SU551-DXT --2-way speaker

[gornir]

Hi everyone,

It’s time for me to share yet another of my designs. I have earlier posted the SAL451 loudspeaker SAL451 - a small 2-way loudspeaker and it’s a wonderful sounding small speaker.

This time I’m going up a notch in box size and driver unit size, but it’s still a relatively small 2-way stand mount loudspeaker.

Be aware! There will be a lot of pictures and diagrams presented here. Even though there are a lot of skilled designers out there I’m always missing more details from measurements and simulations etc. in their designs. It’s hard to evaluate a design showing only one simulated on-axis frequency measurement with a smoothed frequency plot in a 100db/scale and perhaps only a reverse polarity simulation to show phase response etc.

Even though the language barrier, I will try to be more informative in my design and I think a picture can tell you more than a thousand words.

Measurements:

I can not overstate the importance of accurate measurements. In my opinion, without it, it’s no use to continue with a loudspeaker design. It would all be guesswork and the result will be accordingly. I will here shortly describe how my frequency measurements are set up.

1. All measurements are done with the drivers mounted in the actual box used in the design.
2. The loudspeaker box is elevated on a stand mounted on a turntable so that the tweeter centre is halfway between the floor and the ceiling.
3. The microphone is setup at a 1m distance from the tweeter centre.
4. Individual measurements are done for the tweeter and woofer from this position. A measurement with both tweeter and woofer connected simultaneously are done as a reference curve to import in LspCAD to get the correct acoustical offset and phase between the drivers.
5. All above measurements are repeated without moving the microphone for on-axis, 15, 22.5, 30, 45 degree off-axis angle. These measurements are done for both loudspeaker boxes.
6. The measurements are supplemented with near field measurements at 0 and 25cm distance from the woofer cone.

The measurements are valid down to approximately 270Hz in this setup. Are there other ways to measure? Of course, but this method is easy to setup and is fast and accurate. I don’t have the space to measure semi infinite baffle measurements etc.

I use the following tools for my design work:

In order to get accurate measurements you need some reliable tools and you need a calibrated microphone and a calibrated measurement rig. Previously I used a Clio measurement system, but nowadays I use the great free tool “HolmImpulse” for frequency measurements. I highly recommend this software. It’s accurate and very easy to use; give it a try for yourself. For box and cross-over simulation I use LspCAD. It’s not for free, but is a great tool. I have earlier used Calsod and Sound Easy, but in my opinion LspCAD is far superior software to use.

Here’s a list of tools I use:
• Dayton WT3 tester for impedance and T/S parameter measurements. (Dayton Audio - WT3 Woofer Tester)
• HolmImpulse with M-Audio Audiophile 24/96 soundcard for frequency measurements. (HOLM Acoustics)
• LspCAD 6 Pro/5.25 for crossover design and simulation. (Untitled)
• EMM-8cal and MP-1r calibrated mic and pre-amp. (www.ibf-akustik.de)

Driver units:

Yet again I use drivers from my favourite speaker manufacturer, SEAS. I think SEAS drivers have a great price/performance value and great build consistency and quality to back it up. I’m aware that SEAS prices are a bit higher in the USA than here in Europe, but I still think SEAS drivers are nice drivers for its price range.

The drivers are:

SEAS U16RCY/P (H1520) build week/year 39/09 Picture1
SEAS 27TBCD/GB-DXT (H1499) build week/year 7/08 Picture2

Woofer:

I chose a woofer driver that I haven’t seen in a DYI design before. SEAS released a design “Idunn” with its bigger brother U18RNX/P (H1571) and the same Tweeter. Zaph Audio has done some measurements of this U16RCY/P driver.

Here is how SEAS describes the driver:

“U16RCY/P is a 5” High Fidelity woofer with an injection moulded metal chassis, intended for bass reflex and transmission line designs.

New Curve cone, a woven polypropylene with excellent internal damping together with perfectly matched moving parts gives a smooth, extended frequency response.

A unique radial reinforced low loss rubber surround reduces radial resonances and prevents surround break up at large excursions.

Bullet shaped phase plug reduces compression due to temperature variations in the voice coil, avoids resonance problems which would occur in the volume between the dust cap and the pole piece and increases the long term power handling capacity.

Extremely stiff and stable injection moulded metal basket, keeps the critical components in perfect alignment. Large windows in the basket both above and below the spider reduce sound reflection, airflow noise and cavity resonance to a minimum.”


This time SEAS has published specification sheets that are far off the actual measurements. See Picture3. The voice-coil doesn’t seem to be same between the official specification and the real driver unit. The consistency between the measured drivers is great.

