Echoes and Eardrums: Exploring the National Physical Laboratories

In Bushey Park, Teddington lies the National Physical Laboratory, the largest applied physics organisation in the UK. Funded by The Department for Business, Innovation and Skills, NPL has provided the primary national measurement standards for physics since 1900 as well as using its three atomic to serve as the National Time Reference for the UK.

NPL have a number of outstanding acoustic facilities which conduct research into ultrasonics as well as how sound behaves in air and water. The facilities are used for a number of governmental and commercial calibration purposes, which range from testing recording equipment to safety equipment and human hearing ability.

Inside the hemi-anechoic chamber

Inside the hemi-anechoic chamber

Among the facilities are hemi-anechoic chambers, full anechoic chambers and reverberation chambers which were all built in the 2000s when new state of the art laboratories were constructed on site. Research Scientist Dr. Ben Piper showed me around these facilities to get an idea of the work they do and to experience the effects these rooms have on the human body first hand.

The full anechoic chambers are rooms designed to have zero echo, the walls are lined with synthetic acoustic foam wedges which absorb all sound. The metal floor of the room is retractable and more foam prisms lay underneath reducing all possible reflective surfaces.
“The full anechoic chamber is mostly used for free field calibrations of microphones,” Piper said. “There’s two methods, comparison, where you have one sound source and you put a microphone with a known calibration there and then you compare it to one that you’re testing. The other is the reciprocal method which is the primary standard, the highest level. You put a microphone in two spots and you can drive one of them as a speaker instead of a mic, then you swap them round in pairs. That allows you to get an absolute calibration of the microphone”

The hemi-anechoic room has fibreglass wedges lining the walls which are covered in a cloth and chicken wire to keep their structure solid. Unlike the full anechoic chamber, this room has a permanent solid floor. In this room Dr. Piper and his team have recently been testing using optical methods for measuring acoustics.
“There’s one technique where we can scan a laser through a sound field and look at the refractive index change and that can give us the magnitude of the sound field,” he said. “You can create these really nice maps of the way sound is propagating out of a speaker or what happens when it hits a complicated structure. The room is really good for that because it’s big, so we go along the longest axis.”

NPL also get commissioned to do types of hearing research for the Ministry of Defence, one of the big focuses being on soldier’s hearing loss. Often soldier’s hearing defenders don’t work so they’re taken out, this however means that if an explosive goes off nearby this risk of hearing loss is far greater.
“We did a lot of testing last year where we put a blindfold on a subject and spun them around,” Piper said. “We then get them to point where they thought a noise source was coming from in the room and you look at the errors.

Microphone calibration testing taking place inside the full anechoic chamber

Microphone calibration testing taking place inside the full anechoic chamber

“The main thing they were looking at is the front back error,” he pointed out. “If you hear a sound you look at if so if it’s to your left or right it doesn’t really matter, but if it’s behind or in front of you and you look the wrong direction, then you’re going to get shot.”
With a hard surface in the hemi-acoustic chamber the sound flows away from its source, due to this the room’s original use was for doing sound power testing. To do this you place a machine in the middle of the floor and measure a number of points around it, which allows you to calculate the sound power parameter.
“It’s what gets quoted on things like ‘how noisy is your washing machine’”, Piper said. “That’s the original use of this room but we use it more of a research base as well, because having the floor makes it easy to put heavy equipment and stuff in here to mess around with different set ups.”

The anechoic rooms are built on resilient mounts with a cavity all around them, separating them from the rest of the building in a ‘room within a room’ type set-up.
The full anechoic chamber has no lights inside to avoid unnecessary background noise, the darkness combined with the complete lack of echo creates a very surreal atmosphere. Due to this, prolonged time spent in the chambers can result in being able to hear the blood flowing around your head, as well as reports of hallucinations occurring. Even after a few minutes it was clear that there is definitely some truth to this, with my head feeling hot and heavy very quickly.
“I’ve spent quite a lot of time in the one at Salford university which is very similar but with a mesh floor, and yeah after half an hour or so all you can hear is the bass hum going round in your ear it’s very strange,” Piper said. “I’m not sure about the science behind the claims of hallucinations, but i’m sure you would start to feel a bit weird. I mean, there’s all these anecdotal stories of little anechoic chambers people got put in as torture or for mental patient treatment in the Victorian ages.”

The reverberation chamber

The reverberation chamber

The reverberation chamber’s function is exactly the opposite of the anechoic chambers, its design is to allow it to have as long an echo as possible. The design is so effective that it has a longer echo than St. Paul’s Cathedral and given the relatively small volume of the space, it’s quite an impressive feat.
The room has five walls of different angles and reflective panels hanging from the ceiling. The idea is to have as many reflections as possible to give an even sound distribution as possible so you have a diffuse sound field.
“Again microphone calibration is the main thing we do in here, so because we’ve created this diffuse field, you can assume the sound is the same in a number of places so you put two or three microphones in and they should all be measuring the same sound,” Piper explained. “The other thing is absorption testing, so if you want to know how good an acoustic tile or acoustic foam is you can put it on the floor and measure the reverb time it takes away.

The only decay of sound in the room is through air absorption, the walls are all made from hard concrete coated with a special plaster which helps diffuse sound along with the calculated angles the walls have.
“One of the things we on open days is project a spectrogram onto a wall and have people try and spell their names. Using a microphone we get people to make noises using the decay to draw pictures by singing at different pitches, it’s quite fun.”

Other work the scientists at NPL do includes performance testing for MEMs microphones which are used in telephones. Recently the team began looking at their performance up to ultrasound frequencies (up to 100kHz), to try and improve the current models and solves challenges posed by current models such as self generated noise and environmental dependency.

NPL have an open day on the 20th May giving the public an opportunity to take a look around the site including the acoustic facilities to learn more about the work that goes on at the labs.


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