The characteristics of the box (phase inverter) directly affect the sound of the speaker. In automotive acoustics, this is often not given due attention, they use the principle  the more the speaker in the box, the better. Phase inverter requires careful adjustment, rather than the use of scrap materials. To whom laziness is engaged in calculations and measurements, use a closed box.
To calculate the phase inverter, apply simulation programs (bass port), but to get the result, you need to enter a lot of parameters. And even if you know them, there is often a big discrepancy with the final result. With the help of a simple method of calculating the phase inverter, you do not need to know the data for your speakers, drawers, without complicated mathematical calculations and measuring devices. The technique exists for 30 years, the error is only 5%.
Content
 1 Phase inverter differences

2 Phase inverter calculation
 2.1 Calculation of indicators using the program Bass Port
 2.2 Calculation phase inverter according to the methodology of the journal "Radio"
 2.3 Video: how to calculate round ports of the phase inverter
Phase inverter differences
Each speaker has a resonant frequency. When working above this indicator, a good sound is obtained, and lower, the pressure level drops by 12 dB per octave (frequencies are reduced by 2 times). The lower limit of reproducibility, consider the level of 6 dB. By mounting the speaker into the box, the resonance purity is increased due to the additional elasticity of the air. Increasing the resonant frequency, pulls up and lower limit. The less air in the box, the better the elasticity and the greater the performance.
You can make a “big box” without increasing its size. For this use material with damping properties (cotton). The more it is in the box, the lower the frequency of the speaker. But when the filler is too much, it gives the opposite effect. For inexperienced people, the quality of the box and its dimensions are not important. In most cases, the size of the column is optimal.
A phase inverter is a pipe, not necessarily round, of a certain length, which has a resonance. Thanks to the “second resonance”, the sound output of the speaker is raised. The frequency of oscillation of the dynamics, located in the box, should be lower than in the normal state. So, the recession is compensated and the sound expands. These indicators at the phase inverter will be higher by 24 dB than that of the buried box. It expands the lower frequencies of the speaker.
To avoid barrel sound, the resonance performance should not be higher than that of the closed box. And if the frequency is too low, the characteristics of the speaker fall. This is the essence of the phase inverter settings in order to get a positive effect and not spoil the sound. And at home you can achieve a good sound with an error of 5%.
Phase inverter calculation
At resonance, the resistance of the voice coil increases. For measurement, a resistor is connected in series to the speaker; its resistor is higher by an order of magnitude than the speaker, from 100 to 1000 Ohm. When measuring the voltage, you can estimate the resistance of the voice coil. At frequencies where there will be high resistance  the voltage across the resistor is minimal and vice versa.
We are not important absolute values, only the maximum resistance on the coil (the minimum on the resistor). To do this, we use a multimeter in the mode of measuring AC voltage. As a source, professionals use an audio frequency generator. And for our problem a special compact disc is suitable.
The measurement process looks like this:
 Phase inverter hole shuts up a piece of plywood.
 The audio recording disc is turned on at an acceptable volume.
 Switching along the tracks, we monitor the voltage on the resistor as soon as it jumps to the minimum, and that’s the desired frequency.
Sidebyside, the optimal volume of filler is selected for the speaker, gradually adding a small amount and tracking the oscillations of the resonant frequency. And finding this parameter, you need it multiply by 0.63and it will turn out the necessary frequency for the phase inverter. But we still need to measure the length, to do this, open the hole, turn on the test disc with the recording. And look at the resistor. But now we are looking for not the minimum resistance, but the maximum. The frequency of the phase inverter will be very different from the desired one. To increase it, the long tunnel is shortened or its diameter is increased.
Calculation of indicators using the program Bass Port
The program interface is simple and clear, all the fields and the mood are signed.
Required Enter these parameters:
 The speed of sound (by default 344 m / s, humidity 50% temperature 20 ° C).
 The volume of the box.
 Phase inverter frequency.
 The diameter of the speaker.
 Low frequency speaker.
 The course of the diffuser.
 Section of the port.
 The number of holes in the housing.
After that the button is recalculated and you get the result. Keep in mind that the air velocity should not exceed 13 m / s.
Calculation phase inverter according to the methodology of the journal "Radio"
We assemble a circuit with an audio frequency generator and a resistor of 1000 ohms; it is not recommended to take less power. The speakers are placed away from the ceiling and walls. Connect the voltmeter and measure the voltage at 500 Hz. And we find the maximum (Fs) and minimum indicators (Us). To find out the required volume of the box (V), we take the same size box with a hole for the speaker, but not from cardboard. Install the speaker and seal all the holes. We measure and calculate Fs. The obtained data is substituted into the formula: Vas = ((Fs ’/ Fs) ^ 21) * V.
To adjust the phase inverter, close the tunnel hole and calculate the maximum rate (Fs), add soundabsorbing material and measure it again. The result is added in the formula Fb = 0.63 * Fs. The length of the tunnel is calculated: LV = 31 * 10 ^ 3 * S / (Fb ^ 2 * V), where S is the area of the bass reflex port (in cm ²), and V is the volume of the box (in liters).
Phase inverter directly affects the sound quality of acoustics. There are several methods for calculating the phase inverter, they have the same first stage  measurement of indicators. Using software often gives the wrong result. You can also use online services, but they have the same disadvantages.