Latest Update – October 2012

This article has been updated to cover more about the 2012 switch off of Channel 69 (854 MHz to 862 MHz) – some points to note are:

• You need to check if your radio mics can use the allowed frequencies.
• You need to use Channel 38 (606.5 MHz to 613.5 MHz, with a license) or Channel 70 (863 MHz to 865 MHz for free, if usable for you)
• Systems like the Sennheiser G2 don’t cover channel 38 – Boo! Meaning new systems needed, possibly the Sennheiser G3?
• You maybe able to get a rebate – money back from your old system. There are terms and conditions, one of which is that you needed to have been a licence holder prior to the switch over. -registrations for claims has now closed though – info on Shareview.co.uk

VHF or UHF?

Firstly, this page is about UHF microphones rather than VHF microphones. UHF (Ultra High Frequency) radios operate between 822 MHz and 870 MHz whilst VHF (Very High Frequencies) operate between 173 MHz and 220 MHz. Manufactures are tending to put less effort in to VHF systems, and pretty much your baseline system (eg from Shure, Sennheiser etc) will be using UHF. Cheaper systems may be using VHF, do be careful when buying cheap systems, as you really do get what you pay for – at least purchasing the entry level systems from the top brands is a good place to start – eg Sennheiser G2 series…

If you are using VHF, then all I can say is that these frequencies: 173.80 MHz, 174.10 MHz, 174.50 MHz, 174.80 MHz, 175.00 MHz will be the ones to use, as they are license exempt and can be used free of charge. (be careful with 174.80 as it tends to have intermod problems. If you want to use frequencies other than these then you will need a license (more info about Intermod and licenses below)

Radio Mics or IEM?

Another thing to note, is that in this page I tend to talk about Radio Microphones, but the concepts apply to In Ear Monitoring systems too – as these use UHF. (IEM is where, rather than a ‘wedge’ speaker for the people up front, they have ear phones that are connected to a radio receiver, and they can monitor themselves that way). So, when your thinking about how many channels you need for your radio equipment, don’t just count your microphones, but count your in ear monitors too!

Radio Microphone Frequencies:

The  Sennheiser G2 series of microphones are fairly common in Churches – they are solid, reliable microphones that are in the professional bracket, all be it at the cheap end. A Sennheiser G2 (now legacy) can operate on 1,600 tunable frequencies, from 518-866MHz  and the newer Sennheiser G3 has 1,680 frequencies from 516-865Mhz- so how do you choose with frequency to use?

Frequency Channels:

When radio frequencies are talked about, they are often referred to as their channel number, eg "channel 69". Here is a simple table describing the channels that are often used for wireless microphones:

Data correct as of October 2009

As you can see, there are lots of frequencies, but apart from channel 70 all of them require a license.

Free frequencies:

Channel 70, 863 MHz to 864.99 MHz are license exempt. This frequencies can be used without a license – remember though that lots of your neighbours may well be using these same frequencies so watch out for picking up other people’s audio!

Purchasing Frequencies

You can purchase license to use frequencies. For about £75/year (or £135/2 years) you will be licensed to use up to 14 channels… The organisation that manages frequencies in the UK is JFMG. They will assign you frequencies to use that are specific for your area – this ensures that even if your neighbour purchases a license they will be given a separate set of frequencies and you won’t clash. If you use frequencies out side of Channel 70, then you are committing an offence under the Wireless Telegraphy Act! More info from http://www.jfmg.co.uk

Intermodulation

When multiple frequencies when used together, eg if you have more than one radio microphone, the frequencies can sometimes form additional frequencies that are outside the original ones used and cause problems – it’s best to avoid this and therefore pick your frequencies carefully. This is called intermodualtion, ot intermod for short! It’s worth noting that you can mix UHF and VHF systems without having them interfering with each other as there is a big enough gap between the UHF and VHF frequencies.

Intermodulation checkers:

There are software programs that you can purchase, however Audio Technica have a free online checker that is simple to use from their web page. Also, Sennheiser have a intermodulation software program that you can download: Sennheiser.

