S16 (M, MS, MSXT, MSTi) - Constant Directivity Dipoles

The S16 being measured in free space condition.
Roger, Paul, and Daniel are seasoned loudspeaker enthusiasts and builders.
Melbourne 15/1/2011

Work in progress to improve high frequency directivity of S12.

Summary of Midrange and Tweeter variants:
  • S16 - 10/12/10
    Seas L21, Dayton 10" wg, D220Ti compression driver
    Also B&C DE250 compression drivers later on
  • S16/M - 27/9/11
    M=Magnesium - Seas W22 woofers
  • S16/MS - 30/10/11
    Symmetrical tweeter response - Seas W22, DX25 + Monacor WG300
  • S16/MSXT - 27/11/11
    Replaced DX25 with XT25
  • S16/MSTi - 27/11/11
    Replaced XT25 with Vifa DQ25SC16-04


Jaycar CW2119

- Two MiniDSP 2x4 Kit
- Advanced 4-way xo plugin

8-Channel Sure Electronics Class D

Follow the build discussion at DIYAudio Forum.

Measurement Data:

Directivity plot, Native frequency response, 1m:

Directivity plot, Normalised to 0deg, 1m:

In-room frequency response (1m ungated), also with reversed tweeter.

Why the fuss ?

You can compare the directivity plots above with typical dome tweeter implementation:

S9 OB:

We see narrowing of loudspeaker polar response, followed by "tweeter blooming". This is an expected response as the tweeters would behave omni-directionally at transition frequency. Humorously known as the "Klingon bird of prey" polar response :)

Recently a combination of large woofer and waveguide are becoming very popular, similar to the S15 Econowave build. While these types of speakers can provide constant directivity down to 800-1khz, unfortunately they soon transition to omni-directional at lower frequencies:

S15 DSP:


There is a certain correctness about the sound of speakers with such smooth directivity from 20Hz to 20khz. It is immediate and dynamic. However with any loudpeakers they have plus and minuses:
Very dynamic presentation. Willing to play effortlessly at very high SPL. Sound is very 'direct' and make dome tweeters sound diffused in comparison. Very natural.

Lack of rear HF radiation (arguably ok or even necessary), some degree of horn and compression driver coloration make them not as smooth as dome tweeters. Flat dipole bass is unable to pressurize the room effectively below 40hz, although these can easily be substituted with H or W frame dipole radiators.

Room response analysis

Elias Perkonen kindly performed wavelet analysis of the in-room response of these speakers.

Native frequency response, left and right:

Normalised, left and right:

Longer time windows, 150ms and 300ms:

As comparison, here are the bark wavelets of Elias' Dipole Line Array, 20ms and 150ms:

Update 21/5/2011
ps. no rapture yet!

Tried Selenium D220Ti compression drivers with B&C DE250. These drivers use Poly cone material and have a smooth but 'mellow' sound. Read about my burst signal test investigation here. (note: update 22/9/11, I prefer the cheaper D220Ti as a more accurate transducer).

The polar response is excellent and comparable to D220Ti although mounting adapters have to be used.

Update 27/9/2011

Replaced L21RNX/P with Seas Excel W22, Magnesium cone

The Excels are interesting. When I tap the cone interestingly they don't sound like typical metal drivers. They sound like polycones! Magnesium are lighter than aluminium and apparently Seas were able to make a thicker cone for the same weight, providing better damping.

Replacing the Aluminium L21RNX/P there were only 2 differences. The cone resonance peak (W22 is about 100hz higher) and region between 1khz-2khz. Frequency response-wise they track within 1-2db. It was also interesting to note the behaviour of the resonance. It's different. The Aluminium has periodic resonance after the main one, while W22 has only one major peak and drops after that. Both L21 and W22 have serious drop about 2khz and really the 1.5khz XO is really pushing it.

Sonics of S16/M

Now, after only minor adjustment, which is the metal cone notch I sat down and listen.... Tonally they are inseparable. They sounded very much the same. However the Excel have this specific character that is quite difficult to explain. Basically it sounded mellow and laid back, but with resolution and transparency. It projects refinement and practically makes any music with overtones sounds......beautiful?? They reminded me of the Orions.

I think, however that the Excels are lacking the gusto and immediate transients of the Aluminium L21 especially playing percussive instruments.

Interesting approach to solve dipole tweeter directivity by roflynn. Using Scanspeak tweeters and Monacor Waveguide WG300

Building the 16/MS (magnesium, symmetrical)
The objective is to provide front-rear controlled directivity of tweeter region, with symmetry. This was not able to be done with Waveguide/compression driver combination.

To the rescue is Vifa DX25 loaded with Monacor WG300. The dispersion is respectable (not as good as 16/M) but more importantly the small dimension allows back-to-back setup.

The polar response is not as smooth as large waveguide in S16, but it is sufficient to match dipole's directivity of -6db @60deg. When moving around the speakers I could hear the level drops uniformly. It is the best compromise so far between directivity and front-rear symmetry.

0-90deg, 10deg increment, normalised.

Replaced DX25 with XT25 ring radiator. The benefit is elimination of 12khz null thanks to the phase plug. This version was shown at Melbourne round-trip GTG.

Deep reverse null showing excellent time alignment and phase tracking. Also displayed is the difference between DX25 and XT25 when loaded with the waveguide. The lack of phase plug caused ~12khz null with DX25.


I wanted better HF resolution which only rigid/metal dome tweeters can provide. A quick experiment mounting a Titanium domeVifa DQ25SC16-04 to the Monacor waveguide resulted in good result. I removed the phase plug of these drivers.

When loaded with the waveguide a 10+dB peak appears. This is excellent as we can notch this peak and resulted in lower distorion (distortion figures of these drivers are already excellent). It also resulted in excellent phase tracking, despite the non-matching diameter between waveguide throat and the dome. To my surprise the lack of phase plug does not exhibit a dip in the response (that 18khz dip is inherent to the driver).

The smallish waveguide loses directivity at 2khz. The polar response is not as good as a proper 10" waveguide, but definitely much better than normal dome tweeter on a baffle.

I prefer the sonics of these drivers compared to XT25 and earlier DX25. They are very transparent and detailed. YMMV. (Update 8/1/2012: They are not nice visually. The dome is light-reflective and draws attention. I liked the black XT25 better and have reinstalled them).

Update 7/2/2012
Experimentation with Linkwitzlab "Watson"

rich31td's build:

Update 31/1/2015
A 'released' build of this prototype is being commissioned: Gainphile R16