Hi folks,

Your average music school practice hall has a typical hall which can be very lively and reverberant.

Whereas some practice halls are acoustically designed and treated - leaving a natural sounding reverb.

With such location recording what mic or processing techniques may be employed to cope with rooms with too much reverb?

Moving the mikes closer?
Moving the musicians around?

Ive done both of these...but is this a way to control the problem?

Im curious because the May 2006 issue of the mag shows what I would consider to be an untreated hall. Yet the recordings have a little less reverb on them then I would expect.

Any thoughts?

When I'm recording classical music, I try to record it in the same sort of acoustic that the music would be performed in.

The microphones are then put at the place which gives the best balance between the acoustic and the music.

Preferably just a single stereo pair.

My mixer, which I bought about 8 years ago has never been used in anger, because I have, so far, never come across a situation where I needed more than two mics.

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John - Sound-Link ProAudio
President - Federation Internationale des Chasseurs de Sons

I've found the following tricks useful:

(i) Avoid aligning the ensemble with the walls of the room, in either direction. That said, don't have them backed into an exact 90 degree corner, either - see if you can get them set up at some odd angle between the two.

(ii) This done, see if you can then identify the surface that produces the most reverb (there will still be one) and move any extranous objects against it e.g. piles of spare chairs, an upright piano if there is one and you're not using it. Getting people to put their instrument cases against the same wall also helps, particularly if you've got cellists and bassists - a line of upright cello cases is great for breaking up reflectable sound.

(iii) Try to bear in mind the acoustic properties of both the instruments and the bodies wrapped around them. Sorry to come back to double basses by way of example, but a section of, say, four of them sitting on stools is a more effective anti-reverb device than a pair of seated trumpets. It's useful if you can work this kind of thing into your positioning.

(iv) Close microphones tend to pick up less reverb, but too close and you start losing bits of the ensemble, leaving you with no recourse but the old CBS technique of using individual microphones for pretty much everything. Not ideal.

(v) Grouping the musicians closer together can help, but often there's a trade-off in that they won't be comfortable and the sound can become congested. Avoid if possible.

I should add that these ideas happen to work in the rooms I've recorded in - other experiences may well differ.

That said, there can be advantages to reverb. On several occasions I recorded an amateur orchestra in a London church which was very reverberant. The orchestra was pretty good but not up to professional standards, so I recorded them from the rear balcony. The cathedral-like atmosphere of the recorded results (a bit like the old Nimbus Records sound) (a) sounded deliberate - as if I'd spent hours tryng to get this 'very live' sound - and (b) neatly distracted attention from the odd wobbly note!

Quote Barry Pearce:

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With such location recording what mic or processing techniques may be employed to cope with rooms with too much reverb?

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You can't really remove reverb from a recording without doing a lot of damage to the recording at the same time. So if there is an excess of reverb you either have to modify the room to reduce it, or mic closely to maximise the direct/reverberation ratio.

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Im curious because the May 2006 issue of the mag shows what I would consider to be an untreated hall. Yet the recordings have a little less reverb on them then I would expect.

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It was an untreated hall, but it was largely a wooden construction whcih made it less reverberant than an equivalently sized brick and plaster structure. I was also miking rather closer than would be preferable in a better acoustic. I was miking well inside the critical distance for the mics' polar patterns.

hugh

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Technical Editor, Sound On Sound

Quote Hugh Robjohns:

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It was an untreated hall, but it was largely a wooden construction whcih made it less reverberant than an equivalently sized brick and plaster structure.

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That explains a lot! I was indeed comparing the audio to my own experiences with concrete block!

Quote Hugh Robjohns:

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I was also miking rather closer than would be preferable in a better acoustic. I was miking well inside the critical distance for the mics' polar patterns.

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Now that is interesting. Ive never come across the concept of a 'critical distance for the mic's polar pattern' - and ive read a LOT of SOS Can you please explain further Hugh?

Whilst we are talking about mics etc a few of years ago I started recording guitar...typical project studio - wanted it all direct, amp modelling + speaker sim blah mics -oooh scary blah bad rom acoustics etc etc...and just felt disappointed. Having reverted to real tube guitar amps (pre & power) and decided micing was the only was to go (and using easy room treatments for the project studio such as rockwool and the good old duvet!) my mike collection has grown and I now own 16 mics of various qualities (my Rode K2 being my pride). The more I use mics the more I find them fascinating things and a rather more enjoyable way of recording. Instead of being the recording technique that many people seem to shy away from as 'scary' I actually find myself in eager anticipation of the next session Ive a lot to learn but hey using mics is great - and the sound you get is so much more pleasing than direct methods - and has led me into recording other aspects of music (classical) which I play and love as well.

