Please help in designing speaker box : measure TS params

arunraj.in

Member
2009-09-03 12:58 pm
Hi all, I am planning a construct a pair of bookshelf 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, want to measure TS params to calculate box volume. I have downloaded Audiotester but their connection diagram is confusing to me(attached with this post).

Here they have not shown any mic which picks up sound from speaker, then how will it measure ? And my laptop only has headphone out and mic in, can I use mic in as audio in? and what sort of AMP should I use in this test?

I will be using TDA2050 amp for powering this, I already build them :)

Thanks in advance....
 

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Henry8

Member
2010-07-22 1:28 pm
This setup looks the same as speakerworkshop. The diagram shown will measure the impedance and the software will calculate t/s params from this...you dont need any mic's.
Any amp will do for this test, but to get good results you need to be able to setup your s/card levels, laptop in built usally dont work.
Its best to use a reisitor for the DUT say 22R, once you can measure it properly with flat phase from say 5hz up, your setup is ready.
Took me 3 years with SW...........
 
I looked at heaps of spreadsheets for measuring speaker parameters a while ago and ended up writing my own where you add a known mass of blu-tack to the speaker cone. Click on the link in my signature. One VERY important thing when you are measuring speakers that hardly ever gets mentioned is to put the back of the speaker against the end of a 500mm or more length of 100mm square wooden post. This holds the speaker frame from vibrating the opposite direction to the cone and gives a higher Q figure than if you just hold the speaker in mid air. Try it both ways and see for yourself. If there is a vent at the back of the magnet make sure you don't block it off though.
 

Henry8

Member
2010-07-22 1:28 pm
Thanks for the replay :) but watz DUT ? Can you pls explain this a bit more: "Its best to use a reisitor for the DUT say 22R, once you can measure it properly with flat phase from say 5hz up, your setup is ready."

DUT=the speaker you are testing, Device Under Test.

Instead of try to test you speaker straight away, if use use a known value resistor in its place, when you test it should show its resistance and phase, its resistance should be at the same value for all frequencies and it phase should be at 0 for all frequencies. When you can do this, and you can then test a speaker, and you known the results are accurate.

Use 0.1% resistors if you can, for your setup and for a test resistor.
 

arunraj.in

Member
2009-09-03 12:58 pm
At last calculated TS params

At last 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 ? :confused:

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

Please help... :)

Thank you all