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Room mode at 700hz and 850 hz

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Re: Room mode at 700hz and 850 hz

Postby MOF » Fri Nov 01, 2019 5:48 pm

Ah I see, thanks. Can you tell me then where is the baseline for measuring peaks and dips? Surely there is a point somewhere where dB values are + or - the base level
John I think you’re confusing this graph with an audio waveform.
There are no negative values, they are all relative to absolute silence of 0dB (at the bottom line, Y axis).
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Re: Room mode at 700hz and 850 hz

Postby MOF » Fri Nov 01, 2019 5:54 pm

As I tried to explain before - I wasnt trying to demonstrate the performance of my room, I wouldnt take a reading like this normally as Id usually be around 3ft from my speakers for example, and be in my monitoring position and have all my furniture and equipment back in the room. I stripped the room bare(ish) and placed the mic in a place miles from where I would monitor, and in a place that I knew had large bass peaks, simply to demonstrate the graph I was talking about and also to demonstrate that the large peak around 800Hz is still there. In the hopes that all this would help someone help me understand what is going on..

MOF wrote:
You could clearly do with some bass absorbers to tame the low end

Taking measurements from where you mix is really all you should be doing, unless it’s a commercial facility and even then you’re unlikely to avoid bass tilt at the Producer’s sofa at the back of the room.
If you get similar results in the mixing position then I would be concerned that your bass absorption isn’t working since it’s increasing in value at the point where it’s going lower, not gently rolling off.
I presume these measurements were taken close to a wall, where bass frequencies always increase.
Please do a real world test i.e. the mixing position and post the results. I think you’ll find much better results.
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Re: Room mode at 700hz and 850 hz

Postby MOF » Fri Nov 01, 2019 6:03 pm

As I tried to explain before - I wasnt trying to demonstrate the performance of my room, I wouldnt take a reading like this normally as Id usually be around 3ft from my speakers for example, and be in my monitoring position and have all my furniture and equipment back in the room.

Your original post didn’t explain this.
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Re: Room mode at 700hz and 850 hz

Postby jodaki » Fri Nov 01, 2019 6:24 pm

MOF wrote:John I think you’re confusing this graph with an audio waveform.
There are no negative values, they are all relative to absolute silence of 0dB (at the bottom line, Y axis).

:oops: ok now I do feel a bit silly :)

Thanks for your other points. I think my hurry in getting my graph up earlier was to demonstrate that the peak is still there. I will definitely upload a reading when Ive moved the furniture back in.

In the meantime. MOF why is the sound card calibration ‘curve’ an inverse of my measurement wave/graph? And can you think of a reason REW would think that reading was performed at 120db?
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Re: Room mode at 700hz and 850 hz

Postby Wonks » Fri Nov 01, 2019 7:17 pm

jodaki wrote: In the meantime. MOF why is the sound card calibration ‘curve’ an inverse of my measurement wave/graph?

Because (in theory) its the EQ curve you'd need to apply to the DAW output to get a flat measurement response . If you've got a 3dB peak at one frequency then it will counter it with a -3dB dip.

In reality you can't counter room mode dips with just EQ boosts.
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Re: Room mode at 700hz and 850 hz

Postby Hugh Robjohns » Fri Nov 01, 2019 8:38 pm

Measurements are only as good as (a) the equipment, (b) using it correctly and (c) nderatanding and interpreting the realts accurately.

REW is great, but it relies entirely on the accuracy of the measurement mic and/or it's calibration file.

...and then using it correctly and understanding what it's telling you are far from trivial... And as a result it is incredibly easy to chase ghosts...
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Re: Room mode at 700hz and 850 hz

Postby jodaki » Fri Nov 01, 2019 8:45 pm

Wonks wrote: Because (in theory) its the EQ curve you'd need to apply to the DAW output to get a flat measurement response . If you've got a 3dB peak at one frequency then it will counter it with a -3dB dip.
In reality you can't counter room mode dips with just EQ boosts.

I got the wrong end of that too then. The sound card is calibrated prior to measurements so I assumed the sound card calibration was an offset used by REW when drawing measurement graphs to compensate for the imperfections/biases of the card.

I accept what you say but if I’m being honest then if the sound card calibration is simply an inverse of the measurement then I dont really understand the point of calibrating my soundcard, surely REW can just invert the response reading to generate an eq ’recommendation’.

I dont have a device or much of a desire to fix the room modes by eq so I’d prefer to get as far as I can by altering the space characteristics - by which I mean my position in the space and the response characteristics of the space.
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Re: Room mode at 700hz and 850 hz

Postby Hugh Robjohns » Fri Nov 01, 2019 8:49 pm

I fear there's some major misunderstandings here...

If the sound card is anything other than ruler flat from 20Hz to 20kHz it's broken.

The calibration is a correction for the measurement microphone, and any inaccuracies will obviously give false room measurements.
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Re: Room mode at 700hz and 850 hz

Postby jodaki » Fri Nov 01, 2019 10:11 pm

Hugh Robjohns wrote:I If the sound card is anything other than ruler flat from 20Hz to 20kHz it's broken.

Are you talking about REW’s soundcard cal graph? As in the SPL plus phase graph IO posted?

As you can see mine is clearly an inverse of the measurement. So it is not flat, but as far as I can tell my 18i20 works perfectly.

Are there any other possibilities?
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Re: Room mode at 700hz and 850 hz

Postby jodaki » Tue Nov 12, 2019 2:30 pm

Hugh Robjohns wrote:Measurements are only as good as (a) the equipment, (b) using it correctly and (c) nderatanding and interpreting the realts accurately. REW is great, but it relies entirely on the accuracy of the measurement mic and/or it's calibration file.

I read an article last week somewhere on the pointlessness of using an ECM8000 with a generic calibration file as the response of those mics varies wildly. In particular noting that the example mic in the article displayed a set of peaks culminating around 800Hz.

After which I ditched the behringer and bought a UMM-6 and Dayton Audio gave me the calibration file for it.

Since then Ive spent the last week moving speakers and mic around my room measuring the spots Im willing to work in.

This is what I had as a bare measurement (mic and speakers, no other gear) in my favourite spot.

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Not knowing what a good REW SPL graph looks like I'm not in a position to judge this reading, though it looks more even than some Ive had.

Looks like some problems at 163Hz and 370Hz but more absorbers didn't seem to help.

And I don't know whether I have 2 peaks at 50Hz and 63Hz, or whether I have a cancellation at 56Hz. The un-smoothed graph is almost identical sub 100Hz to the 1/24 smoothed graph. I don't pretend to understand but I think the waterfall graph shows peaks so Im guessing these are peaks at 50 & 63Hz.

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Adding more mass to the bass traps (pushing them up to nearly 200kg with extra bales, 140kg/m3 RW6 rockwool and techsound 5.8mm MLV) makes little to no difference to the sub 100Hz region. Not sure what else I can try to reduce the bass there. Perhaps the problem is leakage into the room or reflections from the ceiling or the floor.

Then I added my desk and gear back in, resulting in this measurement. This is a real world example of the response at my listening position:

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My desk seems to have solved the problems at 163 and 370Hz. I don't know why or how.

But has it in turn created a new problem around 97Hz?
Is that mid-range dampening between 600 and 1.6kHz or is that an improvement?
And what do I make of what look like periodic high range resonances from 3.25kHz upwards?
Help understanding these readings would be appreciated..

And perhaps most importantly I don't know whether this is a good result. And what should I try and do from here?
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