All Hevos cabinets are vented box types. A vented-box is a cabinet with a so called vent, usually placed at the front side of the cabinet. This vent connects the interiour of the box with the outside. Here you see a schematic drawing of a Hevos cabinet. The vent is at the bottom and is an integrated part of the cabinet.
Wrongly, many people think that the purpose of this vent is to lead the air inside the cabinet to the outside. Reality is a bit more complicated. The air mass inside the vent is triggered at a specific frequency by the moving air energy from the back of the loudspeaker frame and is going into resonance. The whole secret is to make this air mass move at the right time and at the right frequency. Crucial in this matter are the position and the dimensions of the vent. Within certain limits the cabinet designer can choose at which frequency the vent is tuned.
Because of the the resonance of the air mass in the vent extra sound energy is freed. Here are some graphics that show how things work:
In the upper illustration frequency range of a loudspeaker in a closed box is shown. The so called roll offof the sound pressure levels at the left of the illustration (at lower frequencies) is 12 dB/octave for closed cabinets. This roll offstarts at the blue arrow.
In the second illustration you see the frequency range of the same loudspeaker , but now mounted in a vented-box. As you can see the vent delivers within a small range sound energy, with a maximum at the F-vent, the blue arrow. This is the frequency at which the vent is tuned. The total sound pressure can be seen at the next illustration:
The dotted line is the sum of the direct sound pressure at the front of the loudspeaker and the sound pressure coming from the vent. As one can see the sound pressure is straight from the F-vent upward, and only below the frequency at which the tunnel is tuned there is a steep roll offof 24 dB/octave. Such a tuning is normal for cabinets used in hifi sound reinforcement. At other tuned vents the sum line gets a totally different and irregular character, as you can see at the illustration below. This is sometimes applied in cabinets to create a pseudo-bass. Extra sound energy at frequencies between 70-100 Hz make non-critical listeners believe there is a good bass performance. Such a tuning is often used to hide a bad loudspeaker performence at low frequencies.
The tuning of the vent depends strictly on the use of the cabinet. For a cabinet that will be used for sound reinforcement of bass guitars and double basses a straight course of the sound pressure from the F-vent upward is less necessairy as it is with hifi cabinets. For bass reinforcement a more important feature is the power handling at lower frequencies, especially for basses with a low B string. And it is this specific tuning that makes the ‘secret’ of a well tuned vented-box.
The advantages of the vented box loudspeaker system are:
- The cone of the loudspeaker makes a smaller excursion at the same sound pressure, causing a smaller modulation distorsion and a better power handling.
- The cabinet has an extended frequency range at the bass side compared to a closed box.
- A higher output (theoretically 3 dB)
- Steeper roll offof 24 dB/octave.
Quoting a well known Dutch ex-soccerplayer: every advantage has its disadvantage. This is also true for vented-box systems. The disadvantages and the Hevos solutions are:
- Below the tuning frequency there is a very fast decrease of power handling. That is why Hevos tunes its vents at such a low frequency that Hevos cabinets can handle even low B strings with ease.
- Vented boxes allow only sparse damping. No problem for Hevos cabinets because the optimal cabinet dimensions are calculated with a special Hevos computer application. Internal damping is therefore hardly necessary.
- Standing waves in the vent. Vents in Hevos cabinets have a wedge or conic form and therefore disable standing waves.
- Possible moving air noise in the vent. Hevos vents have large dimensions. The edges of the vents are round shaped to prevent moving air noise.
In brief, with the right dimensions and other precautions, the vented box loudspeaker system is superiour for the sound reinforcement of both bass guitar and double bass.
Moreover, many manufacturers use a plastic port, of the type as shown below. This is done in order to save expenses. One of the disadvantages is that the cabinet cannot be tuned properly.