Ask an Acoustic Engineer (me)

[Ge0]

The Tymphany LAT (linear array transducer)?

I got to play with those recently. Kinda neat but outrageously heavy for what they were.

Attachment 10781

Yes, that's it

Outrageously heavy though. Hmm. Is that necessarily a design limit or just due to materials they chose to build prototypes.

Ge0

[Grinder]

Justin,

I have a question (or two) about passive radiators.

From the mid 80s through the late 90s, I ran a pair of homemade (by a highly knowledgeable friend) passive radiator speakers in my home system - for the first year or two, as the sole speakers of a 2.0; and then for a year or two as satellites of a 2.1; and then thereafter as the center of a 3.1/5.1.

Back then, I simply assumed that as the active drivers (a pair of 4" midbass in the above case) moved in and out, the passive radiator (a simple 8" flat foam panel and surround, with construction adhesive applied to the cardboard backing for additional mass) moved accordingly, due to the positive and negative pressures created by the piston-like in/out movement of the active drivers.

Since then, I've heard it said that passive radiators are analogous to the column of air in the port of a bass reflex. And having built and played around with a few bass reflex subs, I've seen how the cone moves least while the port is most active.

So, what causes the movement of a passive radiator? <edit> How much of a factor is active driver excursion in moving the passive radiator, and does great active driver excursion potential matter (assuming there were no passive radiator excursion limitation)?

Also, having recently heard horizontally mounted passive radiator sag dismissed as "myth," I wonder if you might shed some light on the subject ...and perhaps on the subject of cone sag in active drivers as well.

[geolemon]

So, what causes the movement of a passive radiator?

It's the same dynamic, really - I do want to add to this question if possible:

How does the efficiency of a PR vs port compare - meaning, a 'moving mass of air-with-friction' [I'm interested both in a) an "equivalent diaphragm", meaning 10" PR = 10" diameter port, and b) a "minimal port", or smallest diameter without port noise at full power] compare to the 'moving mass of diaphragm-with-suspension-compliance'?

I ask because Earthquake makes some pretty bold claims - 6dB gains. Obviously tuning factors in to gain as it does with traditional vents - but for equivalent tuning, noting that the inherently heavier-than-air mass makes PR's pretty exclusively low-tune beasts - I'd think an efficiency comparison could be made...

I actually have an experiment that I'm setting up for (though there's an installation in my way) - see this post for more detail.
Basically, using both high-output and standard 12's, I'm going to compare:

• output in a small sealed box
• output in an equivalent small box with two PRs
• output in a medium-size vented box with a medium-low tune (both specifics still TBD)

Also, having recently heard horizontally mounted passive radiator sag dismissed as "myth," I wonder if you might shed some light on the subject ...and perhaps on the subject of cone sag in active drivers as well.

Guilty-
I may be the one you are referring to, who made that comment. Just wanted to add the same historical context that I did there...

Back in the day, PRs were just subs with the motors removed, and then mass added - so the suspension was often over-compliant, since it wasn't built for that mass. And I think also, intuitively - people thought a higher-compliance PR was a more efficient PR (and that's exactly my question to Justin above ), so that basic design didn't change much over time.
...and in fact back in the day - the same was true for speakers in general. Today, we have far heavier subs with lower compliance suspensions that support their mass without issue, even mounted horizontally.

So I was saying that today "PR sag" is no longer a given - that there's PR's (like my PRs shown in my link above) that are designed to be PRs, that have less suspension compliance, and can support their own mass.

Just raising my hand for the "guilty" charge as at least having said that somewhere, recently.

[Justin Zazzi]

Back then, I simply assumed that as the active drivers (a pair of 4" midbass in the above case) moved in and out, the passive radiator (a simple 8" flat foam panel and surround, with construction adhesive applied to the cardboard backing for additional mass) moved accordingly, due to the positive and negative pressures created by the piston-like in/out movement of the active drivers.

This is exactly how it works at very low frequencies, below the tuning frequency of the box. This is what happens when a box "unloads" and the excursion of the woofer becomes uncontrolled so the woofer's excursion increases dramatically. This is why a subsonic or infrasonic filter is used on amplifiers so that any content below the tuning frequency of the box won't explode the woofer.

Since then, I've heard it said that passive radiators are analogous to the column of air in the port of a bass reflex. And having built and played around with a few bass reflex subs, I've seen how the cone moves least while the port is most active.

Yep, exactly right!

So, what causes the movement of a passive radiator?

Short answer: angry air pixies. The woofer moves, compresses the air in the box which acts like a spring, and the compressed air in the box pushes on the passive radiator which acts like a moving mass. I get to share one of my absolute favorite animations! Behold, the paddleball!

The paddle in the kid's hand is like the woofer.
The rubber string acts like the air in the box, acting as a stretchy mechanical link between the paddle and the ball (or between the cone and the passive radiator).
The red ball acts like the passive radiator or the slug of air inside the port.

