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Designing port at peak driver power
Designing port at peak driver power
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Old 15th February 2019, 11:07 PM   #1
jimmyjazz is offline jimmyjazz  United States
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Default Designing port at peak driver power

I'm toying with a ported design in WinISD, and in an effort to get the port length down to something even close to reasonable my port velocity is going up well beyond the rule of thumb I've heard of ~ 5% of sound speed. However, my input power into the system is the peak rating of the woofers. I'm seeing about 33 m/sec in the low bass near the tuning frequency, or almost 10% of sound speed.

Am I worrying for no good reason? Under what conditions would I see peak power into the box, playing real world music? In this case peak is 2X the drivers' RMS power rating. Would such a velocity actually lead to audible noise on a quick transient? Would the ear likely pick up on it? Would it be better to characterize port velocity at the RMS rating of the drivers?

Thanks in advance for any advice.
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Old 16th February 2019, 09:22 AM   #2
David Morison is offline David Morison  Scotland
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Depends a lot on what you'll be listening to on the speakers.

In lots of genres of music, the extreme LF around tuning frequency is definitely not the loudest part of the tracks, so with such music you'd likely get away with the kind of airspeed modelling you describe.

You could try putting a few tracks through a spectrum analyzer to see what LF content they have - the free software Audacity has a perfectly useable module for that, for example.
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Old 16th February 2019, 02:50 PM   #3
jimmyjazz is offline jimmyjazz  United States
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That makes sense. I have Audacity -- I can do that.

To get a feel for worst-case scenario, do you have any suggestions as to what types of music would actually have a fair amount of sub-40 Hz content? Pipe organ? EDM? I'm pretty sure most standard rock 'n roll is above that, since a low E on a bass guitar is 41 Hz, and kick drums don't tend to go real deep.
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Old 16th February 2019, 04:46 PM   #4
TBTL is offline TBTL  Germany
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Some woofers do not work in vented enclosures because a port with reasonable velocities simply does not fit. This is especially true for high excursion subwoofers in small enclosures. What woofers are you using?
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Old 16th February 2019, 05:14 PM   #5
Jerms is offline Jerms  United Kingdom
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Quote:
Originally Posted by jimmyjazz View Post
That makes sense. I have Audacity -- I can do that.

To get a feel for worst-case scenario, do you have any suggestions as to what types of music would actually have a fair amount of sub-40 Hz content? Pipe organ? EDM? I'm pretty sure most standard rock 'n roll is above that, since a low E on a bass guitar is 41 Hz, and kick drums don't tend to go real deep.
Evanescence - Bring Me To Life, full orchestra plus organ. Sheffield Labs sound disk, AV files of locomotives starting up etc....
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Old 16th February 2019, 07:02 PM   #6
GM is offline GM  United States
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Look at worst case Xmax peak SPL and use this power as max @ ~4% as this way you have an extra bit of an audible distortion that the driver is on the fast track to bouncing off the back plate.

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Old 16th February 2019, 08:22 PM   #7
jimmyjazz is offline jimmyjazz  United States
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Quote:
Originally Posted by TBTL View Post
Some woofers do not work in vented enclosures because a port with reasonable velocities simply does not fit. This is especially true for high excursion subwoofers in small enclosures. What woofers are you using?
I'm looking at a few of the small Tang Band products.
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Old 16th February 2019, 10:08 PM   #8
David Morison is offline David Morison  Scotland
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Originally Posted by Jerms View Post
Evanescence - Bring Me To Life, full orchestra plus organ. Sheffield Labs sound disk, AV files of locomotives starting up etc....
Interesting - have had that Evanescence album for ages, never thought to check it - looks like 30Hz is only 7dB down from the peak, which is pretty high for pop/rock.

Haven't checked out too many of these myself, but there have been various threads on various fora discussing the topic over time, herewith a couple from one forum I hang out at a lot:
Songs that are strong below 35Hz
dB Technologies S30N vs RCF SUB 8006-AS Comparison
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File Type: png Evanescence spectrum.png (85.5 KB, 42 views)
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Old Yesterday, 12:59 AM   #9
jimmyjazz is offline jimmyjazz  United States
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Thanks for the suggestions, everyone.
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Old Yesterday, 06:33 PM   #10
TBTL is offline TBTL  Germany
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If cost is no object, you could consider using a passive radiator instead of a port. Dual opposed passive radiators prevent large vibrations from being transmitted to the floor.
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