Using a subwoofer as a pulse air pump - Home Theater Forum and Systems -

Thread Tools
post #1 of 10 Old 10-28-11, 09:23 AM Thread Starter
New Member
Join Date: Oct 2011
Location: Cologne, Germany
Posts: 3
Using a subwoofer as a pulse air pump

Hi everyone. I've been an audio enthusiast for quite some time and built all my own stuff, both car and home, some time ago but it's been a while. This forum has seemed to be the most technically advanced over the years so I figured I would post this here and see what you all think.

This is somewhat of a 4th order bandpass port velocity question, but much more complicated.

I am attempting to make an air pulse pump with frequencies of 20-250Hz to study pulsing air heat transfer effects on a few different sensors for a research project. Basically attempting to simulate exhaust flow in an engine but at lower temperatures. Normally a piston machine would be used in the lab environment, but 250Hz is 12800 RPM which would take something akin to a 2 stroke engine driven by an electric motor to achieve along with an oil supply, cooler, contamination issues to deal with, etc. In my opinion the best thing to use with minimum infrastructure would be a subwoofer, not only for the higher freq needed, but also because freq and amplitude can be adjusted independently of each other.

What I've thought of is in the diagram below. The back side of the subwoofer would be enclosed in a box and ported (if needed) into the front chamber because I need to minimize the sound energy that gets into the lab, and the free end would be vented outside through an exhaust stack with muffler. The front chamber would be made as small as possible to maximize the volume change of the chamber and pump as much air as possible with the reed valves acting as a combination one way valve setup similar to a standard air compressor. The tubing with the measurement sensors is stainless steel with 3mm wall so sound radiation shouldn't be too much of a problem, and I can get my hands on anything from 25 to 100mm diameter to adjust flow and velocity. I can also get my hands on as many reed valves as I need. I'm hoping to create peak velocities of 150-160 m/s and a flowing average of 60-70 m/s in the test section.

I would probably be looking at a 15" subwoofer and can drive it up to 1200 watts given what our lab has on hand.

I've looked at quite a few programs like WinISD and pulled out my acoustics textbook from my first round of school but haven't been able to find much that will give me an idea of performance. Everything I find is for two way port or free air flow, which I expected because this is a completely different setup. I know this isn't a normal sound in air problem, but wondering if anyone had insights on what this might be capable of for airflow. I know it will be somewhat trial and error until I get it tuned in, but any pointers would be appreciated.
Attached Files PulsePump.pdf (2.4 KB) 
justin.rajewski is offline  
Sponsored Links
post #2 of 10 Old 10-28-11, 03:47 PM
HTS Senior Moderator
robbo266317's Avatar
Join Date: Sep 2008
Location: Newcastle Australia
Posts: 5,623
Re: Using a subwoofer as a pulse air pump

Welcome aboard.

I don't see why it wouldn't work and I would be interested to see the finished project.


Silence is golden but duct tape is silver.

DIY completed:
robbo266317 is offline  
post #3 of 10 Old 10-28-11, 04:49 PM
Elite Shackster
Mike P.'s Avatar
Join Date: Apr 2007
Location: Chitek Lake, Sask.
Posts: 19,397
Re: Using a subwoofer as a pulse air pump

I can't give you any pointers but I'm looking forward to seeing how this develops.
Mike P. is offline  
post #4 of 10 Old 10-29-11, 06:29 PM
Join Date: Sep 2011
Location: between here and there
Posts: 46
Re: Using a subwoofer as a pulse air pump

Your concept is sound...(pun intended )

Personally I'd build a motor driven piston using UHMW and stainless steel sleeve vs a subwoofer but if you want to try it you'll need a large driver 15" or so with a very strong cone. (this will be the point of failure.) some titanium reed valves (semi loose flap) and duct tape (kidding)

How large does the output pipe need to be, diameter? flow can be calculated in WinISD so you can figure necessary excursion. i would but the output reed valves in a larger tube so you can have a lot of surface area of reed then a reduction coupling to the final diameter.
revboden is offline  
post #5 of 10 Old 10-29-11, 07:18 PM
Join Date: Sep 2011
Location: between here and there
Posts: 46
Re: Using a subwoofer as a pulse air pump

