# Airspace/chamber for mid/tweets



## Zeitgeist (Apr 4, 2009)

So, I'm pretty familiar with modeling subs and woofers with WinISD, and I can easily see the difference in the chamber size when it comes to QTC/rolloff (response). I know that some tweets have sealed backs - and for others it's recommended to build a very small chamber to keep it isolated from lower frequencies. I'm not sure about mids? I know that even the term "midrange" can be construed rather broadly - in terms of what frequencies it's actually covering.

Is there any rule of thumb when it comes to chamber sizes for mid/tweets? "As small as possible? --- since the drivers really get no benefit from the airspace and if anything would likely pick up unwanted resonances from the chamber?

I know my question sounds rather stupid, but I've seen so many different approaches when it comes to designs. No chamber, small chamber, moderate size chamber..

I would think that for a driver covering above a few hundred hz a very small chamber would be ideal? Is that correct or totally off base?

Thanks in advance!


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## fusseli (May 1, 2007)

If it's a sealed-back mid or tweet then it needs no chamber. For cone midranges (typical woofer as a midrange) then it'd need a chamber, yes. Size needs to be as big as it will model flat in the midrange's passband. You don't want to choke it off so much that it has a peak (really high Qtc) as to color it's frequency range, then that'd would not be desired. Summary: you still need to model it unless it has a sealed back.

In my Monkey Coffins I'm using an RS150 in a little under 2.5L. This is so small to the point it has about a 2dB peak at 200Hz, but it's crossed up above 400Hz so it doesn't matter. The chamber is also fully filled with Acousta-Stuf. The midrange is much much clearer with actual Acousta-stuf, for a while I ran them with just a little pillow fiber in there and wow what a big difference fully filling with the real stuff made. Keep in mind that fill will also alter the Qtc, in the case of a small enclosure it's a desirable effect to slightly flatten the lowend.


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## Zeitgeist (Apr 4, 2009)

fusseli said:


> If it's a sealed-back mid or tweet then it needs no chamber. For cone midranges (typical woofer as a midrange) then it'd need a chamber, yes. Size needs to be as big as it will model flat in the midrange's passband. You don't want to choke it off so much that it has a peak (really high Qtc) as to color it's frequency range, then that'd would not be desired. Summary: you still need to model it unless it has a sealed back. fill will also alter the Qtc, in the case of a small enclosure it's a desirable effect to slightly flatten the lowend.


OK, B&C 8CXT. It's a coax driver. Mid is cone, tweet is horn. I've modeled it before but the results make no sense. I've verified the T/S of the driver in WinISD against the datasheet..

MFG is this black and white response of both the mid and tweet. Graphs are from WinISD.

I'm half waiting for someone to chip in and say "WinISD isn't accurate!"


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## fusseli (May 1, 2007)

That's just how winISD models inductance. WinISD is also meant for box modelling, oh which is only useful for up to 200-300Hz. It's pretty meaningless to look at anything winISD reports above that range.


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## Zeitgeist (Apr 4, 2009)

fusseli said:


> That's just how winISD models inductance. WinISD is also meant for box modelling, oh which is only useful for up to 200-300Hz. It's pretty meaningless to look at anything winISD reports above that range.


I had a feeling..

Any recommendations on what to use to model higher ranges?


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## fusseli (May 1, 2007)

Get Jeff Bagby's PCD 7.0 (Passive Crossover Designer) and RM (Response Modeller) to go with it. A quick rundown on modelling and Xover design: trace impedance and FR plots from driver specsheets (or measure them if you have a measurement rig), process them in RM to get your baffle model and choose desired BSC and extract minimum phase, then load the processed .zma and .frd into PCD. PCD is capable of active and passive Xover modelling.


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## Moonfly (Aug 1, 2008)

fusseli said:


> That's just how winISD models inductance. WinISD is also meant for box modelling, oh which is only useful for up to 200-300Hz. It's pretty meaningless to look at anything winISD reports above that range.


Out of interest, why is this. Surely the maths will hold up above the sub bass range? The WinISD forum does have a section for full range speaker design construction. Their own forum is seldom used, but the fact they created that section when they set up the forum says to me that it can be used for designing any speaker system, you can after all model all kinds of filters.


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## Zeitgeist (Apr 4, 2009)

Thanks! Will do.

I actually heard Jeff speak at the last Parts Express DIY Speaker Competition. He kept joking about his naming schemes. Funny guy. I've been meaning to check out his software.


