3-way WMTMW speakers

Rohit Balkishan Dubla

Introduction

This page describes the construction of my latest speakers using the WMTMW layout. My earlier speakers were large floor-standers and a constant source of irritation to the better-half, which finally resulted in me deciding to build slimmer speakers, plus an opportunity to again do some DIY stuff.

Design goals

• • Spousal acceptance (prime directive ;-) )

  • Small size, especially width

  • Reasonably good bass

  • Low cost

These design goals basically meant that the speaker be thin, yet produce reasonably good bass. This simply means either a very deep box, or a tall box. I personally like tall boxes - and floor-standers, so after a bit of bargaining with SWMBO, came up with a good compromise between size and performance: A tall (but short enough) box with a vertically symmetric arrangement of drivers. This increases the total radiating area, especially for the woofers thereby helping with bass response. To keep in line with my philosophy of 3-way active speakers, it had to be a WMTMW configuration. To keep costs low, I opted for off-the-shelf car speaker drivers: One advantage of these is ruggedness plus (I feel) better tolerance between units of the same model. Also, being 4 ohms drivers they naturally lend themselves to be connected in series when used in pairs - so to the amplifier they still present an 8 ohms load.

Driver details

The following are the T/S parameters as measured by me for the drivers I chose for woofer and mid-range duty.

Woofer

Make: Pioneer TS-G1312R

Dia: 13 cm (10.5 cm effective)

Re: 3.5 ohms

Fs: 110 Hz

Qts: 1.036

Vas: 3.71 litres

Box: 35.5 litres vented (tuning at approx. 68 Hz) with 2 drivers in series.

Though this driver is not a woofer in the true sense of the word, I nevertheless used it as one. I elected to use a vented box for the woofers to extract as much bass as possible for the particular box volume. With this, the simulated -3dB frequency is about 53 Hz.

Mid-range

Make: DSD 410 (manufacturer unknown)

Dia: 10 cm (8 cm effective)

Re: 3.6 ohms

Fs: 153 Hz

Qts: 1.081

Vas: 1.23 litres

Box: 10.8 litres sealed with 2 drivers in series

Results

In all it turned out pretty well. These speakers definitely need a sub but even without it they sound quite respectable. Bass is (as expected) only till about 50-55 Hz which is quite good given the rather small drivers used for the woofers.

The sound is a bit bright in the mid-range - probably because of the mid-range drivers having a 2dB peak before rolling off. However, vocals sound very clear and natural. So much that from another room, voices sound very real...sort of like being there.

There is room for improvement though - especially in the MTM section where introducing a 90° phase lag in the tweeter's signal will potentially improve vertical dispersion at the mid-tweeter crossover frequency. It is recommended in the original D'Appolito design for the mid and tweeter to be in quadrature at the crossover frequency which basically means a 90° phase difference between them - easily achieved by a 3rd order Butterworth crossover. In this case, however the crossover is LR2 - to bring the drivers in quadrature, it has to be done externally via a phase shift network before the tweeter. That said, the lobing effects are not pronounced at all even without phase shifting and I have let it be as it is.

The subwoofer is a sealed box with isobaric loading of drivers and re-uses the drivers from the older floor-standers and also all the wood used to build it came from them. This is described separately here.