I'd do something like this to keep the input air spinning and balanced
Attached Thumbnails
Using a subwoofer as a pulse air pump-pump.jpg  

revboden is offline  
post #6 of 10 Old 10-31-11, 06:24 AM Thread Starter
New Member
Join Date: Oct 2011
Location: Cologne, Germany
Posts: 3
Re: Using a subwoofer as a pulse air pump

The cone strength kind of concerns me too. I was planning on trying to find a driver made of carbon sandwiched honeycomb for the bending stiffness, or just go all out and use something like a Soundstream XXX or JL 15W3. This thing may be running for long periods of time, so I think if may be best to use a ported box (rear chamber ported into front chamber) to keep the heat from building up as well. Otherwise I would need some sort of cooling system inside a sealed enclosure.

The reed valves are for commercial diesel engine EGR systems with stainless flaps and full silicon seals.
justin.rajewski is offline  
post #7 of 10 Old 11-06-11, 06:42 AM
Join Date: Jan 2010
Location: Southern IN
Posts: 96
Re: Using a subwoofer as a pulse air pump

Performance wise, wouldn't the volumetric flow simply be excursion*Sd*freq. roughly? You would have to account for nozzles and such to get velocity but that should give a rough approximation. Whats the sectional area of the test section?

What type and displacement engine are you trying to simulate? You mentioned that the reeds were for a diesel EGR system but I've never seen a diesel go over 5000 rpm. Can't imagine 15K would be possible in a diesel since it's limited by flame speed. Seems that would be a strain on anything but a bike or short stroke racing engine. The reason I mention is because the shorter the passband you seek the easier the driver box combo will come. Also comparing the approximation above with the engine displacement*speed should give you an idea of level of excursion and SD you need. Might be able to get away with a high xmax 10" sub if your test section is small.

Will you need to modulate speed while running or will you set and let it run a while? Seems you would need to have the excursion curve loaded in your control system to modulate output in situ. But if set and let run wouldn't be a problem.

also as a thought since it sounds like your working at a unversity. could this be acheived in a windtunnel with some motorized flaps in front of the intake? Kinda like intake louvers that open and close and would pulse the stream in the test section. Could power with an electric motor and a fourbar linkage , just like a windshield wiper mechanism, but 250 Hz would prolly pose a problem. Also the noise issue might be a problem depending on the size of your tunnel (extremely large subwoofer!). This would allow for flow study and you might see some interesting effects if you could mate the main flow pulsing with the vortice shed frequency of the sensor. A wind tunnel would already have the pitot tubes, etc. also.

Also a variable displacement piston pump would be fairly simple to construct. all you have to do is incorporate a turnbuckle on the throw of the crank. Wouldn't be able to modulate while running though with out some real complicated mechanisms though which is why you never see variable displacement engines of this type, they always work via cylider deactivation.
memarcus is offline  
post #8 of 10 Old 11-10-11, 03:28 AM Thread Starter
New Member
Join Date: Oct 2011
Location: Cologne, Germany
Posts: 3
Re: Using a subwoofer as a pulse air pump

The volume flow will be a function like you said, but the reed valves will have a certain transfer, or volumetric efficiency function proportional to the pressure required to open them at a given frequency. I'm using around 80% as a starting point. Remember that the reed valve opens and closes with every pressure pulse, not every engine revolution, so up to even 400 Hz is not much of a problem for them.

The engine size doesn't really matter as I can adjust flow velocity to achieve the equivalent Reynolds number by changing the pipe diameter after the nozzle. I'm just limited to 25-100mm because the sensors are only accurate in that range in free stream flow.

My biggest problem right now is trying to model "port" resistance. I've figured out how to somewhat trick WinISD into the operational scheme I want, but there is no way to input asymmetrical port resistance. The inlet valves will have the atmosphere as a big pressure reservoir, but the outlet valves to the tube will have to deal with the pressure differential between the tube with the associated pumping loss through it and the front chamber, plus the overall streamline will be different between the two. The flow resistance won't be the same in both directions. Any ideas on what the end correction should be if the inlet direction of a port has much lower resistance than the outlet direction?

