Headset Radio Mics - problems

[unclesteve]

Hi,

Advice please. If a soloist is standing at the back of a stage using a headset radio Mic and is surrounded very closely by maybe 50+ other cast members, is this going to mask the signal to the receiver to any extent ? Also any advice as to using more than one at a time on stage are there any problems as to proximity of one Mic to another ?

[Bobbsy]

A headset mic in the situation you describe WILL pickup sound from the chorus around the soloist, but the good old "inverse square law" should be your friend and tend to give you much more level from the soloist (with the mic inches from their mouth as opposed to the chorus who will be a couple of feet away. The other thing to consider, though, is the pickup pattern of the headset mic you're using. These can be anything from omni (i.e. pick up from any direction) through directional mics like a cardioid up to very directional hyper cardioid. The latter two types of mic will offer more rejection of the surrounding singers but also require more careful positioning to make sure the soloist is in the sweet spot.

 

However, don't necessarily assume that the directional mics are better for you. Even directional mics have lobes to the side or rear where there is pickup from their surroundings, and depending on lots of factors (how loud the soloist sings, the staging around them, etc) sometimes an omni with a more general pickup can be a more natural sound.

 

As for problems with using multiple headsets, there are two major areas to watch out for. First off, each time you double the number of open mics you get 3dB closer to feed back. Therefore, 2 mics will give you 3dB less gain before feedback than 1 mic...and 4 mics will be 3dB less GBF than 2 mics...and so on. This is not necessarily a huge problem unless you're pushing things near the edge, but something to be aware of.

 

Second, when two mics get into close proximity...for example the traditional "love duet"...you'll find that both mics pick up both performers almost equally. This can (and usually does) lead to nasty comb filtering effects due to phase differences in the two signals. Some people swear by inverting the polarity of one of the mics to help this, but the real solution is careful mixing by the op. I quite often pick up the "close" bits from a single mic, carefully taking the other out as the actors get close and bringing it back as they separate.

 

Bob

[Malcolm Gordon]

I think the OP's original question relates less to the audio pickup than to the attenuating effect on the RF signal of a scrum of chorus between the bodypack transmitter and the receiver, the answer to which I would also find interesting.

[Solstace]

In short - yes, the close proximity of flesh will hinder radio mic transmission. For this very reason, the JFMG licence terms tend to give higher limits on the transmitter power of body-worn transmitters than for free-standing or handheld devices.

 

As for how this might affect you, my own guestimate (based on similar previous experience) is:

 

Keep the antennae hidden in the wings (so on stage, but out of sight) and cable these back to the receivers in a quiet corner. So long as you keep the antenna cables relatively short, and send the audio signal to the desk via the mic multicore, you should be fine. Please don't expect to be able to keep the antennae and receivers with you at the sound position at the back of the theatre!

 

For the mixing aspects of using mics on stage in close proximity, then I think Bobbsy is right on the money. I'm very much a fan of only using one mic if that one mic is close enough to 2 or more of the talent to make them all heard evenly. Comb-filtering is not a pleasant thing to hear, especially when it's so easily avoided!

 

As for the more scientific answer - I'd love to read up on any more formal research that may have been carried out elsewhere!

 

c.

[Bobbsy]

Ah okay.

 

Well, on RF there IS some signal attenuation just from absorbion by soft bodies. However, with a properly specced diversity system this has never caused me a problem despite having big choruses. Frankly, I'm in the camp that likes to have the receivers close to hand for the mixer, and uses high quality (pain in the butt to work with) low loss cable of the proper impedence. One thing I find important though is to get the best out of the divesity system by separating the antennae by a long way. Typically I'll have one either side of the theatre and also vary the height. I'd never rely on the sort of antenna you mount on the front panel of the receiver only six inches apart...this sort of nullifies the reason for diversity reception in the first place!

 

Bob

[Graham Threader]

It is worth remembering that dipole aerials do have an 'up' and a 'down' element. The 'down' element should be marked with an 'E'. We mark or hire stock with green/yellow earth tape.

 

Transmitter aerials should be vertical, as should the receiver aerials; then they are polarised in the same plane.

 

RF losses by body absorbtion can be minimsed by keeping packs and aerials away from the skin. Sweat ingress into body packs is not only unpleasant to think about, but will create performance problems. Never let anyone tape aerials to the skin, disaster awaits!

 

Poor RF reception can often be improved by experimenting with aerial placement. Often a small change can transform performance. Spend some time experimenting. When you have a layout that works; make notes for next time.

 

Aerial boosters (head amps) should be used with great care. Often they boost the interference beyond the filtering capacity of the receiver. The cheaper the receiver, the more likely this is to be the case. The corollory of this is that sometimes interfernce acn be reduced by lessening the gain in the aerial system.

[Lamplighter]

Some very interesting views on aerials. My favoured arrangement for UHF is a pair of circularly polarised yagi aerials, of opposite hands, at the rear of the auditorium with about 1 wavelength spacing. This is actually a modified panel array from a mobile phone microcell. When used with a decent pair of filters and preamplifiers it will handle 10 to 16 channels without any nasties. The signal levels from multiple transmitters have a smaller dynamic range at moderate distances from the stage than is experienced on stage particularly against the scenario where a transmitter can be almost next to the receive aerial. It is most unlikely that a null would affect two spaced aerials with 90 degree difference of polarisation or opposing hands of circular polarisation. It is not good practice to have both receive aerials vertical if they are in the same area as a null is often caused by the signal components (direct and reflected) effectively being horizontal. Wide spaced aerials are probably best kept vertical. The only time I have had intermodulation problems has been with the receivers on stage. Finally treat the manufacturers information with some healthy scepticism, it is not always correct. I would strongly recommend doing your own calculations if possible, there are plenty of decent intermod calculators free on the web. One useful quick check, put all your beltpacks in a plastic bucket with a transistor radio, switch all on but unplug the mic from one, listen to this receiver at the working distance. Now switch it off and listen again. Try each channel in turn. If you don't hear the radio then you have a pretty robust system.

Brian

 

PS with the above setup a densely packed stage is not a problem.

[unclesteve]

Many, many thanks for all of your replies. I understand the Mics I will be using are being hired in and I will get some technical support from the suppliers ( Lighting and Sound Services , Ellesmere Port ). I will discuss your suggestions with them. Also now I know, after some research what "comb filtering" is, I will also get better results with my fixed Mics as well.

Thanks again guys,

 

Steve