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Air Stream Bass Technology
The biggest challenge to subwoofer design: The acoustic short circuit
If you were to take a bass driver out of its enclosure and play it, you would notice that the driver doesn’t produce any bass tones. This is because, although the large circumference of most bass drivers is capable of shifting a lot of air, the back of the driver sucks the same amount of air back in. This phenomenon is known as the acoustic short circuit and happens whenever a driver is employed without an enclosure and baffle. Technically, sound is being radiated on either side of the driver but 180 degrees out of phase which leads to a cancelation of the sound.
We are only able to hear frequencies that have a wave length that is shorter than the path from the front side of the membrane to the back. For a driver with a 30 cm diameter, this path is about 32 cm. The frequency that corresponds to this wave length can be easily calculated by dividing the speed of sound (approx. 343 m/s) by the wavelength. A wavelength of 0.32 meters will produce a frequency of 1062 Hz, which lies in the upper midrange.
When constructing a subwoofer, it’s important to find a way to avoid the acoustic short circuit so that the bass can be heard. One way of doing this is to mount the woofer directly on a baffle that is so large that it prevents an easy exchange of air from the front to the back. This will prevent the air pressure in the front from being compensated for by air from the back. The baffle will need to be large enough so that only the very deepest frequencies with wave lengths capable of extending from the front of the driver on the baffle to the back will cancel out. In real terms, this means that if one wants to reproduce a 30 Hz frequency using this method, the baffle will have to have a diameter of 11.20 meters.
Clearly, it’s not practical to have a subwoofer wider than your living room and thankfully there are other options. One simply encloses the entire back side of the driver. This effectively prevents any air being exchanged between the front and back sides of membrane, thereby preventing an acoustic short circuit. The other consideration here is that the enclosure needs to be large enough. How large depends on various aspects of the driver and will differ from woofer to woofer.
A sealed enclosure can be relatively small, but the bass frequency will then also have a correspondingly high low end. The 12 dB per octave roll-off is too flat. If one wants to expands the lower range, acoustic tricks need to be employed.
The bass reflex enclosure
The bass reflex enclosure is a loudspeaker cabinet with a carefully constructed opening in one location. This opening usually takes the form of a tube-like tunnel with a diameter and length determined by the volume of the enclosure and the attributes of the woofer used.
But won’t an opening lead to an acoustic short circuit? A bass reflex enclosure actually turns this phenomenon to its advantage. Through the tunnel-like opening, the enclosure becomes a so-called Hemholtz resonator, or resonating cavity. The resonating frequency for the enclosure can be precisely tuned by the length and diameter of the tube. This resonating frequency allows the sound that is radiating from the backside of the membrane into the enclosure to exit the enclosure as air flowing through the tube. The length of the tube ensures that the sound produced by the tube and by the membrane do not cancel each other out in a certain range but rather reinforce each other. If the tube and the enclosure are tuned so that the resonating frequency falls approximately there where the enclosure reaches its low end, one can achieve a new, deeper low end for the entire system.
The acoustic short circuit does comes into play under the resonating frequency of the enclosure. In this range, the frequency from the back of the membrane and the tube cancel each other out. The frequency response of bass reflex enclosures therefore falls more sharply at 24 dB per octave than a sealed enclosure.
Teufel’s proprietary Air Stream Bass Technology
Since the resonating frequency of a bass reflex enclosure can cause the air to stream rather quickly through the tube, wind noises can develop and disrupt the music or movie playback. These can only be prevented by enlarging the opening through which the air stream exits. Unfortunately, this also alters the enclosure’s tuning frequency. In order to maintain the original tuning frequency, one needs to alter not just the diameter, but also the length of the tube, yet this length is limited by the size of the enclosure.
To solve this problem, Teufel’s engineers devised a way to use a vent in the place of a tube on our subwoofers. This elongated opening has a markedly larger diameter than a tube, and since it runs along the outside wall of the enclosure, it does not need to be so long as a tube to accomplish the same task. In this way, it was possible for us to create our proprietary Air Stream Bass vent in which practically no wind noise is produced.