Why do subwoofers have a hole?

Bass reflex tube. Strong sound from all pipes.

Have you ever wondered why some speakers have openings at the front, back or bottom? These are usually the inputs of a so-called bass reflex tube. They serve a strong purpose: more bass! We explain the principle.

The bass reflex tube clears the way for sound

Sound transducers in the loudspeaker housing are usually installed in such a way that they radiate the sound forward into the room. The air is pushed by the vibrating speaker membrane and spreads out in the room. But that's just one side of the coin. Because sound waves are also generated in the loudspeaker housing. The membrane, driven by a voice coil, always moves back and forth, i.e. in two directions - and thus also sets the air on its back in motion.

This backward radiated sound component remains locked in a closed housing and is lost. Put simply, the bass reflex tube is there to use this part for the sound as well - practically like a valve for full sound. Precision work is required to make the whole thing sound the way you want it - so don't think of drilling or sawing a hole in a closed loudspeaker housing anywhere!

In this blog article we explain the in detail How a loudspeaker works.

The name bass reflex tube already reveals the frequency range for which the sound device is intended. Such an amplifier for the bass is particularly useful because a lot of energy is normally required to generate low-frequency sound waves. The tube makes it possible to reproduce strong bass even with small speakers. But how can frequency ranges be directed through the bass reflex tube in a targeted manner?

Looking deep into the pipe - the process in detail

From a physical point of view, a bass reflex loudspeaker is a vibratory system - comparable to a swing spring and a weight attached to it or a swing and a person pushing the swing. In the case of the bass reflex loudspeaker, the total volume inside the main housing corresponds to the spring and the volume inside the tube corresponds to the vibrating component.

Every oscillatable system also has one Resonance frequency (tuning frequency) (among other things depending on the length and diameter of the pipe). This is the frequency of the energy pulse at which the system oscillates with the greatest deflection. This can be easily understood using the example of a pendulum. This vibrates best when the energy pulses are supplied with a very specific frequency.

Teufel subwoofer with bass reflex technology (selection)

In other words: The air in the bass reflex tube can be used as a second loudspeaker membrane for a certain frequency range. With the same deflection of the loudspeaker membrane, more intense bass (higher sound pressure level) achieve. Ideally, the resonance frequency of the bass reflex tube is in a range that the loudspeaker in a closed design could no longer cover. This shifts the Cutoff frequency so further down.

DPU technology: Teufel bass on all channels

Bass reflex loudspeaker in perfection. - With the Teufel DPU technology, we have further refined the resonator principle. Instead of round openings, we gave the loudspeaker housing slots. On the one hand, these allow very precise tuning to the desired frequency range and on the other hand, minimize flow noisethat can occur with pipes.

  • Theater 500 5.1 set: A system that can play anything: quiet, loud, high and low. The active subwoofer S 6000 SW is responsible for the bass; with an extra large bass reflex channel and a lower cut-off frequency of 25 Hz. You can also control the sub wirelessly, making it particularly easy to accommodate it in the room.
  • T 4000 flat subwoofer: You can hardly see him, but hear him all the better. This 250 watt subwoofer can be easily accommodated under the sofa thanks to its flat design. This is made possible by the downfire principle with downward radiating sound converters and the integrated DPU system.

Attenuate the bass reflex or not?

Some people ask themselves whether a kind of bass reflex shutter is suitable for making the bass sound softer and less massive or more unobtrusive. However, we advise against overly creative solutions à la sock in the pipe. If necessary, special foam plugs are suitable for correcting a slightly shifted fundamental frequency, as this allows the effective diameter of a bass reflex tube to be reduced. A bass reflex tube should work as unhindered as possible when there is resonance. That's why it is built in at all. Attenuation in the bass reflex tube has a fast and often massive negative effect on the actual basic function of the bass reflex. Therefore, one should not experiment here without ado.

Bass reflex tube or bass reflex duct?

Are you a hi-fi fan and technology pro and want to build your own speakers? Then you should first deal fully with the physical requirements and consider whether a bass reflex tube or a bass reflex duct is more suitable for your purpose. The best way to do this is to use the physical formulas to calculate a bass reflex and to calculate a reflex channel. However, that would go beyond the scope of this blog post. However, the following should be said: If you decide on a duct with a bass reflex function, you should definitely note that it is not only the volume of the duct that matters, but above all the ratio of duct cross-section and length. In addition, you should not plan the canal too flat, otherwise flow noises may occur. Of course, you can always choose your favorite Teufel speaker at any time! We have something for every sound lover in our program.

Teufel home cinema systems with innovative bass reflex technology

Conclusion: an effective bass amplifier

  • A bass reflex tube is used to amplify sound waves with low frequencies.
  • The sound waves emitted backwards are used to amplify the bass.
  • The volume of air in the loudspeaker housing and in the opening form a system that can oscillate.
  • The resonance frequency of a bass reflex loudspeaker should extend the lower cutoff frequency.