Acoustic Advisory

This section provides important guidance on the physical limitations of DSP-based room correction and loudspeaker design.

The Limits of DSP Correction

Understanding Physical Nulls

Deep cancellations (nulls) from room modes or driver boundary interference (SBIR - Speaker Boundary Interference Response) are physical destructive interference patterns and generally cannot be “fixed” by boosting with FIR correction or IIR PEQ.

Why Nulls Cannot Be Corrected:

1. Destructive Interference: Nulls occur when direct sound and reflected sound arrive out of phase, canceling each other
2. Spatial Dependency: The cancellation exists at specific locations but not everywhere in the room
3. Energy Conservation: You cannot add energy where none exists due to phase cancellation
4. Headroom Waste: Boosting into nulls consumes massive headroom without improving the actual acoustic situation

Example: A 20 dB null at 80 Hz cannot be filled by adding 20 dB boost at 80 Hz. The boost will:

• Waste amplifier power
• Increase distortion
• Worsen the response at other listening positions
• Create excessive excursion demands on drivers

EQ Best Practices

What EQ Can Fix

Effective Uses of EQ:

• Broad, minimum-phase peaks: Driver resonances, baffle diffraction effects
• Overall target shaping: House curve, tilt, broad spectral balance
• Crossover integration: Blending drivers at crossover region
• Driver mismatch correction: Sensitivity differences between drivers

Example: A 5 dB broad peak at 3 kHz from a tweeter resonance is an ideal candidate for PEQ correction.

What EQ Cannot Fix

Ineffective Uses of EQ:

• Deep, narrow nulls (> 6-8 dB depth): Room modes, SBIR, comb filtering
• Path length cancellation: Nulls from multiple drivers or reflections
• Non-minimum-phase dips: Cannot be corrected without introducing pre-ringing
• Spatial nulls: Cancellations that vary significantly with listener position

Example: A 15 dB null at 50 Hz from a room mode should be addressed with:

• Subwoofer repositioning
• Multiple subwoofers (spatial averaging)
• Bass traps and acoustic treatment
• Listener position adjustment

Not with: A 15 dB PEQ boost at 50 Hz

Boost Guidelines

Conservative Boost Limits

General Recommendations:

• Avoid large boost into deep nulls: Limit boost to ~6-8 dB maximum
• Focus on peaks, not nulls: Cutting peaks is acoustically sound, boosting nulls is not
• Relatively broad adjustments: Wide Q filters (0.5-6.0) for gentle shaping
• Preserve headroom: Every dB of boost reduces available headroom

Why 6-8 dB Limit?:

• Beyond this level, you’re likely boosting into a physical null
• Distortion increases exponentially with excursion (especially at low frequencies)
• Power requirements increase by 2× per 3 dB boost
• Thermal compression reduces actual output gain

Boost vs. Cut Philosophy

Prefer Cutting over Boosting:

• Cut peaks: Removes excess energy without headroom penalty
• Boost sparingly: Only for broad, gentle shaping
• Net gain reduction: Apply global gain to compensate for cuts

Example Workflow:

1. Identify a broad peak at 2 kHz (+4 dB)
2. Cut with PEQ: -4 dB at 2 kHz, Q = 1.0

Room Correction Strategies

Spatial Averaging (Multiple Mic Positions)

Why Multiple Positions Matter:

• Room modes and nulls vary significantly with position
• Single-point measurements can be misleading
• Averaging reduces emphasis on position-specific anomalies
• Produces more robust correction that works across a larger listening area

Recommended Approach:

1. Measure at 3-5 positions within the listening area
2. Use Room Calibration mode in LinFIR to average responses
3. Apply correction based on averaged response
4. Focus on fixing common peaks across all positions
5. Ignore position-specific nulls (cannot be fixed for all positions)

Physical Solutions First

Order of Priority:

1. Speaker and Listener Placement:

   • Wall distance: Keep speakers 5 cm to 1 m from rear wall
     • Avoid placing directly against walls (especially with rear-ported designs)
     • Don’t exceed ~1 m distance to prevent comb filtering issues in bass/low-midrange
     • Closer placement (<50 cm) shifts SBIR null higher in frequency (less problematic with speaker directivity)
     • Farther placement (50-100 cm) shifts SBIR null lower in frequency (more problematic for bass)
   • Adjust listening position to avoid room mode nulls
   • Use the “rule of thirds” as a base for room placement
   • Experiment with toe-in and speaker spacing

2. Bass Management:

   • Multiple subwoofers for spatial averaging
   • Distributed Bass Array (DBA) for modal smoothing
   • Subwoofer crawl technique to find optimal placement
   • Phase and time alignment between subs and mains

