manage NOM dynamically.

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32. Notch Filter

• In Plain English and How to Use It: A notch filter is a very precise type of EQ cut. It "notches out" or removes a very narrow range of sound frequencies.
  

  • How to Use It: This is most often used to surgically remove a specific feedback frequency. If you have one particular squeal that keeps happening, you can use a parametric EQ with a very high "Q" (narrow bandwidth) to create a notch filter right at that squealing frequency, cutting it significantly without affecting the sounds around it too much.

• Advanced but Still Usable: A notch filter is an EQ filter that applies a deep attenuation to a very narrow band of frequencies, while having minimal effect on frequencies outside this band. This is achieved using a parametric EQ with a high Q factor (typically Q > 8-10, sometimes much higher). Notch filters are primarily used for feedback suppression, targeting specific resonant frequencies that are causing oscillation, or for removing specific unwanted tones like hum or a persistent instrument resonance. The goal is to be surgical, addressing the problem frequency with minimal impact on the overall desired program material.

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33. Null Point (of a Microphone)

• In Plain English and How to Use It: Directional microphones (like cardioids, which are common for vocals) are designed to pick up sound best from the front and reject sound from other directions. The "null point" is the direction where the mic picks up the least sound. For a typical cardioid mic, this is directly at its rear (180 degrees off-axis).
  

  • How to Use It: To reduce feedback and bleed, try to aim the null point of your microphone towards the sound source you don't want to pick up. For example, the back of a vocalist's cardioid mic should point towards their floor monitor speaker. For a hypercardioid mic, the deepest nulls are off to the sides/rear (e.g., around 110-126 degrees).

• Advanced but Still Usable: The null point(s) of a directional microphone's polar pattern represent the angle(s) of incidence where the microphone exhibits maximum rejection of sound.
  

  • Cardioid: Deepest null is at 180° off-axis (directly to the rear).

  • Supercardioid: Deepest nulls typically around ±126° off-axis, with a smaller lobe of sensitivity at 180°.

  • Hypercardioid: Deepest nulls typically around ±110° off-axis, with a more pronounced rear lobe than supercardioids. Strategically orienting a microphone so that its null point faces a prominent unwanted sound source (e.g., a stage monitor, a nearby loud instrument) is a key technique for minimizing bleed and maximizing gain-before-feedback. Understanding the specific polar pattern and its nulls is crucial for effective mic placement.

34. PA Summation (Mains, Subs, Front-fills)

• In Plain English and How to Use It: This refers to how the sound from all your different types of main PA speakers (main left/right arrays, subwoofers for bass, front-fill speakers for people close to the stage, delay speakers for the back of the room) all add up and work together in the room. If they aren't properly aligned in terms of timing and level, they can fight each other and create weird dead spots or boomy spots.
  

  • How to Use It: This is advanced system setup. The key is that all these speaker components need to be timed (using delays) so their sound arrives at listeners' ears coherently, especially where their coverage overlaps. If you notice areas in the room where the sound is very uneven, especially in the bass or low-mid region, it could be a PA summation issue that needs an experienced technician to look at with measurement tools.

• Advanced but Still Usable: PA summation refers to the acoustic interaction and combination of sound waves from multiple loudspeaker elements within a sound reinforcement system (e.g., mains, subwoofers, front-fills, out-fills, delay speakers) in the listening area. For optimal summation, especially at crossover frequencies or in overlap zones, these elements must be aligned in terms of:
  

  • Time/Phase: Achieved through precise delay settings (often using DSP) to ensure coherent wavefront arrival. Misalignment causes comb filtering.

  • Level: Ensuring appropriate relative levels between elements for balanced coverage.

  • Polarity: Ensuring drivers are wired with correct polarity. Vector summation of complex acoustic pressures (considering both magnitude and phase) determines the resulting SPL at any given point. Measurement tools (e.g., dual-channel FFT analyzers) are used to verify alignment and optimize summation for smooth frequency response and consistent coverage.

