Ed DeGenaro
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I know I should know this, but I don't.
Why is it that a close miked source is altered by room sound?
Why is it that a close miked source is altered by room sound?
Stupid question about room sound...
- Thread starter Ed DeGenaro
- Start date
Help Support GroupDIY Audio Forum:
it depends on what you are close micing. A super loud guitar cab isnt going to be influenced by the room very very much, but for anything at a reasonable volume, you are always going to get some kind of reflection in the room. Remember, with a cardioid, the rear lobe of the mic is on axis as much as the front is, yes, its directional, but the rear of the mic still hears the room. As you go from card to hyper, the focus of the rear lobe axis gets more intense. So pretty much, no matter what mic you use, its gonna be hearing the room one way or another.
dave
dave
Ed DeGenaro
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[quote author="soundguy"]it depends on what you are close micing. A super loud guitar cab isnt going to be influenced by the room very very much, but for anything at a reasonable volume, you are always going to get some kind of reflection in the room. Remember, with a cardioid, the rear lobe of the mic is on axis as much as the front is, yes, its directional, but the rear of the mic still hears the room. As you go from card to hyper, the focus of the rear lobe axis gets more intense. So pretty much, no matter what mic you use, its gonna be hearing the room one way or another.
dave[/quote]
What about say a 609?
dave[/quote]
What about say a 609?
mathtracks
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stand in the middle of the room and clap your hands.the echo or reverb in the room is what your room basically sounds like.now go to another room and do the same thing.you will notice a difference in every room.go outside and try it also. controlling the echo in the room is basically tuning the room.try a parachute,big couch,comforters,etc..... :guinness: :?
BradAvenson
Well-known member
http://www.dpamicrophones.com/Images/DM00650.pdf
Check out the DPA article about how mics interact with the room. Its very well written and informative.
Check out the DPA article about how mics interact with the room. Its very well written and informative.
MikoKensington
Well-known member
One trick I used for toning down a room is to hang some thick painter's drop cloth made of canvas. I get huge piece like 20X20 or something like that. Fold it over in half so it's half as tall and double thick. Then I hang it in front of the wall. It should be enough to run along a wall or two. Get it to cover at least one corner though. Experiment with different distances from the wall. Works ok and it's cheap, cheap, cheap. :wink:
pucho812
Well-known member
don't forget mic pickup/polar patterns, placement. Depending on which one you use you will have more or less room sound as well as distance from source. In some cases you may want more room sound. I recall reading an artical about how Frank Zappa would place his amp on the floor with the speakers pointed at the celing(SP) and then put the mic at the celing(sp) pointing down towards the amp speakers. If you are trying to get your room sound out use either a cardiod or hyper cardioid . using a mic in omni will usually have more room sound in it due to the way it picks up. The 609 which was modled after the 409 is I believe a cardioid so you should be good. Just keep the mic close to the speakers. close micing usually means less the 12 inches away. If it is still too much, nothing beats good old packing blankets. They are relatively cheap arouind 5.00 american and can be picked up at any packing store. they work real well at damping sound.
Ed DeGenaro
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I'm aware of the difference a room imparts on the source. But my quesyion is with a dynamic mic up aganst the grille and high SPL the room should not matter...
Ed DeGenaro
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I just go the reminder why it's different. It's not what the mic pick-ups but rather how the source (amp into cab) reacts to its surroundings...as in room compression. DOH!
what is room compression? never heard that one before...
IF you are recording with a lot of compression, even on a loud source and a card crammed against a speaker, you can still suck the room in through the rear lobe of the mic with LOTS of compression...
The room shouldnt matter, and even if you dont hear some kind of echo, the room always matters. The coupling of the amp to the floor, what the floor is made out of and how far the amp is from a wall always seems to change the bass response of the amp. In a perfect world, you could balance a cab on a nail and record it that way, you wouldnt have to deal with any of those variables. The kick back legs on fender cabs is a stroke of genius and I rarely see people using them.
