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Old 1st June 2015, 11:51 PM   #1
mirekti is offline mirekti  United States
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Default 2way monitor with passive radiator

I wondered if anyone has experience in building 2way monitor that uses a passive radiator instead of vent.
I am building (in my head) a speaker that would accommodate RAAL 70-20XR and hopefully soon released C153 Cell series bass/midrange driver.
The idea is to put a single P173 or P220 radiator in the back instead of the vent. I understand I would loose some LF, but the bass would be tighter.
Eventually, I will hand of my idea to someone of the custom speaker designer, but would like see whether I am on the right track i.e. I wouldn't like to try to persuade designer into something that doesn't make sense.
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Old Yesterday, 02:38 PM   #2
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Not sure why or how a PR should give 'tighter' bass than vented.

Both work on the same principle: The driver couples to the PR or the air in the vent via the volume of air enclosed in the cabinet and causes them to resonate in sympathy.

However a PR will be 100x heavier than the air volume in the vent.
Thinking about that and how momentum works I would expect a vent to provide 'tighter' bass than a PR.

Where PRs come into their own is with small cabs & small drivers tuned low.
A vent tends to be quite large/long to obtain this, some times larger than the box itself.
A PR can be tuned arbitrarily low just by making it heavier without taking up extra space.
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Old Yesterday, 04:51 PM   #3
PLB is offline PLB  United Kingdom
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Hi Mirekti,

Mackie has a model HR626 studio monitor that employs a passive radiator http://www.mackie.com/pdf/hr626_om.pdf
Personally, I've always been a little concerned about passive radiators because my simulation software shows an unusually high peak in the group delay response. This may not be audible though because IIRC it peaks right at the systems low frequency null.
As for tight bass, I've always considered this to be entirely dependent on the frequency response. I say this because to my mind tight bass implies a fast attack time which, by definition is defined in the time domain and the time domain can be mathematically translated into the frequency domain via a fast Fourier transform.

Peter
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Old Yesterday, 06:03 PM   #4
infinia is offline infinia  United States
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look at stored energy eg passive radiator will be at a disadvantage compared to a column of air.
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Old Yesterday, 06:13 PM   #5
Lojzek is offline Lojzek  Croatia
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Let's examine what numbers are telling us in this simulation of
Seas U22 REX/P-SL vented and PR. Peak excursion of PR is 10 mm.
Vented, passive, vented...
Attached Images
File Type: gif Vented FR.gif (18.7 KB, 78 views)
File Type: gif PR FR.gif (19.1 KB, 75 views)
File Type: gif Vented Excursion.gif (9.5 KB, 75 views)
File Type: gif PR Excursion.gif (9.7 KB, 75 views)
File Type: gif Vented GD.gif (9.8 KB, 75 views)
File Type: gif PR GD.gif (10.7 KB, 15 views)
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Old Yesterday, 06:55 PM   #6
mirekti is offline mirekti  United States
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My assumption was that the steep rolloff of LF would make the bass tighter, and there would not be "mushy, boomy" sound as the low part where the driver would struggle wouldn't be to hear.
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Old Yesterday, 07:04 PM   #7
Face is offline Face  United States
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That's not quite how it works.

Fyi, bass roll off is a 12db slope below the passive's FS (FP), which can be work well with the average room gain curve. The problems are finding a passive with the correct FS to take advantage of this and group delay.
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Old Yesterday, 07:08 PM   #8
infinia is offline infinia  United States
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steepness of high pass filters implies overshoot and more group delay.
slower roll off implies drier bass combined in small rooms tighter bass.
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Old Yesterday, 07:39 PM   #9
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Do you want a small 2-way monitor.... OR do you want magic sound from expensive RAAL + Accuton drivers?
A sealed C158 would offer the best transient response and lowest phase shift.

Have you considered a narrow floor stander with side woofer for full range 30-20Khz bandwidth?

The C158-8-085 in a 0.35cuft sealed volume has a Qtc=0.7 and Fc=78Hz.
The C158-8-085 in a 0.53cuft ported volume with 2"D x 8" long port has Fc ~ 42Hz.

For a small monitor, I would use a front port and spend my money on the best drivers I could afford. Small monitors are often put close to a rear wall/box which affects rear ports.
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Old Yesterday, 07:50 PM   #10
mirekti is offline mirekti  United States
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Esthetics are definitely important as the speakers will be in the living room so they have to pass wife's expectations ;-)
The room is 13x20x9 (4x6x2.7 in meters) and that's why I thought something like radiators would help to cut off the bass. The room can only be moderate treated (one or two bass traps, and 1st reflection diffusers).
Ideally I could build something like Tidal's Piano Cera where one can use it as a 2way monitor or a 2.5way floor standing speaker. I could get away with floor standing speakers, but they would have to be slim. Acctually something like Cera copy would work (my wife likes that simple design).
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