How to design a loudspeaker in 3 easy steps using WinISD.

[Wella]

Gentleman,

I am trying to get my head around the basic of loudspeaker design.

I am trying to take the published specs of drivers which also have published recommended plans, place the specs into WinISD and try to understand why the recommended plans have been recommended. If you understand what i mean...

I have taken the specs for the CHR-70 Driver | Markaudio and am trying to understand the rational for suggesting the CHR-70 Mini BR plan | Markaudio Mini Base Reflex.

I understand that an spl v's frequency graph should be basicially flat without any major dips or blips and nice smooth roll offs.

I also understand that the plot gained from modeling the drivers in WinISD is going to be very different from the real world but as i see it its a good place to start.

When I entered all the data into WinISD i got this...



The program suggested 11.5 litres @ a frequency of 49.8 Hz.
It suggested a vent diameter of 0.102m and a length of 0.769m
This gives a nice flat graph to a steady roll off at 50Hz

The suggested plan above is for a 4.5 L vented box.

If i change the volume to 4.5 L then the plot starts a roll off below 100Hz.

Questions.....

1. Have the designers of the 4.5 L box decided that the roll of from 100Hz is fine
2. Have they changed the frequency to bring the line back up? If so how is done in real life. how do you tune a speaker to x Hz in the real world.
3. Have they done something else im not spotting?

Thanks

A confused Paul

[Scottmoose]

Paul,

I understand that an spl v's frequency graph should be basicially flat without any major dips or blips and nice smooth roll offs.

If we all lived in anechoic chambers this would be true (although you always want a nice smooth roll-off). However, in practice, things aren't always that simple. There are such things as baffle-step-loss, and room gain to consider, as well as enclosure shape, the location of driver and vent within the enclosure, the location of the cabinet within the room, and the nature of the system (particularly the amplifier and its output impedance). Unfortunately, WinISD does not account for any of these, nor for considerations of taste, size of cabinet for a specific purpose etc., and compromises in either direction to achieve this.

The little 4.5 litre box Mark appears to have designed is obviously just a quick & easy box for people who don't have much space or a small room (in which case the balance should be reasonable), rather than being an 'optimal' xyz alignment.

[Wella]

Scottmoose,

thanks for taking the time to reply.

I understand that what I am attempting to model is in no way going to reflect what a measured version of the speaker is going to measure like. I also understand that many many other things are going to effect the design. I sort of knew that i would have to take into account position of drivers and ports, I knew that the size and shape of the front baffle effected the overall sound (plus probably the overall shape of the box).

To be honest i thought those things came a little later in the design... At the tweeking stage (i suspected that the tweeking stages quite often forced you back to rejig your original design and then retweek then rejig...and so on)

You get a basic box size, you then play with other software to position drivers and figure baffle sizes etc, you then design a crossover if multiway etc, you then build a box and take a listen????

I do understand the limitations of WinISD but it must be good for something? The first step maybe to be followed by other software to tweek the design.

Does anyone then have any basic steps that the average DIYer can take to design a loudspeaker that is going to sound good.

Paul

[Trapshooter]

Originally Posted by Wella

Does anyone then have any basic steps that the average DIYer can take to design a loudspeaker that is going to sound good.

Paul

I'll start off with this very basic list:

1.) list your design expectations (keep it simple for a 1st project, FR and SPL requirements)
2.) choose a driver and enclosure type (for a 1st project a CB or BR is probably a safe bet)
3.) enter the T/S parameters and tweak the box volume/tuning to get a desirable FR plot.
4.) check that you can attain your SPL design requirements by entering different signal input values while checking that you haven't exceeded the excursion limits of the driver.

I've probably ventured out onto the thin ice by trying to answer your question, but it's at least a start.

[KrammeAcoustics]

EBP = Efficiency Bandwidth Product, is also a good factor to know. EBP tells something about what kind of cabinettype there are suitable for the driver. A simple way to find the EBP of any driver, simply calculate following: Fs / Qts x 1.1 = EBP

EBP value ( cabinettype ):
0 - 50 = transmissionline
50 - 100 = closed
100 - 150 = reflex
150 - 250 = horn

Info about EBP: Enclosure Dilemma

EBP calculator: Efficiency Bandwidth Product Calculators - EBP, Speaker Resonance Frequency, Speaker Electrical

[Wella]

Great stuff and thanks for all the replies.

Ive spent the day reading http://www.quarter-wave.com/General/BSC_Sizing.pdf & Baffle Diffraction Step which have been most helpful in understanding the effect of Baffles

more questions...

Trapshooter said

4.) check that you can attain your SPL design requirements by entering different signal input values while checking that you haven't exceeded the excursion limits of the driver.

How would i go about that?

Paul

[Godzilla]

>>> There are such things as baffle-step-loss, and room gain to consider, as well as enclosure shape, the location of driver and vent within the enclosure, the location of the cabinet within the room, and the nature of the system (particularly the amplifier and its output impedance). Unfortunately, WinISD does not account for any of these...

