At what frequency can a sub be located in the room? ; 60? 80? 100? 120Hz ?

Discussion in 'Speakers' started by Arthur S, Apr 18, 2004.

  1. Arthur S

    Arthur S Cinematographer

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    On another forum a very high ranking employee of a very well known sub maker states that "bass below 120Hz is non- directional (as long as it's low distortion"). Interestingly enough their High-Pass crossover is fixed at 80Hz with a 6db per octave roll off.

    This is not what I have heard for decades. The upper limit has always been 80Hz for non-directional bass. I believe the lower the crossover the better. That is why I threw in 60Hz. The sub I am considering uses 51Hz as its standard, but chips can be purchased for many other high pass frequencies including:28;34;43;62;75;91;109;134;and 155.

    Thanks

    Artie
     
  2. John F. Palacio

    John F. Palacio Supporting Actor

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    I believe that test have been done but some people dissagree with the results anyway.

    I would agree with "the lower the better" to a certain degree and with a couple of limitations:

    A) The main speakers must be able to go low enough and with low harmonic and intermodulation distortion for this to work.

    B) The amp powering the mains must "cut the mustard" as well.

    C) Not for implementation on HT where the LFE channel it s preset to higher settings.
     
  3. Kevin C Brown

    Kevin C Brown Producer

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    The "going number" is 80 Hz. But... I had a Yamaha receiver with a crossover of 90 Hx (and S&V/Stereo Review measured it to actually be 92 Hz), and I never noticed a problem. The last center channel speaker I had was purposefully bandwidth limited (to minimize reflections from the TV), and I used a 120 Hz crossover with it. Again, never heard any problems. (But I didn't do any specific tests for it either.)

    I think in general, people set their crossovers too low. One rule of thumb is that the crossover should be set 1/2 to 1 full octave above the -3 dB point for the speaker so that the imposed slope of the high pass filter applied to the signal going to a particular speaker is not interferred with by the natural roll off of the speaker. Otherwise, you get a dip (or a hole) in response which I personally think is more of a problem.
     
  4. Bob McElfresh

    Bob McElfresh Producer

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    Think of this:

    - You are in a large, open field on a swivel stool. Your eyes are closed and a subwoofer is playing 50 hz sounds. As you spin, can you tell where the sound is coming from?

    I think the answer is "Yes". You CAN tell the rough direction of low-frequency.

    But in a room you get a lot of subwoofer sounds reflected from the walls. The walls have natural frequencies they are better at reflecting the 80/70/60 hz sounds (or their harmonics).

    So I think "Bass is non-directional below XX hz" is a little too simple. A more accurate statement might be: "Bass frequencies below XX hz are so well reflected in most rooms, the human ear cannot distinguish the location of the direct vs the reflected sound."

    Or have there been studies in a anachonic chamber (no reflections) that show human hearing cannot locate sounds below some frequency?
     
  5. Kevin C Brown

    Kevin C Brown Producer

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    There was that 2 part sub test that S&V did a while back. The reviewer actually did his testing in an open field. I bet he could answer that. [​IMG]
     
  6. Lewis Besze

    Lewis Besze Producer

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    I believe TN did that test and his conclusion was that below 50hz you won't be able to tell the source when playing by it self,but if higher frequencies are present I.E. playing music or movies,then the threashold goes way higher around 100hz or so.
     
  7. Chu Gai

    Chu Gai Lead Actor

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    I'd have enjoyed Bob's scenario were there a woman running towards me with dubious morals.
    Consider Richard Pierce's reply which further embellished upon Tom Nousaine's reply that subwoofers below about 80 Hz are omnidirectional.
    Reply by DP
     
  8. Bob McElfresh

    Bob McElfresh Producer

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    So are frequencies below 50 hz "Locatable"? The link clarified the term we should be using, but does not answer the psycho-acoustic question.

    I do remember taking my daughter to a concert at a stadium and some radio station had a van full of woofers and blasting music in a large open parking lot. I cannot say it was not the higher-frequency sounds giving the positional clue, but the low-frequency sounds were like ocean-waves beating against my body. I could tell the rough direction the waves were coming from (but I cannot say it was because I heard the sound).
     
  9. Chu Gai

    Chu Gai Lead Actor

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    I think the ability to get a handle on the direction of LF sounds, especially in rooms, is pretty difficult. However, it's my opinion, that this needs to be expanded upon somewhat in order to shed additional light on the matter.

    Localization, or the ability to point one's finger and say the sound is coming from 'there' can be related to a number of reasons or issues. A couple are as follows:
    1) Let's say you've got a 7.1 setup and are watching some action movie with a fair amount of special effects which include explosions. These scenes don't necessarily have just 50 Hz or 35Hz information. There's also a lot of other frequencies, much higher and much more complex, that can come from behind, the left side, etc. that provide spatial cues that allow us to perceive the deep, low frequency subwoofer rumblings as actually coming from those directions. Well done, it's a pleasing illusion. In this case, it's not that we can unambiguously say the sub output is coming from a particular location, it's that we're guided by the skill of the recording engineers to 'think' they're coming from there.
    2) Consider the case where you're listening to music with a lot of low frequency information...maybe organ music. In this case not only do we have the strength of the fundamental but we also have the richness of the overtones that are produced by the main speakers. Again, our brain integrates all of this and creates the illusion that the sound is coming from a particular location even though were we just dealing with a 40 Hz fundamental, we'd have a tough time to say if it was coming from the left or the right.

