Har-Bal as room analyser?

[chams]

It just occured to me, looking at some spectrum analyser software, that I might use Har-Bal for this.
If I record some white noise at my mix position, and analyse with Har-Bal, am I looking at my room noise fairly accurately?

[Gordon Gidluck]

Yes. This is one of the creative uses of Har-Bal.

Paavo and Earle may have more to say on this, but I dug up one thread on it ---I'm sure there are others.
http://www.har-bal.com/ipw-web/bulletin ... c.php?t=99

[HarBal]

Yes, you can do that, although you should be using pink noise and not white noise. If you use white noise you will have a spectrum in Har-Bal that rises with frequency with a slope of +3dB per octave. With pink noise it will be flat across the band. Also, if you use white noise you will be in danger of damaging your tweeters as they will be exposed to far more power than they would normally see. Not only that, you'll find it increadibly difficult to be in the room the same time you are making the measurement because white noise is very fatiguing.

One thing you should be aware of is that that sort of spectrum analysis will only give you a partial assessment of your room acoustics. Probably what is more important is the shape of your room impulse response. I have written another application many years ago (for which parts of har-bal are based) called AtSpec that I've use to measure the room impulse response of my setup. It was very helpful in demonstrating the effectiveness of the diffusers in my room in improving imaging. I'd almost be tempted to give it away free to registered Har-Bal users though the big issue is how to use it. It is very much a scientific instrument and unless you are familiar with 2 channel spectrum analysis (nothing like RTA's that recording engineers are typically familiar with) you'll be struggling to figure out what to do exactly. I'd really have to write up the entire process in detail for you to make sense of it and that is something I don't have time for at the moment. When I do I'll structure it around the process of tuning a room using mine as an example.

Cheers,


Paavo.

[chams]

Thanks guys.
Pink noise it is. I forgot that pink noise is even power across the spectrum of freqs.
That application sounds interesting Paavo. No hurry right now
I'm just trying for optimal placement of the bass traps I built, and some diffusion.
I am a registered, happy user of Har-Bal.
I wil try Har-Bal for this with my flattest mic. I guess an Omni pattern would be best?

[HarBal]

Yes, an omni mic is the most appropriate.

Pink noise has a power spectral density proportional to 1/f. It looses intensity at higher frequency. Human hearing perceives loudness in through what is roughly equivalent to constant Q filter bands. Constant Q means the logarithmic frequency scale bandwidth is constant but the absolute bandwidth increases with centre frequency. That increase in bandwidth with increasing freqency is what cancels out the falling intensity of pink noise with frequency to make a flat spectrum. Most audio RTA's use constant Q octave band or third octave band filters. Hence the use of pink noise...

Cheers,


Paavo.

[chams]

Thank you Paavo

[timbo]

Hi,

I tried out the analization using pink noise in my mix position.

Here is a pic of room over direct (ref).

http://www.geocities.com/tim_33431/pinknoise.jpg

Is this pretty bad?

Can you tell me what do I do now with this info?

Thanks,
Tim

[HarBal]

Hi Tim,

I gather the reference trace is the "pink noise" you used to driver your speakers and not the microphone output? It doesn't seem very pink, more red Id' say. In any case from the look of your recorded mic signal it looks pretty flat overall. Certainly flat enough for me to say don't bother trying to EQ it out. The only thing I'd be tempted to do is experiment with your speaker positioning to see how it affects the bottom end response. In a sense, it looks too well decoupled from your room below 80Hz, but then again that may simply be a limitation of your monitors. It is hard to say from where I stand. The large dip a 200Hz is a bit of a concern but I'm guessing that is cancelation from the floor reflection so it is probably beyond your control. By the way, is your set up near field, far field or something in-between and was this test with one speaker only or both simultaneously? I'd recommend driving one only as it will give you a more representative response of what you'll be hearing, otherwise you'll pick up a strong interference pattern between the two sources which you normally don't hear that well because of head blocking.

The thing I'd be more interested in seeing is what the impulse response is doing. Actually , I can do the analysis for you if you like and if you're interested. All you need to do is do a two track recording of the pink noise feed and the microphone output. Put the pink noise feed on the left channel and the mic out on the right channel and record about a minutes worth. then send it to me and I can do the analysis and give you an impulse response to look at. Then you might see if you have early reflection issues to deal with.

Cheers,


Paavo.

PS - by the way, don't compress the file. It has to be PCM. Using mp3 encoding will ruin it and the results will be far from accurate.

[timbo]

Hey Paavo,

Thanks for the response.

Yes the reference is a direct print of the pink noise generated by a Panasonic DA7 console.
And, the source is that pink noise played thru both speakers (haflers) with a Rhode mic at listening position in omni mode. I would say near field (triangle). Maybe 3 feet.

I'll be the first to admit that the room is terrible. Very small at about 10X12 but not square either. HAs a little nook on one side that probably is wreaking havoc.

As far as low end, it is my opinion (probably wrong) that without at least 20 feet, I'm not going to hear much of it. As far as the dip at 200hz, yea, that is quite a dip in my view. Maybe why everything I do sounds muffley.
Har Bal has helped quite a bit with this.However, as I'm sure you know, it is really hard sometimes to accept what you then are hearing.

I will try it with just one speaker.

I'm a little unclear about what you need for an impulse analysis:
"All you need to do is do a two track recording of the pink noise feed and the microphone output. Put the pink noise feed on the left channel and the mic out on the right channel and record about a minutes worth"

So, I think you mean just give you what Is shown in that pic except blend (copy/paste) them into a stereo file? (hard left and right)

And, by PCM do you mean *.wav ?

Thanks a bunch,
Tim

[HarBal]

All I mean by doing an impulse response analysis is use your console to record 1 minute pink noise source (ch 1) and pink noise reproduced by your speaker (ch2, recorded simultaneously with ch1) and then mix down to a stereo wav file of 1 minute duration with ch 1 panned 100% left and ch 2 panned 100% right. You can mix it down to 16 bit wav to keep the file size down (for my benefit). Then I can analyze it and send you back the analysis.

Cheers,


Paavo.

[timbo]

Cool.

Ok, er uh, where do I send it?

Thanks,
Tim

[HarBal]

Email to me care of support@har-bal.com .

Cheers,


Paavo.

[timbo]

Ok Paavo,

I sent you the file via yousendit.com.
It won't be there long.

Thanks
Tim

[HarBal]

Timbo,

Could you re-send the notification to to support@har-bal.com again. I didn't receive it so I don't have the yousendit link to click on.

Thanks,


Paavo.

[timbo]

Here is the link to the file (in case anyone else wants to follow along:)

http://www.yousendit.com/transfer.php?a ... 376903722D

I'll send it email too.

Thanks
Tim