AKG C747 Sound quality advice

[ace]

Hey Guys!

 

Now I am generally a lampie at heart, but tonight I had a go at some sound to help a charity out. Based on my experiences, perhaps someone could offer some advice to some of the problems I had.

 

I was using several AKG C747 mics both on a lectern (a pair) and four on table stands, as they were all we could lay our hands on. Firstly I found that every time someone tapped the lectern or table whilst talking I got a thud, are there any neat ways to avoid this? (We used the basic shock absorbers supplied with the mics).

 

Secondly whenever I turned the mics up, I got a feint ringing sound starting to appear, even though the mics were behind the PA. What is the best way to identify this problem and solve it?

 

Finally regardless of asking the speakers to turn off their mobiles, someone left their blueberry on the desk and caused a lot of inteference. I read on this forum that the mics are very prone to RF. It was suggested that this can be helped by shortening the cable - does this help and is it safe todo?

 

Many thanks for your thoughts!

[Charlie Jeal]

You should have high passed the mic channels(goes a long way to removing the low frequency content of any knocks and shocks etc) and used eq to get rid of the feeds also a proper gain structure in your system helps.

 

Charlie

[david.elsbury]

No John, mobiles are commonly known to cause havoc with the C747's. Its a known issue. And good luck on having EVERY person at a conference actually obey your "no mobiles" sternness...

 

Also, a shotgun is going to pass just as much mechanical noise/ vibration thru its mounting than any other mic, unless its properly shockmounted. And suspended mics aren't going to help one bit- people don't talk with their heads facing the ceiling...

 

and used eq to get rid of the feeds also a proper gain structure in your system helps.

Get rid of *which* feeds?

[Charlie Jeal]

No John, mobiles are commonly known to cause havoc with the C747's. Its a known issue. And good luck on having EVERY person at a conference actually obey your "no mobiles" sternness...

 

Also, a shotgun is going to pass just as much mechanical noise/ vibration thru its mounting than any other mic, unless its properly shockmounted. And suspended mics aren't going to help one bit- people don't talk with their heads facing the ceiling...

 

and used eq to get rid of the feeds also a proper gain structure in your system helps.

Get rid of *which* feeds?

 

 

You know very well what I meant Dave or did you want me to call it ringing, howlround or its full name of feedback.

 

;) ;)

 

Charlie

[tall_phill]

Hey Guys!

 

Now I am generally a lampie at heart, but tonight I had a go at some sound to help a charity out. Based on my experiences, perhaps someone could offer some advice to some of the problems I had.

 

I was using several AKG C747 mics both on a lectern (a pair) and four on table stands, as they were all we could lay our hands on. Firstly I found that every time someone tapped the lectern or table whilst talking I got a thud, are there any neat ways to avoid this? (We used the basic shock absorbers supplied with the mics).

 

Secondly whenever I turned the mics up, I got a feint ringing sound starting to appear, even though the mics were behind the PA. What is the best way to identify this problem and solve it?

 

Finally regardless of asking the speakers to turn off their mobiles, someone left their blueberry on the desk and caused a lot of inteference. I read on this forum that the mics are very prone to RF. It was suggested that this can be helped by shortening the cable - does this help and is it safe todo?

 

Many thanks for your thoughts!

 

Often you can improve - though not necessarily end or solve - mechanical coupling issues between a lectern and microphone by adding mass to the lectern. I have seen many that are off a light construction, futhermore the main panel, upon which notes/laptop/hands might be rested opens up revealing a little compartment beneath it. Adding some mass or dampening in this area, can help things somewhat. Shock mount and HPF the mics in question.

 

Jim Brown and David Josephson wrote this excellent paper titled "RF Susceptibility of Condenser Microphones" I hope it helps.

http://www.audiosystemsgroup.com/RF_in_Microphones.pdf

 

Best,

 

phillip

[p.k.roberts]

Finally regardless of asking the speakers to turn off their mobiles, someone left their blueberry on the desk and caused a lot of inteference. I read on this forum that the mics are very prone to RF. It was suggested that this can be helped by shortening the cable - does this help and is it safe todo?

 

The cable between the mic and the pre-amp is unbalanced which will make it even more susceptible to picking up interference. So when setting them up, the routing and positioning of this cable can make a difference - getting the pre-amp close to the mike and so keeping the the connecting cable 'out of the way' can often help.

[Chris Adam]

ah the old C747, haven't seen one of these on a gig for a while. We have had many battles.

 

Lectern mics in general will suffer very badly from what is know as the proximity effect. This is caused and affected by how close a person is to the mic in question, which being on a lectern, could be quite far (as lecterns tend to be at leaning height). This will not only result in a really low amount of audio going into the mic in the first place, but it will sound thin and bodyless. Trying to EQ some fullness in will just give you grief.

