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Fitting HT subwoofers in a pre-defined space

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  • billfitzmaurice
    replied
    I advise damping direct radiating subs even though it might not be needed, based on how cheap damping is, and that while it may not help it can't hurt.

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  • BertusS
    replied
    Thank you billfitzmaurtice and devnull for taking the time to respond. Much appreciated. My main concern is if these resonances are excited by higher order harmonics caused by driver distortion, especially if these resonances, being so close to each other, have a combined effect. I don't have a gut feel for what to expect. I looked at my enclosure dimensions where I made a 2" provision for feet. Initially had cones on felt pads in mind to rest on the laminate floor. I have now decided to rather go for lower profile rubber feet and that would allow for some additional volume, resulting in requiring a shorter port length and a greater separation of the resonance frequencies. I am still planning on lining at least the back wall and the wall opposite the port opening with something like a mattress cover or cotton batting, (still reading about this part) which would hopefully further reduce the risk.

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  • devnull
    replied
    I wouldn't worry about it. This is a subwoofer so your AVR is going to low pass the signal going to it. I'm going to guess that it's using at least a 12dB an octave filter, if not an 18 or 24. Let's guess again that you're using 100 Hz as the crossover freq to the subwoofer. That means that at 400 Hz you are down at least 24dB which shouldn't be audible. If you have problems you can always set a low pass filter on the subwoofer amp.

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  • billfitzmaurice
    replied
    Those resonances would be of concern if they were within, or at least close to, the speaker pass band, but they aren't. A potential resonance only resonates if it's excited.

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  • BertusS
    replied
    May have found a way to increase the net box size to 5.8ft^3 resulting in a shorter port. 5.8 ft^3 is coincidentally also parts express' recommended net volume. I will also certainly follow billfitsmaurice's recommendation from another post to line the inside with sound absorbing material - especially the risky surfaces. Thanks again to billfitsmaurice and devnull for taking the time to respond to my posts.

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  • BertusS
    replied
    The required length of the port and the distance between the baffle and enclosure backwall was a concern. So I entered the enclosure dimensions in "BoxNotes" https://www.subwoofer-builder.com/freesoft.htm and it calculated numbers that are a bit too close for comfort to my newbie mind. The side to side resonances in the calculation are incorrect because of the angle of the one side, but that is not what I was after.


    Click image for larger version  Name:	SubWooferDrawing3.jpg Views:	1 Size:	87.2 KB ID:	1419524

    The question: How far should the 1st port resonance be from any internal enclosure resonance? The graphic at the bottom of the screenshot suggest that I may be okay: - No?

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  • BertusS
    replied
    Originally posted by devnull View Post

    Lowest frequency in the Behringer software is 20Hz. There is a way around this. Set a steep high pass filter, something like a 24 or 48dB Butterworth at 20Hz, Then set a narrow Q 3dB peq filter at 20Hz to bring back the rolloff of the high pass filter down to 16Hz. I doubt you'll hear or feel anything adding it back in.

    You won't be able to tell until you measure and don't forget you can EQ out peaks but you can't EQ up nulls.
    Thank you devnull - I'm looking forward to play with the future toy!

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  • devnull
    replied
    Originally posted by BertusS View Post
    Do you know if the high pass filter in the NX3000 can be tuned? Ideally I would prefer something around 16Hz

    Yes, I am also going to investigate if I could increase the surface area of heatsink.
    Lowest frequency in the Behringer software is 20Hz. There is a way around this. Set a steep high pass filter, something like a 24 or 48dB Butterworth at 20Hz, Then set a narrow Q 3dB peq filter at 20Hz to bring back the rolloff of the high pass filter down to 16Hz. I doubt you'll hear or feel anything adding it back in.

    You won't be able to tell until you measure and don't forget you can EQ out peaks but you can't EQ up nulls.

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  • billfitzmaurice
    replied
    The port exit needs to be less than 1/4 wavelength from the cone. Since the shortest wavelength in the port pass band is roughly 28 feet you don't have a problem.

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  • BertusS
    replied
    Thanks to billfitsmaurice's welcomed suggestion there are now more options for fitting a significant shorter single 16" port. Unfortunately, in order to keep a 4" clear area around the inside opening it may still require an elbow, either 45 or 90 degrees.

    Theoretically the port can be placed on any location on the front baffle as indicated by the green area in the attached drawing. What are the guidelines for placing the port in this case? I have read about coupling between the driver and having the port too far from the driver not being good for the sound at the listening position. So close may be good but not to close? 1 port diameter? Closer to the floor or higher up (we are talking about a port tuned to 18 Hz). Probably away from ear level (i.e. closer to the floor)

    Also, I assume it is preferable that the inside opening should not face the back of the driver. My thoughts are that the inside port should face some sound absorbing material to reduce the chance of any higher order component exiting the port resonance. Any other suggestions?

    Edit: This has kind of been discussed before:

    http://techtalk.parts-express.com/fo...-tube-location

    Last edited by BertusS; 08-14-2019, 11:02 PM.

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  • BertusS
    replied
    Originally posted by devnull View Post
    1. Take advantage of the DSP and use a high pass filter to limit cone excursion
    Do you know if the high pass filter in the NX3000 can be tuned? Ideally I would prefer something around 16Hz

    2. If you replace the fan don't change the air flow direction of it. It blows across the power transistors and won't work as effectively if it's turned around to exhaust the air from inside
    Yes, I am also going to investigate if I could increase the surface area of heatsink.

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  • devnull
    replied
    1. Take advantage of the DSP and use a high pass filter to limit cone excursion
    2. If you replace the fan don't change the air flow direction of it. It blows across the power transistors and won't work as effectively if it's turned around to exhaust the air from inside

    Leave a comment:


  • billfitzmaurice
    replied
    At the levels required for chuffing to occur I can't imagine being able to distinguish it.

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  • BertusS
    replied
    Originally posted by billfitzmaurice View Post
    If you have room for the ported boxes by all means do it. If it turns out that your room gain makes the additional extension of ported unnecessary you can always seal the cabs. On the subject of the ports, a single 4" is probably sufficient. It's unlikely that you'd ever run with more than 200 watts actual program power, and that's only potentially problematic below 20Hz, where it's even more unlikely you'll put more than 50 watts actual program power into it.
    Thanks, that will certainly free up some net volume and reduce the required port length with the associated increase in the first port resonance frequency. You are correct, feeding the driver with 100W results in an SPL between 102 and 104 dB and a max airport velocity of 26m/s at the 16.5 Hz (port is tuned to 18 Hz). The SPL is almost at maximum THX levels considering that there will be some room gain and there are 2 drivers in the room. I was wondering about under what real life (music or movies) any potential chuffing (in this kind of setup) will actually be heard over the other sound content.

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  • billfitzmaurice
    replied
    If you have room for the ported boxes by all means do it. If it turns out that your room gain makes the additional extension of ported unnecessary you can always seal the cabs. On the subject of the ports, a single 4" is probably sufficient. It's unlikely that you'd ever run with more than 200 watts actual program power, and that's only potentially problematic below 20Hz, where it's even more unlikely you'll put more than 50 watts actual program power into it.

    Leave a comment:

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