A few conceptual questions about a tapered QW pipe design

[sujat]

Dear Stalwarts & Enthusiasts,

I couldn't find anything on the internet yet (or probably i missed recognising it on Martin King's wonderful site) to resolve a few conceptual questions on QW pipe design.

* I have read in more places than one, that when a driver is mounted in a QW pipe (or an open-ended terminated line of any geometry), the system resonance goes lower than the driver Fs (unlike in any other enclosure). On basis of this:

1. Is it safe to assume that the effective Mms that the driver's motor "sees" is greater than that as measured in free air and mentioned in the driver datasheet?

2. If yes, would the higher Mms also lower the high frequency extension as well as the sensitivity of a wide-band driver?

3. Does it have any indication that the column of air through the line/pipe acts as a constant load against the driver cone effectively helping control its excursion? Is this air column load effective only around the 1/4 wavelength tuning frequency, or consistent throughout the whole frequency band?

4. On basis of 3. above, for a given 8" driver, let us assume two boxes - a 54 litre simple closed box and a Tapered QW pipe with the same internal volume, i.e. 54 litres (with 1/4 wavelength tuning at approx. 45 hz.) - * Would the driver gain any advantage in the QW pipe (over the simple closed box) as far as its excursion-limited power handling in the lowermost operating frequency band is concerned (i.e. 100 hz and below)?

These questions have remained unresolved in my mind for a long time. I would be greatly thankful if anybody can shed some light on this.

Thanks in advance! ��
Sujat

 

[krakatoa]

I'm going to hope to answer your first (and second...) question: the pipe (or enclosure if you actually used is as an acoustic enclosure with a driver unit mounted on it) is an independent acoustic system and has it's own resonant behaviour independent of any source of sound; it's true that if a driver is placed in different places around the length of the pipe you get slightly different results, but the fundamental resonance remains where it was. The Mms of the driver does not change... and the resonance of a pipe can be at a frequency lower or above the Fs of the (any) driver because it's an independent acoustic system. The matching between the parameters of the pipe (an acoustic system) and the driver (another acoustic system) will result in the placement (how low, how high),power (how strong) and broadness (how wide is the freq band) of each of the harmonics of the pipe. Mms does not change so therefore the high freq extension of the driver cannot change. As far as SPL is concerned if you lower the tuning enough you might get the sensation you lost overlall SPL, this I know from practice (no measurements)... I don't know the theory behind it.

 

[sujat]

Krakatoa, i get the idea from what you have explained. Thank you for your insight, energy, and your well-considered inputs. I feel grateful for them.

This will help me better understand and visualise a QW pipe design. Thanks again!

 

[sujat]

As far as SPL is concerned if you lower the tuning enough you might get the sensation you lost overlall SPL, this I know from practice (no measurements)... I don't know the theory behind it.

Does a 1/4 wave tuning lower than the driver Fs have any advantages as far as control of cone excursion is concerned? Will it help the driver take more wattage before it reaches Xmax?

 

[Andrew LB]

I'm not sure if it will help, but i learned a lot about these cabinet designs from this video from Tech Ingredients.



YouTube


Even went ahead and built a pair of scaled down (folded variety) that i just finished today. I've only listened to them for about an hour so far but they are amazing. Two friends couldn't believe what they were hearing was coming from only two 4" full range Dayton RS100 drivers.

 

[SemperFi]

Making an electrical analogy, does a pipe model like L-C in series (very low impedance at resonance) or LC in parallel (very high impedance)?
If as LC in series one would think the air cushion behind the driver acts as if the box becomes huge at resonance, and if in parallel the box seems to get small as the air impedance goes up.
Not sure I am describing my question logically here...

 

[krakatoa]

Does a 1/4 wave tuning lower than the driver Fs have any advantages as far as control of cone excursion is concerned? Will it help the driver take more wattage before it reaches Xmax?

Since you "directed" the question to me I'm going to try to answer but my initial response was there because I have noticed nobody was willing to step in (that's not how I remembered this forum from 2012 when I was reading it regularly). I'm having the impression you are worried about tuning under Fs - that's normal, there is nothing wrong with that, but as you go lower (example: Fs=60 hz, tuning is 40 hz) you start losing from quality, the delay of the output of the pipe is larger and will loose speed or impulse response performance as it's called. I never worry about excursion because I don't like to push speakers like that (high SPL) but as you tune lower (play lower) you need more excursion; the fluffy thing in the pipe helps with this (if I remember correctly) but I don't care much about using it, it tends to absorb energy... from the sound, in the subwoofer world this is not a problem but when dealling with fullrange speakers you need to worry about "killing the energy" of the speaker.
If you are just making research and that's it let make a sugestion - build a TABAQ type enclosure and make the port adjustable in length, you will get an idea what happens if you tune lower. Ideally you would need to make it also larger as you tune lower but that's complicated to build (an adjustable enclosure).

 

[Scottmoose]

To a point. Re lack of response, it is possibly worth noting that not all of us can always be here 24/7; there are life factors beyond internet forums.

Be that as it may, there is no single answer to that question as it depends heavily on the pipe design. A vented box whether QW or Helmholtz derived will suppress cone motion at resonance, but this is by definition assuming a resonant pipe design rather than a heavily damped TL where acoustic output is suppressed and the system impedance & driver deflection behaviour will be rather more linear. While by nature related, they are basically different animals, designed differently, for different objectives (or should be for optimum results).

'Killing the energy' is largely speaking a myth resulting from poor design practice rather than damping in & of itself; far too many QW / TL designs have been acoustically undersized, killing gain, which is further reduced when damping was added to suppress unwanted harmonic modes. Excessive quantities of material in close proximity to the drive unit can also cause a mass-loading of the suspension / diaphragm, preventing both from moving / resonating as designed which can lead to a similar effect. This is relatively uncommon though since the densities required are usually somewhat higher than most people use in most designs. Still, the policy of not allowing a great deal of damping material to be in very close proximity to the drive unit is a fairly sound one.