Group Delay and Damping Factor - IB Application

Edward J M

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Sep 22, 2002
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Maybe I should post this over at the "Cult Of The Infinitely Baffled", but I know some of you frequent both forums.

I'm helping a friend design a twin Tempest IB subwoofer in his basement (he has an adjacent and completely isolated storage cellar that is perfect).

We've been discussing transient response and group delay and I'm not sure what these characteristics are in an IB application as compared to a sealed acoustic suspension design.

Since the IB driver is essentially operating in free air with no enclosure loading, does it take longer for the drive motor and suspension to stop moving after the signal stops? If this is true, would it manifest itself as "bass ring" or "overhang" and is this a common characteristic associated with IB subwoofers?

Would an amp with a really high damping factor (like a Crown) help minimize this, if it really is concern? Thanks.

Regards,

Ed
 

Rick Guynn

Second Unit
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Mar 23, 1999
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If I understand the concept correctly, (I'm sure Thomas will be along to comment on this anyway
) The damping you are referring to is the system Q. With IB's, the system Q is the same as the Qts of the driver being used. A 'ringing' would seem to indicate an underdamped system. As long as you use a driver with a reasonable Q value, you shouldn't have a problem.

Oh, and the key to doing this with the Tempest is to run each driver on one voice coil only. You can even 'dial in' the Q by putting a resistor across the second voice coil. Check out the white paper on the subject at Adire's website.

RG
 

Edward J M

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Thanks, Rick. I'll look in total system Q and how it relates to the damping of the driver motor. Appreciate the tip on the single coil - we were going to do that, but I like the idea of a variable Q to optimize.

Ed
 

ThomasW

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Nov 6, 1999
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Edward

IB and dipole designs have the best transient response of all subs that use conventional cone drivers. The only speakers with better bass transient response are planars.

IMO amps with higher damping factors improve the SQ from subwoofers.
 

Edward J M

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Sep 22, 2002
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Thanks, Thomas. Appreciate you getting back to me on this.

We are proceeding with a basic IB transition box mounted to the wall with twin opposed Tempests. The amp selection is not confirmed, but we were looking at a 2 channel Samson S1000 touring amp rated at 500WPC into 4 ohms or 340 WPC into 8 ohms.

Ed
 

Javier_Huerta

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Mar 9, 2002
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About RDO - something interesting to remember is that, if the Qes of the driver doubles, efficiency drops by 3dB (if I remember correctly).
 

Rick Guynn

Second Unit
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Mar 23, 1999
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Javier, you are correct. The problem arises (for the Tempest anyway) in that you don't really want to run both voice coils, because then your Q would be ~0.36... way overdamped. So you have to give up some headroom for the SQ.

RG
 

Brian Bunge

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Also keep in mind that you can get drivers specifically designed for IB use from Parts Express. IIRC, the Qts is around .65.
 

Jack Gilvey

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Mar 13, 1999
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So you have to give up some headroom for the SQ
Actually, headroom will be determined by the Vd, and not Pe, in the deep bass. IB loading requires very little power to reach Xmax down deep, so the "loss of efficiency" from RDO is a non-issue as I see it. The "efficiency rating" seems not to apply too well down deep, anyway.
 

ThomasW

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Yes lower "Q" designs are by defintion over damped. And in theory a "Q" of 0.7 is the 'ideal' driver for IB or dipole use. However I strongly suggest that people compare/experiment to see whether or not they think that there is a significant audible difference between high and low "Q" setups.

My personal experience is that lower "Q" systems (overdamped systems) sound fine. If a person wants/needs more 'boom' just turn up the volume and or dial in a touch of EQ.
 

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