AFMG Network Forum

charles · Member

Hi guys, I’m designing a system for a large workshop, the surfaces are concrete with a steel roof so the acoustics are awful. I was initially going to use 10 Bosch LH1-UC30E Music Horns (I think they “sound” better than conventional horns). A colleague suggested using Bosch LBC3482 Horn speakers instead on the basis that the STI would be better because of the roll off at lower frequencies. This did not appear to be the case, the music horns produced a slightly better STI. I decided to investigate further with a Test room, a simple 10m x 10m box. I locked the RT and set all bands to 0 second apart from 100Hz - 200Hz bands which I set to 10 seconds. Once again the Music horns produced a better STI even though they produce output at the bands with an RT of 10 seconds. Why is this?

thomas · Member

STI is calculated from the MTF. Look at the MTI-values in the View window. There you can see what happens.
The conventional horn (sensivity =0dB for low and for high frequencies) produces MTI values of 0 in these frequency bands. In all other bands MTI = 1 (without masking or noise). This leads to an STI < 1 (=0.87).
The music horn produces MTI = 1. Here, with T=0s, only direct sound is considered. Therefor all MTI = 1 and STI = 1.
Inserting some reverberation into the lower bands has influence on the music horn only. But the MTI is still higher than 0 (the value for the conventional horn).
Took a while, to see that.

The question is, is it rigth to set MTI=0 for bands that aren't feeded with a signal? Hmm, anyone?

thomas · Member

charles · Member

Thanks Thomas but I still don't understand, forget EASE for a moment, if this were a real room with high Rt at the lower bands, 20 seconds at 100, 125, 160 and 200Hz and 0 seconds at all other bands, then surely the LBC3482 would produce a better STI because it is NOT producing any signal in the lower bands and the LH1-UC30E would produce a worse STI because it IS producing signals in the lower bands.
So why does EASE produce the opposite result?

thomas · Member

STI is a weighted sum of the MTI's (definition of STI!).
If you want a higher STI, you have to improve one MTI at least.
Since MTI is roughly speaking a SNR, without a signal (only noise left) it has to be 0 or undefined (no signal, no noise), I think.
Decreasing some MTI's (with band limiting) cannot improve the STI.
In real world you can often increase the level after band limiting and improve the SNR to the background noise (not to reverberation-"noise"). This can lead to a net profit for the MTI and an improved STI.

charles · Member

Thanks Thomas, but unfortunately it still doesn't answer my question.

thomas · Member

Ok, perhaps my answer was to short.
I think the conventional horn wouldn't improve the STI in comparison with the music horn in the room you described (explanation above). And therefor I don't see, that EASE produces an opposite result.

charles · Member

If the Rt between 100Hz & 250Hz is locked at 18.3 seconds and all the other bands are locked at 0 seconds then the STI throughout the Audience Area is mapped as follows:-

Music Horn 0.75
Conventional Horn 0.61

The conventional horn is producing zero output in the reverberant bands, the music horn is producing lots of output in the reverberant bands yet produces a better STI, that can't be right, can it??

I can send you the packed project if you want.

thomas · Member

I made some tests with charles' model (varying RT's, noise, levels, checked
Mapping vs. Aura) - there is nothing unexpected. In general.
In general - that means there is on little strange thing that happens.
For the band limited speaker in the Area Mapping (pure statistic, no simulation)
a strange and unexpected MTI > 0 occurs in frequency bands without signal,
but with reverberation. That's not right, for shure (no signal <-> MTI = 0).
With Masking or noise included in the STI calculation, this dissapears and
the MTI = 0 as to expect.
Using Aura this effect doesn't occur.

thomas · Member

I made some tests with charles' model (varying RT's, noise, levels, checked
Mapping vs. Aura) - there is nothing unexpected. In general.
In general - that means there is one little strange thing that happens.
For the band limited speaker in the Area Mapping (pure statistic, no simulation)
a strange and unexpected MTI > 0 occurs in frequency bands without signal,
but with reverberation. That's not right, for shure (no signal <-> MTI = 0).
With Masking or noise included in the STI calculation, this dissapears and
the MTI = 0 as to expect.
Using Aura this effect doesn't occur.

AFMG Frank Siegmann · Member

Hi Charles, Hi Thomas,
could somebody forward us the example project ( support@afmg.eu ). Then we can check the MTI issue here once more.
Frank Siegmann
AFMG