    I kind of suspect that part of what you heard or experienced Bob was due to considerations such as I mentioned. Also, one has to consider that those low frequencies might not have been as low as you think they were which improves our ability to discriminate their direction.
     
  10. Ryan Schnacke

    Ryan Schnacke Supporting Actor

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    This is what makes the most sense to me:

    Sound tones seem to become difficult to localize at the same frequency where the wavelength approaches the dimensions of your room. Take for instance an 80Hz tone:

    1sec/80 * 1hr/3600sec * 700mi/hr * 5280ft/1mi = 12.8ft

    And now a 50Hz tone:
    1sec/50 * 1hr/3600sec * 700mi/hr * 5280ft/1mi = 20.5ft

    I don't know the exact speed of sound at sea level, but 700mph is close enough for govt work.

    The 80Hz wavelength might be just a little shorter than your room dimensions. So you might hear an entire cycle before it bounces of the walls. Or maybe not.

    But the 50Hz tone will almost definitely bounce off at least one wall before you hear a full cycle. And we all agree that it will be much more difficult to localize.

    So it seems to me that there's probably some significant relationship between the size of the room and the frequency range that is non-localizable (or at least difficult to localize).

    Now this assumes you've got a sub that completely suppresses all harmonics. If you're using the $19.99 Rump-Thumper special you found at the First-Monday flea market with 3/8" panels and no cross-bracing then its probably best to put it directly next to (preferrably between) your main speakers.
     
  11. ChrisWiggles

    ChrisWiggles Producer

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    First, you can't use many common consumer subs to do this, because many many subs are boomy, distorted, and generally poor, such that they emit a lot of sounds at other frequencies due to poor design, cabinet vibration, port chuffing, etc. Things in the room will also be vibrating/rattling.

    If you listen to a good subwoofer outside, you likewise will have difficulty locating the source. From what has been explained to me, it has much less to do with the reflections in the room, rather, diffraction around your head. High frequencies are blocked by your head and pinna as you move your head around, so that arrival time varies, and also intensity. Bass has a very long wavelength and will diffract around your head easily to reach your other ear at a loud enough volume that it is difficult to distinguish from the SPl in the other ear.

    I do not think that the room would have an appreciable effect on the localization of bass. The room affects high frequencies too (and the delay is the same, they all travel the same speed 1130 ft/sec), so I'm not sure your emphasis on the room is an issue. Bass is difficult to localize outside too(if you have a good sub), which is nature's giant anechoic chamber.
     
  12. ChrisWiggles

    ChrisWiggles Producer

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    Lord Raleigh's explanation (turn of the century):

    1) sound shadow (of your head) for high frequencies causes severe SPL difference in one ear to the other. Low freq, however, difference is very small due to diffraction around your head.

    2) later: because low Freq can still be localized, albeit usually much poorer compared to many high freq, hypothesized that phase difference between the two ears due to slight timing differences will allow some degree of localization.

    "experiments have confirmed the fact that for freq. up to about 1,000hz, localization occurs mainly through detection of the phase difference at the two ears (for steady sounds) or the difference in arrival time (for clicks). Above 4,000hz, localization by intensity takes over. Between 1,000 and 4,000hz, the accuracy of localization declines, wich a high error rate around 3,000hz demonstrating that the two mechanisms do not overlap appreciably.

    At high frequencies (5,000hz and upward) the pinna aids in localization of a sound, particularly in distinguising between sound coming from the front or back, because it receives sound with slightly greater efficiency from the front.

    An important corrolary to sound localization is the precedence effect (haas effect), which applies to efforts to localize a sound source in a room. If similar sounds arrive within about 35ms, the apparent direction of the sound source is the direction from which the first arriving sound comes."

    The room is not discussed as an important part in the ability to localize a particular frequency, though the room acoustics are important in the specific ability to localize things correctly in that space.

    quotes from: the Science of Sound 3rd ed.
     
  13. Kevin C Brown

    Kevin C Brown Producer

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    I like that explanation. The longer the wavelength, hence the lower freq, the smaller a phase shift you get because of the distance bewteen you ears. (Smaller portion of the wavelength.)

    I don't believe the size of the room has a lot to do with it either. That would mean then that *depending* on the room size, then the cutoff for being able to localize low freqs would change. And I've never heard that stated.

    But then some interesting things follow:

    1) A person with a bigger head would theoretically be able to localize freqs lower than a person with a smaller head. [​IMG]

    2) But seriously, if the phase shift idea holds, then it would also mean it's easier to localize low freqs if they are to either side of you vs in front or in back. Bigger phase shifts to the sides.

    Some people say that they can localize low freqs based on the direction of air pressure changes. I don't know if I believe this one or not though. Could be tested easily enough though.
     

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