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Below is a design in progress for a Seas all metal monopole 3 way implemented via a passive crossover. Feedback welcome.
Drivers
Tweeter – Seas 27TBFC/G
Midrange – Seas L15
Bass – Seas L26
Enclosure
The goals of the enclosure shape are (in no particular order):
1. Provide listening on (or possibly slightly below) tweeter axis
2. Provide front mounting of all drivers to suit the intended crossover points
3. Minimise baffle width for mid range and tweeter to provide best imaging (I’m not convinced of this benefit)
4. Minimise the visual footprint from the front, whilst still being wide enough to accommodate the L26
5. Provide non-parallel sides to reduce standing waves
6. Smooth diffraction ripple for midrange / tweeter given no rounder will be used on the final enclosure (for veneering ease)
7. Provide something interesting / different to look at
All drivers will be flush mounted. No roundover will be done on the final enclosure as this will make veneering difficult due to the non-parallel baffle edges.
The enclosure is modelled using ¾” (18mm) thick material (including braces) with a 1.5” (36mm) thick baffle. I intend to build using MDF. Also attached is a cutlist to try and minimise fence settings for the old table saw. I estimate approx 3.5 sheets of 8’ x 4’ (2400mm x 1200mm) MDF will be needed. (I’m too scared to cost the veneer I need!) Braces in the woofer enclosure are placed using approximate golden ratio distances to break up panel sizes to spread resonances. Damping material will probably comprise foam on the walls with Dacron stuffing. Dacron will be used over fibreglass since I want to minimise any fibres from coming free and finding their way into the open voice coil assemblies of the L15 and L26.
Midrange tuning
I decided to go for a very large sealed enclosure (approx 25L) for the L15. Reason being, I am hoping the depth of the enclosure (20.5” or 522mm) suitably stuffed will absorb plenty of backwave. The larger enclosure should reduce the box pressure therefore reduce the resistive / restoring force on the cone.
Bass tuning
The L26 seems usable in either sealed or vented enclosures. A 0.7 Qtc is achieved in a 47L sealed box or flat 90L vented alignment with a 30Hz Fb. A 4” diameter port, 4” long will reach the desired tuning. The 90L vented box will give a flat response and does not allow for room gain. A lower tuning via longer port may be required to cater for room gain. I don’t want to increase enclosure size as 90L enclosures are big enough.
I’m leaning toward the vented enclosure purely to minimise excursion since the L26 has 7mm of xmax and I want to run these full range with normal music. For low frequency effects (ie. the odd movie), I will use a sub and probably apply a 30Hz line level 2nd or 3rd order high pass full time to protect the L26. For aesthetics, I intend to mount the port at the back as shown.
Crossover simulation
The crossover below targets 4th order acoustic LR slopes at 240Hz and 1760Hz. The average sensitivity will be approximately 85dB. Impedance minimum is 3.3 ohms at 85Hz.
The below simulations were done using SpeakerWorkshop based on factory amplitude and impedance curves. The curves were imported into the FRD tools and had the box and baffle effects added in. I used the HBT transform in the FRC spreadsheet to extract minimum phase. The following driver offsets were used to approximate the acoustic centre offset placing the mic at a 1m position:
27TBFC/G = 0”
L15 = -1.1”
L26 = -7.7”
A deficiency with the above method is that the FR curves are on-axis measurements. SpeakerWorkshop cannot simulate vertical off-axis power response due to beaming for the L15 and L26 drivers (nor did I try and apply this before importing the data into SW). I’m hoping the lower crossover points for the L15 and L26 will reduce simulation inaccuracies. After all, it is just a simulation.
Listening room
My lounge is 18’ x 24’ with a cathedral ceiling going from 9’ at the sides to approx 15’ at the peak. The speakers will be placed along the 24’ wall. For serious listening I can place the speakers up to 3’ / 1m from the back wall. This would still give approx 10’ / 3m to the listening position hopefully allowing enough distance for acoustic summation from drivers at the listening position.
Next steps
I obviously intend to build a test box to the exact dimensions of the target enclosure and measure in place. I am able to measure down to approx 30Hz using an outside balcony approx 13’ from the ground. My mic measuring distance will be approx 2m (or greater) to provide proper summation of individual driver responses.
I also have not yet taken any non-linear distortion or impedance measurements which I will do in box (to highlight any resonance or other box effects). Using Mark K’s site for the L26 and some observations from Zaph’s work (eg. looking at W15 trends), I think the crossover points will minimise non-linear distortion in the drivers’ passbands. The 27TBFC/G is certainly capable of a 1760Hz LR4 slope. I also intend do so some excursion modelling to see which driver maxes out first and at what SPL.
I might have over-estimated:
a) my box construction capabilities with the approx 17 degree angle required
b) the L15 output on the low end (that there might not be enough SPL to blend well with the L26 at 240Hz). Once in box, I might therefore not need such a “heavy handed” 2nd order electrical HP on the L15. An alternative might be some impedance compensation on the low end (if possible) and a single cap. (In summary, I don’t trust the source FR curves and whether I have correctly subtracted / added measured / modelled box responses).
c) The $ budget to complete this project It certainly doesn’t aim to rank high in the bang for the buck stakes
Down the track, I might do a matching centre channel MTM using the L15 and 27TBFC/G (assuming I can get this done before the drivers go out of production!)
I’d appreciate your feedback.
