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Bass Frequency Myth!

#21
Consider a sine wave.
Consider your ear.
Theoretically, your ear is a single point, and the wave moves past the point. So you perceive only one part of the wave at a time.The frequency, or how long it takes the wave to pass, is the pitch. But you're only "hearing" the wave at one point: what's in front or behind doesn't matter. A low frequency wave is FORMED at the same speed as a high frequency, but it takes longer for one wavelength to pass, because the wavelength is greater. But it doesn't matter because you've only got it at one point. The compression of air begins moving outward immediately: even if you're only micrometers away from the speaker cone, the wave will hit you with the same frequency. The distance the wave travels to get to you has zero effect on it's properties, neglecting environmental things like aforementioned reflection and scattering.

So that was me trying to explain it in a way that made sense.
 
#25
I think you run into this situation at its extreme in large venues (outdoors being the extreme case) with a cabinet that's too short. There are no reinforcing waves to speak of (just physical coupling vibrations) and the extreme changes in air pressure are happening right at your knees. To me, the best remedy if you must stand that close is to move more air at ear level (ie, taller/stacked cabs).

But I thought I should mention this:
There is an interesting phenomenon called "standing waves" which can reak havoc on bass players in small rooms (especially squarish rooms) because notches of frequencies in the bass guitar range are getting extreme cancellation and amplification because of the relationship of room dimension to wavelength. There's lots of good reading out there on this subject - especially in terms of home theater subwoofers. Often called room "modes". Key takeaway: try not to audition bass cabinets in a small room, when you get it in a concert hall the freq response curve will be totally different.

Hope this is helpful.
 
#26
Interesting ........ a question I never asked my self !


I'll bring some gas to the discussion ;

I theory , for a speaker to create a lower frequency waveform , it has to move , say, 3 inches (voice coil) , isn't that slower than creating a higher frequency ?
(maybe in the nano second range ??? )
Will a woofer voice coil move faster to create a lower frequency than a tweeter will for higher stuff ?


I wonder .......
 
#27
according to bill fitzmaurice, all voice coils respond at the same speed and there is no such thing as slow or fast response in a driver.

3" really? xmax is measured in millimeters ;)
 
#28
The physics in it are so interesting and complicated! So while the speaker is moving in/out slowly for the fundamental it's vibrating very fast at the same time to reproduce harmonic content. And that creates such terrible distortions that you have to separate the frequency bands and put them through separate drivers. But then you get into crossover issues, arggg.
I have nothing but respect for you guys out there designing cabs - requires lots of effort.
 
#29
There is a lot that needs to be said about the physiology of the ear. Part of what makes bass frequencies harder to hear —at any distance really— is that the design of the ear (and therefore our ability to perceive sound) lends itself to optimal hearing in and around the 1000-5000Hz department. As far as I can tell, this explains why cutting your mids makes you all but disappear in the mix! :P

So, ultimately, it is true that we tend to rely more on the upper harmonic content of a bass note for perception. Furthermore, this also explains why we need to push a lot more intensity (watts :P) to cut through a busy mix.

Source? Studying psychophysics. If you're interested in learning more I suggest starting here: http://en.wikipedia.org/wiki/Equal-loudness_contour as my explanation is a little bit on the simpler side. :)
 
#30
I wonder if part of the confusion here isn't the result of confusing different ideas of speed. Specifically, (1) speed as how fast a sound wave travels vs (2) speed as the frequency with which the wave cycles.

Maybe a crude analogy will help. Imagine two 1000-foot-long chains of cars, emerging from a tunnel at a constant speed of 60 mph, exactly in parallel. Imagine that each vehicle is moving with its nose glued to the tail of the preceding vehicle (ie, no gaps). However, one chain consists of a bunch of Mercedes stretch limousines, and the other consists of a bunch of SmartCars. Since the former are much longer than the latter, but the two chains are the same length (1000 ft), clearly there are many fewer vehicles in the Mercedes chain than in the SmartCar chain.

OK, imagine that you are standing at a fixed point 200 ft away from the mouth of the tunnel, and the two chains have emerged from the tunnel (in parallel, remember) and are approaching you in such a way that one will pass to the left and the other to the right.

So what happens? The first vehicles in each chain will pass by you at the same time, as will the last vehicles, because each chain is 1000 ft long and each one is going at a constant 60 mph. (Note, BTW, that you don't have to be standing >1000 ft away from the mouth of the tunnel to experience the whole 1000-ft chain passing by.)

