dealing with modal resonances in midrange transmission lines

[thadman]

As long as the line is stuffed, the resonances along the length of the line should be attenuated significantly due to the high particle velocity at the open end of the tube...but what about the resonances that propagate along the radii?

Is there anyway to deal with the radial resonances other than building appropriately sized diffusors in the interior? Wouldn't the particle velocity be 0 at the walls and the pressure be at maximum, thus negating any absorptive effects of foam for radial modal resonances.

[planet10]

I stuff a midrange line "backwards"... heaby at the terminus getting lighter as you approach the driver. Get it right and you have an aperiodic line (or near to it) with little or no resonant structure.

dave

[thadman]

How does this effect the radial modes? Varying the density of stuffing along the length of the line would intuitively only effect those modes who propagate down the length, not the radial modes.

[GM]

What radial modes? If the TL is a round pipe there will be an infinite number of standing waves, but all at theoretically the same frequency that the stuffing will attenuate.

If you're referring to the pipe's physical vibration modes, obviously it must be rigid/damped enough for them to either be below or above its pass-band, so normally it would be 'U' clamped (mass loaded) to the back of the baffle.

GM

[thadman]

Originally posted by GM
What radial modes?
GM

Lets assume we're using a tube that is 7 inches in diameter, to accommodate a 6.5" midrange. Without stuffing, the first radial resonance should be present at 957hz (14") just as the first room mode occurs 1 octave below the longest dimension of the room. If we desire a 2khz crossover point, this will not suffice since there will be resonances present throughout our desired bandwidth.

[planet10]

Originally posted by thadman
we're using a tube that is 7 inches in diameter, to accommodate a 6.5" midrange.

That tube is not likely big enuff...

dave

[thadman]

Originally posted by planet10


That tube is not likely big enuff...

dave

thats independent of the principle being outlined

[planet10]

Originally posted by thadman
Lets assume we're using a tube that is 7 inches in diameter, to accommodate a 6.5" midrange. Without stuffing, the first radial resonance should be present at 957hz (14") just as the first room mode occurs 1 octave below the longest dimension of the room. If we desire a 2khz crossover point, this will not suffice since there will be resonances present throughout our desired bandwidth.

2 things you can do

1/ what you do damping wise in any other box

2/ taper the line -- in both directions if you want to minmize side-to-side resonances.

dave

[thadman]

Originally posted by planet10


2 things you can do

1/ what you do damping wise in any other box

2/ taper the line -- in both directions if you want to minmize side-to-side resonances.

dave

1. Enclosure related (mechanical) resonances are not the issue, I'm worried about acoustic resonance modes created by the transmission line itself.

2. Tapering the line merely redistributes the resonance modes and makes them simply harder to predict. If you tapered the walls of your listening room, would that get rid of your bass modes?

[planet10]

Originally posted by thadman
1. Enclosure related (mechanical) resonances are not the issue, I'm worried about acoustic resonance modes created by the transmission line itself.

I wasn't talking about mechanical resonances (adding damping material to box walls that is intended to counter any mechanical ringing is, IMO, often self-defeating).... you (attempt to) deal with them by lining the enclosure walls and a bit of damping in the centre of the box

2. Tapering the line merely redistributes the resonance modes and makes them simply harder to predict. If you tapered the walls of your listening room, would that get rid of your bass modes?

The same amount of energy is spread across a much wider range of frequencies, reducing any modes magnitude, snf letting the damping in 1/ habe an easier go at killing them.

The TL itself concentrates the main modes into the length of the line.

dave