What is the key to the DDD driver’s exceptional phase linearity?

Welcome to the June 2022 German Physiks newsletter.

We have often talked about the importance of the DDD driver’s very wide frequency response in achieving a high level of sound quality, but so far we have not explored an equally important property of the DDD driver, namely its excellent phase linearity.

Part 1: Wide bandwidth

Figure 1. Carbon fibre diaphragm DDD driver

If an audio device is phase linear, it accurately preserves the phase relationships of the individual frequencies that make up the complex audio signal. If an audio device is not phase linear, then its output is no longer an accurate reproduction of the input signal.

The DDD driver has exceptional phase linearity and very importantly, it provides this over an extremely wide frequency range. To understand how this is achieved, we need to look at how the DDD driver is constructed.

The basic construction of the DDD driver is very similar to a conventional loudspeaker driver. It uses a conical diaphragm with a voice coil attached to the narrow end and the open end is attached to the driver frame using a flexible surround.

There are some major differences compared to the pistonic drivers used on most loudspeakers and these differences are crucial to how the DDD driver works:

• Unlike the diaphragm on a pistonic driver, which is intended to be rigid, the DDD driver’s diaphragm is deliberately designed to be flexible. To facilitate this, the diaphragm on the current DDD driver is made from 0.15 mm thick carbon fibre.

• The DDD driver’s diaphragm is taller and narrower than that on a pistonic driver.

• The DDD driver’s surround is stiffer than the surrounds generally used on pistonic drivers.

• The DDD driver is oriented facing downwards rather than facing forwards, as is the case with the pistonic drivers on most conventional loudspeakers.

Achieving a wide bandwidth
The conventional drivers used in most loudspeaker designs only operate over a limited frequency range, because the diaphragm of a driver that is able to move enough air to provide a useful output at low frequencies, will be too heavy to withstand the acceleration necessary to reproduce high frequencies. A low moving moving mass is key to solving this problem.

The DDD driver uses three modes of operation:

• At low frequencies it is purely pistonic, like the drivers used in conventional loudspeakers.
  

• Above this is an over-lapping band of frequencies where it changes to radiating using bending-waves.
  

• Finally, at the dipole frequency, where a standing wave is established on the diaphragm surface, it changes to modal operation.

The bending wave and modal radiation modes provide a very low moving mass and allow the DDD driver to operate over an exceptionally wide operating frequency range. This is typically 200Hz to 24,000Hz. This is almost the entire audio range – taking the usually accepted range for human hearing as 20Hz to 20,000Hz.

The lower roll-off frequency of the carbon fibre DDD driver depends on which German Physiks loudspeaker it is used on and this varies from 220Hz for the HRS-130, to 170Hz for the PQS-302. On all but one model, the very low frequencies are handled by a bass system using one or more conventional woofers. On the German Physiks Unicorn Mk II, a single DDD driver is combined with a horn and operates from 40Hz to 24kHz

The upper roll-off frequency of the carbon fibre diaphragm DDD driver is always 24kHz.

Part 2: Phase linearity

Figure 2. Cross-section of the DDD driver*

When the DDD driver is working in bending-wave mode, the force of the voice coil pressing down, causes the thin and deliberately flexible carbon fibre diaphragm to flex and a wave travels down the diaphragm from the point A in figure 2, where it joins the voice coil, to the open end of the diaphragm.

As the wave travels down the diaphragm, it pushes out an air pressure wave. This is shown by the horizontal arrows emanating from the diaphragm surface in figure 2. The velocity of the wave in the carbon fibre is very much higher than that of sound in air, and by carefully selecting the shape and dimensions of the DDD driver diaphragm, the air pressure wave radiated from the bottom of the diaphragm perfectly lines up with the waves radiated from all the regions of the diaphragm above it. The consequence of this is that despite the DDD driver having a conical diaphragm, it behaves like a pulsating cylinder.

In the frequency range covered by modal radiation, the shape and dimensions of the DDD driver’s diaphragm and the high velocity of the wave travelling in the diaphragm material ensure that the DDD driver behaves like a pulsating cylinder, as it did in the bending wave range.

At the very low frequencies covered by pistonic motion, because of the long wavelengths involved and the shape of the diaphragm, the driver will tend to act as a pulsating cylinder.

This is how the DDD driver preserves the phase relationships of the individual frequencies that make up the complex audio signal.

Another way of expressing this is that there is no time delay between any of the frequencies covered by the DDD driver.

Most importantly, as mentioned above, the DDD driver does this over a very wide frequency range: typically 200Hz to 24,000Hz. Consequently, the waveform of the air pressure wave that reaches your ears is closer to the electrical waveform that drives the loudspeakers, than it is with loudspeakers that are not phase linear.

*We must give credit for the idea behind figure 2 to a review of the Ohm F loudspeaker originally published in the November 1973 issue of Stereo Review magazine.

What is the benefit of excellent phase linearity?

It is easier to explain the benefit of excellent phase linearity if we consider a conventional loudspeaker and talk in terms of the differences in the times it takes for the signals from the various drivers to reach the listener’s ears.

Music is a series of transients. These are sounds that very rapidly go from a low level, to a high level, such as a string being plucked, struck or rapidly bowed, a percussion instrument being struck, or a wind instrument being blown hard. A transient signal is composed of a very wide range of frequencies. In a 2-way loudspeaker, some of these frequencies will be reproduced by the woofer and some by the tweeter. In a 3-way loudspeaker, the transient’s component frequencies may be be spread over three drivers. Unless the contribution from each driver arrives at the listener’s ears at exactly the same time, the transient will not be accurately reproduced. It will be softened to some extent, depending on how many different drivers the loudspeaker has and the degree of delay between the signals arriving from them. The delay will depend on the differences in the distances between the various drivers and the listener’s ears. Some loudspeaker designers try to minimise these delays by using techniques such as angled baffles, however they cannot control the position of the listener’s ears, as this will depend on the height of their seat and their body dimensions.

Even a small delay will soften the transient to some degree. From this, it is clear that a loudspeaker using the DDD driver, which has minimal delay across a very wide frequency range, will offer a better transient response than a loudspeaker with two or more drivers, where significant portions of the audio range are reproduced by each driver and where it cannot be guaranteed that the the distances between each driver and the listener’s ears are exactly the same.

The advantage of the DDD driver’s wide bandwidth and excellent phase linearity was very clearly described by the US reviewer Dick Olsher, when he reviewed the German Physiks Unlimited loudspeaker for the magazine The Absolute Sound.  This uses the same carbon fibre DDD driver as all German Physiks loudspeakers.  He wrote:

"It only took me a few seconds to realize that this is a superbly coherent loudspeaker whose wavelaunch more closely mimics live music than the disjointed presentation of a typical multi-way speaker. While I don’t intend to condemn all multi-way designs, the truth is that the great majority simply fail to coalesce into a coherent whole. That’s a direct consequence of slicing and dicing the music spectrum into pieces for consumption by tweeters, mids, and woofers arrayed on a large baffle. Such speakers may integrate reasonably well at a few points in space but move the measuring mic or your head a few inches and the balance shifts significantly."

You can read the full review here.

The DDD driver’s excellent phase linearity particularly benefits German Physiks loudspeaker’s ability to resolve fine detail. You will hear this on spectrally complex instruments such as percussion, especially cymbals and on pieces of music where a number of instruments are playing together and it is easier to follow each thread. The ability to resolve fine detail is maintained even at high sound levels, which is where other loudspeakers may suffer.

• An online review of the Emperor integrated amplifier.

• Announcements of the appointments of new distributors.