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BR simulation vs experimental results.
BR simulation vs experimental results.
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Old 3rd June 2019, 02:37 AM   #1
oon_the_kid is offline oon_the_kid  Malaysia
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Default BR simulation vs experimental results.

Hi,

I am wondering if anyone here has tried to compare their simulation result and actual experimental results.

In my latest project, I simulated a BR speaker with a 4" driver. I chose a box frequency much lower than Fs. The simulation goes from 100Hz to about 50Hz with a slow decline, maybe about 6dB before it drops off rapidly for going below port tuning frequency. Port tuning frequency about 60Hz. But when I built and measured it, I have a massive drop in frequency between 100Hz to 50Hz.

Any idea what could have caused it?

Several theory of mine, hopefully somebody with more experience can chime in.

1) The measurements are off - I am doing it in room with a quasi anechoic technique using Dayton omnimic. Quasi anechoic is notoriously unrealiable in the room due to room modes

2) My port is located at the bottom, I suppose the simulation may assume that the port in in front.

3) The simulation doesn't take into account the rise in impedance, assume that the power delivery is always constant, whereas in real circumstances, there is two impedance peaks where power delivery is minimal

Thanks in advance for all your thoughts and suggestions.

Oon
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Old 3rd June 2019, 04:21 AM   #2
GM is offline GM  United States
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Right, basic sims assume that the vent, driver are emanating from the same point in space.

GM
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Old 3rd June 2019, 05:34 AM   #3
wonderfulaudio is offline wonderfulaudio
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Quote:
Originally Posted by oon_the_kid View Post
2) My port is located at the bottom, I suppose the simulation may assume that the port in in front.
The port location does not matter at those frequencies as long as there is nothing obstructing the port. If your port is at bottom, you may need to lift the box to get sufficient clearance between port and ground.

Quote:
Originally Posted by oon_the_kid View Post
3) The simulation doesn't take into account the rise in impedance, assume that the power delivery is always constant, whereas in real circumstances, there is two impedance peaks where power delivery is minimal
The simulations assume a constant voltage source and thus takes are of impedance variation, the impedance varies quite a lot as compared to Re. If impedance variations were not taken into consideration then the simulations would be useless.

Little bit of result variation between simulation and reality is expected and of not much consequence.
Measure outdoors on ground and compare with a half space simulation.
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Old 3rd June 2019, 05:43 AM   #4
oon_the_kid is offline oon_the_kid  Malaysia
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Hi GM,

Thanks for the information, so there would be some difference if the port is below, since the so called front component is significantly less. In which case the increase in amplitude due to constructive inteference when both port signal and driver signal combined is also be significantly less. Would that be correct?

That aside,

Does anybody know what perceptible difference in bass quality in front firing port, rear firing port and bottom firing port.

Theorectically, they should be the same, as the wavelength is very long, and it should be the same everywhere. But of course that is only theory.

Oon
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Old 3rd June 2019, 06:38 AM   #5
GM is offline GM  United States
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Greets!

It depends as wonderfulaudio noted; the actual vent output WLs are huge [344/pi/60 = ~1.825 m diameter], so unless the woofer is further away than typical or the floor is padded/carpeted it will depend on how you measured it, room interaction and how accurate your mic is down low.

Also, did you sim with published specs and if so, is it a brand known for accuracy?

FWIW, I was taught to measure drivers at < ~1/4" [.635 cm] from the dust cap, vents, horns, etc., with the mic centered on axis inside ~1/4" [.635 cm], i.e. a 1/2 WL of ~27,120 Hz, to take the room out of the 'equation'.

GM
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Old 3rd June 2019, 10:56 AM   #6
Michael Chua is offline Michael Chua  United States
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Oon

Can you post the Omnimicmeasurement and the simulation?

Also, the driver you are using.

Regards
Mike
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Old 4th June 2019, 03:09 AM   #7
Brett is offline Brett
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Quote:
Originally Posted by oon_the_kid View Post
That aside,

Does anybody know what perceptible difference in bass quality in front firing port, rear firing port and bottom firing port.

Theorectically, they should be the same, as the wavelength is very long, and it should be the same everywhere. But of course that is only theory.

Oon
My experience is that if the port and driver are relatively close, eg, <1/4 wavelength at tune, it generally doesn't matter with conventional designs. My first preference based upon a number of factors is bottom ported, but it's been a while since I did a BR speaker. The next one I have designed on paper is bottom firing.
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Old 4th June 2019, 04:12 AM   #8
diyuser2010 is offline diyuser2010
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Quote:
Originally Posted by oon_the_kid View Post
Hi,

I am wondering if anyone here has tried to compare their simulation result and actual experimental results.

