Air gap height
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URL: https://forum.speakerplans.com/forum_posts.asp?TID=98207
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Topic: Air gap height
Posted By: stevie
Subject: Air gap height
Date Posted: 03 April 2017 at 2:40pm
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I notice that cheaper long-excursion drivers tend to have a smaller air gap height (like 7mm, for example) whereas more expensive long-excursion drivers tend to go for bigger air gap heights (like 10mm) even though this reduces the theoretical xmax and makes them heavier. What's the engineering benefits/drawbacks of having a short or long air gap? |
Replies:
Posted By: snowflake
Date Posted: 03 April 2017 at 2:56pm
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smaller air gap means you have to reduce the motor strength or increase the moving mass. |
Posted By: stevie
Date Posted: 03 April 2017 at 3:28pm
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That seems logical. What's the advantage of having a longer gap in that case? |
Posted By: mobiele eenheid
Date Posted: 03 April 2017 at 5:21pm
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Better cooling and longer gaps often stabilise excursion better beyond the gap. It might reduce the mathematical Xmax but also might increase the Xmax, defined by 70% of BL or Klippel. Today's high excursion (overhung) PA subwoofers tend to have an gap height near 15 mm. |
Posted By: stevie
Date Posted: 03 April 2017 at 6:06pm
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Thanks. That's very helpful. |
Posted By: gen0me
Date Posted: 03 April 2017 at 7:11pm
Why increase the moving mass? |
snowflake wrote:Posted By: snowflake
Date Posted: 03 April 2017 at 10:55pm
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other things being equal you have to wind a longer length of wire onto
the voice coil to keep the BL the same and it has to be thicker to keep
Re the same. halve the air gap length and you have to have four times as much copper. |
Posted By: gen0me
Date Posted: 04 April 2017 at 8:45am
| True, but when you wind more layers of wire than gap will be wider and B will drop even more. |
Posted By: Teunos
Date Posted: 04 April 2017 at 9:15am
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Motor strength per unit wire is BL/Re^2, and is the parameter you should optimize. Higher motor strength eventually leads to a higher efficiency and higher SPL. Designing a motor is always a balancing act of gap height, gap width, coil wire thickness, winding height, number of layers in and outside the gap (think 18sound split winding) as well as keeping in mind you require a certain Xmax, power handling as well as cost to actually make the driver affordable and keeping it producable. Did i mention magnet geometry yet? Some of today's magnets are marvels of engineering being completely FEM simulated to optimize the flux density inside the gap whilst minimizing the size and weight of the magnet making the magnetic material being used much more efficiently and producing less stray fields. There are many factors to consider and these aforementioned do not even include the cone or suspension that eventually make up a majority of the mechanical parameters. Different approaches can lead to different results, just look at the new 21DS115 from B&C. It uses a whole different coil geometry than the typical 21SW152, yet still provides similar or even better results. There is no simple answer. ------------- Best regards, Teun. |
Posted By: Crashpc
Date Posted: 19 July 2017 at 8:16pm
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As said, it depends. It might be good to think about extremes. 20mm coil with 18mm gap gives you 1mm of mathematical Xmax. But it will work well at excursion of 3mm, as most of the coil will still be in the gap. now 7mm long coil with 3mm gap will give you 2mm of Xmax, but more % of the gap will be without the coil inside at excursion of 3mm. Therefore less efficiency, more distortion etc... ------------- Nikon and Canon people should not be married to each other. Why did you let this happen? |
Posted By: Hemisphere
Date Posted: 20 July 2017 at 2:22am
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It's almost as if the parameter 'xmax' is not fit for purpose. It sound like something you'd print in neon red on the front of a boombox that looks like a spaceship.
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