"Critical distance" is the crossover point (the borderline if you wish) beyond which the reverberation becomes louder than the source. The "for the polar pattern" Hugh mentioned has to do with the fact that each polar pattern allows for different amount of room sound to be captured and thus the perception of direct VS reverberated sound balance depends on the pattern used as well.

Archdake has given you a quick description that pretty much sums it up. I'm about to dash out or a Studio SOs session, but I'll try to expand on it when I return (unless some else beats me to it.)

Critical Distance is something that we all know intuitively, but it doesn't seem to be taught as a concept, or how to use it to place mics accurately.

hugh

hugh

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Technical Editor, Sound On Sound

Lately I have had good results with mics above the main part of the orchestra, resembling a Decca tree. In some acoustics this can remove some excessive reverb. Currently I simply run omnis up there, guess next step is to make some kind of acoustic balls or similar. Additionally I usually have outriggers and sometimes a spot here and here.

I place a high stand right in front of the conductor (not all agree to that) and on top i have this home-made T construction with mics at the ends. A picture may help
http://trombonisten.se/images/4051c.jpg

The T is made from three parts of aluminium U profile I found in a hardware store, bolted together.

Gunnar

Quote ghellquist:

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Lately I have had good results with mics above the main part of the orchestra, resembling a Decca tree. In some acoustics this can remove some excessive reverb. Currently I simply run omnis up there, guess next step is to make some kind of acoustic balls or similar. Additionally I usually have outriggers and sometimes a spot here and here.

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Aha! Ive been looking for something similar - could you enlighten me on the construction details - particularly how you mounted the T on the mic stand and how you mounted the mic holders? Ive been thinking about making exactly this sort of thing to try out a decca tree myself - I think the commercial products are hugely overpriced.

Now I show yet more ignorance - can you please explain 'outriggers' - I think I understand but it has never been explained...

Oh and 'acoustic balls?' ??? What are these?

DPA info on their 'acoustic balls' (and other shapes)

The research paper that got people more interested in them. (on DPA site) Though the basic phenomena associated with capsules mounted in spheres and other shapes have been known about for decades (e.g. Neumann M50), afaik, this was the first really comprehensive (i.e. more than just spheres) academic research published on the subject.

DPA 'acoustic ball' kit (also available individually) They once made a kit containing several other shapes as well as spheres but I can't see it in their current range.

Schoeps 'Acoustic Balls'
Neumann 'Acoustic balls'

And then of course there's the Neumann M50-type mics (M50, M150, TLM50) with a small diaphragm capsule embedded in an 'acoustic ball'.

And, taking things a stage further, (and integrating spherical diffraction phenomena between both individual mics and pairs there are stereo mics like the Neumann KFM 100 and Schoeps KFM6 (and several other similar devices from other makers). And going even further (and with limited/variable success) surround micing systems based upon combining these things with other mics, Schoeps KFM360.


'Outriggers' = additional mics spaced (normally quite widely - metres rather than cm - but precise distances vary according to application) positioned across the front of an ensemble either side of a main array. They could be in line with the main array, in front of or behind it (no rules - just do what works).

Typically these are omnis or subcardioids but could be anything at all depending upon the requirements of the job and characteristics of the venue/performance. These can be added into the mix to widen the image, introduce a bit of movement (aka instability/correlation error so use very carefully), help to extend the LF response in a recording based upon a comparatively bass light main pickup like a Blumlein pair using mics whose LF rolls off comparatively high, and perhaps as accent mics to help instrumental tone/balance, especially on wider ensembles (though they're usable on anything - in the right circumstances) like enhancing the string balance on an orchestral recording.

In addition to the possible advantages of using them, disadvantages include increased noise/pickup of rumble in noisy venues, especially omnis on things with close audiences in live situations, correlation problems (mono incompatibility errors) and, if not positioned carefully, they can cause imaging problems with soloists stood in a 'conventional' position either side of the conductor (if the outriggers pick up too much soloist it can smear the sound and drag them off way too far to one side). All of these are easily (normally) overcome by careful selection of mic, polar pattern, positioning and mixing.

I normally rig a pair (or two) by default (particularly on larger ensembles), though I don't always end up using them.