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## fusseli (May 1, 2007)

Moonfly said:


> Out of interest, why is this. Surely the maths will hold up above the sub bass range? The WinISD forum does have a section for full range speaker design construction. Their own forum is seldom used, but the fact they created that section when they set up the forum says to me that it can be used for designing any speaker system, you can after all model all kinds of filters.


If there was a way to get raw driver .frd and .zma into winISD then it could be used for fullrange I suppose... to my knowledge that isn't possible. Even then you would still need to process the frds for a specific baffle, and the woofer's impedance for a specific enclosure alignment.

Jeff's software is immensely powerful  all you need is Excel and to take the time to understand what you're doing and how to use the software properly.


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## Moonfly (Aug 1, 2008)

fusseli said:


> If there was a way to get raw driver .frd and .zma into winISD then it could be used for fullrange I suppose... to my knowledge that isn't possible. Even then you would still need to process the frds for a specific baffle, and the woofer's impedance for a specific enclosure alignment.
> 
> Jeff's software is immensely powerful  all you need is Excel and to take the time to understand what you're doing and how to use the software properly.


I figured the way you would have to do it with WinISD is model each driver, in its own cab, then look at the combined responses on the graphs, and tweak filters to suit. I guess its not ideal though as like you say there is nothing in the way of baffle step comp etc etc in there.

This program you mention does sound much better for speakers, but I dont have excel so couldnt really mess with it anyway.


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## fusseli (May 1, 2007)

And it's only capable of active filters and doesn't allow combining responses to observe phase / off-axis response. It's just not developed for fullrange speaker design.

The Bagby stuff on the other hand...


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## Zeitgeist (Apr 4, 2009)

But if I understand Bagby's software - you need FRDs?

Since there don't appear to be any FRD's for the 8CXT... Sounds like I'd have to create one.


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## fusseli (May 1, 2007)

Zeitgeist said:


> But if I understand Bagby's software - you need FRDs?
> 
> Since there don't appear to be any FRD's for the 8CXT... Sounds like I'd have to create one.


Ya unless you can find someone else's measurements or make your own, you'll need a program called SPL Trace which you click along a screenshot to trace the manufacurer's specsheet FR. Then you'd need the same for the impedance plot to make a .zma. There is also the option in RM to model the enclosure with the woofer via T/S parameters, which is the way to go since you can model sealed and vented. You'd do that to generate the .zma, then you splice on the box response at 100-300Hz. The only trick is to adjust the parameters carefully so that the woofer's rising impedance closely matches that of the specsheet.


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## Zeitgeist (Apr 4, 2009)

PCD looks great for passive crossover design... but I don't see how it takes into account the box/chamber sizes.

Does Unibox handle full range any better than WinISD?

I'm thinking about just building 2 chambers and then measuring FR...


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## fusseli (May 1, 2007)

You account for box alignment in Response Modeller (RM), then load the result into PCD.

Modelling the enclosure will not effect frequencies above a few hundred Hz. Modelling the baffle WILL effect FR due to edge diffraction and baffle step. That's a huge advantage of using RM to model the baffle, enclosure, and extract minimum phase from .frd and .zma data for proper modelling.

At very high frequencies the baffle can have an effect, but the larger contributors are edges such as the tweeters (if not flush mounted), having a grill in place, etc.


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## Zeitgeist (Apr 4, 2009)

fusseli said:


> You account for box alignment in Response Modeller (RM), then load the result into PCD.
> 
> Modelling the enclosure will not effect frequencies above a few hundred Hz. Modelling the baffle WILL effect FR due to edge diffraction and baffle step. That's a huge advantage of using RM to model the baffle, enclosure, and extract minimum phase from .frd and .zma data for proper modelling.
> 
> At very high frequencies the baffle can have an effect, but the larger contributors are edges such as the tweeters (if not flush mounted), having a grill in place, etc.


Ahhh. HM. OK. Some day I'll wrap my head around Jeff's suite. 

Well, OK, perhaps that's where I'm confused. I'm not planning on using the mid until a few hundred hz. 400-600hz perhaps. When I hear midrange, I always think of it starting at few hundred hz. So, "If the midrange starts above above a few hundred hz, the enclosure doesn't matter (much)". Is that accurate?

I'm not really concerned about BSC and other issues you'd normally need to integrate into a crossover design - since I'm going fully active and using a MiniDSP and planning on using some biquad filters for EQ.

The 8CXT is going to be flush mounted. 

Thanks for your help by the way!


