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Multi-Way Conventional loudspeakers with crossovers

Crossover frequency for transmission line
Crossover frequency for transmission line
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Old 12th April 2018, 06:19 PM   #1
Cliveinuk is offline Cliveinuk  United Kingdom
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Question Crossover frequency for transmission line

Hi there
I am wanting to build a 3 way speaker with a transmission line as bass maybe from about 30 hz tuning frequency using a Morel Ultimate UW1058 (25hz to 800hz)

with a conventional sealed box for a midrange - using a Morel Supreme SCM634 ( 50Hz -7kHz)

and a tweeter ( not concerned with that yet)
My question is 2 fold
1) Where would I look to cross the bass to midrange , there is no info out there as to how wide (octaves) the bass band can be with a TL speaker

2) if I cross to the midrange at say 150 hz do I tune the midrange box to the 50 hz resonant frequency as claimed by the speaker manufacturer or do I tune the box closer to the crossover frequency I want?

thanks in advance
Clive
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Old 14th April 2018, 12:55 AM   #2
altie is offline altie
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Join Date: May 2010
Location: Portland, OR
My experience from recent simulations has been that there's fairly continuous behavior between vented/ported/bass reflex configurations and TLs where you can trade off size (vented -, TL +) for port air speed (vented +, TL -).

You could start by simming a recommended vented box configuration for that woofer, see what happens when you extend the port to a quarter wave length (286cm by my calc), and then adjust the port and box dimensions to where the response (which will be very wonky) starts to nudge towards what you would want it to be.

Another possible approach: start with the most naive possible transmission line as a simulation and add stuffing to get the resonances somewhat tamed. This approach has the advantage of giving you a more direct picture of what kind of bandwidth you can expect. From the graph attached, the first null generated by pipe resonance centers around 90Hz.

So far in my sims and reading I've run into four main ways you might improve that, in rough order of how often they're talked about here: mass loading (MLTL), tapering, driver offset, and a coupling chamber. Look those up if it looks like getting rid of that first null might be getting you in the ballpark of the response you're looking for.
Attached Images
File Type: png morel-uw-1058-striaght-pipe.png (13.0 KB, 151 views)
File Type: png morel-uw-1058-striaght-pipe-2.png (6.1 KB, 144 views)
File Type: png morel-uw-1058-striaght-pipe-4.png (15.4 KB, 139 views)

Last edited by altie; 14th April 2018 at 01:03 AM.
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Old 14th April 2018, 01:08 AM   #3
altie is offline altie
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Location: Portland, OR
To answer your questions more directly:

1) You could cross as high as let's say 500Hz or higher if you wanted, if that would serve other design goals like reducing midrange excursion. At 500Hz crossover and 30Hz tuning it looks like you'd be exposing the listener to 4 nulls. Those nulls get progressively shallower as you move up in frequency, although group delay around them will get worse. The depth and the group delay can be addressed by more stuffing, at the cost of reduced bottom end efficiency.

2) You could probably tune the midrange box higher than the recommended 50Hz and get a slight efficiency boost, at the possible cost of phase and group delay issues (higher Q/boominess).
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Old 14th April 2018, 01:40 AM   #4
altie is offline altie
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A correction: group delay doesn't get worse as you go up in frequency but may still be pretty bad near those nulls.
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Old 14th April 2018, 01:27 PM   #5
pkitt is offline pkitt  United States
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If the TL is designed properly, optimized for the intended woofer and built as designed the TL itself will not limit where you can cross the woofer to a midrange anymore than it would be limited if the woofer were in a sealed or typical vented box. Of course coming up with a proper and optimized design can only be accomplished if you have accurate software to use for modeling the design, like Martin King's software.
Paul
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Old 14th April 2018, 05:43 PM   #6
Cliveinuk is offline Cliveinuk  United Kingdom
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Hi Altie

looking at your diagram 3 those null frequencies start at around 80 hz about one and a half octaves above the tuned frequency ( these definitely do not look good on sound quality)
so maybe one needs to look at a 4 way setup and cross over at the 90 hz mark as per
Speakers: Principles

Paul and Altie will a sealed bass show these same nulls ( I am sure not)

thanks again
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Old 14th April 2018, 09:16 PM   #7
PeteMcK is offline PeteMcK
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"Where would I look to cross the bass to midrange" - at the baffle step frequency
"tune the midrange box" - so it gives the flattest response in the passband, combined with the rolloff you want below the LF crossover frequency
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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 14th April 2018, 10:52 PM   #8
pkitt is offline pkitt  United States
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Of course a sealed box will not have nulls like this but that's not the point. If you correctly design an optimum TL, most likely a tapered one to minimize its length along with appropriate stuffing, the nulls will be optimally smoothed out and all but negligible, allowing crossing as desired. With the Morel woofer the OP wants to use and its fS and Qts, the optimum tuning frequency (1/4-wavelength resonant frequency from the line's length and taper ratio) is going to be at 18-20 Hz, BTW.
Paul

Quote:
Originally Posted by Cliveinuk View Post
Hi Altie

looking at your diagram 3 those null frequencies start at around 80 hz about one and a half octaves above the tuned frequency ( these definitely do not look good on sound quality)
so maybe one needs to look at a 4 way setup and cross over at the 90 hz mark as per
Speakers: Principles

Paul and Altie will a sealed bass show these same nulls ( I am sure not)

thanks again
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Old 14th April 2018, 11:25 PM   #9
Cliveinuk is offline Cliveinuk  United Kingdom
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Join Date: Nov 2017
Pete
excuse my ignorance here , just recently bought the loudspeaker cookbook
but what is the baffle step frequency and how do I get this from the specs ? https://www.morelhifi.com/wp-content.../11/UW1058.pdf

""tune the midrange box" - so it gives the flattest response in the passband, combined with the rolloff you want below the LF crossover frequency" so you are saying make my midrange box to suit an Fs of the crossover frequency and not of the midrange speaker which is lower than the crossover

Paul are you saying with a tapered line that is well padded those null areas will not be apparent? what frequency would you cross over at?
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Old 15th April 2018, 12:19 AM   #10
altie is offline altie
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Thank you. That 18-20Hz tuning sounds plausible to me from the sims I've been doing lately, but for my own education, how'd you arrive at it?

@Cliveinuk, like pkitt is saying, yes those nulls would destroy midrange qualities, no you don't have to tolerate them. Look up some details on mass loading, tapering, driver offset, coupling chambers and stuffing, and try running sims so you can see how those approaches would help. The numbers and graph I posted are for the most basic possible box you could call something like a transmission line for the driver you posted - *not* one that's optimized like pkitt is talking about, or one you would actually want to use. It's a starting point.
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