Filby1
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Registered: 12th Dec 03
Location: Brighouse
User status: Offline
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Im going to buy one of the Alpine Letterbox Ported sub boxes for my type R sub. Do i need the subsonic filter turned on? and at what setting?
[Edited on 26-07-2005 by Filby1]
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cdcool1
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Registered: 9th Jun 02
Location: Scunny
User status: Offline
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Yes you do. What settings does your subsonic filter come with on your amp and what frequency is the box tuned to?
Subsonic filters are designed to cut out frequencies below the tuning of your ported box. If you didn't use a subsonic filter, and your box was tuned to, say, 40Hz, and a tone of 30Hz is played at any significant power, your sub may get seriously damaged.
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myke
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Registered: 7th Feb 01
Location: High Wycombe, Buckinghamshire
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i thought sub sonics were any frequencies lower than what the human ear can hear?
so that you use the filter to cut out any tones below what is audible to allow the sub to work harder playing the notes that you can hear?
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mark87
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Registered: 13th Jul 05
Location: Essex
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Well human hearing goes down - at its best - to 20Hz - so I guess so too.
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cdcool1
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Registered: 9th Jun 02
Location: Scunny
User status: Offline
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No, its a common mistake that poorly trained dealers often say.
As a simple demonstration, listen to a sub in a sealed enclosure with the sub sonic filter on and off. If what you say is right, you won't beable to hear a difference, but you can.
Speakers depend on air pressure in the enclosure to support the cone. In sealed boxes this is easily explained. In ported boxes, this is also true at and above tuning, however below tuning, the air cannot support the cone of the speaker and therefore the cone will flap dangerously. By cutting out frequencies below the tuning point of the box, the sub will not get damaged.
I believe the subsonic filter was probably named that due to boxes being tuned to the point at which our hearing is effective so at that point it is cutting out frequencies we can't hear, however for boxes tuned to 40hz, it will also need to cut out frequencies that we can hear
Its more accurately called a high-pass filter 
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