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Comb filtering INSIDE YOUR HEAD #3004025 08/19/19 06:09 PM
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matthew mcglynn Offline OP
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This post is brought to you by the guy who built a 3-pattern mic from one of my kits, many years ago, and wanted to return it because "the rear side of figure-8 is defective." He said it sounds hollow and thin.

"Does it sound hollow and thin on recorded tracks?" I asked.

"No," he wrote back, "that's the weird thing. It only sounds thin when I'm monitoring my voice live and singing into the back side of the mic."

For anyone who hasn't done this test on a Fig-8 mic before, this is a pretty interesting and maybe surprising phenomenon. The way figure-8 works is that the rear side has reversed polarity as compared to the front. (If the rear was at the SAME polarity as the front, you'd get Omni... and if you're interested in polar pattern math, write a comment to that effect and maybe we can make another thread about it.)

Therefore when you live monitor your own voice through the back side of a Figure-8 mic, the sound coming into your headphones has reversed polarity as compared to the sound inside your throat and head. This causes cancellation: the sound waves coming into your ears partially cancel the ones coming from your throat. The cancellation makes your voice sound hollow and thin. The effect changes with proximity. If you recite a phrase as you move the mic from about 12 inches to about 3 inches and back, you'll find a spot that makes you feel like you've got the audio equivalent of a blind spot somewhere inside your head. I personally find it sort of unsettling.

Has anyone here recorded group vocals through a Fig-8 mic, with artists wearing headphones? If so, did you have complaints from the singers on the back side of the mic? Did you have to reverse the polarity on the headphone feed for them?


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Re: Comb filtering INSIDE YOUR HEAD [Re: matthew mcglynn] #3004207 08/20/19 04:53 PM
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Mike Rivers Offline
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Many years ago at a NAMM show, I was listening to a mic (cardioid) on headphones at an exhibitor's booth. It sounded fine when pointed around the room, passed the key-jangle test, but when I listened to my own voice, it sounded thin. I looked at the preamp it was connected to and found the polarity (phase) switch was set to the reverse polarity position. I flipped it back where it should be and the mic sounded fine. I pointed it out to the guy manning the booth, who didn't seem to know what that switch did. I wonder how many other people listened to the mic that way.

The same thing occurs when monitoring through a DAW, though for a different reason. The delay (latency) going from the mic to the A/D converter, the guts of the software, the D/A converter, and back to your ears via headphones causes the headphone sound to be somewhat out of phase with the sound reaching your ear drum through the natural organic path. If they're close to being the same volume (at your ear), the 1/2 to 2 millisecond typical monitoring latency added to the natural sound creates a few comb filter notches in the important part of the vocal range.

When I've pointed this out in the past, I was surprised at the number of people who say "I've never noticed that." I believe that the reason why is that they have the headphone level turned up to ear bleeding levels so they can hear their vocal over the drums or guitar. The headphone signal dominates the sum of the two sources and the comb filter notches are small enough not to be noticeable. I tell them to start talking with the headphone level all the way down, then bring it up slowly while talking.

Re: Comb filtering INSIDE YOUR HEAD [Re: matthew mcglynn] #3004400 08/21/19 09:38 PM
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matthew mcglynn Offline OP
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Mike, I'm cringing that the sales rep at the booth didn't understand the basics of the gear he's using to demo his own products. Ouch.

Thanks for the comment regarding monitoring levels. It is a really good point that all DAW monitoring has some latency, and that that can affect the perception of vocal sounds.


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Re: Comb filtering INSIDE YOUR HEAD [Re: matthew mcglynn] #3004806 08/24/19 11:41 AM
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Mike Rivers Offline
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Originally Posted by matthew mcglynn
Mike, I'm cringing that the sales rep at the booth didn't understand the basics of the gear he's using to demo his own products. Ouch.


Well, he was demonstrating the mic, not the preamp. wink However, I'm surprised that nobody else noticed it before I did. It's also possible that someone listening to the mic was fiddling with the polarity switch to see what it sounded like when flipped and inadvertently left it in the wrong position. People like me fiddle with stuff on the show floor whenever we can.


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