Nothing like a wind tunnel is available, but we do have a few flow benches that I may be able to adapt to it once I prove out the basics. That would allow additional massflow with flow that doesn't come to a complete stop with each pulse. But before I can do that, I need to get a pressure/velocity trace and compare it to an engine to prove I need it. My time is "free" but any dedicated stuff the project gets charged for.

If I get this thing built, I plan on doing a frequency vs power map to create a 3D massflow profile. Then I want to compare that to the predicted cone excursion in WinISD and come up with a volumetric efficiency profile. It's not really pertinent to the project, but would be good to know. Now if only I could track down a LASER interferometer to measure real time cone excursion...
justin.rajewski is offline  
post #9 of 10 Old 11-11-11, 06:16 PM
Senior Shackster
Mark Seaton's Avatar
Mark Seaton
Join Date: Jun 2006
Location: Chicago, IL
Posts: 107
Send a message via AIM to Mark Seaton
Re: Using a subwoofer as a pulse air pump

Hi justin,

While not really my area of expertise on the flow behavior and resistance in the exhaust pipe, the point where I would focus some attention is the asymmetrical loading of the driver on each 1/2 cycle. I would think you would need to match the cone's resistance for compression and rarefaction, else the driver will bias one way during use, most likely inward. One way around this might be to make the rear chamber of the box rather small so the box compliance swamps the function, but this will affect the required input power.

The first question I would have is do you need to adjust the frequency on the fly, or could you make adjustments per frequency test? If discrete frequency testing is fine, you could have more efficient options.

With the acoustic setup you have sketched, I would think your primary concern is the inlet valves and tubing to have similar behavior, thresholds and resistance to those at the entrance to the pipe to keep the resistance similar on each 1/2 cycle. Any difference in the compression/rarefaction could cause the driver to bias, most likely inward.

I would also recommend changing your configuration to use 2 drivers that are face to face, so you don't have to deal with rocking forces and physical vibration, or at least place them with less than a 45 deg angle between the mounting baffles. Doubling drivers also helps greatly with power handling. Do also be conscious of the impedance curve of the driver, as you can pump a lot of Voltage into a driver at an impedance peak without much heat dissipated, but at a minimum and at frequencies well below the lowest peak, you have to be careful.

Check with LMS and Klippel on the laser measurement devices, but you can probably work something up for much cheaper after looking over what they sell. For a simple means to observe the behavior, especially any biasing of the driver, I would just make one side clear acrylic or similar and install a strobe light with a fine variable frequency control. Adjusting the strobe frequency makes it easy to view motion or freeze the operation.

Just some quick thoughts on what you might face there, but certainly an interesting concept. Good luck!

Mark Seaton
"Make no little plans; they have no magic to stir men's blood..." - Daniel H. Burnham
Mark Seaton is offline  
post #10 of 10 Old 11-12-11, 03:15 PM
Elite Shackster
bambino's Avatar
Join Date: Feb 2010
Location: Iowa
Posts: 4,418
Re: Using a subwoofer as a pulse air pump

Mike P. wrote: View Post
I can't give you any pointers but I'm looking forward to seeing how this develops.
bambino is offline  


air , pulse , pump , subwoofer

Quick Reply

Register Now



Confirm Password
Email Address
Confirm Email Address
Random Question
Random Question #2

User Name:
Please enter a password for your user account. Note that passwords are case-sensitive.


Confirm Password:
Email Address



Activation requires you reply to an email we will send you after you register... if you do not reply to this email, you will not be able to view certain areas of the forum or certain images... nor will you be able download software.


See our banned email list here: Banned Email List

We DO NOT respond to spamcop, boxtrapper and spamblocker emails... please add @hometheatershack DOT com to your whitelist prior to registering or you will get nowhere on your registration.

Email Address:


Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.

Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page

Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML is not allowed!
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


For the best viewing experience please update your browser to Google Chrome