Subwoofer technical question

Discussion in 'Archived Threads 2001-2004' started by marc paolella, Jan 22, 2003.

  1. marc paolella

    marc paolella Auditioning

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    I'm trying to understand this.

    The best sub-woofers can output down to below 20 Hz. However, it is my understanding that with very low frequencies, the wave length of the produced sound can exceed the dimensions of some typical rooms.

    So let's say for argument that a $2,000 sub outputs usable volume at 18 Hz. However, let's assume the wavelength of sound at 18 Hz is 30 feet. If the subwoofer is located in a 10 x 10 room:

    Can I even hear the sound since a full wavelength of 18Hz sound is never realized?

    If the answer is yes, I'd like to know why. Any acoustical engineer types hang out here?

    If the answer is no, then isn't it a waste of money to spend megabucks on a sub if the room is smaller then the wavelength of the lowest frequencies that the sub can produce?
     
  2. Anthony.Lin

    Anthony.Lin Stunt Coordinator

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    Don't quote me specifically, but here are a few things I'd like to point out:

    The wavelength of a 18Hz sound is about 19 feet (which is still unable to fit into a 10X10X8 room).

    The human ear cannot hear any frequencies below 20Hz (and even 20Hz is questionable).

    It's to my understanding that waves can reflect off of walls and the floor, so a 18Hz wave can be completed after reflecting off a few walls. It's in this loss of energy from the reflection that I'm presuming the walls vibrate.

    I'm not too sure if my third point is correct, but that's what I'm assuming. Maybe others can comment further.
     
  3. Dustin B

    Dustin B Producer

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  4. John F. Palacio

    John F. Palacio Supporting Actor

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  5. marc paolella

    marc paolella Auditioning

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    Thanks guys. Sorry to bring up something that's been covered. Next time I'll try some searches first. Anyway, at least I can rest easy knowing that spending megabucks is in harmony with the laws of physics. That's a relief.

    -Marc
     
  6. Anthony.Lin

    Anthony.Lin Stunt Coordinator

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    ohh.. correction on my part.. that was 19 meters, not feet. (used 340m/sec as speed of sound, but didn't change the units around)
     
  7. John Garcia

    John Garcia Executive Producer

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    The wave may be longer than the room, but it will reflect and remain in the room for the most part, so you will still get low frequencies. The room shape and size will determine how that wave is bounced, and will affect how well a particular frequency is heard (or in the case of 18Hz, felt). The way the wave bounces within the room, based on location and other factors, will cause some cancellation or standing waves, for given frequencies, otherwise known as room modes. This is why placement is far more important than many people seem to understand.
     
  8. Doug BW

    Doug BW Stunt Coordinator

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    Your ear does not directly sense the entire sound wave at a single moment in time, which would somehow require the entire sound wave to be present and in tact at any given moment. Instead, what you hear is the sound wave moving the air as the wave passes by your ear. So the only part of the sound wave that "matters" is the tiny bit of the wave that is pushing or pulling on your ear drum at any given moment. What's going on with the rest of the wave...for example, whether or not it fits in the room...doesn't matter.

    Here's another question to think about. Is it possible to drive a car at 60 miles per hour if you don't drive for an hour?
     
  9. Edward J M

    Edward J M Cinematographer

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    I have run a 1/12 octave frequency sweep from 10-100 Hz on my 20-39PC+. I used a factory certified and recently calibrated B&K Model 2205 SPL meter mounted on a tripod near the couch of my HT room.

    With one port plugged and the SS filter set to 12 Hz, my FR is essentially flat to 11 Hz. In fact it was 2 dB louder at 11 Hz than it was at 13 Hz! I have very fortuitous room acoustics.

    At any rate, my playback level for the sweep was around 80 dB. I can feel a pressure on my ears from 11-16 Hz. I can definitely hear the tones at 17 Hz and above.

    Regards,

    Ed
     
  10. Mark Seaton

    Mark Seaton Supporting Actor

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    I am glad someone remembered to point out the headphone logic. If the assertion was true, we could not hear anything from headphones.

    "Sound" is percieved through fluctuations in air pressure. Waves and wavelengths are important to how sound *propigates* and interacts with boundaries, but not in how it is heard.

