Hi,
I have just built my first active sub and it sound good. I'm about to fine tune the sound, but I don't understand some stuff.
Background: I'm using it for Home Theater and music in my Living room. I'm using a 15" 300W RMS driver (quoted anyway...?) with a 150Watt RMS amp. The box was supposed to be 380 liters in volume, but that was too much for me, so I changed it to 131 liters. What difference will this make?
Firstly, when designing the port length, you have to give a tuning frequency. What frequency is this, and what is best to use?
I used this app to calculate my port length, and it’s still making a noise when I play it at high volume.
http://www.linearteam.dk/default.aspx?pageid=ventcalculator
I used these values:
Vb = 131 liters
Tuning frequency: 23Hz
1 vent
Vent diameter: 10.8 cm
Acoustic power of 0.72
What did I do wrong? What is the port suppose to do to the sound, and how do you know you've got the right setting?
One last question. Does anyone have a link where I can download sound files that play different low frequencies so I can test my Sub with that?
Thanks
Ernie
I have just built my first active sub and it sound good. I'm about to fine tune the sound, but I don't understand some stuff.
Background: I'm using it for Home Theater and music in my Living room. I'm using a 15" 300W RMS driver (quoted anyway...?) with a 150Watt RMS amp. The box was supposed to be 380 liters in volume, but that was too much for me, so I changed it to 131 liters. What difference will this make?
Firstly, when designing the port length, you have to give a tuning frequency. What frequency is this, and what is best to use?
I used this app to calculate my port length, and it’s still making a noise when I play it at high volume.
http://www.linearteam.dk/default.aspx?pageid=ventcalculator
I used these values:
Vb = 131 liters
Tuning frequency: 23Hz
1 vent
Vent diameter: 10.8 cm
Acoustic power of 0.72
What did I do wrong? What is the port suppose to do to the sound, and how do you know you've got the right setting?
One last question. Does anyone have a link where I can download sound files that play different low frequencies so I can test my Sub with that?
Thanks
Ernie
.Hi
This is my second post here. I'm a pro cab builder specialising in the unusual.
I've taken a look at the limited info you supplied regarding the driver and the port. The problem is the air speed in the port is reaching around 21.5m/s which is producing the noise you hear.
The solution is to fit a second port and make sure both ports are 103mm long and 108mm in diameter. The second port can be fitted anywhere, even the back of the sub it makes no difference.
Hope that helps
Rossco
This is my second post here. I'm a pro cab builder specialising in the unusual.
I've taken a look at the limited info you supplied regarding the driver and the port. The problem is the air speed in the port is reaching around 21.5m/s which is producing the noise you hear.
The solution is to fit a second port and make sure both ports are 103mm long and 108mm in diameter. The second port can be fitted anywhere, even the back of the sub it makes no difference.
Hope that helps
Rossco
ernieretief said:
Not sure where these came from but here anyway:
http://www.users.bigpond.net.au/wirelinx/frequency/
ernieretief said:
There seems to be agreement that flared tubes are the equivalent of a tube of twice the crosssection.
For a 4" tube, try www.partsexpress.com part# 268-352.
Those flared tubes are available other places, as well.
The tuning frequency is the frequency where the port will produce the most sound. This frequency is not something you can choose how you would like it but it has to fit with the rest of the speaker. A good tuning frequency is where the frequency-response curve keeps in a straight line and in the end falls off strong (24dB/O is common for a basreflex), not a bump in the curve and a long slowly lowering curve is not the best also.
A flared tube helps a bit to reduce the port noise but I strongly doubt that it is equivalent of a tube of twice the cross-section. A flared port mostly helps to reduce the port non-linearity. this is the chance of tuning frequency to a higher frequency. This happens because the turbulence at the ends of the port increases with the airspeed, this will make effective part of the port shorter, that results in a higher tuning frequency. The flared port reduces the turbulence and its noise a bit but a bigger port is most of the time the best solution, or making more ports.
I would do what Rossco suggested.
Good luck getting it better!
A flared tube helps a bit to reduce the port noise but I strongly doubt that it is equivalent of a tube of twice the cross-section. A flared port mostly helps to reduce the port non-linearity. this is the chance of tuning frequency to a higher frequency. This happens because the turbulence at the ends of the port increases with the airspeed, this will make effective part of the port shorter, that results in a higher tuning frequency. The flared port reduces the turbulence and its noise a bit but a bigger port is most of the time the best solution, or making more ports.
I would do what Rossco suggested.
Good luck getting it better!
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