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Old 20th March 2007, 04:03 AM   #11
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ttan98,

I am not an expert but I will try to answer your question the best I can.

The baffle step can be attempted to be implimented in various ways. I guess the most flexible is to have an extra woofer. But this too can present problems with filter interactions. Ideally the baffle step frequency would occur at a crossover frequency but this usually does not occur unless the drivers and width of the cabinet are both planned out.

If the baffle step frequency occurs at a crossover you can just pad the drivers above the baffle step so they output less SPL.

If you double drivers in the same frequency range (assuming they are similar) then you should get around a 3 db higher sensitivity in that area so hence more output. This can be done with a second woofer which is low passed at the baffle compensation frequency instead of the standard crossover frequency. If the baffle step frequency happens at the crossover frequency you could also add two woofers or mid/basses instead of pading down the upper frequency drivers. Thus you would be increasing the sensitivity of the system at frequencies below the baffle step frequency.

This is all theory and filter interactions can make the extra woofer method difficult to accomplish if any of the crossover points are too close. So ideally you would want the baffle step frequncy at the crossover point.

What constitutes too close for filter poles I am not knowledgable enough to say. I know that the rule of thumb for parallel networks is minimum two octaves apart and three or more is better. I know very little about other filter topologies like series or cascaded or hybrid types.
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Old 20th March 2007, 09:23 AM   #12
sreten is offline sreten  United Kingdom
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Default Re: Baffle Step compensation

Quote:
Originally posted by ttan98

I thought when you apply Baffle Step(BS) compensation I think you have to apply at least to mid-bass as well as woofer, not just the mid-bass. No need for the tweeter I believe.

Hi,

Pretty much correct. Output into Half space should fall 4 to 6 dB
from bass area to mid band area well before the treble range.

You cannot just BSC the mid bass, bass output must be correct.

/sreten.
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Old 20th March 2007, 11:18 AM   #13
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by Hezz
Sreten,

Perhaps my use of the term "Ultimate" was too general. I am talking about getting the "ultimate sound from these drivers with this cabinet. Not in an absolute sense. And getting relatively deep bass without BR alignments.

True, the drivers are not the best, but these are what I have and they are capable of very good sound. At least I know the Vifa drivers are. In fact, all to often I see high end speakers used in the DIY community in inferior designed cabinets. In fact, this is more the norm than the exception. It doesn't make sense for me to intentionally compromise the cabinets and make them of lower quality than I know how to make them since this is a DIY project and to downgrade the only thing that I have the most control over does not make much sense to me.

There is compromise. There is overdoing it.

As for the 5 inch not being used as the BS driver I ask "why not" ?. Not because I want so much to use it as a BS driver but because I want the 6 inch as the mid/bass driver because it has a smoother top end roll off.

http://www.zaphaudio.com/audio-speaker13.html
one way of handling the c/o but note this c/o has BSC.


Can you give me any further reasons why I should just wire the bass drivers in parallel and not attempt to intitiate some sort of baffle step. There is also another good reason for using the 5 inch as the BS. Because the speakers will only be about 26 inches out from the wall I don't want more than 1-2 db of BS. I though the smaller driver would not excite as much air and might help to moderate the effect a bit.

Only 26" ? = only 1 to 2dB BS ? really ......

It is possible that at the distance from the walls the speakers will be used I may not even need the BS. But since I want to cross the woofer at a low Fs I figured that crossover point would be far enough away from the BS frequency to reduce some of the problems of crossover interaction. I'm thinking if the lower crossover is at 100 Hz and the baffle step is around 400-600 Hz that I am at minimum two octaves apart at these two filter points. I can see that if the two points were very close there could be problems.

You said a 3.5 way. what you now seem to mean is a 3-way with a 0.5 way mid section. 100Hz c/o point ? Price up the compoments. Be better off as a 3 -way with P21/W14/D27.

AMV8,

I think that you have misunderstood my design intentions. The tweeter and the mid will be in the upper cabinet. I have already ruled out a sealed alignment for this project and I only like critically damped sealed alignments. That required too big of a cabinet for this bass driver.

I'm not going to really comment on dogmatic statements about
bass alignments, room matching is far more important, and for
critically damped sealed alignments (=q=0.5) generally bass can
be massively improved by the addition of a correctly tuned port.


Also, the cabinet sizes cannot be any bigger or taller than these are so I have basically decided that the cabinet shape will not change. It's only a matter of how the innards are arrainged.