Ive never tried it, but those auralex pads that people put their monitors on, would be worth trying to place them under a guitar cab to see how the botttom tightens up.
dave
IF you are recording with a lot of compression, even on a loud source and a card crammed against a speaker, you can still suck the room in through the rear lobe of the mic with LOTS of compression...
The room shouldnt matter, and even if you dont hear some kind of echo, the room always matters. The coupling of the amp to the floor, what the floor is made out of and how far the amp is from a wall always seems to change the bass response of the amp. In a perfect world, you could balance a cab on a nail and record it that way, you wouldnt have to deal with any of those variables. The kick back legs on fender cabs is a stroke of genius and I rarely see people using them.
Ive never tried it, but those auralex pads that people put their monitors on, would be worth trying to place them under a guitar cab to see how the botttom tightens up.
dave
Ed DeGenaro
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[quote author="wilebee"]I think he is refering to how the guitar cabnet at a set SPL will compress dut to the air in te space being acted upon . Wil
Wilebee[/quote]
Yup, that's what I'm talking about.
Wilebee[/quote]
Yup, that's what I'm talking about.
> with a dynamic mic up aganst the grille and high SPL the room should not matter...
It matters. Room sound is the sound that bounces ALL around. Room sound is everywhere. It is the same (statistically) at the source as in the far corners.
Going "close" to the source gets more source. Or if you turn-down to keep the same VU reading, you get less room. But it never goes away.
For an omni mike and small source, the room sound is the same all over but the direct source sound drops with distance from the source.
In a living room or small studio, they are "equal" at about 4 feet from the source. (You hear mostly direct sound at further distances because of Haas effect: first arrival locates the source, and room sound is all later arrival.)
For a small source, going from 4 feet/50" to 5" should increase direct sound 20dB. So direct is now 20dB more than room sound. This is "mostly direct", but the direct sound fades instantly, the room sound fades slow, and there is a "tail" of audible room sound behind each note. (Compression tends to bring-up the tail: "compression adds reverb" is not exactly true, but something to remember.)
Guitar speakers are not small. Anywhere inside about a 12" distance is all the same acoustic power, since it is spreading from a 12" cone and at 12" distance has hardly begun to spread. So you really can't get direct sound more than about 12dB above room sound.
Directional mikes are NOT directional when they face close to a large reflecting surface. A speaker cone is not 100% reflective, but sure is not "dead", and room sound that comes "from the mike's backside" gets bounced into the mike's frontside. There is some use in directional mikes to shade the sound, but they can't give laser-aim in this situation.
In a large concert hall, or a very dead studio, direct and room sound are equal out around 10 feet, so close-mike will give direct/room ratio around 20dB. In a small not-dead room, the direct=room distance may be under 2 feet and you can't get more than about 6dB direct/room ratio.
A very-very-small room won't have a normal "room sound". Say 3'x4'x5', the "room sound" is so short it can't be told from the direct sound, it all sounds direct. If you need pure-direct sound, put the speaker in the closet. Or put it outside, where there is no room-sound (just trucks, birds, angry neighbors...).
SPL really does not matter, as long as it is much more than the background noise, say 25dB. Say noise is 20dB SPL. Sounds over 45dB SPL will have clear room-sound tails. And sounds under 85dB SPL tops are not generally used in music (string quartet beats 85dB SPL; harpsichord is the main soft musical instrument and the exception that confirms the rule). Since a "teeny" guitar-amp is 1 Watt, and speakers are 88-90dB SPL/W @4', even a teeny lectric-guitar makes over 85dB SPL peaks and WAY more than enough to raise its room-sound "tail" out of the background noise.
It matters. Room sound is the sound that bounces ALL around. Room sound is everywhere. It is the same (statistically) at the source as in the far corners.
Going "close" to the source gets more source. Or if you turn-down to keep the same VU reading, you get less room. But it never goes away.
For an omni mike and small source, the room sound is the same all over but the direct source sound drops with distance from the source.
In a living room or small studio, they are "equal" at about 4 feet from the source. (You hear mostly direct sound at further distances because of Haas effect: first arrival locates the source, and room sound is all later arrival.)
