i am taking a first year physics course in high school. my friend and i got into a debate about subwoofers one day…
she claimed that for really low (but frequency within human range of hearing) bass notes, the wavelength is longer than the car, so you are not able to hear the note (but presumably you could if you were standing outside the car). i retorted with a smart “uh… i don’t know why, but no”. i want to hear some explanations about this…
The lowest frequency that humans can hear is around 20Hz., which has a wavelength of roughly 17 meters. That doesn’t necessarily mean it can’t be heard inside a car, though. I have long since forgotten all my knowledge of acoustics but I’m sure some smarter Dopers will fill in the rest of the details for you shortly.
The short answer is you’re right, she’s wrong.
The fact that the wavelength is longer than the space it’s in does not keep the air from compressing and rarifying, which is all a sound wave is.
Think about this: If she were correct, you could not here low bass notes using headphones, which I can guarantee is not true. In fact, you couldn’t here much of anything as your ear canal is much smaller than the wavelength of most audible frequencies.
You’re a guest and cannot search, but we had a pretty good discussion of this a while back. See this thread:
It seems to be just a sort of Urban Legend.
Welcome to the boards.
Wait a minute. Unless I’m missing something, you could find out for yourself. Stand outside the car. Listen. Get a feel for how it sounds. Get inside the car. Listen. Compare. Furthermore, if you can hear (not “here,” RJKUgly) the sound while you are only a few feet away from the car, wouldn’t that disprove her theory? Is this a case of trying to prove something from the academic or intellectual point of view while disregarding personal experience? How much do you trust your schooling? Wow.
The wavelength doesn’t really affect your ability to hear a sound anyway. You can adjust the wavelength of a sound by propagating it through a different medium, but as long as the frequency is unchanged, you’ll still hear it as the same pitch. A 20 Hz note played through a piece of steel held up to your ear, for example, has an even longer wavelength than in air, no matter how big the piece of steel is.
I think the wavelength is important for EM waves being picked up by an antenna. That’s probably what she’s thinking of.
]I don’t think this is necessarily a case of “disregarding personal experience”, although it may be, I obviously don’t know the people involved. But this is a fairly common “fact”, often heard when people talk about loud car stereos.
In fact, It may be sort of the opposite of disregarding personal experience, in that it seems to an attempt to explain why anyone in their right mind would play their music at such obviously ear damaging levels. I’m certain many here have had the experience of being near such a vehicle in full “boom” mode, and wondered why the people inside put up with it.
But it is easy to disprove with personal experience, which I have done myself, as talked about in the linked thread.
You’re right and she’s wrong. Now, there are some similar sounding situations she may have confused with this. If the wavelength is much longer than your head, then you can’t judge its direction by sensing the difference between what your two ears hear. That’s why you only need one subwoofer whereas more generally in stereo you need two of everything.
>I think the wavelength is important for EM waves being picked up by an antenna.
This is true but it doesn’t keep your transistor radio from picking up AM radio waves that are thousands of feet long. It just dictates some relationships in the antenna’s design.
Do you have a cite to support this? I thought it was the frequency that changes. One example is the change in one’s voice when sucking helium. The frequency changes, not the wavelength.
You cannot change the frequency without changing the wavelength. The frequency is inversely related to the wavelength. They are different ways of expressing the same thing:
(wavelength in meters) = (speed of light in meters per second) / (frequency in Hz)
(quote corrected for typos and punctuation errors by me)
I’m no expert, but isn’t this only true if the speed of propagation remains constant, as TJdude825 said?
For example (using completely wrong numbers for simplicity) if you 100 pulses per second moving through a medium at 1000 feet per second, those pulses are 10 feet apart. This is a 100 Hz sound with a wavelength of 10 feet.
Now change the medium so the speed of propagation is 1200 feet per second. That same sound now has a wavelength of 12 feet, but pulses are still passing any given spot at a frequency of 100 per second.
Similarly, you could change the frequency and the speed of propagation, and wind up with the same wavelength.
The frequency of a sound wave cannot change in this way - sound is a series of pressure waves, and they can only come along at the rate they were originally generated, regardless of any changes in the velocity of propagation along the way. The wavelength is what changes, as **TJdude825 ** said.
The change in your voice when inhaling helium is due to the resonant frequency of the human throat cavity increasing in the presence of helium.
When waves move from one medium to another, the frequency stays the same, but (if they have different speeds), the wavelength changes. However, most sources of waves produce them at a particular wavelength, not a particular frequency. When you inhale helium, your voice produces the same wavelength as normal (in your voicebox), but at a different frequency, because the speed of sound is different in helium from in air. When the sound waves leave your helium-filled respiratory tract, they stay at that high frequency, but change wavelength.
And sound waves won’t resonate in something significantly smaller than their wavelength (smaller than half or a quarter of the wavelength, depending on whether it’s open), but they can still be there, just non-resonantly.
I have wondered why this is true, thanks. The usual anwer I hear is that “bass is not directional”, now I know why.
maybe this is related to the idea that microwave radiation can’t escape the window on microwaves because the wavelength is larger than the holes?
wow, thanks for the great and timely responses! in answer to you, <b>CC</b> , i based my assumption on my common sense, and what i asked for here was the physics reason to back it up. thanks again
Helmholtz resonators can be physically much smaller than the wavelength of thier resonant frequency. A beer bottle, for example, sounds like about 220Hz (maybe lower?) or so to me, which has about a 4.5’ wavelength.
I actually don’t believe that there are many car stereos that can come close to producing a 20Hz signal at a respectable SPL (sound pressure level). Definitely not stock premium level stereos like Mark Levinson or Bose, although some extreme aftermarket competition systems can probably do so. As for the wave length, the max SPL will be 17m from the source and will degrade as you get closer to the source. You might be able to hear it, but it will be at quite a low output volume compared to 17m away and most likely will have a lot of distortion. In a sense she is correct because the point of reproducing sound in a car is for the people inside the vehicle to enjoy the music. Unless of course you are trying to piss people off in the next car or win a professional sound competition. 