The following reference contains commonly used formulas and unit conversions for speaker design, crossover calculation, and electronics. To convert a value, multiply it by the number in the multiplier column. Want to skip the math? Use the MTX Car Audio Calculator to solve these formulas interactively.
Length Conversions
| From | To | Multiply by | |
|---|---|---|---|
| inch | → | millimeters | × 25.4 |
| inch | → | centimeters | × 2.54 |
| inch | → | meters | × 0.0254 |
| inch | → | feet | × 0.0833 |
| millimeters | → | centimeters | × 0.100 |
| millimeters | → | feet | × 0.00328 |
| millimeters | → | inch | × 0.03937 |
| millimeters | → | meters | × 0.00100 |
| centimeters | → | millimeters | × 10.0 |
| centimeters | → | feet | × 0.0328 |
| centimeters | → | inch | × 0.3937 |
| centimeters | → | meters | × 0.0100 |
| feet | → | centimeters | × 30.48 |
| feet | → | inch | × 12.0 |
| feet | → | meters | × 0.3048 |
| feet | → | millimeters | × 304.8 |
| meters | → | centimeters | × 100 |
| meters | → | inch | × 39.37 |
| meters | → | feet | × 3.28 |
| meters | → | millimeters | × 1000 |
Area Conversions
| From | To | Multiply by | |
|---|---|---|---|
| inch² | → | centimeters² | × 6.452 |
| inch² | → | feet² | × 0.006944 |
| inch² | → | millimeter² | × 645.2 |
| inch² | → | meter² | × 0.0006452 |
| centimeter² | → | inch² | × 0.1550 |
| centimeter² | → | feet² | × 0.001076 |
| centimeter² | → | millimeter² | × 100 |
| centimeter² | → | meter² | × 0.0001 |
| millimeter² | → | inch² | × 0.001550 |
| millimeter² | → | centimeters² | × 0.0100 |
| millimeter² | → | feet² | × 0.00001076 |
| millimeter² | → | meter² | × 0.000001 |
| meter² | → | inch² | × 1550 |
| meter² | → | centimeters² | × 10000 |
| meter² | → | feet² | × 10.76 |
| meter² | → | millimeter² | × 1,000,000 |
| feet² | → | millimeter² | × 0.000929 |
| feet² | → | centimeter² | × 929.0 |
| feet² | → | inch² | × 144.0 |
| feet² | → | meter² | × 0.0929 |
Volume Conversions
| From | To | Multiply by | |
|---|---|---|---|
| inch³ | → | feet³ | × 0.0005787 |
| inch³ | → | centimeters³ | × 16.39 |
| inch³ | → | meters³ | × 0.00001639 |
| inch³ | → | liters | × 0.01639 |
| feet³ | → | centimeters³ | × 28,320 |
| feet³ | → | inch³ | × 1728 |
| feet³ | → | meters³ | × 0.02832 |
| feet³ | → | liters | × 28.32 |
| centimeters³ | → | inch³ | × 0.06102 |
| centimeters³ | → | feet³ | × 0.00003531 |
| centimeters³ | → | meters³ | × 0.000001 |
| centimeters³ | → | liters | × 0.001 |
| liters | → | centimeters³ | × 1000 |
| liters | → | inch³ | × 61.02 |
| liters | → | feet³ | × 0.03531 |
| liters | → | meters³ | × 0.001 |
Electronics Formulas
In many electrical circuits, component values are specified using SI prefixes to avoid long strings of zeros:
mega
× 10⁶
kilo
× 10³
milli
× 10⁻³
micro
× 10⁻⁶
nano
× 10⁻⁹
pico
× 10⁻¹²
Variable key
E = Potential Difference (volts) · I = Current (amperes) · R = Resistance (ohms) · P = Power (watts)
Power (watts)
P = E × I
P = I² × R
P = E² / R
Potential Difference (volts)
E = I × R
E = √(P × R)
E = P / I
Current (amperes)
I = E / R
I = P / E
I = √(P / R)
Resistance (ohms)
R = E / I
R = E² / P
R = P / I²
Resistance combinations
RT = R1 + R2 + R3 + ... + Rn
Resistances in series — add them
1/RT = 1/R1 + 1/R2 + 1/R3 + ... + 1/Rn
Resistances in parallel — reciprocal method
RT = (R1 × R2) / (R1 + R2)
Two resistances in parallel — shortcut
Capacitance combinations
1/CT = 1/C1 + 1/C2 + 1/C3 + ... + 1/Cn
Capacitances in series
CT = C1 + C2 + C3 + ... + Cn
Capacitances in parallel — add them
Want to solve these formulas with your own values? Use the Ohm's & Watt's Law calculator →
Crossover Formulas
6dB High Pass Filter
Series Cap = 0.159 / (Impedance × Frequency)
Result in Farads. Impedance in ohms, frequency in Hz.