If you imagine hanging the ball from the paddle and moving the paddle up and down slowly then the ball will just follow the motion of the paddle inch for inch. This is equivalent to the air rushing into and out of the box when the cone moves slowly, like you first mention above, where the frequency is below the tuning frequency of the box.

If you move the paddle up and down at exactly the right frequency then you only have to move the paddle a tiny bit and you can send the ball flying with ease. This is the resonant frequency of the system where the frequency is a perfect match for the combination of the spring tension in the string and how heavy the ball is. This is also when the cone in the speaker moves the minimum amount but the passive radiator moves a tremendous amount (or the air in the port is moving a ton).

If you move the paddle up and down really fast then the ball tends to not move very much and remain mostly stationary. This is what happens in a box at well above the tuning frequency: the passive radiator essentially stops moving and the box starts behaving like a regular sealed box. The same thing happens with the slug of air in the port, it mostly stops moving.

How much of a factor is active driver excursion in moving the passive radiator, and does great active driver excursion potential matter (assuming there were no passive radiator excursion limitation)?

A ported box can leverage the resonance to get similar output by using a woofer with less excursion, so high excursion is not required as much as it would be like in a sealed box.

However! The air pressure inside the box at resonance is really high and so the forces on the cone are really high. In theory you would want a speaker that can push really hard near the rest position. Some folks call "push" the ratio of BL^2/Re or sometimes Klippel calls it the "motor force factor". Others call it "shove" but I'm fond of skookum.

Also, having recently heard horizontally mounted passive radiator sag dismissed as "myth," I wonder if you might shed some light on the subject ...and perhaps on the subject of cone sag in active drivers as well.

I cannot say for absolute certain, but I think this a product of materials from long ago that were more susceptible to things like sag. I want to say this is a myth for speakers made this century. If you're concerned, just flip your woofers every time you flip your mattress.

[Justin Zazzi]

It's the same dynamic, really - I do want to add to this question if possible:

How does the efficiency of a PR vs port compare - meaning, a 'moving mass of air-with-friction' [I'm interested both in a) an "equivalent diaphragm", meaning 10" PR = 10" diameter port, and b) a "minimal port", or smallest diameter without port noise at full power] compare to the 'moving mass of diaphragm-with-suspension-compliance'?

There's a neat twist on the question! The efficiency of a passive radiator or a port is proportional to the losses (friction) they have. In a passive radiator this can be somewhat quantified as the Qms value. A higher Qms value will be more efficient because there are less mechanical losses (less friction within the surround and spider materials).

The losses of a port are related to the surface area of the inside walls of the port. If you zoom in to the inside surface of the port then you would see the air touching the surface is going to be stationary and then as you zoom out further away from the surface of the port the air will begin to start moving. The air velocity will be maximum near the center of the port, away from the interior walls.

This effect is called a boundary layer. It can also be seen in a river where the water along the shore will be moving very slowly compared to the water moving in the middle of the channel which will be the fastest (because it is the furthest away from both the shore and the bottom of the river where the earth is). Sometimes the fluid nearest the boundary can turn into swirling patterns as seen in the image below. Notice the particles near the top of the image are moving fastest. In this case, the stationary surface or boundary (or wall) is along the bottom of the image.

Boundary layers also exist on your skin so if you sit still in a room you can feel warmer than if there is a breeze. This is how windchill works: you can exchange more cool air against your skin and it will wick away heat faster. The opposite is true in a convection oven, it cooks faster.

You can minimize the losses in the boundary layer by reducing the surface area of the port, and by ensuring the central area of the port is as far away from all surrounding surfaces as possible. If your port is a long narrow slit then you will have a ton of surface area and not much space in the "middle" for the air to move freely. A circular port is the most efficient for this. Did your math teacher in elementary school ever give you a puzzle like this:

"Karen has 100 feet of fence and she wants to make a cage for her animals. How long and how wide should the cage be to make the most room for her animals?"
And then you're like "I hate word problems Karen can figure it out herself".

Well that's the same problem here. You want maximum cross-sectional area for the air to flow through and you want the minimum wall surface area for it to rub against.

I ask because Earthquake makes some pretty bold claims - 6dB gains. Obviously tuning factors in to gain as it does with traditional vents - but for equivalent tuning, noting that the inherently heavier-than-air mass makes PR's pretty exclusively low-tune beasts - I'd think an efficiency comparison could be made...

I actually have an experiment that I'm setting up for (though there's an installation in my way) - see this post for more detail.
Basically, using both high-output and standard 12's, I'm going to compare:

• output in a small sealed box
• output in an equivalent small box with two PRs
• output in a medium-size vented box with a medium-low tune (both specifics still TBD)

This would be a very interesting question. I think the efficiency can also be related to the surface area of the "radiator" whether is it a passive radiator or an air port. We know that if you double the number of subwoofers you have then you will get about +3dB of sound output, even if you use the same amplifier and don't add any extra watts. This is because doubling the surface area of the radiator will increase the radiation efficiency of the system.

