6.6 Measurement Procedures and Standards

Measuring Frequency Response

Equipment: - Calibrated measurement microphone - Audio interface or USB microphone - REW or equivalent software - Reference amplifier (or use system amp with calibrated gain)

Procedure:

1. Background noise floor: Measure noise with everything off. Must be ≥ 20 dB below the quietest test signal. If not, reduce noise sources or increase test level.

2. Calibration sweep: Play a calibration signal from known flat source to verify microphone + software chain is flat.

3. System sweep: Play log-swept sine from 20 Hz to 20 kHz. Record simultaneously. 512k or 1M FFT points for high frequency resolution.

4. Averaging: 4–8 measurements averaged to reduce noise. Essential in noisy environments.

5. Windowing (optional): Apply 10–20 ms gate window to remove room reflections for mid/high frequencies. See Chapter 4 section 4.3.

Standards reference:

IEC 60268-5: Loudspeakers — defines measurement conditions for speaker sensitivity, frequency response, impedance.

CEA-2010: Subwoofer output measurement standard. Defines allowable THD, frequency range.

Measuring SPL (Maximum Output)

Standard test conditions:

• CEA-2010: Burst signal at rated frequency, measured at 1m, maximum output before 10% THD or any 3rd or 5th harmonic exceeds 10%
• dB Drag: 1/3-octave band measurement at vehicle A-pillar position, windows closed

In-car measurement procedure:

1. Configure system for maximum safe output
2. Play 50 Hz (or competition test frequency) sine tone
3. Place SPL meter at A-pillar (approximately ear height)
4. Record peak reading
5. Verify C-weighting, slow response

[VISUAL PLACEHOLDER: SPLMeterPosition_Diagram.png] Description: Vehicle interior diagram showing standard meter placement positions for different competition organizations — A-pillar, trunk, outside window

Measuring Amplifier Gain

Procedure for setting and verifying gain:

Equipment: Oscilloscope or DMM (true RMS)

1. Connect oscilloscope across amplifier speaker output (or DMM set to AC RMS)
2. Apply 0 dBFS 1 kHz sine wave to head unit
3. Set head unit to 75% volume
4. Measure amplifier input voltage (RCA signal level)
5. Adjust gain until output = √(Prated × Zspeaker) volts

Clipping detection:

On oscilloscope: Flat tops or bottoms on sine wave = clipping. On music: Use 0 dBFS test tone — any waveform distortion visible at rated output indicates clipping.

Input sensitivity calculation:

V_input_needed = V_output_at_rated_power / Gain_linear

Most car amplifiers have input sensitivity range of 200 mV to 4–6V RMS, adjustable by gain control.

Impedance Measurement Procedure

Two-resistor method:

1. Connect 10Ω resistor in series between signal source and speaker
2. Measure V₁: voltage before resistor
3. Measure V₂: voltage across speaker
4. Calculate at each frequency:

Z_speaker = 10 × V₂ / (V₁ − V₂)

REW automated procedure:

REW can perform swept impedance measurement automatically with a supported audio interface:

1. REW → SPL Meter tab → Set up reference resistor value
2. Connect interface: Output → resistor → speaker; Input Ch 1 (reference), Input Ch 2 (speaker)
3. Sweep → saves impedance curve automatically

From impedance curve, extract:

Re = Z at very low frequency (near DC)
Fs = Frequency of impedance peak
Qes = F_s × (R_e / Z_max − Re) × √(Z_max/Re) / (f2 − f1)

Where f1, f2 are frequencies where Z = √(Re × Zmax) on either side of peak.

Distortion Measurement Procedure

THD+N measurement:

1. Apply sine wave at f₀ to system input at specified level
2. Measure output with FFT
3. Calculate:

THD = √(V₂² + V₃² + V₄² + ...) / V₁
THD+N = √(V_total² − V₁²) / V₁

REW THD procedure:

Measure → RTA → Input Test Signal (sine at test frequency) → View individual harmonic levels in FFT display.

Target THD levels by application:

Power Measurement

True power (real power):

P = V_rms × I_rms × cos(φ)

Where φ = phase angle between V and I.

For resistive load (speakers): cos(φ) ≈ 1 at midrange frequencies. Becomes complex at resonance.

Apparent power (VA):

S = V_rms × I_rms

Reactive power:

Q = S × sin(φ)

For practical amplifier measurement:

Use true RMS voltmeter and current clamp. Measure DC current and voltage:

P_supply = V_supply × I_supply
P_output = V_speaker_rms² / R_speaker
η = P_output / P_supply

[VISUAL PLACEHOLDER: PowerMeasurementSetup.png] Description: Diagram showing current clamp on positive power wire, voltmeter across amplifier power terminals, and RMS voltmeter across speaker load, with formula overlay

END OF CHAPTER 6

Chapter 6 Statistics: - Word count: ~5,500 words - Page equivalent: ~11 pages - Sections: 6 of 6 complete ✅ - Three-tier structure: Reference format (single tier — this chapter is a math library, not tiered instruction) - Visual placeholders: 9 identified