Advanced Smaart & Spatial Averaging
1. The Mechanics of the Phase Trace and Time Alignment In the Transfer Function (TF), the Phase Trace is the fundamental metric for determining the temporal relationship between a reference signal and a measurement signal. Mathematically, phase is the expression of time delay as a function of frequency: $\Delta \theta = 360 \cdot f \cdot \Delta t$.
When the Delay Tracker in Smaart is accurately set, the internal reference signal is delayed to match the physical arrival of the sound at the microphone. If the timing is perfectly synchronized ($\Delta t = 0$), the phase trace appears as a flat horizontal line at 0°. However, if the Delay Tracker is mismatched—meaning the reference signal and measurement signal are not synchronized in time—the phase will "wrap" or "spin." This happens because a fixed time difference ($\Delta t$) represents a different number of degrees for every frequency. Higher frequencies complete cycles faster; therefore, even a microsecond of mismatch causes high frequencies to accumulate phase shift rapidly, resulting in a steep, spinning slope. A "fast spin" in high frequencies indicates a significant temporal discrepancy between your tracker and the actual acoustic arrival.
2. Coherence as a SNR and Linearity Metric Coherence is a value between 0 and 1 that represents the squared magnitude of the cross-spectral density divided by the product of the auto-spectral densities of the input and output signals. In simpler terms, it is a measure of the "linearity" and "purity" of the relationship between what the console is sending and what the mic is hearing.
Low Coherence in Low Frequencies: This is often caused by "Room Modes" (standing waves) or strong boundary reflections. These acoustic events add energy to the room that the console didn't send, corrupting the comparison.
Low Coherence in High Frequencies: This is typically a result of a low Signal-to-Noise Ratio (SNR). Environmental factors like wind (causing air turbulence and temperature gradients) or background crowd noise will introduce "uncorrelated" signals into the microphone, causing the coherence to drop.
3. Spatial Averaging: The Law of the Common Denominator A single measurement microphone provides a "truth" that is only valid for one cubic inch of space. Due to local reflections and comb filtering, a microphone placed at Seat A might show a massive notch at 400Hz that doesn't exist at Seat B. If an engineer EQs the PA based on Seat A, they are "fixing" an acoustic reflection that is unique to that spot, potentially making the rest of the room sound worse. Spatial Averaging involves taking the Transfer Function from 4 to 8 different microphones across the venue and mathematically averaging them. This reveals the "Systemic" problems—the peaks and valleys that every seat shares—allowing the engineer to apply EQ that improves the experience for the entire audience.
4. The Impulse Response (IR) and Reflexive Analysis The Impulse Response is the "Time Domain" view of the PA. By analyzing the IR, we can identify the arrival of the direct sound followed by secondary "spikes" known as reflections. By measuring the time difference between the main arrival and a secondary spike, we can calculate the physical distance of the reflecting surface (e.g., a balcony front or a back wall). This allows the systems engineer to adjust the vertical splay of the array to "steer" energy away from reflective surfaces, improving intelligibility and coherence.
5. Measurement Sources: Pink Noise vs. Program Material While Smaart can perform a Transfer Function using music, Pink Noise is the preferred professional standard for system tuning. Pink noise contains equal energy per octave, ensuring that every frequency bin of the FFT is adequately "excited" with a constant, predictable signal level. In contrast, music is dynamic and spectrally "gappy." If a song has no energy at 60Hz (e.g., between bass notes), Smaart will have nothing to compare at that frequency, leading to a total loss of coherence in that bin. Pink noise ensures the Transfer Function remains stable and highly coherent across the entire audible spectrum, allowing for precision EQ and alignment decisions that music cannot reliably support.
II. Practical Lab: The Master Alignment
Tool: Smaart v9.
Tasks: * Configure 4 Virtual Mics and create an "Average Group."
Find the "Delay Tracker" value for the Main PA.
Perform a Main-to-Sub alignment using the Phase Trace overlap method.
III. Daily Assessment (Friday)
Q1: If your Phase Trace is "spinning" quickly in the high frequencies, what is wrong with your Delay Tracker?
Q2: Why do we use Pink Noise for Transfer Function measurements instead of music?
WEEK 2 FINAL ASSESSMENT: The Master Exam
(RF) Calculate the IM3 products for frequencies 500.0 and 501.0 MHz.
(Consoles) Define "Automatic Delay Compensation" and why it matters for parallel processing.
(PA) What is the "Power Alley" and how can you mitigate its effect?
(Network) What is the standard DSCP value for Dante Clock (PTP) traffic?
(Smaart) Describe the difference between a "Magnitude" dip caused by EQ vs. one caused by a Reflection.