Sound system problems .
On the Monday morning I had an email from the a church.
Hi John,
I just thought I would pass on to you the very positive comments I received from the congregation on the quality of the sound at this mornings service. I did a couple of very minor ‘tweaks’ to levels during the week and it now seems to meet everyone’s expectations. Many thanks for your help.
Best Regards Bob
This is what led up to it.
A local church had been having progressively more comments from their congregation that the sound system in the church was not meeting their needs.
After talking to the original supplier , and buying a new mixing desk at their recommendation, which did not provide a satisfactory solution to their sound system problems; The person in charge of the technical equipment in the church contacted me with a view to solving the problems for them. I arranged to visit the church along with some sophisticated equipment I am very familiar with, so I could compare this equipment with the church’s system. The equipment I brought with me included a Midas M32R digital desk which has brilliant tone control capability and a pair of IMG LRAY 1000 miniature line arrays which have very controlled dispersion and a transparent sound
To get a feel for the system I played some music from a known recording through the system.
The first impressions were:-
- the system has a time alignment problem , when you are on the balcony or the rear of the church the rear speakers are heard before the sound coming from the front speakers . This causes an echo effect and smears the sound .
- The coverage of the system is not uniform from a level point of view .It also varies with frequency. No amount of tweaking of the tone controls can correct this.
- There are areas in the church which are in the speakers direct sound field . In these areas the direct sound from the speakers dominates what is heard and the reverberation heard is negligible. As a result intelligibility tends to be better here. In the other areas where the proportion of direct sound to reverberation is lower intelligibility is much poorer . Unfortunately a large proportion of the church which is not in direct field.
- The system does not sound good.
Further tests were done with the test equipment and microphones alone to investigate the resonances in the building and feedback . The results of these test showed:-
- There is a significant resonance in the building
- The lectern and plinth are resonant, this is mechanically coupled to the lectern microphones .
- The result of the interaction of the building and system does produce some peaks in the frequency response and these do vary dependent on where you are listening from .
The conclusions from these tests with our systems are that the building has very reflective surfaced which causes sound systems to be prone to feeding back and no amount of tone control can prevent this from happening. The only way to deal with the problems is to selectively target the sound into the areas it is required and away from the places which cause problems
The churches system was turned back on and the test system switched of
Something which has already been noticed was the way the churches speakers were poorly directed which did not help with the direct verses reverberant sound ratio in the majority of the church. This was corrected and an improvement to the intelligibility immediately noticed.
After experimenting with various set ups of the churches system’s eq and feedback suppressor the church’s system sounded better, although still far from ideal.
These results still did not account for some other resonances and the higher susceptibility to feedback of the current system.
During the tests I had noticed that the lectern and pulpit microphones sit on there respective podiums which are in the middle of the staging block , like being on a drum skin making them act like microphones on a bass drum. This problem was reduced by re positioning them on a wall of the staging, which is much stiffer, before the tests were continued.
To look at this problems further. I looked at how the other additional features of the churches mixing desk, besides the tone controls, were set up .
The first thing that jumped out (not all that obvious from first inspection ) was the ambiance microphone used for recording and to feed the loop, was actually being routed to the main system , this was because to get the ambient microphone’s signal to the new memory stick recorder on the desk forces you to do it this way, as the record input to the recorder is derived from the master left and right outputs!. This defiantly is a bad thing to have to do, so to stop this happening, I stopped the ambient microphone feed going to the record / main output by switching the main out routing switch for the ambient microphone off. now it only goes to the auxes.
The next thing which was not immediately obvious and not in the desk’s manual, is that the post fade aux 5 and 6 are still sent to the desks internal digital effects even when an external device (in the case of the churches system ,the loop amp and the feed for the external rooms) are connected to the output jacks! . So when I started my investigations the churches desk’s auxes 5 and 6 not only sent their signals to the loop and external room feeds, but also sent anything on these bus ( and that’s every channel including the ambiance microphone) to the effects processors and as these send their outputs to the main, outputs all sorts of unexpected sounds result . In particular reverbs and strange feedback. To overcome this I turned all the outputs and repeats from the effects processors right down
After all this had been completed the churches system’s eq was set up for its optimum performance and left in this state so its performance could be examined on the coming Sunday
it should be noted that all the work done so far does not address the problem of the lack of time alignment of the balcony fills or that their coverage is not good.
