Audio calculator
Wireless Mic Intermod Calculator
When two transmitters key up near each other, their signals mix in any nonlinear stage and create third-order intermodulation products at 2f1 - f2 and 2f2 - f1. If a product lands on a third channel, that channel takes hits even though its frequency looks free on paper. Enter 2 to 4 carrier frequencies and this checker computes every IM3 product and flags collisions.
Products closer than this to a carrier are flagged. 0.25 MHz is a common working margin.
Formulas
Two-transmitter IM3
f = 2 × f1 - f2 and f = 2 × f2 - f1- f1, f2:
- transmitter carrier frequencies
Three-transmitter (triple beat)
f = f1 + f2 - f3 (all permutations)- f1..f3:
- any three active carriers
How it works
Intermodulation happens wherever two RF signals meet a nonlinearity: transmitter output stages in close proximity, overloaded receiver front ends, even corroded metal junctions. Third-order products are the ones that matter because they land close to the original carriers and are strongest.
The products only exist while both source transmitters are keyed, which is why an intermod hit is intermittent: the compromised channel works fine at soundcheck with half the packs off and falls apart during the show.
Even spacing is the classic trap. Carriers at 500.000, 500.500, and 501.000 MHz put 2×500.5 - 500.0 exactly on 501.000. Compatible sets are deliberately unevenly spaced so all products fall in the gaps.
Worked example: Three handhelds at 518.200, 519.150, and 520.100 MHz
- 1.Pair products include 2 × 519.150 - 518.200 = 520.100 MHz.
- 2.That product lands exactly on transmitter 3.
- 3.Moving TX3 to 521.700 MHz clears every product by more than 0.25 MHz.
The evenly spaced set self-destructs; the uneven set is compatible.
Products generated vs transmitter count
| Transmitters | Pair IM3 products | Triple beats |
|---|---|---|
| 2 | 2 | 0 |
| 3 | 6 | 3 |
| 4 | 12 | 12 |
| 8 | 56 | 168 |
Field notes
- Keep transmitter packs physically apart; two body packs in one pocket is a portable intermod generator.
- Scan on site before trusting any plan; the RF environment at load-in is the only one that counts.
Frequently asked questions
Why can I not just space wireless frequencies evenly?
Because 2f1 - f2 for two evenly spaced carriers lands exactly on the next carrier in the series. Every added transmitter multiplies the products; compatible sets are computed, not gridded.
How far apart should wireless mic frequencies be?
Adjacent carriers generally want at least 0.3 to 0.4 MHz of spacing, and every IM3 product wants a working margin (commonly 0.25 MHz) from every carrier. Manufacturer coordination software encodes tighter, model-specific rules.
Does this replace full frequency coordination?
No. This checks IM3 math for a handful of channels. Real coordination also accounts for TV stations, other rentals on site, receiver selectivity, and antenna systems.