Trevor holds his Eddy award

Precision HiFi Loudspeakers:
'Ground Effect Omnidirectional'

Compact design: ideal for small, complex, rooms

author: Trevor Marshall

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Discussion / Questions on the DIYaudio Loudspeakers, or the HomeTheaterShack forums, please

Design of the matching Power Amp modules can be found on my Class-D tutorial webpage.

TrevorMarshall ground-effect omni speakers

How small can you make a set of HiFi speakers?

A few months ago I realized that I was going to be traveling a lot over the next few years, and I started to explore how tiny I could design a HiFi system without losing fidelity. My goal was to produce a set of small speakers, with a tiny matching amp, that could travel in a suitcase with me, my laptop, and my FLAC collection. What I developed is the 8" high black speaker you can see in the photo. It uses the magnificent Peerless Tymphany 830893 widerange driver, covering 50Hz-20Khz. It stands on the floor, using ground-effect reflection to help radiate low frequency energy.

However, in all but the smallest rooms, the 2" widerange needed a little more deep bass power, so I added a woofer to cover 20Hz-200Hz, using the SEAS driver from my Linkwitz Pluto (the white tube).Suddenly I found I had not only a portable system, but a top-notch set of HiFi speakers capable of holding their own in any 'average living living room'.

True, the 30Hz foot-pedal pipes of the Theatre Royal organ during the opening phrases of "Phantom of the Opera" (original cast recording) do definitely exercise the 12mm maximum linear excursion of the SEAS cones wink  As does the bass drum on Nirvana's "Lithium." wink But the 200Hz cross-over minimizes intermodulation, with the clean wide-range treble and middle of the Peerless masking the  contortions of the SEAS woofer, so that the overall system sounds just great wink

Key to high performance is the integration of a MiniDSP board, to handle the crossover, smoothing and ka precompensation functions which are so necessary with omnidirectional speakers. There is no need to change capacitors in a circuit board to tweak a cross-over, you just change the filter parameters in the MiniDSP's GUI. I even get the MiniDSP to provide "tone controls." The four channels of bi-amp power amplification are provided by two of my modded SURE TDA7498 boards, with standard computer power-bricks to provide the needed 'oomph.'

But the real advantage of these speakers is that they radiate in all directions, unlike the Pluto, which morphs into a direct radiator above 1KHz.  The Peerless drive unit  is pretty flat, especially at listening angles of 30-45 degrees, when the listener is only a few feet from the speakers. Indeed, it is in small rooms that these speakers really shine. They disappear into complex arrangements of furniture, bookcases, and computers. It is quite amazing to walk around a room and not be aware of the speakers, the sound field is truly realistic, and moves with the listener.

TrevorMarshall ground-effect omni speakers

The Peerless 830983 widerange driver.

Quite early on, I realized that any small, widerange, speaker had to either avoid ground-effect (the reflection from the ground between the listener and the speaker) or embrace it. If one could build a speaker where the path difference between the reflected and direct signal was small, then the destructive interference from the ground would not be so much of a problem. In practice, this means placing the speaker within a few inches of the floor. I found 8" to be a practical maximum distance.

Simultaneously, I needed to provide a critically damped cavity to the rear of the speaker to properly load the cone at low frequencies. This is tough to do. I eventually chose a length of 2 inch ABS pipe, cut to 6.5 inches (plus top and bottom caps). You have to cram in every ounce of high-density padding to totally fill the container. At least 4 ozs of Madisound's, preferably more. If you don't cram it tight then the reflections from the bottom of the pipe will mess up the radiated sound.

I compensated the MiniDSP precomp curves (boost below ka=1) with a combination of ear and sound-field (microphone) measurements. You will not find any system frequency response curves here, I didn't figure out how to measure the response in a complex real-room environment. There are so many resonances to make a response curve look bad, yet the ear compensates for them, the brain knows how a real-room sounds, and when an omnidirectional field is injected into that room, the ear is quite happy. Can you inject the 30Hz organ notes into a small room? Well, the brain sure thinks it can feel them... even if the microphone can't measure them...

