In reference to Session Documentation #1 http://www.electrical.com/phpBB2/viewtopic.php?t=152, why are the ambient floor drum mics delayed? I understand why delay may be used when there are several sources and several mics used at different distances, but a drum kit is essentially a single source. Why not move the ambient mics back 2 feet (effectively a 20ms delay), or, more appropriately, delay all the other mics on the kit so that they align with the ambient mics. What am I missing here?

brew wrote:In reference to Session Documentation #1 http://www.electrical.com/phpBB2/viewtopic.php?t=152, why are the ambient floor drum mics delayed? I understand why delay may be used when there are several sources and several mics used at different distances, but a drum kit is essentially a single source. Why not move the ambient mics back 2 feet (effectively a 20ms delay), or, more appropriately, delay all the other mics on the kit so that they align with the ambient mics. What am I missing here?

There are several functions served by delaying the mics.

The ambient microphones receive both a direct signal from the sound source (this is the first arrival of sound at the mic) and a diffused omnidirectional reverberation from the room reflections. The direct signal is acoustically slightly delayed from the close mic signal (as determined by difference in distance from the sound). Notice also that the delay is different for each part of the drum kit, as there is a non-trivial (at audio wavelengths) distance between the hi-hat and the floor tom, for example. These differences create a complex comb-filtering effect which can muddy or hollow-out the sound quality when the two signals (close and far) are added together.

Since sound travels (very roughly) at 1000 feet per second, one foot of distance roughly equals 1 millisecond of delay.

Now imagine a stereo pair of ambient mics on a drum kit at an equilateral distance of 10 feet from the center of the bass drum. The mic on the left side would be only 6 feet from the snare drum, while the ambient mic on the opposite side of the kit would be 12 feet away, but only 8 feet from the rack tom. This creates a mess of overlapping short delays specific to each instrument, and different for the left and right ambient mics.

You proposed delaying the close mics to match the acoustic delay, which might work for a physically small point source and a single ambient microphone, but would entail running the principal signal through a delay unit (which could degrade its sound quality), and would require monitoring and recalibration of the delay whenever climatic conditions changed. This is also ruled-out when using stereo ambient mics, as time differences are inherent in stereo recording, and it is also impractical when using several (many) close mics, as it would be a total pain in the cock.

Much easier is to delay the two ambient mics by a few milliseconds, which moves them out of the haas effect (the delay range where delays affect sound quality and localization rather than being perceived as ambience) and into the range of perceived ambience.

There is a logic to this from acoustic perspective as well. If there is a boundary 20 feet from you, its reflected delay time (from your position) is double the physical distance to the sound source (approximately 40 milliseconds), because the sound has to make a round trip out-and-back. Adding a few milliseconds to the acoustic delay inherent in a room mic seems to mimic this effect, making the ambient sound seem less muddy.

-steve

Hey brew,
I have no full explanation but I've delayed ambient mics myself on some session and here is what I recall:
There are basically two things that you're misunderstanding.
1. Sound travels much faster than you think. It travels more than 22 feet in 20ms (depending on the room temperature). The live room in studio B is quite large but just pulling the ambient mics back some additional 20 feet on each side of the kit might be difficult.
2. Those mics on the floor are ambient mics that means to provide an additional ambience they mustn't be time-aligned with the close mics on the kit. To delay ambient mics is a psycho acoustic trick to make the room sound bigger.
I've never recorded in a room where I could spread mics 40 feet to each side of the kit, but I think it wouldn't sound that cool because the amount of indirect sound into the mics would be much bigger than the amount of direct drum sound. If you place them 22 feet away from the kit, you'll get a nice balanced mix of room reflections and direct drum sound, that can contribute to your whole drum mix.
Delaying room mics has a great effect on your snare sound. You have your initial close miced snare signal and before this signal is over the delayed room mic brings the snare in again and makes it sound much longer and bigger. The close miced signal and the room signal don't overlap so you should not have phase problems (assuming you have the right delay time).
A different and annoying thing is the Hihat. It's a very short signal so the delayed Hihat signal from the room mics can be heard . The human brain can seperated two signals from each other when the are at least 14 ms appart from each other (Haas effect).
I know that this was a very rude and uneducated explanation, so please correct me if I was wrong.
Max.

Sorry, I didn't see Steve's answer when I was typing my reply. So you better stick to his explanation.

Steve,

Which outboard delay do you prefer for this type of work?
Also, what factors do you take into consideration when determining the length of delay? Is 14ms the minimum or should the delay be tailored to the room size?


Thanks.

Dan

Thanks for the reply. I goofed on the 20ms/20ft thing, but...thinking about the Haas effect makes a lot of sense for separating the ambient signal from the close mics.

danmaksym, I believe the length of the delay in this case is mostly based on the limits of the Haas effect, which says that short reflections (<30ms about) are not perceived as distinct ambience but are blended with the direct sound. Therefore your delay should be at least 20ms, if not greater...but only experimentation will yield the best answer in any given situation.

What stereo delay can anyone recommend for delaying the ambient mics? I have an AMS A/V Delay Sync that I like, but alas only the one and they're hard to come by. It'd sure be nice if Eventide released the Electrical Audio delay, but since that's unlikely to happen, what's the next best thing?Thanks

I use a TC Electronics D-2. It's fine, even though it's got way too many other parameters that you just have to zero out. You can probably grab one for less than $200 used.= Justin

Every time I've added mere delay to my room mics, I haven't ended up using it, but I have put a little delay on my reverb returns to good effect. Steve has the advantage of having a great drum room that allows for a lot of manipulation with room mics. Not really going to work with every room, I think. Balance between room ambiance and direct sound is going to be a huge variable when futzing with adding delay to the signal. Still fun to try.

My usual stereo pair was in the shop this past week and I wound up trying something new, a SDC over the snare similar to where it would normally be in a spaced pair and I experimented with a few different mics out in front of the floor tom kind of facing in towards the snare. I then delayed the mic in front about 15 ms and panned the two hard right left. It's not any kind of true stereo sound but I liked it. Anyone else ever try this or have any other suggestions on some different things to try?