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Early reflections and comb filtering
Old 24th December 2013
  #1
Gear Addict
 

Early reflections and comb filtering

I just read Bruce Swedien's recording method book and now I'm reading Bob Katz's mastering book. Bruce talks about having delays of 120-132ms for reverbs so that transients and (especially) early reflections have time to shine through and affect the listener. Bob's book talks about the dangers of comb filtering and says to talk into your cupped hands or talk walking towards a window and to notice when the coloration starts. So, are those important early reflections affected by comb filtering? And are great rooms the ones that comb filter in the most beautiful ways? Kind of "one man's garbage is another man's treasure"? What's the sauce here?
Old 24th December 2013
  #2
Registered User
Delays create a comb filter because they are copies of the original wave, so at various points along the wave the energy either sums and boosts the frequency, or it nulls and cancels the frequency - and all shades in between.

The effect is very frequency dependent, which is why the end result is a series of boosts and cuts at regular intervals, which looks like a comb with many teeth.

It's generally not a nice sound - but it's something that happens all the time in the real world, and our brains are fairly good at ignoring it. Until it gets recorded and played back through speakers, where it becomes more obvious and annoying.

The best rooms absorb and diffuse the delays, so there are less of them, and they are randomly spread out evenly across the spectrum. A well designed room sounds good because the early reflections are designed so they don't reinforce each other.

The worst room in the world would be a sphere, or a dome is nearly as bad. The bulk of early reflections would all be the same distance from the centre, so these tend to ring like a bell. That's a very nasty comb filter.

The best rooms are very random - some natural caves have the most beautiful reverb you will ever hear. Well designed cathedrals can sound very nice too.

Small rooms tend to suck quite a bit. Some of the greatest "small room" sounds are created in large rooms were the wavelength of the distances between walls is sub-bass and therefore not a problem. The small room sound is created with a lot of absorbant and will closely placed baffles. Have a study of some of the photographs of Abbey Road in the golden days ... some very cool acoustic stuff going on there. They also made very interesting sounding echo chambers from small rooms filled with sewer pipes - because convex surfaces are the opposite of concave reflectors - they diffuse the sound in all directions and make a very nice smooth room sound.

But depending on the source and mic placement, sometimes less than optimal rooms can still have a good sound. But there are basic principles of acoustics that you can study and it's fairly easy to change a room sound just by moving furniture and placing absorbant material carefully.
Old 24th December 2013
  #3
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Quote:
Originally Posted by brassmoose View Post
I just read Bruce Swedien's recording method book and now I'm reading Bob Katz's mastering book. Bruce talks about having delays of 120-132ms for reverbs so that transients and (especially) early reflections have time to shine through and affect the listener. Bob's book talks about the dangers of comb filtering and says to talk into your cupped hands or talk walking towards a window and to notice when the coloration starts. So, are those important early reflections affected by comb filtering? And are great rooms the ones that comb filter in the most beautiful ways? Kind of "one man's garbage is another man's treasure"? What's the sauce here?
132 ms is kind of long time for comb filtering. Yes, if you have a source that perfectly repeats after that long of a period you will get comb filtering, but that's pretty unlikely. In fact in the usual sources 30ms is plenty to get rid of the effect (that's what the K-stereo processor uses). And this is especially true for transient heavy sounds...

You could also use a short reverb, they often utilize modulation and other tricks to get rid of the combing.
Old 24th December 2013
  #4
Registered User
A lot of digital reverbs have fake Early Reflections, and usually these are a cluster of short delays which emulate sound bouncing off walls. I find that they are nasty comb filters and really smeary or clattery on percussive stuff. They might work on vocals or smooth stuff, but I find them very annoying and cheap sounding (including high end Lexicon etc). This is where the Bricasti M7 shines - it takes a massive amount of CPU power to generate enough delays so that the ER's sound far smoother and more like a high quality room rather than a nasty cheap room. That's why in the 80's when Lexicon reverbs ruled, people were still using stone rooms and stuff to get good drum sounds. At least for the early reflection part, and still current practice today to use real room sound as much as possible because it is very hard to fake convincingly.

Convolution reverb using IR's taken from real rooms actually work quite well though.

All you can really do is use your ears. Rather than avoid all phasing or comb filtering (pretty much the same thing), try to get it to work to your advantage. It's all part of the real world of sound, and if you try to remove it all sound can be very sterile and boring.

When I was younger I used to be in awe of the guitar tones of Brian May (Queen) and I find a lot of what he did had an awesome phasey room sound that I still love. It's hard to get quality information about what he was doing on the records, but there were a lot of very non-standard tricks. Possibly exotic microphone placements, unusual amps (Pete Cornish describes little battery amps he made for them). For his live arena rig he used multiple amps and chorus and delay - these are all ways to increase the phaseyness of the sound you hear. So not all comb filter sounds are bad - you can really make it work for you.
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