Took some quick pics. Not time to do anything serious about it.
http://trombonisten.se/Decca/index.htm

Gunnar

heheh. Fantastic - reminds me of something i made last year for the precision flying I do.

Kinda obvious really - excellent. I feel a decca tree coming on!! (I assume the 3/8" bolt was easy to pick up?

Cheers very much indeed.

Cheers for the notes about balls and outriggers!

Quote Barry Pearce:

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I assume the 3/8" bolt was easy to pick up?

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3/8 nuts and bolts are getting harder to find as metric takes over. Canford Audio have them here and a few proper old hardware shops will stock them. Otherwise, you can also use the threaded mic holders which come on K+M stereo bars (they can be unscrewed and removed) and the threaded washers that go on the top of mic stands (to stop things screwing on too far) make a good 3/8" nut substitute. Or you could try to find a friendly engineering/metal fabrication shop and ask them to make something up (if they still have the threading kit).

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heheh. Fantastic - reminds me of something i made last year for the precision flying I do.

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Ah - you're that Barry Pearce. I did wonder

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heheh. Fantastic - reminds me of something i made last year for the precision flying I do.

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Ah - you're that Barry Pearce. I did wonder

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uh-oh. That sounds ominous!!!

Another flyer? Lately I have not done much though, mostly towing glide planes.
http://www.ssfk.se/

The bolts can be found, I am happy to have a store in town that has just about any dimension made.

Gunnar

Quote Barry Pearce:

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Quote 0VU:

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heheh. Fantastic - reminds me of something i made last year for the precision flying I do.

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Ah - you're that Barry Pearce. I did wonder

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uh-oh. That sounds ominous!!!

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Not at all. I'm just leaping to the conclusion that you're the Barry Pearce who is/was an instructor at Benson and received the CAA commendation a few years ago? (Apologies if not - the precision flying comment seemed too coincidental.)

er...yes, and still is.

Following on from the earlier qeustion about critical distance...

Critical Distance or Dc is the distance from a sound source where the level of direct sound equals that of any reverberant sound.

Critical Distance does not tell you where to put a microphone, but it does give a useful prediction of how it will sound at any particular point realtive to the sound source(s).

For the recorded sound to be reasonably "tight" and to have good intelligibility, the microphone needs to be closer than Dc as a minimum specification. Beyond Dc all sound will essentially be from the reverberant field, and will therefore be unsuitable for clear speech or sung words (although obviously ideal for ambience and atmosphere).

Critical distance can be measured with simple tools: a long tape measure, a broad spectrum sound source ("noise") and a meter that indicates sound levels. I generally use a small portable FM radio with built in speaker for the noise source by tuning it to the gaps between FM stations to get white noise/hiss, and the sound level meter is usually the same cheap (£30) SPL meter I use to align surround sound speaker arrays. You could use an omni mic and the meters on your mixer or recorder, if you want, although it's not as convenient.

Reverberant sound is essentially the same level at any point in a space since it is reflected randomly off all surfaces. If you walk around with the sound meter outside Dc then the levels measured hardly change at all. On the other hand, direct sound obeys an inverse square law, in which the direct level drops with distance (down 6 dB for every doubling in distance).

So, all you need to do is turn up the radio hiss, lay out a long tape measure, and note the sound level at increasing distances from the radio speaker. I start at 10cm, then 20, then 40, then 80 and so on. You can start further out in a very dry room to save time, as long as you double the distance betwen each measurement. However, by starting in as close as 10cm the ~6dB drop with each double distance point is very obvious and reassuring that the test is working.

When you start measuring the drop in level will be close to 6dB. In some situations it may be a little less -- maybe as little as 4dB -- but as long as the change is of that order (ie 4-6dB) and consistent, you are still within Dc.

At some point as you keep repeating these measurements there wont be the same amount of level drop. There will be a negligible change in level from the previous reading. That shows that you have now moved into the purely reverberant field, and the measurement point at which it happens is Dc -- the Critical Distance.

If you carry on beyond it you'll find no appreciable difference in level measurements. Critical distance is fairly constant for anywhere within the venue space as long as you stay well away from strongly reflective surfaces (which act like local sources all of their own) -- such as windows, stone pillars and so on.

If you measure the Dc when you check out a new recording venue, you can then draw on the floor plan a circle of that radius around the orchestra, singers or whatever. If it is a big group of musicians, you might want to draw separate circles for the front and back or extreme left and right elements to make sure that your intended mic placements can actually cover the portions of sound sources in the way you planned.

