Subwoofer Servo

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#2
Theres a thread or three on the subwoofer servo stuff in this forum.(search for motional feedback also)

2ways
- Using 2nd Voice coil as pickup (see electronics world 1990s)

- Using a transducer that measures the subwoofer movement.

http://www.diyaudio.com/forums/sear...62684062&sortby=lastpost&sortorder=descending

The EW article I used to have scans of,stated that output from two 10" DCV average sorts of car woofers,with servo on,had <5% THD at 15hz. SPL wasnt stated,but it was probably audible (if they are decent guys lol) - 105dB +.
 
#3
I don’t know if it has been discussed here before, but there is a possible principal weakness with servo-controlled bass speakers, using sensors on the driver cone. May be worth considering:
When the speaker and a node (of one of the standing waves in the room) are spatially coinciding, the speaker cone movement will be reduced because of the higher acoustic impedance. (In other words, here the pressure is high and the movement of air small). At such frequencies, the cone’s coupling to the air is at its most effective, and if the servo tries to compensate for the reduced cone movement by increased input from the amp, the standing waves will only reach new heights, degrading the final result as never before…

(just something I read somewhere).

But after all, the standing waves in the typical listening room, isn't this the only remaining big problem? I mean, anybody can build or buy a subwoofer which gives the wanted level and frequency range etc.
 
#4
Mr. Erath has a servo of sorts but it forms a loop from pre-amp signal to amp output. The actual driver is in a sealed enclosure that resonates at 70Hz and have a "known" response.

He has a couple of patents on it but if your DIY'ing it you can build it yourself.

I bought a couple of the car units(one for me and one for my brother) and a home unit. I used a Bryston 4b and a pair of 15" woofers in a 1.25 cu ft. ea. enclosures. Flat, tight and responsive down to 20Hz.

Crossed over @125Hz to a JRDG Model 2 driving a pair of B&W Matrix 805's. Sounded great but the front port on the B&W's was not ideal. @ 125Hz it was more of a bi-amped 3 way instead of a sat /sub system.

He has just "micro-ised" the unit and made it very small. He is building my new one now. This one will be active speakers with 2 x 10" Aluminum woofers, B&W 6.5 " mid(robbed from the 805's) and SS Revelator tweeters bi-amped with UcD 180AD on top and ST on bottom.

Sorry to digress.

Anyway I mentioned this since it is a "third type" of servo. But this one's loop only encompasses the processing and amp.
 
#5
sletol said:
I don’t know if it has been discussed here before, but there is a possible principal weakness with servo-controlled bass speakers, using sensors on the driver cone. May be worth considering:
When the speaker and a node (of one of the standing waves in the room) are spatially coinciding, the speaker cone movement will be reduced because of the higher acoustic impedance. (In other words, here the pressure is high and the movement of air small). At such frequencies, the cone’s coupling to the air is at its most effective, and if the servo tries to compensate for the reduced cone movement by increased input from the amp, the standing waves will only reach new heights, degrading the final result as never before…

(just something I read somewhere).

But after all, the standing waves in the typical listening room, isn't this the only remaining big problem? I mean, anybody can build or buy a subwoofer which gives the wanted level and frequency range etc.
Click to expand...

Check this reply from me, I used a microphone and you can see a 3 db improvement in room response at 70 hertz.
Because a microphone senses air pressure instead of acceleration, the room modes get somewhat equalized.
The effect is not very high because the room modes are not very high at the cone.
 
#6
bobo1on1 said:


Check this reply from me, I used a microphone and you can see a 3 db improvement in room response at 70 hertz.
Because a microphone senses air pressure instead of acceleration, the room modes get somewhat equalized.
The effect is not very high because the room modes are not very high at the cone.
Click to expand...

Still a nice result, bobo.
I wonder how the curves would look if the mic was mounted an inch or two before the cone. The same, more or less?
 
#8
Sub Servo

The goal of this project is to see what I can build and have it work. Too me the more complex the bigger challenge and the bigger reward.

I'm just trying to push what I'm able to do and have it produce sound.

The quality of the sound is secondary. In my next DYI projects I will worry about that.

At the moment I'm trying to learn as much as I can and the only way to learn is to build some thing.

Thanks to All.
 
#10
Just a thought: how about a pressure transducer measuring the pressure inside the cabinet? There are little semiconductor strain gauge pressure sensors that are fast enough to be used as microphones, and presumably the pressure inside the box can be related to sound output. And that may even be more accurate than just measuring cone movement.
 
#11
troystg said:
Mr. Erath has a servo of sorts but it forms a loop from pre-amp signal to amp output. The actual driver is in a sealed enclosure that resonates at 70Hz and have a "known" response.

He has a couple of patents on it but if your DIY'ing it you can build it yourself.

I bought a couple of the car units(one for me and one for my brother) and a home unit. I used a Bryston 4b and a pair of 15" woofers in a 1.25 cu ft. ea. enclosures. Flat, tight and responsive down to 20Hz.

Crossed over @125Hz to a JRDG Model 2 driving a pair of B&W Matrix 805's. Sounded great but the front port on the B&W's was not ideal. @ 125Hz it was more of a bi-amped 3 way instead of a sat /sub system.

He has just "micro-ised" the unit and made it very small. He is building my new one now. This one will be active speakers with 2 x 10" Aluminum woofers, B&W 6.5 " mid(robbed from the 805's) and SS Revelator tweeters bi-amped with UcD 180AD on top and ST on bottom.

Sorry to digress.

Anyway I mentioned this since it is a "third type" of servo. But this one's loop only encompasses the processing and amp.
Click to expand...

Troystg, I am interested in anything Erath. I have a pair of his LWE IVs that have 4X 15" woofers per cabinet! Hooked to a sub-harmonic synth I can pop your ears.

Where did you buy your Erath gear? Is there a web site with products for sale? I am familiar with lwespeakers.com but that seems to be more of a personal site than a sales outlet.

Thanks.
 
#12
Mr. Erath is well and good.

He just finished my new home units and I am adding the high freq amps to the case now.

I'm using 4 Hypex UcD modules (2 highs, 2 lows) with his servo controller to bi-amp my home speakers.

He is in Abbeville and the sight you saw was his "official" sight as he is not a PC or web guru but an audio and electronics guru. 😀

He has a new pwred 8" two way speaker called the "Trout" that sounded pretty good, and if your room is on the large side he has a pair of jacks on the rear of the built in amp to drive a passive 10" unit.

For all, Mr. Eraths unit is not really a servo but more of a feedback /sampling design that incorporates the amp and woofer VC into a loop.
 
#13
dangus said:
Just a thought: how about a pressure transducer measuring the pressure inside the cabinet? There are little semiconductor strain gauge pressure sensors that are fast enough to be used as microphones, and presumably the pressure inside the box can be related to sound output. And that may even be more accurate than just measuring cone movement.
Click to expand...

I heard about this idea from a friend, but Google only turned this thread up, not the project. Can anyone find any examples of it online? What are the relative merits and pitfalls of this scheme? It sounds pretty feasible and to be able to use any driver without modification in a servo sub is attractive.
 
#17
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