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Space needed for sound waves to develop

P

projectMalamute

Jan 9, 2008
2,126
27
52
Westfield, MA, USA
#1
This is a debate going on in the recording section of the forum. The issue pertains more to amplification in general I think, so I am posting it here for discussion.

It appears that this is being taught at one of them 'recording arts' colleges:

sonic assassin said:
take 440hz. thats roughly a 2 1/2 ft wave. you will still hear it at 3 inches away, but it wont be exactly 440hz and it wont be a very good signal. if you are 2 1/2 feet away, you will suddenly hear this signal better. this is a common mistake when setting one's amp at a show. we monitor it for 6ft away at most (unless you're playing stadiums or something), and at that distance, you are only getting a true representation of 186hz and down. you can HEAR other frequencies, just not as well, and not accurately.

now if you walk onto the crowd, say 28 ft, you will be hit with a FLOOD of 40 hz because that wave is fully developed.
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thoughts?
 
#5
I have some very nice studio-quality headphones - it's a shame they're not "accurate" - clearly I can't be hearing the low frequency response I paid so much money for as there's so little distance between my tympanic membrane and the headphone transducer.
 
#8
I don't think the headphone analogy ever worked with bass speakers. [Edit] That doesn't make since now that I've re-read it. Anyway, ...[/Edit] I remember numerous occasions where the audience and other talent would tell the bass player to turn down, I'd walk over to his position and sure enough, he could barely hear himself. Walk away from him and he was loud enough to make your ears bleed. I have someone in the crowd who tells me how high to turn up and I stand approx three feet away from my speaker and my monitor. I sound much louder than if I was standing next to either of them and the frequencies and their harmonics sound different. Headphones are a different matter since they can't be anywhere else but near the ear.
 
#9
lindseyp said:
I don't think the headphone analogy ever worked with bass speakers. [Edit] That doesn't make since now that I've re-read it. Anyway, ...[/Edit] I remember numerous occasions where the audience and other talent would tell the bass player to turn down, I'd walk over to his position and sure enough, he could barely hear himself. Walk away from him and he was loud enough to make your ears bleed. I have someone in the crowd who tells me how high to turn up and I stand approx three feet away from my speaker and my monitor. I sound much louder than if I was standing next to either of them and the frequencies and their harmonics sound different. Headphones are a different since they can't be anywhere else but near the ear.
Click to expand...

I believe this phenomenon has more to do with standing waves and room effects than the purported 'takes space to develop' theory.

Not to mention that at 3' the speakers are pretty much pointed at the back of your knees.
 
#11
Total bunk. Sound transmits in air as compression waves, not like you're flipping a jump-rope.

If that argument were true, nobody would ever be able to close-mike a cabinet. I seem to recall lots of records being made with the mike right at the speaker cabinet.

The "flood of 40Hz" is most likely due to room modes and boundaries.

Miking from a decent distance vs. close-miking has its merits, but it isn't due to that phony physics. It's because the floor and room play a part in the reproduction of the instrument.
 
#14
A general rule of thumb for miking acoustic instruments is to mike from a distance roughly equal to the size of the instrument. This is because the entire surface of the instrument creates the sound.

So you might mike a violin from one foot, an acoustic guitar from two feet, and an upright bass from as much as four feet away. Naturally, you listen and place the mike where you get the recorded tone you want.

An electric guitar or bass just blows through a speaker. If you want a simple tone, you mike the speaker closely. If you want some of the room, you move back. If you want a lot of room, you move way back.

I was wondering whether you need a long cable when recording with a DI, so the waves have space to develop in the cable! ;)
 
#15
projectMalamute said:
This is a debate going on in the recording section of the forum. The issue pertains more to amplification in general I think, so I am posting it here for discussion.

It appears that this is being taught at one of them 'recording arts' colleges:



thoughts?
Click to expand...
Its a bit of a shame that the people who teach this stuff usually dont know enough about it, or they would probably have a full time job doing it rather than spouting to students.
The bit they forgot to mention is that this only applies to cancellation of frequencies due to standing waves in rooms.
If you stand a loudspeaker in a room that is less than 29 odd feet from a parallel reflective wall thats when you start loosing VLF but place a mic on axis with the speaker at usual mic distance you wont loose much if at all.
Thats why headphones do in fact work, although they are not too good to mix on.
:ninja:
 
#17
Code: 
A general rule of thumb for miking acoustic instruments is to mike from a distance roughly equal to the size of the instrument.

I recorded and mixed a lot of records, jingles, and demo's in my time and have never read or heard that before. Also have not seen that rule applied by any top notch engineers.

I can just imagine what a 12' grand would sound like miked at 12'.

I would agree that room acoustics play an improtant role for humans when listening to low frequencies in a large room, a room with poor acoustics, or as the SPL increases.

Would also suggest that huge generalizations in mike placement are irrelevent without discussion of mike design, type, proximity effect, and the type of sound one is looking for. Different mikes can react differently as they are moved away from the source.

Interestingly enough I was at a rehearsal Thursday night where the bass amp was an ancient Yamaha B100 (15" speaker with a scoop). Originaly started with the EQ flat with a rock band and couldn't make it through the first chart without pulling tons of 80 hz from the amps graphic. Boomy as can be as I stood 3 feet away.

Forty minutes later I grabbed a chair and the tone of the bass changed 180 degrees; unbelievably tinny; stood up and everything was back. Later in the evening I moved accross the rectangular, wood panneled room, to read a chart with the keyboard player; again big change in tone.
 
#18
xgator4u said:
wow, who is teaching this crap ? and who is paying the teachers???
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The students(or more likely the students' parents) are paying for this kind of thing. I do not know the specific school in question.

It is one thing for craftsmen to develop practical techniques that work in the real world despite incompletely or incorrectly understood underlying mechanisms. It is another thing entirely to charge a great deal of money to teach this stuff as fact under the auspices of a (presumably accredited) college program.
 

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