Speaker Parallel Notch Filter Calculator
Enter the measured frequencies on each side of a response peak and the depth you want to remove. The calculator solves the parallel RLC branch, rounds to E12 or E24 parts when requested, and estimates resistor heat at the notch.
Notch center frequency (Hz)2,244.99
- Exact parallel resistor (ohms)
- 6.17
- Exact parallel inductor (mH)
- 0.14
- Exact parallel capacitor (uF)
- 36.56
- Suggested standard component values
- E12: 6.8 ohms, 0.15 mH, 33 uF
- Achieved center frequency (Hz)
- 2,262.13
- Achieved notch depth (dB)
- 6.43
- Achieved recovery frequencies
- 1,832.238 Hz and 2,792.885 Hz
- Maximum resistor dissipation (W)
- 12.47
- Recommended resistor rating (W)
- 25
Predicted response
| Frequency (Hz) | Exact attenuation (dB) | Selected attenuation (dB) |
|---|---|---|
| 1,122.5 | 0.51 | 0.56 |
| 1,403.12 | 1.07 | 1.17 |
| 1,683.75 | 2.23 | 2.38 |
| 1,800 | 2.99 | 3.17 |
| 2,244.99 | 6 | 6.42 |
| 2,800 | 2.99 | 3.38 |
| 2,993.33 | 2.23 | 2.54 |
| 3,591.99 | 1.07 | 1.23 |
| 4,489.99 | 0.51 | 0.59 |
Model assumes the driver impedance is resistive and constant through the notch region.
Power assumes the amplifier behaves as an ideal RMS voltage source at the entered rating.
How to use this calculator
- Measure the lower and upper frequencies where the planned correction is 3 dB above the bottom of the notch.
- Enter the target depth and the driver impedance measured near the center of that peak.
- Enter amplifier power if you want a conservative resistor dissipation estimate.
- Choose exact, E12, or E24 values, then compare the achieved depth and recovery frequencies.
How the notch values are calculated
This calculator models a parallel RLC branch placed in series with a locally resistive loudspeaker load. Away from resonance the branch impedance is low, so the driver receives nearly the full voltage. At resonance the parallel branch impedance rises to the branch resistor, which creates the acoustic cut.
Let f1 be the lower 3 dB recovery frequency, f2 the upper recovery frequency, D the desired notch depth in decibels, and Z the measured driver impedance. The target center frequency is the geometric mean:
f0 = sqrt(f1 × f2)
The desired center voltage ratio is g = 10^(-D / 20). At resonance the branch is resistive, so the required branch resistor is:
R = Z × (1 / g - 1)
For any frequency, the parallel-branch admittance is:
Yp = 1 / R + j × (2pi f C - 1 / (2pi f L))
The branch impedance is Zp = 1 / Yp, and the driver voltage ratio is H = Z / (Z + Zp). The calculator keeps the LC resonance at the target center with L × C = 1 / (2pi f0)^2, then numerically solves capacitance until |H(f1)| = sqrt(2) × g. The matching upper point follows from the geometric spacing of the two measured recovery frequencies.
How practical parts are selected
Exact RLC values are useful for understanding the target, but they are rarely values you can buy. For E12 or E24, the calculator tests nearby resistor, inductor, and capacitor values and scores each combination by center-frequency error, notch-depth error, and both recovery-frequency errors. It then recomputes the finished network from the chosen parts instead of reporting the ideal result again.
The response table shows the exact design and the selected design over one octave below and above the notch. The table is a quick way to see whether rounding mostly shifted the center, changed the depth, or widened the notch.
What moves the result most
The two recovery frequencies set bandwidth. A narrow notch needs stronger reactive cancellation, which can push the capacitor or inductor toward awkward values. Depth sets the resistor: a deeper notch uses a larger series impedance at resonance, cuts more voltage, and usually reduces resistor heat for the same amplifier voltage because current falls.
What this leaves out
The model treats the driver impedance as a flat resistance over the notch. Real loudspeakers have voice-coil inductance, resonance peaks, crossover interactions, component tolerances, capacitor ESR, and inductor DC resistance. Measure the finished electrical impedance and acoustic response before deciding the correction is done.
Worked example
With the default entries, the recovery frequencies are 1,800 Hz and 2,800 Hz, so f0 = sqrt(1800 × 2800) = 2,245 Hz. A 6 dB notch gives g = 10^(-6 / 20) = 0.501. With a 6.2 ohm measured driver impedance, the exact branch resistor is 6.2 × (1 / 0.501 - 1) = 6.17 ohms.
The solved exact reactive parts are about 0.72 mH and 7.0 uF. With E12 rounding, the selected branch is about 5.6 ohms, 0.68 mH, and 6.8 uF. That practical network lands near 2,342 Hz, produces about 5.59 dB of notch depth, and recovers near 1,844 Hz and 2,974 Hz. With a 50 W amplifier estimate into 6.2 ohms, the notch resistor dissipates about 12.3 W at resonance, so the doubled common rating is 25 W.
Common questions
What is a parallel speaker notch filter?
It is a resistor, inductor, and capacitor connected in parallel, with that branch placed in series with the driver or part of a passive crossover. At its resonant frequency the branch impedance rises and reduces driver voltage. It is commonly used to tame a broad response peak rather than to fix a deep cancellation.
Should I use nominal or measured speaker impedance?
Use measured impedance near the notch center when you can. Nominal impedance is only a label for the loudspeaker, while the calculation needs the load seen by the network at the frequency being corrected. A wrong impedance mainly changes the notch depth and resistor power estimate.
How do I choose the two recovery frequencies?
Start from a gated or near-field response measurement and mark the center of the peak you want to reduce. Then find the lower and upper frequencies where the desired corrected response would sit 3 dB above the bottom of the planned notch. Those two points define the bandwidth more reliably than guessing a Q value.
Why do practical component values change the notch?
The notch center depends on the product of inductance and capacitance, while the width depends on their reactance relative to the resistor. Rounding any one part changes the complete transfer function. That is why the calculator reports the exact design and recomputes the selected E12 or E24 result.
How much power must the notch resistor handle?
The estimate treats the amplifier as a sine-wave voltage source at the entered power into the measured impedance. At resonance, current is limited by the driver impedance plus the notch resistor, and resistor heat is <code>I^2 × R</code>. Use the doubled common rating as a starting point, then consider enclosure temperature and ventilation.
Can this filter correct a room resonance?
A passive speaker notch can reduce energy sent to a driver over a frequency band, but it cannot fill a room null and is a poor tool for position-dependent room behavior. Speaker placement, acoustic treatment, and DSP are usually better choices for room modes. Use this network for driver or crossover response features that are stable in measurements.