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Old 24th January 2009, 01:02 AM   #1
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Default Passive subwoofer help

Hi,

I've built a small, approx 30litre, passive subwoofer. It uses 2 x 8inch drivers in isobaric configuration. It is ported by a single port 3inches in diameter and 10 inches in length.
Both drive units have been heavily modified and have fs of 20hz.
Just need some suggestions as to how I can alleviate the bass hump between 35hz-50hz (please see attachment)

Thanks in advance
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Old 24th January 2009, 06:07 AM   #2
GM is offline GM  United States
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Without driver specs, it's hard to say, but a double hump is usually due to a high inductance (Le) combined with a too high a tuning.

GM
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Old 26th January 2009, 01:06 PM   #3
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Thanks GM.

Sorry I couldn't provide driver specs, as the drivers are very heavily modified and are pretty old.

Just general suggestions to get me started. Both of which are very good.

Thanks
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Old 27th January 2009, 12:42 AM   #4
simon5 is offline simon5  Canada
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I could be caused by the room you are using it in.

Try moving it around the room, let it play in the room, find where bass sounds best in your room and put it there.
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Old 27th January 2009, 12:58 PM   #5
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Thanks Simon.

The measurements were taken in an anechoic chamber.
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Old 27th January 2009, 02:27 PM   #6
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by delphiplasma

..... The measurements were taken in an anechoic chamber .....

Hi,

Unless it is a seriously big and very expensive
anechoic chamber the bass end will have errors.

Vented tuning of the box is ~ 40Hz. If driver Qts is too high you will
get a peak around/above the vent tuning frequency whatever you do.

Only if the bass is dry and overdamped with the port sealed will
vented work. In that case simply keep tuning the port lower
and lower until any sign of a peak disappears, possibly go
a little lower for better transient response / room matching.


/sreten.

The above assumes normal design parameters.
FWIW 30L / ~ 1 cuft is very large for isobaric 8" drivers
and this suggests something is very amiss somewhere.
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Old 28th January 2009, 12:38 PM   #7
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Thanks, Serenten.

I have the system tuned at 20Hz.

I've tested manufactured active subwoofers in this Chamber and they have measured as per the manufacturer’s specs, no humps or bumps.

I may try varying the inductor value to see if that will affect the overall response.
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Old 29th January 2009, 11:22 PM   #8
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Ok, it seems removing the capacitor and leaving the inductor removes that hump. Not good, as plenty of midrange seepage!

However, there is still a slight dip between 25hz-40hz. Guess I will need to experiment with different drive units.

I might decide to just go active!

The subs do sound great, with great extension and solid upper bass. However, can be a bit overwhelming (Due to the hump)
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Old 30th January 2009, 12:34 PM   #9
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by delphiplasma


I have the system tuned at 20Hz.

Hi,

A 3" diameter port would need to be 40" long for a 30L box.
These are WinISD numbers, not exactly accurate but ballpark.

Your box is tuned to ~ 40Hz.

/sreten.
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Old 2nd February 2009, 08:43 PM   #10
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Hi,

I measured the system tuning and the impedence dip centred at 20hz.

Thanks
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