Advice about ports - avoiding chuffing

Hello! Long time lurker, first time poster, etc

I have a question about ports. Excuse me if I get the terms wrong or I’m barking up the wrong conceptual tree. I’m pretty new to this

I have an approximately 62L vented cab that I’m building for a Seas FA22 8" full-range. The design that I’m cloning uses a single reflex port in the front with a diameter of around 50-55 mm max

I’ve just today learned all about port velocity and what the design goals should be and when I model it in WinISD I get a velocity of around 40m/s. I understand I should be aiming for around 17m/s or less with a bit of wriggle room for flaring.

What I can’t work out is, what is the effect of the measured frequency on the port velocity. It peaks at 40m/s around 20Hz which is -18dB on the Transfer Function Magnitude plot and is just under 20m/s at 40Hz which is about my F6.

Does the fact that there’s so little energy at the peak velocity mean that I’m less likely to hear chuffing or am I thinking about it wrong? I have plenty of flexibility in the length of the port but I don’t want it any larger at the front from a aesthetic perspective. The only other way I can see around it is a 2nd port on the back but I don’t know how that would go.

I’d appreciate any advice/recommendations from the more experienced builders. Thanks in advance.

Here’s a pic of where I’m up to so far and the speakers that I’m drawing inspiration from, the Fern & Roby Ravens
 

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Last edited:

freddi

Member
Paid Member
2005-08-16 4:21 pm
what's the vent length and power rating of your amplifier? With 62 liters and a 10cm long by 52mm diameter vent, hornresp (2 pi, 2.83v, Rg = 0) indicates Fb ~28.5Hz and small signal F6 around 31Hz. With this low tuning, FA22's 3mm one-way Xmax will be hit with ~3 watts input at 44Hz. Maximum port velocity with 3 watts input occurs around 20Hz at ~14 meters per second. There really won't be anything useful at that frequency and rarely if ever excited unless by turntable rumble and record warp - which should be highpass filtered anyhow.

If the vent is shortened to obtain Fb close to 46Hz, hornresp says 1.8cm length (typical wall thickness) will tune ~43,5Hz and the response now has a 5dB peak around 55Hz.

Perhaps other programs are more accurate with vent length than hornresp ?

Anyhow, SEAS application notes recommend 70 liters tuned to 33Hz for vented box (QB3 alignment) and 60 liters closed. It looks like a wonderful driver.
 

RankStranger

Member
2019-09-22 10:41 am
what's the vent length and power rating of your amplifier? With 62 liters and a 10cm long by 52mm diameter vent, hornresp (2 pi, 2.83v, Rg = 0) indicates Fb ~28.5Hz and small signal F6 around 31Hz. With this low tuning, FA22's 3mm one-way Xmax will be hit with ~3 watts input at 44Hz. Maximum port velocity with 3 watts input occurs around 20Hz at ~14 meters per second. There really won't be anything useful at that frequency and rarely if ever excited unless by turntable rumble and record warp - which should be highpass filtered anyhow.

If the vent is shortened to obtain Fb close to 46Hz, hornresp says 1.8cm length (typical wall thickness) will tune ~43,5Hz and the response now has a 5dB peak around 55Hz.

Perhaps other programs are more accurate with vent length than hornresp ?

Anyhow, SEAS application notes recommend 70 liters tuned to 33Hz for vented box (QB3 alignment) and 60 liters closed. It looks like a wonderful driver.

Thanks, Freddi

The recommended vent length at 52mm was around 57mm, from memory (I don’t have my laptop with me but I remember lengths I tried were rarely about 10% more than width). My current amp is 40W per channel but I think I’ll ultimately run it off and 8wpc ACA (when I get around to it). Still, I want it to have a bit of versatility.

I haven’t tried hornresp, I’m trying to get my head around WinISD and I’ve had a play with SpeakerBox but it seems to be less versatile, with respect to port design.

I think I followed your post, do you think I should be fine with the peak velocity that low in the frequency range?
 
I haven’t tried hornresp, I’m trying to get my head around WinISD and I’ve had a play with SpeakerBox but it seems to be less versatile, with respect to port design.

From someone who use to have many different programs including WinISD, et al and some you've probably never heard of plus many custom Excel designers, calculators have been reduced to just Hornresp and links to a relatively few specialized calculators on line, so believe your time is better spent learning HR; the Wizards alone replaced most of what I had and some that didn't/don't exist except for possibly AkAbak, which is considered to be the hardest by far to master.

GM

edit: You may find these easier to use: Advanced Speaker Box Calculator
 
Last edited:

RankStranger

Member
2019-09-22 10:41 am
From someone who use to have many different programs including WinISD, et al and some you've probably never heard of plus many custom Excel designers, calculators have been reduced to just Hornresp and links to a relatively few specialized calculators on line, so believe your time is better spent learning HR; the Wizards alone replaced most of what I had and some that didn't/don't exist except for possibly AkAbak, which is considered to be the hardest by far to master.

GM

Thanks for the tip. I’ll give it a look 👍