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## Brewski (Jan 8, 2010)

Zeitgeist said:


> Thanks! Will do.
> 
> I actually heard Jeff speak at the last Parts Express DIY Speaker Competition. He kept joking about his naming schemes. Funny guy. I've been meaning to check out his software.


I didn't know you were at MWAF Did you bring a set of speakers? I had the Ruinations and the Levitations there this year and was there for Jeff's discussion after lunch.

Take it easy
Jay


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## Zeitgeist (Apr 4, 2009)

Brewski said:


> I didn't know you were at MWAF Did you bring a set of speakers? I had the Ruinations and the Levitations there this year and was there for Jeff's discussion after lunch.
> 
> Take it easy
> Jay


I actually didn't have a set of speakers. I really came to check out the tent sale (having never been - and only living 1.5 hours away I figured why not). I came part way through the day to MWAF - not fully understanding that it was an all day event. I was there for 2-3 hours and had a nice time. 

I distinctly remember seeing your speakers and being blown away. I loved the baffle design on the Ruinations. I believe I heard the Ruinations -- and was surprised by the bass!

Some day I'll have to build a pair and bring em. I feel lucky to live so close to PE.

My current project is the AE TD12S/B&C 8CXT fully active 3-Way (LCR).. Hoping to get the cabinets mostly finished tomorrow.


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## Brewski (Jan 8, 2010)

Zeitgeist said:


> I distinctly remember seeing your speakers and being blown away. I loved the baffle design on the Ruinations. I believe I heard the Ruinations -- and was surprised by the bass!


I unfortunately heard some conference room boom which I haven't heard at local DIY gatherings. In general though folks that have heard them are surprised by the bass the Founteks put out. 

Here are a couple links that will help when you opt to tackle PCD I think of them as the quick start quide vs. Jeff's more detailed manuals.

Paul C
http://sites.google.com/site/undefinition/simulated-measurements

Wolf
http://techtalk.parts-express.com/blog.php?b=74#comment234

Take it easy
Jay


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## Mark Seaton (Jun 22, 2006)

Zeitgeist said:


> Ahhh. RM. OK. Some day I'll wrap my head around Jeff's suite.
> 
> Well, OK, perhaps that's where I'm confused. I'm not planning on using the mid until a few hundred hz. 400-600hz perhaps. When I hear midrange, I always think of it starting at few hundred hz. So, "If the midrange starts above above a few hundred hz, the enclosure doesn't matter (much)". Is that accurate?


Not really, or at least "it depends..."

The models that aren't making sense are correct, they are just incomplete. T/S models are looking at simple acoustic output, and take no account for changing directivity or spacial radiation. While not free, LspCAD has some tools to approximate these factors such as directivity due to cone size and simulation of baffle diffraction. you can find separate models in different shareware tools that cover most of the factors; the work is in combining them together to a minimum of views and models with the ideal being one.

You can model a dome tweeter as a sealed speaker and get a better grasp of the excursion requirement vs. frequency for low crossovers, as well as directivity effects. The models scale just fine, but additional effects which are non-issues at low frequencies become significant factors at higher frequencies.

The sizing of the mid enclosure will be determined by your design philosophies and priorities, and unfortunately there is no absolutely correct answer. You have to consider reflections in the chamber, sensitivity over the frequency range of interest, electrical efficiency (related to impedance curve), as well as the response shape you are targeting or willing to work with...


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## Zeitgeist (Apr 4, 2009)

Hey Mark, 
Thanks as always for sharing your wealth of knowledge. Your explanation clears up much of my confusion (correct models, just incomplete).

I've looked as LspCAD before - because it seems to be the only software that does any of the more advanced simulations (and optimizations). Not sure if the standard version would satisfy my curiosity or not. Big difference between $220 for std and $1100 for pro!

I've decided on a chamber design already (built). I never would have imagined that mid/tweet chamber sizing would be so muddy. I'm sure it's crystal clear with some good models - but much more challenging to model than just a simple subwoofer.

I guess that explains why most (DIY?) designs don't seem to have much engineering behind the mid/tweet chambers. Seems like by far most just leave the backs open.


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## Zeitgeist (Apr 4, 2009)

Brewski said:


> I unfortunately heard some conference room boom which I haven't heard at local DIY gatherings. In general though folks that have heard them are surprised by the bass the Founteks put out.
> 
> Here are a couple links that will help when you opt to tackle PCD I think of them as the quick start quide vs. Jeff's more detailed manuals.
> 
> ...


Thanks for the links. I've started playing with PCD more (using an SPL trace program to generate FR and Impedance files). 

