What is a port? (correction TechQuickie, sorry Linus ;p)

[Lutkeveld]

Quote Linus in the last Techquickie vid (https://www(dot)youtube(dot)com/watch?v=jhvXWpRmhvM):

"Enclosures increase sound clarity and distortion by preventing sound waves created at different locations from crossing and ensuring that the woofers always can receive the right amount of air for all the movement they need to do, usually a couple of strategically placed holes"

 

Sorry Linus, but that's not right. It's a common misconception, so I shall explain it so others here know the correct explanation:

 

Between the cone moving forward and the cone moving backward, there is half a cycle (hertz). You can say the backwave is 180 degrees out of phase with the front.

An enclosure prevents the two waves from meeting, because otherwise they would cancel each other out. Then you might say: why is there a hole in the enclosure then?

Well, it's not a hole, it's a chamber that acts as a so called 'helmholtz resonator'. There is a certain frequency where the air inside starts to resonate and creates sound but 180 degrees as seen from the backwave.

Because the back was +180deg, but is now flipped +180deg, it is in phase with the front (altough 1 cycle, or 360deg, behind). So now it doesn't cancel, but increase the output of the front wave.

The Helmholtz Resonance Frequency is often called "Tuning Frequency"

 

Let's say this is the response of our closed box:

 

And here the output of the port is added:

 

Then this is the total response, which has deeper/lower bass as you can see.

 

After this resonance the port is indeed a hole, letting the backwave through without change (so out of phase).

This causes destructive interference so the output drops much quicker. Also: the cone doesn't have the springiness of the air anymore and can move more freely.

Resulting in quickly rising excursion of the cone below resonance, this is why you need a sub sonic filter to cut the low bass with ported subs.

 

If you have any questions about this matter, don't hesitate to ask. 

Hope this helped

[Zeke]

I can't be arsed to real all that but there are graphs so that must be true.

[ImBleu]

A port is where you park your boat. Generally next to a body of water.

 

@LinusTech @nicklmg @Slick might wanna check this thread out.

[NeatSquidYT]

I can't be arsed to real all that but there are graphs so that must be true.

Same

[NeatSquidYT]

*le edit

[LarsReviews]

 

 

CoC! and the OP makes sense to me. Linus was wrong! *shocked*

 

[werto165]

https://www(dot)youtube(dot)com/watch?v=jhvXWpRmhvM this made me laugh don't know why, you know you can post URL's. 

[othertomperson]

To be fair it is TechQuickie and not a crash course in basic wave theory

[Lutkeveld]

To be fair it is TechQuickie and not a crash course in basic wave theory

 

No, it's supposed to be short, not wrong ;p. No problem if it doesn't go in depth, but I just wanted people to know that a port is not a 'hole' that lets air out for movement of the cone. 

 

 

https://www(dot)youtube(dot)com/watch?v=jhvXWpRmhvM this made me laugh don't know why, you know you can post URL's. 

 

Otherwise it will post the whole video with thumbnail and all. I don't like unneccesary long posts ;p. 

[SuperPug]

Quote Linus in the last Techquickie vid (https://www(dot)youtube(dot)com/watch?v=jhvXWpRmhvM):

 

That video looks like it was produced by a 13 year old. Whats with all the spazzy shit going on?

[othertomperson]

No, it's supposed to be short, not wrong ;p. No problem if it doesn't go in depth, but I just wanted people to know that a port is not a 'hole' that lets air out for movement of the cone. 

 

There are always going to be inherent inaccuracies with the format because there's always a deeper, more correct level of detail that you could have gone to. At some stage you have to decide "ok this isn't perfect, but this is enough for now." People can always read more later about constructive and destructive interference if they care to. And those kinds of videos are better left to Teran anyway

[Lutkeveld]

There are always going to be inherent inaccuracies with the format because there's always a deeper, more correct level of detail that you could have gone to. At some stage you have to decide "ok this isn't perfect, but this is enough for now." People can always read more later about constructive and destructive interference if they care to. And those kinds of videos are better left to Teran anyway

 

Then don't mention it or give a brief, but right, explanation. Something like 'some have ports which contain air that resonate at a certain frequency to increase bass output'.

No blame on Linus and his crew, it's difficult to be right on all topics. Just wanted to correct it so people know what it's really meant for.

[NeatSquidYT]

CoC! and the OP makes sense to me. Linus was wrong! *shocked*

 

Am I breaking CoC?

[LarsReviews]

Am I breaking CoC?

Tldr is not allowed. I just don't want you to get any warning points

[NeatSquidYT]

Tldr is not allowed. I just don't want you to get any warning points

Oh, wow thanks I didn't know that

[SSL]

That video looks like it was produced by a 13 year old. Whats with all the spazzy shit going on?

 

Probably because it was intended to be consumed by 13 year-olds.

[h264]

http://www.planetdj.com/i--B1500DPRO

 

>State-of-the-art 1400-Watt Class-D amplifier with comprehensive over-excursion, thermal and clip limit protection 

 

NOW, BIG subs and tweeters have a feature called over-excursion protection to keep the woofer from "bottoming out" or damaging the driver when it's pushed too hard.  This is handled with an electrical circuit, not a "port".

[Lutkeveld]

This is handled with an electrical circuit, not a "port".

Are you replying to my OP? Then I think you misread my post.

[creatip123]

A port is where you park your boat. Generally next to a body of water.

 

@LinusTech @nicklmg @Slick might wanna check this thread out.

 

Actually, port is the left side of a boat's deck, starboard is the right side.

[nekro]

 

Well, it's not a hole, it's a chamber that acts as a so called 'helmholtz resonator

 

Isn't it the effect that happens when you blow on a glass bottle of beer? . very nice analysis

[Lutkeveld]

@nekro Spot on! That comparison is often used because a lot of people can relate ;p

[Lutkeveld]

nevermind

[h264]

Are you replying to my OP? Then I think you misread my post.

 

no directly.  More of a note about Linus mentioning driver over-excursion in his quote in a funny way.

[Lutkeveld]

no directly.  More of a note about Linus mentioning driver over-excursion in his quote in a funny way.

 

With the holes? I think Linus was under the impression that a driver couldn't make enough excursion because of the air-pressure on the backside of the driver.

In fact; sealed and ported behave the same on the cone excursion side of things, except at and under the tuning frequency.

At tuning frequency the cone won't move much, because the port is creating almost all of the sound.

Under tuning freq sealed still has the air compliance, while ported just has a hole where air will be flushing in and out.

It has nothing to push against and thus excursion will increase rapidly

You can prevent bottoming the driver out by using a static subsonic filter, or a flexible (often DSP controlled) protection circuit.

 

For people that don't know what 'bottoming out' is and why it can cause damage:

The voice coil should remain in the magnetic gap to produce a linear sound (as in: the same as the input but louder)

When you push it further than its linear capability (called Xmax), it will eventually reach its damage point (Xmech).

At this point the voice coil slams against the backplate of the magnet assembly, leaving the VC deformed and unable to move in the magnetic gap.

[SSL]

With the holes? I think Linus was under the impression that a driver couldn't make enough excursion because of the air-pressure on the backside of the driver.

In fact; sealed and ported behave the same on the cone excursion side of things, except at and under the tuning frequency.

 

If not excursion, does this affect damping?