However, the frequency with which you see a new vehicle go by (= the frequency with which a cycle of a sound wave passes your position) is different for the two chains, even though the two chains are moving at the same speeds. You'll see a new SmartCar go by more often than you will a new Mercedes stretch. This is because there are fewer cars, and thus a lower "change-of-vehicle frequency," in the Mercedes chain. Yet the Mercedes chain is not moving any slower than the SmartCar chain.

Admittedly not the most rigorous analogy, but....
 
#31
I like Richard Lindsey's analogy above. <geek>I am not an audio expert, but I did study oceanography in grad school, especially very low frequency waves on the beach (a family of wave like motions with very long periods driven by the energy of wind waves, but not the wind waves themselves, wind waves are the ones everyone knows and sees at the beach). So I agree that the "bass waves take longer to form because of a longer wavelength" is a myth, or perhaps more correctly a canard. More importantly I can discuss standing waves, modes, and nodes with pretty good authority.

Standing waves are most commonly caused by reflection in a container, and bass waves reflect more cleanly off of walls than treble waves, so in a small room you can get standing waves and they mostly effect the bass frequencies. Ever sloshed the water back and forth in your bathtub, and set up a standing wave where the water is high at one end, shallow at the other, and then switches back and forth? Notice that in the middle of the tub the water level stays roughly the same? This is a standing wave caused by reflection at the ends of the bath tub. The ends of the tub are the anti-nodes of the standing wave, and in the middle of the tub is node of this standing wave- if you are considering the height of the surface of the water (or depth, leaving oceanography aside) as your variable of interest. (Note that the longitudinal water velocity, however, is the exact opposite- nodes at the end of the tub, anti-node in the middle.)

The problem for us bass players with standing waves is that since the effective waveform no longer progresses - it is standing - the areas of maximum excursion, called anti-nodes, and areas of NO excursion, called nodes, are fixed in space. (In music we're concerned with the sound pressure level, rather than the height of the water surface, but the same physics and math applies.) So at the node of a standing wave, you won't hear anything! The initial wave and its reflection are destructively interfering at that point to sum to zero. Since each frequency has it's own wavelength, node and anti-node positions in a room are different for each frequency, so when you're standing in one place you hear some bass notes as greatly attenuated, while others are boomy and over-blown, depending on the frequency of the note and where you're standing.

Modes: standing waves often can break up into a number of different modes. Going back to bath tub example, the first mode is the most common, with anti-nodes at both ends of the tub and a single node in the middle (variable of interest is water depth). If you really excite the wave motion by sloshing the water back and forth very hard and fast you may be able to excite higher modes, the next higher mode would involve three anti-nodes- one at each end and one in the middle of the tub, and two nodes, at the 1/3 and 2/3 of the tub length. Theoretically there are an infinite number of modes available for excitation.

Notice the mathematical relationship between these standing wave modes and the modes of harmonics on a plucked string? Excellent, because the harmonics of a plucked string are *actually due to a longitudinal standing wave in the string* progressing down the length of the string, reflecting at the end, and moving in the opposite direction (it reflects at both the bridge and the nut, which is why not have good contact in either place, and allowing the energy to escape the string rather than reflect, sucks the sustain in strength out of a note).

So don't confuse modes with nodes, or standing wave modes with diatonic scale modes. Name-space clash! Warning! </geek>
 
#32
Good stuff. It follows that the design of indoor spaces should strive to avoid standing waves. The latter's existence and characteristics are determined by the shape and size of the room, and by the absorptive and refractive materials placed therein.

The worst possible room is a sphere, followed by a cylinder or a 3-D oval. In a traditional room with parallel and perpendicular surfaces the worst is a cube. The better choice has all three dimensions different, and not in the ratio of small integers.
Invalid Link Removed

Better yet is to also have no parallel surfaces, which implies a sloping ceiling and an odd quantity of perimeter walls, although it can be done with an even number of walls if some of them can slope (i.e. not perpendicular to the floor). Note that many drop ceilings have no effect on vertical standing waves, which will be determined by the shell of the building. Hence, the sloped ceilings must be structural (e.g. Sydney Opera House).

Once the geometry is correct, the rest consists of providing the right absorption, so that sound decays fast enough. Preferably, the decay time is similar over a broad range of frequencies, which usually requires the incorporation of sizable bass traps in the design of rooms that will be used for music. Since intelligibility of speech and enjoyment of music are not maximized with the same decay time, some rooms are designed to have adjustable decay time.
 
