Non "acoustic" timing reference

[Chuck Gerlach]

I need to near-filed measure some subs so that I can check their near field phase response. And I need to so this for multiple subs in different locations in the room - so I can ultimately align all of those sube near field (don't ask why I'm doing it that way as it is just what needs to be done). The current approach of using another speaker as the timing reference won't work since the distance between that signal and the sub being measured will not be consistant as I move from sub to sub around the room. What I need, I think, is an internal to REW reference but don't know if that exists - and I using the Mac version of REW.

Thanks

 

[sm52]

If you measure in the near field, that is, the microphone will always be 1 cm from the membrane, then wherever you move the subwoofer, the measurement will be the same.
If you still want to do this, the reference speaker, if small, can be attached to the top of the subwoofer so that it moves with the subwoofer.

 

[Chuck Gerlach]

If you measure in the near field, that is, the microphone will always be 1 cm from the membrane, then wherever you move the subwoofer, the measurement will be the same.
If you still want to do this, the reference speaker, if small, can be attached to the top of the subwoofer so that it moves with the subwoofer.

Interesting idea, but not practical. I won't be adding a speaker, amp and speaker wire to my travel kit. This ability already exists in products such as Scarlett 18i8 USB Audio Interface, but I am hoping there is some way to get it accomplished in REW.

 

[dotnet]

Not sure about the timing reference, but I wonder how practical near-field phase measurements of subwoofers are? No matter how close to the membrane you put the microphone, during the time a single cycle of a 50Hz tone takes to complete (20ms) reflections from the nearest wall or floor would have arrived, mixed with the direct signal and shifted the phase.

How would one window measurements in the subwoofer range? Wouldn’t one need to have a large venue, or be out in the open?

 

[John Mulcahy]

You could use a loopback connection as a timing reference if you are not using a USB mic, or you can have REW estimate any delay in the IR of each measurement and remove it if you just want to see the responses without any delays.

 

[Chuck Gerlach]

I'm using the Focusrite Scarlett 18i8 3rd Gen USB Audio Interface. So to implement the loopback timing reference, I could use the input and output interfaces on the rear of the unit - but how do I tell REW where to look for the timing reference input ?

Is it a two step process? That is, select the proper input and output on the USB interface to obtain the timing reference and then perform the actual speaker measurement?

 

[John Mulcahy]

You can select the loopback connections on the soundcard preferences:

or on the Measure dialog:

Then you just measure as normal and REW will use the loopback as the timing reference.

 

[sm52]

John Mulcahy​

Is this a screenshot of your screen? You have selected devices with 'EXCL', but volume control is not available. If you select the same device but without 'EXCL', volume control will be available. You can use both options, but which one is more reliable?

 

[John Mulcahy]

WASAPI exclusive drivers generally bypass Windows volume controls.

 

[Chuck Gerlach]

I'm having issues with my Focusrite. I also have a PreSonus Audiobox USB. So would I connect a cable from the "main out" on the rear to one of the inputs on the front?

 

[John Mulcahy]

Yes. The mixer knob on the Presonus should be turned all the way to Playback.

 

[JonPike]

Not sure about the timing reference, but I wonder how practical near-field phase measurements of subwoofers are? No matter how close to the membrane you put the microphone, during the time a single cycle of a 50Hz tone takes to complete (20ms) reflections from the nearest wall or floor would have arrived, mixed with the direct signal and shifted the phase.

How would one window measurements in the subwoofer range? Wouldn’t one need to have a large venue, or be out in the open?

This is technically correct, you would need a longer window, and the room would be "too small" to properly exclude. Some have raised the speaker very high in the air, or made a hole in the ground and used the ground as an infinite baffle, with the sky as the rest of the "room". Difficult.

Another aspect of near field measuring, which could make it worth doing, you can get a high signal to noise ratio in the measurements. In other words, the driver signal being very close to the mike can be so much stronger than the room reflections, they aren't as noticeable in your measurment. Now, this works well with higher frequency drivers, I'm not sure how much less it might work with a subwoofer in a "small" to the wavelengths room. Anyone have any knowledge here? (I'm still getting my test gear back together)

 

[thothsong]

You might have a look at that post.

 

[thothsong]

There's also the Keele paper.

 

[Chuck Gerlach]

Yes. The mixer knob on the Presonus should be turned all the way to Playback.

So doing it that way, would I still use the measurement signal created in REW sent to the subs over HDMI or since the timing signal and the measurement signal (apparently) both need to come from the same place, I would need to run a single ended interconnect from the Presonus to the Trinnov? If that is the case, not sure how I would be able to measure SPECIFIC speakers without moving the interconnect to the various inputs on those processors that would allow me to do that.

OR is there a way to get the Presonuse generated timing reference back into REW?

 

[John Mulcahy]

Winding back to the start, you wanted to use a timing reference other than the acoustic reference. The alternative is a loopback connection. The loopback connection goes from an interface output to an interface input, so you would use the same device for input and output. I'm not really clear why you need a timing reference for a near field sub measurement, but since you say that's what you need the loopback can provide it.

 

[Chuck Gerlach]

so you would use the same device for input and output.

And doesn't that mean that I can not use the REW HDMI out for generating the signal but rather must use an output from my Presonos since it is the "timing reference" (single ended interconnect) to an input on the processor (which may be problematic in many cases)? Or am I still missing something?

 

[John Mulcahy]

Yes, that's what it means. If you are measuring individual subs wouldn't you just connect directly to each sub?

 

[Chuck Gerlach]

Yes, that's what it means. If you are measuring individual subs wouldn't you just connect directly to each sub?

Thanks. In the case of the Trinnov Altitude 32, there is an LFE input which I can use, but in other cases, your approach could work. For the vast majority of the work I do, the acoustic reference works perfect (and is a lot less convoluted than the this approach).