How-To: Build a MMM rig for RTA purposes

[erinh]

This is my second video and I'm learning. It isn't perfect (I need better garage lighting and a better mic) but it's the journey, right?...

Anyway, in this video I'll teach you how to DIY your own Moving Microphone Measurement (MMM) setup to use with an RTA system to EQ your stereo system's response.

Due to the reflective nature of car and home stereos it is often recommended users take multiple RTA measurements of their system in the "head space" (where they sit and listen). To do this you are left with two options: 1) Place the mic at different locations, measure and then average all the results together. However, this can be time-consuming and not very repeatable. 2) Sit in the listening position and hold the microphone in front of you while moving the mic around the head area. This is complicated by your own body's influence (even your hand can change the result). This is especially detrimental if you are trying to use a "target curve".

Therefore, I am providing a low-cost alternative: an oscillating fan to move the microphone. I am definitely not the first person to do something like this and I'm not pretending to be. Also, while not perfect, this solves the above two issues of time and in-accuracy and will be an improvement over the typical methods we use.

Make sure to watch the entire video for some tips on things I learned when building mine. I tried to answer all questions in advance so don't tell on yourself by asking something until you've watched.

[Jdunk54nl]

So what is the downside to having your body there? Obviously it blocks some stuff, but a body will always be there when listening. Wouldn't we then want to tune for a body being there instead of a body being absent?

I do realize that this could then mean you could be outside your car tuning, which would be quite a bit cooler temps for most. Which would be a benefit.

[erinh]

So what is the downside to having your body there? Obviously it blocks some stuff, but a body will always be there when listening. Wouldn't we then want to tune for a body being there instead of a body being absent?

I do realize that this could then mean you could be outside your car tuning, which would be quite a bit cooler temps for most. Which would be a benefit.

I'll answer your question with a question:
What's your target curve built upon? (I can guarantee you that whatever it is, it isn't built with a human body being part of it.)


AFAIK, the only curves that incorporate human anatomy are ones built for headphones. And you can't apply those to a standard free-field measurement. There might be others that use HRTF to do some stuff that's beyond typical stereo use, though.

[Jdunk54nl]

Fair enough.

Cars obviously have more impact by a human body than home theaters due to our placement in front of the speakers. So tuning HT with or without a body isn't as big of a deal.

I guess this makes me question, if we tune for a curve without a human body, and the human body impacts that curve, how does this relate to our listening. If we place a human body in the position after tuning, obviously we have changed the curve and we hear something different than what we actually tuned for, and every body is going to change it slightly differently depending on the size of the body. I know just moving my leg in front of my door speaker impacts the FR to a measurable amount and I can hear a difference. I don't remember the frequencies it impacted or if that is still the case now that I have 3 way, but I know I measured this impact when I was still running a 2 way.

Is this a good thing to hear a different curve than what we tuned for?

[erinh]

Funny, someone on FB misunderstood my comment about taking the body out of the equation so I'm just gonna copy/paste myself here as it applies.



If you take a target curve that was generated without a human body as part of the measurement and then you try to hit that same target curve *while you are holding the mic* then you've introduced a variable that wasn't part of the original target curve: your body. You should try to match a target curve under the same conditions it was generated.

Here's an example... all of these measurements were taken with the volume at the same level. However, what I changed was how I measured. The first 4 are single mic measurements. #5 is the average of those 4. 6 - 11 are variants of the MMM with the mic either facing forward or up. 12 & 13 are left and right ear binaural measurements, respectively. You'll notice that below about 160hz all the results are relatively the same. However, above this the response varies greatly.





Now if I simply take the above and cut it down to: Average of the single mics (bold green) vs MMM with the mic facing forward (brown), you can see the response is quite different at various points.




I did all this last year... I meant to do a write-up about it. If I can get some time I'll do a quick video this week to discuss it a bit more.

[Notloudenuf]

I would not have thought of using a fan for this.
This is a helpful DIY device with repeatable results.

[erinh]

I would not have thought of using a fan for this.
This is a helpful DIY device with repeatable results.

Thanks for the feedback. I hope people can take this and actually use it. Maybe one day I'll got a show or meet and see everyone putting fans in their front seats. LOL

[bbfoto]

Thanks for the feedback. I hope people can take this and actually use it. Maybe one day I'll got a show or meet and see everyone putting fans in their front seats. LOL

Erin, when you have a moment and do another "fan" measurement, try weighing down the based of the fan in the seat. I have relatively small sand bags & shot bags from the film/video lighting industry that I use to do this. Let me know if you find a measurable difference, particularly in the low end?

[dumdum]

Erin, when you have a moment and do another "fan" measurement, try weighing down the based of the fan in the seat. I have relatively small sand bags & shot bags from the film/video lighting industry that I use to do this. Let me know if you find a measurable difference, particularly in the low end?

This is very credible and probably will alter the measurement as it depends on the mic not resonating as it will then effectively cancel the diaphragm movements at certain freqs I would think

its the same reason we used to put scores up that were 1db more on hard standing vs grass at 160db plus in dB drag I think, the car was more likely to move even if by tiny amounts

[erinh]

I noticed some odd things in one measurement where the response dropped below 20hz. But I believe that was because I hadn’t closed the door all the way. So as far as loss due to transmission, I didn’t have any that I recall. That said, I also wedged the fan in between the seat cushions to hold it in place. It’s possible that on a different surface (like a hollow stage riser in a theater room) or non-secured seat you could get some form of loss via vibration.

That said, in some ways you could consider this as decoupled from the seat; as opposed to wedging the microphone in between the headrest/seat and having vibration cause issues with measurement (another video I have plans to discuss; not enough hours in the day even with corona).