Unlicensed/Free Frequencies in 2012

As already said, channel 70 (863 -864.99MHz) is unlicensed and will be remaining so after 2012. The debate Ofcom are having at the moment is regarding Channel 69, and selling those ranges off… the question is, what happens to wireless microphones that are currently licensed to use Channel 69? – the answer is that you’ll need to use Channel 38 (with a license from JFMG).

Example Sets of Usable Frequencies in Channel 70:

Here are 2 example sets of 4 usable frequencies in the free/unlicensed bands (channel 70) that have been tested for intermodulation:

If you are wanting to use more than 4 radio microphones then you have no choice apart from purchasing a license and use Channel 38.

You would only use one of these groups – choose either the blue or the green group! – or find your own set of frequencies that don’t have an intermod problem.

Having said this, Sennheiser state that their G3 systems can use the following frequencies without intermodulation problems:

Other Links:

This page is an amalgamation of other resources I have collected and compiled and tried to re-write in an easy to understand way, here are some other links to further your reading:

• A Discussion: http://www.worshipcentral.org/forum/topic/microphones?page=2
• Intermodulation: http://en.wikipedia.org/wiki/Intermodulation
• Channel 69 sell off comment by Sennheiser: http://www.theaudiofiles.net/2009/05/uhf-frequencies-what%E2%80%99s-happening/

Closing Notes

This information was compiled from various resources and is thought to be correct at the time of writing (October 2009, updated in October 2011) – this information doesn’t come with any guarantees, but do use it for informational purposes.

Please do leave a reply below with other information, feedback etc!

What is Feedback?

Lots of ways to help you reduce audio feedback in your PA system

Feedback is the screeching/squealing/hooting sound heard when there is an accidental loop in the PA system. That is when the sound from a microphone is amplified and fed to the speakers and then picked up again by the microphone, this sound then carries on round and round resulting in the nasty noise. The feedback frequency depends on a number of factors such as: the resonant frequency of the room, distance of microphone and speakers, frequency response of the microphone, characteristics of musical instruments.

To reduce or eliminate feedback this loop needs to be broken, here are a few ways of doing that.

Speakers & Microphones

Change Speaker and Microphone Position

If the speakers are behind the microphone ie:

Speaker –> Microphone –>  Audience

Then moving the speakers forward, so that they are either in-line with the microphones or in front or them will mean that there is less sound entering the microphone from the speaker.

Microphone –> Speaker –> Audience

Also remember about your Foldback/Monitor speakers (see below).

Turn the Speakers down

Simply turning the speaker volume down will mean there is less sound entering the microphone.

Careful Use of Monitors /Foldback speakers

Monitors (aka Foldback speakers) are the speakers that the band use to hear themselves and the band leader. Monitors can be the cause of the feedback.
Position of Monitors

As with the main speakers, try not to point the monitors into the microphone! Place monitors behind the microphone:

Singer –> Microphone <– Monitor

The singer will be using directional microphone which is best at picking up sound coming from directly in front of it, so in this arrangement it won’t pick up the sound from the monitor.

Volume of Monitors

Keep the monitors as low as the band are happy to work with.

Equalise the Signal

EQ Dials

A feedback squeal probably consists of one pure tone, ie, a single rogue frequency. By using the EQ controls on a sound desk, the frequency that is feeding back can be ‘dialled out’, leaving the rest of the audio material unaffected. A sound desk will typically have for each microphone channel:

• A High Frequency control
• One or Two sweeping mid-range controls, giving adjustment of frequency and gain
• A low frequency (bass) control

By finding the frequency that is feeding back and cutting it will suppress the feedback. This is called ‘dialling out’ or ringing out’…

On a per-microphone basis, the basic steps to do this are:

1. Ask the singer/preacher to sing/talk at their normal volume.
2. Turn the gain up on the channel to just under clipping
3. Turn up the master volume control until you hear the ringing
4. Whilst it’s ringing, use the EQ sweeping controls to sweep through the frequency range with the relevant gain control cutting (ie not boosting) until the ringing stops. With experience you will learn to guess roughly which frequency is squealing.
5. Put the faders back to normal and check the singer/speaker still sounds good, and push the volume to check for feedback again.

