[Paul Spencer]

There are two main things to look at which will tell you if your vent will have audible turbulence:

• peak vent velocity at maximum excursion (simulate with WinISD)
• aerodynamic factors

Reducing peak vent velocity

You can do this in a number of ways:

• larger total vent cross sectional area
• lower tuning
• rumble filter

Choosing a larger vent diameter only works to a point, as you will find the length required becomes large. Tuning a little lower can help, but you will find that this also requires a longer vent. Sometimes making the box larger can help as well, since the required vent length is reduced for a given tuning and vent diameter. It should now be apparent that an EBS makes a chuff-free vent easier to implement, and with some very high excursion drivers, this may be the only way you can reach its SPL potential in a conventional alignment without a passive radiator.

Aerodynamic factors

There are a number of aerodynamic factors which determine the maximum acceptable vent velocity before turbulence becomes audible. These include:

• air flow around both ends of the vent (obstructions will create problems)
• surface to volume ratio - keep as low as possible (a number of small vents combined will have more surface area compared to a single larger vent for a given total cross sectional area)
• flares on each end of the vent

Flares are critical.

Highly recommended: See experiments conducted by Bill (Collo) on this forum:
[vent flare testing]

There is a downloadable calculator. Enter vent diameter and flare radius and it will return the maximum acceptable vent velocity. You can then simulate in WinISD and find out the required size of your vent and the vent velocity that is actually achieved.

Conclusion

With drivers now being manufactured with very large excursion capability, it is getting more and more difficult to design vented subwoofers that can use all the excursion without chuffing. The simple but costly solution is to add a passive radiator. However, I suggest you may first try to get a vent that won't chuff. You will need to juggle factors like box size, vent tuning and diameter, while considering the vent flare radius you can build and the acceptable velocity. Without a rumble filter you will likely find it an impossible task unless you have a very large box or not much excursion. Ignore the standard recommendations of acceptable vent velocity recommendations (typically 17m/s or 34m/s) as they don't take account of enough factors to be of any use. Keep the velocity below Bill's recommended velocities and you should achieve good performance.

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