Flush mounting nearfields again

  • Thread starter Thread starter Bob Mould
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Bob Mould

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I've read all the threads here i could find on this subject and i think i pretty much have understood the concept. Just to be sure i would highly apprecieate if some of you knowledgable people could confirm this for me. As im about to shop for new monitors (so i still have the choice on what to get) im interessted making it easy for me :)

Flushmounting speakers with rear bass ports is generally not a good idea, Although its possible using Barefoots designs.
Speakers with Passive radiator design like Mackie 824(i really dont know what passive radiator is, if someone could explain it to me that would be nice). fall into this category aswell.

Speakers with front bass ports like Event 20/20, Krk V8, Genelecs 1031s would benefit from flushmounting more easilly with some bass adjustment on the speaker. Normal flush mounting methods could be used, like the designs at Johns site.

is this correct..

thanks
/mike
 
Yeah, that’s right.

A passive radiator is basically a 2nd speaker cone with no magnet or voice coil. It works on a very similar principle to a port. Both the passive radiator and the air inside a port act as tuned low frequency resonators. The low frequency acoustic energy produced by the speaker driver is coupled to either the passive radiator or port via the air inside the cabinet which acts as sort of a spring. The majority of deep bass is transmitted into the room by the port or passive radiator while the driver cone is heald relatively still by the “reactance” of the system.

The term “spring” and “resonator” should give you a clue about the tradeoff made in achieving the lower bass extension of these types of designs. The thing you trade is transient response. Because of the inertia of the resonators in these types of speakers, they cant react to signal changes as quickly as a well-damped closed box design. Passive radiators tend to be worse in this respect than ports since the mass of the false cone is much greater than the mass of air inside a port.

Hope this helps.:)

barefoot
 
A passive radiator is basically a woofer without a driving coil. What makes the woofer cone move is the air inside the cabinet, from the powered woofer.

Powered woofer moves in, passive radiator moves out, and vice versa. This design produces some interesting peaks in bass response, as the sacrifice of others. Also, like Barefoot stated above, you often lose transient information because both woofers move slower.

Without a port or a passive radiator, the woofer is "sprung" forward or back with the combined strength of the surround (the foamy stuff around the edge of the woofer) as well as the compression, decompression of the air inside the sealed box.

Though... passive radiators often increase the db of the speakers - but again, with tradeoffs.
 
frederic said:
Powered woofer moves in, passive radiator moves out, and vice versa. This design produces some interesting peaks in bass response, as the sacrifice of others. Also, like Barefoot stated above, you often lose transient information because both woofers move slower.
Click to expand...
It’s not quite that simple. Your explanation holds at very low frequencies close to DC where you’re essentially just pumping air back and fort like a plunger. At audio frequencies you need the acoustic wave equation to explain exactly what’s going on.

The transient degradation isn’t due to a reduction in speed. It’s due to a reduction in the ability to react to changes in speed.
Without a port or a passive radiator, the woofer is "sprung" forward or back with the combined strength of the surround (the foamy stuff around the edge of the woofer) as well as the compression, decompression of the air inside the sealed box.

Though... passive radiators often increase the db of the speakers - but again, with tradeoffs.
Click to expand...
The surround normally provides very little springiness to the driver. Its primary job is to help keep the cone centered and absorb cone resonances. The corrugated spider behind the cone, close to the voice coil and magnet, provides most of the springiness.

The air in a bass reflex (port/PR) cabinet also helps “spring” the driver back and forth. The difference is what the spring is attached to. In the case of a closed box it’s as if the air spring has the woofer cone attached to one end and a brick wall attached to the other. So the only reaction the woofer feels is from the spring itself. In the case of a bass reflex it’s as if the air spring has the woofer cone attached to one end and some light weight movable object attached to the other. So it’s a complicated jiggling back and forth between the two, but the springiness of the air is still the mode transmission.

Lastly, the efficiency (dBs) isn’t significantly different between closed and ported designs over most of the frequency range. Ported systems are only more efficient in the lowest octave or so – hence the extended bass response.

Sorry to get so picky frederic. I design speakers. I can’t help it.:D;)

barefoot
 
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big thanks for that detailed explanation... :)

Another question but this time about freestanding rearported nearfields:
If i have a low, low-mid absorbing wall behind the nearfields, while this should reduce the phase problems with sounds reflecting from the wall. wouldnt it make the speakers bass-lite?



thanks /mike
 
Nice post barefoot - thanks for that.