 

This mate, is the bain of all conference audio guys. Nervous speakers will wander away from the lectern, lean back and can quite often be a little scared of having a mic in front of them. You will instantly spot a good public speaker among the many bad ones. The best way to overcome this is to try and get the mic closer to the person's mouth, or better still, the person closer to the lectern and speaking in a clear, loud voice. (ha ha ha ha)

 

The ringing can be reduced with a 31 band graphic EQ and someone who knows how to use it. It will colour your sound if EQ'd too much though, but it could give you that tiny extra bit of volume you need. (most clients will care more about everyone hearing what is being said, rather than the quality of the audio - but that's not to say you shouldn't try your best to get it right)

 

The shock mount noise is an issue with all lectern mics. Only last night we had one get hit by a whiskey glass during the toast to the haggis (don't ask, it's a scottish thing).

 

RF, P.K is right. get the cable well away from the person speaking if you can. If it's an executive /management / director / doctor etc, they will not switch off their mobile. It's their lifeline. It doesn't help us, but we have to deal with it. If you ask them to switch off, then you have done your bit. The worst offenders even sit their mobiles on the lectern!!

 

I'll admit that some of my most stressful gigs have been the tiny ones with 5 lectern mics and some playback. When it's your job to make bad mic technique sound good, it can really do your head in. The C747 isn't too bad a mic, but unless you can get it up near the speaker's gob, you have to live with what you've got. No amount of turning it up will help.

[soundo26]

I won't dwell on the EQ & Gain structure issues too much, it's all been said here already!

 

One thing I find is that with using two mics on a lectern, they tend to cancel each other out! I tend to place one mic only on the screen side of the lectern, this is the direction the speaker will turn to look toward the screen (even when youpainstakingly supply comfort monitors)! I do often put pairs on the lectern but only using the outer mics as backup. I tend to favour using lapel mics in preference because they go where the presenter goes, so if they do step back two feet from the lectern you still have them! Lectern mics are always a problem with nervous presenters tapping the lectern or gesturing and slapping the mics! As far as the thump is concerned, the lapels negate that problem too but most better desks have a bass cut on each channel which can help a bit, other than that your graphic EQ is your best friend! Also with the issue of ringing it's worth remembering that for each open mic used you will lose about 3db of headroom so you will need to take the faders down by about this much to stop the feedback building up!

Mobile phones, I don't care if he is an MD, he can't take a call while he's on stage, I tell them they MUST remove mobile phones or pagers whilst on stage, after all, it is they who look stupid when the phone goes off!

 

Finally, There are better mics out there than 747s, they do look good but of all the mics I have had problems with in the past they are the worst offenders! Sorry AKG but they feed back easier than any of their peers!

 

That's my 2p worth, Good luck!

[j_b]

Most condensor mic capsules have shunt filter capacitors in them to reduce the effects of mobile phone interference. However the problem usually travels up the cable toward the phantom power adaptor, which is not necessarily protected. In fact you can move the phone along the cable and find hot spots where the system is more sensitive than others, repeating at intervals related to the wavelength of the RF signal causing the interference.

 

Some manufacturers have a special 'RF' series of microphones, e.g. http://www.clockaudio.com/tabid/216/tabid/289/Default.aspx.

[revbobuk]

Finally, There are better mics out there than 747s, they do look good but of all the mics I have had problems with in the past they are the worst offenders! Sorry AKG but they feed back easier than any of their peers!

 

So what would you choose? The C747 has some 'rifle' characteristics - would you rather use a gooseneck, and have less directionality but a longer body/closer to the mouth? Interested to know...

[Chappie]

C747s are very hypercardioid and their feedback characteristics can vary plenty, depending on what they are mounted on or in front of. If they are mounted on a reflective (acoustically) lectern or table, their sharp rear-lobe can cause problems. If they are in front of a flat set that is close to the speaking position, the same thing can happen.

 

As for the physical noise, if you are using them with the same surface over and over (your own lectern for example) you can experiment with different mounting options that you can leave permanently installed. There are various anti-vibration mounts that might work. You could also try a semi-hard foam (the softer the better, 747s are so light it doesn't take much to support them) or anything that decouples them from the surface.

 

I believe the only reason 747s are still in use is their physical size and the fact they work well on TV etc. Its certainly not because of their sound. We stopped using them about 10 years ago, switching to Shure MXs (with additional windshielding), which are also starting to show their age now. These are susceptible to RF interference as well and Shure have recently re-designed the pre-amp to help alleviate this problem (which also meant that many other mics that we have wired to work with them stopped working, grrr...). We have recently started using Audix M1245s on goosenecks which are excellent, causing me to receive complaints from engineers who no longer want to use the MXs.

