PMC at the NPL

Feature

PMC at the NPL
Wednesday, July 10, 2013

When PMC launched the fact.12 speaker they mentioned that it had benefited from work done at the National Physical Laboratory or NPL in Teddington, London. Last week I found out just what they had been up to in this vast facility that has been measuring things with utmost precision for over a hundred years.

 

Richard Jackett in the anechoic chamber trying to convince us it was that big. Note retro reflective screen behind the speaker.

 

The acoustics division of NPL have come up with a way of measuring sound waves using light and PMC took advantage of the opportunity to see if this technique could be used to advance the state of loudspeaker engineering. Richard Jackett works in the Quality of Life Division of acoustics, which can’t be bad, he has been working with PMC’s Ollie Thomas to find out what could be done with the techniques they have developed. They have been using laser interferometry to detect changes in air pressure and temperature in front of a loudspeaker as it emits frequencies from 500Hz upwards. This is achieved by placing a speaker on its back in a hemispheric anechoic chamber, a chamber with a hard floor but non reflective surfaces on walls and ceiling. This particular chamber is 5m square and because the whole room is suspended within a box noise levels are sub 0dB. That’s quiet. In here they use a laser interferometer to scan the sound field coming off the speaker by sending a light pulse through the sound field, bouncing it off a retro reflective sheet and measuring changes in the speed and direction of the light. The results are recorded on a computer and can be turned into a video which shows how the wave front changes with frequency.

 

Ollie Thomas with a computer displaying the progress of the laser across the sound field.

 

On the one hand it’s very clever stuff but when you see the way that the wave changes as it approaches the crossover point it’s easy to appreciate differences in behaviour with frequency. What’s difficult is using that information to make a better loudspeaker. Ollie used it for two elements in the design of the fact.12, it allowed the crossover slopes to be fine tuned for optimum dispersion and had a significant effect on the shape of the midrange flange, which was developed to offset some of the cabinet edge dispersion effects. This technique is also useful for confirming theories that the engineers at PMC had about the dispersion of sound as it leaves the speaker but they expect to find other uses for it in future.

 

The wall of the anechoic chamber, each wedge is about 80cm deep.

 

As well as the anechoic chamber we also visited a room built for the opposite purpose; reverberation. This room within a room has a background noise level of 5dB and is 7m long by 6m high with a sloping ceiling and no parallel surfaces. The reverb time at 200 Hz is over 30 seconds when the room is empty, even when there are a dozen people in there and the door is open normal speech bounces around for an age. The acrylic panels are to remove standing waves. This room is used to calibrate microphones and test the sound absorbency of materials, a pair of large doors opens to reveal nothing but a place to put the material to be tested so that they can compare the decay time of a signal in both conditions.

 

The reverb room with acrylic panels to kill standing waves, it sounds a lot weirder than it looks.

 

There is a British Standard living room at the NPL which as you might imagine looks nothing like a domestic environment, there is a window but it has a picture behind it. Nonetheless this gave PMC the opportunity to play a few pieces on the fact.12, not enough to get to grips with this intriguing new model but certainly sufficient to inspire further requests for a review sample. Watch this space.

 

Results from the NPL which show the more precise nature of dispersion from a PMC Wafer with and without absorption material on the baffle.

 

These images represent the dispersion patterns of on the left a second order passive crossover and on the right a 4th order active crossover. The greater phase aligment of the active crossover results in a smoother wave front and less lobing effect in the right hand images, the black areas on these representing cancellations cause by the crossover.

 

The laser scanning head.

 

With enough of this stuff around literally no one can hear you scream.