Low freq. audio

[Matheous]

I remember reading that notes around 30-40 c.p.s. have a wavelength of two or three feet ? -and have always wondered how we can hear such low frequency notes with good headphones?

[nightmare]

Firstly, the wavelength is more like 20 or 30 feet.

velocity = freq. x wavelength

The speed of sound is approx. 332 metres per second, so the wavelength at a frequency of 30 -40 Hz is around 8-10 metres i. e. around 20 -30 feet.

Secondly, why should the wavelength have any bearing on why headphones can reproduce sounds ?
The important ability is for the headphones to be able to oscillate at the required frequency.

[heathfield]

Whilst several feet may be the wavelength, it's the speed the waves are reaching your ear that counts. At 40Hz, your eardrum is receiving a positive pressure on the first half of the wave, then a negative pressure on the second half. And this is happening 40 times a second, which is why you hear it in your headphones.
For sound wavelength, have a look here

[mibn2cweus]

When the calm surface of a pond is disturbed by dropping a pebble, as waves travel across the surface of the pond the water itself moves mostly up and down in place as can be observed in the motion of a leaf floating on the surface. This simulates the way sound travels through the air but in three dimensions.

It might help to think of sound as being absorbed from the air laterally, (as consecutive adjoining slices) rather than longitudinally (as in a continuous sheet)?

Frequency is sensed as the rate of change in the density of the air as it impacts the eardrum. The amplitude (loudness) and frequency (rate) are decoded in the cochlea and this information is sent to the brain.

[Gef]

Because both your eardrum and the speakers in the headphones can vibrate at 30 - 40 Hz.

[nightmare]

To mibn2cweus or whatever your name is.
Please do not confuse the issue by referring to water waves, which are transverse and hence completely different from sound waves , which are longitudinal.

[mibn2cweus]

Nightmare, that woke me up!

Actually water �surface waves� are a third type of wave composed of a combination of transverse and longitudinal wave motions. I agree that coherence is lacking in the water analogy when strictly adhered to. The point I had hoped to make is that while sound is propagated through the air from the point of origin in expanding concentric spheres , the individual molecules of air that propagate the sound waves are only temporarily displaced within their locality and are not required to travel along with the wave for the sound to reach a destination. This analogy was intended to help the questioner visual by means of common experience the relationship between wavelength and the perception of a sounds frequency. We do not perceive wavelength directly. What we sense is the sequential effect of sound, as it arrives, on the local air pressure which displaces the eardrum.

I will continue to work on improving this analogy and any helpful advise is welcomed. Thanks for pointing out that my explanation was all wet and does not hold water. Sweet dreams!

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