Simulation Results: Dayton RS180 / Seas 27TBFC 2-way Bookshelf

[Jay_WJ]

Up here is my first (hopefully last, but I know it's addictive ) speaker building project. The design goal is to build excellent cost-to-performance 2-way bookshelf speakers for HT/music use with a subwoofer added. For drivers, I chose Dayton RS180S-8 metal cone woofer and Seas 27TBFC/G metal dome tweeter, based on their low price/big dollar performance reported in DIY circles. Their distortion measures are excellent.

I'm currently in a simulation stage using FRD Consortium tools and Speaker Workshop. Unfortunately, I don't have a measurement setup yet and won't be bothered to have one for this project---I know it's reckless but that's what I can afford at this time.

For consistent measurements between the woofer and tweeter, I used the same Zaph's data for them---measured on an infinite baffle using the same method and tools.

Using FRC tools, I simulated the frequency responses of measure box and baffle, and target box and baffle. And they were combined to obtain a frequency response to be used in Speaker Workshop for crossover design. I found FRC tools very sophisticated and easy to use.

I'll use PE .5 cu 2-way cabinet for vented design with a port tuned at 47Hz.

I used Speaker Workshop to design a crossover network. As far as XO design is concerned, I think this software supports all necessary things, including acoustic offset adjustment for a driver on the baffle. Very easy to use, too.

I started from two existing network topologies for metal cones. One was used in Zaph's L18/TBFC and the other used in Roman JB's modified Dr. K RS180/RS28 MTM design. After a lot of tweaking trials, I decided on two different designs (see figures). Their resulting frequency responses with individual driver responses are also shown.






As you can see, their FRs are virtually identical. The crossover point occurs at around 1.65k. Their impedance plots are also almost identical with minimum 5 ohms around 3.5k (figures not shown here).

A key difference between the two designs is the type of a notch filter used to attenuate the cone breakup nodes of RS180. Design 1 uses two series LC notch filters parellel with the driver (Zaph's L18 design bechmarked) to manage twin peaks at around 6.5k and 8.5k. Design 2 uses a parallel LC notch serial to the driver (RJB's RS180 design benchmarked).

Although Design 1 does not completely kill the breakup nodes, both designs are quite effective in notching them out. A minor difference in their performance is their modeled reverse nulls when the tweeter polarity is flipped (figures shown below). Design 2's reverse null is deeper that Design 1's.




My question is, which XO design is better? If you prefer one to the other, why? I'd appreciate any helpful comments.

[sreten]

Hi,

There is no point in keeping high frequency impedance lowish.
For nearly all amplifiers the higher the hf impedance the better.

Part of good c/o design is maximising impedance and minimising phase angles.
Large phase angles are not good, particularly at low impedances.

You need to investigate vertical and horizontal responses.
In a sim you can simulate driver spacing interactions.
Ideally you also need 15, 30, 45 degree responses.

Note : your acoustic target is 4th order L/R but modified due to driver offset.
2nd order L/R acoustic is unworkable with the drivers and c/o point.

note : rounding of the baffle edges can be approximated by making
the effective size of the driver larger in the baffle simulator.

Using Zaphs measured RS180-S 8 parameters :
(considered 4 ohm version ? 3dB more voltage sensitivity ?)

47Hz is too high a tuning frequency, say 40Hz, I like the look of 36Hz.

Regarding c/o topology - whatever meets your targets.
Have you tried one notch (Zaph style) centred at 8.5kHz ?

note : Zaph uses the LC to notch the driver and give part
of the required c/o high pass slope to reach his target.

Seems you are well on the way to building a very good speaker.

/sreten.

[Dave Bullet]

Hi Jay,

would you mind posting your SWD file when done? I'd be keen to see how you've spliced in / simm'd the low end response for the enclosure (can't seem to figure this out in SW).

Cheers,
David.

[Jay_WJ]

Originally posted by Dave Bullet
Hi Jay,

would you mind posting your SWD file when done? I'd be keen to see how you've spliced in / simm'd the low end response for the enclosure (can't seem to figure this out in SW).

Cheers,
David.

I didn't use SW to model box and baffle effects. I used Unibox, BDS and FR Combiner at FRD Consotium for that purpose. SW was used only for the purpose of XO design.

