Substituting 8'' woofer with greater Fs - diyAudio
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Old 27th December 2014, 06:57 PM   #1
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Default Substituting 8'' woofer with greater Fs

Hello! I've built a pair of loudspeakers using the woofer Vifa M21WO39-08 in a 60 litres bass reflex cabinet tuned at about 35 Hz. The towers have a very good low extension but lacks of incisiveness and impact in the bass region of the "acoustical" musical instruments, as bass guitar and drums, like saying they are a little limp and not very engaging on that yield. In other words I'm looking for a crispier and more detailed response in the 50-150 Hz band.
For more than one year I've tried to improve this decreasing internal volume, shortening the vent tube, attenuating tweeter, but what I got was always boomy effect and need to return to the original configuration.
My question is now: can I expect improvement in the sense I want by replacing the woofer with one characterized by a lighter cone and a stiffer suspension (and thus greater Fs) but equal Qts and efficiency? I'm referring in particular to a Seas Excel W22NY001E0045, which has a Fs of 33 Hz and a Vas of 72 liters against the corresponding 28 Hz and 105 liters of Vifa, while the Qts values of 0.30 and the sensitivity of 90 dB are identical for both drivers.
In case, what may I expect from the different inductance value Le, that goes from 1 mH of the Vifa to 0.61 mH of the Seas? Must I change something in the crossover?
Thank you!
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Old 27th December 2014, 07:19 PM   #2
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Vas, Fs and Qts define the bass alignment (volume and tuning). For the crossover to act equally on a new woofer you need equal FR and impedance. A lower inductance means a lower rise in impedance, so probably not an easy drop-in replacement.

About your issue, are you sure that the crossover is dealing correctly with the baffle step effect? Ignoring the baffle step effect makes the bass thin, as is some dB (6 or slightly less) lower than the mid and tweeter.

Ralf
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Old 27th December 2014, 07:54 PM   #3
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What 'style' is the woofers main inductor?

I have found iron core low Rdc inductors to work best.

Air core inductors are obviously best for the midrange.
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Old 27th December 2014, 08:39 PM   #4
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assuming you've dealt with baffle step correctly,
re:"shortening the vent tube - how much did you change the box tuning? Tuning the box higher will give you more bass level, at the expense of low extension. The trick is not to over-do it (a 1-2 dB hump) to avoid the boominess (assuming everything else is OK, the box is not too big, is properly damped etc.) This is easily simmed in Unibox. A quick sim shows me that the peak bumps up quite quickly, so you can't tune much higher than ~40Hz, which will give a hump about ~1.75dB

re:"looking for a crispier and more detailed response in the 50-150 Hz band" - you're looking in the wrong place, detail is higher up
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Impedance varies with frequency, use impedance plots of your drivers and make crossover calculations using the actual impedance of the driver at the crossover frequency
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Old 27th December 2014, 11:23 PM   #5
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Please would you explain what is the baffle step effect? And how can I verify if it happens in my system?
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Old 28th December 2014, 11:13 AM   #6
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google it
first match: Baffle Step Compensation
fourth match: bafflestep
Forget about the equalization as explained in those links, it is properly done with an appropriate oversized low-pass inductor. If your speaker has a text-book crossover then surely the baffle step has not been taken into account. For an example on how to properly deal with the effect see this project: Zaph|Audio and in particular the transfer function: http://www.zaphaudio.com/audio-speak...erfunction.gif (look at the 200-1500Hz region).

Ralf
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Old 28th December 2014, 08:10 PM   #7
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Thank you, I'll study well. However the system I've built is based on The New Vifa Tower by MurphyBlaster, what do you think about its low-pass filter?
Inductors I've used are Intertechnick Tritec type, the main has ferrobar inductor body, the other hasn't.
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Old 29th December 2014, 02:47 PM   #8
Dissi is offline Dissi  Switzerland
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Quote:
Originally Posted by andreaemme View Post
The New Vifa Tower by MurphyBlaster, what do you think about its low-pass filter?
Based on a simulation the New Vifa Tower seems to have a hump of 3 dB around 100 Hz. Cause is the large woofer coil of 3 mH in the crossover. I'm not sure if that explains the issue with your speaker, perhaps someone else can tell more.

The Excel woofer certainly is not a drop-in replacement for the Vifa, you would have to redesign the woofer filter completely IMO.
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File Type: jpg SimulationAndreaemme.jpg (106.7 KB, 179 views)
File Type: jpg CrossoverAndreaemme.jpg (59.7 KB, 176 views)
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Old 30th December 2014, 07:44 AM   #9
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This could explain the negative sensation I have just for that field of frequencies.. So can I resolve this problem without substituting driver?
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Old 30th December 2014, 10:38 AM   #10
Face is offline Face  United States
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Have you taken any measurements?

Are you sure that the issue isn't the room itself?
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