All mics intended to pick up primarily direct sound have to be within the circle(s). Bear in mind also that this is a three dimensional thing. The real shape is a sphere centered on the sound sources -- so the height limits of your mics can also be determined using Dc.

Once that is done, you need to factor in the directivity of the mics you intend to use. Omni microphones have to be placed at less than 30% of Dc to achieve useful intelligibility and direct/reverb balance. So if the Dc is 3 metres, then the omni mic has to be within 90cm of the source.

Cardioids have to be placed at less than 50% (they can go further out because their directionality rejects more of the reverberant sound). So in the case above with a Dc of 3m, a cardioid could be placed up to 1.5m away from the source and still give the same direct/reverb balance and intelligibility as the omni at 90cm.

More directional mics can go further out again, some approaching the Dc boundary. Outside that boundary the sound from the microphones -- whatever polar pattern they have -- will be reverberant and ambient.

Note, you don't have to place the mics at their corresponding Dc distances. This is only a guide and isn't designed to remove the art from the task of miking an orchestra or whatever... but it does provide useful information about where a mic definitely wont work, and a useful basis on which to argue for putting the mics where you want to put them. You cannae argue with the physics

There same principle applies to the placing of speakers in PA situations, where you want to achieve clarity and intelligibility of reproduced sound within a reverberant space.

Hope that helps demystify the Dc thing for you. Sotrry it got a bit long winded. it really is easier than it might sound, and a very useful skill to have up your sleeve.

Hugh

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Technical Editor, Sound On Sound

Wow. Cheers Hugh. Fantastic. Sounds like for my next recording session I need to get to the hall at 10am for a session starting at 14:00 so I can play

Only couple of further Q though...I can see how this information relates to forward facing mics, outriggers picking up the ambience typically reside on the outside of the Dc stereo arrays need to be inside. How does this relate to an M+S setup? Is the critical factor here the mid cardoid?

Also you have given rough figures for omni and cardoid how do figure of 8s figure - do they come more under the heading of omni in this regard?

Cheers,
Barry

Quote Barry Pearce:

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How does this relate to an M+S setup? Is the critical factor here the mid cardoid?

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Yes. Decoded MS with a cardioid M equates to coincident XY cardioids (ish). Think of the MS array in the same way as an XY pair at the same place.

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Also you have given rough figures for omni and cardoid how do figure of 8s figure - do they come more under the heading of omni in this regard?

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A sub cardioid can be 0.4Dc

A Fig8 has the same directivity factor as a cardioid, so should also be within 0.5Dc

A Hypercardioid has a directivity factor of 2 and can be at 0.6Dc.

hugh

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Technical Editor, Sound On Sound

This Critical Distance stuff is really intruiging and something that I've not come across before, so thanks for the eye-opener, Hugh!

As the vast majority of my work revolves around close-mic'ing situations it's probably not something I'd use on a daily basis, but I'm all about expanding my skillset - you never know when a call may come through for something other than the usual technical metal and various sub-genres of prog that most of my clients perform!

I have one question though concerning the measurement process.

Am I right in assuming that the level of the noise source (whatever that may be) will not affect Dc? I'm basing this assumption on the fact that the physics of the inverse square law apply irrespective of inital level, so although the actual measured levels of the direct sound from the noise source will be less the reflected sound will still be proporional in level, resulting in the same Dc.

Have I got this right, or is there in fact an optimum SPL which should be used in determining Dc in these situations?

Quote Matt P:

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Am I right in assuming that the level of the noise source (whatever that may be) will not affect Dc?

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Correct. It only needs to be loud enough that you can measure its level change reliably over ambient noise at the Dc. I find a small portable radio is fine for most purposes.

Hugh

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Technical Editor, Sound On Sound

For additional information on Critical Distance, check out Shure's "Microphone Techiques for Recording" in the educational portion of their website. A direct link to this free booklet is here: http://216.64.193.140/literature/pdf/booklets/mics_for_music_studio.pdf Go to page 30.

Also here: http://shure.custhelp.com/cgi-bin/shure.cfg/php/enduser/std_adp.php?p_faqi d=82

There is also an on-line calculator here: http://www.mcsquared.com/critical.htm

Best wishes,
Dave

Edited to correct broken links

Edited by Hugh Robjohns (24/06/07 01:30 AM)