Port noise

[mgboy]

What's the suggested 'limit' on how much air velocity is too much? Is there one? What is audible, and what isn't? etc etc..

[SteveCallas]

It varies based on the diameter of the port. Download Collo's "Flare It" program to quickly calulate at what speed chuffing and compression will occur with different diameter ports at different frequencies.

[brucek]

Steve,

I believe his testing (and the tool) only go as far as 6 1/2" ports.

My observation from members with LLT's is that the large 8" and 10" ports don't chuff or compress enough for anyone to be complaining. Why is that?

Whenever I have taken the specs from someones LLT and looked at the Air Velocity charts at various power inputs, the values always exceed the recommendations.

Is the reason because of the low frequency that max velocity occurs in an LLT won't likely ever happen?

Case in point. bnw's post shows the Air Velocity chart. I don't think 1000 watts is out of line as a power input, yet shows slightly high port speed. In comparison to many other LLT's that have been built here, I can almost guarantee he wouldn't hear port noise with this design. Why is that?

brucek

[Blaser]

brucek wrote:

Is the reason because of the low frequency that max velocity occurs in an LLT won't likely ever happen?
brucek

I think this is the real reason, the lower the tune the less you will ever likely meet any port shuffing. For ex. my LLTs have a 13.5 Hz tune, and I can never hear any shuffing or so with 8 inches ports....And FYI, I don't hesitate to fire 125 db+ in my room when I'm in the mood....

[mgboy]

Well what do you guys think about having a couple elbows on the port, on the inside of the box? Bad?

[Blaser]

The larger the elbow Radius the better, the larger the port diameter the better, the smaller the elbow angle the better. But I think there is no major problem if you can build it properly.

[SteveCallas]

With the 15" driver LLTs using 6" diaeter ports, yes, technically the port air speed can get too high and cause compression, and I have mentioned this before, as I don't find those designs to be "ideal". They are optimized, but not ideal. When you get into 8" and 10" ports however, I have rarely seen a design where compression will occur. An 8" diameter port begins reaching compression when air speed gets ~32m/s based on Collo's work, and depending on how you feel air velocity should be predicted, I haven't seen any designs get that high. There are several ways to display air velocity in WinISD Pro, and I don't necessarily agree that "peak" is the best representation unless you plan on only playing straight sine waves. Actual media is mainly comprised of short bursts at various frequencies, therefore I feel "rms" is a better representation - you can change this under File, Options. The max speed when using rms will be lower than peak, below ~32m/s with any 18" driver I have seen using a healthy 1000-1500 watts.

As far as chuffng, based on Collo's work, flare radius plays a large role, yet those who have built LLTs with no flare on the port have not heard chuffing - I think this happens for many reasons. First, the playback levels required to reach chuffing are quite high, and not often reached. Second, when such a level is reached, any chuffing that might be going on would easily be drowned out by the actual bass otput. Third, as has been mentioned, the tuning is low enough that frequencies in that range aren't heavily called upon. Factor all these things together and chuffing is basically a non issue.

Compression is the issue I would worry about more, and as we'll likely see from Ilkka's testing of the TC 2k LLT using a 6" diameter port, there should be a few dbs of compression at max playback levels near tuning. With an 8" diameter port for an 18" driver or 10" diameter port for dual 15"s, you should be quite safe from any compression.

[brucek]

Yeah, thanks for that post Steve - good information. The reality that the velocity will ultimitely be determined by program material is a good point. I guess it would be smart to take a look at both peak and RMS values of velocity and excursion to get a feel if you're within the ballpark with your design...

brucek

[collo]

I couldn't have put it better myself, Steve!

[SteveCallas]

brucek wrote:

I guess it would be smart to take a look at both peak and RMS values of velocity and excursion to get a feel if you're within the ballpark with your design...

You know I never even realized you could set excursion to rms as opposed to peak. That sheds a lot of light on the other discussion going on about how much power to feed a LLT and the amount of power I thought was needed for amp limiting. Seems you can get by with a lot more power for a given amount of excursion if it's not a continuous tone. Very interestng.

[brucek]

Yeah, I think you could look at peak to see the real worse case, knowing that this would only occur at that frequency with a continuous tone. Then look at the RMS value and know that this is more representative of standard program material around that very low frequency. Interesting indeed. Seems that amplifier clipping (with cheap amps) is a bigger problem actually from that other discussion...

brucek

[Chrisbee]

If the port chuffing is frequency related then it would chuff more on inaudible frequencies around the tuning point rather than the audible range.
So "drowning out" will not take place except on high level, harmonically rich signals.
That said, finding pure sinewaves at high levels might be difficult outside the classical organ music genre.
Very few films seem to use pure VLF sinewaves for LFE.
There is usually some harmonic content for the average film fan to latch onto.
Only bass junkies (like us) would have equipment capable of doing VLF sinewaves reasonable justice.