Ask an Acoustic Engineer (me)

[Justin Zazzi]

I wonder if you could help me with this question...

https://www.caraudiojunkies.com/show...asound-Whisper

...seeing as I haven't made any practical progress yet, due to work

Thanks in advance

I'll reply to your thread directly.

[Jscoyne2]

I like this question!

I don't think there would be any difference if the speaker is spinning at a slow speed like 1 revolution per second, and if you are directly on-axis. If you were off axis by quite a bit then you might (maybe?) hear a kind of warbling in the higher frequencies where cone breakup occurs. I think this would be true since cone breakup modes are not always symmetric with respect to rotation.

If you were listening to a horn then you would certainly hear a difference if you're off-axis since horns are usually designed with a very specific vertical and horizontal off-axis response that are different. For example horns for live sound can have a wide horizontal response and a narrow vertical response.

Neat, thanks for answering.

Another question. Does the shape of a sealed enclosure matter as long as the airspace needed is provided? I know that the amount of air inside is supposed to act like a spring but i feel like the shape of an enclosure could have pressure zones within it. For example, if one were to build a kickpanel enclosure that had enough room to set the speaker into and then the enclosure followed the left of the driver seat and then led to a large box underneath the seat. Technically, you would have enough airspace for a driver but there would be sections of small, tubular, and large air spaces all linked.

And on that note. When it comes to kicks venting outside of the vehicle. Is there a simple way of understanding how big of a hole is needed? I had 10"s drivers in kicks that were vented through the firewall by 2 4in circular holes. I still had a huge rise in the FR that would be reminiscent of an undersized enclosure. I'm curious if there is any math or lamens terms to understanding what realistic venting is needed? The only way i can think of is using a DATS device(https://www.parts-express.com/dayton...ystem--390-806) to test QTC of a driver in free air and then adding more or larger holes in the firewall until the qtc of the enclosure matched.

[dumdum]

Neat, thanks for answering.

Another question. Does the shape of a sealed enclosure matter as long as the airspace needed is provided? I know that the amount of air inside is supposed to act like a spring but i feel like the shape of an enclosure could have pressure zones within it. For example, if one were to build a kickpanel enclosure that had enough room to set the speaker into and then the enclosure followed the left of the driver seat and then led to a large box underneath the seat. Technically, you would have enough airspace for a driver but there would be sections of small, tubular, and large air spaces all linked.

And on that note. When it comes to kicks venting outside of the vehicle. Is there a simple way of understanding how big of a hole is needed? I had 10"s drivers in kicks that were vented through the firewall by 2 4in circular holes. I still had a huge rise in the FR that would be reminiscent of an undersized enclosure. I'm curious if there is any math or lamens terms to understanding what realistic venting is needed? The only way i can think of is using a DATS device(https://www.parts-express.com/dayton...ystem--390-806) to test QTC of a driver in free air and then adding more or larger holes in the firewall until the qtc of the enclosure matched.

I’m sure he will come along and chime in, but yes if there is a restriction then it will have an effect on the sound of the driver as it will effectively load up the driver and restrict airflow a fair bit

as for the last part, I do know of people who use a dats for that exact purpose to test the Q of enclosures and also IB drivers as well ���� So it’s certainly a thing

[jtrosky]

Ok, I have an easy one for you (I think?). A lot of people seem to look "down" on 6x9 speakers. Personally, I've found that they make fantastic midbass speakers. What are you thoughts on "non-round" speakers? To me, for something like midbass, a 6x9 seems perfectly fine and has the advantage of more cone area so it can provide deeper bass than 6.5" midbass speakers.

Just curious what an engineer things about 6x9s. :-)

Thank you!

[Jdunk54nl]

Ok, I have an easy one for you (I think?). A lot of people seem to look "down" on 6x9 speakers. Personally, I've found that they make fantastic midbass speakers. What are you thoughts on "non-round" speakers? To me, for something like midbass, a 6x9 seems perfectly fine and has the advantage of more cone area so it can provide deeper bass than 6.5" midbass speakers.

Just curious what an engineer things about 6x9s. :-)

Thank you!

The only real downside (that I know of) to a 6x9 is that you have a 6" speaker properties and a 9" speaker properties so things change based on how you orient the speaker in the door.

[jtrosky]

The only real downside (that I know of) to a 6x9 is that you have a 6" speaker properties and a 9" speaker properties so things change based on how you orient the speaker in the door.

But would that really matter for the freqs that the 6x9 speakers play (400hz and under, in my case)? I can't imagine that "turning" the speaker would really change the sound at all?

[Jdunk54nl]

You can look at pg 33 here
https://testgear.audiofrog.com/wp-co...y-it-Works.pdf

This is the picture that goes along with it.



If we use that and back out 4 x diameter of a 9", that would give us a frequency of ~375 hz to be in the green area. 2x Diameter = ~750hz. This is in air at 20 degrees celsius.

This depends on how the factory oriented the speakers though. Is it behaving like a 6" speaker or a 9" speaker or somewhere in between?

[erinh]

But would that really matter for the freqs that the 6x9 speakers play (400hz and under, in my case)? I can't imagine that "turning" the speaker would really change the sound at all?

No, because the 1/2 wavelength of the longest dimension is still > 400hz. 13500/2/9 = 750hz. (Speed of Sound ÷ 1/2 wavelength ÷ 9 inches)

[Holmz]

I like this question!

