Room Measurement Tutorial for Dummies Part 1

[TimoJ]

When measurement range starts below 20Hz, REW sweep always includes linear sweep from DC to 10Hz. This is a documented feature, but why does it do that? Is there a way to avoid this?
@JohnPM

 

[JohnPM]

When measurement range starts below 20Hz, REW sweep always includes linear sweep from DC to 10Hz. This is a documented feature, but why does it do that? Is there a way to avoid this?

REW's sweep starts at half the start frequency, so a measurement from 20 Hz starts its sweep from 10 Hz. As sweeps follow an exponential frequency profile the sweep spends equal time in each octave, maintaining an exponential (aka log) sweep with a start lower than 10 Hz would spend excessive time at very low frequencies, so REW substitutes a linear portion to reach 10 Hz then exponential thereafter, ensuring the sweep has sufficient low frequency energy without devoting too much of its total energy to that range.

 

[TimoJ]

REW's sweep starts at half the start frequency, so a measurement from 20 Hz starts its sweep from 10 Hz. As sweeps follow an exponential frequency profile the sweep spends equal time in each octave, maintaining an exponential (aka log) sweep with a start lower than 10 Hz would spend excessive time at very low frequencies, so REW substitutes a linear portion to reach 10 Hz then exponential thereafter, ensuring the sweep has sufficient low frequency energy without devoting too much of its total energy to that range.

But if I select 15Hz as the start frequency, REW sweep seems to start from DC instead of 7.5Hz. Is that correct?
This near DC part seems to trigger some amplifiers protection circuit at the start. Not in my case but a fellow user reported this on his system.

 

[JohnPM]

But if I select 15Hz as the start frequency, REW sweep seems to start from DC instead of 7.5Hz. Is that correct?

Yes.

 

[TimoJ]

Yes.

Why does it start from DC instead of half of 15Hz i.e. 7.5Hz?

 

[RPG]

Hi,

I'm starting to explore using REW, but I'm not sure if my current setup will even allow me to do a room measurment. First of all, I only have a basic Surface Pro laptop. I also have an old Omnimic V2 microphone that I can connect to the Surface Pro via USB.

I don't have the ability (no HDMI, audio outs, etc.) to connect the Surface Pro to my stereo system in order to play the measurement sweeps back through the speakers. I'm using Logitech Media Server (now Lyrion) and an RPI4 as a streamer connected via USB to an RME ADI-2 dac which is, in turn, connected to a Benchmark AHB-2 amplifier via XLRs.

My question is: Can I save/export the REW sweep file and place it on the LMS/Lyrion server, play it back through my speakers and then, with the Omnimic plugged into the Surface Pro have the ability for REW to do a room measurement??

Not sure if I've provided all the info needed to see if this is possible, so if any additional information is needed, please ask. Help is much appreciated!

Rick

 

[dc655321]

Hi,

I'm starting to explore using REW, but I'm not sure if my current setup will even allow me to do a room measurment. First of all, I only have a basic Surface Pro laptop. I also have an old Omnimic V2 microphone that I can connect to the Surface Pro via USB.

I don't have the ability (no HDMI, audio outs, etc.) to connect the Surface Pro to my stereo system in order to play the measurement sweeps back through the speakers. I'm using Logitech Media Server (now Lyrion) and an RPI4 as a streamer connected via USB to an RME ADI-2 dac which is, in turn, connected to a Benchmark AHB-2 amplifier via XLRs.

My question is: Can I save/export the REW sweep file and place it on the LMS/Lyrion server, play it back through my speakers and then, with the Omnimic plugged into the Surface Pro have the ability for REW to do a room measurement??

Not sure if I've provided all the info needed to see if this is possible, so if any additional information is needed, please ask. Help is much appreciated!

Rick

In principle, what you want is an "offline measurement" - https://www.roomeqwizard.com/help/help_en-GB/html/offlinemeasurements.html

 

[RPG]

In principle, what you want is an "offline measurement" - https://www.roomeqwizard.com/help/help_en-GB/html/offlinemeasurements.html

Ah, thanks. So, if I understand correctly, for the offline measurment procedure, I would first generate a sweep file from the REW signal generator, save it, and then copy the saved file to LMS/Lyrion. Then, with the mic hooked up and REW loaded, play the sweep and save the result as a response file (?). Then I would import the original sweep file as the stimulus, import the saved response file as the "response" and proceed from there?

Is there any way to do this live---in other words just play the sweep file that I saved to LMS/Lyrion and have the room response measurment done real time without needing to do the offline measurment procedure?

 

[Keith_W]

Connect your Surface Pro to your ADI-2 via USB. Connect to your Omnimic V2 via another USB port. If you don't have enough USB ports, use a port expander.

Set REW to Java mode. Set the ADI-2 as the output, and the Omnimic as the input. Play a test tone SOFTLY to confirm that all is well, then increase volume, and check levels. Set REW to use an acoustic timing reference, then do a sweep.

(edit) Oh yes, the Omnimic comes with a mini tripod. Throw it away and get a proper mic tripod if you are serious about taking good measurements.

 

[RPG]

Connect your Surface Pro to your ADI-2 via USB. Connect to your Omnimic V2 via another USB port. If you don't have enough USB ports, use a port expander.

Set REW to Java mode. Set the ADI-2 as the output, and the Omnimic as the input. Play a test tone SOFTLY to confirm that all is well, then increase volume, and check levels. Set REW to use an acoustic timing reference, then do a sweep.

(edit) Oh yes, the Omnimic comes with a mini tripod. Throw it away and get a proper mic tripod if you are serious about taking good measurements.

Thank you, Keith. Yeah, the Surface only has one USB port, but I do have an expander. Appreciate the advice on the trip. I will definitely give this a try.

 

[TimoJ]

Why does it start from DC instead of half of 15Hz i.e. 7.5Hz?

@JohnPM I'm still wondering this, could you please explain. If I select linear sweep and set it to start from 7Hz, it starts from 7Hz. And if I select logarithmic sweep, it also starts from 7Hz.
So why doesn't measurement sweep start from e.g. 8Hz if I select 16Hz start frequency? Why does it always start from DC i.e. 0Hz when start frequency is set to below 20Hz?

 

[JohnPM]

why doesn't measurement sweep start from e.g. 8Hz if I select 16Hz start frequency? Why does it always start from DC i.e. 0Hz when start frequency is set to below 20Hz?

That's the way I designed the measurement sweep to ensure sufficient low frequency energy to obtain a good impulse response and hence an accurate low frequency result.

 

[RPG]

Thank you, Keith. Yeah, the Surface only has one USB port, but I do have an expander. Appreciate the advice on the trip. I will definitely give this a try.

Turns out the 10-yr old Omnimic had either gone defective (the likely problem, I suspect) or it just would not work with REW---even using the proper mic cal file. So, I replaced it with a UMIK-1 which worked perfectly with REW using the ADI-2 dac connected to the USB port of the Surface (which actually does have two USB ports!). Thanks again for putting me onto the solution. Along with my tripod mic stand I was able to get some excellent room measurements.

 

[abemud]

Pictures seem missing in the article?

 

[jamescarter1982]

just found this guide ..so useful thankyou been struggling with rew on and off for ages