QTS value???

Hi, boys.
What´s driver QTS value that you recommend for a closed box?? And what about a vented box???
I believe a Qts of .7 is best for free air, but I don´t any idea about the other cases.
Qts below 1 is better for vented box???
Someone please correct me if I'm wrong.
Thanks in advance!!!
Marcelo
From Argentina
 
New questions...

Sandro, thanks for your answer.
I have other questions now. I want to use a vented box.
I have a woofer of 8" with :
Fs 48Hz
Qts 0.67
Qes 0.86
Qms 2.95
SPL 90

According to this if I have a SLP of 90 then Vas=50 litros
(from SPL=112+10*log n0 and n0=9.64*10E-10*Fs^3*Vas/Qes)
If a use a vented box then Vas=Vb ((FB/FS)^2 -1) with Vb 45 Lt then FB=69Hz.
Someone please correct me if I'm wrong.
The T/S parameters were measured according to www.sound.au.com/tsp.html.
I used Speaker Workshop too, but I don´t trust it in the values that give me.
Thanks in advance.
Marcelo
from Argentina
 
Higo,

the woofer you have seems not good for bass reflex enclosure, QTS is too high and you risk to end up with something 'slow and muddy' in the bass range.

Also from your previous post you seem not confident about the measures, and calculating Vas from an estimated SPL level is not the 'maximum'.
Since you seems to have an audio generator and a good voltmeter, try the following measurement procedure (coming from Renato Giussani, a big reference in loudspeaker design):

1 - Setup
- take a resistor around 10 Ohms and measure it to have the right value. Let's suppose it is exactly 8 Ohms.

- Connect the measure system normally with this resistor in place of the speaker (generator to amp, amp to Device Under Test, voltmeter in parallel to DUT)
- now select a frequency around 100 Hz and trim the amp output until you read exactly 0.8 volts on the instrument (of course, if the resistor is 10 Ohm, you should read 1 volt)

Your set is now calibrated, and you will read 0.1 volts for each Ohm, that makes the reading very simple.

2 - Measure

a ) DC resistance. Put the speaker face down, make absolute silence and measure DC resistance.

b) Frequencies
Place the speaker face-up, connect it in place of the cal resistor,maintain the silence and run the measures of FS, F1 and F2 as you did before.

c) Since you have the speaker connected, measure the impedance at 1Khz and 10 Khz: it allows us to calculate the Le and the impedance behavior at high freq.

d) Add Info - Most Important
Take a non magnetic weight between 10 and 30 grams (wood, aluminium, plastic etc.), and measure it accurately Let's suppose it is 22 grams.
Put it on the speaker as close as possible to the centre (on the dustcap i.e.) and fix it to avoid vibrations (... in a non-destructive way of course!)
Now measure the resonance of the speaker with this added mass. This way it is possible to determine cms and recalculate the other parameters.

I actually don't remember the formulas, but if you post Fs, F1, F2 Fs(added mass) and the weight I will put them in a sw that calculates it.

3 - Others
It would be great if you can get a copy of the response graph.

Little hint: try to repeat 4-5 times the measurement runs, and check if the results are consistent.

hasta bueno
sandro
 
Thank again Sandro.
You are right. In the begining I wasn´t sure about the measurement procedure and the T/S values. But now I´am.
I tell you my way:
I put a R of 10,5 Ohms in serie with the woofer and I started to change the frequency of the generator from 20 Hz to 10 Khz. In each step I measured the tension value in the R and woofer.
Like you say I repeated the measure in the resonant frequency range, only to be sure that all was right, ´cause I know that If you want to build a vented box the T/S parameters should be exact. I use a better tester in this stage. I attached a Excel file with the measures and calculations.
If you notice diference in T/S values it´s due to yeterday I did it again (4 times)
This weekend I´ll do the measurements of the Fs with mass in the way that you say.
I´m not interested in bass range, ´cause I´m thinking in build a SubWoofer (15"). This is the reazon why I wanted to build a vented box with Fb=70Hz.
I have the intention of building a HT system.
This pair of speakers will be the L and R until I build a better pair. Then I move this to SurroundSound.
I"ll expect your comments.
Thank you very much.
Marcelo.