In my latest project, I simulated a BR speaker with a 4" driver. I chose a box frequency much lower than Fs. The simulation goes from 100Hz to about 50Hz with a slow decline, maybe about 6dB before it drops off rapidly for going below port tuning frequency. Port tuning frequency about 60Hz. But when I built and measured it, I have a massive drop in frequency between 100Hz to 50Hz.

Any idea what could have caused it?

Several theory of mine, hopefully somebody with more experience can chime in.

1) The measurements are off - I am doing it in room with a quasi anechoic technique using Dayton omnimic. Quasi anechoic is notoriously unrealiable in the room due to room modes

2) My port is located at the bottom, I suppose the simulation may assume that the port in in front.

3) The simulation doesn't take into account the rise in impedance, assume that the power delivery is always constant, whereas in real circumstances, there is two impedance peaks where power delivery is minimal

Thanks in advance for all your thoughts and suggestions.

Oon
The drooping response will sound like less than a quarter as loud ( due to the way our hearing at low frequencies works at low sound pressure levels ) See: Robinson–Dadson curves - Wikipedia Robinson–Dadson curves - Wikipedia

At an 80dB midband level around 1khz, you need the bass at 60 hz to be 100 dB, or 20dB louder to be perceived at the same level. For highest efficiency, tune at Fs or higher. Not to say that one can't tune below that, though you are in the compliance controlled region ( not mass controlled ) and if the suspension is tight, you may not get near what the simulation says.

Also, you must use a proper high pass filter with a ported enclosure, below tuning the driver will be unloaded, and oscillate without control. Quasi-anechoic, can you elaborate? Are you remote from walls, floor and ceiling, with the microphone 1/4" to 1/2" or so from the dustcap? This method will not show the port contribution, the output from the woofer would be much louder. Forget the mic for a moment, place the box with the driver face up on the floor, and play some music through it. Does it have any bass? You may have other issues with signal chain, or simply being in a null in the room. Check the port tuning frequency with a signal generator, you may be way off target. ( if you haven't measured it and are assuming )

Did you account for the lost volume of the port in the actual construction size? 60 hz tuning with a 4" driver is an optimistic, limited SPL application only. Like boombox or small bluetooth speaker application, where nearfield listening takes place. Sealed, a 4" driver is good for 75 dB @1 meter @ 1.08 mm excursion ( one way ) or 2.16 mm p-p, driven with a 60 hz sine. Piston Excursion calculator In a ported enclosure, the excursion maxima above tuning is ~1.5x your tuning frequency.

Choosing 60 hz tuning, ~ 90 hz would be your excursion maxima, assuming roughly 2 mm P-P clean excursion ( 1 mm one way) limits your maximum output to approximately ~ 81 dB peak @ 1m. No allowance for dynamic range head room is made, nor is there a reduction in the LF. ( drooping response, possible coil offset, rock hard suspension, motor asymmetries, etc ) If you have a drooping anechoic response, placement near intersecting boundaries will add back some gain. ( floor / wall, ceiling / wall, etc ) Placed in free space, the small driver with drooping response will seem very underwhelming. Hope that helps.
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Old 4th June 2019, 05:59 AM   #9
celef is offline celef
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What does your ears say? Relying on measurements only is not advisable. For low frequencies, near field measurements are good
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Old 9th June 2019, 09:12 AM   #10
oon_the_kid is offline oon_the_kid  Malaysia
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Quote:
Originally Posted by Michael Chua View Post
Oon

Can you post the Omnimicmeasurement and the simulation?

Also, the driver you are using.

Regards
Mike
Hi,

Sorry for the late reply. Have been a bit busy lately.

The speaker driver is a Faitalpro 4Fe32

The freq response by Omnimic measurement shows a 12db drop from 150Hz to 50Hz. Distance from mike to speaker is about 50cm. The simulation (woofer box model and circuit designer by Jeff Bagby) shows a drop of about 6db only.There is also a further drop of 6db at port tuning frequency (circa 60Hz). I had also verified by impedance measurement using the DATS system the port tuning frequency is at 60Hz.

The volume of the cabinet is about 3l and is a 9inch sphere.

the cabinet is a wooden sphere of about 9 inch in diameter. The driver is facing the front and the port is at the bottom. The distance between port and bottom surface is about 1 inch.

Thanks.

Oon
Attached Images
File Type: jpg Freq response.JPG (39.6 KB, 223 views)
File Type: jpg Simulation of freq response.JPG (63.5 KB, 223 views)
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