# Uncalibrated dbx RTA-M Microphone Advice



## Math (Aug 17, 2015)

I am planning to EQ my home studio where I mix orchestral music. My personal research has lead me to two uncalibrated microphones that have a relatively, flat response curves; I would say, flat enough to use to find major problems in my room, the dbx RTA-M and the Presonus PRM1. I am very aware of the popularity of the MiniDSP UMK-1 but I don't trust USB microphones because there is always noise that is introduced into the signal because motherboards are not all the same. So, the only true "quiet" inputs are XLR inputs on quality external sound cards. Like it or not, that is just the way it is; ask any audio engineer with a degree. As such, I'd be no better off with the UMK-1 with noise than I would be with the dbx mic on a Mackie VLZ3 input. Again, this is just my opinion and I could be completely wrong!

_Now before this turns into a dogpile, please take my opinion with a grain of salt. This is my opinion. It's just my previous experience with this type of technology. I'm NOT Eq'ing my home studio for calibrating instruments...just to mix music. I know where the problems are but I would like to be a little more in the ballpark. _

From what I've been reading and viewing, in my head it appears the "quality vs dollar" graph appears logarithmic when it comes to the $100-$1000+ reference microphones; meaning, the more expensive a mic is the quality improvement is only fractional. It seems that they all are relatively flat up to the 1kHz where the dbx is still relatively flat up to about 5kHz and the expensive ones are just a bit flatter above that with minimal deviation from 0bd (with the exception of being calibrated - let's not forget that!)

When it comes to mixing orchestral, the higher after 5kHz freq's are not so much of a problem for me to tame, so the dbx is a clear choice for me. The PRM1 does have a bit more stability in the upper highs but when I sweep for harsh freq's a little A/B comparison on Yamahas then on a pair of Klipsch, I find what to cut without an issue. So that's my reason for sticking with the dbx.

I've noticed my main issues, acoustically, are the lows and somewhere about the 3kHz range. Now, if I decide to go with the uncalibrated dbx, does anyone have any suggestion from experience, or if you just know, what should I be paying close attention to? What should I not assume?

_Sorry for the long intro but I thought it was necessary to justify why I'm not following the standard protocol with the suggested USB microphone. I can foresee someone's suggestion to getting the UMK-1 if I just ask simply about tips on using an uncalibrated microphone._


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## Wayne A. Pflughaupt (Apr 13, 2006)

First I’ve heard of the USB mic noise issue, but you seem to know what you’re talking about so I’ll take your word for it. :T

We haven’t had many come through here using an uncalibrated dbx mic, so I doubt anyone will be able to answer your question specifically.

Nevertheless, while I understand the importance of a quiet system for recording, it isn’t an issue when taking measurements. After all, you’re measuring a signal generated by the speaker. That noise is going to swamp any noise the USB mic might generate.

You should be fine using the dbx, depending on what you want to do with the measurements. However, if you intend to apply any manual equalization, I’d highly recommend a calibrated mic. You don’t want to equalize based on a mic that doesn’t have certifiable flat response, else you don’t know at what point you’re equalizing for the speaker’s deficiencies or the mic’s.

Regards, 
Wayne


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## Math (Aug 17, 2015)

So far, I'm finding the dbx RTA-M quite accurate using either REW or pink noise from within a DAW and observing a spectrum analyzer. Ultimately, the cuts I've needed to make are consistently between 100 and 300Hz with either REW or manually in the DAW.

It seems that the EQ settings in REW really need to be experimented with for each individual system; meaning, I've really made some drastic changes to certain sections and the best room EQ setting I was able to get was one that had only 4 filters...again, in between 100-300Hz. i must have tried at least two dozen combinations before I started to see consistency in one particular average, which is the 4 filter EQ I ended up with.

So that's my experience with the dbx and REW. So far, so good.


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## Wayne A. Pflughaupt (Apr 13, 2006)

There is virtually no way four filters should legitimately be needed between 100-300 Hz. I’d suggest maybe you’re over-equalizing? Filters that are narrower than ~1/4-octave and/or only 1-2 dB up or down are virtually inaudible

Regards, 
Wayne


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## Math (Aug 17, 2015)

There was one particular spot that kept giving me a spike. The four REW-generated filters are:

Freq: Gain: Q
75.17Hz: -5: 0.167
141.6Hz: -17: 0.750 <---- This is where the major problem is, mostly.
292.4Hz: 6: 0.250
313.4Hz: -2: 0.250

Here is my goal: Try to do what I can acoustically and use REW to assist in finding major spikes and offset them with an algorithm-based inverse EQ curve to compensate for what I can't control physically in the room. 

As such, I want to avoid having to place any EQ filters on my master bus but if I understand correctly, you're implying that I can just manually set one filter and use REW merely as a guideline. Is that correct?

Unless I've completely misused the software, I understand that the algorithms will produce inverse curves where appropriate and so the program seems to be trying to get a particular curve such that the predicted response if flatter and closer to the target line.

The Target Settings shown in the attached picture, essentially got the program to "look at a particular" spot and placed the target curve just about where I wanted it, to avoid generating filters anywhere else. In trying to be somewhat precise, for my own peace of mind, it took quite a few combinations that didn't deviate very much from the settings you see now.

