Plz help w/ ported box tunig

johnbomb · 21st February 2011, 07:23 PM

Hello,

I'm trying to measure the resonance frequency of my ported box, using the methods listed here:

The Subwoofer DIY Page - Box-Building FAQ

I tried measuring both the voltage across a 10 watt 5.5 ohm resistor as well as directly measuring the current output. However, my multimeter, a RadioShack digital from probably 15 years ago, won't read a stable voltage or amperage and instead skips all around as I input a fixed frequency to the sub.

Any ideas? Do I need a better multimeter? Is there another method?

I've built a large/low tuned box, w/ a goal tuning of 10 hz (home theater), so I doubt measuring db output will be accurate at 10 hz.

Thanks,
John

Cokewithlime · 09:10 PM

If you slowly sweep through the LF region until the cone is moving the least(minimum travel) that is the frequency at which the enclosure is tuned to.

Also If you provide the following it should be simple to fairly accurately predict what the tuning is ?

btw - 10Hz tuning is very low even for a HT application.

What is the internal volume of the cabinet ?
What is the number and size of the drivers you are using ?
What is the number and size(diameter and lenght) of the port(s) you are using ?

johnbomb · 22nd February 2011, 12:31 AM

Thanks for the reply, Cokewithlime. The box is 73cft, with 2 Elemental Designs 160v.2 drivers and a single 16x5x29 inch port. I'm using WinISD for simulation.

So, I got close to the sub at the expected resonance (about 8.9 hz) at a low volume to watch the drivers, and noticed something interesting: the dreaded port chuffing! However, I decided to put it to use: it's loudest at resonance, which of course WinISD shows.

I took my db meter and set it to "slow", and placed it inside the port, thinking that it would pick up the noise of the air movement within it. Sure enough, as I slowly swept through the expected resonance, the meter showed the highest reading at about 8.8 hz, which was w/in 0.1 hz of predicted- not bad.

There you have it, another way to measure port resonance. Hopefully, someone else will find this useful.

John

Cokewithlime · 22nd February 2011, 04:42 AM

Those 160v.2 drivers do move a lot of air - lol

Have you tried to flare at least the exit end of the port. might help a bit

johnbomb · 22nd February 2011, 09:31 PM

Actually, I was mistaken about the max air velocity and box tune in WinISD. The air velocity maxes just below tuning according to that program.

Cokewithlime · 22nd February 2011, 11:58 PM

With the info you provided, I guestimated the actual box tuning to be between 9.9 and 10.4Hz so you are well within the ballpark.

21st February 2011, 07:23 PM	#1
johnbomb diyAudio Member Join Date: Jan 2011 Location: Nashville	Plz help w/ ported box tunig Hello, I'm trying to measure the resonance frequency of my ported box, using the methods listed here: The Subwoofer DIY Page - Box-Building FAQ I tried measuring both the voltage across a 10 watt 5.5 ohm resistor as well as directly measuring the current output. However, my multimeter, a RadioShack digital from probably 15 years ago, won't read a stable voltage or amperage and instead skips all around as I input a fixed frequency to the sub. Any ideas? Do I need a better multimeter? Is there another method? I've built a large/low tuned box, w/ a goal tuning of 10 hz (home theater), so I doubt measuring db output will be accurate at 10 hz. Thanks, John

21st February 2011, 09:05 PM	#2
Cokewithlime diyAudio Member Join Date: Oct 2009 Location: Toronto	If you slowly sweep through the LF region until the cone is moving the least(minimum travel) that is the frequency at which the enclosure is tuned to. Also If you provide the following it should be simple to fairly accurately predict what the tuning is ? btw - 10Hz tuning is very low even for a HT application. What is the internal volume of the cabinet ? What is the number and size of the drivers you are using ? What is the number and size(diameter and lenght) of the port(s) you are using ? Last edited by Cokewithlime; 21st February 2011 at 09:10 PM.

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22nd February 2011, 12:31 AM	#3
johnbomb diyAudio Member Join Date: Jan 2011 Location: Nashville	Thanks for the reply, Cokewithlime. The box is 73cft, with 2 Elemental Designs 160v.2 drivers and a single 16x5x29 inch port. I'm using WinISD for simulation. So, I got close to the sub at the expected resonance (about 8.9 hz) at a low volume to watch the drivers, and noticed something interesting: the dreaded port chuffing! However, I decided to put it to use: it's loudest at resonance, which of course WinISD shows. I took my db meter and set it to "slow", and placed it inside the port, thinking that it would pick up the noise of the air movement within it. Sure enough, as I slowly swept through the expected resonance, the meter showed the highest reading at about 8.8 hz, which was w/in 0.1 hz of predicted- not bad. There you have it, another way to measure port resonance. Hopefully, someone else will find this useful. John

22nd February 2011, 04:42 AM	#4
Cokewithlime diyAudio Member Join Date: Oct 2009 Location: Toronto	Those 160v.2 drivers do move a lot of air - lol Have you tried to flare at least the exit end of the port. might help a bit

22nd February 2011, 09:31 PM	#5
johnbomb diyAudio Member Join Date: Jan 2011 Location: Nashville	Actually, I was mistaken about the max air velocity and box tune in WinISD. The air velocity maxes just below tuning according to that program.

22nd February 2011, 11:58 PM	#6
Cokewithlime diyAudio Member Join Date: Oct 2009 Location: Toronto	With the info you provided, I guestimated the actual box tuning to be between 9.9 and 10.4Hz so you are well within the ballpark.

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