Please help in designing speaker box : measure TS params

2009-09-03 12:58 pm
Hi all, I am planning a construct a pair of speakers. 40 + 40 watts. As I have a tight budget and due to unavailability of quality drivers, I bought 2 cheap 8 inch woofers (boston make) and 2 piezoelectro tweeters (dapic).

The problem - woofers dont have much specs given by the manufacturer.
Given specs - Impedance 8ohms, SPL (1W/1m) 88dB, RMS 40W.
And for tweeter: Freq response: 1.8-20 Khz, SPL 91dB, RMS 75W.

I am completely new to this, tried to measure TS params to calculate box volume.

using the following :
T-S Parameter Calculator

I calculated TS params for my 8inch Boston woofer

Re: 7.4 ohms
Diameter: 17.27 cm
Rated power: 40 W

Fs: 50.5 Hz
Rmax: 25 ohms
F1: 35.6 Hz
F2: 71.1 Hz

With added mass of 27gms Fs: 29.4Hz

Qms: 2.615
Qes: 1.0994
Qts: 0.7739
cms: 0.000717512 m/N

Vas: 55.1195 lit
EBP: 26.743

Inductance of voice coil: 14.275 mH

Any comments on these? I used an average rms multimeter(not true rms: Model UNI-T UT70), will it have any effect on these ?

Inductance value seems wrong??

And now how can I go forward and design my speakers ?

Please help...

Ron E

2002-06-27 10:41 pm
average responding meter should be ok if you are using sine waves. One gotcha is with non-flat frequency response from inexpensive meters. They will read wrong typically below ~40 and above ~500Hz. If you check that the reading of the output voltage of the signal generator is the same at all frequencies between ~30 and 100Hz, you should be fine - if not you will have to recalibrate your setup for each frequency.

VC inductance is probably 1/10 that figure.

With a Qts of 0.7+, you will have to settle for a bit of midbass emphasis - I probably wouldn't put an 8" driver in a box more than 40 liters.
I used an average rms multimeter(not true rms: Model UNI-T UT70), will it have any effect on these ?

Please help...

hi there. I have the same multimeter as the OP.

theres times when i wish id just bought a cheap scope......

when measuring my TS params, with a 1kR resistor, and using the 200mVAC stop, i had similar problems. ALthough it wasnt extreme the frequency response (as stated in user manual) for this DMM is 40-400hZ at -3dB.

have no fear. just set your mV readind to something conveniant using the volume on the amplifer, say 100mV at 50Hz, across a dummy load. i used a 10R res for calibration so it made sense to use it here. then if at 50Hz youre reading 100mV, you can give each frequency a decimal as a calibration k factor.

ie 100mV = 1.0
then with a lot of patience and a little math you can take points at every 2Hz (or 1hz if you need to), below 50hz and above it. use your common sense here....396hz, 398hz ........skip that, using your discretion. as RON E stated, average RMS reading makes no difference on a sinewave, they are the same anyway, unless you have gross THD on the oscillator.....

easy. just alot of patience required. hope ive been of some help.
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