Speakerplans.com Homepage
Forum Home Forum Home > General > Advanced Discussion
  New Posts New Posts RSS Feed - Air gap height
  FAQ FAQ  Forum Search   Events   Register Register  Login Login

Air gap height

 Post Reply Post Reply
Author
Message
stevie View Drop Down
Registered User
Registered User


Joined: 16 March 2008
Status: Offline
Points: 319
Post Options Post Options   Thanks (0) Thanks(0)   Quote stevie Quote  Post ReplyReply Direct Link To This Post Topic: Air gap height
    Posted: 03 April 2017 at 2:40pm
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?
Back to Top
snowflake View Drop Down
Old Croc
Old Croc


Joined: 29 December 2004
Location: Bristol
Status: Offline
Points: 1684
Post Options Post Options   Thanks (0) Thanks(0)   Quote snowflake Quote  Post ReplyReply Direct Link To This Post Posted: 03 April 2017 at 2:56pm
smaller air gap means you have to reduce the motor strength or increase the moving mass.
Back to Top
stevie View Drop Down
Registered User
Registered User


Joined: 16 March 2008
Status: Offline
Points: 319
Post Options Post Options   Thanks (0) Thanks(0)   Quote stevie Quote  Post ReplyReply Direct Link To This Post Posted: 03 April 2017 at 3:28pm
That seems logical. What's the advantage of having a longer gap in that case?
Back to Top
mobiele eenheid View Drop Down
Young Croc
Young Croc


Joined: 15 August 2004
Location: Netherlands
Status: Offline
Points: 1292
Post Options Post Options   Thanks (0) Thanks(0)   Quote mobiele eenheid Quote  Post ReplyReply Direct Link To This Post Posted: 03 April 2017 at 5:21pm
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.


Edited by mobiele eenheid - 03 April 2017 at 5:22pm
Back to Top
stevie View Drop Down
Registered User
Registered User


Joined: 16 March 2008
Status: Offline
Points: 319
Post Options Post Options   Thanks (0) Thanks(0)   Quote stevie Quote  Post ReplyReply Direct Link To This Post Posted: 03 April 2017 at 6:06pm
Thanks. That's very helpful.
Back to Top
gen0me View Drop Down
Registered User
Registered User


Joined: 20 February 2016
Location: UK
Status: Offline
Points: 154
Post Options Post Options   Thanks (0) Thanks(0)   Quote gen0me Quote  Post ReplyReply Direct Link To This Post Posted: 03 April 2017 at 7:11pm
Originally posted by snowflake snowflake wrote:

smaller air gap means you have to reduce the motor strength or increase the moving mass.

Why increase the moving mass?
Back to Top
snowflake View Drop Down
Old Croc
Old Croc


Joined: 29 December 2004
Location: Bristol
Status: Offline
Points: 1684
Post Options Post Options   Thanks (0) Thanks(0)   Quote snowflake Quote  Post ReplyReply Direct Link To This Post Posted: 03 April 2017 at 10:55pm
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.
Back to Top
gen0me View Drop Down
Registered User
Registered User


Joined: 20 February 2016
Location: UK
Status: Offline
Points: 154
Post Options Post Options   Thanks (0) Thanks(0)   Quote gen0me Quote  Post ReplyReply Direct Link To This Post 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.
Back to Top
Teunos View Drop Down
Old Croc
Old Croc
Avatar

Joined: 23 November 2008
Location: The Netherlands
Status: Online
Points: 1684
Post Options Post Options   Thanks (0) Thanks(0)   Quote Teunos Quote  Post ReplyReply Direct Link To This Post Posted: 04 April 2017 at 9:15am
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.

,,Diamonds are forever''
Back to Top
 Post Reply Post Reply
  Share Topic   

Forum Jump Forum Permissions View Drop Down

Forum Software by Web Wiz Forums® version 11.10
Copyright ©2001-2017 Web Wiz Ltd.

This page was generated in 0.031 seconds.