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35. Path Length Differential (Δx)

• In Plain English and How to Use It: This is simply the difference in distance that sound travels from a source to two different points (like two microphones, or from a speaker to a mic vs. a direct sound to a mic). If one path is longer than the other, the sound arrives a little later through that longer path.
  

  • How to Use It: Understanding this helps you understand phase cancellation. If two mics are picking up the same singer, but one is 1 foot away and the other is 3 feet away, there's a path length difference of 2 feet. This difference causes a time delay, which leads to some frequencies cancelling out when the mic signals are mixed. You fix it by trying to make path lengths similar, using fewer mics, or sometimes with electronic delays.

• Advanced but Still Usable: Path Length Differential (Δx) is the difference in the distance sound travels from a source to two distinct reception points (e.g., two microphones) or from two sources to a single reception point. This difference in path length results in a time-of-arrival difference (Δt=Δx/c, where c is the speed of sound). This time difference, in turn, creates a frequency-dependent phase difference (Δϕ=(2πΔx)/λ=(2πfΔx)/c, where λ is wavelength and f is frequency) between the two signals. When these signals are combined, this phase difference can lead to constructive or destructive interference (comb filtering), impacting the tonal balance and spatial characteristics of the sound.

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SUNDAYMIX ACTION STEPS

3 THINGS TO IMPLEMENT THIS NEWSLETTER TODAY

1. Build Your Next-Level Feedback Vocabulary.

• Action: From this week's list, let's focus on 3 critical terms for controlling feedback: NOM (Number of Open Microphones) (#31), Notch Filter (#32), and Null Point (of a Microphone) (#33). Create your flashcards for these. On the back, write a simple "what it is" and one practical tip you can use this Sunday. For example: "For Null Point, my tip is: 'Point the back end of the pastor's mic directly at the floor monitor'."

• Why: If Gain is your first line of defense against feedback, these three terms are your elite special forces. They give you specific, targeted strategies to prevent feedback before it starts and kill it surgically when it appears, often without having to make big, tone-killing changes to your EQ.

2. Hear It, See It, Fix It - Stage & Console Connection.

• Action: Next time you are setting up, think in terms of these concepts:

  • A. The NOM Test: Before anyone arrives, turn up your main speakers to a normal level. With all your vocal mics unmuted, listen to the ambient hiss and how sensitive the system feels. Now, mute all but one of them. Notice how the background noise drops significantly. You just lowered the NOM (#31). During the service, make it a habit to mute mics that aren't being actively used.

  • B. The Null Point Check: Look at the lead vocalist's microphone and their floor monitor. Is the monitor speaker pointing at the front of the mic? If so, you're aiming sound right where the mic is most sensitive. Physically reposition the monitor or the mic stand so the speaker is aiming at the back of the microphone, into its Null Point (#33). This is a free, powerful way to reduce feedback.

  • C. The Notch Filter Mission: If you get a stubborn, single-note feedback ring, you can create a Notch Filter (#32). On your mixer's EQ, turn the "Q" or "Width" knob to make the cut as narrow as possible. Sweep the "Frequency" knob to find the exact pitch of the ring, then cut that frequency's gain until the ring stops. Notice how this removes the problem squeal without thinning out the whole vocal.

• Why: This gets you thinking beyond just the faders. It connects these new terms to physical actions on your stage (Null Point) and disciplined habits during the service (NOM), which are some of the most effective ways to improve your sound. The Notch Filter is your surgical tool to clean up what's left.

3. Unlock a Secret to Audio Clarity: "Path Length Differential" (#35).

• Action: The name sounds complex, but the idea is simple and powerful. Spend 10 minutes this week watching a video explaining Path Length Differential (#35) or searching for "what is microphone comb filtering easy explanation." Don't worry about the math; focus on the visual of how two sound waves arriving at slightly different times can cancel each other out.

• Why: This concept is the secret why behind many common sound problems. It explains why using two mics on one acoustic guitar can sometimes sound thin and weak, or why that choir sounds "hollow." Understanding this basic idea will help you make smarter decisions about how many mics to use and where to place them, leading to a clearer, fuller mix with fewer "weird" audio issues.

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