For a small source, going from 4 feet/50" to 5" should increase direct sound 20dB. So direct is now 20dB more than room sound. This is "mostly direct", but the direct sound fades instantly, the room sound fades slow, and there is a "tail" of audible room sound behind each note. (Compression tends to bring-up the tail: "compression adds reverb" is not exactly true, but something to remember.)
Guitar speakers are not small. Anywhere inside about a 12" distance is all the same acoustic power, since it is spreading from a 12" cone and at 12" distance has hardly begun to spread. So you really can't get direct sound more than about 12dB above room sound.
Directional mikes are NOT directional when they face close to a large reflecting surface. A speaker cone is not 100% reflective, but sure is not "dead", and room sound that comes "from the mike's backside" gets bounced into the mike's frontside. There is some use in directional mikes to shade the sound, but they can't give laser-aim in this situation.
In a large concert hall, or a very dead studio, direct and room sound are equal out around 10 feet, so close-mike will give direct/room ratio around 20dB. In a small not-dead room, the direct=room distance may be under 2 feet and you can't get more than about 6dB direct/room ratio.
A very-very-small room won't have a normal "room sound". Say 3'x4'x5', the "room sound" is so short it can't be told from the direct sound, it all sounds direct. If you need pure-direct sound, put the speaker in the closet. Or put it outside, where there is no room-sound (just trucks, birds, angry neighbors...).
SPL really does not matter, as long as it is much more than the background noise, say 25dB. Say noise is 20dB SPL. Sounds over 45dB SPL will have clear room-sound tails. And sounds under 85dB SPL tops are not generally used in music (string quartet beats 85dB SPL; harpsichord is the main soft musical instrument and the exception that confirms the rule). Since a "teeny" guitar-amp is 1 Watt, and speakers are 88-90dB SPL/W @4', even a teeny lectric-guitar makes over 85dB SPL peaks and WAY more than enough to raise its room-sound "tail" out of the background noise.
Scodiddly
Well-known member
Room modes are going to be an issue... some (lower) frequencies are just going to go "boom" on you, and it doesn't really matter how close you can get to the source.
Ed DeGenaro
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Now that's a hella explanation. Thanks.
> high SPL the room should not matter...
An "obvious" point that isn't obvious to everybody (even if they done a lot of sound):
The room-sound SPL is proportional to the direct and near-field sound. Play louder, the room sound is louder. Logical, since the room sound IS the direct sound after many bounces inside the room.
There is compression in a tube guitar-amp at high power: short-term compression from grid overdrive and longer term compression as the voice coil gets overheated. I don't see how this changes the direct to reverb ratio. There is another kind of compression as you begin to approach 194dB SPL, but the cone of a super-power speaker isn't much over 135dB SPL so that does not apply. (194dB SPL only happens in the near-field of a bomb or sonic-boom, and you don't want to play there.... the throat of a horn can hit 150dB SPL and compression causes distortion in the throat even if the driver is perfect, but we don't mike horn-throats for music recording.)
The fact that the room-sound is the "same all over" isn't "obvious", but very good to know, and even to prove to yourself.
Get a small sound source, like a computer speaker, micro-monitor, or one of those $29 9-volt "guitar amps". If it has weak bass, that's good: most rooms get erratic below a few hundred Hz so we'd like to avoid exciting bass-modes for experimental clarity. Find a sound that is constant-volume: pink noise (or FM noise) is traditional, but a heavily-compressed pop track works fine. Something where a VU meter hardly moves. Then get a sound-level meter or a mike into anything with a good meter.
The "same all over" effect is easiest to see in a "hard" room, a concrete garage or cellar. You can observe it in any room live enough to sing well in, and should try your music room, but a hard room is easier to get the idea.
Set the speaker somewhat off-center in the room. Put the mike very-near, and adjust the speaker for pretty loud (about as loud as you or the speaker can stand, for best S/N), and adjust the mike-meter for a reading near the top of the scale. Be sure the source-sound is constant-volume enough so the needle doesn't waggle much. (On a sound level meter, use the Slow setting, and A-weight emphasizes the midrange where we hear best.)