3. Acoustic Treatment:

   • Bass traps in corners for low-frequency modal control
   • Absorption at first reflection points
   • Diffusion on rear wall for controlled reflections

4. DSP Correction (Last Step):

   • Correct broad, minimum-phase peaks
   • Apply target curve shaping
   • Light smoothing of overall response
   • Do not aggressively boost nulls

Frequency Region Strategies

Below Room Transition (Schroeder Frequency)

Characteristics:

• Strong modal behavior (room modes dominate)
• Deep nulls and sharp peaks
• Response varies dramatically with position
• Typically 80-200 Hz depending on room size

Recommended Approach:

• Repositioning and treatment are most effective
• Use multiple subwoofers for spatial averaging
• Apply bass traps to control modal ringing
• Light EQ for broad peaks only (avoid boosting nulls)
• Expect imperfect results (physics limits DSP)

Why EQ Struggles:

• Nulls are spatial (different at every position)
• Boosting nulls creates problems at other positions
• Time-domain ringing (long decay times) cannot be fixed with EQ

Above Room Transition Frequency

Characteristics:

• Direct sound dominates
• Room modes are no longer the primary issue
• Speaker-room interaction remains important (perceived sound = direct + reflections)
• Two design philosophies emerge:
  • Wide directivity: Uses room reflections for spaciousness (requires good crossover design)
  • Controlled directivity: Limits room interaction for consistency across spaces
• Typically 200-300 Hz and above

Recommended Approach:

• Gate/window measurements to emphasize direct sound
• Focus on crossover design and driver integration
• EQ for target curve shaping and driver correction
• Time-align drivers for proper summation

Measurement Technique:

• Use impulse response windowing to exclude reflections
• Set window to capture direct sound only (e.g., 5-10 ms after arrival)
• This removes room influence and focuses on loudspeaker design

Speaker Boundary Interference Response (SBIR)

What is SBIR?

SBIR occurs when direct sound from a speaker combines with its reflection from a nearby boundary (floor, wall, ceiling).

Characteristics:

• Deep null at \(f_{null} = \frac{c}{4d}\) where \(c\) = speed of sound (343 m/s), \(d\) = distance to boundary
• Example: Speaker 1 meter from wall → null at ~86 Hz
• Cannot be fixed with EQ (phase cancellation)

Solutions:

1. Move speaker closer to boundary: Drecreases \(d\), shifts null to higher frequency (less problematic with speaker directivity)
2. Subwoofer integration: Crossover below SBIR null frequency
3. Accept and avoid: Don’t boost the null, work around it

Why EQ Doesn’t Work:

• Null is caused by phase cancellation (180° out of phase)
• Adding energy (boost) cannot fix phase relationship
• Boosting wastes headroom and creates distortion

LinFIR Design Intent

Primary Use Case: Loudspeaker Design

LinFIR is optimized for:

• Driver and crossover design using anechoic or quasi-anechoic data
• Windowed measurements to isolate direct sound
• Crossover filter design (HP, LP, correction)
• Driver integration and phase alignment
• Target curve shaping

Best Results With:

• Outdoor measurements (no room reflections)
• Anechoic chamber measurements
• Gated far-field measurements (remove reflections)

Secondary Use Case: Room Correction

LinFIR can be used for room correction with realistic expectations:

Appropriate:

• Broad spectral balancing (house curve, tilt)
• Correction of broad peaks from room modes
• Light smoothing of overall response
• Spatial averaging across multiple measurement positions

Inappropriate:

• Aggressive boosting of deep nulls
• Single-point correction without spatial averaging
• Expecting perfect flat response in a typical room
• Relying solely on DSP instead of acoustic treatment

Realistic Expectations:

• DSP has fundamental limits
• Room acoustics require multi-faceted approach (placement + treatment + modest EQ)
• Perfect correction is impossible in modal region
• Best results come from combining all techniques

Common Mistakes to Avoid

1. Boosting Nulls Aggressively

Mistake: Applying 15 dB boost to fill a room mode null at 80 Hz.

Why It’s Wrong:

• Null is caused by destructive interference (cannot add energy where none exists)
• Boost increases distortion and thermal compression
• Worsens response at other listening positions
• Wastes amplifier headroom

Correct Approach:

• Reposition subwoofer or listening position
• Add second subwoofer for spatial averaging
• Use bass traps to dampen modal ringing
• Accept a small null (better than aggressive boost)

2. Single-Point Measurement for Room Correction

Mistake: Measuring at one position and correcting for that spot only.

