Car subwoofer is designed to play lower frequencies. To get good low frequencies on conventional speakers (without a subwoofer), large (6.5-inch speakers) and powerful speakers are required.
The use of a subwoofer will relieve the speaker system at low frequencies. Since the human ear can not recognize the direction of the low-frequency sound, only one subwoofer will be sufficient, and it can be placed anywhere in the car. The sound quality will be improved because you do not have to overload the speakers with bass, and thus reduce the amount of distortion.
There are three types of design:
- sealed box,
- phase inverter,
Subwoofer housing type – Closed Box (CB)
This is the simplest in a production type of acoustic design. Fluctuations in such a box are in a closed volume and eventually extinguished. Since the sound wave is energy, then fading it is converted into heat. And although the amount of this heat is small – it still affects the characteristics of the speaker system.
To prevent this effect, CB is filled from the inside with a sound-absorbing material (cotton wool or padding polyester), which absorbs sound and heat. The increase in air temperature becomes much smaller and the speaker “seem” to have behind it a much larger volume than it is. In practice, in this way, it is possible to achieve an increase in the “acoustic” volume of the box compared to the geometric by 15-20%.
Despite the simplicity of the design of the closed box, it has many advantages – first, the simplicity of the calculation. There is only one parameter which is volume. Secondly, it significantly reduces the likelihood of overload of a speaker and mechanical damage. Third, with the right choice of parameters of the subwoofer head and the volume of the case – the closed box has no equal in the characteristics that determine the subjective perception of bass frequencies.
If everything is so good, why do we need other types of acoustic design? There is only one catch – efficiency. In a closed box, it is the smallest in comparison with any other type of acoustic design. The lower the volume of the box, the less effective it will be.
Subwoofer housing type – Phase Inverter (PI)
In the phase inverter, part of the energy that is “put to the wall” in a closed box is used for peaceful purposes. To do this, the internal volume of the box is communicated with the surrounding space tunnel, comprising a certain mass of air. The value of this mass is chosen in such a way that, in combination with the elasticity of the air inside the box, a second oscillatory system is created that receives energy from the rear side of the diffuser and emits it where necessary and in the phase in the radiation of the diffuser. This effect is achieved in a not very wide frequency range, but within its limits, the efficiency increases significantly.
In addition to the higher efficiency, the phase inverter has one more advantage – the presence of a massive gap in the speaker housing can restrain the movement of the diffuser, and the amplitude of the diffuser oscillations is significantly reduced near the tuning frequency. In the operating range, the phase inverter creates absolutely hothouse conditions for the speaker, and precisely at the tuning frequency the oscillation amplitude is minimal, and most of the sound is emitted by the tunnel. The allowable input power is maximum, and the distortion introduced by the speaker – on the contrary, minimal.
The phase inverter is careless to the choice of parameters and adjustment since three parameters are subject to selection for a specific speaker: the volume of the box, the cross-section and the length of the tunnel. The tunnel often is done in a way to be able to adjust the length of the tunnel, changing the tuning frequency.
Subwoofer housing type – Bandpass Speaker
If a closed box and phase inverter – acoustic filters of high frequencies, bandpass – combine the filters of the upper and lower frequencies. The simplest bandpass is a single of 4th order. It consists of a closed volume, the so-called rear chamber and a second, equipped with a tunnel, like a phase inverter (front chamber). The speaker is installed in the partition between the chambers so that both sides of the diffuser operate at fully or partially closed volumes – hence the term “symmetrical load.”
Among traditional designs, bandpass – performance champion. Efficiency is directly related to bandwidth. The frequency response of the bandpass loudspeaker looks like a bell.
Bandpass is capricious in calculating and time-consuming to manufacture. Since the speaker is buried inside the case, it is necessary to go to the tricks of assembling the box, so that the presence of a removable panel does not violate the rigidity and tightness of the structure. Pulse characteristics are also not the best, especially with a full band.
How is this compensated? First of all – the highest efficiency. Second – all sound is emitted through the tunnel, and the speaker is fully closed. With the layout of such a subwoofer opens up considerable opportunities to install it in the car. It is enough to find a small place at the junction of the trunk and the cabin, which can accommodate the mouth of the tunnel – and the path to the most potent bass is open.