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Multi-Way Conventional loudspeakers with crossovers

BM-D446 on PH-4220
BM-D446 on PH-4220
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Old 9th March 2019, 11:16 PM   #1
kipman725 is offline kipman725  United Kingdom
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Default BM-D446 on PH-4220

I have been measuring this driver and horn combination:
Product - Pro Audio Components
Product - Pro Audio Components
My measurement environment is far from ideal, as the horn exhibits directivity I am assuming no reflection from the rear wall, looking at the impulse response my first major reflection is at 8.5mS which corresponds to 3m (distance from front of horn to wall and back to microphone). Its too late right now to measure loudly and take the light shade off.

I have measured two of driver+horn to see if they are consistent. For all measurements input voltage was 2.83V and the microphone was 1m away on axis.

I'm not sure on a few things:
1. Are the harmonic distortion figures I'm getting normal? I see around 2-3% THD in the operating band. I know that psycoacousticly this is fine but I don't know how this compares to other 1" compression drivers.
2. The horn has an abrupt transition from square profile to round this causes diffraction, is this the likely cause of the boost in the 4-10k band? should I do something about smoothing this? I was thinking wood filler.
3. looking at the distortion curves I think I want to cross using a 24dB/octave filter @ 1.5Khz to my mid range, is this sensible? this is for a PA application as well as for use at home, however I won't ever need to go over 120dB peak/1m.
Attached Images
File Type: jpg test_configuration.jpg (193.6 KB, 337 views)
File Type: png Distortion1.png (47.6 KB, 338 views)
File Type: png Distortion2.png (48.0 KB, 322 views)
File Type: png Freq_responses.png (36.6 KB, 326 views)
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Old 10th March 2019, 12:24 AM   #2
Sonce is offline Sonce  Macedonia
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1. Yes. they are normal. All compression drivers have dominant 2nd harmonic. 3th harmonic less than 0.5% is OK.
2. Find smaller and better horn.
3. Cross it at 1.8 kHz, better yet at 2 kHz.
Quote:
Originally Posted by kipman725 View Post
this is for a PA application as well as for use at home, however I won't ever need to go over 120dB peak/1m.
Ha, don't make me laugh! Of course you will!
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Old 10th March 2019, 02:46 PM   #3
kipman725 is offline kipman725  United Kingdom
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I would actually like a bigger horn as I am going to drill some holes for 4*faital pro 3FE25 to fire into the horn. I'm planning to use them down to 400Hz, the horn is far too small to load them this low but there will be efficiency gain higher in frequency, the combination should behave as a point source and allow me me the get the horn acoustically close to a 15" midbass. I do own some larger ZXPC 18 horns for 2 drivers but am saving them for another project.

I have added some wood filler to smooth the transition, the only notable change Im seeing is increase output 18-20kHz. I would guess if I could measure the polar response this would correspond to reduced off axis output in this range as I have reduced a source of diffraction. If this is true I would ***** this as a positive change as the directivity will be monotonic which is considered desirable. If I were not modifying the horn anyway this would probably not be worthwhile doing as you can see it now requires repainting.

I don't anticipate having to provide sound beyond 10m with this speaker which would still be 100dB peak which is loud enough. By 18kHz sensitivity is down to 90dB/2.83V/1m anyway so hitting that kind of level is only going to happen in the mid band without over powering the speaker. Most of the time this will be used indoors with less than a watt.
Attached Images
File Type: png filller.png (38.8 KB, 307 views)
File Type: jpg IMG_20190310_143342815.jpg (152.8 KB, 73 views)
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Old 21st March 2019, 09:05 PM   #4
kipman725 is offline kipman725  United Kingdom
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I have a WT3 now so time to begin work on the Faital pro 3FE25. I am using 4 of them per horn and would like to cross them to the compression driver as high as possible.

To do this I need to minimize the rear sealed volume. The plan with this is to construct the rear chamber using paper mache. The rear chamber volume can be found for simulation purposes by observing the change in resonant frequency. I have prototype'd this with painters tape: Free air: 114=Hz, tape=572Hz. You can see from the impedance sweep there are some anomalies caused by incomplete sealing, however I am pretty confident this is going to be a viable method for getting a high quality sealed back midrange.
Attached Images
File Type: jpg IMG_20190321_204417407_BURST000_COVER_TOP.jpg (186.4 KB, 71 views)
File Type: png leaky_sealed.png (125.4 KB, 66 views)
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Old 27th March 2019, 09:25 PM   #5
kipman725 is offline kipman725  United Kingdom
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I added 3 layers of paper mache on top of the glue and some instant nails adhesive. As you can see from the impedance sweep the sealing has improved with no evidence of the free air resonance. The next stage is to model the midranges on the horn, after working backwards to find the rear chamber volume.

The closed mic measurement isn't calibrated. The -3dB looks to be around 380Hz which is great as this is my target crossover frequency to my LF driver.
Attached Images
File Type: png sealedback.png (112.7 KB, 38 views)
File Type: png sealed_back.png (398.2 KB, 48 views)
File Type: png seald_back_close_mic.png (193.4 KB, 54 views)
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Old 28th March 2019, 09:44 PM   #6
kipman725 is offline kipman725  United Kingdom
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I have simulated the midrange mounted on the horn. What I am looking for here is that the cancellation null will be high enough in frequency. For some reason in simulation my drivers output is ~ 80dB/1W in hornresp (for a large box) while FaitalPRO give sensitivity as 91dB/1W. I do hope I can add 11dB to these sims otherwise I'm not going to hit my SPL target! (120dB/1m). The sims look OK I think to cross at 1.5kHz, the cancellation notch will be a bit lower in frequency as I haven't included the compression driver.

