Assignment #3 Sound 2/17/11

1. Vocal cords of frog involve two separate membranes

1. How do A and C differ? High frequency (1-2 kHz) is generated by vocal cords (lighter upstream membranes) which are gated on and off by glottis at 100-200 Hz - amplitude modulation Use glottis to amplitude modulate sounds from vocal cords C) Leave glottis open while frequency modulate vocal cords

2a) Sound losses in water Water absorption = -0.008dB / 100 m So between 100 and 200 m the sound amplitude decreases by 0.008dB Relative amplitude = -0.008dB = 10 log(I200m/I100m) -0.0008 = log10(I200m/I100m) I200m/I100m = 10-0.0008 = 0.998 So only 0.2% loss due to water absorption

Alternative way to solve Between source and 100m 0.008dB * 100m = 0.008dB = 10 log (I2/I1) 100m I2/I1 = 100.0008 = 1.0018 = Isource/I100m Think about what is I1 and I2 in this case.

Alternative way to solve Between source and 100m 0.008dB * 100m = 0.008dB = 10 log (I2/I1) 100m I2/I1 = 100.0008 = 1.0018 = Isource/I100m Between source and 200m 0.008dB * 200m = 0.016dB = 10 log (I2/I1) I2/I1 = 100.0016 = 1.0037 = Isource/I200m

Alternative way to solve Between source and 100m 0.008dB * 100m = 0.008dB = 10 log (I2/I1) 100m I2/I1 = 100.0008 = 1.0018 = Isource/I100m Between source and 200m 0.008dB * 200m = 0.016dB = 10 log (I2/I1) I2/I1 = 100.0016 = 1.0037 = Isource/I200m So from this we can figure

2b) Sound losses in air Air absorption = -1.2dB / 100 m So between 100 and 200 m the sound amplitude decreases by 1.2 dB Relative amplitude = -1.2dB = 10 log(I200m/I100m) -0.12 = log10(I200m/I100m) I200m/I100m = 10-0.12 = 0.758 So lose 24.1% due to air absorption

2c) Which media transmits better Water loss is 0.2% Air loss is 25.4% So water transmits far better

2d) Spreading loss Since intensity falls off with 1/distance2 In going from 100 to 200 m So in terms of relative amplitude So -6dB per 100m

2d) Spreading loss Since intensity falls off with 1/distance2 In going from 200 to 400 m So in terms of relative amplitude So -6dB per 200m which is -3dB per 100m

Compare losses in air at different distances So close to sound source, spreading loss is most important Further away, absorption is biggest loss

Sound losses will depend on the sum of spreading loss and absorption - as well as any other losses sum

3a. Talking around a campfire Factors that might impact getting sound from one person to another Sound absorption Sound refraction - density gradients of cold air and hot gas of fire will bend sound Sound scattering Noise from fire

Losses are a function of frequency Lecture 4 slide 28 Losses are a function of frequency Higher frequencies have higher losses Loss at 10 kHz is 10x loss at 1 kHz Loss Frequency

Scattering summary Lecture 4 slide 40 Fig 2.6 Bigger objects, more scattering. Shorter wavelength, more scattering. For 1 kHz, wavelength is 33 cm. So for objects < 5 cm, Rayleigh and objects>3.3 m, simple scattering. In between get complex scattering pattern. Fig 2.6

Scattering summary Fig 2.6 Lecture 4 slide 40 Large Wavelength Small Bigger objects, more scattering. Shorter wavelength, more scattering. For 1 kHz, wavelength is 33 cm. So for objects < 5 cm, Rayleigh and objects>3.3 m, simple scattering. In between get complex scattering pattern. Large Wavelength Small Small Frequency Large

1. Global attenuation All frequencies decrease equally Lecture 4 slide 22 1. Global attenuation All frequencies decrease equally Close to source Further away

3b+c. Frequency effects Absorption is higher for higher frequencies Scattering is worse for higher frequencies Mosquito’s sound won’t carry as well as friend Bear will carry better Spreading loss is the same for all frequencies

4. Wavelengths vs body sizes Since we know the sound frequencies, we can convert these to wavelengths using the appropriate sound velocity c / f Where I used c = 344 m/s for air 1513 m/s for water Guesstimate the body sizes:

Animal Frequency Hz Wavelength,  (m) Body length (m) /body length Range? Mosquito 500 0.69 0.002 344 Short Cicada 2000 0.17 0.02 8.6 Bat 100,000 0.0034 0.1 0.03 Long Frog 4000 0.086 0.05 1.72 Human 400 0.86 1.5 0.6 Fish 100 15.1 0.3 50.4 Whale 20 0.76 Dolphin 50000 3 0.01

4. Animal sounds So many animals do use frequencies that they can generate at high intensity for long distance communication However some do not Mosquito - have you ever heard a mosquito across a room? Cicada - I think they overpower us with their numbers since any one of them can’t generate a lot of sound Fish - generally communicate over short distances

5. Aquatic animals In aquatic habitat, high freq can be better Avoid notch due to surface reflection Avoid low frequency noise Attenuation is worse at high freq but not too bad since low overall attenuation in water Fish - exception in using low freq Whales - also use low freq for long range