# Waves and barriers

1. Jul 18, 2007

### neworder1

Any barrier passes more low frequencies than high (i.e. the amplitude of transmitted wave is higher when the frequency is lower). This can be derived form wave equations, but I wonder whether there is any qualitative explanation for this, not resorting to mathematics?

2. Jul 18, 2007

### mgb_phys

Not necessarily, low pass filters are probably easier to think about mechanically where a damping system will reduce high frequencies.
In electroncis it's often more common to filter out lower frequency eg 60Hz interference while transmitting a high frequency signal.

3. Jul 18, 2007

### Staff: Mentor

What barrier and what kind of waves are you asking about? Some barriers will resonate at particular frequencies, and hence pass a band of frequencies and reflect the others.

4. Jul 18, 2007

### neworder1

I mean barriers like a capacitator in RC high pass filter (change of voltage) or an ordinary building wall (sound wave). When a wave passes from one medium to another some frequencies are reflected more than the others - I wonder whether in the example above it can be shown "qualitatively", without wave equations.

5. Jul 18, 2007

### Staff: Mentor

You don't use the wave equation to solve for the frequency response of an RC filter.

On the wall example, you can intuitively see how at the fundamental resonance frequency, the middle of the wall will alternately bulge a bit out and back in, at the sound frequency that matches that wall resonance (which is determined by the rigidity and size of the wall). Since the wall is physically moving back and forth, it transmits some of the longitudinal sound wave's energy through to the other side. Is that the kind of non-equation intuitive explanation that you are looking for?