Speakers come in all shapes and sizes, but that’s not what really sets them apart from each other. Driver design choices and the way they are integrated into a speaker cabinet play a huge role in the sound that is reproduced.
In some cases, one or more passive radiators, ports, or vents are used in addition to the speaker drivers. In this article, we take a look at speakers that incorporate ports or vents, which are called bass reflex design.
What is a Bass Reflex speaker
A bass reflex speaker combines a tube, port, or vent with a traditional speaker driver to produce the desired sound reproduction.
When placed inside a speaker cabinet in combination with the traditional speaker, the bass reflex assembly produces sound by pushing air generated by the main speaker through a port or vent into the listening area.
The main driver pushes the air both forward and backward. Air exhausted from the front of the cabinet is dispersed directly into the listening area, while air exhausted from the rear of the speaker is directed through a tube to a port or vent and then to the listening area. Vent ports or grills may be located on the front or rear of the speaker cabinet and may vary in depth and diameter.
The port or vent that releases the air pushed into the listening area adds to the speaker by providing more sound, with an emphasis on the lower frequencies, without requiring more amplifier power.
Another benefit of a front-ported speaker is that they can be placed closer to the wall behind them, minimizing the impact of that surface and room corners on the speaker’s bass response.
Where Bass Reflex Speakers Are Used
You may find bass reflex assemblies used with the following types of speakers.
subwoofers: The purpose of a subwoofer is to provide deep, room-filling bass. By using one, two, or sometimes three ports in combination with the main subwoofer’s speaker driver, more air can be pushed out at the desired low frequencies while keeping the subwoofer box smaller.
Floor standing or bookshelf speakers: Similar to the ports or vents used on a subwoofer, a floorstanding or bookshelf speaker can complement the already included midrange/woofer or woofer in the low-frequency department, delivering fuller sound without requiring more amplification power.
Advice: Some portable and smart speakers may include a bass reflex array, but they will most likely use a passive radiator for that purpose as it takes up less interior space.
Optimization of a Bass Reflex speaker
On the surface, the concept of a bass reflex speaker is easy to understand, but manufacturers don’t randomly place a tube inside the speaker cabinet and plug it into a port or vent.
When loudspeaker designers make the decision to incorporate one or more ports or vents into a loudspeaker or subwoofer, the following must be considered:
Outdoor speaker box design: The outside of the box, like the type of baffle used, has to withstand the sound coming out of the port or vent into the room cleanly and without interference.
Speaker box interior design: The interior of the cabinet must support air movement from the main speakers so that air moves through the inner tube and out of the port or vent at the desired frequencies.
The choice of prime movers: The main drivers must reproduce the audio input signal at the desired frequencies and must move the proper amount of air to take advantage of the bass reflex capabilities of the speaker.
tube design: The tube feeding the port or vent should complement the main speakers and cabinet so they all work together to deliver the desired listening experience.
Tuning: In addition to assembling all the necessary components, the speaker designer must fit the bass reflex assembly to the main speakers to prevent unwanted distortion and ensure that the desired frequencies are emphasized with sufficient audibility.
Some things that are tuning obstacles include puffing or hissing sounds from the port as airflow increases. Also, unlike a sealed speaker cabinet, some bass-reflex speakers may not be as fast, accurate, or distortion-free as a subwoofer or speaker with more or more traditional speaker drivers or one speaker driver in combination with a passive radiator.
If the tuning process is successful, the bass reflex assembly absorbs energy from the main speakers, reducing their movement (damping). A byproduct of using the bass reflex solution is that damage (also known as pop) to the main speakers is reduced. This means that a bass reflex speaker (depending on make/model) can safely operate at higher volume levels.
Advice: While there are audible and physical design benefits to considering using ports or vents, another benefit is that they are less expensive than adding larger traditional speaker drivers to a speaker setup.
Examples of Bass Reflex speakers
Examples of Bass Reflex Subwoofers
Advice: Another method of increasing the sound output of the speaker is to use a passive radiator instead of using a bass reflex design, which we will cover separately.