ADVANCED RECORDING PRIMER

CHAPTER 3 - PART 4,  BY ROBERT DENNIS

DELAY FOR TIME CORRECTION

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.

1 Foot = 1 Millisecond

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.
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.

drumtime.gif (26149 bytes)

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

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.
Delay In Sound Reinforcement
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.

Figure 1 - Delay Times For Sound Reinforcement

Copyright 2001, Robert Dennis, ALL RIGHTS RESERVED