The official specification shows a frequency peak at 5000Hz that doesn’t show up so pronounced in my measurements. There is however irregularities between 800-1200Hz most likely due to cone surround resonances and diffraction issues. These suck-outs and peaks are also clearly shown in the impedance plots. In my design the cut-out for the woofer is chamfered, otherwise it would bee even more noticeable in the frequency response.

It’s hard to tell if these irregularities can be heard. Zaphs distortion measurements show a 2nd order peak at 1200Hz. 2nd order distortion isn’t as bad as odd order distortion which in this case is under control.

Since the cone breakup is so gentle, it’s very easy to get a smooth frequency roll-off with this woofer without the need of any notch filter. This is more like a 5.5” driver, but fitted in a 5” driver frame.

Tweeter:

I have used this tweeter successfully in an active design before and it has gained some reputation in the DIY community thanks to its performance and the DXT technology. Both Zaph and Mark K have done extensive tests of this driver.

Here is how SEAS describes the driver:

“27TBCD/GB-DXT is a High Definition aluminium/magnesium alloy dome tweeter with a DXT lens. An optimally shaped dome and a wide SONOMEX surround both manufactured by SEAS, ensure excellent performance and consistency.

The compensation magnet increases the sensitivity and reduces the magnetic stray field and allows use in close proximity to CRT screens.

A fine mesh grid protects the diaphragm. Stiff and stable rear chamber with optimal acoustic damping allows the tweeter to be used with moderately low crossover frequencies.

This revolutionary DXT tweeter addresses the major issues regarding directivity control in traditional loudspeaker designs. DXT solves several well-know issues regarding; directivity control, off-axis response, integration with midrange units and baffle diffractions.”

My measurements don’t show a significant change from the official specification sheets. It’s a very sensitive driver; around 95-96db and thanks to its DXT lens it has a remarkable off-axis response.

Next, design goals……….

[gornir]

Design goals:

When I initially started to plan the new design I only had one absolute demand, the design should fit the Parts Express 14liter curved cabinet. Although I like woodworking, nowadays I don’t have the time or patience to build new boxes for each new design and the Parts Express boxes are really nicely built.

By choosing a mid-sized stand-mount cabinet the most logical is to build a 2-way speaker, but what drivers should I use in this relative small cabinet? The box is a bit to big for most 5” drivers and to small for most 6.5-7” drivers. I could of course use a 5” driver and fill up the box with something solid to reduce the volume. At the same time I wanted to have a design that could play bass without the need for a subwoofer. The choice fell to use a 5.5” driver. First I looked at the ScanSpeak drivers from the 15W series. These drivers have a solid reputation and sound great. I didn’t choose them, primarily because they are very commonly used in different DIY designs and they are in my opinion a bit to low in sensitivity.

I finally decided to use a 5.5” driver from SEAS. There are three different flavours of SEAS drivers in this size, the L16, W16 and the U16. I wanted to use the woofer a bit up in frequency, around 2500Hz and that unfortunately rules out the W16 and even more so the L16 due to nasty cone break-up. They both require low cross-over frequencies around 1400-2000Hz. I might however use the Excel W16 driver in a different design in the future.

Ok, the U16 is the driver I selected. It has a very smooth frequency roll-off and should be easy to mate with a tweeter around 2500Hz.

What about the tweeter? I looked around for a new tweeter to buy, but decided to use a tweeter I had in stock. I have several nice tweeters to choose from like the 27TDFC, 27TBFC/G and a couple of different ribbon tweeters from Fountek. Finally I decided to use the 27TBCD/GB-DXT driver with its unique DXT-lens technology.

This driver with its unique frequency response isn’t the easiest beast to tame and I like the challenge to make it work in the design without overcomplicating the cross-over design. The goal is to build as a simple cross-over as possible and to use standard component values with as few parts as possible.

Summing up the design goals:
• Parts Express 14liter box “302-721s”.
• Smooth system power response and off-axis frequency response.
• Cross-over frequency around 2500 Hz.
• Cross-over slopes, 4th order Linkwitz Riley, acoustical.
• Bass extension down to low 40ies.
• Simple crossover with standard component values.
• Adjustable tweeter level.

Next, Box simulation and design…….

[gornir]

Box simulation and design:

There is nothing special here since I have already chosen the 14liter enclosure from Parts Express. 14liter is just about what it takes for this driver when a coil resistance of 0.3-0.5 is included in the equation. Don’t stare blindly at what some simulation software might suggest for box volume. These suggestions are mostly theoretical text-book table lookups and have little to do with real world scenarios. In my experience a closed enclosure volume with a total Q of around 0.55-0.65 often fits to use for bass-reflex enclosures.

Picture1: show how a simulated 14liter box with a 44Hz port tuning and an added resistance of 0.5 Ohm looks like.