I think the problem here is the way some near fields are designed. Some near field designers give an increased low end response to their systems to allow for the fact that they are free standing and so potential clients will say "hey check out the low end of these" ( a few JBLs come to mind here) When they are flush mounted the low end rerponse is toooo great and the system sounds bass heavy.

I had that problem at Cloud Studios when I flush mounted their JBLs. The moment they replaced that series of JBL with the LRS series the problem was solved.

any comments barefoot??

cheers
JOhn
 
Good point John.

Yeah, this could definitely be a problem. I think I’ve explained in previous threads that diffraction loss, bass spreading, or whatever you want to call it, causes a –6dB step in the low frequency anechoic response. Some models may compensate for this with a low shelf filter (the wrong thing to do IMO). You don’t want to use monitors like this for flush mounting unless you can bypass that filter. Personally I would argue that you don’t want to use monitors like this at all unless you happen to be mixing in an anechoic chamber.

For bass frequencies inside a normal sized control room on axis response doesn’t mean much. The bass has filled the room by the time your ears receive the several wavelengths required for full tonal and loudness perception. So I think having a flat power response rather than a flat anechoic response is the right thing - i.e. no filtering.

barefoot
 
Now you've gotten me confused Bearfoot :) (might be cause im swedish and having some trouble will all this technical terms in english:)

If using the speakers filter for adjusting bass when flush mounted is a bad idea how would u compensate for the bass lift, I mean if u have a 3db lift at 100hz for example that would seriously affect your low end mixing?


/mike
 
Mike,

It’s easy to see why there is confusion because manufacturers have no standard (or sometimes even sensible) way of dealing with this situation.

This graph describes a normal free standing monitor with no bass compensation filter. “a” is the on axis anechoic response and “p” is the power response.
Code: 
No filter/Not mounted 

+6|
  |
  |
 0+-pppppppppppppapapapapapapapa---
  |            a
  |          a
-6| aaaaaaaa
  |____________________________
If we flush mount this speaker we then get the situation where both the anechoic and power response are flat.
Code: 
No filter/Flush mounted 

+6|
  |
  |
 0+-papapapapapapapapapapapapapa---
  |           
  |
-6|
  |____________________________
So, you see, flush mounting doesn’t really cause a bass boost. It simply flattens out the on axis anechoic (the speaker independent of the room) response by forcing the bass frequencies to radiate into the forward hemisphere just like the high frequencies are forced to do by the speaker baffle. The actual bass power emitted into the room remains unchanged and flat with respect to the highs.


This graph describes the response of a free standing monitor with a bass compensation filter that is designed to flatten out the anechoic response. The anechoic response is flat alright, but the power emitted into the room is not.
Code: 
Shelf filter/ Not mounted 
  |
+6| pppppppp
  |          p
  |            p
 0+-aaaaaaaaaaaaapapapapapapapap---
  |           
  |
-6|
  |____________________________
Then if we flush mount this monitor the anechoic response follows this poor power response curve.
Code: 
Shelf filter/ Flush mounted 
  |
+6| papapapa
  |          p
  |            a
 0+--------------papapapapapapap---
  |           
  |
-6|
  |____________________________
What makes it really confusing is that some monitors which incorporate a low shelf boost illustrate disengaging the filter or setting it to 0dB as a cut. It’s not a cut. It’s removing a boost.

And since low bass frequencies are essentially omni directional in a room the on axis response is meaningless and the actual power response is what determines the spectral balance. So a flat power response is most important in my opinion and bass low filters should not be used in any case.

In short, the graphs show that the best situation is obviously a flush mounted uncompensated monitor.

Hope this helps clear things up.:)

barefoot
 
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ahhh....i get it!
humungous thanks barefoot. :D

But how do you know if the monitors uses bass compensation , can u tell by looking at the monitors specs or by its design?


(The monitors i have in mind are KRK V8 and E7 www.krksys.com the Event 20/20bas, Genelec 1031's and JBL's)


thanks mike
 
I'd contact the manufacturer of the speakers Bob. I was interested in the JBL LSR series and I contacted them via email and they sent me the Low F adjustments for flush mounting. When we got the genelecs they had the flush mounting specs in the manual.

cheers
JOhn
 
Sorry to get so picky frederic. I design speakers. I can’t help it.:D;)

barefoot [/B]
Click to expand...



No sweat man, I'm more than happy to be corrected, because it means 1) I learned something and 2) I'm no longer passing out incorrect information.

So thank YOU :)
 
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