A discontinuity (such as the screen) will reflect sound back to the diaphragm. So if the screen is a half-wavelength from the diaphragm, the reflected wave will return to the diaphragm in phase, thus creating a peak on the sound level. This applies for all integer multiples of the half-wavelength frequency, resulting in a "comb filter" or "picket fence" spectrum.
So a 2.5" path length would have peaks around 5" wavelength, which is 2700 Hz, and multiples (5.4kHz, 8.1kHz, 10.8kHz, etc. Most horns have other resonances, too, so it's sometimes difficult to be sure what you are seeing - just a thing to be aware of.
This description ignores minor effects, such as reactive acoustic impedances including mechanical impedance of the diaphragm/amp, etc. but it should be close for a hard reflector like a metal screen. If the reflection is caused by a low acoustic impedance (relative to the plane wave impedance) it might reflect in reversed phase, giving resonances at 1/4 wavelength, 3/4, 5/4, etc. This is what gives the clarinet its characteristic sound - the reed end is high impedance, the open key is low impedance. But the spacing is the same.
