Help in measuring response of 4th order BP box

[venki]

Hi,

Yesterday I completed building my first 4th order BP box. Can nebody help me out measuring and tuning the box. I would like to get info like
How to measure the port resonant frequency ()

Which software to use ()

I connected the speaker to my TV using amp amp and a 20hz HPF (just testing to see if it works at all. ). While doing this i noticed that at some point of time i heard some whistling sound (like the sound when you blow thru a pipe) is it because my design is wrong or something else the driver has a high Qts (Abt .6) and BP box has a freq range from 30hz - 80hz (according to design).

Thanx in advance,
Venki.

[Volenti]

Post your drivers TS parameters and your enclosure volumes and vent size/length and I'll run it through bassbox and see what it looks like.

[venki]

Sorry for the delay Volenti, my pc stopped working 2 days back . The T/S specs are as follows

fs - 30hz
Qms - 2.95
Qes - .76
Qts - .6
Vas - 46.16

Box parameters
s=.6
fb=48.3hz
Qbp=.966
Fl = 30hz
fh = 77.8hz
Vr = 30lt (+ or - 5%) (including driver vol)
Vf = 25lt(+ or - 5%) (including pipe volume)
Vent dia 8.5cm
Vent lenght 24 cm

Thanx for the help i advance

Venki.

[simon5]

Just a quick check here:
http://www.linearteam.dk/default.aspx?pageid=newdriver

That says your box got a peak of +8 dB at ~22 Hz. Probably your 8.5 diameter port is too small to prevent vent noise with that peak.

In this faq, at the point 1.03 there's two methods for measuring real Fb:
http://www.diysubwoofers.org/faq.htm

[Ron E]

Originally posted by venki
The T/S specs are as follows

fs - 30hz
Qms - 2.95
Qes - .76
Qts - .6
Vas - 46.16

Box parameters
fb=48.3hz
Vr = 30lt (+ or - 5%) (including driver vol)
Vf = 25lt(+ or - 5%) (including pipe volume)
Vent dia 8.5cm
Vent lenght 24 cm

What are:
Re - coil DC resistance
Xmax - max linear excursion
Sd - cone projected area (or frame diameter, 8", etc...)
Pe - power handling

Are your parameters self-measured or factory parameters?
Your response shape looks good in BPBoxmodel, not anything like what simon5 got with winISD.

I can't model excursion or vent velocity without the added parameters.

[simon5]

Sorry I used the Fl to tune it in WinISD online instead of the vent length because you can't tune with vent length on WinISD online.

I used WinISD Pro and I can confirm the findings of Ron E, that enclosure looks right, perfectly flat.

Ignore my previous posting about peaking.

[venki]

Hey thanx Ron and simon,
I did measure the T/S param using audio tester (latest version)
I dont have the Xmax for the driver since the driver manufacturer doesnt provide anything (no specs ).

I have some additional parameter which i got from measuring the T/S specs using the audio tester

Zmax = 25.8 ohm
Rdc = 5.3 ohm
the driver is 8" in dia.
Power rms = 50W
Power Max = 100W

Thanx again,

Venki.

[Ron E]

I think I understand your question now.

I doubt you need a bigger port or anything, at least from the models I did. I think your problem is that you are trying to run a bandpass fullrange.

At a frequency given by F=speed of spound/(2*L), so about 700Hz for your BP box, the port will resonate like a pipe. The way to prevent this is to filter any signal going to the BP box such that frequencies above 100Hz or so are attenuated.....

The pipe resonance is such that it can equal or exceed the output of the sub.

[venki]

Hi again, thankyou all for the help, I tested the box with a 4th order active BP filter. i did find out certain problems in my design.

1. I have not yet covered the inner part of the box with damping material
2. Some air was leaking from the front volume which was causing the distortion at high amplitude ( the whistling sound still exist the only problem in increasing the size of the vent is it compromises the net front chamber volume ). Yesterday i went to one local shop in search of damping and stuffing material i found two materials . one is for damping the shop keeper showed me thing what looks like a carpet made of jute fibre (i think this what is called felt but no idea). second i asked him to show something for stuffing he showed me something which looks like cotton but is synthetic it looks like the one used for making pillows . he says these are the stuff ppl buy for stuffing/damping. If anyone of you have used these stuff please suggest if these are good candidates for stuffing/damping.

Can anybody suggest me a good measuring software.

Thanx in avance,
Venki.

[Ron E]

I honestly doubt you need any damping in either chanber unless you accounted for it in your design. Adding damping to the front chamber will reduce output. Adding damping to the rear chanber will change the response shape.

The sound from a vent too small is usually more like a car window partially rolled down when driving at high speed. Sort of a turbulent rumble (chuffing) rather than a whistle. Really bad leaks can cause whistles and I suppose the chuffing could excite the organpipe resonance, but I don't hear this on my own speakers because I tend to use rear ports.

The only good free PC measurement package out there is speaker workshop. It has a lot of capability, but it can be difficult to make high resolution low frequency measurements. You will have to close mike the port which means you do it at low volume. If you measure at high volume in your room, your room resonances will strongly affect the measurement.

BTW, it is not uncommon that the organpipe resonance is many dB higher than the bandpass. If that turns out to be the issue you could redesign your box so the port output is directed downward or backward so spurious noises are diffused, if you have not done this already.