Can someone please explain what is meant when for example a box is tuned to 30hz with a Q of 2? What is meant by 'with a Q of 2'? Much appreciated.
Basically "Q" defines the shape of the response in the transition area to the roll-off. A Q=2 ("underdamped") indicates the response is peaked above the passband response before it ultimately rolls off. Normally you would see lower Q values as a target response. Example: 0.707 indicates a maximally flat passband response that extends as low as possible with no peaking above the nominal level.
There are many references on this if you Google.
Dave.
There are many references on this if you Google.
Dave.
The Wikipedia link delt mostly with Q as it's used in electronics. The OP asked about box tuning and it's relationship with Q.
Below are some "Q" curves plotted as dB speaker box response vs freq. Q is short for Qtc A Thiele/Small paramater. Box Q can be measured with a DMM by feeding a varying test tone to the box and finding the point along the freq. spectrum where measured impedance initially peaks as freq. is increased. That will be pretty close to Qtc.
Here is a link the includes an explanation of Q or Qtc and other T/S parameters.
Speaker Box Calculations
Below are some "Q" curves plotted as dB speaker box response vs freq. Q is short for Qtc A Thiele/Small paramater. Box Q can be measured with a DMM by feeding a varying test tone to the box and finding the point along the freq. spectrum where measured impedance initially peaks as freq. is increased. That will be pretty close to Qtc.
Here is a link the includes an explanation of Q or Qtc and other T/S parameters.
Speaker Box Calculations
Attachments
Thank you for responding to my query. The difficulty I'm having is how to implement a particular QTC. I'm reading references to the application of alternate QTCs and tuning to a set box size when the definition of QTC itself is suggestive of a static/fixed outcome dictated by said box size. Does this mean that for a given box size, a driver with particular parameters must be selected in order to achieve the desired QTC? Or vice versa in that by choosing a specific box size for a particular driver, the desired QTC can be achieved?
If you scroll to the bottom of the link I provided, there is a calculator there that you can 'play with' by varying the driver T/S parameters and see the outcome on a closed or ported box. It doesn't work in reverse.
I think you need a good introductory text on the subject of speaker box design. One I bought years ago got me into the hobby. It's still available. The author is David Weems.
http://www.amazon.com/Designing-Building-Testing-Speaker-Projects/dp/007069429X
All you need in the math dept. is a basic knowledge of algebra. You'll soon find yourself manipulating the design formulas to get the answers you seek.
Good luck and enjoy the hobby....
I think you need a good introductory text on the subject of speaker box design. One I bought years ago got me into the hobby. It's still available. The author is David Weems.
http://www.amazon.com/Designing-Building-Testing-Speaker-Projects/dp/007069429X
All you need in the math dept. is a basic knowledge of algebra. You'll soon find yourself manipulating the design formulas to get the answers you seek.
Good luck and enjoy the hobby....
May I ask where this information came from, or is it general? By itself, an enclosure can't really have a Q factor unless you're referring to the physical wood itself. The driver can have its own Q factor, and the driver + box system has its own Q factor, but not j ust the box.
For example, if you have a .5cu ft ported box and put an 18" woofer in it, that is going to be an extremely high Q system. If you then put a 4" full range driver in the same box, the Q is going to be extremely low. The main determining factor here is the mass of the driver and equivalent stiffness of the driver vs. the stiffness of the air in the box. In the case of the 18", the air in the box appears to be much stiffer because you have a huge cone compressing a tiny amount of area, and a huge mass (the cone and moving parts) that is doing the compressing. That is going to make the system underdamped. Likewise, if you put in the 4" driver, the box will appear to have much less stiffness to the 4" driver. You have less cone area trying to compress all of the air in the box, and much less mass to keep under control. This system is likely to be overdamped.
Just as a tip on how hairy this can get: in electronics, we call critically damped Q = 1. In loudspeakers, critically damped is Q = 0.5. It gets dicey pretty quickly. 🙂
For example, if you have a .5cu ft ported box and put an 18" woofer in it, that is going to be an extremely high Q system. If you then put a 4" full range driver in the same box, the Q is going to be extremely low. The main determining factor here is the mass of the driver and equivalent stiffness of the driver vs. the stiffness of the air in the box. In the case of the 18", the air in the box appears to be much stiffer because you have a huge cone compressing a tiny amount of area, and a huge mass (the cone and moving parts) that is doing the compressing. That is going to make the system underdamped. Likewise, if you put in the 4" driver, the box will appear to have much less stiffness to the 4" driver. You have less cone area trying to compress all of the air in the box, and much less mass to keep under control. This system is likely to be overdamped.
Just as a tip on how hairy this can get: in electronics, we call critically damped Q = 1. In loudspeakers, critically damped is Q = 0.5. It gets dicey pretty quickly. 🙂
It was implied, but not explicitly stated, that box Q or Qtc is the result of the box AND woofer working together. The link and it's calculator I provided in Poat #4, show how woofer properties will affect box response.
A song written many years ago stated it nicely: "You can't have one without the other"....
Leman23 was absolutely correct about the confusion between electronics Q and loudspeaker Q. IIRC, in electronics, Q has always been just Q. In speakerbuilding, Thiele and Small's Qtc has been shortened to Q in many discussions.
A song written many years ago stated it nicely: "You can't have one without the other"....
Leman23 was absolutely correct about the confusion between electronics Q and loudspeaker Q. IIRC, in electronics, Q has always been just Q. In speakerbuilding, Thiele and Small's Qtc has been shortened to Q in many discussions.
Thank you Lemans23 for the practical explanation, just what I was hoping for. And the others as well.
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