Correcting your room acoustic environment.

There are many features of Har-Bal we still haven't discussed in this forum. Below we will start sharing a few items. Please feel free to add yours.
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HarBal
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Correcting your room acoustic environment.

Post by HarBal »

I've noted that some people have difficulty in obtaining good results and wonder about the effectiveness of HarBal. In many of these cases I wonder about the quality of the listening environment, especially if you are making EQ decisions upon what you hear. Here's a few suggestions to consider before discarding HarBal as a failed product.

How good are your speakers and how good is your acoustic environment. Both of these, and especially the later, can have a huge impact on what you will achieve if using speakers to review your EQ changes. A simple way to get an idea of the quality of your acoustics and speakers together is to listen to a large collection of commercial music and rate the quality of what you hear for all of them. If less than 50% of the CD's you listened to sound good then it is more than likely that you've got a problem with your acoustic environment. If 80% sound good then you've probably got an excellent environment.

Other queues as to the quality of the acoustics is how long you can listen to music before the quality of your perception is diminished. If you can listen to music at moderate levels (80-85dB) without any degradation in percieved quality for an hour or more then you're environment is good. If it is less than this then you probably have problems. Your hearing can be physically fatigued by tonal imbalances and strong resonances and your hearing can also be mentally fatigued by ambiguity in the stereo image. The later is caused when early reflections from the walls of your studio reach the listening position giving spatial queues that don't match the recorded queues in the music being reproduced. As an analogy, try visualising looking at a framed picture and interpreting it when there's a strong reflection of what's behind you in the proctective glass in front of the picture. You can fix early reflection problems with diffusers and tonal problems with frequency selective damping.

A simple and robust way to test your room performance is through a simple reverberation test. Take a measurement microphone (like the Behringer ECM8000) and put it in your normal listening position with the front face in the same orientation as your ear would be. Hook it up to your preamp/mixing desk with no applied EQ and minimum preamp gain. Blow up a balloon, set your DAW to record the mic signal, put on some ear muffs, grab a pin and walk up to where your speaker sits. Now hold the balloon above the speaker with you to the side and a clear path between the balloon and the Mic and pop it while remaining as still and as quiet as you can. After 10 seconds you can then move and stop your DAW from recording. Now you should have a clean recording of the impulsive reverberation decay of your room. In your DAW find the point where the level decays to 60dB bellow the initial peak. The time between that point and the peak is the reverberation time. Depending on the size of the room it should be between 200ms to 500ms. In my room I have 300ms. If you want an idea of the spectral balance of the decay, edit the file to give a waveform that is around 1.5 seconds long and leave about 200 milliseconds of silence from the start of the file to the initial impulse and if there's any noise here just blank it out. Then open the file in HarBal. You should have something that looks bumpy but generally flat across the mid-range and drooping down at the top end a bit. If you have any regions that stand out then you've got a resonance problem to deal with. In my case I have two resonance issues to deal with. One at around 550Hz and one at 3kHz.

Since moving in to my new house I've had this issue to resolve. At first I was blaming my speakers but I noted that I had the same colourations in sounds reproduced by instruments and not through the speakers so it was clear that something else was wrong. After doing a series of reverberation tests the problem became clear. I even went to the extent of removing all the damping from the room and adding it one by one, doing a balloon test at each step, to see if the damping was adding the colouration. It wasn't. I'm now designing helmholtz panel absorbers to correct the colouration in the reverberation response and this should hopefully fix the problem. I fixed a early reflection issue with home made quadratic diffusers.

In short, it's easy to blame Har-Bal if things don't sound right when you're CD doesn't transport well from one environment to the next, but if you've got a problem in your acoustics then it's not a fair judgement, particularly if you are adjusting on the basis of what you hear. I'd encourage everyone serious about music spending more time on getting the acoustics right. If you are reasonable at carpentry it need not be that expensive either. Just a little time consuming, but the improvements are well worth the effort.

Regards,


Paavo.
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