Thanks to 5th for the work documenting his experiment. We can also test the inverse horn idea. A possible next step would be to calculate the volume of what I would call a dunce cap (not a squid). Replace it with a tube of equal volume. Test and compare responses.
From what you posted to date, it does not seem that the shape of the rear chamber extension is critical.
Now, I hope you don't mind, but I am adding a little bit that only fits with your thread because of "When I was young....."
Circa 1974 or 1975, T/S was still fairly new and quite the talk of the town. I had a pair of Panasonic 1.5 inch dome midranges with a vented pole piece and rear chamber. At the time it seemed obvious that modifying that chamber was the thing to do.
I still have that pair of drivers and because of your thread I unpacked them and retested. Revisiting them is interesting for several reasons.
1. Dome design has not advanced much since the mid 70s. The Panasonics have a nasty dip a little over 4 kHz (internal reflection between dome, front of magnet structure, and the dense damping material placed behind the dome), but the top end response is acceptable. Extension even to 20 kHz.
2. The pole piece vent, if small (as pictured below) is the critical dimension. The rear chamber is a variable of little import. Consistent with your testing, the volume of the rear chamber only slightly extents the LF response.
3. Treated cloth domes hold up very well over time. They seem to suffer very little from the effects of age (as long as they are kept away from strong UV).
Anyway, here is a photo of the vent. You can just barely see the perferated metal sheet that holds the behind dome damping material.
From what you posted to date, it does not seem that the shape of the rear chamber extension is critical.
Now, I hope you don't mind, but I am adding a little bit that only fits with your thread because of "When I was young....."
Circa 1974 or 1975, T/S was still fairly new and quite the talk of the town. I had a pair of Panasonic 1.5 inch dome midranges with a vented pole piece and rear chamber. At the time it seemed obvious that modifying that chamber was the thing to do.
I still have that pair of drivers and because of your thread I unpacked them and retested. Revisiting them is interesting for several reasons.
1. Dome design has not advanced much since the mid 70s. The Panasonics have a nasty dip a little over 4 kHz (internal reflection between dome, front of magnet structure, and the dense damping material placed behind the dome), but the top end response is acceptable. Extension even to 20 kHz.
2. The pole piece vent, if small (as pictured below) is the critical dimension. The rear chamber is a variable of little import. Consistent with your testing, the volume of the rear chamber only slightly extents the LF response.
3. Treated cloth domes hold up very well over time. They seem to suffer very little from the effects of age (as long as they are kept away from strong UV).
Anyway, here is a photo of the vent. You can just barely see the perferated metal sheet that holds the behind dome damping material.
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This post is about the reponse of the Panasonic.
The Panasonic has a round mounting plate 13 cm in diameter. The pole piece vent is 15 mm in diameter and 20 mm long. The unit was tested free air far field using a quasi anechoic technique.
While advertised as either a dome midrange or midrange/tweeter, the Panasonic is a much better tweeter than midrange. The above 6 kHz response is acceptable and like the Morel is unaffected by the rear chamber.
Below 4 kHz, the rear chamber does impact the response, but only by small amounts. A couple hundred Hertz extension at most. These very small changes even when moving from small sealed enclosure to dipole (open back) suggest that rear chamber shapes are not critical for vented pole piece dome tweeters.
The LF extension does not seem desirable for either driver. Even with the LF extension, both still need to be crossovered to a woofer at what is considered a particularly critical region. A region generally avoided when designing loudspeakers and crossovers.
Still, rear chambers of dome tweeters are excellent projects for first projects for novice designers. While the possible gains are small, so are the possible pitfalls.
Good designing and good building,
Mark
The Panasonic has a round mounting plate 13 cm in diameter. The pole piece vent is 15 mm in diameter and 20 mm long. The unit was tested free air far field using a quasi anechoic technique.
While advertised as either a dome midrange or midrange/tweeter, the Panasonic is a much better tweeter than midrange. The above 6 kHz response is acceptable and like the Morel is unaffected by the rear chamber.
Below 4 kHz, the rear chamber does impact the response, but only by small amounts. A couple hundred Hertz extension at most. These very small changes even when moving from small sealed enclosure to dipole (open back) suggest that rear chamber shapes are not critical for vented pole piece dome tweeters.
The LF extension does not seem desirable for either driver. Even with the LF extension, both still need to be crossovered to a woofer at what is considered a particularly critical region. A region generally avoided when designing loudspeakers and crossovers.
Still, rear chambers of dome tweeters are excellent projects for first projects for novice designers. While the possible gains are small, so are the possible pitfalls.
Good designing and good building,
Mark
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Wow thats quite a dip those panasonics have, I was thinking you meant about a 5dB dip not 18dB!
Have you tried removing/altering the material behind the dome to sort out the null? or is access behind the dome and top plate non existant?
Cheers for your response.
Have you tried removing/altering the material behind the dome to sort out the null? or is access behind the dome and top plate non existant?
Cheers for your response.
The "damping" material is not a key variable in this drivers' response. Not all domes are shaped the same. While this dome does well above 7 kHz, the very nasty dip just over 4 kHz is the result of a combination of the volume of air behind the dome, the "tuning" of the air in the pole piece vent, and reflection off the magnet pole piece.
In this situation, no material yet in existence is capable of damping the acoustic vibration mode that produces the 4 kHz dip.
Let me say as clearly as possible, the Morel tweeter is far superior to the Panasonic because Morel has controlled for dome to magnet front plate problems.
Good designing and good building,
Mark
In this situation, no material yet in existence is capable of damping the acoustic vibration mode that produces the 4 kHz dip.
Let me say as clearly as possible, the Morel tweeter is far superior to the Panasonic because Morel has controlled for dome to magnet front plate problems.
Good designing and good building,
Mark
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