Spectral Analysis & Smaart v9
1. The Fast Fourier Transform (FFT) Decoded Spectral analysis is the process of translating sound from the Time Domain (what we hear) to the Frequency Domain (what we see). The FFT is the mathematical algorithm that makes this possible. However, the FFT has a trade-off: The "Uncertainty Principle."
High Frequency Resolution: Requires a long "Time Window."
High Time Resolution: Requires a short "Time Window." In Smaart, we use "MTW" (Multi-Time Window) which uses different window lengths for different octaves, allowing us to see high-frequency detail and low-frequency transients simultaneously.
2. The Transfer Function: The Engineer's Truth An RTA (Real Time Analyzer) shows you the "Absolute" energy in the room. This is useful for seeing a feedback frequency, but it is useless for system tuning because it includes the "source" material (the music). The Transfer Function is a "Relative" measurement. It takes a "Reference" (the clean output of the console) and compares it to a "Measurement" (the microphone in the room). By dividing the Measurement by the Reference, Smaart "removes" the music and shows you only the PA system's influence. This allows us to tune a PA during a show while the band is playing.
3. Coherence: The "Trust" Metric Coherence is a value from 0 to 1 (or 0% to 100%) that tells you how much the Measurement signal resembles the Reference signal.
High Coherence: The PA is the dominant sound source. You can trust the data.
Low Coherence: The data is "corrupted" by room reflections, wind, or background noise. If coherence is low, you should not use EQ to "fix" the trace, as you would be EQing the room's noise, not the speaker's response.
4. The Phase Trace: Seeing Time The Phase Trace is often the most intimidating part of Smaart, but it is simply another way to look at time. Phase is "Time expressed in degrees."
A flat phase line means all frequencies are arriving at the same time.
A sloping phase line means some frequencies are "late" compared to others. We use the phase trace for System Alignment. To align a subwoofer to a main speaker, we look at the "Phase Slope" of both at the crossover frequency. When the slopes overlap perfectly, the two speakers are "In Phase" and will sum constructively. If they are 180° apart, they will cancel each other out, creating a "hole" in the low-end.
5. Averaging and Smoothing In a real venue, sound is chaotic. A single "snapshot" of audio is often too jagged to read. We use "Averaging" (combining multiple FFTs over time) to smooth out the data. "Infinite Averaging" is great for tuning a quiet room, while "Fast" or "1-second" averaging is better for tracking changes during a live performance (like when the temperature rises and the HF starts to drop).
II. Practical Lab: Alignment & Verification
Objective: Align a "Fill" speaker to a "Main" speaker using the Phase Trace.
Tasks:
Configure Smaart with a Reference loop and a Measurement mic.
Use the "Delay Finder" to find the distance between the mic and the Main speaker.
Measure the "Fill" speaker. Note the time difference (e.g., the Fill is 15ms "earlier" than the Main at the mic position).
Apply 15ms of delay to the Fill speaker.
Observe the Phase Trace. Adjust the delay in 0.1ms increments until the Phase lines of both speakers overlap perfectly at the crossover frequency (e.g., 1 kHz).
III. Daily Assessment (Friday)
Theory: Why is the "Transfer Function" superior to the "RTA" for tuning a PA system during a soundcheck?
The transfer function gives us an accurate depiction of how sound is behaving in the room, while an RTA is simply an accurate representation of the soundwaves in the room right now. You couldnt use an RTA to tune a room because an RTA includes the source audio that would mean you are EQing the noise of the room, not the speakers to the room.
Identification: You see a sharp "drop" in Coherence at 250 Hz. What are two possible acoustic causes for this?
At 250Hz it is most likely the Room modes, Standing waves or Boundary Interference, but there could alos be a lot of reflections and outside noise occuring at that frequency causing coherence to drop.
Application: If your Phase Trace is "wrapping" (spinning) very quickly in the high frequencies, what does that tell you about your "Delay Tracker" setting?
It tells us the delay tracker value is mismatched with what is really happening in the room. If your speaker is 30ms away but your Smaart delay is set to 25ms, the high frequencies will "wrap" because they are arriving "late" relative to the reference. The faster the wrap, the bigger the time discrepancy. You need to hit "Find" on your delay tracker to sync the internal clock of Smaart with the physical arrival of the sound.