That conference room for the meet seemed...... less than great. I was sitting near the rear and it seemed like there was a huge amount of distracting ambient noise from the HVAC.


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## Brewski (Jan 8, 2010)

Zeitgeist said:


> I guess that explains why most (DIY?) designs don't seem to have much engineering behind the mid/tweet chambers. Seems like by far most just leave the backs open.


If you're using Jeff's software you'll want to model the chamber in RM if you aren't using cabinet measurements. If you're using measurements of the of the driver in the enclosure you don't need to simulate the enclosure and the baffle effects since they are already in your measurements. If you have phase data in the FRD and ZMA file you can put it in PCD directly. If not you'll want to use RM to extract the phase data for both files.

Take it easy
Jay


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## Zeitgeist (Apr 4, 2009)

Brewski said:


> If you're using Jeff's software you'll want to model the chamber in RM if you aren't using cabinet measurements.
> Jay


I used FRD/ZMA data based on the Mfgr. graphs.

I loaded that data into RM and then tested some different cabinet sizes and 400-500hz was the point at which I started to see rolloff. Above that, it looked flat.

Is there still more magic that I'm missing?

It makes sense that you need to get the response in RM and then pull that into PCD.


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## Brewski (Jan 8, 2010)

It doesn't sound like it since you aren't going to splice in box bass effects in RM since it's rolling off above 300. If I was using a driver with more extension I'd splice the box effects to the FR. 

The next step in RM would be to apply the baffle step and diffraction effects from the bottom of RM for baffle step and diffraction effects. I'd expect that once you enter the drivers position data, round over data you'd see a ripple that you'll want to apply to the FRD file before saving it and extracting the phase data. How big the ripple will be would depends on round over size, placement of the driver on the baffle, and how large the driver is itself. Tweeters have the highest sensitivity to diffraction effects and RM allows the user to change the tweeters location on the baffle for the least impact. I'd expect there to be maybe a ~2 db ripple for a mid range.

If you decide on wanting less than 100% baffle step you can apply the full baffle step and then choose to enter the baffle width at the top and inverse the baffle step effects. If you do this you'll want to apply the same value to all of the drivers in the design.

Take it easy
Jay


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## fusseli (May 1, 2007)

Thanks for picking up where I stepped out of the convo, brewski. These sound like good instructions to me :yes: If only the whole process were so easily explained as a few forum posts... well I guess it really is, it just takes a lot more effort to understand what is being done.


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## Zeitgeist (Apr 4, 2009)

Thanks for the pointers Brewski!

BTW, noticed that you had your speakers on the PE site in the Speaker Projects section for the MWAF.
http://www.parts-express.com/project-gallery/midwest-audio-fest/

Nice job.


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## Brewski (Jan 8, 2010)

Thanks for the plug I couldn't wait to get them up when PE sent an invitation to those that went to MWAF to post thier designs.

One thing I didn't touch on above... Once you've entered the box data and TS parameters into RM I'd extract the zma file and then extract the phase for that file. I'm not sure how much of an impact it will have on chaning the mids impedence in RM vs. manufacture's spec but the model should yield a value very close to what the drivers impedence value is in the cabinet. If the mid is a sealed back design I'd measure imp phase for the mfg spec like I would with a tweeter and only solve for baffle and differaction effects.

Take it easy
Jay


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## Mark Seaton (Jun 22, 2006)

Zeitgeist said:


> Hey Mark,
> Thanks as always for sharing your wealth of knowledge. Your explanation clears up much of my confusion (correct models, just incomplete).
> 
> I've looked as LspCAD before - because it seems to be the only software that does any of the more advanced simulations (and optimizations). Not sure if the standard version would satisfy my curiosity or not. Big difference between $220 for std and $1100 for pro!
> ...


If you already have the chamber constructed, do a quick model of the chamber for the LF behavior and relative Xmax/SPL vs. power & frequency and get on to measuring. No amount of modeling will change what you already have constructed! 

If you are interested in better correlation to your models, impedance measurements are key. This is how you can confirm if you have the Fb correct and the low frequency loading. Once you confirm that, you can then examine the baffle, boundary and directivity variations from the simple model of acoustic output (assumed omni and in constant spacial loading). From the measurements you can then decide if you want to do anything else in the mid chamber. You can always make it smaller or stuff with materials with differing effects. Stuffing/damping reflection off the back of the chamber in some manner is most critical though.

There is no "correct" sizing, as the preferred choice depends entirely on the designer. In passive crossovers, I typically do not want a flat raw response for a mid in a 3 way, but many other designers would never want to start with that... different strokes and all that.


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