Hi THere
I was wondering if you clever people out there could help me.
I have built a sub and using a computer program set the tuning frequency of the port to 35Hz.
My question is this, how can i check that i have got this tuning frequency in the built cabinet? I know there is a way to do it but i cannot remember how 😕
Thanks in advance
Moondust
I was wondering if you clever people out there could help me.
I have built a sub and using a computer program set the tuning frequency of the port to 35Hz.
My question is this, how can i check that i have got this tuning frequency in the built cabinet? I know there is a way to do it but i cannot remember how 😕
Thanks in advance
Moondust
You check it via measuring the system impedance, which requires testing equipment that you probably don't have or you'd already know the answer to the question.
what equipment do you need?
I have built the sub amp into the cabinet. I have got it hooked up to a signal generator. What else do i require?
I have built the sub amp into the cabinet. I have got it hooked up to a signal generator. What else do i require?
Generate a whole heap of wav files of say 5 seconds duration each, at 50Hz, 49Hz, 48Hz, etc and burn them to an audio CD as separate tracks. Play them through your system and watch the woofer cone. At the (real life) tuning frequency the cone movement will decrease to maybe a quarter of what it was, and the port will really start to pump a lot of air. Don't put your hand any closer than ~200mm from the port otherwise the tuning will be affected somewhat.
You can. The tuning frequency is approximately the point of minimum impedance.
Simple, quick, but only approximate guide to Fb:
1. Set your oscillator to 1.2 times expected Fb.
2. Hook up your speaker as normal and set the level so you can just hear the tone.
3. Insert your meter on 200mA range. Watch polarity if it's an analogue (needle type) meter!
4. Begin sweeping down the frequency, slowly.
5. When you reach a peak current, that's your Fb.
If you want to be more accurate you need to do some more involved testing.
Simple, quick, but only approximate guide to Fb:
1. Set your oscillator to 1.2 times expected Fb.
2. Hook up your speaker as normal and set the level so you can just hear the tone.
3. Insert your meter on 200mA range. Watch polarity if it's an analogue (needle type) meter!
4. Begin sweeping down the frequency, slowly.
5. When you reach a peak current, that's your Fb.
If you want to be more accurate you need to do some more involved testing.
richie00boy,
Ive tried this technique before, and wound up with a fried meter.
Suggestions as to what I can to do avoid this in the future?
Ive tried this technique before, and wound up with a fried meter.
Suggestions as to what I can to do avoid this in the future?
Kittle said:
All meters should have fuses in them, Im assuming you replaced it?
Try setting the meter on a higher amperage scale.
moondust said:
This might actually be enough. Tune the frequency in the range 20-50 Hz and put the hand on the cone. A minimum in the cone vibration will occur at fh.
Another way of doing it is to connect a series resistor (~100 ohms) and measure the voltage across the speaker. You will find two peaks (at roughly 20 and 60 Hz, typically for a system tuned to 40 Hz) and a minimum between them. This minimum occurs at fh.
Yet another way would be to connect a current meter in series with the speaker, then you will find two minima at ~20 and ~60 Hz, and a maximum at fh.
The port equations are pretty accurate and there should be very little need to double-check unless you have a weird situation. The port minimum can be hard to find with a meter, it can be fairly broad. The most accurate way with simple measurements is to measure the SPL in front of the cone and find the port minimum.
The Golden Ear method...
If you have a fairly good ear and can hear reasonably low (down to 25hz or so) and are not "tone deaf" (you can tell the difference between musical notes). I would suggest this method. It is very simple, it only requires one thing you may not have, a test tone at your calculated tuning. What you do is this...
Play the test tone at the calculated resonant frequency through the sub, 35hz.
Listening closely to the pitch of the tone.
Turn the amplifier off, then put your ear over the port and tap the cone of the speaker near its outer edge (the part nearest the surround)
You should hear the port resonance when you tap the speaker and if it is the same picth as the tone you played earlier, your good to go.
This method may not be as exact as some other methods and it is definately useless when using tunings below your hearing range, but I have used it to guess the tuning of many of my friends' car audio enclosures with a fair amount of accuracy.
If you have a fairly good ear and can hear reasonably low (down to 25hz or so) and are not "tone deaf" (you can tell the difference between musical notes). I would suggest this method. It is very simple, it only requires one thing you may not have, a test tone at your calculated tuning. What you do is this...
Play the test tone at the calculated resonant frequency through the sub, 35hz.
Listening closely to the pitch of the tone.
Turn the amplifier off, then put your ear over the port and tap the cone of the speaker near its outer edge (the part nearest the surround)
You should hear the port resonance when you tap the speaker and if it is the same picth as the tone you played earlier, your good to go.
This method may not be as exact as some other methods and it is definately useless when using tunings below your hearing range, but I have used it to guess the tuning of many of my friends' car audio enclosures with a fair amount of accuracy.
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