Now go all around the room, watching the meter. The level almost everywhere in the room is within a dB or two of any other place, and about 15-25dB down from the near-field level. There will be some rise near walls and corners, especially in the bass. There is rise as you approach the source, of course. But in a hard room, ignoring bass and the area near the source, the level all over is pretty darn constant.
Why? Look at the sound. It comes off the speaker, spreads, and weakens with spread. 6dB every time the distance doubles. But then it hits a wall and comes back the other way, still weakening. And then it hits the opposite wall and comes back again, overlapping. If you sketch it with "pressure" on a vertical scale and the left-right layout of the room, you have a lot of slanting lines, half slant down-left, half slant down-right. If you figure the power-sum of these slanted lines, you get a level line: same pressure everywhere. And a live room will bounce sound for about 0.3 seconds before it decays 20dB. If the room is 20 feet, there are about 15 bounces before the decay is 20dB. Stand in one spot and hear the same thing 15 times, weaker each time. And more reflections from the other walls and the ceiling/floor, 50 audible end-end bounces in a typical "nice" room. So it really IS the "same all over".
Now walk from a far point toward the speaker. Note where the level seems to rise 3dB. That distance is called "critical distance" by some acousticians. Keep going, and there is a range where every time you are half as far, you are 6dB louder, simple direct-sound inverse-square relation. For large speakers, when you get within about one cone-diameter, the level stops rising: 1" is about the same as 2", and for a 12" speaker a mike at 5" or 10" from the center may not show much change of level.
That "critical distance" or Dc is where the direct and reverberant sound levels are equal. Going closer is louder, going further is all the same level. The direct sound continues to drop: at 3 times Dc the direct sound is 10dB below the room-sound.
This is "critical" in speech PA systems: listeners should not be closer than Dc to avoid blasting, nor further than Dc because room-sound starts to smother speech clarity. Also the potential acoustic gain from talker to listener depends on Dc (and some other things).
For true stereo recording os an acoustic act, like orchestra or choir, where the room is part of the act, this same range of Dc to 3*Dc is a good place to set the mikes. Closer than Dc will "close-up" the band and often be too dry. To a live listener, music can be good well beyond 3*Dc because most music does not demand the spit-clarity that communicating speech needs. But for record-playback where the listener does not have sight and head-turn clues to the acoustic, 3*Dc is normally rather far, 2*Dc is about as far as I'd go unless the room is very nice and the music can be "room-lush". (I guess that organ gig posted in the Pub was done about 8*Dc, but organs evolved for such spaces.)
Acoustic theory assumes the source and mike are omnidirectional. A very directional source like a Fender Twin will show a larger Dc in front and shorter Dc on the side than a true omni source, and directional mikes also distort effective Dc.
Dc is short in a live room, long in a dead room. Outdoors, Dc is infinite (no room reverb at all).
Dc in a typical US livingroom is 3 or 4 feet. Many home-studios will be similar, unless they over-do the wall-foam which will increase Dc. Concert halls 150-800 seats tend to cluster around Dc of 11 feet, and Dc does not rise much with size because if Dc were too large, levels in most of the seats would be low. Large concert halls have a lower percent absorption than small halls to keep room-sound levels high. Pro studios will range between livingrooms and concert halls, with wide variation at similar size depending on flavor "live" or "dead". Whether the main use is musicians who want reverb for blend and self-feedback, or players who need to see each other without hearing each other or cross-talk into each other's channels and reverb is added later.
Oh, and for the public performance situation, there is a fairly simple way to estimate Dc at a glance. Most for-money playing happens in rooms where the walls and ceiling are hard (plaster, mirrors, hard paneling, beer-signs) and the floor is "soft" because it is "covered with people" (can't get rich in a big room with a small crowd). In a concert hall with about 75% of floor covered in soft bodies (auduences have high and pretty predictable absorption) the Dc is about 1/4 of the ceiling height. Clubs can be a little less with tables, or a little more when the joint is packed butt-to-belly. (A crushing-crowd absorbs less than an elbow-room crowd, but a crush-crowd is a disaster waiting to happen. Someone here has a link to a soccer-disaster.) You see that in a long low club, much of the crowd is beyond 3*Dc and you need a lot of "throw" (directionality) in speakers to punch clarity to the far corners. (But maybe those folks picked the corners because they don't want to hear the band so well.)
