Chapter 6: Field Measurement & Calibration Protocol

Measurement is the only way to move from "I think it sounds good" to "I know it is accurate." This chapter details the specific, repeatable procedures used by Ohmic Audio to verify the performance of a mobile audio system. We prioritize Empirical Consistency across different measurement environments.

6.1 Gain Setting: The Oscilloscope Method

Using a multimeter to set gains only measures average voltage. To detect the onset of distortion (clipping), a high-speed oscilloscope is required.

1. Source Setup: Disconnect all speakers. Set the head unit volume to 75% or the maximum "clean" level.
2. Signal Generation: Play a 0dBFS sine wave (1kHz for mids/highs, 40Hz for subwoofers).
3. Probe Placement: Connect the oscilloscope probe to the amplifier's speaker output terminals.
4. Adjustment: Slowly increase the gain until the peaks of the sine wave begin to "flat-top" or square off.
5. Back-off: Lower the gain until the sine wave is perfectly rounded. This is the maximum clean power point.

6.2 Ground Resistance: The 4-Wire Protocol

Chassis ground resistance is the leading cause of system noise. A standard multimeter probe adds too much error for accurate readings.

• The Measurement: Measure between the amplifier's ground terminal and the negative terminal of the battery.
• Pass/Fail Target: Resistance must be less than 0.1Ω. Any reading higher than this indicates a poor mechanical connection or inadequate wire gauge.
• Correction: If resistance is high, remove the bolt, grind the metal to a mirror finish, and re-torque using a star washer.

6.3 RTA Analysis: The Moving Microphone Method

Static microphone placement in a car leads to "comb filtering" errors caused by reflections. To get a true average of the cabin's response:

• Motion: Move the microphone in a slow, continuous "8" pattern around the listener's head area (the "Ear Zone").
• Averaging: Set the RTA software to "Infinite Averaging."
• Timeframe: Sweep the zone for at least 30 seconds per channel.
• Result: This provides a spatially averaged response that correlates much better with what the human brain actually hears.

6.4 Time Alignment: Impulse Response Validation

Distance-based alignment is only a starting point. Physical driver offsets and DSP latency require impulse-based correction.

1. Play an acoustic "Click" or "Chirp" through the Left and Right tweeters simultaneously.
2. Analyze the impulse response peaks in the software.
3. Adjust the delay in 0.01ms increments until the peaks align perfectly on top of each other.
4. Repeat for Midranges and Woofers using their respective crossover bands.

6.5 Enclosure Verification: The Impedance Sweep

How to tell if your box is actually tuned to the right frequency (Fb).

• The Plot: Use a DATS or similar impedance analyzer to sweep the driver in the box.
• Identification: In a vented box, the Fb is the "dip" between the two impedance peaks.
• Correction: If the dip is at 40Hz and your target was 32Hz, the port is too short or the box is too small.