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Discussion Starter #1
Something I came across recently that have me concerned, not knowing what's causing it. I recently finished building a pair of speakers, and the first set of measurements I took I was using my preamp and power amp so I had to use the headphone jack on a windows 10 laptop. The FR graph has a gentle ramp up from 5K Hz. I then do the same using HDMI on my Marantz AVP with the same power amp. Then suddenly the familiar looking FR graph with a gentle slope down from around 9 Khz is back!!

I then decided to try my MacBook Pro to see if there was something caused by the windows laptop. The REW Mac version I have on it seemed goofy (I knew that from wayback), but it is dual boot, so I booted it to windows 10 and plotted the graphs again using both headphone jack to RCA and HDMI. The results are the same, that is, the HDMI has the high end slope down while the headphone jack to RCA connection/cable would ramp up. When using the HDMI I obviously had to use the Marantz AVP and the power amp, while when using Jack to RCA I was using it with a separate pure amp, but in both cases, no EQ was applied.

The two graphs are attached as I am hoping someone may have ideas as to what might have been the cause of the difference. I really don't know what else to do and which curve to trust, but I am thinking of trying the jack to RCA connection using a different jack/RCA cable, in case there is something funny go on with one cable.

Another question I am hoping someone can help me is, how do I use REW to measure the impedance of speakers?

Thank you in advance.
 

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Premium Member
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Discussion Starter #3
Re: REW V5 Tips

Are you using any cal files?

Try making loopback measurements from the chosen output (a line level output from the AVR if using HDMI) to the line input to see how they differ.

REW is updated quite frequently, so I wouldn't base your decisions on something that was "goofy" from "wayback".

V5.19 impedance measurement
V5.20 beta impedance measurement
Yes I always use the cal file supplied by Umik. I was too lazy to download the newer Mac version, I am sure if I do it will run fine. I'll try the "loopback" thing if I can figure out how. I should also double check to make sure the cal file was in fact selected in both cases.

Thank you for the links to impedance measurement.
 

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Discussion Starter #4
Hello John, based on my understanding of the diagrams from the links, I don't think my laptops have the necessary I/Os to do the impedance measurements even if I have the Rsense.
For my purpose, I just need to find the impedance saddle to confirm it is at around 32 Hz that the port is supposed to be tuned to based on length of the port tube. So do you think I can simply use REW's tone generator, a voltmeter and a clamp on ammeter to measure V, I vs frequency from say 20 to 50 Hz in order to find the saddle frequency? For this purpose I think +/- 3 to 5 % accuracy should be good enough anyway.
 

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Why not put the mic by the port and make a measurement to see where the port is tuned directly?
 

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Generally aim for the mic tip to be at the centre of the port. The peak should be at the port tuning, so looks to be around 27 Hz.
 

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Discussion Starter #8
Generally aim for the mic tip to be at the centre of the port. The peak should be at the port tuning, so looks to be around 27 Hz.
Thank you very much. I actually did just that, but also tried moving the mic around slightly back and forth and always near the center of the port hole. The peak seemed consistent at around 27-28 Hz. I was aiming for 32 Hz, so I am going to have to try shorting the port tube.

By the way I forgot to tell you I found the reason for the difference result of the sweeps between using HDMI and the headphone jack of the laptop via a jack to rca cable. When I used the same jack/rca cable but plug into the analog input of the AVP, I got the same results as using its HDMI input. So then I took a closer look of the preamp that I used the same jack/rca cable last time when I got the ramped up high frequencies starting at around 5-6 kHz in case I had tone control on. Sure enough, that input#5 I randomly picked for the sweeps had tone control enabled, with a +6 dB setting. I must have done it long ago when play around with something, and forgot to zero it disable tone control after.
 
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