Alan,
> Glad to see you are now measuring things ( having got your Samson USB mic to work with REW ) .
A few things ;
(i) You will still need to acquire a test mic that comes with a unique calibration file ( that is then loaded into REW ) to correct for the mic's deficiencies ( if you want to believe that what you measure is accurate ) .
- Test mics typically use small diameter ( 1/2" & smaller ) , omni-direction capsules .
- I looked briefly at the available Samson USB based microphones and all the ones I saw don't suit the needs for accurate data acquisition ( they're all aimed at the M.I. market having large capsules & bodies ) . Samson USB Mics
- Here's the MiniDSP mic previously mentioned .
- One ( apparent ) weakness ( of this unit ) is the need to have the users computer quite near the mic . Another is the maximum sampling rate of 48K ( which limits HF distortion measurements more than anything ) .
(ii) The exaggerated HF roll-off shown might be there ( or may be less ) . It's all dependent on the linearity of your test mic ( as well as your testing methodology, ie ; whether the mic was pointed directly at horn & was within it's coverage angles ) .
(iii) Display standards ( for test data ) is a bit of a moving target . Much of it is referenced to the older paper print-outs produced by B&K ( who used a 50db vertical axis ) .
- Your latest pic well displays the HF roll-offs ( though the horizontal length of the pic might tend to exaggerate their severity ). See;
vs this one lengthened by 50% ;
> There is a bit of art & craft to data display ( when all is said & done ) .
(iv) For the small cost of making up the Altec 30923 HF compensation network, I would encourage you to do so ( build at least one ) . Insert it just before the compression driver ( you'll likely need to fully turn up the Lpad on the horn ) and then remeasure & take a listen ( you should see & hear that you have reclaimed a lot of missing HF & UHF in that part of the circuit )
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