Multiple Small Subs - Geddes Approach - Page 157 - diyAudio
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Old 30th January 2013, 06:56 PM   #1561
gedlee is offline gedlee  United States
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Quote:
Originally Posted by Omholt View Post
What studies show that lower sensitivity drivers, for let's say frequencies below 200 Hz, leads to inferiour sound quality?
Do we know this for sure?
Isn't it possible to avoid power compression with multiple high quality smaller drivers and suited amplifier power?
Below 200 Hz I don't think that anyone is saying that lower sensitivity leads to inferior sound unless the design limits are exceeded. But this later issue is far more likely to occur with a low efficiency design than a high efficiency one.

It is possible to "avoid power compression with multiple high quality smaller drivers" and this is fine when interactions between drivers is not an issue like it is <200 Hz. At 2 kHz this is not at all the case and multiple drivers create more problems than they solve. Fewer is better above about 500-1000 Hz.
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Old 8th February 2013, 04:19 AM   #1562
AllenB is offline AllenB  Australia
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Quote:
Originally Posted by Gedlee
A poorly damped room would have a very high spatial variation - since your assumption is incorrect, the rest of the discussion is moot.
OK, I guess I've been off the game for a time. I'm implying that multiple sources can (even should) offer damping as a package. If this is correct I guess the only questions I have remaining from this is: Can the sources compensate for a lack of passive room damping and if not completely: could a room with solid walls permit low spatial variation that is damped through the sources and at the risk of digging further into the same hole, would these walls then increase reverberation time and hence level?
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Old 8th February 2013, 02:15 PM   #1563
gedlee is offline gedlee  United States
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When multiple subs are setup properly, it can be shown the turning one off usually raises the response in the room in some band of frequencies. This means that this source is acting as active damping in this region. I am not sure this is a complete answer to your question, but it is part of it. It is still always best o have as much LF damping as you can.
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Old 8th February 2013, 04:17 PM   #1564
AllenB is offline AllenB  Australia
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Ah yes, and damping is related to level is related to reverberation time and of course a sub can only damp what it can see.

BTW I thought you might like to see what I'm up to.
Attached Images
File Type: png 1.png (467.2 KB, 306 views)
File Type: png 2.png (442.6 KB, 308 views)
File Type: png 3.png (421.9 KB, 301 views)
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Old 8th February 2013, 04:26 PM   #1565
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I think the term "active damping" should be limited to motional feedback "traps".

Problem here in the land of engineering wannabees, is that displays are rarely better than two- or maybe three-factorial.

Like many little amateur HiFi enthusiasts, I have spent time reversing poles on woofers, by choice or through ignorance. I'd say it is the exceptional super-symmetrical set-up when it makes any over-all clear difference and ultimately a matter of taste in sound.

What I mean is that - even with the simplest of test tones - the sound varies inch by inch and freq by freq around the room. Granted, there's some very low freq when it does matter, maybe, but even so the range above may favor the other polarity. But that kind of thinking doesn't show up on simple graphs.

BTW, nobody could be as enthusiastic about the real-room value of multiple HETEROGENEOUS kinds of subs in heterogeneous locations as I am.

Ben
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Old 8th February 2013, 04:27 PM   #1566
gedlee is offline gedlee  United States
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You need a radius at the outside edge. That is very important.
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Old 10th February 2013, 12:31 AM   #1567
AllenB is offline AllenB  Australia
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Thank you. The picture shows only half of the length constructed.

For a given axial length (throat to mouth), increasing the radius decreases the length of the waveguide proper. Is both this length and the radius a tradeoff against the lowest frequencies to be produced? When I try to prove this I always find inconsistencies.
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Old 17th February 2013, 07:40 PM   #1568
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Dr Geddes - I know you've done a lot of research into bass traps and room treatment. You were the first person to clue me into the idea of using Helmholtz bass traps.

I have a question -

Can one use unpowered sealed subs as bass traps?

For instance, one could construct a helmholtz bass trap, but calculating the volume of the box and the volume of the helmholtz resonator is a mystery to me. (Is it the same math as a vented box? I do not know.)

But it occurred to me that one could simply build a sealed box, pick a woofer with the proper FS, and leave the box unpowered. It seems to me that the woofer cone would act like a passive radiator, and would cancel out sound in the room at that frequency.

Click the image to open in full size.
For instance, a Pyle PPA 15" costs $50.99 delivered, and has an FS that's a good match for the room mode I'm fighting.

Thoughts?
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Old 17th February 2013, 07:43 PM   #1569
Omholt is offline Omholt  Norway
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Quote:
Originally Posted by Patrick Bateman View Post
Dr Geddes - I know you've done a lot of research into bass traps and room treatment. You were the first person to clue me into the idea of using Helmholtz bass traps.

I have a question -

Can one use unpowered sealed subs as bass traps?

For instance, one could construct a helmholtz bass trap, but calculating the volume of the box and the volume of the helmholtz resonator is a mystery to me. (Is it the same math as a vented box? I do not know.)

But it occurred to me that one could simply build a sealed box, pick a woofer with the proper FS, and leave the box unpowered. It seems to me that the woofer cone would act like a passive radiator, and would cancel out sound in the room at that frequency.

For instance, a Pyle PPA 15" costs $50.99 delivered, and has an FS that's a good match for the room mode I'm fighting.

Thoughts?
Take a look at this:
Untitled Document
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Old 17th February 2013, 07:46 PM   #1570
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Quote:
Originally Posted by Patrick Bateman View Post
Thoughts?
The Q is too low to be a good absorber IMO.

Now, if you design a TH with massive peaks in the same locations as the room modes...

I've actually measured dips in FR caused by my test TH being left unpowered and unconnected in my living room. Even published them somewhere, I think. Purposely designing one for this purpose may work.
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