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can a vent which doesn't support sine play music?
can a vent which doesn't support sine play music?
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Old 12th June 2010, 02:53 AM   #1
freddi is offline freddi  United States
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can a vent which doesn't support sine play music?
Default can a vent which doesn't support sine play music?

back in the 1950's and probably into the 1960's it was not uncommon to see distributed vents in the form of slits or small diameter holes cut into a speaker panel. Vent velocities could be high - - support of sine-wave from what I've seen rather poor. I once tuned a 3.6 cubic foot reflex loaded with a 15" woofer to around 40Hz small signal with 42-3/8" holes at at perhaps 10 watts input around Fb, it made a pretty good "fan".

I don't know if the technique is still used. Can the result on music be subjectively good with distributed port within a speaker's passband? What's happening dynamically with the vents friction and turbulence effects? What's happening in music vs steady state to allow such a thing to play reasonably well and how does that depend upon the music genre and types of instruments? How often does music reach steady state or sine condition?


here's a wheezy 6 sawblade slit vent Karlson 12 copy blowing out a candle with 70Hz sine

YouTube - High air velocities in a Karlson 12 distributed slit vent exhausts candle
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Old 12th June 2010, 03:45 AM   #2
planet10 is offline planet10  Canada
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can a vent which doesn't support sine play music?
Seems pretty similar idea to what i'm doing in my miniOnkens... works really well. I do make the ports long as part of the receipe. It is like adding an R to the vent.

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Old 13th June 2010, 03:23 AM   #3
fudce is offline fudce  United States
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I've seen many small holes with foam behind them used as an aperiodic vent. You would have to model the 42-3/8" holes through whatever thickness of material to determine if they were acting as a resonant tuned vent. Maybe someone else knows of box modeling software that lets you set an arbitrary number of vents.
As to whether high vent velocities are good or even okay, I can say no with pretty high certainty. The caveat is whether blowing out birthday cake candles is "high" velocity. I seem to recall that bad velocity is defined as some fraction of the speed of sound, you can probably blow out candles well below that threshold. IIRC, the issue is laminar flow through the vent. If the vent velocity gets too high, you loose laminar air flow and start to get non-linear turbulence. Non-linear turbulence is going to mean non-linear resistance at the port/vent and, one would expect, non-linear distortion of the audio.

This has got me thinking though... enough small holes of the right length should be able to give a resonant tune with reasonable vent velocity. If these holes were spaced uniformly along the back (or front, or maybe all sides...) of the enclosure, the port output would be relatively diffuse. While diffuse is the opposite of the holy grail point source, it is a lot like a planar transducer which many people enjoy. With a conventional driver and diffuse port design, you might have a little of both worlds. So many ideas, so little time .
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Old 13th June 2010, 06:11 AM   #4
freddi is offline freddi  United States
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can a vent which doesn't support sine play music?
George Augspurger's "ventwrk" that ran in dos would allow that kind of calculation. I would imagine the large signal tuning changes from small signal.

here's two Karlson 12 I compared some years back with 50Hz sine outdoors and around 1 watt input, each with an Eminence Beta 12cx and having the same overall bulk and similar tunings. The 6-slit vent version produced 15dB more 3rd and ~11dB more 6th harmonic than the earlier Karlson with a single vent.

many speakers in those days had higher Qts than what might be desired and Karlson although with recommendations were sold as mix and match with whatever driver as with other empty cabinets.

the question I have - does music hit the steady state enough to have these overtones present with sine to appear with the music? Some folks say the distributed vent sounds "better"
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