| Sound
travels 1130 feet per second at normal temperature, humidity and air
pressure. If the speed of sound was 1000 feet per second, it would
take 1 ms for sound to travel 1 foot. Although this is 10% off, this
approximation is useful in setting delay times to compensate for time
delay cause by the sound traveling through air. |
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| The actual
speed of sound is about 10% faster, but the above works in most cases. |
| Close and Distant Micing |
| The close
microphone on instruments contributes to the "right there"
presence of the sound pickup, where distant microphones help make the
sound "big." It is possible to achieve both the presence
and the huge aspect to the sound by compensating for the distance between
the microphones. The best example is a technique commonly used to
get a leadf guitar sound in recording Rock. |
| A good rock lead guitar should sound "right
there" and "big" (read that as "Huge"). There should also be a
clarity of the notes being played. These factors create a conflict in how you place the
microphone to pick up the instrument. |
| The "big" sound of the guitar comes from
placing a microphone several feet away (10-15 feet). The clarity comes from a microphone
being placed very close to the speaker (2-5 inches). The distant microphone is not clear
and the close microphone is not big-sounding. |
| If you tried to use both a close and a distant
mic, the
clarity would get worse because of the time difference between the microphones. Sound
travels at a speed of about 1/1000th of a second per foot. This makes the distant
microphone get its signal 10-15 ms. later than the close mic. This time difference blurs
the attack and clarity of the guitar. |
| A technique which
is effective, is to use both microphones but to delay the close microphone before blending the
two mics. You would want a delay unit that would delay the signal 10-15 ms and insert this
in the channel you use for the close mic. Now when you blend two mics, you will get a
sound that is both clear and big. |
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| Multiple microphones on a set |
| Whenever
you have multiple microphones placed in a small area, there will be
leakage between the microphones. Since the microphones are spaced
apart, the time difference between the microphones tends to blur the
attack. There isn't a better example of this than the drum
kit. |
| Attack Time In Drums & Multiple Microphone Blurring |
| The attack of a drum hit is established in the first 2-3 milliseconds of
the generated sound wave. This means that when a drum is picked up by multiple
microphones that are spaced apart by more than 2 milliseconds, the attack becomes
"blurred" (less definite). Thus leakage of the snare into cymbal and tom
microphones more than 2 feet away will lessen the snare attack. This same problem
would occur for all of the drum kit instruments. |
| Putting The Drums In Time |
Sound travels at 1130 feet per second. Thus it
takes about 9/10 of a millisecond for sound to travel a one foot distance. Recording
engineers conveniently round this off to the timing formula of 1 ms = 1 foot. |
When you put all of the microphones in time with each other, the attack
of the drums is astoundingly better. Ben Blau, an RID instructor, first introduced
this idea some 8 years ago when he was managing one of our recording studios. I had
been using delay to compensate for spaced microphones on guitars but had never tried it on
the relatively small time differences of the microphones on a drum kit. I expected
somewhat of a difference when Ben explained his idea to me but I was unprepared for
how
much of a difference it made.
| To put all of the microphones in time with each other you select a
"focal point" say 6 feet from the floor over the drums. This could be a
place that you used X-Y microphones as overheads, but you use this focal point even if you
don't have overhead microphones there. You measure (or estimate) the distance of
each microphone to the focal point and then delay each microphone by that amount using the
formula of 1 ms = 1 foot. |
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APPROXIMATE
DELAY TIMES TO FOCAL POINT |
FOOT DRUM |
5.0 milliseconds |
SNARE DRUM |
4.0 Milliseconds |
LOWER TOMS |
4.0 Milliseconds |
UPPER TOMS |
3.5 Milliseconds |
HIGH HAT |
3.5 Milliseconds |
| OH CYMBALS |
2.0 Milliseconds |
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| In Practice |
| Delaying all those microphones by the indicated amount will take 8
channels of delay. If you are using a digital console, there is a delay function on
each channel of the console that can be used. If you have an analog console it is
harder because you need individual delay lines. If you are using an analog console
but recording onto an MDM (ADAT or DA-88), you can use the "track delay"
function of the tape machine during mixdown. If you are recording onto analog tape,
you can get partial attack improvement (with less delay lines) by delaying the foot and
the snare track by the time difference between that instrument's mic and the cymbal
mics.
In the above example, this would be foot delayed 3 ms and the snare delayed 2 ms. |
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| Delay In Sound Reinforcement
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| Sometimes
at live performances, extra speakers are placed down the walls of the
auditorium. If these speakers are not delayed, it will appear that
the sound is coming from the walls rather that from the state. The
delay of the sound coming from the state, mixing with the un-delayed
signal coming from the speaker, can also muffle the attack and clarity of
the sound. To prevent this from happening, side speakers used for
sound reinforcement are delayed by the number of milliseconds equal to the
number of feet that the speaker is away from the stage. Figure one
below shows two sets of speaks on the side wall. The closest set is
20 feet away from the stage so the signal sent to these speakers is
delayed 20 ms. Since the second set is 40 feet away from the stage,
their signal is delayed by 40 ms.
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Figure 1 - Delay Times For Sound
Reinforcement |
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