Stagerred L/R reflex tuning? - diyAudio
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Old 11th November 2012, 06:16 PM   #1
ro9397 is offline ro9397  United States
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Default Stagerred L/R reflex tuning?

Boundary effects largely contribute to overall performance. The most significant negative contribution of boundary effects below the Schroeder frequency is timing: a bass note (or notes) continue reflecting between boundaries after the succeeding note appears. Often the result is two simultaneous egregiously discordant bass notes where should be one perfectly harmonious bass note. Obviously the transient attack of the new proper note is weakened or destroyed.

Some (including myself) say the above phenomena negatively affects performance more than anything else in the entire reproduction chain (for chains assembled with moderate care).

Bass is the music's foundation. Imagine music written with one bass line (the quantity of bass players is irrelevant): instead of one bass music line, for some notes the listener hears two bass lines, one line is exactly as the author intended, the other bass line wrongly increases the time signature of certain notes, with fixed, repeated relationships between pitch and the length of the time signature distortion. What music author, what conductor or band leader, what recording engineer, would tolerate such a state of affairs with live music? None that are sober.

Yet "audiophiles" accept it hook line and sinker as SOP. To the present point.

Toward the goal of minimizing bass mode affects: Why is it not SOP to stagger the reflex tuning of reflex enclosures? IMO full range single column loudspeakers are anathema. But they exist and audiophiles love them. Though it generally detracts from mid/treble performance (different reflection path lengths), some suggest offset L/R boundary distances when siting speakers for the purpose of minimizing boundary effects in the bass.

Not only does staggered L/R reflex tuning minimize modal effects in general, it also minimizes the effect of dreaded bass drone occurring when port tuning matches a room mode.

For symmetrical L/R speakers, the user could swap sides, employing the setup with less modal effects. Alternately, and for non-symmetrical L/R speakers, the user could employ different port lengths.

I generally dislike passive radiators, but staggered mass loading could accomplish the same goal.

The only potential downside I can imagine is minimized power handling. But power handling is a function of reflex tuning and program content. For instance: a sub tuned for 16 Hz performance has more output and more power handling at 16 Hz vs. the same driver tuned for 20 Hz. But the sub tuned for 20 Hz has more power handling (though maybe not more output) higher up in frequency where is more frequently greater program content. Surprisingly, the sub tuned for 20 Hz is more reliable for most program material.
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Old 12th November 2012, 05:56 PM   #2
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Quote:
Originally Posted by ro9397 View Post
Why is it not SOP to stagger the reflex tuning of reflex enclosures?

Not only does staggered L/R reflex tuning minimize modal effects in general, it also minimizes the effect of dreaded bass drone occurring when port tuning matches a room mode.
Most designers are after a target "Q" or frequency response, change Fb (tuning) and both are changed.
Cabinets of the same volume and drivers using different Fb will have different phase and frequency response, an additional sonic problem independent of the smearing of bass lines caused by room echos and modal effects.

A "dreaded bass drone" from room modal effects will happen regardless of Fb or cabinet design.
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Old 12th November 2012, 06:20 PM   #3
sreten is offline sreten  United Kingdom
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Hi,

Staggered bass tuning for ports simply does not work. Closely staggered ports
behave moreorless the same as one port at the median frequency. For widely
staggered ports the lower tuned port is simply ineffective (as it is "shorted"
by the higher tuned port) and being a lot bigger the only thing it does is take
up useful box volume for no useful effect. Its simply a very bad idea.

Proper vented box design is a very good idea. Properly designed overdamped
quasi first order rolloff vented boxes have none of the problems alluded to.

rgds, sreten.
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Old 12th November 2012, 06:22 PM   #4
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I know a designer who only puts the reflex duct on the bottom ( downfiring ).
He says that once set the correct distance from the floor ( with ...feet )every
customer won't find any problem in positioning the speakers in the room .
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Old 12th November 2012, 09:47 PM   #5
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Originally Posted by weltersys View Post
... Cabinets of the same volume ...
There is no reason why this should be a requirement. They may look the same but have different internal volumes.

Horst Moller has experimented with this. Go here:
subfanfare
His main reason for doing this is to interleave the impedance peaks of the two bass enclosures to ease the load on the amplifier. He also mentions the response smoothing effect, but in my opinion this is spurious. My main reason for saying so is that the phase of the output of a reflex (or tapped horn) speaker reverses below the fb. It tends to cancel the output of the other speaker which is still above fb. It would be especially noticeable in his examples which show the speakers located next to each other. It would probably be less noticeable if the speakers were well separated, ideally located on room nodes well suited to their tuning.
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Old 12th November 2012, 10:26 PM   #6
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re:'Closely staggered ports behave moreorless the same as one port at the median frequency' - Sreten, as I read it the OP is talking about the L & R cabinets being tuned differently...
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Old 12th November 2012, 11:10 PM   #7
sreten is offline sreten  United Kingdom
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Originally Posted by PeteMcK View Post
- Sreten, as I read it the OP is talking about the L & R cabinets being tuned differently...
Hi,

Perhaps but that is not going to work well with otherwise identical speakers.

Two different bass alignments will add to a median bass alignment in room,
and cause asymmetric power handling issues where neither speaker can
use its strengths, being limited by the overload in the other speaker.

Its a bad idea and doesn't work, that is why it is not done commercially.

rgds, sreten.
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Last edited by sreten; 12th November 2012 at 11:12 PM.
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Old 12th November 2012, 11:50 PM   #8
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whatever the technical arguments, the idea offends my prejudice that the stereo chain needs to be symmetrical. After all, the source material mix is unpredictable...
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Old 13th November 2012, 10:32 PM   #9
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Speakers with different frequency responses in the low bass can be quite useful when trying to reduce room modes. There's quite a lot of work been done (and documented on diyaudio) on positioning of multiple subs in a room, and pointing out that the subs do not have to have identical frequency responses.

The problem with doing this in the two main enclosures is that you can't move them around to optimise their room mode excitation. The best you can do is swap left for right and see which works better. I would not expect it to be done commercially because it assumes a level of knowledge in the customer that may not exist, and is too application dependent (room acoustics and speaker postioning) to be universally applicable.
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