Why It’s Wrong:

• Room response varies dramatically with position (especially below 200 Hz)
• Correction optimized for one spot often worsens others
• Modal nulls and peaks are position-dependent

Correct Approach:

• Measure at 3-5 positions within listening area
• Use Room Calibration mode for spatial averaging
• Correct only features common across all positions
• Ignore position-specific nulls

3. Ignoring Physical Solutions

Mistake: Relying only on DSP to fix all acoustic problems.

Why It’s Wrong:

• Physical placement and treatment are more effective for many issues
• DSP cannot fix phase cancellation, time-domain ringing, or spatial nulls
• Some problems have no DSP solution

Correct Approach:

1. Optimize speaker and listener placement first
2. Add acoustic treatment (bass traps, absorption, diffusion)
3. Use multiple subwoofers if needed
4. Apply DSP as final polish (not primary solution)

4. Over-Damping the Room

Mistake: Covering all walls with absorption to eliminate reflections.

Why It’s Wrong:

• Rooms need some reflections for spaciousness and envelopment
• Over-damping creates a “dead” sound
• Low frequencies are unaffected (absorption ineffective below 200 Hz without massive thickness)

Correct Approach:

• Treat first reflection points only
• Use bass traps in corners for low-frequency control
• Add diffusion (not just absorption) for controlled reflections
• Preserve room liveliness (some reflections are good)

5. Excessive FIR Correction Gain

Mistake: Applying FIR correction with 15 dB boost at low frequencies to “flatten” response.

Why It’s Wrong:

• FIR correction amplifies everything, including distortion and noise
• Massive boost wastes headroom
• Likely boosting into nulls that cannot be fixed
• Thermal compression reduces actual gain achieved

Correct Approach:

• Limit FIR correction to reasonable ranges (±6-8 dB)
• Focus on correcting broad peaks (cutting, not boosting)

Summary: Best Practices

DO:

✅ Focus EQ on broad, minimum-phase peaks
✅ Use spatial averaging (multiple mic positions) for room correction
✅ Combine placement, treatment, and modest DSP
✅ Window measurements above transition frequency to isolate direct sound
✅ Limit boost to ~6-8 dB maximum
✅ Prefer cutting peaks over boosting nulls
✅ Use LinFIR for driver and crossover design (primary use case)

DON’T:

❌ Boost deep, narrow nulls (> 6-8 dB)
❌ Rely solely on DSP for room correction
❌ Expect perfect flat response in modal region
❌ Apply single-point correction without spatial averaging
❌ Ignore physical solutions (placement, treatment)
❌ Over-damp the room (preserve some reflections)
❌ Boost nulls caused by SBIR or room modes

Physical Acoustics Primer

Speed of Sound

\(c = 343 \text{ m/s}\) (at 20°C, sea level)

Wavelength Formula: \(\lambda = \frac{c}{f}\)

Examples:

• 20 Hz → 17.15 m wavelength
• 100 Hz → 3.43 m wavelength
• 1 kHz → 0.343 m (34.3 cm)
• 10 kHz → 0.034 m (3.4 cm)

Implications:

• Low frequencies have long wavelengths → difficult to control with treatment
• High frequencies have short wavelengths → easy to absorb, control

Room Modes

Axial Mode Formula (between parallel walls):

\[f_n = \frac{nc}{2L}\]

where \(n\) = 1, 2, 3… (mode number), \(L\) = room dimension

Example (5m room length):

• 1st mode: \(f_1 = \frac{343}{2 \times 5} = 34.3 \text{ Hz}\)
• 2nd mode: \(f_2 = \frac{2 \times 343}{2 \times 5} = 68.6 \text{ Hz}\)
• 3rd mode: \(f_3 = \frac{3 \times 343}{2 \times 5} = 102.9 \text{ Hz}\)

Modal Density: Number of modes per Hz increases with frequency. At high frequencies, modes overlap (modal smoothing).

Schroeder Frequency (Room Transition)

Approximate Formula:

\[f_s \approx 2000 \sqrt{\frac{RT_{60}}{V}}\]

where \(RT_{60}\) = reverberation time (seconds), \(V\) = room volume (m³)

Typical Living Room (50 m³, RT60 = 0.4s):

\[f_s \approx 2000 \sqrt{\frac{0.4}{50}} \approx 179 \text{ Hz}\]

Below \(f_s\): Modal behavior dominates (discrete modes, deep nulls/peaks)
Above \(f_s\): Statistical behavior (modal overlap, smoother response)

Related Documentation

• Room Calibration - Multi-position measurement workflow
• Sweep Measurements - Measurement techniques and best practices
• IR Management - Windowing impulse responses to isolate direct sound
• System Processing - Global FIR correction parameters and limits
• Troubleshooting - Common measurement and processing issues