Avoiding port noise -- churf and resonance.

[mrstampe]

Assuming both port ends are flared, is there a port velocity below which churfing can be avoided? I've seen this addressed in different ways and with different answers in several threads.

On a related question, how far above the low pass freq is it safe to have the first port resonance in order to avoid turning your sub into a trumpet?

[avaserfi]

mrstampe wrote:

Assuming both port ends are flared, is there a port velocity below which churfing can be avoided? I've seen this addressed in different ways and with different answers in several threads.

Different people try to keep air velocity through the port below certain speeds to minimize chances of port chuffing. Depending who you ask you will get different answers, but in general lower is better. I generally try to keep max airspeed below 25m/s, but depending on application can allow for it to go higher.

mrstampe wrote:

On a related question, how far above the low pass freq is it safe to have the first port resonance in order to avoid turning your sub into a trumpet?

This depends on a couple factors mainly crossover order and crossover point. The lower frequency first resonance starts at the higher order crossover you will need to use to avoid reaching this band. I have heard and am currently building a pair of subwoofers whose first port resonance is about 120Hz. Due to this resonance happening at a pretty low frequency I will be using an extremely steep crossover (8th Order - 48dB/octave) and crossing the subwoofers over around 60-80Hz.

[brent_s]

Also, keep in mind when you're looking at WinISD or Unibox port velocity graphy, you're usually looking at a worst case scenario. Most of us use the max sustained amp power as the input power for the sub. This is great for making sure you won't destroy the driver if someone cranks the master volume to '11'. However, in the real world, you're unlikely to drive your sub with that much steady state power. Drop the power by half, -3dB in SPL, and you'll see air velocity drop also.

I once read a white paper published by Peerless, the driver manufacturer, that recommended 34 m/s as a max....I think that's 0.1 Mach. And at one time, that was the recommendation included with WinISD Pro. Later versions have dropped the recommendation to 17 m/s, which seems to coincide with some of the research that Collo did. But note, that even his research suggested there are variables that can't be completely accounted for in actual usage, such as masking effect of real content vs. sine waves and port orientation (toward listener vs. firing into a carpet pad).

I like to point to a real world example. The SVS PB10 has a high excursion 10" woofer and only a 3" dual flared port. If you model something similar, say the Dayton Titanic 10", you'll see some really high port velocity numbers with "max" power. However, none of the reviews, paid or owner testimonials, I've read have noted port noise as a problem.

For me personally, I tend to stick with Peerless' 34 m/s number for now, but I generally never push my subs near max excursion. I've got a small ported sub planned with a RS315-HO that has a worst case scenario that gets into the low 40s near 20hz...I don't expect a problem with my real world volume requirements for that sub, but it'll be an interesting experiment to see how easily I can induce noise.

-Brent

[Ricci]

I basically try to stay below 34m/s as well. Good luck trying to stay below 17m/s at max power with a long throw subwoofer design. I'm not totally sure about this, but I think that the larger the diameter, or area of the port, the higher the vent speed before it becomes audible, or overloads. An 8inch port should be able to handle 25m/s air speed better than a 2inch port right? I could be completely off base here, but I believe that I read that somewhere.

[SteveCallas]

Maximum port velocity achievable without producing chuffing is not a concrete number for all port diameters - it varies by port diameter and the flare radius. The larger the diameter of the port and flare radius, the higher the speed you can get away with without experiencing chuffing. Collo's "Flare It" program (which I believe can be found on this forum) does the math for you, you just input your flare properties. His program will also tell you when compression starts setting in.

As for first resonance, based on experimenting done by various LLT owners, I'd recommend you keep it higher than 190hz when using an 80hz crossover. This roughly equates to a port no longer than 36". The shorter you can make it while still having enough port area to avoid chuffing and compression, the better, which gets back to using large enclosures.