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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => Audio Measurement and Testing => Topic started by: Lyle Williams on December 26, 2013, 03:04:04 am

Title: VNAs in audio applications.
Post by: Lyle Williams on December 26, 2013, 03:04:04 am

Does anyone use VNAs ( Vector Network Analysers ) for audio system testing?

This would be the standard tool for measuring phase response at RF frequencies.

Title: Re: VNAs in audio applications.
Post by: Lyle Williams on December 27, 2013, 07:34:38 pm
In the absence of an answer, I gave it a go.

All the bits of the signal path that are meant to be linear test out just fine with a VNA.  From the mic cable through to speaker terminals (which obviously need quite a bit of attenuation when driven by an amp).

Once I go out an "air interface" into a room and mic the results are horrible.  It takes a huge number of sweeps to start to see what is going on. 
Title: Re: VNAs in audio applications.
Post by: Jason Lavoie on January 08, 2014, 01:25:29 pm
In the absence of an answer, I gave it a go.

All the bits of the signal path that are meant to be linear test out just fine with a VNA.  From the mic cable through to speaker terminals (which obviously need quite a bit of attenuation when driven by an amp).

Once I go out an "air interface" into a room and mic the results are horrible.  It takes a huge number of sweeps to start to see what is going on.

I'll give it a go..
they are both essentially doing the same thing: applying a stimulus to the input of a system and watching the response at the output, at multiple frequencies

the biggest difference is that your typical lab VNA will measure a single frequency at a time, and if you like it will do a sweep and log the results. These results are accurate, but noisy so they are useful for seeing trends or letting you know if your circuit is unstable.
Although most dual channel FFT software packages can be made to work exactly like a VNA nobody does because as you've found out it's a PITA
Smaart and others have options to use averaging (from small averages right on up to many seconds) and smoothing. as well as many other little tricks like time windows to avoid unwanted signals from getting into the measurement.

FWIW your results would be the same with Smaart.. cables, electronics etc all measure great with nice clean responses (as they should) and speakers/rooms measure comparatively terrible.
If you were in an anechoic chamber the differences would be more subtle.

Jason
Title: Re: VNAs in audio applications.
Post by: Lyle Williams on January 08, 2014, 03:49:43 pm
Another issue with swept measurements is that the VNA is expecting speed-of-light type latency through the circuit rather than speed-of-sound.  The sweep rate needs to be turned right down to let the VNA find it's input signal in the output.
Title: Re: VNAs in audio applications.
Post by: Jason Lavoie on January 09, 2014, 09:51:46 am
Another issue with swept measurements is that the VNA is expecting speed-of-light type latency through the circuit rather than speed-of-sound.  The sweep rate needs to be turned right down to let the VNA find it's input signal in the output.
Right. other than measuring sound through air a VNA will always have the response come back way faster. this may make your VNA useless
if you have a DSP handy or a digital delay try setting a delay of 20, 50, 100ms and see if the VNA gets a proper measurement.

Jason
Title: Re: VNAs in audio applications.
Post by: Lyle Williams on January 09, 2014, 03:03:31 pm
I need the time machine variant that lets me set negative delays.  :-)
Title: Re: VNAs in audio applications.
Post by: Tim McCulloch on January 10, 2014, 11:07:45 am
I need the time machine variant that lets me set negative delays.  :-)

Mr. Peabody's Wayback Machine is available... :)
Title: Re: VNAs in audio applications.
Post by: Lyle Williams on January 10, 2014, 02:13:00 pm
Sadly I am old enough to have watched a lot of the original on TV...