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Does Qtc value matter if we use DSP?

ppataki

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I have designed a few speakers already (mainly fullrange models) and I always use DSP (Dirac Live 3.x) to correct them
I was wondering that:
- if I design my speaker to have a Qtc value of 0.707 then use DSP to boost the low end accordingly
- then if I design the same with Qtc=0.5 for example and use DSP again to boost the low end
What will be the difference?
Obviously the cabinet size for the Qtc=0.5 model will be considerably bigger but both cabinets will be DSP corrected anyway
For the bigger cabinet less low-end boost will be needed for sure
I guess what I try to understand is if there is a point to design with lower Qtc...
The bigger cabinet might be able to reproduce bass instruments better? (e.g. doublebass or big drum)

Any builds/comments would be appreciated
Thank you
 

Trdat

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I am really interested in this one as well so I'm going to eagerly wait for the experts to chime in. But, on that note I will say that I have asked this question on many occasions in various forums.

My understanding is that as soon as you change the roll off regardless of the cabinet size it will be hard to discern between two cabinets with the same roll off(different sized cabinets). The person who mentioned this said that on the premise of double blind tests in subwoofers. Of course hard to believe at first as an amatuer but becoming more versed on the inadubility of sub QTC, transients and group delay figures you get to understand that the comment has some truth to it, if not is entirely true. Then also understading that the sub drivers motor structure has probably more do to with quality sound then the QTC itself and once DSP is applied the cabinet doesn't end up mattering as much anyway. I digress and will let the experts comment on the particulars that need science behind it.

My subwoofer cabinet is .4 QTC and I use DSP to give me a flat response to 30hz and automatically I feel some difference and also when rolling off my .7QTC sub to a roll off of one similar to .4 I also feel the difference. This tells me that using DSP gives you the desired roll off and not the cabinet(when using DSP) but I still prefer making a larger cabinet(lower QTC) "if you prefer the dry side of things when it comes to bass" then using DSP and not relying on the DSP to give you the desired bass response. this is preferable in my view. Now, what that concludes I have no idea just something I have become conscious of.

I have also come across many who claim the benefits to larger cabinets(midrange as well), again I prefer them to but not sure if there is any science behind it.

Ultimately, I always go larger(if you prefer what is related to the subjective side of larger cabinets that is) and then use DSP.
 
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SIY

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If I understand correctly, it's a matter of power handling. I use EQ on second order subs with low Qtc but am thermally limited.
 

Wolf

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With regards to Qtc, it is the amount of damping the box alignment has. According to the old equations, the higher the number, the more there is a chance of overshoot and having peaks in the rolloff. While I understand that DSP can change the rolloff via EQ, I do not think that the Qtc of the system changes, and therefore overshoot is still possible. We are speaking sealed 12dB/oct rolloffs, then 0.707 is the lowest F3 possible, while the larger 0.5 Qtc is critically damped and at a slightly higher F3. I would avoid Qtc of above 0.707 even with DSP, so as to avoid overshoot.
 

Wolf

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Also of note- If you have a sealed box, and EQ/DSP up the bass to reflect the extension of what the driver would do in a vented alignment, then the group delay that would normally be present in the vented alignment will be equal to that of the sealed box with EQ/DSP.
 

antcollinet

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You are going to consume headroom on your amplifier. If you have to boost the SPL by (say) 10dB, then you are going to need to x10 your amp output power in that frequency region.

(I think :))
 

dc655321

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While I understand that DSP can change the rolloff via EQ, I do not think that the Qtc of the system changes, and therefore overshoot is still possible.

While this is possible with a pole/zero compensation filter (aka Linkwitz Transform), that tool seems most often used to change both rolloff frequency (Fc?) and Qtc.

See here for an example (that thread may be of general interest to the OP).
 

Trdat

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Also of note- If you have a sealed box, and EQ/DSP up the bass to reflect the extension of what the driver would do in a vented alignment, then the group delay that would normally be present in the vented alignment will be equal to that of the sealed box with EQ/DSP.

Just to clarify, the DSP/EQ vented alingment will now have group delay matching a vented alignment in the sealed box is that what your saying? Cause my investigation into this topic has led me to undertand exactly that. Once you EQ you change the alignment including its symptoms.
 

voodooless

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If I understand correctly, it's a matter of power handling. I use EQ on second order subs with low Qtc but am thermally limited.

