🔧 INSTALLER LEVEL: Advanced Measurement Techniques

Multi-Point Spatial Averaging

A single measurement at the driver's seat tells you what that position hears — not the whole vehicle.

Illustration in preparation Description: Top-down car diagram showing 9 measurement positions: driver, passenger, four rear positions, and three intermediate points along center console

Recommended positions: 1. Driver's left ear 2. Driver's right ear 3. Driver's seat center (between ears) 4. Front passenger 5-8. Each rear seating position 9. Rear center

Procedure: - Measure all positions sequentially, saving each - In REW: All SPL tab shows overlay of all measurements - Calculate arithmetic average or use REW's averaging function

Goal: No single seat should be more than ±6 dB from the average response at any frequency. If it is, the system has a severe localization or resonance problem that EQ at one position will make worse at another.

Impulse Response and Time Domain Analysis

Every REW swept-sine measurement contains not just frequency response, but the complete impulse response — essentially how the system responds to an instantaneous click.

Illustration in preparation Description: Impulse response graph with labels: initial arrival spike, room reflections following, decay region, and the gate window for anechoic extraction

What to look for:

Initial spike: Sharp and narrow means clean, time-aligned sound Multiple peaks: Different drivers arriving at different times — time alignment needed Long tail: Resonance or reflections (cabin acoustics, enclosure ringing)

Time alignment from impulse:

1. Measure each driver in isolation (disable others via DSP)
2. Note time position of each driver's impulse peak (in milliseconds)
3. The driver that arrives latest is your reference — no delay
4. All other drivers get delay equal to the difference

Example: - Tweeter peak at 1.8 ms - Midbass peak at 3.2 ms - Subwoofer peak at 5.5 ms

Subwoofer is reference (latest). Add: - Midbass: 5.5 - 3.2 = 2.3 ms delay - Tweeter: 5.5 - 1.8 = 3.7 ms delay

Waterfall / Cumulative Spectral Decay (CSD)

The waterfall plot adds a third dimension to frequency response: time. It shows how quickly sound decays at each frequency after being produced.

Illustration in preparation Description: 3D waterfall plot viewed at an angle, showing frequency on X-axis, time (0–500 ms) on Y-axis, level on Z-axis, with two visible ridges indicating panel resonances at 85 Hz and 140 Hz

Reading a waterfall:

• Quick decay everywhere: Clean, well-damped system
• Ridge extending in time at a specific frequency: Panel or enclosure resonance — that frequency rings long after the signal stops
• Slow decay across a broad band: Heavy room interaction or poorly damped enclosure

Identifying panel resonances:

Ridges between 40–200 Hz usually indicate vehicle body panels resonating. Apply sound deadening to the panels responsible and re-measure — you'll see the ridge shorten or disappear.

Ridges between 200–800 Hz may indicate enclosure panel resonance. Brace the enclosure walls.

Distortion Measurement

Why distortion matters:

A speaker playing 5% THD doesn't sound like 5% distortion in any intuitive way — it sounds slightly coarse, edgy, or fatiguing. The ear is more sensitive to certain harmonics (3rd, 5th — "odd order") than others (2nd — actually somewhat musical).

Illustration in preparation Description: FFT spectrum showing a 100 Hz fundamental at 90 dB SPL, with 2nd harmonic at -42 dB, 3rd at -54 dB, 4th at -68 dB, THD calculation shown

REW distortion measurement:

1. REW → Generator → Sine wave at test frequency (e.g., 80 Hz)
2. Set level to typical listening SPL
3. REW → Measure → Distortion
4. View harmonic spectrum and THD percentage

Reference thresholds:

Common causes of high distortion:

• Amplifier clipping (gain too high) — measure gain, reduce if clipping
• Speaker over-excursion (too much power or too low crossover) — raise HPF or reduce power
• Poor connections (intermittent, corroded) — inspect all connections