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Old 7th August 2006, 08:40 PM   #1
alexcd is offline alexcd  United States
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Default Port air velocity

Are there any rules of thumb for designing a subwoofer port when it comes to port air velocity? I use Bassbox Pro 6. It's not a superfancy program but it does show me air velocity giving the box I design, driver, and power. I notice is changes with port area which seems obvious enough. How do I know how low is low enough that I wont be listening to that "chuffing" sound instead of my music?
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Old 7th August 2006, 10:42 PM   #2
ScottG is offline ScottG  United States
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between .15 and .2

This IS dependent on power input as well - so whats true for 1 watt will not be for 64 watts.

BTW, most people will never supply more than 64-128 watts to a subwoofer. So a look at 128 watts should be more than sufficient.

(..note that this doesn't mean that you shouldn't have more power for your amplifier. The reason for more power in a sub amp has nothing to do with the wattage, but rather it suggests a higher current power supply which should be more capable of dealing with driver created back emf.)
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Old 8th August 2006, 01:19 AM   #3
Ron E is offline Ron E  United States
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The generally accepted rule of thumb for preventing turbulence and port compression is to design for a max velocity of 5% of the speed of sound, or about 17 meters per second (~55 feet/sec). This may be increased (some suggest doubled) if a flare of a large enough radius is added to both entrance and exit.

If you are not afraid of a (very) little math, do a search on the site for a formula I posted that can calculate port diameter based on a specific SPL goal and max desired air velocity.

BTW, ignore the back emf nonsense in the previous post.
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Old 8th August 2006, 01:42 AM   #4
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55ft/sec, wow, that's a relief. I was getting nervous being at about half that.

Sounds good. I just hope my software is reliable. (cost $100 so it shouldnt be crap)

I should start another thread and ask if anyone can vouch for it.
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Old 8th August 2006, 07:24 PM   #5
ScottG is offline ScottG  United States
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Quote:
Originally posted by Ron E
BTW, ignore the back emf nonsense in the previous post.
Oh really? Pray tell?

Btw, you forgot to mention that the suggested flares effectivly reduce port length by the amount of the flare. I personally would think this *more* than a little important.
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Old 8th August 2006, 08:08 PM   #6
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I have to agree with Scott here. Back EMF is very important and can change the tuning drastically.

Is the part about a flared port being able to handle double the airflow true? That would be very handy in high power applications.
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Old 8th August 2006, 08:51 PM   #7
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Hmm, no offence but if you were able to say with authority that back EMF is/isn't a problem then you wouldn't be asking about vent velocities.

The issue with back EMF is a kind of red herring. The actual issue is that phase shift due to voicecoil inductance around the tuning frequency can give misleading measurements if you simply take the point of minimum impedance (as measured on a simple meter, not a complex impedance meter) to be the tuning frequency.
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Old 8th August 2006, 10:14 PM   #8
ScottG is offline ScottG  United States
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Quote:
Originally posted by richie00boy
The issue with back EMF is a kind of red herring. The actual issue is that phase shift due to voicecoil inductance around the tuning frequency can give misleading measurements if you simply take the point of minimum impedance (as measured on a simple meter, not a complex impedance meter) to be the tuning frequency.
Oh, its not a "red herring". However, go back and look at the context from where I first mentioned it - (i.e. we are talking about something different here).
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Old 9th August 2006, 12:20 AM   #9
Ron E is offline Ron E  United States
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Quote:
Originally posted by ScottG
between .15 and .2
Hmm. 0.15 to 0.2 - wouldn't it be helpful to quote units?

Quote:
Originally posted by ScottG
This IS dependent on power input as well - so whats true for 1 watt will not be for 64 watts.
So what you are saying is that if I put more power into my speakers, the port velocity will go up? Glad you cleared that up for me.

Quote:
Originally posted by ScottG
BTW, most people will never supply more than 64-128 watts to a subwoofer. So a look at 128 watts should be more than sufficient.
There's a lot of assumptions in there. Hard to say if that is good advice without knowing the application.

Quote:
Originally posted by ScottG
(..note that this doesn't mean that you shouldn't have more power for your amplifier. The reason for more power in a sub amp has nothing to do with the wattage, but rather it suggests a higher current power supply which should be more capable of dealing with driver created back emf.)
Aside from the complex power (capacitive/inductive load) issues, there isn't that much going on here. Good amplifiers are designed to run into loudspeaker loads.

It is nonetheless good advice to have some headroom.
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Old 9th August 2006, 01:33 AM   #10
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"The reason for more power in a sub amp has nothing to do with the wattage, but rather it suggests a higher current power supply which should be more capable of dealing with driver created back emf."

High current is good, but not for the stated reason. Back EMF is maximum at the driver/box resonance, which would be called Fc in a sealed system (but which is still there in vented), giving an impedance peak and a *minimum* in current draw.

And driver velocity and back-EMF is at a minimum at Fb.

"Back EMF is very important and can change the tuning drastically."

How so?

In any case, the whole issue of measurement anomalies can be sidestepped by putting your finger on the cone and feeling for minimum displacement, which tells you the true freq (OK, you have to measure that with a meter) of Fb.
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