Again, I could be completely misusing the software but fundamentally, if the final EQ curve you use doesn't deviate much from the average EQ curves generated, then the curve with the least filters would be the most ideal, right?

By the way, thank you for your insight.


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## Math (Aug 17, 2015)

I made some changes to the slopes and freq intervals and let REW decide the appropriate filters. It took two different independent slopes in two different intervals for me to smooth out the major spikes along the natural line my system inherently has.

After this is all over, I'll have to listen to some reference tracks to get a feel of the changes...that will take a little of time.

Essentially, the files uploaded are the same. I just named one Highs for the high filter and one for the lows.

I'll have to test these new EQs out later as I did this analysis late.


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## Wayne A. Pflughaupt (Apr 13, 2006)

Math said:


> There was one particular spot that kept giving me a spike. The four REW-generated filters are:
> 
> Freq: Gain: Q
> 75.17Hz: -5: 0.167
> ...


Something seems really amiss here. You’re trying to clean up between 100-300 Hz, which is only an area only 1-1/2 octaves wide. Yet the filters you’re posting there are huge: 0.167Q translates to 5 octaves. That filter will affect five octaves on _each side_ of the center frequency. Likewise, 0.75Q is ~1-3/4 octave and .25Q is ~3-3/4 octave.




> Here is my goal: Try to do what I can acoustically and use REW to assist in finding major spikes and offset them with an algorithm-based inverse EQ curve to compensate for what I can't control physically in the room.
> 
> As such, I want to avoid having to place any EQ filters on my master bus but if I understand correctly, you're implying that I can just manually set one filter and use REW merely as a guideline. Is that correct?


Not necessarily a single filter, it depends on the nature of the deviation in response you’re looking at. For instance, the peak shown at ~140 Hz in your picture is asymmetrical, falling off sharply on the low side and more gradually on the high side. 

Problems like this are difficult to address because filters are symmetrical. So, an appropriate filter applied at the center of the peak will drag down frequencies on the low side that aren’t a problem. So, in addressing one problem you create another one. The now-deficient area on the low side of the peak would then have to be addressed with a second filter to counter the effects of the first. Make sense?

Alternately, a filter could be applied not at the peak center, but above it (i.e. a higher frequency). This would keep from dragging down the area on the low side of the peak previously mentioned, but will not adequately address the peak itself.

Bottom line, asymmetrical peaks aren’t going to be perfectly resolved with equalization. The best you can hope for is an overall improvement in sound quality in that area, as there will still be a measurable peak or dip even after equalization. Problems like this are most likely best addressed seat-of-the-pants with manual EQ, as any auto-EQ platform is only looking at peaks and troughs with no regard for overall sound quality. It’s much easier to equalize if you apply some smoothing to the graph. 1/3 or 1/6 octave smoothing will get you a graph that better resembles what you’re actually hearing.

Regarding your .mdat files, those are with equalization in place? I ask because I see no improvement in the 140 Hz region.

Regards, 
Wayne


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## Math (Aug 17, 2015)

I've assessed all your input and I believe you're right about just doing this manually and increase the smoothing.

I am not an acoustics specialist but I clearly understand the Q to Octave ratio and it's becoming more obvious that perhaps I should use this microphone with REW, run the RTA and place some Rockwool panels and bass traps in the appropriate locations.

Thank you for investing your time in the feedback you've provided. You have inspired me to do some serious research in acoustics. I'll probably go and audit a class in acoustics on campus if it doesn't coincide with work.

I have to admit, the filters did target the problem because sounds I couldn't clearly hear were much clearer but to your point, they solved one problem and created another. As far as the graphs go, not that it matters anymore, one graph addresses one problem in the high end and does not address the low end, hence the lack of change. I will get this figured out successfully...I am certain of that. Tomorrow should be interesting...


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## Wayne A. Pflughaupt (Apr 13, 2006)

Bass traps could possible make an improvement in response in the 140 Hz range. Other types of treatment probably won’t show much improvement in frequency response, as they main absorb or diffuse the sound waves. Improvements with treatments show up better on other REW graphs such as RT30 and ETC.

Here's a good article on small room acoustics.
http://www.acousticfrontiers.com/room-acoustic-measurements-101/

Regards, 
Wayne


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## Math (Aug 17, 2015)

That was extremely useful information...thank you so very much. Some things stated really boosted my confidence in what I was doing.

So, with the RTA running, I positioned some Rockwool panels and the result is attached. The bass issue I had before, is essentially gone; everything is clear and crisp. I'm getting used to the sound of my reference monitors. What a difference! They definitely sound much clearer and have absolutely no boomy bass! I did it manually, as you suggested, and I'm satisfied with the result as I do not have to use an EQ on my master bus. Originally, this is what I was going for.

Originally, this post was for advice on the dbx mircrophone, which I was hoping to get some feedback on. Now, after all that has transpired I found this microphone to be more than satisfactory and accurate. What the graph shows and what I hear now, all coincide positively with the result I have now. I would suggest anyone reading this to follow the link you provided above.

I'll continue to study more about this and most likely, ask a few people I know, that own home studios, if I can analyze their rooms. I think with a little more hands-on experience, I would like to add this to my list of skills.


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