I don't think there would be any difference if the speaker is spinning at a slow speed like 1 revolution per second, and if you are directly on-axis. If you were off axis by quite a bit then you might (maybe?) hear a kind of warbling in the higher frequencies where cone breakup occurs. I think this would be true since cone breakup modes are not always symmetric with respect to rotation.

If you were listening to a horn then you would certainly hear a difference if you're off-axis since horns are usually designed with a very specific vertical and horizontal off-axis response that are different. For example horns for live sound can have a wide horizontal response and a narrow vertical response.

Well it may not be heard, but I think it would the same as transverse Doppler.
A fellow I know has tried explaining it to me a few times... I remind him of his dog, with its head cocked over.

http://www.conspiracyoflight.com/Tra...er_Effect.html

[Justin Zazzi]

Neat, thanks for answering.

Another question. Does the shape of a sealed enclosure matter as long as the airspace needed is provided? I know that the amount of air inside is supposed to act like a spring but i feel like the shape of an enclosure could have pressure zones within it. For example, if one were to build a kickpanel enclosure that had enough room to set the speaker into and then the enclosure followed the left of the driver seat and then led to a large box underneath the seat. Technically, you would have enough airspace for a driver but there would be sections of small, tubular, and large air spaces all linked.

Hey you're welcome!

I've had some debate about this with other engineers and it's fun to think about. I believe you're right that given enough "small" passages and air chambers then a sealed enclosure can start to behave differently. The pressure zones you mention could be thought of like standing waves or resonances in the air chamber. Below a certain frequency the sealed box (and all of the connected passages and whatnot) will behave like a pressure vessel that is very evenly pressurized. Above that certain frequency, the air will be able to slosh around and compress and expand unevenly which I think is the "problem" you're asking about.

What if we built a box that had small air passages and air chambers that were meant to be restrictive like this on purpose? Could we learn something about it? Sure! A 4th-order bandpass box has an air chamber and a narrow passage in front of the cone. A 6th-order bandpass box has more air chambers or ports. More complex enclosure designs have even more chambers and ports.

All of those fancy enclosures have something in common too: multiple humps in the electrical impedance, such as what the Dayton DATS can measure. If you have one, you can easily see if your enclosure is working the way it should be by sweeping the impedance and counting the number of humps in the response.

The graph below is a 4th-order bandpass box. See the two humps? Those are obvious.



The impedance graph below is ... I assume a sealed box or maybe a 2-way system. There is one large hump around 150hz and a smaller one around 60hz. Do you see the smaller one? That is an additional smaller hump that, in the context of this question, could be a misbehaving air chamber or small passageway (maybe). I can't say for sure since I haven't done this kind of experiment, but it's what I imagine it could look like.



So yeah, your "sealed" box could misbehave but it's really hard to predict or notice unless you have a tool like making an impedance sweep.

Here's a fun experiment I just tried. Model a woofer in a ported box with an enclosure that is very small like 0.01cuft. Then add a port 10in in diameter and 1in long. The frequency response won't make any sense but look at the impedance. You should see one peak in the impedance somewhere around the fs of the driver like 50hz or whatever. This is kind of like having one large air passage in your sealed box. Kinda.

Then reduce the port diameter to something smaller like 3in dia and make it longer like 10in. Look at the impedance graph again. You might see a tiny 2nd peak emerging higher in the frequency rang elike 300hz or something. This is similar to having a smaller air passage in your sealed box that goes somewhere else in the car.

Then reduce the port to a small diameter like 1in and make the length longer like 20in. Now you should certainly see two peaks in the impedance plot. This is like if you have a small restricted passage in the "sealed" box to somewhere else in the car. The extra hump in the impedance plot is showing the other resonance in the system.

The above experiment is not really how things would behave since subwoofer modeling programs aren't designed to model a labyrinth sealed box like we're talking about, but it's close enough to the idea that it's fun to play with.

Are there some rules of thumb we can use to make a "good" sealed enclosure? Maybe. When I was dreaming about installing midbass drivers in my kick panels and venting them to the outside I kept reading people recommending certain things. The common wisdom was something like "keep the air passage no smaller than half the size of the woofer". This seems like a good plan. If you don't choke down the air passage on the rear of the cone too much, then it will be able to breath to the atmosphere and you'll have properly vented kick panels. The same concepts should apply to the narrow air passages and chambers if you want to make a labyrinth sealed enclosure too. See the question below for a more exact formula:

And on that note. When it comes to kicks venting outside of the vehicle. Is there a simple way of understanding how big of a hole is needed? I had 10"s drivers in kicks that were vented through the firewall by 2 4in circular holes. I still had a huge rise in the FR that would be reminiscent of an undersized enclosure. I'm curious if there is any math or lamens terms to understanding what realistic venting is needed? The only way i can think of is using a DATS device(https://www.parts-express.com/dayton...ystem--390-806) to test QTC of a driver in free air and then adding more or larger holes in the firewall until the qtc of the enclosure matched.

I kinda answered it above by happy accident, but yes using something like DATS is immensely powerful. Measure the Qts of the driver in free air or on a flat baffle with no enclosure in front or behind. Then install in the car. If your air passages are large enough then the Qts of the woofer in the car should closely match the Qts when in free-air. The Qts in the car might be a little higher than free-air no matter what you do, but a substantial increase would be easy to detect.