To see how the widerange speaker works on its own, just bypass the MiniDSP's 200Hz high pass cross-over filter, disconnect the woofer, and you will have a quite amazing wide range speaker. Its limitation is volume (and distortion) at low frequencies. In a quiet environment (a hotel room) it is OK, but its 1.5 mm cone excursion doesn't really allow a clean bass response wink That is where the SEAS L16RN really shines

The SEAS L16RN-SL woofer.

I spent quite some time playing with various woofers, both large and small, and came to the conclusion that the L16RN which SEAS designed for the Pluto, is really one of the very best compact woofers available. High efficiency, good power handling, and the long 12mm linear excursion, all combine to make it a top-notch woofer. At one point I explored designing a sub-woofer (for a 3-way system), and decided that, for more bass, it is better to add additional L16RN (and amplifiers) than to use a larger speaker. For example, the 10" Peerless 830668 used in the Pluto+ sub can only push 2.2 times the amount of air around as the L16RN, and has a similar f0.  So, if you really need to emulate a disco, use twice as many L16RN wink

The power response (at right) shows about 90dB levels are possible at 30Hz, and that is pretty well what I am seeing. My ears start to collapse at about 85dBC SPL, and the Peerless 830983 / SEAS L16RN combination will happily produce that level in an a small room. The widerange can handle even more level without blowing apart, the woofer becomes distressed first...

SEAS L16RN response in 16x6 inch tube

SEAS L16RN power handling in 16x6 inch tube

The MiniDSP.

MiniDSP overview

The MiniDSP is in many ways the heart of this speaker system. It allows precise adjustments to be made to the perceived frequency response, as well as providing all the basic functions - including crossovers and ka compensation. The outline of a MiniDSP system is shown above. There are multiple circuit blocks, and each has a number of computational units which can be used to create the filters we need. Luckily, the GUI  which they sell as the "Advanced" interface contains most of the basic functions, and we only have to enter a few Biquads by hand. The input and output gain blocks are left flat-out, with the exception of the gain control, which I recommend you use as your master Volume Control. There is a header to wire a potentiometer to the MiniDSP, any pot will do (eg 5K linear) as it is a voltage controlled function (you don't have to shield the cable). the programming for each of the two stereo channels is identical, and there is a handy button on many screens to clone the settings to the opposite channel.

You can download the parameter file I created, and read it into your MiniDSP Advanced software using the "System Settings" "Load Configuration" tabs.

MiniDSP Tweaking section

The first section which has to be programmed is the Parametric EQ section. I used a Biquad here to provide about 10dB of gain (voltage gain=3). This reduces the dynamic range a little, but also allows me to plug MP3 players directly into the MiniDSP inputs. The inputs are set for true Line input level, much higher than the average headphone-type source puts out. So the spare Biquad gives us a volume boost. I also use the Parametric EQ section to provide Tone Controls. I have programmed in the Bass Boost and Treble Peaking which I have used, but you should feel free to change these to match your personal preferences. The combined response of the gain and peaking is shown on your MiniDSP screen (at left)

MiniDSP Crossover sectionThe next MiniDSP section implements the Crossovers. For channels 1 and 3, the two SEAS woofers, we need a 20Hz 48db/octave Linkwitz Riley highpass filter to stop the speaker cones from hitting the ceiling as you place the stylus onto a record, and  a 200Hz 24db/octave Linkwitz Riley highpass filter to keep low frequencies out of the Peerless 2". I chose to implement the 200Hz crossover lowpass for the SEAS in the Parametric equalizer, as using Biquads in the Crossover stage is more complex than in the Parametric. At left is the display from the Crossover.

MiniDSP Precomp section

Next is the "Parametric EQ 1" section, which implements the 200Hz 24db/octave Linkwitz Riley lowpass filter, and the low shelf to extend the woofer frequency response below the enclosure cut-off. I have rolled off the low-shelf about 4dB by 30Hz, you may wish to extend it more than I. The thinking here was that the amplifier I am using to drive the SEAS puts out more than 70Watts, and this can really move the cone at the lower frequencies. This is especially true when deep bass drum (for example on Nirvana's 'Nevermind') coexists with music which is normally played quite loud. In any case, the MiniDSP allows you to change these choices based on your own music, and your own preferences. And that's pretty neat...

