Thin backplate material for 24" x 24" 3-way speaker

mattstat

Member
2009-10-19 7:47 pm
This one models pretty well in 10 liters with your woofer. EDIT: I didn't look at the depth when I posted this originally. It's too deep for your design. I'll look at another and post in few minutes.

If you only use one, the Xmax of the passive is exceeded at full power below about 55 Hz. Depending on how you intend to use the speaker this may not matter to you (typical listening volume and subwoofer use). If you want maximum output and low frequency performance, two of them would be better with about 30 grams of added mass per passive radiator.

You could start with one passive radiator to do basic tests and add the second later if you like how things are working together. The sound won't be significantly different between 1 and 2 passive radiators except at high levels with bass around the tuning frequency (about 40 Hz).

Dayton Audio DS175-PR 6-1/2" Designer Series Passive Radiator
Dayton Audio DS175-PR 6-1/2" Designer Series Passive Radiator

The problem with using smaller passive radiators is that compared to the woofer you typically need around 2x the volume displacement for the passive. You'd probably need 4 or so smaller radiators to get that.
 
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mattstat

Member
2009-10-19 7:47 pm
Both the woofer and the radiators already have to be recessed about 5mm to achieve that (I don´t know if that generates any acoustic problems ?)

You should be OK with recessed low-frequency drivers. Things like this typically cause some response anomalies around a few kHz and higher, but your woofer should be rolled off significantly by then, and it's typically a only a couple dB squiggle that doesn't significantly change the character of the speaker. A roundover on the recess typically helps.
 

mattstat

Member
2009-10-19 7:47 pm
This one also looks OK with your woofer in 10 liters with a single radiator and 5 grams of added mass for a resonance around 55 Hz. One issue with these is the protruding bolt for added mass. That can be cut off though. Also, the mass mount extends beyond the basket at high excursion. Given your previous descriptions, that might exclude this one from consideration also.

Dayton Audio ND140-PR 5-1/4" Aluminum Cone Passive Radiator
Dayton Audio ND140-PR 5-1/4" Aluminum Cone Passive Radiator

This one has the same bolt and mass mount issue, but less depth. 2 of these look reasonable in 10 liters, with a resonance of around 60 Hz. The passive's Xmax is exceeded by about 25% at full power at resonance, but realistically, I'd probably ignore that instead of adding a third radiator. It has a 2 dB response peak with no added mass, but I'd try them that way. Adding mass decreases the passive's resonance frequency, which increases its excursion at resonance.

4 of these with about 10 grams of added mass each pushes the resonance into the lower 50 Hz range.

Dayton Audio ND105-PR 4" Aluminum Cone Passive Radiator
Dayton Audio ND105-PR 4" Aluminum Cone Passive Radiator
 
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mattstat

Member
2009-10-19 7:47 pm
55 hz, but I think the LW150-4 has a resonance frequency of 62 hz?

Yes, the driver on its own in free-air has a resonance of 62.6 Hz.

But with that driver in a box with a passive radiator (or port), the normal tuning for the system is lower than the free-air resonance of the driver.

If you look at the suggested vented (ported) box size/response here, they suggest an F3 of 40 Hz in roughly 9 liters. To get that, you need a tuning frequency of about 48 Hz for the box volume/port. When using a passive radiator, your overall response shape target is normally the same as you'd get with a ported box.
Dayton Audio LW150-4 6" Low Profile Woofer 4 Ohms

Again, because of the passive radiator's own parameters, this isn't always possible. Or a slightly de-tuned alignment has a benefit (like needing less passive radiators).
 

mattstat

Member
2009-10-19 7:47 pm
With the ND-1005PR, you need at least 2 passive radiators for 1 woofer.

In post 43, all of the commentary above each link is about that particular radiator. In both cases, with the LW150-4.

So, the following information is about the ND-105PR, as modeled in Unibox:
"This one has the same bolt and mass mount issue, but less depth. 2 of these look reasonable in 10 liters, with a resonance of around 60 Hz. The passive's Xmax is exceeded by about 25% at full power at resonance, but realistically, I'd probably ignore that instead of adding a third radiator. It has a 2 dB response peak with no added mass, but I'd try them that way. Adding mass decreases the passive's resonance frequency, which increases its excursion at resonance.

