Ohmic Audio

🔧 INSTALLER LEVEL: SPL Measurement and System Design

SPL Measurement Techniques

Equipment needed: 1. SPL Meter or smartphone app - Type 1 (precision): ±1 dB accuracy - Type 2 (general): ±2 dB accuracy - Smartphone apps: ±5 dB (calibration helps)

  1. Calibrator (optional but recommended)

    • Generates reference tone (usually 94 dB or 114 dB)
    • Ensures meter accuracy

  2. Test tones or music

    • Pink noise (most common)
    • Sine wave sweeps
    • Test tracks with known content

Weighting Filters:

SPL meters have different weighting curves that filter frequencies to match human perception:

A-weighting (dBA): - Mimics human hearing at moderate levels - Reduces low and high frequencies - Used for general environmental noise - Most common in regulations

C-weighting (dBC): - Flatter response, less filtering - Better represents actual acoustic energy - Used for peak measurements - Common in car audio SPL competition

Z-weighting (dBZ) or unweighted: - No filtering, flat response - True acoustic pressure - Used for technical measurements

For car audio: - Use C-weighting or Z-weighting for fullrange measurements - Use slow response for average levels - Use fast or impulse for peak levels

Measurement Procedures

Basic Interior SPL Measurement:

  1. Positioning:

    • Place meter at head position (driver's headrest level)
    • Point microphone toward source
    • Keep windows and doors closed
    • Engine off (unless testing with engine)

  2. Background noise:

    • Measure ambient noise first
    • Music should be at least 10 dB above background
    • Subtract background noise if necessary

  3. Frequency response:

    • Play pink noise or frequency sweep
    • Measure at multiple frequencies
    • Note peaks and dips
    • Create response curve

  4. Peak SPL:

    • Play bass-heavy test tone or music
    • Note peak reading
    • Repeat multiple times for consistency
    • Record highest stable peak

Competition SPL Measurement (IASCA/dB Drag Standards):

Standardized procedures ensure fair comparison:

  1. Meter placement:

    • Specific location defined by sanctioning body
    • Usually driver's side B-pillar or windshield
    • Microphone faces specific direction
    • Secured to prevent movement

  2. Test conditions:

    • Doors/windows sealed
    • Engine running (alternator charging)
    • Specific test frequency (typically 40-63 Hz)
    • Measurement time (typically 30 seconds)

  3. Multiple runs:

    • Usually best of 3 runs counted
    • Reset time between runs
    • Note voltage and temperature

SPL System Design Goals

Sound Quality Focused: - Target: 95-110 dB peaks - Emphasis: flat frequency response, low distortion - Listening level: 75-90 dB continuous

Daily Driver with Impact: - Target: 110-125 dB peaks - Balance: quality and output - Listening level: 80-95 dB continuous

SPL Competition: - Target: 140-160+ dB peaks - Emphasis: maximum output at specific frequency - Brief demonstration only (not for listening)

Calculating Required Power:

Starting reference: 90 dB with 100 watts

To reach target SPL:

Required Power = 100W × 10^[(Target_dB - 90)/10]

Examples: - 100 dB: 100 × 10^1 = 1,000 watts - 110 dB: 100 × 10^2 = 10,000 watts - 120 dB: 100 × 10^3 = 100,000 watts - 150 dB: 100 × 10^6 = 100,000,000 watts (theoretical)

Real-world factors: - Speaker sensitivity (±3-10 dB variation) - Cabin gain (typically +6 to +12 dB below 80 Hz) - Enclosure efficiency - Bandwidth (narrow = higher peak SPL)