Speaker Related Projects

(A T-line and ribbon tweeter 2-way. August-2012)

(A tall, thin, upwards firing omnidirectional speaker. May-2010)

(A powered subwoofer using a 12" driver and 15" passive radiator. Jan-2010)

(A computer speaker; redux. December-2005)

(A computer speaker in a light canister. Jan-2005)

(10" vented subwoofer in a cardboard tube, powered by a Parapix amp. May-1999)

   MTM Center Channel Speaker
(A Madisound design. Nov-1997)

   2-way Surround Speakers
(5" woofer and 1" tweeter. July 1997)

   3-piece mini system
(6" DVC bass module mated to 4" car speaker. June 1997)

   3-way Vented Floorstanding Speaker
(vented 10" woofer, 5" mid and 1" tweeter in a 4 ft tower. Summer 1995)

   NHT1259 Subwoofer
(A 12" woofer in a sealed architectural pedestal. Winter 1994-95)

   Inexpensive Speaker Stands
(Particle board, sand and spray paint. Fall 1994)

   2-way satellite
(6.5" woofer and 1" tweeter. Summer/Fall 1994)

Electronics Related Projects

  900 MHz Audio Receiver
(Better use for bad headphones. Jan-2008)

  Buster - A Simple Guitar Amp
(Perfect for the beginner. Jan-2010)

  A PC-based Audio Console
(Use a PC to play tunes. Jan-2010)

  LM-12 Amp
(Bridged LM-12 opamps. Aug-2003)

(A CD player and FM tuner from spare computer parts. Oct-2002)

   Quad 2000 4-Channel Amp
(Premade modules by Marantz. May-1998)

   Zen Amp and Bride of Zen Preamp
(by Nelson Pass. Apr-1997)


  Backing-up LPs to CD-R
(Whiningdog.net 10-Dec-2002)

  Using Wood in Speakers FAQ
(Work in progress)

   MDF FAQ for speaker builders

   Woodworking Tools for the DYIer
(HomeTheaterHiFi.com Oct-1998)

  Some Thoughts on Cabinet Finished for DIY Speakers

   Large Grills Made Easy

   Some Parts Suppliers

Other Useful Stuff

   DIY Audio Related URLs

  Veneering Primer
(by Keith Lahteine)

   How to get a Black Piano Finish
(by DYI Loudspeaker List members)

   Sonotube FAQ
(by Gordon McGill)

   Excerpts from the Bass List
(Oldies but Goodies)

DIY Loudspeaker List

   Current DIY Loudspeaker Forum Home

   Former DIY Loudspeaker List Subscription Page

  DIY Loudspeaker List Archives

Excerpts From the Bass List

Here are some old posts from the Bass List that some might find useful.

Subject: How Stuffing Increases Box Size: Adiabatic vs. Isothermal [Was: F illed Box Effective Volume]
From: owner-bass <owner-bass@lunch.engr.sgi.com>
Date: Fri, 31 Mar 1995 04:56:00 -0500
To: bass <bass@lunch.engr.sgi.com>
From: Douglas Purl

>> How exactly does the filling "effectively" increase the box volume? I
>> can imagine the filling increase the box's absorptive losses, thereby
>> increasing the damping. But what exactly causes the lower resonance
>> frequency of filled vs. unfilled? Does the filling cause an
>> increased air load on the driver diaphragm and/or add moving mass
>> itself? Perhaps I should break down and buy a copy of Beranek? :-)

I will attempt to answer this question so that no technical knowledge or mathematical background is necessary to its comprehension. Because I believe two formulas clarify the description, I do include and refer to them.

A sealed box, unless a vacuum has been created within it, contains a gas in the form of air. The gas is composed of a number of particles randomly distributed. If one or more walls of the box contains a movable boundary in the form of a loudspeaker diaphragm and motor, the means will be available to alter the pressure within the box. When the diaphragm presses into the box, pressure rises because the air particles are being violently forced into one another and compelled to occupy a smaller volume. (When the diaphragm moves outwards from the box, a vacuum compared to the ambient pressure--ambient pressure being zero, the neutral pressure of the box--is created and all the reckoning above likewise applies, but with a negative value.)

