[ctviggen]

If my info is correct, the speed of sound at 70F is 1129 ft/s.  Because speed of sound = frequency times wavelength, a frequency of 20 Hz would mean a wavelength of 56.45 feet.  How does a bass driver produce a wavelength of this amount?

[MikeCTM2]

i think you're right with your math there.   i remember my DIY cabinet maker saying that a bass freq can be 50ft long.   i can tell ya good bass is produced by a sub being able to move a lot of air like my 15 inch adire audio tempest.   try your question at diyaudio.com maybe

[TG]

If my info is correct, the speed of sound at 70F is 1129 ft/s.  Because speed of sound = frequency times wavelength, a frequency of 20 Hz would mean a wavelength of 56.45 feet.  How does a bass driver produce a wavelength of this amount?

What the speaker does is create alternate areas of compression and rarefaction of the elastic medium (air) by oscillating about a resting point at the appropriate speed.  To make a 20Hz tone (20 cycles/second) it has to oscillate 20 time/second.  The actual length of the wave is not important with regard to initial creation of the tone, however it becomes important with regard to phase cancellations, propagation, standing waves, internal room reflections, harmonics, and many other factors.

[Shamrock Audio]

As the driver moves forward it compresses the air in front of it. The driver then moves backward and to a point as far behind its resting position as it had moved in front of it and back to its resting position. If the driver repeats this cycle 20 times each second it is moving at 20Hz, or 20 cylces per second.

Now, the compression of air doesn't just stay there in front of the driver. It moves out away from the point of its origin, and at a particular speed. In this case, it moves at the speed of sound in air (roughly 1130 feet per second). Since  the driver is compressing the air in front of it at 20 times per second, and those compressions are moving in air at 1130 feet per second, there is 56.5 feet between the peaks of each compression.

Remember, it's the compressions and rarefactions that cause our eardrums to vibrate as they pass through and around us at 1130 feet per second.  

[ctviggen]

Thanks, that makes sense.  I was originally thinking that a woofer would have to move in and out distances near the wavelength, but that would be hard.

[Tyson]

Also you have to realize that the amount of air that a driver can move will limit how loud it can play the lower frequencies.  Generaly as you step down an octave, you have to move 4x the amount of air to keep the same SPL level.  So 20hz would requre 4x more air to be moved than 40hz, which in turn requres 4x more air movement than 80hz, etc.  That is why smaller woofers never really do 20 hz with authority or low distortion.

[nathanm]

Thanks, that makes sense.  I was originally thinking that a woofer would have to move in and out distances near the wavelength, but that would be hard.

You'd sure sell a lot of woofers to the SPL freaks if you had a driver with a 56 foot XMAX!

[Carlman]

Thanks, that makes sense.  I was originally thinking that a woofer would have to move in and out distances near the wavelength, but that would be hard.

You'd sure sell a lot of woofers to the SPL freaks if you had a driver with a 56 foot XMAX!

Yes, and then moved at the speed of sound!  I think tripath would have to get better to perform such a feat!