I also included a gain Biquad (set to 1) to compensate speaker/amp combinations. For example, if you add another pair of SEAS woofers (and amps) you would probably want to change that coefficient to 0.5 wink

MiniDSP Precomp section

Finally comes the "Parametric EQ 2" section, which implements some complex auditory shenanigans, as well as the boost for the Peerless 830983 when it is operating below its ka=1 cutoff (about 3KHz). Based on extended listening, I empirically put a 2dB notch at 3500Hz, a 4dB notch at 600Hz and a 3dB notch at 180Hz (approximately the enclosure resonance). In order to limit cone excursion, I rolled off the bass boost so that 50Hz is about 3dB down.

The gain Biquad is set to 0.4 , which gives pretty good balance around the 200Hz cutoff point (to my ear, at least) wink

Random thoughts.

It is not (IMO) a good idea to bolt the enclosures to each other. I usually scatter them around the obstacles which are always in my own rooms, and find that if you put the Peerless and woofer enclosures too close, then you lose some of the benefits of omnidirectionality, as the woofer tubing will reflect a larger proportion of the HF energy, by comparison with separating them by 6 inches (or so). You do need to keep the pathlength difference between the SEAS and Peerless cones to less than a foot, or so, to minimize phase issues. Remember that you are listening at an angle, though, and a foot between the enclosures may give a lesser difference in distance between the speaker cones. However, you definitely can't get away with one common woofer. You need one woofer per channel (or two if you want more 'oomph').

Making the speaker 'enclosures'

I found it hard to get the 6 inch I.D. PVC piping. Do not buy the 6" ABS piping, it is a different internal diameter. I ended up finding 2 foot lengths of "Valencia Pipe 6" PVC 1120" at our local Home Depot. The cheapest caps I could find were from Amazon, "Plastic Trends Cap P1606." These also have the advantage that they are flat-topped, making it easier to cut the speaker holes. I used a 15 inch length of piping to give a final assembled height about 16 inches. I did not have to glue the end caps of either enclosure in place, friction was enough to hold everything together.

I teased out about 12 ozs of lightweight polyester fiberfill (from my  local craft store) to damp the woofer enclosure.

There are two types of 2" endcaps for the Peerless enclosure. You can see the more common type at the top of my speaker 'enclosure' (above) and the shorter caps at the bottom. The shorter caps are  flat, which makes them easier to work with. I bought mine from Lowes, but Amazon lists them. The 2" ABS pipe is stocked pretty well everywhere. I cut a 6.5 inch length, so the final enclosures size is about 7.5 inches, depending on how hard you push on those end caps :) As I emphasized above, stuff them absolutely full of high density (eg Madisound) padding.

MiniDSP Precomp sectionMiniDSP Precomp section

Cutting large holes in plastic is a dangerous endeavor.
You must cut slowly, so as not to overheat the cutter, and make sure that the cutter is firmly in the chuck, and the various screws on the cutter are absolutely tight. I have owned my own "Circle Hole Cutter" for decades, but, to my surprise, I see that they are still selling them (on Amazon). I thought they would have been banned by now. Life is so safe these days wink

Note that I have bolted the plastic top down to the drill press. You can see the holes in my finished speakers at the top of this page if you think I am being over dramatic here. Bolt it down. If the cutter catches in the workpiece you want the drill to stall, not hurl the cutter, vise, and plastic top right at your stomach.

In any case, once the precautions were put in place, I had no trouble cutting any of the speaker holes. These photographs were taken cutting some 4" piping I used for smaller woofers, which I eventually discarded in favor of the SEAS L16RN-SL.

I sealed the speakers to the tubing endcaps with silicone adhesive. Black silicone is available to match the black ABS. I guess, one day, I will get around to painting my woofer enclosures black to match the Peerless...wink I did look for covers to protect the speaker cones, and I found an excellent wire-mesh cage to protect the Peerless. I got mine from Home Depot, called "Expanded galvanized strainer" for gutters, by Amerimax Home Products, part no 29059. When I get around to painting (and shaping) them, I will put a photo here  wink

I don't have a good solution for protecting the SEAS yet, all suggestions are welcomed...

Please make those suggestions, and ask any technical questions, on the DIYaudio Loudspeakers or the HomeTheatreShack forums, please


Copyright (C) 2012 Trevor G Marshall, All Rights Reserved