Good design is the art of intelligent compromise.

/sreten.
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Old 20th March 2007, 06:48 PM   #14
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Steten,

I guess if I do what I am planning the BS will actually be accomplished by the combined output of the mid/bass and the BS driver since the woofer-midrange crossover is so low. Techinically this would not be a 3.5. But I don't really know what to call it. THe BS driver would be overlapping the woofer and the three bottom octaves of the midrange driver.

I read on I think Rod Elliot's site that since speakers are not used in real 4pie space in normal rooms that only around 3 db is needed for speakers that are way out in the room and some lesser amount the closer the speaker is to the wall. Mine will be about 26 inches from the front of the cabinet so are even closer to the wall then I may have lead you to believe.

Thus my estimate of 1-2 db of BSC.
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Old 20th March 2007, 07:38 PM   #15
sreten is offline sreten  United Kingdom
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http://www.rjbaudio.com/Audiofiles/FRDtools.html

And don't forget to run excursion limited power handling sims on your 5" woofer.

You can use the above to design your crossovers.

/sreten.
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Old 30th March 2007, 01:54 AM   #16
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Ok guys,

After discussing this project with those who responded I have given this speaker design some more consideration.

In the interest of ease of building and ease of designing I have decided to go with a bass reflex bass alignment. I have also decided to use the 6.5 inch Silverflute and the 8.5 inch Vifa as dual woofers covering the same range.

In order to achieve some baffle compensation I will move the lower crossover point up in the 375-425 Hz range which is about where I will need the baffle compensation to kick in.

The lower cabinet is large enough so that each woofer can have it's own seperate space and be tuned Butterworth.

So now the question I have is since I have no way to measure the acoustic center between the two woofers should I just estimate it to be halfway between them and use the baffle width at this location to calculate the baffle step and crossover frequency.

I think because of the sloping bass cabinet sides I will try to use a 6db crossover between the woofers and the mid. Or will the acoustic centers between the woofers and the mid be too far apart in this critical region. THe reason for the 6 db is the low phase shift and faster rise time in this critical lower midrange area.

Thoughts?

Here is a picture of the new overall concept. Upper woofer has 2 inch port in rear of cabinet. Also, would it be better to put both ports on the back. THe only problem I see with this is the lower port will be too close to the wall. My concern is phase shift between the ports but the staggered tuning should actually be useful in my estimation.

I think putting both ports on the front would crowd the upper port in too close to the drivers.

More thoughts?
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Old 30th March 2007, 12:34 PM   #17
sreten is offline sreten  United Kingdom
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Hi,

It would make sense to me to implement BS by making the bass
section a variant of a 0.5 way, basically different roll-offs for the
two bass drivers - you can use more of the SFs good midrange
and it should of course blend better with the SF mid unit.

/sreten.
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Old 30th March 2007, 12:44 PM   #18
sreten is offline sreten  United Kingdom
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Hi,

And now the bad news,
the W14 is not efficient enough to be use with both bass units in parallel,
bi-amping with active bass to mid and passive mid to treble is an option.

/sreten.
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Old 30th March 2007, 01:18 PM   #19
sreten is offline sreten  United Kingdom
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Hi,

Possible options are probably to ditch the W14, use W17 as mid.

Then either a 2.5 way type affair, top sealed bottom reflexed ?

Or add another P21 for watt/puppy clone ?

/sreten.
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Old 30th March 2007, 05:11 PM   #20
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Looking at the effeciency levels of these drivers is does look like I may have a problem with the W14. I suppose I am going to need a mid with at least a 90-91 db effeciency or I may have level matching problems.

I guess the woofers can be padded down a little but probably not by much and since the mid ends up having more passive crossover components in it's path it's sound level will be reduced even more than the woofers.

Alternatives:

Use W17 as mid and do 2.5 way.

Use W17 as upper woofer and get a more efficient mid driver.

I have a few Dynaudio 8.5 inch drivers model 21W54 that fit in a small volume but have terrible bass extension. So poor that I wouldn't want to use it as a woofer. THe Silverfulte W17 has significantly deeper bass extension.

I really would prefer to have some kind of carbon fiber mid as I love the smooth carbon fiber sound. I could reduce the box size a little if I went with only three drivers.

Either way, I think I could solve a lot of problems by going with a 2.5 way and reduce the cost of the crossover at the same time.
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