    We can most certainly hear very low frequencies, but the threshold of audibility increases quickly at low frequencies. So, we can hear 20Hz, and even lower, but it has to be at high levels for it to be more than a pant-leg flapping experience.
     
  11. Cees Alons

    Cees Alons Moderator
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    Apparently this is a difficult subject to many. But actually it should be simple.

    The ears do not perceive "waves", but periodical differences in air-pressure.

    It doesn't matter that a room is shorter than a full wave: the sub will higher and lower the airpressure in the room like a pump and you will be able to perfectly hear that. This has nothing to do with reflection to the walls. In fact, for the best results, you generally can use as little reflection as can be realized in your room!

    In practice it's good that the room is smaller than the wavelelength, because (1) it helps diminishing the directivity (that's why a sub is any good: most of the wavelengths below 100Hz are substantially larger than the room and your head) and (2) it helps avoiding standing waves and (3) it eventually makes the presence of multiple subs easier because it diminishes the chance of interference.

    Cees
     
  12. John Garcia

    John Garcia Executive Producer

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    Ideally, you want little or no reflection. In an untreated room, you will almost certainly have standing waves somewhere. The "wave" does bounce and remain in the room for the most part. When a particular frequency is activated, the air in the room resonates. As the wave bounces off of a wall in a symetrical pattern (as it intersects itself), standing waves will occur. I did not say that the wave itself is what is heard, I said that how the soundwaves interact with the room will affect the resulting sound.
     
  13. Cees Alons

    Cees Alons Moderator
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    In practice, when the wavelength is greater than the dimensions of the room, a standing wave is not possible.

    Cees
     
  14. John Garcia

    John Garcia Executive Producer

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    Standing waves are possible in ANY room, short of a room with treatments or an anechoic room (or specifically designed room). They are a result of reflected sound, and the most common place will be in corners where sound, particularly bass, tends to "collect" due to it's wavelength.

    Ever seen a professional recording studio that did not have wall treatments such as this? (specifically for ceiling corners) :

    [​IMG]
     
  15. Cees Alons

    Cees Alons Moderator
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    Of course John, standing waves are possible in any room, but the wavelength has to be shorter than the largest room dimension (distance between two reflective surfaces).

    Or else, the wave has no room to stand.


    Cees
     
  16. Dustin B

    Dustin B Producer

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    What about multiples as well as 1/2 and 1/4 fractions?
     
  17. John Garcia

    John Garcia Executive Producer

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    Not to be too argumentative, but if the wave cannot form to create a cancellation pattern, then it would be reasonable by this same thinking, that it would not exist as sound at all. The low extension of a sub is affected, but not completely limited by, room dimensions. A sub tuned to 20Hz, should still be able to reach that frequency in a typical room, regardless of the longest dimension of the room (within reason).
     
  18. Richard Greene

    Richard Greene Stunt Coordinator

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    In answer to the original question:

    The driver inside the enclosure vibrates, which "vibrates"
    the air in your listening room, which vibrates your eardrums. Sometimes there is also a vibrating column of air (port) in the loop.

    The smaller the room, the louder the deep bass due to room pressurization (aka "room gain" or "cabin gain") below a certain frequency related to room dimensions.
    For a rectangular room, room gain begins at a frequency roughly equal to 565/largest room dimension in feet and increases roughly 12dB/octave below that frequency. That means the smaller the room, the louder the output at 20Hz., for one example.

    Whether you can hear 18Hz. or not depends on your hearing ability -- at reasonable volumes, some people can, but others can't. Fortunately, all people can FEEL the pressurization at 18Hz. even if they can't HEAR very well at that frequency. Even if your ears were able to hear 18Hz., you would probably need a sound pressure level of about 75dB to exceed the threshold of audibility.

    It's difficult to measure SPL at 20Hz. and below because
    a sound meter can not differentiate between the sounds caused by 20Hz., 40Hz. harmonic distortion, walls resonating, objects in the home rattling, ports chuffing and the wife hollering: "turn it down". So the reading on the sound meter may reflect far more than the SPL at 20Hz.
    ... not to mention that sound meters tend to be less than accurate at frequency extremes ... and program content under 20Hz. is rare.
     

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