This is exactly correct. A DSP let’s you shape the response in any way you want. A bigger box only leads to better power handling. In case xmax is the limiting factor and not thermal power, the box size is largely irrelevant, as long as you have enough power.
 

Wolf

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You are going to consume headroom on your amplifier. If you have to boost the SPL by (say) 10dB, then you are going to need to x10 your amp output power in that frequency region.

(I think :))

Yes, this is true!
 

Wolf

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Just to clarify, the DSP/EQ vented alingment will now have group delay matching a vented alignment in the sealed box is that what your saying? Cause my investigation into this topic has led me to undertand exactly that. Once you EQ you change the alignment including its symptoms.

The sealed box with DSP that emulates the vented alignment, will have matched group delay to the actual vented box.
 

Wolf

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This is exactly correct. A DSP let’s you shape the response in any way you want. A bigger box only leads to better power handling. In case xmax is the limiting factor and not thermal power, the box size is largely irrelevant, as long as you have enough power.

A bigger box does not yield better power handling. Power handling as it is is a thermal spec. Mechanical power handling is usually a lot lower number, varying with box size and alignment. Smaller boxes usually can handle more power due to the tighter air spring in the cabinet. Above tuning, vented boxes usually operate more efficiently, and can reduce the amount of xmax a driver is required to yield. I would never say larger boxes have better power handling.
 

voodooless

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The sealed box with DSP that emulates the vented alignment, will have matched group delay to the actual vented box.

Yes, group delay is a function of frequency response. Add some room gain, and the group delay of the sub is mostly meaningless anyway.

There is little point I obsessing over these things, best to fix the in-room response. I usually tune for extension and max output, response you can (and must) alway fix using DSP.
 

voodooless

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A bigger box does not yield better power handling. Power handling as it is is a thermal spec. Mechanical power handling is usually a lot lower number, varying with box size and alignment. Smaller boxes usually can handle more power due to the tighter air spring in the cabinet. Above tuning, vented boxes usually operate more efficiently, and can reduce the amount of xmax a driver is required to yield. I would never say larger boxes have better power handling.

I think we mean the same thing. You need less power for the same excursion. So if you are power limited, the larger box can have an advantage. That is not always the case however.
 

Wolf

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I think I understand your meaning now. I was referencing how much power the driver can handle in situ, while you were saying there was only so much power available.

I always look at it from the driver's perspective, as that keeps it happy, and let's the builder know its capabilities. I also shoot for output level of 105 to 110 in a sub design if possible, and feel 100dB should be minimum capability. This keeps peaks as attainable and not painful for the driver, and not clipping the amp.
 

bigjacko

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Decrease box volume will increase stiffness of air inside the box, this will reduce sensitivity primarily at low frequency where stiffness dominate the mass spring system. If the driver has very high BL and Kms, the stiffness from the air will contribute less to the whole stiffness, so sensitivity does not reduce that much. Those are all theory stuff, I don't have any real world experience with lots of drivers, so I can't really say those theories are the whole picture.

From playing with WinISD, I found that the excursion and group delay will be the same if transfer functions are the same, regardless of ported or closed box or dsp. Closed box does not exchange air with outside so the heat will built up where ported will reach equilibrium.
 
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ppataki

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Thank you @digitalfrost this article is very useful (to me at least)
I definitely do not want more distortion so I will go for larger box size (= lower Q)
I guess I got my question answered
 

fluid

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Thank you @digitalfrost this article is very useful (to me at least)
I definitely do not want more distortion so I will go for larger box size (= lower Q)
I guess I got my question answered
Distortion in drivers used within their linear region tends to come from excursion. The limit of a drivers acoustic output is it's volume displacement Vd. This is excursion (xmax) multiplied by radiating area (Sd). A bigger box allows less amplifier power to be applied to get the same SPL level, that is the trade, Hoffman's Iron law, small, loud, low you can only pick two.

When speakers are placed in a room and used below the transition frequency the only thing that matters is output and position. The rest can be equalised and will almost always have to be equalised to get the best response. As frequency goes down the room is ever more the dominant factor. Q of the speaker becomes meaningless.

If this relates to the corner speaker you were planning, concentrate on how the baffle and drivers will interact with the wall as this will matter much more than how much volume is inside the cabinet.
 
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ppataki

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@fluid Yes it relates to that specifically but it was also a more generic question that I wanted to clarify for future projects as well
Can you please elaborate your last sentence? Do you mean acoustic treatment on the walls around the speakers?
Thank you
 
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