I've digressed from your close-mike cabinet to smokey clubs. But the sound waves don't know the difference, and the general relation between direct sound and room reverberant sound follows the same general trends anywhere. Put your ear to the jukebox and you still hear the murmur of the crowd in the whole room. In that case turning-up the jukebox should overwhelm the crowd (except real crowds talk louder when the jukebox is turned up). When the room sound is the decaying direct sound, turning-up or -down won't change the ratio of direct to room sound (unless you turn down so very far that background noise downs the room reverb).
An "obvious" point that isn't obvious to everybody (even if they done a lot of sound):
The room-sound SPL is proportional to the direct and near-field sound. Play louder, the room sound is louder. Logical, since the room sound IS the direct sound after many bounces inside the room.
There is compression in a tube guitar-amp at high power: short-term compression from grid overdrive and longer term compression as the voice coil gets overheated. I don't see how this changes the direct to reverb ratio. There is another kind of compression as you begin to approach 194dB SPL, but the cone of a super-power speaker isn't much over 135dB SPL so that does not apply. (194dB SPL only happens in the near-field of a bomb or sonic-boom, and you don't want to play there.... the throat of a horn can hit 150dB SPL and compression causes distortion in the throat even if the driver is perfect, but we don't mike horn-throats for music recording.)
The fact that the room-sound is the "same all over" isn't "obvious", but very good to know, and even to prove to yourself.
Get a small sound source, like a computer speaker, micro-monitor, or one of those $29 9-volt "guitar amps". If it has weak bass, that's good: most rooms get erratic below a few hundred Hz so we'd like to avoid exciting bass-modes for experimental clarity. Find a sound that is constant-volume: pink noise (or FM noise) is traditional, but a heavily-compressed pop track works fine. Something where a VU meter hardly moves. Then get a sound-level meter or a mike into anything with a good meter.
The "same all over" effect is easiest to see in a "hard" room, a concrete garage or cellar. You can observe it in any room live enough to sing well in, and should try your music room, but a hard room is easier to get the idea.
Set the speaker somewhat off-center in the room. Put the mike very-near, and adjust the speaker for pretty loud (about as loud as you or the speaker can stand, for best S/N), and adjust the mike-meter for a reading near the top of the scale. Be sure the source-sound is constant-volume enough so the needle doesn't waggle much. (On a sound level meter, use the Slow setting, and A-weight emphasizes the midrange where we hear best.)
Now go all around the room, watching the meter. The level almost everywhere in the room is within a dB or two of any other place, and about 15-25dB down from the near-field level. There will be some rise near walls and corners, especially in the bass. There is rise as you approach the source, of course. But in a hard room, ignoring bass and the area near the source, the level all over is pretty darn constant.
Why? Look at the sound. It comes off the speaker, spreads, and weakens with spread. 6dB every time the distance doubles. But then it hits a wall and comes back the other way, still weakening. And then it hits the opposite wall and comes back again, overlapping. If you sketch it with "pressure" on a vertical scale and the left-right layout of the room, you have a lot of slanting lines, half slant down-left, half slant down-right. If you figure the power-sum of these slanted lines, you get a level line: same pressure everywhere. And a live room will bounce sound for about 0.3 seconds before it decays 20dB. If the room is 20 feet, there are about 15 bounces before the decay is 20dB. Stand in one spot and hear the same thing 15 times, weaker each time. And more reflections from the other walls and the ceiling/floor, 50 audible end-end bounces in a typical "nice" room. So it really IS the "same all over".
Now walk from a far point toward the speaker. Note where the level seems to rise 3dB. That distance is called "critical distance" by some acousticians. Keep going, and there is a range where every time you are half as far, you are 6dB louder, simple direct-sound inverse-square relation. For large speakers, when you get within about one cone-diameter, the level stops rising: 1" is about the same as 2", and for a 12" speaker a mike at 5" or 10" from the center may not show much change of level.
