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Old 7th December 2011, 12:59 PM   #1
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Default Speaker "Q" factor for open back enclosure

This is an offshoot of another discussion I had started. The question is this... There are three configurations I've seen used for a speaker.

1. a Sealed enclosure
2. an bass reflex enclosure with a ducted port,
3. An open back box, such as often used om guitar amplifiers.

I have an idea of what speaker specifications to look for in the first two cases. But what kind of specifications (especially in the total Q factor) should I be looking for in that third case?
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Old 7th December 2011, 01:36 PM   #2
oshifis is offline oshifis  Hungary
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Quote:
Originally Posted by Peter Pan View Post
This is an offshoot of another discussion I had started. The question is this... There are three configurations I've seen used for a speaker.

1. a Sealed enclosure
2. an bass reflex enclosure with a ducted port,
3. An open back box, such as often used om guitar amplifiers.

I have an idea of what speaker specifications to look for in the first two cases. But what kind of specifications (especially in the total Q factor) should I be looking for in that third case?
Xmax is the most important factor here. Qts should be between 0.8 and 1. And of course fs as low as possible.
There is an other factor not in the specs: When the loudspeaker is driven by a high-amplitude low-frequency signal, it should not produce any creaking sound.
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Old 7th December 2011, 02:55 PM   #3
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My answer on the open baffle woofer specs is 'it depends'.

If you wish to use a passive crossover then a higher Q (0.7 or some greater) would be advantageous. On the other hand if you use an active crossover with EQ then the woofer's Q can be lower and more driver possibilities are available. Likewise for open baffle, you will likely need a woofer with higher sensitivity than your mid or tweeter as you will use some of that woofer sensitivity to account for the dipole rolloff on the lower end of the band. It is worth noting that most successful open baffle speaker designs use active crossovers with EQ.

You can read some of the design trades on various websites from Martin J. King, John K, and Sigfried Linkwitz.

Quarter Wavelength Loudspeaker Design
Music and Design
Linkwitz Lab - Loudspeaker Design
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Old 7th December 2011, 05:47 PM   #4
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@Jim: It is my fault here that I didn't express at the outset that the project I have in mind calls for a single full range speaker, and a small one in this case. The reason I mentioned the guitar amp was because the project is both intended for a guitar, and because by necessity I will likely be dealing with the equivalent of a small open back guitar amp box. In any case, there will be no separate tweeter in this case, not any passive crossovers. I've run across lots of catalogs with full range speakers, and many will usually recommend whether the speaker's specs make it best suited for sealed enclosure use, as opposed to a tuned port. But for an open back design, and a full range speaker with no crossover, shouldn't I be looking for very LOW total Q?
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Old 7th December 2011, 06:25 PM   #5
GM is offline GM  United States
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Assuming the old rules still apply, the lower the Fs, the lower the Qts and vice versa for open back amps. With all the various EQ available today though, seems like the desired power handling, break-up modes BW for the app would be the determining factors.

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Old 7th December 2011, 08:21 PM   #6
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Why aren't you just looking for guitar speakers? That is a specific application that would be better addressed in other forums.


Quote:
Originally Posted by Peter Pan View Post
@Jim: It is my fault here that I didn't express at the outset that the project I have in mind calls for a single full range speaker, and a small one in this case. The reason I mentioned the guitar amp was because the project is both intended for a guitar, and because by necessity I will likely be dealing with the equivalent of a small open back guitar amp box. In any case, there will be no separate tweeter in this case, not any passive crossovers. I've run across lots of catalogs with full range speakers, and many will usually recommend whether the speaker's specs make it best suited for sealed enclosure use, as opposed to a tuned port. But for an open back design, and a full range speaker with no crossover, shouldn't I be looking for very LOW total Q?
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Old 7th December 2011, 09:08 PM   #7
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@fastbike1 Because this is miniature... they don't make speakers specifically called "guitar speakers" this small. BUT... for future reference, thanks! I do have one bigger and more normal sized guitar amp project with a terrible speaker (budget was very low!). Maybe I can get some better choices for that project there. ! :-)
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Old 7th December 2011, 09:46 PM   #8
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Quote:
Originally Posted by Peter Pan View Post
@fastbike1 Because this is miniature... they don't make speakers specifically called "guitar speakers" this small. BUT... for future reference, thanks! I do have one bigger and more normal sized guitar amp project with a terrible speaker (budget was very low!). Maybe I can get some better choices for that project there. ! :-)
Perhaps you could spell it out, that you want this speaker mounted IN the guitar.

For others' edification, here's the earlier thread you're referencing:

Speaker Inside a guitar: reccomendations

This would indeed be smaller than "guitar amplifier" speakers, and a small full-range or small "someothing" would be appropriate. Frequency response needs to be about 80Hz to 15kHz, and IMHO even being approximately flat to those frequencies is vital.

I think I WOULD suggest "open back" presuming removing a substantial part of the guitar's back doesn't affect the integrity of the guitar. A sealed or bass-reflex enclosure would be too volnerable to feedback.

The speaker's resonant frequency might work well around 80-90 Hz, which would emphasize the lowest notes, as they are being attentuated by the small "cabinet."
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Old 8th December 2011, 01:13 AM   #9
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@benb: Having framed the question that way and not gotten the answers I wanted about speaker specs, I felt the application was too distracting. The only answer I got there about speaker QTS rating was to look for higher numbers, which is only recommended for sealed enclosures on the sites I've been on that sell speakers. So i was trying to re-frame the question, specifically in reference the standard enclosure types.

Back to your recommendation, wouldn't a speaker with a lower Qts tend to be flatter in response? Or at least avoid resonant peaks in the low register?
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