I wonder if a similar claim could be made for passive radiators? If you take one 10" passive radiator that is tuned to 40hz, and you replace it with a pair of lighter 10" radiators that are combined tuned at the same frequency, would you get double the output from them? I would love to try that!

For your experiment, I'd love to see what results you come up with!

[Holmz]

I thought is was just Jazzi... I never knew there were so many acoustic engineers here.

[Justin Zazzi]

I thought is was just Jazzi... I never knew there were so many acoustic engineers here.

Im not sure if it's clear but did you know Jazzi is my old username?

[Maxmil982]

@Justin Zazzi. First off man, major kudos on all that you've shared. I've been reading the thread and the troves of knowledge and excellent questions asked and answered has been truly remarkable. I know this post looks like an essay but my questions are very entry level and theres just a good bit of back drop...

Part 1:
As a total beginner to car audio I have a few basic questions.
I am installing a 2way active system in a 4door Nissan Altima. The sub will be a Sundown SA-12 powered abdundantly with an Orion HCCA 2400 amp that will provide 1200rms clean. I listen to bass heavy music and will be listening very loud at times. The front stage will be powered with a ppi phantom p900.4. The total system will be around 2500w rms.

I understand the basics of efficiency and how the system will not be using the entire 2500w playing music. I plan to just monitor voltages and current once the system is up and running and then plan out electrical upgrades. My stock alt is 110a. It is also time for a new battery and I'm considering a Napa Legend Premuim AGM with 55Ah and 750cca, it's just a mid level East Penn Mfg battery and of course ill be doing the big 3 upgrade.

I've heard so many conflicting tips. Some people say stock alt AND stock battery will be fine. Some people say get caps some say caps are trash. Some say get an XS power AGM and get HO alternator from the beginning; and every imaginable combination of those things.

So my question is just simply, if I have voltage drops, dimming lights etc. once the system is installed, should my next steps be a 2nd AGM in the trunk or a HO alternator?

Part 2:
I've also been having trouble finding a proper front stage that will keep up with the sub. I was considering Crescendo UN65 or PWX65 as the mids and pairing them with a proper tweeter.

I'm now considering the Steven's Audio MB8 to go in the front doors. I talked to Eric Steven's and he said his MB8s will do fine in a 2way active setup with the proper high sensitivity dome tweeter. My goals for this setup are high efficiency, decent sound quality, and good frequency coverage. Being a beginner, my goals and expectations arent pristine SQ just yet, but just loud, clear and acceptable performance. I will be getting a DSP, bomb proofing my doors, tuning, aiming, using a mic to calibrate things and setting gains properly with an o-scope or some other effective method. So I do plan on trying to make this 1st build as successful as possible.

Below is a link to the Steven's Audio MB8 and my question here is, do you have an opinion on these 8"s ? Or do you see any glaring issues with my plans that I need to address? Again I've heard so many conflicting tips on this 2way front stage to pair with the SA12 . I've heard 8s will be unruly in a car door, and how 6.5s wont be loud enough to keep up and how one or other speaker wont play high or low enough to blend, etc. I'm not looking for perfection just yet, and I have a good grasp on how to solve basic issues of door treatment and tuning. I just want to know that I'm on the right track with these MB8s and that they will at least sound pretty good with the right tweeter in the right location.

https://www.facebook.com/10225841351...5408394204672/

[Cutaway]

I thought this might be a great thread to ask this silly question. I am almost embarrassed to ask and have done a lot of googling but cant seem to find an answer that i understand.

Is there a rule of thumb when designing an enclosure in regards to a drivers Fs? How far below the Fs can you tune a ported enclosure? What about a sealed enclosure's F3 response, can it be below the Fs of a driver? And lastly... If designing any enclosure (not and IB) is there any weirdness that happens if the F3 is well below the Fs of the driver...

Thanks in advance

[dgage]

The two amps you’ve selected are class D, which are very efficient compared to class A, A/B, etc. So I doubt you’ll have any issues with the stock alternator.

Sounds like you’re all set on how to identify the better batteries if you need to replace your current battery.

One thing you need to realize on the second battery and capacitors is that sometimes people try to fix something that may not be broken or go overkill just because. A second battery is primarily for listening to the system with the engine off (I’d use a battery isolator so the main battery doesn’t get too low a charge) but I guess those competing in SPL often use batteries to ensure they have a strong enough electrical system. Also realize that a second battery is a second load on the charging system so it is possible it could cause some system issues while the second battery is charging, which might take away from the stereo amplifiers.

Regarding a capacitor, realize that amplifiers have built-in capacitance and capacitor construction has (slightly) improved over time. A capacitor is just a really short term and very fast “battery” using the generic term of a battery being an electrical charge storage device. So I would recommend you look at adding that later IF you have a voltage issue with the amps or lights dimming. Otherwise you are just adding another device in your system that may or may not be needed and I’m all for following K.I.S.S. and keeping the system simple. I wouldn’t add another device in the signal chain if I didn’t have to and would follow that same logic to the electrical system.