#33
Regarding wavelength, it may be useful to be aware that wavelength is a variable that depends, not only on the frequency of the wave, but also on the velocity of propagation of the medium. Hence, the same frequency can have a dramatically different wavelength in different media. The faster the medium's propagation velocity, the longer the wavelength (for a constant frequency).

PropagationVelocity (m/sec) = Wavelength (m) x Frequency (1/sec)

We are most interested in sound pressure waves propagating in air, whose propagation velocity at 20 degrees C is ~343 m/sec. Therefore, C1# (2nd fret on low B string, = 34.65 Hz) has a wavelength of ~10 m (~ 33 ft) in air.

However, sound propagates 4.3 times faster in water. Hence, that same frequency, when driven by an underwater speaker, will have a wavelength of ~43 m.

Inside your amplifier, where electro-magnetic waves propagate at approximately 1/2 the speed of light (~ 150 million meters per second), the wavelength of of your C1# will be ~4.3 million meters, or slightly over 1/10 the perimeter of Earth.

What stimulates our ear is variations of sound pressure versus time. Those variations alternately suck and blow our eardrums. Our inner ear's spectrum analyzer converts that to amplitude versus frequency (changing over time). Nowhere does wavelength enter that picture.

Where wavelength is relevant is in the resonance of mechanical structures at the beginning and end of the signal chain:
- Fretted string length (and equivalent in other instruments)
- Speakers and their enclosures
- Rooms
 
#34
Thanks for all the responses!

1) This was mentioned in passing in a History of Blues and Jazz (not as part of a lesson or anything that is on his curriculum- I should have said that earlier). This guy is a seriously kickin sax player and jazz instructor, not a sound tech.

2) I told him about the headphones and how if a sound wave had to fully develop for it to be audible, there would be no way to mic a bass cab, etc.

I was just hoping someone could point me to an article that explains it better than I could. Maybe I'll just send him the link to this forum and tell him not to take it personally. It *is* a pretty common misconception, so I don't blame someone for believing it. I know I've heard people say that on TB several times (actually the first time I ever heard this myth was when I was asking why my bass sounded like poo 10 away from the amp) and it gets corrected pretty fast.

Thanks for all the help.
-Mike
 
#35
carbonfold said:
Bass frequencies are longer. Not a huge science there but it is science. But they are longer thus take longer to "form" :) you feeling me? Just depends on how you look at it I guess.

But in application issues, usually bass suffers from bass wave cancellation from reflection issues due to its wave form. Hence the "some" truth about traveling and wave formation.

Dunno, I'm not a huge "the science in sound" guy, but I'm sure there should be some solid sound engineers that will chime in on this. I'm sure that how I am saying this could be picked apart against me but I should be close enough (I think haha)
Click to expand...

Ever hear of a little something sound engineers called the "proximity effect?"
 
#36
MichaelSmalley said:
I was just hoping someone could point me to an article that explains it better than I could. Maybe I'll just send him the link to this forum and tell him not to take it personally. It *is* a pretty common misconception, so I don't blame someone for believing it. I know I've heard people say that on TB several times (actually the first time I ever heard this myth was when I was asking why my bass sounded like poo 10 away from the amp) and it gets corrected pretty fast.
Click to expand...
eh, no reason to rub it in ;)
 
#38
Muaguana said:
Ever hear of a little something sound engineers called the "proximity effect?"
Click to expand...
where getting right up on a dynamic mic increases its low end output? what's that got to do with this question? (edit: oh, you mean because low end is happening into the mic with less distance, not more.)

+1 to the mythiness of the myth. bass waves "develop" just fine in the short length afforded by in-ear monitors.
 
#39
Proximity effect is an effect of microphone transducers, not due to the source sound. That's why there is no proximity effect in your ears.

About the often mentioned headphone point, that one might not hold as much water you would want. Just due to the very small size of the speaker diaphragm and the small length of travel of that driver, a headphone speaker doesn't generate very much energy at the fundamental frequency of a low bass note. There is a psycho acoustic effect whereby your brain generates the fundamental given the overtone series of a low note, and this is probably responsible for a significant amount of the bass one hears in ear-bud headphones and in-ear monitors. That is just a supposition on my part, but I can't imagine ear-buds putting out much energy at all into 37 Hz.
 
#40
HolmeBass said:
That is just a supposition on my part, but I can't imagine ear-buds putting out much energy at all into 37 Hz.
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well, they wouldn't have to, would they?

an IEM with a good seal creating even a little 27Hz would make your eardrum vibrate at that 27Hz quite easily.
 

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