Measurement of room acoustics (for the layman)

Tariq, I differ from Simon's opinion in a few points...

1 It's better to use 1/12dB smoothing to get a better appreciation from the graphs as then we know what we're actually looking at.  Var Smoothing is variable, so we can't accurately judge the true height of the peaks and the Q factor of the resonances.

2  The waterfall trace hasn't been generated for a long enough time for your quite resonant room (I suggest 800ms or more will give a better picture); the plot needs to be scaled in time to show the resonances decaying almost to completion (at least -40dB of decay should be shown).  The fact that the entire band 37Hz to 65Hz and the peaks at 28Hz and 74Hz decay so slowly indicates the requirement for bass traps in the room.

3  The resonance is at 37Hz (not 39Hz) corresponds to a room dimension of 4.6m.  However it's indicated as only +8dB wrt the octave surrounding it, and it's actually entirely in line with the amplitude 100Hz - 200Hz, so, in the greater scheme of things it isn't really at all prominent.  In fact the prominence of the 100Hz - 200Hz band looks to me to be more of an issue, causing an overblown 'plumminess' to the bass; rather than the specific 37Hz local peak.

4  The minor 74Hz resonance (corresponding to 2.3m) is most likely to be the primary mode of the floor to ceiling distance, rather than a harmonic of the 37Hz mode.

5  There is another notable resonance at 28Hz, corresponding to a room dimension of 6.1m, but at this frequency the anechoic bass response of the speakers is beginning to fall off quite a bit.  Extending the water fall to 800ms (in fact, even 1s or more may be needed to show the decay) will show how important it is to deal with this resonance.

I started to read the REW stuff but it quickly took me down a rabbit hole.  Maybe someone can answer . . . 

With a Macbook Pro and a commercial mic, can I get a somewhat-accurate picture of my room with this? Or do I need other hardware?  

It seems I need a way to connect the Macbook to my SN2 to generate the sounds that the software is going to analyze?

If this is relatively simple, I'm up for it, as I know my room needs SOMETHING and is holding back my enjoyment of the music.

Huge, not bad for my untrained eye 

The thing that caught my eye was with the 37/39 Hz peak, wasn't so much its magnitude, but its relative long delay which I think would spoil some wide band recordings ... my sub should be arriving in a few days, so I'll be playing around with this more... I am having BK in Southend build me one.. I though best not spend too much until I get some home experience on this.

 

Bart posted:

I started to read the REW stuff but it quickly took me down a rabbit hole.  Maybe someone can answer . . . 

With a Macbook Pro and a commercial mic, can I get a somewhat-accurate picture of my room with this? Or do I need other hardware?  

It seems I need a way to connect the Macbook to my SN2 to generate the sounds that the software is going to analyze?

If this is relatively simple, I'm up for it, as I know my room needs SOMETHING and is holding back my enjoyment of the music.

Hi Bart, It depends what you mean by a commercial mike.  If you mean a studio mike or a stage mike, then you can use it if you have a calibration for it.  However one of the simplest and one of the best options is to use a miniDSP UMIK-1 (about £80), which is a simple USB instrumentation mike calibrated +-0.1dB 10Hz to 25kHz.

As you have an NDS, output is probably simple.  Most Mackbook Pros have a miniToslink output in the headphone jack - just connect this to an optical input on the NDS.  If your Macbook doesn't have an optical out, then you can use a simple USB to SPdif converter (optical or coax) to connect to the NDS.

The REW documentation is rater technical and does need some head scratching, but the information in it is quite sound (pun intended!).

DrPo posted:

Is there an app to get started (all the limitations of the built in microphone and the associated software notwithstanding)?

I don't know of an app for a mobile device that's anything more than a toy.  Have you considered using REW on a laptop?

Simon-in-Suffolk posted:

Huge, not bad for my untrained eye 

The thing that caught my eye was with the 37/39 Hz peak, wasn't so much its magnitude, but its relative long delay which I think would spoil some wide band recordings ... my sub should be arriving in a few days, so I'll be playing around with this more... I am having BK in Southend build me one.. I though best not spend too much until I get some home experience on this.

 

Yep, it's the slow decay that's the issue just there.  That will give an impression of 'slow' bass.

Have fun setting up the sub, it took me several months to get it just right; but now the integration to the main speakers is pretty much seamless.  How are you planning to work out the best position and the filter parameters for the miniDSP?  I agree about not spending too much initially, in some rooms it's almost impossible to achive a good enough integration of a sub.

If at all possible, you should persuade BK to tell you what the group delay is at a few frequencies form 50Hz up to about 150Hz.  This will allow you to get the path length difference almost right before even powering up the sub never mind measuring it.

Huge, I have bought miniDSP and the calibrated mic to go with it to experiment, however I am planning to have forward facing behind outside the stereo pair of speakers and set frequency cut off around 40 Hz, fed from the speaker outs, which is about 10 Hz or so below the ATCs, and I will set the gain quite low.. I won't be planning for flat but rolled off bass down to 20Hz. However I will have to see how I get on... largely trial and error... I have quite a good bass response, with good punch and no prominent single noting, but a small but noticeable peak around 48 Hz... so the phase control might need to be used....

 

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