 

I would recommend a parametric EQ over a graphic for feedback control on a mic, as it gives you far more control and is less destructive.

[Dan Gruner]

Maybe not that relevant to the OP for this case, but for "talking heads" stuff like this one of the best tools to have in your box is a decent fully parametric EQ and sweepable high-pass filter. If you fortunate enough to be using a decent desk with this on then great. Else a KT DN405/410 or Midas XL42 inserted on a group and the relevant mics routed to this. You don't end up hacking the sound to pieces with the "blunt" tool that most EQ's are on small inexpensive desks.

 

If you have two mics on the lectern use one and the main mic and the other as a backup in case the main fails. Using two at the same time will result in phasing and cancellations that are clearly audible when the speaker moves around the lectern!

 

Make sure any high-pass filters on your desk channels are engaged and if you are using a system that has subwoofers - disconnect/turn them off if possible. If you are just providing speech reinforcement you don't need them and they will cause you more problems. Try to isolate the mic from the lectern/stand as best you can and *very importantly* - grab a friend to give you a hand! It is very useful to have someone stand and chat at the lectern whilst you adjust the EQ and walk the room for coverage check etc.

 

Dan

 

P.S - Out of interest, I've always found a Shure SM81 (with a windshield) to be a rather nice lectern mic. Always carry them around in my toolbox and have often used them for this.

[ace]

Hi Guys!

 

Many thanks for your thoughts!

 

I have another opportunity to have a play tomorrow, and want to try to improve upon the sound quality of last time. There will be no mobile phones near the mics this time!! - Or so I am promised!

 

This time I have acquired a couple more items, a Behringer UltraCurve 2496 and a 32 Band EQ. (I know it is low end kit, but it is the best that I have been able to beg steal and borrow!) I am hoping to use the UltraCurve as both a Parabolic EQ, and a compressor. I will be using 5 AKG c747 mics as tabled mics and several lapel and hand held radio mics. Are there any other features on the Ultracurve that may be useful to me?

 

Is my best option to pass the output from my mixing desk -> ultracurve -> EQ -> amps? Or swap the EQ and the UltraCurve?

 

Will using the Ultracurve as a compressor help both take out the loud thuds and help standardize the levels for all of the mics in this way? Is this the best way to do it? Also are there any rules of thumb as to the best starting positions for the compressor settings? I have read a lot about how they work, but when I get to play I will need to start from somewhere...!

 

Also any tips on EQing? I understand the theory as to how they work, I just have very little practical experience as to how to get it right!

 

Finally one last question on gain - I understand how to set the gain levels on my desk for all of our cabled mics. However the radio mics we are using have a seperate gain on the reciever. How should I set that gain relative to the gain on their channels on the mixing desk?

 

Many thanks - I appreciate your thoughts, as I always learn more when I have somewhere to start from!

 

Chris

[revbobuk]

I've used the ultracurve in live situations on many occasions. It has a graphic eq in already. The real difficulty with it is that unless you have played with it a lot, the user interface means quick changes are hard work.

 

I'm not a great fan of compressors on voice channels - they can promote, rather than reduce feedback. A good operator is more use!

 

I find the RTA function helpful, especially with a separate graphic, as you can see ringing and identify the frequency easily, then pull it down in one go on the graphic.

 

The ultracurve has an awful lot in it - delays, feedback killers, parametric eq (not parabolic!) graphic eq, and more. But it isn't a bit of kit you pick up quickly. Make sure you practise with it before the event.

[ace]

I've used the ultracurve in live situations on many occasions. It has a graphic eq in already. The real difficulty with it is that unless you have played with it a lot, the user interface means quick changes are hard work.

 

I'm not a great fan of compressors on voice channels - they can promote, rather than reduce feedback. A good operator is more use!

 

I find the RTA function helpful, especially with a separate graphic, as you can see ringing and identify the frequency easily, then pull it down in one go on the graphic.

 

The ultracurve has an awful lot in it - delays, feedback killers, parametric eq (not parabolic!) graphic eq, and more. But it isn't a bit of kit you pick up quickly. Make sure you practise with it before the event.

 

Hey Revbobuk!

 

I have been playing with it and got used to the user interface. I seem to be able to find and change settings quickly, but it is knowing what to set them to. We aquired it for its RTA feature. As you say it makes seeing and removing the ringing channels quick for a newbie! That is also part of the reason for having the two bits of equipment together, although I cant get my head around the best order to put them in the signal path!

 

Chris