The air inside a sealed enclosure behaves like a simple spring. This spring is said to have a constant, which describes mathematically the compression factor in the air. Now the interesting matter that enters here is that there is a different result when pressure rises from a rapid change in volume and when it rises from a slow change in volume. Rapid changes are adiabatic and slow changes are isothermal (we will define these shortly). I will reproduce their representation here because the figures instruct us:

Adiabatic: Delta P = -1.4K Delta V
Isothermal: Delta P = -K Delta V

Where K = the gas constant, P = pressure, V = Volume, Delta = change in

Notice that the pressure change is greater in adiabatic compression than in isothermal. Why the difference?

An isothermal compression is one that takes place at a constant temperature. An adiabatic compression (or rarefaction, remember) is one in which the temperature rises during compression and falls during rarefaction.

During slow compressions, there is time for the heat generated by the compression of the air particles to be transferred to the walls of the enclosure; during slow rarefaction, the heat is transferred back into the air from the walls of the enclosure, keeping the temperature of the gas constant (and hence isothermal). [Note: "Slow" here does not refer to the rate of change, but to the velocity of propagation of the particle disturbance through the gas medium. The velocity is the same across the audible sound-wave spectrum into both infra- and ultra-sonic propagation.]

During fast compressions and rarefactions, there is insufficient time for the disturbed air particles to transfer their heat to the enclosure surfaces; hence, the temperature of the gas rises during compression and falls during rarefaction. In fact, under such circumstances the instantaneous temperature of the gas could be used to indicate the instantaneous pressure.

It so happens that sound in air observes the laws of rapid-change gas variations. Thus normal propagation-velocity sound in a sealed enclosure behaves adiabatically.

Now if the velocity of the air particles could be slowed, the adiabatic compression could be converted into isothermal. The velocity of gas molecules varies as the square root of the absolute temperature of the gas. In the adiabatic process, the heat of the gas rises and therefore the velocity of the gas molecules increases. The gas particles collide with each other more frequently and more violently, causing more momentum transfer from particle to particle. In the adiabatic process, the gas molecules get hotter, collide with each other and the box walls more frequently as they heat, and having acquired greater momentum, transfer more momentum to the walls.

Compared to adiabatic, in the isothermal process the air molecules are cooler, have less momentum, and collide with the enclosure walls less often. In other words, the enclosure looks larger to the confined gas.*

This is the principle that Edgar Villchur patented (which he later revoked rather than fight Jensen Loudspeakers over it--ironically, Jensen International now owns the company Villchur founded, Acoustic Research) in 1954 (or soon thereafter). Compared to the (virtual) infinite baffle, an acoustic suspension reverses the relation between the spring constants of the diaphragm spider and surround on the one hand and the spring constant of the entrapped air in the sealed enclosure on the other. One great advantage is that the isothermal air mass has a relatively linear compression behavior, whereas the mechanical spider and annulus vary in their stiffness according to position and temperature. The acoustic suspension principle necessarily results in lower distortion than the sealed-box method it displaced.

Isothermal compression is achieved by critical box stuffing. Too much and the enclosure walls are effectively constricted; just right and the enclosure walls are expanded. Thus the impedance contributed by critical stuffing in a sealed box converts the displacement/volume equation into an isothermal process and effectively couples the driver diaphragm to a larger enclosure.

