Why bass reflex?

We are used to talking about bass-reflex and conventional loudspeakers, but we are seldom aware of what this actually means. When we ask, we usually get vague explanations, most of them incomplete: "it increases the efficiency of the loudspeaker because the rear sound wave reinforces the front one", "it increases distortion compared with a sealed enclosure", "they are much more dynamic", "the output of the bass-reflex port and that of the active driver are out of phase", "they are less well defined because they rely on resonances", and so on. These definitions, among many others, are more or less correct, but certainly incomplete when taken individually.

A driver that is not mounted in a loudspeaker cabinet is unable to move air, and therefore unable to "generate sound". Every time it moves its diaphragm and tries to displace air on one side, it creates a depression on the opposite side, so instead of pushing the air in one direction, the empty space at the front is quickly filled. This cancellation of the sound waves generated at the front and rear of the diaphragm must be prevented.

From a technical point of view, the best solution is to build the driver (as a transducer) into a wall separating two rooms, so that one room only receives the waves generated at the front and the other the waves from the rear. This wall would need a minimum radius of 17 meters to be compatible with 20 Hz sound waves. Although this is the "ideal" design, it is also an impossible one, because it is so impractical.

The next solution is to build a sealed enclosure that prevents communication between the two sides of the driver. The basic design is the sealed-box loudspeaker. A well-known development is the bass-reflex design, or bass reflector, which adds an opening and a port to the sealed enclosure, opening it up for improved reproduction of low frequencies. Its design is far from easy, as we will see below, and does not simply come down to drilling a hole at random in some attractive spot on the cabinet. To understand the complexity of a bass-reflex enclosure, we first need to briefly review how a driver works and how it behaves when coupled to a cabinet.

A driver is a transducer, that is, a component that converts electrical energy into sound pressure (exactly the reverse of what a microphone does). The driver's job is to use the voltage (not power) supplied to it to move the diaphragm accordingly. This in turn moves a membrane, which pushes the air that we hear as sound. It should be clear that the diaphragm must move in direct proportion to the voltage. According to Newton's second law (who said those laws would never come in handy?), force equals mass times acceleration or, put another way, acceleration is directly proportional to the force applied, as long as the mass is constant.

To move the diaphragm we need a motor, which in the case of drivers is the voice coil. The voice coil uses the voltage it receives to create a magnetic field, which in turn moves a magnetic element that pushes the diaphragm. The system would be perfect in an ideal world but, in practice, there are variations and problems. One of them is efficiency, which never reaches 100%. Since energy is neither created nor destroyed, all the voltage that does not contribute directly to moving the diaphragm is converted into heat, which helps increase resistance to movement. Although the heat problem might be considered insignificant, combined with other problems it does become a concern. Another factor is that the inductance of the voice coil changes depending on the voltage applied to it, which in turn produces different magnetic fields.

If the aim is a linear design, "linear excursion" must also be taken into account. This value defines which section of the voice coil, together with the magnetic material, follows a linear pattern. It is logical to think that the further the magnetic material moves away from the coil, the smaller the part of the coil exerting a magnetic field, preventing constant motion. Only within the parameters defined by linear excursion (that is, as long as the coil completely surrounds the magnetic material) does the driver work as we want it to. Otherwise, distortion occurs.

To stay within these limits, suspension systems are used, whose job is to prevent the excursion from being exceeded and also to allow a quick return to the optimal position. In exchange, they add a new additional opposing force (necessary, on the other hand, especially in low-frequency drivers) that is not constant, since it will be inversely proportional to that exerted by the magnetic field.

RESONANCE FREQUENCY
There is an exact point, defined in hertz, where the resistance caused by the voice coil is identical to that offered by the suspension mechanism. We know this point as the "resonance frequency".

Above the resonance frequency, the inertia of the voice coil dominates the resistance to acceleration. At high frequencies the diaphragm moves very little, so the suspension does less work. Below it, the effect is exactly the opposite: the suspension force has much more influence than the force exerted by the voice coil. And precisely at the resonance frequency, in addition to presenting its highest impedance value, the acoustic output reaches its greatest efficiency, resulting in superior reproduction of that frequency. But what happens when we place a driver in an enclosure with a bass-reflex port?


BASS REFLEX
The volume or mass of air inside the loudspeaker cabinet does not react instantly to the movement of the diaphragm, even less so when we are above the resonance frequency. This tells us that the bass-reflex port (with or without a passive radiator) is not very effective in this situation. Its behavior is similar to that of a sealed-box loudspeaker in almost every respect, including frequency response.

As we approach the resonance frequency, the greatest excitation of the air is achieved.

Before continuing, we must understand that once a driver is placed in a cabinet, when we talk about resonance frequency we are referring to that of the driver+cabinet+port combination.

In bass-reflex loudspeakers, the design of the port (the length of the duct) and the diameter of the opening are calculated to achieve the best results, taking as a reference the characteristics of the driver and those of the cabinet itself.

At the resonance frequency, the bass-reflex port (when properly tuned) limits the excursion of the active driver (thus reducing possible distortion and sound output, and preventing an exaggerated peak). In return, it extends the response at the port, and the combination delivers more linear results than a sealed enclosure.

Around the resonance frequency, the combination of the active driver and the bass-reflex system results in a notable extension of the frequency response, although never above that of the loudspeaker cabinet itself (in other words, the frequency response is flatter). Below the resonance frequency, the role of the bass-reflex port is almost nil, as it acts as a simple hole. This causes the overall response to roll off at 24 dB per octave, in other words, cancellation.


ADVANTAGES
A well-designed bass-reflex port makes it possible to extend the bass frequency response of a loudspeaker enclosure.

According to loudspeaker design theory (taking as a reference the mechanical and physical elements of the driver's suspension system, the type of magnetic material and the behavior of the air), we can deduce that to improve the efficiency of a sealed loudspeaker we must build a larger cabinet, which will allow greater diaphragm travel (or, put another way, if we reduce the volume of a cabinet, we lose efficiency or extension). We can also deduce that if we improve extension, we either lose efficiency or must enlarge the cabinet volume.

With a bass-reflex design, by comparison, we can achieve +3 dB of efficiency without sacrificing extension (it being clear that we improve the system as a whole, not the individual elements). We can even reduce the size of the cabinet by up to 50% without sacrificing extension. This improves the low-frequency response by up to 1/3 of an octave (sometimes even more).


DRAWBACKS
To achieve all of the above, it is clear that a large amount of air will be "pushed" through the bass-reflex port. This air has to exit through a port, so turbulence can easily be generated (unless we use a passive radiator, although this complicates the design somewhat). That is why some manufacturers design their ports with this problem in mind.

Behind the design of a loudspeaker lies an extensive engineering study that results in a series of tolerance margins. Depending on how these margins are applied, side effects can cause unwanted vibrations (and therefore noise), false frequency responses, the need for much more precise physical placement of the loudspeaker, and so on.

In fact, poor tuning can deliver worse results than a "sealed" version. Along the same lines, using bass reflex causes electrical changes (impedance, inductance, etc.) to occur more often and more quickly, demanding more precise and tolerant electronics (referring to those already fitted in the subwoofer itself).

Be careful!

When you use a floorstanding or bookshelf bass-reflex loudspeaker with a rear port, follow the manufacturer's instructions carefully and respect the "clearance" between the loudspeaker and the next obstacle, usually the wall. The same applies to subwoofers.

If they have the bass-reflex port (air outlet) and/or driver facing the floor, you should leave a few centimeters of clearance around them.