6dB Low Pass Filter
Series Inductor = Impedance / (6.28 × Frequency)
Result in Henries. Impedance in ohms, frequency in Hz.
12dB Butterworth (high and low pass)
Capacitor = 0.1125 / (Impedance × Frequency)
Result in Farads.
Inductor = (0.2251 × Impedance) / Frequency
Result in Henries.
Calculate component values for your specific crossover frequency using the Crossover Frequency calculator →
Attenuation Circuit
The attenuation circuit uses two resistors (R1 in series, R2 in parallel) to reduce driver output by a specific number of dB. Use these values for the most common impedances:
| Attenuation (dB) | 2 Ohms | 4 Ohms | 8 Ohms | |||
|---|---|---|---|---|---|---|
| R1 | R2 | R1 | R2 | R1 | R2 | |
| 1 | .25 | 16 | .5 | 33 | 1 | 60 |
| 2 | .5 | 7.5 | .8 | 15 | 1.6 | 31 |
| 3 | .66 | 5 | 1.2 | 10 | 2.5 | 20 |
| 4 | .75 | 3.4 | 1.5 | 6.2 | 3 | 13 |
| 5 | .83 | 2.5 | 1.7 | 5 | 3.4 | 10 |
| 6 | 1 | 2 | 2 | 4 | 4 | 8 |
| 7 | 1.1 | 1.8 | 2.2 | 3.4 | 4.3 | 6.6 |
| 8 | 1.2 | 1.4 | 2.5 | 2.75 | 5 | 5.2 |
| 9 | 1.3 | 1.2 | 2.5 | 2.2 | 5.2 | 4.3 |
| 10 | 1.4 | .83 | 2.75 | 1.87 | 5.5 | 3.75 |
Zobel Network
A Zobel network (also called an impedance compensation network) is placed across a driver to flatten its rising impedance at high frequencies, making the crossover filter behave more predictably.
Zobel Network Formulas
Resistor = 1.25 × Re
Re = DC resistance of the driver voice coil
Capacitor = Inductance / Resistor²
Inductance = Le (voice coil inductance in Henries)
Series Notch Filter
A series notch filter is placed across a driver to reduce the impedance peak at resonance, helping crossover filters see a more consistent load.
Series Notch Filter Formulas
Capacitor = 0.1592 / (Re × Qes × Fs)
Re in ohms, Fs in Hz
Inductor = (0.1592 × Qes × Re) / Fs
Result in Henries
Resistor = Re + ((Qes × Re) / Qms)
All values from Thiele/Small parameters
Thiele/Small parameters
Re, Qes, Qms, and Fs are all Thiele/Small parameters specified on MTX speaker data sheets. See the Speaker & Audio Glossary for definitions of each.
Frequently Asked Questions
What is a crossover filter and why do I need one?
A crossover filter routes different frequency ranges to the appropriate speaker. A high-pass filter blocks bass from reaching tweeters (which can't reproduce it and can be damaged by it). A low-pass filter blocks high frequencies from reaching subwoofers. Without crossovers, full-range signal goes to every speaker, which wastes power and can damage drivers.
What is the difference between a 6dB and 12dB crossover?
The dB per octave rating describes how steeply the filter rolls off frequencies beyond the crossover point. A 6dB/octave filter halves the signal for every octave past the cutoff. A 12dB/octave filter attenuates twice as fast. Steeper slopes provide better driver protection and less overlap between speaker ranges, but require both a capacitor and an inductor rather than one component.
What is a Zobel network used for?
A Zobel network compensates for the rising impedance of a speaker driver at high frequencies caused by voice coil inductance. Without it, crossover filters designed for a nominal impedance (say 4 ohms) see a much higher impedance at the crossover frequency, which shifts the actual crossover point. A Zobel network flattens the impedance curve so the filter behaves as calculated.
How do I convert speaker enclosure volume from liters to cubic inches?
Multiply the volume in liters by 61.02 to get cubic inches. For example, a 30-liter enclosure is 30 × 61.02 = 1,830.6 cubic inches. To go the other way, multiply cubic inches by 0.01639 to get liters.
What are Thiele/Small parameters and where do I find them?
Thiele/Small parameters (Fs, Qts, Vas, Re, Le, etc.) are physical and electrical measurements of a speaker driver that allow engineers to predict its frequency response in a given enclosure. They are listed on product specification sheets and data pages. MTX provides Thiele/Small parameters for all of its subwoofer products.
