Critical Distance
4.16 ft
D/R Ratio
6.4 dB
Pattern
Cardioid
Critical Distance (Dc)
4.16
feet
D/R Ratio
6.4 dB
SPL Drop
6 dB
3:1 Spacing
6 ft
Source is within critical distance – direct sound dominates. Good clarity.
Inputs
Result
V = 10×10×8 = 800 ft³. A = 0.049×800/0.4 = 98 sabins. Dc = 0.14×√(3×98) = 0.14×17.15 = 2.40 ft. At 1.5 ft, mic is inside critical distance – direct sound dominates.
Inputs
Result
V = 60×40×25 = 60,000 ft³. A = 0.049×60000/1.5 = 1960 sabins. Dc = 0.14×√(3.7×1960) = 0.14×85.16 = 11.92 ft. Stage performers should be close-miked.
Critical distance (Dc) is where direct sound and reverberant sound are equal in level. Closer than Dc, direct sound dominates (clearer). Farther, room reverb dominates. Formula: Dc = 0.14 × √(Q × A), where Q is mic directivity and A is room absorption.
| Mic Pattern | Q Factor | Dc Multiplier vs Omni |
|---|---|---|
| Omnidirectional | 1 | 1x |
| Subcardioid | 1.7 | 1.3x |
| Cardioid | 3 | 1.73x |
| Supercardioid | 3.7 | 1.92x |
| Hypercardioid | 4.3 | 2.07x |
Sound pressure level drops 6 dB every time distance doubles. At 2 ft, SPL is 6 dB lower than at 1 ft. At 4 ft, 12 dB lower. Formula: SPL drop = 20 × log10(d2/d1). This applies only to direct sound in free field.
| Distance | SPL Drop | Relative Level |
|---|---|---|
| 1 ft (ref) | 0 dB | 100% |
| 2 ft | -6 dB | 50% |
| 4 ft | -12 dB | 25% |
| 8 ft | -18 dB | 12.5% |
| 16 ft | -24 dB | 6.25% |
Proximity effect is bass boost that occurs when directional mics are placed very close to a source (within 1 ft). Cardioid, supercardioid, and figure-8 mics exhibit it. Omnidirectional mics do not. Effect can add +6 dB below 200 Hz.
The 3:1 rule states that the distance between two mics should be at least 3 times the distance from each mic to its source. This prevents phase cancellation. If a mic is 1 ft from its source, the next mic should be at least 3 ft away.
RT60 is the time for reverb to decay 60 dB. Longer RT60 means more reverberant room and shorter critical distance. A room with RT60 of 0.3s has ~2x the critical distance of a room with RT60 of 1.2s. Studios aim for RT60 of 0.3-0.5s.
| Room Type | Typical RT60 | Critical Distance (cardioid) |
|---|---|---|
| Vocal booth | 0.2s | ~5 ft |
| Treated studio | 0.4s | ~3.5 ft |
| Living room | 0.6s | ~2.5 ft |
| Classroom | 1.0s | ~1.5 ft |
| Church | 2.0s | ~1 ft |
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Last Updated: Mar 25, 2026
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