That "critical distance" or Dc is where the direct and reverberant sound levels are equal. Going closer is louder, going further is all the same level. The direct sound continues to drop: at 3 times Dc the direct sound is 10dB below the room-sound.
This is "critical" in speech PA systems: listeners should not be closer than Dc to avoid blasting, nor further than Dc because room-sound starts to smother speech clarity. Also the potential acoustic gain from talker to listener depends on Dc (and some other things).
For true stereo recording os an acoustic act, like orchestra or choir, where the room is part of the act, this same range of Dc to 3*Dc is a good place to set the mikes. Closer than Dc will "close-up" the band and often be too dry. To a live listener, music can be good well beyond 3*Dc because most music does not demand the spit-clarity that communicating speech needs. But for record-playback where the listener does not have sight and head-turn clues to the acoustic, 3*Dc is normally rather far, 2*Dc is about as far as I'd go unless the room is very nice and the music can be "room-lush". (I guess that organ gig posted in the Pub was done about 8*Dc, but organs evolved for such spaces.)
Acoustic theory assumes the source and mike are omnidirectional. A very directional source like a Fender Twin will show a larger Dc in front and shorter Dc on the side than a true omni source, and directional mikes also distort effective Dc.
Dc is short in a live room, long in a dead room. Outdoors, Dc is infinite (no room reverb at all).
Dc in a typical US livingroom is 3 or 4 feet. Many home-studios will be similar, unless they over-do the wall-foam which will increase Dc. Concert halls 150-800 seats tend to cluster around Dc of 11 feet, and Dc does not rise much with size because if Dc were too large, levels in most of the seats would be low. Large concert halls have a lower percent absorption than small halls to keep room-sound levels high. Pro studios will range between livingrooms and concert halls, with wide variation at similar size depending on flavor "live" or "dead". Whether the main use is musicians who want reverb for blend and self-feedback, or players who need to see each other without hearing each other or cross-talk into each other's channels and reverb is added later.
Oh, and for the public performance situation, there is a fairly simple way to estimate Dc at a glance. Most for-money playing happens in rooms where the walls and ceiling are hard (plaster, mirrors, hard paneling, beer-signs) and the floor is "soft" because it is "covered with people" (can't get rich in a big room with a small crowd). In a concert hall with about 75% of floor covered in soft bodies (auduences have high and pretty predictable absorption) the Dc is about 1/4 of the ceiling height. Clubs can be a little less with tables, or a little more when the joint is packed butt-to-belly. (A crushing-crowd absorbs less than an elbow-room crowd, but a crush-crowd is a disaster waiting to happen. Someone here has a link to a soccer-disaster.) You see that in a long low club, much of the crowd is beyond 3*Dc and you need a lot of "throw" (directionality) in speakers to punch clarity to the far corners. (But maybe those folks picked the corners because they don't want to hear the band so well.)
I've digressed from your close-mike cabinet to smokey clubs. But the sound waves don't know the difference, and the general relation between direct sound and room reverberant sound follows the same general trends anywhere. Put your ear to the jukebox and you still hear the murmur of the crowd in the whole room. In that case turning-up the jukebox should overwhelm the crowd (except real crowds talk louder when the jukebox is turned up). When the room sound is the decaying direct sound, turning-up or -down won't change the ratio of direct to room sound (unless you turn down so very far that background noise downs the room reverb).
Swedish Chef
Well-known member
:shock:
jensenmann
Well-known member
Another influence on closemiking are combfiltering-effects. When the first direct reflection returns to the mike it will interfere with the directsound erasing some frequencies and boosting others. This comes from the fact that the travelled distance for reflected sound is longer than for direct sound. If the difference of distance is the half wavelength of the frequency it will attenuate the direct sound (hell, english is hard). The reflected sound is damped around 8dB by hitting the surface and arrives therefore with a lower level than direct sound. So you won´t get a total deletion at those freqencies but a still hearable attenuation.
cheers
Jens
cheers
Jens