[Jay_WJ]

Originally posted by sreten
[B]Hi,
Part of good c/o design is maximising impedance and minimising phase angles.
Large phase angles are not good, particularly at low impedances.

System impedance ranges from 5 ohms (at 3.5k) to 27 ohms (900Hz) and the phase angle ranges from -61 degrees (1.5k) to 30 degrees (450Hz). Is this alright?

note : rounding of the baffle edges can be approximated by making
the effective size of the driver larger in the baffle simulator.

BDS at FRDC does have an option for different types of baffle edge.

Using Zaphs measured RS180-S 8 parameters :
(considered 4 ohm version ? 3dB more voltage sensitivity ?)

I talked with a PE technician. The 4 ohm version is not 3dB more loud at 2.83V due to some uncertain individual driver characteristics. So there is no point using a low impedance driver (DCR 2.9 ohms) in that case only to give a heavy load to an amp.

47Hz is too high a tuning frequency, say 40Hz, I like the look of 36Hz.

If I wanted to use these speakers without a subwoofer, I'd go with 40Hz for deeper bass. But I wanted a bit more upper bass for seamless blend with my sub. But your point is well taken. It seems that 44Hz Fb is a good compromise.

Regarding c/o topology - whatever meets your targets.
Have you tried one notch (Zaph style) centred at 8.5kHz ?

Good point. I tried this and got a good result. Will post it below.

note : Zaph uses the LC to notch the driver and give part
of the required c/o high pass slope to reach his target.

Yes, you're right.

Seems you are well on the way to building a very good speaker.

Thank you for your comments! I really appreciate it. I have a question for you. What is the usual amount of driver offset between a tweeter and a 6.5 inch woofer mounted on a flat baffle? My guess is around .7 to 1 inch. But I want to narrow the range down for my particular drivers, because SW takes into account driver offsets in computing drivers' phase interaction.

[Dave Bullet]

Jay,

My guess based on using Zaph's measurements modelled in SW to get the deepest reverse null was -1.13 inches on the woofer relative to the tweeter in the L18/27TBFCG speaker.

Cheers,
David.

[Jay_WJ]

Originally posted by Dave Bullet
Jay,

My guess based on using Zaph's measurements modelled in SW to get the deepest reverse null was -1.13 inches on the woofer relative to the tweeter in the L18/27TBFCG speaker.

Cheers,
David.

1.13 inch is probably greater than the usual amount of offset of a 7 inch woofer from the baffle --- I guess it's normally from .8 to 1 inch. I think this is consistent with Zaph's statement. He says that the best phase matching between woofer and tweeter happens with ears level with the top of the enclosure.

[Jay_WJ]

Here's another XO design with a single serial LC notch in the woofer net:



And the resulting FR:



My only concern with this design is a very small inductor value used in the LC notch. Jatzen 18 ga line has it, but maybe it's too small? How about error tolerance? Or any other problem because it's too small? Not sure.

Actually I tried .05 mH, but the notch was too peaky to manage the twin peaks. Another problem was that the matching capacitance is too small for the woofer's good LR4 slope to be used for my target XO point (should be less than 2kHz).

By the way, the in-phase offset amount (woofer relative to tweeter) of each design is:

Design 1: -1.07 inch
Design 2: -.7 inch
Design 3: -1.11 inch

What do you think?

[sreten]

Hi,

JMO that sealed boxes work far better with subs transient reponse wise.
And remember all things being equal a smaller box sounds better,
due to greater rigidity / lower panel area, and they fit in better.

I'd go for the 0.38 box sealed .........

Consider omitting R10.

/sreten.

Can't you add a small series resistor to control notch width ?
edit : no problem with L being "too small", select the value
and gauge (DCR) that gives you the best compromises.

[Jay_WJ]

Originally posted by sreten
JMO that sealed boxes work far better with subs transient reponse wise.
And remember all things being equal a smaller box sounds better,
due to greater rigidity / lower panel area, and they fit in better.

I'd go for the 0.38 box sealed .........

The reason is probably that in a sealed box bass response decreases at -12dB/oct and with another -12dB/oct filtering of a receiver I can achieve -24dB/oct, which can be matched with the usual -24dB/oct decrease of subwoofer filtering? I think this is a good idea.

Consider omitting R10.

To increase system impedance, right?

Can't you add a small series resistor to control notch width ?

Also an excellent idea. I'll try it.

Thanks for the helpful comments. This is really educational!