what is Free air design?

[bhvm]

hello everyone.
I'm new to audio world and I thought why not ask you experts About some questions buzzing my head.

what is Free air woofer and how is it different from sealed box woofer?

what are physical difference in free air and sealed woofers?

I took out some small woofers from their ported enclosures as I wanted to make sealed boxes for them. I just tried them for curiosity and there was negligible bass even though they were pulsating really well. what could cause this?

will they work well in sealed boxes?

I'll send the pics soon.

 

[Hari Iyer]

Free air data for the woofer are published where the TS parameters are mentioned. All TS parameter for a driver is mentioned with the driver suspended in free air with no reflection or pressure build from any sides.

In real time scenario drivers are mounted on an enclosure and the driver parameters vary when they are mounted on a baffle than if suspended in free-air. Even so free-air data is used for calculation of the box volume and tuning of the port, the amount of damping required in the box etc.

Driver when suspended in free air is not capable of high power handling and all TS parameters for the drivers are small signal parameters measured with around 1watt @ 1 meter.

 

[bhvm]

can you explain me in simpler terms about construction difference among free air and sealed woofers? can a sealed one be converted to free air and vice versa?

 

[Hari Iyer]

can you explain me in simpler terms about construction difference among free air and sealed woofers? can a sealed one be converted to free air and vice versa?

As per my information, the free air data is just for measuring the driver TS parameter and has to be used for designing the speakers when mounted in eithher a Sealed, vented, TL or other enclosure types. You cannot mount a driver in free air and expect it to perform well. Exceptions might be tweeters which can be mounted in free air. Any woofer will require a baffle and/or and enclosure around it for it to sound optimum. Free air means the driver is not mounted but suspended in space with no reflections and the data measured with this is called free air data.

 

[bhvm]

As per my information, the free air data is just for measuring the driver TS parameter and has to be used for designing the speakers when mounted in eithher a Sealed, vented, TL or other enclosure types. You cannot mount a driver in free air and expect it to perform well. Exceptions might be tweeters which can be mounted in free air. Any woofer will require a baffle and/or and enclosure around it for it to sound optimum. Free air means the driver is not mounted but suspended in space with no reflections and the data measured with this is called free air data.

thank you very much for the explanation. may I also know that why a driver when taken out of it's enclosure produces no bass at all? I mean what's the theory here?

also, how should I go about converting ported units into new sealed boxes.? assume I don't have any ts data available. just physical driver and it's old ported box. thanks.

 

[koushik]

When the diaphragm of speaker moves it produces sound waves in both the direction(front and back). The sound waves are exactly similar but in opposite phase. Now when this waves (in opposite phases) interact with each other and due to destruction interference according to superposition theorem, they cancels each other. Hence we don't get to hear bass.

Why do you want to put a driver from ported box to sealed box? Because that that driver may be made to excel in ported enclosure.

 

[bhvm]

hello again,
that's because I don't like the ported sound. it's too boomy and muddy. these woofers were lying around for the same reason. so I thought why not make new box and put them to good use?

I have also heard that sealed boxes can be made little smaller and they're very forgiving. I'm looking forward to make slim sealed boxes so that I may put them under my car seat . how will I know that they'll adapt well to a sealed box?

amp is capable of delivering far more powers than these babies can handle.

 

[koushik]

that's because I don't like the ported sound. it's too boomy and muddy.

It is not because of port. Best of the best speakers in the world are ported design.

I have also heard that sealed boxes can be made little smaller and they're very forgiving.

Yes you have heard it right. They are forgiving but not to the extend that you can make any enclosure. The error tolerance is a bit high for sealed design. You still have to know the correct volume.

Close the port of the existing enclosure with a piece of wood and seal the edge properly with m-seal. See if you like what you hear. Then we can think of the next approach.

 

[bhvm]

It is not because of port. Best of the best speakers in the world are ported design.



Yes you have heard it right. They are forgiving but not to the extend that you can make any enclosure. The error tolerance is a bit high for sealed design. You still have to know the correct volume.

Close the port of the existing enclosure with a piece of wood and seal the edge properly with m-seal. See if you like what you hear. Then we can think of the next approach.

probably the default boxes are crap. tuned too high and give a boomy sound that teenagers seem to like. that's the reason even plugging the hole etc will not help much. I'm thinking to make a 14 inch x 12 inch x 4 inch box. driver is 6 5 inch. please pour your expertise .

 

[koushik]

I am no way a speaker designing expert. Just keep myself informed reading different DIY articles.
As we don't have the T/S parameters, it will always be a guess work. Give your design a shot and see if that works.

Just a rough estimate, build a enclosure about 18 lt in volume. And avoid cube shape.

 

[bhvm]

I am no way a speaker designing expert. Just keep myself informed reading different DIY articles.
As we don't have the T/S parameters, it will always be a guess work. Give your design a shot and see if that works.

Just a rough estimate, build a enclosure about 18 lt in volume. And avoid cube shape.

all the help is welcome no matter what.
my design comes to be 672 cubic inches. it's a slim rectangle, not cube. 14x 12x 4 inches.
what do you mean by 18 lit? isn't that way Too big for 6.5 inch woofer?

 

[koushik]

what do you mean by 18 lit? isn't that way Too big for 6.5 inch woofer?

No, I don't think so. It all depends on the T/S parameter and not the size of the woofer cone.

I have made a Bookshelf with 4" mid/bass driver which is 10 X 12 X 13 = ~ 19.5 lt. I know it is bit large. The calculation estimated 16.7 Lt but after building it became this. Although after internal absorbing materials, port and driver displacement it is close to 18 lt now.

 

[bhvm]

thanks for all the guidance. I'll build the boxes and let you know.

on the side note, the amp installed in my car gives loud buzzing sound from the speakers attached to it. the sound is absent in factory speakers connected to factory hu. also, the buzzing goes away when low pass Filter in engaged on the amp but is very audible in full range or hpf mode. thanks.

 

[keith_correa]

I suggest you don't build anything without the T/S parameters. Measuring the T/S parameters is not rocket science. All you need is a resistor, a PC with a soundcard and REW from hometheatreshack. This software is just one among many that will allow you to do the same thing.

 

[captrajesh]

Driver when suspended in free air is not capable of high power handling

Is it so!! News to me.

 

[Hari Iyer]

Is it so!! News to me.

I think so, because the suspension of the driver will not be able to hold the excrusion of the driver for longer time. In an enclosure the back pressure from the box helps to balance the suspension and keeps the suspension in place. In free air the spring effect is lost and the driver can get damaged soon. You can try this out in any driver mounted on a box and in a free air, the excrusion will be higher in free air. Driver manufactures also caution not to play the driver in free air with high power else it will get damaged beyond repair. Exceptions might be some open baffle speaker which are designed to handle high excrusion and the TS parameters allows such operation.