Why cant you tell where a sub is comming from?

Discussion in 'Speakers' started by Ken Burkstrum, Apr 27, 2006.

  1. Ken Burkstrum

    Ken Burkstrum Stunt Coordinator

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    What is the science behind that?

    And why is it I can barely hear my PB2+ from outside right next to the room it's in but I can hear car subs thumping from half a mile down the road?
     
  2. Jack Briggs

    Jack Briggs Executive Producer

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    Ken, very-low-frequency tones are nondirectional. Hence, you hear the bass but it seems to be everywhere. This has been understood for a long, long time.

    You probably do not remember Infinity's very first speaker, the Servo-Static I. It was two electrostatic panels crossed over to a single subwoofer (a real subwoofer, mind you). This was back in 1970 or so.

    Users can install second subwoofers if they like, but it's not necessary.
     
  3. Chu Gai

    Chu Gai Lead Actor

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    In a general sense, the reason why low frequencies are non-localizable has to do with interaural time differences. IOW, it's the arrival times between your two ears that allows you to localize where the sound came fromm. The wavelength of low frequencies is pretty large and becomes larger the further down you go. Essentially, the distance between your two ears is too small to properly convey the information as to where the bass is coming from since its wavelength is so large. As the frequencies go up it becomes easier.

    As to why you can't hear your sub when you're outside the room, I don't know. Maybe you just happen to be in a null. Try playing some low frequency sounds on your sub (not too loud!) and slowly walk around the room and notice the character of the basss.
     
  4. Ken Burkstrum

    Ken Burkstrum Stunt Coordinator

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    I have noticed that being in a different part of the room can have a different outcome with bass. I notice it when I'm walking around the room while listening to music. There are certain spots where the floor is doing something completely different then another spot lol.

    Can you explain alittle more about "larger wavelength", not sure what you mean by that. I know sound goes out in spherical wave in every direction. I'm just not picturing whats going on that makes you unable to tell the subs is comming from that direction. Or is that, waves have to be closer together for you to hear them and as such tell where they are comming from, and that by each individual wave getting to your ear slower, you are unable to pinpoint that direction? I'm using that theory in relation to people not being able to hear under 20hz, because there's hardly any waves there? I'm still kind of iffy on it, anything you can do to make it clearer would be much appreciated.
     
  5. ScottCHI

    ScottCHI Screenwriter

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    [​IMG]
     
  6. Arthur S

    Arthur S Cinematographer

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    The "prevailing wisdom" is that if a sub is crossed over at 80Hz or lower, you cannot tell where the bass is coming from. In my experience, 80Hz IS localizable. I would humbly suggest that 60Hz is a more realistic cut-off for non-loacalization.
     
  7. Chu Gai

    Chu Gai Lead Actor

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  8. John Garcia

    John Garcia Executive Producer

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    The sound changes in different parts of the room because there are different intractions of soundwaves in different areas. Basically, the length of the lower wavelengths are longer than the typical room, the entire waveform is already partially reflected before it entirely forms. When various waves intersects themselves, it may cause cancellation or exaggeration of a particular frequency and that will vary throughout the room because the intersection points are different as you move around.
     
  9. Leo Kerr

    Leo Kerr Screenwriter

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    As for the "size" of the wavelength, consider our old friend, water.

    throw a small rock in a flat pond. The ripples are pretty small. Now throw a brick in the pond. The ripples are larger - both in amplitude (depth) and size (distance from crest to crest.) Now throw a VW Beetle into the pond.

    Okay, maybe not.

    Sound is like ripples in the water's surface. High frequency sounds are very "short" - the distance between wave-crests is very small. Low frequency sounds are very long - literally feet and yards long.

    The space between your ears is (probably on average) say, 8 inches. Plus or minus. Sound travels that distance in a measurable amount of time - actually, about 0.6 milliseconds. A high frequency sound may have a very short wave-length, so that the pressure "crest" reaches your left ear before your right ear, if it comes from the frontish direction, or also has a distinct "shadow" if it comes from your left side. The sound will "wrap" around your head, but also the echos will also be more significant on the right side, delayed a bit.

    Lower frequency sounds, however, are able to "wrap" more around objects like your head, so even though it comes from, say, your left side, in a fraction of a second, the full amplitude will still "trigger" your right ear, without anywhere near as much of the "shadow" effect from your skull.

    As for subwoofers versus cars, well, there's a different sort of animal.

    Generally, you don't want to be aware of your subwoofer - you just want the sound to extend smoothly down to 5Hz (joke!) To be consious of the subwoofer means it isn't doing it's job right - it should be acoustically invisible.

    Most people with the cars that hop down the street, however, don't want to hear anything else. The entire car becomes a passive radiator, and generally is tuned to a higher frequency that has more physical impact upon the passengers. Except the entire car is radiating this frequency.

    This "one note" sort of bass is highly efficient, but is only good for making cars hop down the road!

    Did any of that help? I'm not sure...

    As an aside, one of the best books I've seen on this sort of subject in general is the "Master Handbook of Acoustics" by F. Alton Everest, and published by TAB Books. I've got a copy of the 3rd edition; it's now in its 4th edition. Generally, it covers more than you really want to know, but some libraries do have it for checkout.

    Leo Kerr
     
  10. Ken Burkstrum

    Ken Burkstrum Stunt Coordinator

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    Yes it helped, thank you very much.
     
  11. Arthur S

    Arthur S Cinematographer

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    Leo Kerr said, "Generally, you don't want to be AWARE of your subwoofer - you just want the sound to extend down to 5Hz (joke!)

    Not really a joke, just an expensive proposition to get bass down to 1, yes, one Hz.

    http://www.sonicflare.com/archives/e...-the-world.php
     
  12. ChrisWiggles

    ChrisWiggles Producer

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    It actually does come from a direction, but rather our hearing acuity is not sufficient (for the various reasons explained above) to localize that sound.

    Keep in mind that this assumes a quality subwoofer that is only producing the sounds it's supposed to. Many MANY subwoofers, especially cheaper and consumer subs and small subs will rattle/vibrate/port chuff/have lots of harmonics higher up in the freq range that will be localizable. But a good subwoofer properly crossed over and integrated with the main speakers will be very difficult indeed to localize.
     

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