Be careful not to wreck the sound by taking out too many frequencies or by cutting too much – be sure to check how it sounds!

Use of Microphones

Distance:

A major reason for feedback is the sound engineer having to turn the gain up high for a microphone as the speaker/singer’s mouth is far away from the microphone. Vocal microphones are meant to be used close up to the mouth – ie within 1 inch. Having the microphone close to the source means means a lot of sound energy is entering the microphone, when this happens the Gain on the sound mixer can be lowered, giving much more head room before feeding back. Another positive benefit of having the singer as close as possible to the microphone is that it will pick up more bass and produce a warmer sound. When used for speech, the microphone needs to be 12 to 18 inches from the mouth so that, when the speaker makes natural head movements, the proportionate change of mouth-to-mike distance isn’t large and the PA operator doesn’t have to ride the faders. This also reduces ‘pop’ sounds on the spoken letters ‘p’and ‘b’. The extra distance (compared to sung vocals) makes the system more prone to feedback, but at least you can probably turn off monitors and all the other mikes. There are other ways to reduce the ‘pop’ sound, using compressors, but that is beyond the scope of this document.

Turning Off

Simply muting the microphone when not in use will mean that it won’t be the cause of any feedback! If the PA operator decides on this policy, he/she then has to be constantly alert to fade that mike up again just before it is needed.

Use Microphone Stands, where possible

A mike on a stand, once sound-checked, behaves predictably for the rest of the event. A handheld mike, used by an animated singer, could be pointed anywhere and its distance to loudspeakers or monitors will vary. If a setup proves to be really feedback-prone, try to minimise the number of moving microphones, eg, persuade all thb backing vocalists to use mike stands.
Hand Held Microphone Tips

When using hand held microphones, either wired or wireless, apart from making sure the singer/speaker holds the mic close to their mouth, make sure they don’t pace their hand over any part of the ‘head’, but use the handle. Even the ‘back’ half of the ball-shaped head shouldn’t be covered.If their hand is around the head part – the pick up part, then that will cause resonance within the microphone itself.
Wireless Microphones

Wireless microphones are useful as the can move around the stage/room easily. This as the potential for causing feedback if they are used near to the speakers or monitors. Try to find out the places on the stage where feedback may occur and then train the users on where they can and can’t go with a radio microphone. If the users are confident is muting the microphone themselves (most have a mute button) then they can be in charge of muting/un-muting the microphone. This means if they wonder around the room with the microphone then they are free to go where they like. Often people will hold a handheld radio mic whilst walking around and at that low height it could end up close to monitors or pointing towards the main speakers.

Standardise on Microphones

Every make/model of microphone is different in the way that it behaves. Different models will feedback at different frequencies If all your microphones are the same model, then this will help you a lot in diagnosing a feedback problem.

Cheaper microphones are also more prone to feedback. Where possible invest well in microphones. Microphones are the key item to quality in a sound system.

Other Things to reduce Feedback

Rehearsals

Rehearsals are the ideal time to resolve feedback problems,especially where you may have a lot of microphones in different positions being used. It far better to spend time at this point eliminating feedback than in front of an audience!

Hearing aids

Just a note on hearing aids – we’ve had couple of time in our church recently where we’ve had what sounds like feedback – but it didn’t seem like it was coming from the PA system at all! The sound was high pitch and at a constant frequency. It came and went through the service. I was on the sound desk and pretty much turned everything off, all the channels were muted and I took everything out of the foldback/monitor speakers – yet the sound kept on coming back! It turned out that it was actually someone hearing aid. He had turned it up so much that it was feeding back on itself! The gentleman was so deaf that he didn’t realise this!

So – just something to be aware of if you are hearing feedback and just can’t fathom where it’s coming from!

More!

Here is a good site/page with further information:

http://www.mediacollege.com/audio/ is a good website offering information/training on media, plus it’s resources are free!