# Lower Frequencies pass through materials easier?

1. Apr 10, 2006

### richnfg

This is a question that has been bugging me for quite and long time and I have been searching on the Internet, but having no luck.

So, why does lower frequency sound pass through materials much easier then higher?

Thanks, Rich

2. Apr 10, 2006

### Astronuc

Staff Emeritus
I am not sure that 'easier' is the correct idea. Low frequency waves attentuate differently than high frequency waves. The are properties of the material such as bulk modulus and density, and structural properties like porosity and crystal structure which affect dispersion of sound. And then there is the geometric effects such as wall thickness in a pipe, which might match a wavelength of the sound.

http://hyperphysics.phy-astr.gsu.edu/hbase/sound/souspe2.html

http://hyperphysics.phy-astr.gsu.edu/hbase/sound/sprop.html#c1

One needs the relationship between dispersion/attenuation and frequency.

3. Apr 10, 2006

### richnfg

Ok, so say you had 2 identical materials

you pass both a high and low frequncy sound through it, yet the lower passes through easier...thats what I meant to ask.

Probabaly didnt make it clear enough :P

4. Apr 10, 2006

### richnfg

5. Apr 10, 2006

### kmarinas86

Longer waves tend to be canceled out over longer distances where as shorter waves are more likely to be canceled out at shorter distances than longer waves.

6. Apr 10, 2006

### richnfg

what do you mean by cancelled out?

as frequency is about how fast the atoms etc. vibrate? and amplitude the size of the vibration?

7. Apr 10, 2006

### kmarinas86

Two waves of identical wavelength and opposite amplitude and direction will level each other.

Yes, but more specifically, its equal to the speed of the sound wave divided by it's wavelength. For sound, the wavelength is the distance between compressed regions of a longitudinal wave.

Amplitude of sound is specified in terms of either pressure (Pa) or decibels (dB), not size.

Last edited: Apr 10, 2006
8. Apr 10, 2006

### richnfg

yeah, i know all that

but that doesn't explain why they cancel out

9. Apr 10, 2006

### Astronuc

Staff Emeritus
What frequencies (Hz or kHz range) or wavelengths are you using, and what is the dimension of the solid through which the sound is propagated?

How are you determining that one frequency propagates more easily - or do you mean one is less attenuated, i.e. more of the sound energy propagate through?

10. Apr 10, 2006

### richnfg

i am using frequencies of 1000Hz and 3500Hz and some cardboard around 2mm thick

by using an oscilloscope to see the amount that comes and reaches a microphone...the 1000Hz gives much bigger results showing more lower frequencies get through.

11. Apr 11, 2006

### richnfg

sorry, I know im annoying

12. Apr 13, 2006

### lurksalot

It might seem daft but is the resonant frequency of the cardboard the issue here ??

13. Apr 14, 2006

### rbj

for high frequencies the wavelength is shorter and the fibers or whatever stuff that is in your material are about the same size (or bigger) than the wavelength and will reflect the waves inside the material in random directions and your unified wavefront will be dispersed.

for low frequencies the wavelength is longer and the fibers or whatever stuff that is in your material will tend to simply be the medium for the acoustic wavefront and will just conduct it further.

Last edited: Apr 14, 2006
14. Apr 15, 2006

### bobbytkc

I haven't done any research,nor do i have much knowledge on the propagation of sound waves, but i have an idea which i would like very much to be verified.

I take it that when you say 'by using an oscilloscope to see the amount that comes and reaches a microphone' you mean the amplitude of the resulting wave after passing through the cardboard

The max velocity of an oscillating particle is given by:

max velocity = (amplitude) x (angular frequency)^2

where angular frequency is directly proportional to frequency.

This implies that the max velocity of the atoms/molecules transmitting the sound waves is higher with a higher frequency.

since the root mean square of the velocity of the molecules is its thermodynamic temperature, it means that when a sound wave passes through, locally, the mean temperature is higher than the surrounding areas, resulting in a net energy transfer to its surroundings and energy loss for the sound wave.

the maximum potential of a particle in simple harmonic motion is directly related to the maximum displacement of the particle from its mean postion (in this case, the amplitude) so supposing that there is little deviation for the frequency, the loss of energy of the wave would result in a smaller amplitude for the sound wave.

can anyone with sufficient knowledge help me find a mistake in my reasoning? thanks.

15. Apr 20, 2006

### pcxmac

Sound Travels through air / gas, liquid, even solid materials. Radio frequencies choose the medium which allows them to travel at the speed of light, elctro-magnetic waves. Earthquakes travel in waves (energy propagation).
Learn about how HF travels through the ionosphere. Its all about the wave length, medium of transportation and what it comes in to contact with. Subs use ULF or ELF to go through water (electromagnetic) because there wave length allows them to penetrate through the water, and even deeper, unfortunately there antennas are gigantanormously long.