Woofer Vents

[RonSSS]

416 and 515 woofers were vented via the dustcap.
The 411 was vented out the rear.
Can someone enlighten me as to the significance of this?
Just curious.
Ron

[Don McR]

Venting though the dustcap was the first method of relieving the pressure of air trapped between the dust cap and pole piece since this was the simplest solution. Venting out the rear is marginally more expensive since it requires machining a hole in the pole piece and pot structure. While more expensive, it was soon discovered that rear venting offered advantages in cooling the magnet structure that increased output and reduced dynamic compression.

I would guess that Altec kept the front vent on the 515 to the very end due to its value as a visual trademark as opposed to any performance issue. By 1990, they had converted all of their other bass drivers to rear vents.

[Todd W. White]

Don is right - the 515's remained unchanged because of tradition. They rear-vented everything else also because the front vent goes "whoosh-whoosh" at high velocities.

However - you really can't hear the "whoosh-whoosh" unless you're in a near-field environment, something the 515, 803, and 416 were not originally designed for. They were designed to go behind a motion picture screen, and be at least 20 feet from the nearest listener. It wasn't until they started going into home systems that the noise became annoying...

All of Iconic Manufacturing Company's woofers are rear-vented, for the same reasons given here.

[ckdf]

This is an interesting subject! I recently experienced some problems caused by this vent hole! I?m currently building 816 cabinets for 515-16Gs and first measurements revealed a significant dip in frequency response from 650 to 700 Hz. That was bad news as I had intended to cross over not earlier than 750 Hz. Almost drove me nuts as I could not find the reason for it. Cabinet bracing and damping did not cure it.
Finally I suspected that it was caused by the drivers. So I umounted them and did free air measurements. And indeed the SPL curve showed the same problem.
I then suspected that it was caused by the vent hole in the dustcap. I put a small round self adhesive felt patch onto the plastic grid that covers this vent hole. It is a bit smaller than the hole, so it leaves some circular gap to not completely seal off the voicecoil compartment. This almost completely cured the problem!
(I would not recommend this if you put lots of power to the driver, as it probably means less efficient cooling)

The reason for this effect is described by Dickason in his book (not for these drivers but in general):
The dustcap acts as a kind of airpump. When it moves backwards it pushes some air from the dustcap cavity out through this vent and when it moves forward it ***** some in. This creates a sound wave that is in anti-phase to the sound coming from the cone. For low frequencies where the whole cone radiates the SPL from the cone is dominant, so no problem. For high frequencies however the effective radiation cone area is getting smaller due to cone breakup modes. And maybe there is also a resonance at some frequency in the dustcap cavity increasing its efficiency. For this frequency the wave from the dustcap hole can obviuosly than cause some significant cancellation of the cone generated SPL.

May seem like a crazy idea, but I actually even thought whether it might be possible to completely remove the dustcap. This of course would only work if the dustcap is not needed to generate sound at higher frequencies. But a 604 e.g. which should have a similar membrane can obviously generate enough SPL up to 1200Hz without dustcap, so maybe the 515gs would do well without too?

So I?m wandering: Has anyone ever measured the frequency response of a 515g without dustcap? Maybe some of the professionals on this forum who also do reconings of drivers could do me a favour and do an SPL measurement of a 515g without dustcap as part of their next reconing job?
Would be interesting to see the results.

[Don McR]

Here's a suggested project for Iconic Manufacturing - a 515/416 recone that eliminates the vented dustcap and problems descibed above. It is possible to do this by venting through the coil former. The most notable examples are the Gauss bass drivers and JBL LE15. These drivers have slug magnets and closed pots just like the 515/416. They vent through perforations at the top of the former between the coil and where it attaches to the cone. Air vents to the space between the spider and the pot and a permiable spider allows this space to be vented to the rear of the cone.

Just a thought.