A local community theater I volunteer at is having trouble with the main speaker cluster. I am one of two Sound Coordinators for this company, so I get called for problems.
Some background, the main sound system consists of a flown cluster of four speaker cabinets: a Main covering the bulk of the audience, a Front Fill (FF) covering the first 4 or 5 rows, a Subwoofer (Sub), and a Stage Monitor (SM) covering the vast majority of the stage. All are ElectroVoice EVF series speakers. The Main is a 15″ 2-way, the FF is a 12″ 2-way as is the SM, all are 90 degree by 40 degree. The Sub is a single 18″ driver. The crossover and house processing is handled by a Rane RPM-88 driving QSC RMX series amps. We are not currently bi-amp’ing the Main, FF, or SM cabinets, but that is the long term plan.
The other night I get a call from the current production’s Sound Designer (who is also the A1 Mixer) and his A2 Deck Sound. They tell me that the Main has distortion compared to the FF or SM. They swap various parts and narrow the problem down to the wiring, passive crossover, or drivers in the Main. I am out of town and can’t stop by immediately.
When I get back into town I stop by on a dark night (they are having auditions for the next show in the season so I can only make noise and work on things for about an hour). I capture the following response using the test oscillator built into the Soundcraft K2 FOH desk. I am using StudioSixDigital’s Audio Tools on my iPhone 5 with the built in microphone.
Just look at all those harmonics!
Just to sure I am not seeing some weird room effect (as I sweep the frequency I can hear the drums in the pit getting excited) I mute the Main and run the same tests on the FF, see below.
That is more like what I expect to see out of a loudspeaker in terms of harmonic content. Just to be certain the problem is with the 15″ cone and not an amp channel I test the horn in the Main.
That all looks reasonable.
So I am thinking that we damaged the 15″ cone at some point. This is the first show that has noticed any issues and is the second production of this season. So something happened in the past few weeks to degrade that driver.
I will continue to update this post as we learn more. We can’t get up to the speaker until after this production closes in another week.
While I had my phone out and the FFT running I stopped in the Equipment Room and grabbed this spectrum. This is with all the gear running but the fans in the lighting dimmer packs were not running as there were no theatrical lights in use.
I am really curious at the spike at 163 Hz. which is clearly audible in the room. Power line is 60 Hz. three phase, but that should get me 60/120/180 Hz., not 160.
I also sat in the middle of the house (it only seats about 250) and grabbed this spectrum of the noise floor.
