I see on a lot of bass cabs that there is a little slot or opening at the bottom just under the speakers. What is the purpose of it?
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Question about cabs
- Thread starter FallingUnknown
- Start date
A port, designed to adjust the functional volume of the cabinet so as to maximise the low-end efficiency of the housed speaker(s) at a given tuning frequency. At least, that is my lay take on the matter. There are a good many professional and semi-professional builders and designers here who will no doubt provide a more detailed explanation in due course.
- Nov 24, 2008
- 4,343
- 7,163
- Disclosures
- Owner & designer, AudioKinesis; Auth. mfg, Big E (Home Audio only)
Growler is on the right track, but as he suspected a more detailed explanation is on its way.
The port + the volume of air in the box forms a "Helmholtz resonator". Now that sounds pretty technical, but I'm betting it's something you've encountered before even if you didn't know the name for it. Ever blow across the top of a beer bottle? Wuuuuuuuh. That's a Helmholtz resonator, formed by the neck of the bottle and the volume of air in the bottle. It resonates primarily at a certain frequency, but also puts out some energy on either side of that resonant frequency. Reduce the amount of beer in the bottle (thus increasing the volume of air), and the pitch goes down. If you could increase the length of the neck while keeping its diameter the same, that would also lower the pitch.
Back to our bass cab. Instead of blowing across the port from the outside of the box, the woofer in effect blows across the port from the inside of the box. Wuuuuuuuh. We designer types "tune" the port dimensions so that the Helmholtz resonance is a bit lower in frequency than where the woofer would normally roll off in that size cab, and thus we get deeper bass than we otherwise would have. We have to keep the port area wide enough for the airflow, and we have to pay attention to fartout, but if we do a decent job of juggling all the tradeoffs then the result is increased usable bass output compared to a sealed cab with the same woofer.
The port + the volume of air in the box forms a "Helmholtz resonator". Now that sounds pretty technical, but I'm betting it's something you've encountered before even if you didn't know the name for it. Ever blow across the top of a beer bottle? Wuuuuuuuh. That's a Helmholtz resonator, formed by the neck of the bottle and the volume of air in the bottle. It resonates primarily at a certain frequency, but also puts out some energy on either side of that resonant frequency. Reduce the amount of beer in the bottle (thus increasing the volume of air), and the pitch goes down. If you could increase the length of the neck while keeping its diameter the same, that would also lower the pitch.
Back to our bass cab. Instead of blowing across the port from the outside of the box, the woofer in effect blows across the port from the inside of the box. Wuuuuuuuh. We designer types "tune" the port dimensions so that the Helmholtz resonance is a bit lower in frequency than where the woofer would normally roll off in that size cab, and thus we get deeper bass than we otherwise would have. We have to keep the port area wide enough for the airflow, and we have to pay attention to fartout, but if we do a decent job of juggling all the tradeoffs then the result is increased usable bass output compared to a sealed cab with the same woofer.
billfitzmaurice
Commercial User
- Sep 15, 2004
- 16,860
- 278
- 76
- Disclosures
- Owner, Bill Fitzmaurice Loudspeaker Design
http://electronics.howstuffworks.com/speaker10.htm
But ignore this phrase, 'The disadvantage is that there is no air pressure difference to spring the driver back into place, so the sound production is not as precise', because it's not true.
But ignore this phrase, 'The disadvantage is that there is no air pressure difference to spring the driver back into place, so the sound production is not as precise', because it's not true.