Maximum SPL is limited by driver input power to ~124dB continuous assuming that the sim 11dB down and multiplying the number of driver by 4 increases the SPL by +12dB.
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File Type: png 3FE25_on_horn.png (37.7 KB, 39 views)
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Old 17th April 2019, 11:07 PM   #7
kipman725 is offline kipman725  United Kingdom
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I don't have time to write up everything I have done right now but I am excited enough about this measurement to post it immediately. This is the 2.83V sensitivity of the nominal 8ohm load of 4 mids mounted on the horn. This is above 100dB/1W from 337Hz to 1.34 kHz. I would have preferred them to get a bit higher in frequency to meet the compression driver as it would make the crossover easier. As it stands if I aimed to an acoustic crossover at 1.3kHz this should (just about) work.
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File Type: png Mids.png (71.9 KB, 45 views)
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Old 22nd April 2019, 08:05 PM   #8
kipman725 is offline kipman725  United Kingdom
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I now have some time so will explain the mechanical attachment of the midranges.

To attache the midranges I constructed two identical mounting plates per horn (top and bottom). These mounting plates are made up of two sheets of ~6mm plywood (more like 5.5 mm in practice), that are glued together using wood glue. I constructed both the top layer and bottom layer of the mounting sheets using the same template that is attached. The procedure for using the template is as follows:

1. Stick on and cut around template and mark with small drill hits the center of the midrange taps.

2. Remove the template from this piece of ply (water effective), this is now the bottom sheet in the lamination.

3. Stick on and cut around template and use 72 mm hole saw (available from downlight kit) to cut out driver holes. This is the top lamination.

3. Align sheets at bottom left corner, clamp and wood glue.

4. Use coping saw to cut out slot.

4b. I think mounting the horn will be easier if 15mm is removed from the front of the template which I have done to one of the mounted plates pictured and plan to do on other future plates. I would advise doing this at this point for maximum mechanical rigidity in earlier steps.

5. Drill through driver mounting hole locations with 6mm drill bit

6. Screw in M4 threaded inserts (10mm depth type)

7. Epoxy plate to horn. The back of the mounting plate should be aligned with the end of the flat part of the horn.

8. Use 19 mm wood bit to complete driver taps using previously marked locations in step1. I did this very slowly and stuffed newspaper in the horn so that when breaking through I didn't gouge the horn.

I measure the midrange tap centers as 55 mm from the horn throat.
Attached Images
File Type: jpg alignment_of_mid_taps.jpg (132.7 KB, 42 views)
File Type: jpg clamping.jpg (352.1 KB, 35 views)
File Type: jpg mids_on_horn.jpg (122.5 KB, 42 views)
File Type: jpg Mid_tap_locations.jpg (171.0 KB, 48 views)
File Type: png WooferPads.png (29.5 KB, 37 views)

Last edited by kipman725; 22nd April 2019 at 08:09 PM.
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Old 23rd April 2019, 07:00 AM   #9
kipman725 is offline kipman725  United Kingdom
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Here are the impedance and frequency responses. I have quite a lot of reflection issues at lower frequencies. But I don't have a better environment to test in easily. This is more noticeable in the mid range as the horn isn't as directive. The mids are wired as series parallel to make an 8 ohm load.

I have also taken a close mic measurement of the mids (at the horn mouth), I think that it shows that there HF bandwidth is been limited by the volume of the air under the cone and so will attempt to build volume filling plates tonight.
Attached Images
File Type: png mid_and_comp.png (84.4 KB, 28 views)
File Type: png 3FE25_close_mic_on_horn.png (80.2 KB, 33 views)
Attached Files
File Type: pdf 3FE25_4series_par_on_PH-4220.pdf (36.0 KB, 6 views)
File Type: pdf BM-D466_PH-4220_With_Mids.pdf (36.9 KB, 6 views)
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Old 24th April 2019, 11:12 PM   #10
kipman725 is offline kipman725  United Kingdom
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I added some volume reduction plates to the woofer pads. Its late so I can only do close mic measurements. They show a modest increase in bandwidth ~100Hz.

The volume under the woofer cone has been decreased to almost the minimum possible it looks to me that that the limit on the mids high frequency output is due to their location on the horn. A few mm reduction in distance is possible but little more so this looks to be the best that this horn will do. It's a good job the compression driver looks to be able to go low.

With plug:
mid level: 106.33
-3dB (high): 1.286k
-6dB (high): 1.35k
-3dB (low): 305 Hz
-6dB (low): 285 Hz

Without plug:
mid level: 106.36
-3dB (high): 1.194k
-6dB (high): 1.254k
-3dB (low): 320
-6dB (low): 305

differences:
+92Hz (-3dB high)
+96Hz (-6dB high)
-15Hz (-3dB low)
-20Hz (-6dB low)
Attached Images
File Type: png fillers_comarison.png (77.8 KB, 25 views)
File Type: png filler_plates.png (3.6 KB, 23 views)
File Type: jpg filler_plates.jpg (191.3 KB, 30 views)
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