4 of these with about 10 grams of added mass each pushes the resonance into the lower 50 Hz range."
 

mattstat

Member
2009-10-19 7:47 pm
What would the FR be in 5 liter ?

With two ND105-PR (with no added mass) and a 5 liter box volume, Fb is 73 Hz, and F3 is 66 Hz. Response peak is 4 dB. At full power (40 watts) and resonance, passive radiator Xmax is exceeded by 20%. Woofer Xmax is exceeded by roughly 50% from 20-40 Hz, and woofer excursion drops down to Xmax at about 64 Hz.

If power is reduced to 20 watts, woofer Xmax is only exceeded by 20% from 20-40 Hz. Passive radiator Xmax is no longer exceeded at this drive level.

Adding about 6 grams to each passive radiator extends F3 to about 57 Hz, and reduces the response peak to 2 dB. As discussed previously, this also increases passive radiator excursion, so it's now 45% beyond its Xmax at a 40 watt drive level, which is why I previously said I'd start with 2 passive radiators and no added mass to save from having to add more radiators.

I realize this seems like a lot of details, but it's really just how it goes when you're dealing with ported/passive radiator designs. There are a lot of related items that all have to be juggled simultaneously. Compared to a sealed box, you get extended frequency response, but you pay for it in complexity and high woofer excursion below the tuning frequency (and some other things).
 

OSLO

Member
2021-02-02 12:41 pm
When I simulate the speaker with a woofer with 10 liter as ported and FR = 62 hz the response looks like this:

254348899_10159619136204636_5658219568177997922_n.jpg
 
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mattstat

Member
2009-10-19 7:47 pm
PS. You know that with 10 liter I have to go 32x32" (or 24x24" if I test the woofer alone)

I have no strong opinion on your precise box volume. The 5-10 liter range seems reasonable for what you are doing. Beyond that, there are too many variables, and ultimately it's your decision how much each factor means to you. Until you build a prototype and see how things sound to you, it's mostly speculation and assumptions, even with modeling in play. My opinions and preferences may be vastly different than yours.

My advice in general is to view the process as iterative, and to build and listen to prototypes as early as is feasible. I see many people/projects languish in the design phase (not just in audio). In my experience, it's very, very rare to get the design right the first time.

Additionally, I'd stick with passive radiators with known parameters and modelable performance initially. Once you know how things behave in that regime, you can introduce the other passive radiators with no specs. If you start with parts with no specs, you'll never really know what's going on if there's a problem.
 
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OSLO

Member
2021-02-02 12:41 pm
You are right. I am unfortunately a guy that regards every attempt as the final version and have very little patience. I very often change my mind about something and most people can't keep up with that. The work is therefore somewhat of a rollercoaster ride with a lot of downs and a few ups. But it is addictive and I love it.

Your advice is therefore very valuable and I greatly appreciate it. And I am surprised that you haven't totally lost your patience with me yet.
 

mattstat

Member
2009-10-19 7:47 pm
Your advice is therefore very valuable and I greatly appreciate it. And I am surprised that you haven't totally lost your patience with me yet.

I'm glad you're getting some value from it.

I understand most of what you're going through with all the decisions that have to be made and trying to optimize things. But I've also built and listened to enough speakers that I really appreciate that the listening is the ultimate test, so I try go get that done very early. Bass quality/quantity is one of the things that seems particularly hard to judge just going by models. The fact that you are mounting on-wall also makes things harder to infer from models, since most of them assume free-space, and that's not where you're going to be operating.

I also have a friend who changes his mind about his car stereo install about 5 times as rapidly as anything you've talked about, so it's all a matter of perspective :)
 

mattstat

Member
2009-10-19 7:47 pm
Is it ok if I mount the ND105-PR4 like this ?

The general idea is OK, but the issue I brought up earlier will be a problem with what you have shown: on this style passive radiator, there's a permanent weight/mounting plate where you add mass if required. This plate moves with the radiator and will extend out beyond the basket at high excursion. Attached image has the permanent bit circled.

The extra weight shown in the picture and wing nut are removable. The mounting bolt will need to be cut off for your application given the current depth restriction.
 

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