I think that's why i like using it so much. I can build a simple box and cut holes into it. Otherwise my carpenter skills are lacking. For me WinISD provides a starting point to get the bass right. Then i build the speaker and listen to it before finishing... play with stuffing next. If i still think things could sound better i change the port length or slot size and listen, play with stuffing some more and repeat. I do this in the room the speaker will live. My basement, living room, den and office all have tools to build speakers (screwdriver, pliers, wire stripper, soldering iron... and a bunch of caps and coils to alter crossover points). Once i feel the bass is tuneful and reaching its potential (by ear) i focus on blending the drivers. This is how i enjoy the hobby.

The 'exotic' enclosures out there are all over my head and so i leave it to others (like generous Scottmoose, GM and MANY others) to offer their thoughts and theories then see what sits right with me in my mind before building. If it were not for the BIB spreadsheet i would never know where to begin building a cabinet like that!

http://www.zillaaudio.com/bib-calculator.htm

If it were not for the backhorn designs already built, tuned and shared by others i'd never have a clue what more ambitious enclosures would sound like.

The H-frame MJK built, documented, measured and shared with us all is probably the best sounding bass i've had in my listening room ever. Thanks sir!

So many thanks to the people on this forum who think and build and play with ideas to help all of us achieve excellent sound for our different budgets!

For me, WinISD is a great starting point when comparing drivers for my 'basic box' projects. It allows me to see which drivers will most likely work better in a given box volume (closed or ported) and calculates a port size that may not be perfect but is usually pretty close to get good results. IMO 'close' is pretty good considering all the variables mentioned at the top of this post.

Godzilla

[Scottmoose]

thanks for taking the time to reply.

You're welcome. That's what forums are for.

To be honest i thought those things came a little later in the design... At the tweeking stage (i suspected that the tweeking stages quite often forced you back to rejig your original design and then retweek then rejig...and so on)

Would that life was as simple as that. Let us assume for a moment that you were designing a 40 litre BR box, and you constructed a tall, slim floorstander. Bass reflex math assumes pure Helmholtz resonance, an even air-particle density in the box and no standing waves. However, because of the configuration / box shape described above, a powerful vertical standing wave is likely to be present, and chances are the cabinet will not actually be tuned to the frequency you (or a lumped parameter programme like WinISD or similar) thinks it is. As a result, the vent dimensions & likely the driver location will need to be adjusted. Which isn't to suggest that WinISD etc. are not useful: they are. But remember to keep their limitations in mind.

Re other matters, FWIW, take Efficiency Bandwidth Product with a very large dose of salt indeed. At best it's an over-simplistic 1st order generalisation, and very successful systems can be created using drivers that the EBP assumptions imply should not be employed in a particular box type. For e.g., EBP, which is generally taken as Fs/Qes implies that the CHR70, with an EBP of 90.667, is not suited to use in a vented alignment, or a horn. Which is drivel. It can be used perfectly well in either alignment. Expanding slightly on this, the concept that horns require a driver with an EBP 150 - 250 is dangerously misleading, especially for back-horns, which require characteristics almost the reverse of what a high EBP implies. So my advice is to forget about EBP; it can cause people to dismiss something that might suit them well, or even lead them right up the garden path.

Martin's step-loss write up is excellent; you'll find some more here: Transmission Line Speakers -- Technical Articles However, keep in mind that these assume that no compensation is built into either the driver's response (which is the case with the MA units) or the cabinet response. Also depends on their position in the room, and the characteristics of the room itself.

Trapshooter's description is basically a good generalised way forward. To enter different signal values in WinISD, go to the 'signal' tab, and increase the power (number of watts), then select cone-excursion from the drop-down menu on the main plot & check it doesn't exceed the driver's Xmax. IIRC, there are some bugs in the software here, so don't take it as exact.

In an ideal world, at the maximum average sound pressure levels you wish to have at your listening position (it will drop 6dB for every doubling of distance, so remember to account for this in your assessments -pull up the SPL graph from the drop down menu) you'll want to have plenty of driver excursion still available to handle dynamic swings in the music.

[Trapshooter]

Originally Posted by Wella

Great stuff and thanks for all the replies.

Trapshooter said

How would i go about that?

Paul

Using WinISD go to the signal tab and enter something other than the default 1W in "System Input Power", then check the "SPL" plot for what that generates, then check the " Cone Excursion" for FR vs excursion. Keep in mind this is just a sim, but it at least gives an estimate of how much power the design can handle per frequency. You might find that a small FR needs bass assistance for your application. I'm usind a single EL70 in a 15l CB and the sim shows about 1 mm excursion at 200 hz (4 mm being the max). with max input 20W. You can also sim filters and there effect on FR and excursion with the EQ/Filter tab.

[skorpion]

Paul,

We actually built your design also from use of Win-ISd. We cheated a bit by bying an enclosure of 12 litres but keeping the 49.8 Hz tuning frequency You can change parameters in Win-ISD so we used a 5 cm diameter tube and got a 15.8 cm long vent. The Enclosure height was 38.5 cm, width 24.5 cm and depth 20 cm, so everything could fit. You can play around a bit with parameters in Win-ISD to make the design fit your bill.

Then you can use the EDGE programme: Home of the Edge to calculate the baffle-step for the enclosure. We used a BSC consisting of an 1.8 mH airwound coil and a 4 ohm MOX resistor, values which we got from EDGE. The end result was very good.

The speaker was used as a High School project and indeed surprised the audience.

/Erling