David.
Drivers
Tweeter – Seas 27TBFC/G
Midrange – Seas L15
Bass – Seas L26
Enclosure
The goals of the enclosure shape are (in no particular order):
1. Provide listening on (or possibly slightly below) tweeter axis
2. Provide front mounting of all drivers to suit the intended crossover points
3. Minimise baffle width for mid range and tweeter to provide best imaging (I’m not convinced of this benefit)
4. Minimise the visual footprint from the front, whilst still being wide enough to accommodate the L26
5. Provide non-parallel sides to reduce standing waves
6. Smooth diffraction ripple for midrange / tweeter given no rounder will be used on the final enclosure (for veneering ease)
7. Provide something interesting / different to look at
All drivers will be flush mounted. No roundover will be done on the final enclosure as this will make veneering difficult due to the non-parallel baffle edges.
The enclosure is modelled using ¾” (18mm) thick material (including braces) with a 1.5” (36mm) thick baffle. I intend to build using MDF. Also attached is a cutlist to try and minimise fence settings for the old table saw. I estimate approx 3.5 sheets of 8’ x 4’ (2400mm x 1200mm) MDF will be needed. (I’m too scared to cost the veneer I need!) Braces in the woofer enclosure are placed using approximate golden ratio distances to break up panel sizes to spread resonances. Damping material will probably comprise foam on the walls with Dacron stuffing. Dacron will be used over fibreglass since I want to minimise any fibres from coming free and finding their way into the open voice coil assemblies of the L15 and L26.
Midrange tuning
I decided to go for a very large sealed enclosure (approx 25L) for the L15. Reason being, I am hoping the depth of the enclosure (20.5” or 522mm) suitably stuffed will absorb plenty of backwave. The larger enclosure should reduce the box pressure therefore reduce the resistive / restoring force on the cone.
Bass tuning
The L26 seems usable in either sealed or vented enclosures. A 0.7 Qtc is achieved in a 47L sealed box or flat 90L vented alignment with a 30Hz Fb. A 4” diameter port, 4” long will reach the desired tuning. The 90L vented box will give a flat response and does not allow for room gain. A lower tuning via longer port may be required to cater for room gain. I don’t want to increase enclosure size as 90L enclosures are big enough.
I’m leaning toward the vented enclosure purely to minimise excursion since the L26 has 7mm of xmax and I want to run these full range with normal music. For low frequency effects (ie. the odd movie), I will use a sub and probably apply a 30Hz line level 2nd or 3rd order high pass full time to protect the L26. For aesthetics, I intend to mount the port at the back as shown.
Crossover simulation
The crossover below targets 4th order acoustic LR slopes at 240Hz and 1760Hz. The average sensitivity will be approximately 85dB. Impedance minimum is 3.3 ohms at 85Hz.
The below simulations were done using SpeakerWorkshop based on factory amplitude and impedance curves. The curves were imported into the FRD tools and had the box and baffle effects added in. I used the HBT transform in the FRC spreadsheet to extract minimum phase. The following driver offsets were used to approximate the acoustic centre offset placing the mic at a 1m position:
27TBFC/G = 0”
L15 = -1.1”
L26 = -7.7”
A deficiency with the above method is that the FR curves are on-axis measurements. SpeakerWorkshop cannot simulate vertical off-axis power response due to beaming for the L15 and L26 drivers (nor did I try and apply this before importing the data into SW). I’m hoping the lower crossover points for the L15 and L26 will reduce simulation inaccuracies. After all, it is just a simulation.
Listening room
My lounge is 18’ x 24’ with a cathedral ceiling going from 9’ at the sides to approx 15’ at the peak. The speakers will be placed along the 24’ wall. For serious listening I can place the speakers up to 3’ / 1m from the back wall. This would still give approx 10’ / 3m to the listening position hopefully allowing enough distance for acoustic summation from drivers at the listening position.
Next steps
I obviously intend to build a test box to the exact dimensions of the target enclosure and measure in place. I am able to measure down to approx 30Hz using an outside balcony approx 13’ from the ground. My mic measuring distance will be approx 2m (or greater) to provide proper summation of individual driver responses.
I also have not yet taken any non-linear distortion or impedance measurements which I will do in box (to highlight any resonance or other box effects). Using Mark K’s site for the L26 and some observations from Zaph’s work (eg. looking at W15 trends), I think the crossover points will minimise non-linear distortion in the drivers’ passbands. The 27TBFC/G is certainly capable of a 1760Hz LR4 slope. I also intend do so some excursion modelling to see which driver maxes out first and at what SPL.
I might have over-estimated:
a) my box construction capabilities with the approx 17 degree angle required
b) the L15 output on the low end (that there might not be enough SPL to blend well with the L26 at 240Hz). Once in box, I might therefore not need such a “heavy handed” 2nd order electrical HP on the L15. An alternative might be some impedance compensation on the low end (if possible) and a single cap. (In summary, I don’t trust the source FR curves and whether I have correctly subtracted / added measured / modelled box responses).
c) The $ budget to complete this project It certainly doesn’t aim to rank high in the bang for the buck stakes
Down the track, I might do a matching centre channel MTM using the L15 and 27TBFC/G (assuming I can get this done before the drivers go out of production!)
I’d appreciate your feedback.
David.
Attached Files
Ahh well - at least I can still stuff till my heart is content.
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