[jules]

I've got a couple of different methods for determining box volume from Thiele Small parameters but no mention is made of "box Q" unless it sometimes has a different name.

Could someone help me out with a method/formula for calculating box Q and if you're feeling a bit expansive, maybe talk about desirable values for box Q [or just direct me to a reputable expert type website]

[poseidonsvoice]

I've got a couple of different methods for determining box volume from Thiele Small parameters but no mention is made of "box Q" unless it sometimes has a different name.

Could someone help me out with a method/formula for calculating box Q and if you're feeling a bit expansive, maybe talk about desirable values for box Q [or just direct me to a reputable expert type website]

Although this is primarily a subwoofer site...the method to calculate box Q, and desirable values for box Q are the same  for various LF alignments even if you are building a regular MT, MTM, etc...so here it is:

http://www.diysubwoofers.org/

There is also plenty of software out there to help you make box calculations based on measured T/S parameters...look at Parts Express: http://www.parts-express.com/cat/measurement-design-tools/25

Best,

Anand.

[andy_c]

I've got a couple of different methods for determining box volume from Thiele Small parameters but no mention is made of "box Q" unless it sometimes has a different name.

Could someone help me out with a method/formula for calculating box Q and if you're feeling a bit expansive, maybe talk about desirable values for box Q [or just direct me to a reputable expert type website]

By "box Q", I'm assuming you mean what Small calls Q_L, which is a measure of how lossy the box itself is.  This loss can be due to such things as air leaks around the driver (such as the dust cap), air leaks in the seams of the box, and power losses in the stuffing material of the box.

As far as design, the situation with Q_L is somewhat of a paradox.  It can only be measured after the box is built, but its value is also a required input to the design of the box!  This means an estimate must be used in the design.  This isn't as bad as it sounds, as it has only a mild effect on system performance (unless the box is extremely lossy).

Small gives these guidelines:
Very large box (such as a very large subwoofer): Q_L = 5
Medium to large box: Q_L = 7
Small box: Q_L = 10

[poseidonsvoice]

Sorry, I probably misinterpreted what you said, and thought you were talking about Qtc which is really the Q as it relates both the box, LF alignment and driver together.

My bad 

Hopefully the links will serve some useful purpose...

Best,
Anand.

[jules]

Thanks Andy and Anand,

the reason I asked was that I'd been browsing through some notes I had from a reputable speaker designer who'd suggested that for a particular pair of 6" mids in MTM [low Q_ts of .25], the following applied:

Volume .4 cubic feet, cab res. 121Hz, vent freq 66.7 ---> Q of .61
volume  .7 cubic feet, cab res. 192Hz, vent freq. 65 ---> Q of .5
volume  1.0 cubic foot, cab res. 88Hz, vent frq.  55Hz ---> Q of .43

It's the "Q" he's referring to here that I was asking about.

I'm studying the info on the sites you mentioned and I'm reading through Vance Dickason on the topic. I'm not sure if he makes it better or worse though it does make my head hurt. I might have to seek professional help in the end [for my head and the box design probably].

thanks again

Jules

[MJK]

Deleted.

[andy_c]

the reason I asked was that I'd been browsing through some notes I had from a reputable speaker designer who'd suggested that for a particular pair of 6" mids in MTM [low Q_ts of .25], the following applied:

Volume .4 cubic feet, cab res. 121Hz, vent freq 66.7 ---> Q of .61
volume  .7 cubic feet, cab res. 192Hz, vent freq. 65 ---> Q of .5
volume  1.0 cubic foot, cab res. 88Hz, vent frq.  55Hz ---> Q of .43

It's the "Q" he's referring to here that I was asking about.

Oops, looks like I'm the one who misinterpreted your question, 

The "Q" being referred to there is called Q_TC, also sometimes known as the "total system Q".  To see its effect, Google "Closed-Box Loudspeaker Systems Part 1: Analysis", and download the PDF file you find.  Have a look at Fig. 4 of that article.  That shows the idealized frequency response (what it would be in an anechoic chamber if the voice coil inductance were zero) with varying Q_TC.  The "T" stands for "total", while the "C" denotes "closed box".  You'll also see Q_TS mentioned.  The "S" denotes free air, so this is a property of the driver alone, while Q_TC is for the driver in a box.  For Q_TC < 0.707, the frequency response has no peaking, but there will be some ringing in the time domain until Q_TC becomes 0.5 (the so-called "critically-damped" condition), below which the ringing goes away.

Q = 0.707 is called "maximally flat" (flattest possible frequency response: no sagging, no peaking)
Q > 0.5 is called "underdamped"
Q = 0.5 is called "critically damped"
Q < 0.5 is called "overdamped"

Note that maximally flat is also considered underdamped, as it has some ringing.

[jules]

It was my question that was not clear!

Thanks Anand, I'll have a look at the site you recommend and weigh things up.

I'm planning an active, ported, three way which simplifies some things though most box plans seem to assume conventional passive Xovers.

Jules