Update usage of bandpass in sound_field_imp()

[hagenw]

This is a follow up of #113.

At the moment we have the default config setting:

conf.usebandpass = true;
conf.bandpassflow = 10;
conf.bandpassfhigh = 20000;

This setting results in an execution of the following line inside sound_field_imp():

d = bandpass(d,bandpassflow,bandpassfhigh,conf);

in bandpass() the function fir2() is used to create the filter and applied by convolution().

sound_field_imp() is the only place where the bandpass filtering is applied at the moment.

This pull request should figure out, why we have enabled the bandpass filtering as the default setting and if we can disable as default or even remove it.

[hagenw]

If we simulate an impulse response at a single listening position, as for example done by time_response_wfs() the bandpass filtering results in a reduced amplitude as shown in #113.

Here, I do sound field simulations in the time domain for WFS and NFC-HOA with and without bandpass filtering.

WFS

conf = SFS_config;
conf.plot.usedb = true;
conf.usebandpass = true;
sound_field_imp_wfs([-2 2],[-2 2],0,[0 2.5 0],'ps',200,conf)
conf.usebandpass = false;
sound_field_imp_wfs([-2 2],[-2 2],0,[0 2.5 0],'ps',200,conf)

NFC-HOA

conf = SFS_config;
conf.plot.usedb = true;
conf.usebandpass = true;
sound_field_imp_nfchoa([-2 2],[-2 2],0,[0 2.5 0],'ps',200,conf)
conf.usebandpass = false;
sound_field_imp_nfchoa([-2 2],[-2 2],0,[0 2.5 0],'ps',200,conf)

Looking at both results I would say that both looks better without the bandpass filtering.

@fietew: do you have an idea for an example that might look better with bandpass filtering? @spors: if I remember the bandpass filtering was included from the beginning in the time-domain scripts, could you comment on the idea behind it

[hagenw]

@fietew: would it be ok with you, if I change the default value of conf.usebandpass to false?

[fietew]

Is it used somewhere else appart from the time-domain sound field computation?

[hagenw]

No.

[fietew]

I will have a quick chat with @spors, whether we are missing any benefit of using the bandpass filtering. I don't see any.

[hagenw]

Ok, thanks.

[fietew]

One meaningful application could be, that the user wants to simulate the sound field using an bandlimited impulse ranging e.g. from 0 to 1 kHz instead of a fullband impulse. However, switching the default of conf.usebandpass to false makes sense imho.