Loop Amplifiers :The good the bad and the downright ugly
Loop Amplifiers are great for improving hearing for people, via their hearing aids using the “T” facility, in a church or conference environment, In these places you tend to have people speaking from a lectern or using radio microphones. This mean the source of the of the sound you are interested in improving is very close to the microphone. This results in the signal for the loop having mainly direct sound, little reverb and back ground noise in it. This signal also has quite limited dynamic range.
What is a hearing aid loop?
The basic operation of a loop system is to use an amplifier to drive a current through the loop of wire which surrounds the area you want to cover. The source of the signal for the loop amplifier is usually a microphone or the pa system output signal. This produces a magnetic field that changes in unison with the signal. The hearing aid has a “T” setting, which turns the hearing aid into an inductive receiver capable of receiving the signal from the loop. For a more in depth description have a look at Ampertronic loop amplifiers and info
Ambient sound and background noise, far from perfect!
I have experienced loop amplifiers in another environment, where the customer believed, all they need to do is have an ambient mic feeding the loop. I didn’t expect this to provide a better listening experience than the hearing aid users own hearing aid microphone provides. From comments from the users this was definitely true and they didn’t bother switching to t
music groups a theatres and live music venues
My other really big problems with loop amplifiers occur when I am working in a church with a music group, a theatre or a live music venue where the dynamic range is huge and the compression of the loop amplifier is insufficient to cope. My best solution to this is to use another compressor between the desk and the loop amp. This is set so it has very little effect on the signal when say the sermon is being preached, but above that level, when the praise band plays and produces a much higher level with more dynamics, has a quite severe compression ratio with a fast attack (as fast as it will go) longish hold (a time longer than 3 or 4 beats of the music at least) and a medium release. (1/2 a second). These time settings are chosen to prevent the music from pumping. The result of this, additional specific compression, is it has the effect of presenting the loop amp with a signal with a much smaller dynamic range, within it’s AGC’s capability.
Can’t get the Loop amplifier microphone in the right place
As far as the problem of the venue where you can’t get a mic closer to the sound source than the hearing aid users own microphone, I am open to any suggestions. I have used highly directional microphones to help with the problem (effectively getting the mic closer to the sound source) but often in these venues the position of the sound source is not fixed and it is not possible to reposition the microphones for each set up in the room.
Here is the really ugly! please read on, it may get you out of some unexpected problems
Why is my pa feeding back when I don’t have any mikes turned up? not easy to twig the problem
There is a further problem with inductive loops. If you are working with a band with single coil pickups on their instruments (bass guitars and other guitars not using humbuckers). These will pick up the loop signal and act
like a microphone to this signal. So anything being transmitted by the loop say a paging call will for no apparent reason come out of the band’s PA . If the loop is driven by the same signal which drives the PA that is amplifying these instruments there is a feedback path. The result is the system is unstable..
All you can do is get the instrument out of the loop’s coverage area, which isn’t always possible or turn the loop off.