Tempest low freq performance questions/concerns

Discussion in 'Home Theater Projects' started by JamesThompson, Dec 18, 2003.

  1. JamesThompson

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    Finally got around the creating a test tone disc and running some low freq sine waves through the new Tempest.
    I have the budget Radio Shack SPL meter and I think I'm having some performance issues with my new sub. I had originally written it off as "bad room", but I'm not so sure now. I created the test tone disc as 5 sec tracks of sine waves starting at 90Hz, dropping 10Hz until 40Hz, then dropping 5Hz until 20Hz.

    The wife is in the house, so I'm running the tests in the ~90db range from about a meter. I'm seeing a tremendous drop-off around 30-35Hz. This kind of fall-off can't be normal. [​IMG]

    I flipped the sub upside down, ran a couple tones and noticed the mother of all air leaks on the bottom panel. I took this opportunity to adjust the poly-fil away from my ports and caulk the bottom panel. I'll let the caulk dry tonight and give it a whirl tomorrow with my fingers crossed.

    I broke out the multi-meter and made sure I saw 4 ohms at the speaker, and that checked out ok. I saw LOR:Two Towers the other day and all sounded well, but I've never had a sub of this size and don't really know what to expect.

    Any ideas on why my sub seems to wimp out below 35Hz? Could jammed up ports and air leaks cause this type of behavior? Do I just need to crank up the amp gain?

    TIA for any help. BTW, this board rocks!

    - JamesT
     
  2. Joseph Sabato

    Joseph Sabato Stunt Coordinator

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    James,

    The RS meter gets inaccurate at very low frequencies and you have to add to the numbers that you get from the RS meter:

    12Hz add 16.5dB
    16Hz add 11.5dB
    20Hz add 7.5dB
    25Hz add 5dB
    31.5Hz add 3dB
    40Hz add 2.5dB
    50Hz add 1.5dB
    63Hz add 1.5dB
    80Hz add 1.5dB
    100Hz add 2dB
    125Hz add .5dB


    There is a list around that is broken down in smaller increments of Frequency, but I do not remember off hand where it is.

    With all that, it may not explain everything. Are you taking your measurements near field (1 meter from the sub) or at your listening position? There can be some pretty big peaks and dips depending on where you sit and the sub's placement. If you are getting that near field it may indicate another problem...leaks in a ported system will change the tune of the box...if your tuning frequency went up because of the leaks you could be seeing the results you are getting. Vented subs fall off at an 18 db / octave rate so if your new tuning frequency had you down 3db @ 35HZ, you would be down 21 db at 17 - another 11.5 db for meter error...well you get the picture. Hope this (plus caulk!) helps.
     
  3. ThomasW

    ThomasW Cinematographer

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  4. Paul Spencer

    Paul Spencer Stunt Coordinator

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    If I'm not mistaken, nearfield measurements are not taken a metre away, but as close as you can get without touching virtually. As I understand, measurements are taken from just outside the port and just next to the driver. This would normally be done with software, which can then put them together into one chart. Speaker workshop is one program which is designed for this, and it is free.

    From the Speaker Workshop site:

    nearfield measurements of a sub:

    combining nearfield port and driver info

    I have not done this yet myself, but this program seems to do all that you would need.

    Keep in mind that at 1m the room acoustics will have a significant impact on the response. To eliminate room gain effect, gated measurements are used, which takes the response before room reflections have a chance to alter the response. This is the cheap way of getting a quasi-anechoic response.

    The impact of the room on the response is such that the only use of in room measurements is to see where you need to use eq to get the bass response right. (this is not to give the sub any more extension than it has, but to smooth out peaks introduced by the room).
     
  5. JamesThompson

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    Thanks for the advice everybody. The air leak I had on my bottom panel was massive. After taking the driver out and sticking my head in the hole, I could see light (!) coming through the spot where the leak was. I caulked up the crack and will try again this afternoon.

    The thing is, I had no idea the leak was there until I turned the sub upside down and played some sine waves through it. Is there a better way to look for leaks than feeling and listening to all seams on the enclosure?

    My biggest concern wasn't neccesarily the meter value dropping around 35Hz, but the driver went from a nice constant/solid (?) tone, to more of a waffing. I could live with things if I had to rebuild the enclosure, but I certainly don't want to damage the equipment.

    I'll try and find some acoustic software to measure and log some data so I can get some real numbers. Any suggestions?

    - James
     
  6. Paul Spencer

    Paul Spencer Stunt Coordinator

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    James,

    play a 10 Hz test tone with the driver moving at xmax or close to it to find air leaks. In a sealed box it is even harder. You might need to glue a small block over that hole on the inside. Down low the sound will change from a constant tone to something you can feel as a pressure on your ears more than a sound.
     
  7. JamesThompson

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    Great idea on gluing in a "filler" block. If my caulk doesn't work I may line every seam with trim on the inside to be double sure all leaks are stopped.

    I'll also give the low freq air leak search a try.

    Thanks for the hints,

    JamesT
     

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