*The matter discussed above is the first topic considered by Leo Beranek in *Acoustics*. I have the 1993 edition (revised in 1986) published--inexpensively--by the Acoustical Society of America (ISBN 0-88318-494-X) for $30 in hardback. Like every good text book, it requires comprehension of preceding matters before subsequent discussions can be understood. It is not for browsers, nor for those allergic to math. Despite its several revisions, it contains errors. For example, on p. 4 Beranek states that isothermal molecules will collide with the container more often and on p. 5 he states that adiabatic molecules will so collide more often. The ambiguity is the product of an insufficiently rendered discussion of the mechanics of alterations in gas pressures. Even so, the book is required reading for those who wish to understand the physics of sound and sound reproduction, and it is a steal at its subsidized price (491 pp. in a quality hardbound).

Subject: Stuffing Stuff
From: owner-bass <owner-bass@lunch.engr.sgi.com>
Date: Tue, 4 Apr 1995 03:41:00 -0500
To: Bass <bass@lunch.engr.sgi.com>
From: Ken Kantor
In light of recent discussions, let me share some thoughts regarding cabinet stuffing. I'll do this from a practical point of view, partly because the physics side has been well articulated by Doug. The other reason I'll stay away from theory is that, in the matter of cabinet fill, theory has proven over the years to be of only limited help in real-world speaker design. I'll also confine most of my comments to issues related to sealed systems. Vented systems do share a few of these same issues, but really the goals and the physics of stuffing a vented box are different.

Most professional designers would agree that practical experience, combined with trial and error, is best way to find the optimum stuffing material, quantity and method for a given design. This is why good designers routinely experiment with fill in the development of a new system, ala Vance's data cited here. This particular information is a valid data point, but it is important not to over-generalize. If you are designing a system that differs substantially in shape or volume or source impedance (passive crossover) from a known you will need to iterate for best performance.

In my practice, adjusting the filling is the last step in getting the bass right, and is used mostly to fine-tune the system Qtc and resonance. As increasing amounts of polyester are added to a sealed box, the resonance and Q gradually go down. This can be shown mathematically to be due in roughly equal parts to the effects of simple resistive damping and isothermal conversion. At some point, a mimimum is reached, and further material simply reverses the trend by taking up volume. During the filling process the impedance curve is constantly monitored, and convergence to optimum usually takes only a short time. Filling also has the important effect of reducing internal reflections, to reduce standing waves and comb filtering. However, the amount of filling has comparitively little effect on its efficacy in this regard.

[Side Note- it is a common misconception, I believe, that professional designers rely heavily on LEAP and SPICE and CALSOD to define their designs a priori. On the contrary, professional designers use these modeling tools mostly to guide and optimize revisions. Unlike DIY designs, a typical commercial 2-way will go through perhaps 3 revs of each driver, 2 to 4 box trials, and easily a dozen+ crossover changes.]

Lining the walls of a vented enclosure to reduce internal reflections, or filling a transmission line to absorb the back wave, highly absorptive wool or fiberglass are ideal. However, these materials will not generally provide the desired results in a sealed system. It is true that they will provide more reflection absorption than polyester, but the later is quite good in this regard in the critical midrange. In a sealed system you don't want absorption at lower frequencies anyway; you want damping and isothermal conversion. I have tried "all-out" efforts using fiberglass lining and polyester fill to achieve the best of both worlds. I found the results to offer little practical benefit over polyester alone, but its worth looking into.

All NHT systems now use polyester fill, of one variety or another. We used to use fiberglass in our vented designs, but found a Danish polyester that mimicked the properties of fiberglass very closely. I don't know if this kind of polyester is available to hobbyists. Excluding this special poly, there are essentially two kinds of fiber available: pillow stuffing, and audio-spec polyester. The later type allegedly has hollow core fibers, but I have been unable to verify this with my keen eyesight! Sorry, but forget the pillow type. Sure, it's easy to get. If you use enough, it will damp the midrange, and that's better than an empty box (by alot). But it will have little effect on the lower frequencies.

Well, that's pretty much all I know about stuffing speakers. I'm anxious to hear about the results of people here. Especially the one's experimenting with the use of small animals and children to fill subwoofers.....



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