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Old 3rd June 2003, 12:09 AM   #1
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Default Back wave vs OB vs Front wave

A newbie question please: For low frequencies, what are the respective objectives for each of the speaker configurations i.e. closed box, OB, TL, front-loaded horns, back-loaded horns, aperiodic, etc???

Seems like each design attempts to control the low frequency waves but aren't some waves out of phase? For example, closed box has no back wave therefore in-phase front wave, right? OB has both front and back wave, opposite phase and cancellation? Horn-loaded back wave attempts to overcome the in-phase front wave with concentrated, coherent wave, no? TL has dampened, delayed (like back horn) back wave but out of phase....

How does all this work?

Thanks,
Rick
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Old 3rd June 2003, 01:44 AM   #2
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By OB, I assume you mean Open Back. These are frequently called dipoles and I have little experience with them, although people are using them for subwoofers. My understanding is that you get cancellation if the baffle the driver is mounted on is less than half the wavelength of the bass note being played. That would mean a board 5 feet wide would have cancellation under 100 Hz. My understanding is that things are not so simple in real world rooms, where the back wave is bouncing off a nearby wall. That's all I can tell you-there are a couple of dipole threads underway-I am sure someone can help you.

I guess we all know that in the Closed Box there is only the front wave. No cancellation.

The Bass Reflex. At the box tuning frequency, the back wave is reversed. That is why the Bass Reflex used to be called the Phase Inverter enclosure. With the back wave reversed, it adds to the front wave. Above or below the tuning frequency, the Bass Reflex is somewhat out of phase with the front wave. Above the tuning frequency though, the responses can blend with the cone output so a well tuned box does not have a big hill with two troughs in the response curve. Below the tuning frequency, the Bass Reflex quickly reverts to being a dipole.

Open end Transmission Line. I am no expert, but I believe the length of line determines the phase inverter frequency, instead of line volume and length of port, like a Bass Reflex. An unstuffed Transmission Line has an impedance curve similar to the Bass Reflex. When proplerly done, the Transmission Line has been traditionally considered to give a smoother, tighter bass than the Bass Reflex.

I have litle experience with horns, except I like the sound of front loaded horns-both high and low frequency.
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Old 3rd June 2003, 02:49 PM   #3
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Thanks Kelticwizard,

Is the phase and delay shift noticable for the back wave designs? Seems like there has been so much emphasis on correct time alignment yet the back wave designs inherently are not time aligned?

Apparently, it is not noticable?

Rick
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Old 3rd June 2003, 06:32 PM   #4
chops is offline chops  United States
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If you want to read up on "OB" designs, (open back, open baffle, or dipole are a few different terms), look at the thread I stated a while back at My first Dipole Subwoofer!!!. You can also find everything there is to know about dipole designs at Linkwitz Labs. Siegfried Linkwitz pretty much sums things up about dipoles and how they work. Up until a few months ago, I knew very, very little about dipoles and how they worked until I joined this great and informative forum.

You will learn plenty here!!
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Old 4th June 2003, 12:41 AM   #5
MJK is offline MJK  United States
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For a bass reflex enclosure, when the port output is at a maximum (system tuning frequency), the phase difference is 90 degrees with respect to the driver. The driver's output is significantly attenuated so that almost all of the response comes from the port, this can be seen as a notch in the driver displacement curve. Below this frequency, the driver and the port are 180 degrees out of phase which leads to the 24 dB/octave low frequency roll-off. Above this frequency, the output from the driver and port approach being in phase but the port output is dropping off rapidly so all you hear is the driver response.

In a TL, when the first quarter wavelength standing wave resonance is reached the output at the open end has a 90 degree phase difference from the driver. Again the driver's output is strongly attenuated as the standing wave in the line controls the driver's motion. Below this frequency the driver and open end are 180 degrees out of phase leading to a 24 dB/octave roll-off at the very lowest frequencies. If the TL is stuffed, the transition from pass band to 24 dB/octave roll-off will be rounded and gradual. This has fooled many people into thinking that a TL has a 12 or 18 dB/octave low frequency roll-off. Above the fundamental there will be a series of higher harmonic standing waves which will again attenuate the driver output and produce open end output that is +/- 90 degrees out of phase with the driver.

As far as time alignment problems, when the SPL is being produced by the open end of a TL there is virtually no output being generated by the driver so there will not be a time alignment issue. But there can be echos produced by a TL if the stuffing is not adequate due to part of the sound wave coming from the open end being reflected back into the line, traveling to the closed end and being reflected again, then traveling back down the line to exit through the open end some time later. The second echo will be attenuated due to the fiber damping. Oh, this second echo goes through the same cycle again and again until is has been completely attenuated.

Hope that helps,
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