Hi-fi enthusiasts interested in speakers and subwoofers are well aware of bass-reflex technology. However, how familiar are we with the working principles of a bass-reflex enclosure? Here are a few explanations.
The challenge of reproducing low frequencies
Low frequency restitution has always been a challenge for audio engineers. Due to their longer wavelength, it is necessary to place wide drivers within a large volume of air to obtain proper bass delivery. In the early years of hi-fi, the shape and size of speakers were not exactly compatible with typical living room esthetics. Back in the day, speakers with large back horns were actually quite common as this design was necessary to mechanically amplify low frequencies delivered by early, low-wattage tube amplifiers.
The French take on bass-reflex technology
The invention of the transistor in the 50s changed the game and the high power of solid state amplifiers made horns obsolete, leading to a new approach to designing bass drivers. In France, Elipson based its research on the work of Hermann von Helmholtz, a German physicist who studied the phenomenon of air resonance within a cavity (chimney, barrel, etc.) and established the theory of the acoustic resonator (Helmholtz resonator). The French brand was the first to use this type of resonator for their hi-fi speakers, a technology better known as the bass-reflex port.
Bass-reflex technology: the benefits
Using a bass-reflex port will boost the volume of low frequencies over a certain frequency range. This is especially important as the driver’s sensitivity is greatly reduced over this range. Bass-reflex technology provides a few extra decibels, which increases the overall sensitivity of the speaker without affecting that of the medium/high driver. The use of a bass-reflex port also helps limit the number of passive components in the circuit, thereby preventing significant alterations of the signal. The bass-reflex port acts as a mechanical low-pass filter and prevents the driver from attempting to reach very low frequencies. This limits the cone’s movement and results in an increased power handling. The filter roll-off under the resonance frequency reaches 24 dB/octave. Lastly, using a resonator allows for more compact speakers.
Bass-reflex technology is the reason why compact models and slimline tower speakers constitute the major part of most manufacturers’ catalogs.
Bass-reflex technology: working principles
It is both simple and complicated. Simple because the basic principle is to cut a hole in the speaker through which to place a tube (resonator). The sound created by the vibrations coming from the back of the driver is picked up by the bass-reflex port, which resonates and amplifies these low frequencies. It is also a complicated process as the size of the speaker and that of the resonator are directly linked, meaning that a certain ratio must be carefully respected so that the bass-reflex port works over the proper frequency range. This “proper” frequency range depends on the mechanical specifications of the woofer(s). For example, a bass-reflex port should not be tuned at 30 Hz with 4” (10 cm) drivers as the resonator would be inefficient and frequencies between 40 and 100 Hz would be muffled. Finding the right tuning frequency is an art and some manufacturers even design their drivers with this element in mind while carefully considering the size of the speaker’s cabinet.
Bass-reflex technology: drawbacks
Bass-reflex technology is extremely efficient and can deliver low frequencies with a moderate amount of air, but it is not without drawbacks. The word “resonator” itself raises an important question for a speaker: can a resonating tube be used to produce music? Several problems are inherent to this type of enclosure.
First of all, the air resonates with a delay which can be neither shortened nor controlled, and in such a way that the transient response is affected. In other words, the lows “drag,” creating a delay effect which can be easily heard.
Another shortcoming is the rise in impedance caused by resonance over the frequency range amplified by the resonator, mostly in the higher bass range. An amplifier powering a pair of ported speakers will therefore have to provide more power than if it were working with sealed enclosure speakers. The last drawback is that phase rotations happen at the resonance frequency, which disrupts sound placement. While this is nothing terrible as low frequencies do not require a precise orientation, it can make setting up a subwoofer a bit complicated.
Bass-reflex technology: the influence of the shape and number
The shape of the resonator only has a slight influence on the port’s behavior. Whether the resonator is circular or square-shaped, it will resonate at the same level and frequency. Nevertheless, the resonator’s surface has an impact on the airflow. The larger the surface, the more slowly the air will flow. Meaning that a port with a smaller surface will imply a faster airflow, which can cause noise. Using two, three or even four resonators will slow down the airflow and prevent noise. Some manufacturers, such as Monitor Audio and Bowers & Wilkins, have carried out research in order to optimize the structure of their resonators’ surfaces and regulate soundwave behavior.
Bass-reflex technology: resonator placement
The location of the resonator has to be considered with regard to the layout of the listening room. For a back-firing port, a wall or a corner behind the speaker can be used to maximize the efficiency of the bass-reflex port. This mechanical amplification is called room gain and can even double the volume of low frequencies delivered by the bass-reflex port. Front-firing ports are ideal for compact speakers as they may be placed on a bookshelf. A front-firing port also has the advantage of offering direct diffusion toward the listening area, therefore avoiding the usual latency between the lows and the rest of the sound spectrum. Sometimes, the bass-reflex port can even be placed underneath the speaker for a more harmonious sound diffusion. Some models by Focal (such as the Focal Aria 900) are fitted with both frontal and vertical bass-reflex ports.This post is also available in: French