Picture2: show how a simulated 14liter box with different port tunings looks like compared to a text-book simulation with driver data from SEAS original specification sheet. All three examples have an added resistance of 0.5 Ohm.

I suggest using a port tuning between 42-44Hz or even a bit lower depending on room interaction and loudspeaker room placement. It’s up to the builder to decide which port tuning to use based on their personal preferences. I would say its ok to adjust +/- 10% to box volume, port tuning and box filling. I use about 20% sheep wool box filling.

Picture3: show the loudspeaker baffle layout. Don’t forget to use the same baffle round-over if you are building the loudspeaker enclosure yourself. It’s absolutely critical that the driver unit baffle placements are kept as the described picture, otherwise the frequency response and the cross-over won’t be correct.

Next, frequency measurements…..

[Loren42]

Impressive start!

I really like the way you list your design goals, methods for testing, and the design process. It makes it very clear what you are doing, how you plan to do it, and how you will verify that you achieved the original goals. Bravo!

[ultrakaz]

Thank you for sharing and comprehensively documenting your design. I just read your prior speaker build and am amazed at your logical and detailed approach to speaker design. I especially like that you compared your work to another respected diy builder, Zaph. Please keep us posted on your progress.

[gornir]

Thank you both for your nice comments. I will do my best not to disappoint you.

[gornir]

Impedance measurements:
The pictures show the two driver unit samples, named 1L and 2R. The measurements are made with the drivers mounted on the baffle.

Woofer:
The consistency between the two woofer driver samples is great. There are some wrinkles in the impedance plot at 850Hz and 1200Hz. These wrinkles also show up at the frequency plots and are most likely caused by surround resonances and cone diffractions.

Tweeter:
The consistency between the two tweeter driver samples is also good. The 2R sample has slightly higher impedance than the 1L around the resonance frequency. According to SEAS specification the resonance frequency is at 900Hz. It’s hard to tell due to the double impedance bump at lower frequencies, but my guess is around 550-600Hz a bit lower than the official specification.

Picture1: U16RCY/P closed box, blue = 1L, green = 2R
Picture2: U16RCY/P bass-reflex box, port tuning = 44Hz, blue = 1L, green = 2R
Picture3: 27TBCD/GB-DXT, blue = 1L, green = 2R

Next, frequency measurements…..

[gornir]

Frequency measurements:
All measurements are done at tweeter height and at 1m distance. No frequency smoothing applied. Measurements are valid down to 250Hz.

Woofer:

Picture1: Tweeter-axis, 0 degrees.
Picture2: Tweeter-axis, 15 degrees.
Picture3: Tweeter-axis, 22.5 degrees.
Picture4: Tweeter-axis, 30 degrees.
Picture5: Tweeter-axis, 45 degrees.
Picture6: Tweeter-axis, 0, 15, 22.5 degrees combined.
Picture7: Tweeter-axis, 22.5, 30, 45 degrees combined.
Picture8: Tweeter-axis, 0 degrees, box 1L (blue) and 2R (red).

Comments:
Very smooth frequency response and roll-off. There is some raggedness between 800-1200Hz caused by most likely surround resonances and cone diffractions. No nasty cone break-up and a usable range up to 3000Hz with maintained power response and off-axis behavior. Good frequency consistency between the two driver units.

[gornir]

Frequency measurements:
All measurements are done at tweeter height and at 1m distance. No frequency smoothing applied. Measurements are valid down to 250Hz.

Tweeter:

Picture1: Tweeter-axis, 0 degrees.
Picture2: Tweeter-axis, 15 degrees.
Picture3: Tweeter-axis, 22.5 degrees.
Picture4: Tweeter-axis, 30 degrees.
Picture5: Tweeter-axis, 45 degrees.
Picture6: Tweeter-axis, 0, 15, 22.5 degrees combined.
Picture7: Tweeter-axis, 22.5, 30, 45 degrees combined.
Picture8: Tweeter-axis, 0 degrees, box 1L (blue) and 2R (red).
Picture9: Tweeter-axis, 0 degrees, cone break-up.

Comments:
Smooth frequency response and thanks to the DXT technology, crazy good off-axis frequency response and behavior. Nasty cone break-up at 27000Hz, but high enough in frequency to not cause a problem. Slight frequency dips between 3000-4000Hz caused by baffle diffraction and at 13000Hz caused by the DXT lens. Good frequency consistency between the two driver units.

Next, cross-over design and simulation….

[JvS]

Looks nice. I Really like that tweeter! Can't wait to see the result.

Is the speaker allready 'done' and are you feeding us bits and pieces of information in a movie-like tension build-up?