Category Archives: Wildlife Recording

The MEMS Microphone. The Biggest Selling Microphone Ever!

History lesson

If we go back 100 years or more, every telephone on the planet transmitted speech via a carbon microphone. This was a small container of carbon granules polarised by a low voltage and having a diaphragm at the front. As the diaphragm vibrated, the granules were subjected to varying pressure causing small changes in the voltage. This signal passed through a transformer and went off down the telephone line to be picked up by the receiving phone. Carbon microphones continued to be used in telephones all way up to the 1980s. Millions upon millions of carbon microphones were in daily use right across the world! The audio quality was a bit shit to say the least, with plenty of crackling and unwanted noise, BUT……. speech was intelligible, they were dirt cheap to manufacture AND they were utterly reliable over many years of use.                                                                             

End of history lesson.

Scroll forward to the present day.

Exit the carbon mic, enter the MEMS mic. Today, mobile phones, laptops, tablets and other portable communication devices are everywhere. Most of these devices contain at least one MEMS microphone. Recent sales figures from US manufacturer Knowles Acoustics show sales for their SiSonic silicon based MEMS microphones in excess of 1 billion units!! 1 Billion units!! That is a lot of undercover microphones.

So What Is a MEMS microphone?

Knowles SiSonic MEMS microphone SPU0410LR5H-QB

A MEMS microphone is manufactured using micro-electromechanical systems processing techniques. These microphones are etched into a semi-conductive silicon wafer. A pressure-sensitive moveable membrane (diaphragm) is etched behind a stationary perforated backplate. The perforated stationary plate and the diaphragm act together to form a capacitor (similar in design to a condenser microphone). The MEMS mic is designed to be mounted on a circuit board along with preamp and other application specific components.

MEMS microphone motor

Sketch of a MEMS microphone motor (not to scale). The diaphragm has a thickness of 1 µm, the gap between the backplate and the diaphragm is 4 µm, the diameter of the perforations in the backplate is 10 µm, and the thickness of the backplate is 2 µm. The distance across the motor from support post to support post is 590 µm. Sketch courtesy of Knowles Electronics.

Just like the carbon mics of the last century these MEMS microphones are extremely cheap to produce and are highly reliable. However, what is very different is the audio quality. The Knowles SiSonic MEMS microphone pictured above has a near flat frequency response from 100Hz to 10kHz which is pretty good for such a tiny, inexpensive, microphone. (<£1)

Knowles SPU0410LR5H-QB MEMS mic frequency response 100-10kHz

But what is really mind blowing is the Ultrasonic response!

The graph continues on up to 80kHz !!! (and beyond)

Knowles SPU0410LR5H-QB MEMS mic frequency response 10kHz-80kHz

This brings me to my current project Ultrasonic Recording,

This MEMS microphone is perfect for recording the activity of bats, amphibians, crickets and other wildlife whose calls are in the ultrasonic range high above our rather limited human hearing.

AudioMoth https://www.openacousticdevices.info/ is a miniature recording device currently being developed to record and collect data on the activities of a variety of wildlife. The latest version of AudioMoth uses the Knowles SPU0410LR5H-QB MEMS microphone featured above. 

Open Acoustic Devices AudioMoth

So what does it sound like?

CLICK HERE to hear ‘BATS with Guitar’ (Yes, you heard right! )

N.B. The BATS have been slowed down to bring them within the range of human hearing

Conclusion

Hidden away out of sight on a circuit board inside a mobile phone or a laptop or some other electronic device the MEMS microphone is never going to gain any awards for stylish good looks. Even though its audio quality is amazing (considering its price) it isn’t about to replace the Neumann U87 in the recording studio! Nevertheless it is an extremely versatile tool that will be with us for many years to come and is without doubt the biggest selling microphone ever.

JrF ‘D’ Series Hydrophone.

JrF D Series Hydrophone and matching transformer

The optional matching transformer is well worth buying to use with XLR inputs.

JrF D Series Hydrophone

Expanding on my recent forays into the wonderful world of outdoor recording I am now starting to explore the possibilities of recording underwater using my newly purchased JrF D-Series Hydrophone. These are handmade to order in the North of England by the acclaimed sound artist Jez Riley French. I was looking for a high quality, lightweight device that is quick and easy to deploy. It is also very reasonably priced!

Hydrophones and the world underwater.

Hydrophones utilise piezoelectric transducers to detect underwater vibrations and pressure differences. A piezoelectric transducer is a device that produces an electric current when a mechanical force is applied to it. No external power source is required. Piezoelectric materials are able to flex under pressure, converting mechanical energy into electrical energy. Because sound is a form of pressure wave that physically moves particles, it produces a mechanical force as it comes in contact with a hydrophone. The first widespread use of hydrophones was during WW1 when they were used to detect enemy submarines. This 100 year old technology is still alive and well! Today they also have wide ranging use in underwater exploration, seismology, aquatic research, and deep sea recording. Apart from these important scientific functions, hydrophones can also simply be used to capture fascinating and unusual sounds with which to delight the ear!

My first Hydrophone Recording

Some friends of ours have a pond in their garden and this seemed like a good place to start. Dropping the hydrophone into clear open water the initial results fell somewhat short of the whales and dolphins I had been imagining…………….  absolute silence!!

However, we persevered. Moving the hydrophone into a patch of water weed suddenly produced this strange sound!  Perhaps some alien life form sending code?   CLICK On Photo

The Ford

This next recording features a delivery van driving through a shallow ford across a stream. The hydrophone is positioned in the middle of the weir about 2 feet out into the stream. As the van passes we hear the initial waves caused by the wheels followed by a strange ‘phasing’ sound produced by the reflected waves bouncing back off the wall on the opposite bank.

The Duck

This final recording is a strange one from another local pond. I dropped the hydrophone into the water close to a large rock just below the surface about 3 feet out from the bank. Along came a duck and started to rub it’s beak on the stone only inches from the hydrophone. I thought at first it was sharpening it’s beak but I am informed by knowledgeable folk that this is in fact a sort of conditioning that ducks carry out. Who knew?

To Conclude

The JrF D-Series hydrophone is a great addition to my microphone collection and has opened up a whole new world of sound exploration. Thoroughly recommended.

 

 

An Audio Postcard From The COVID-19 Lockdown 2020

The Parabolic Microphone.

In the Beginning.

The notion of using a parabolic reflector to pick up sounds from afar has been around for many years.

With the advent of aerial warfare in the First World War, the British military started developing acoustical devices to provide early warning of incoming enemy aircraft. The first ‘sound mirror’ was erected in Maidstone in April 1915. During the 1930s, in the run-up to World War 2, a number of these enormous concrete parabolic reflectors appeared along the South coast of England and at other strategic locations. The largest of these structures could detect aircraft at a distance of 25 miles. However, the whole project was abruptly abandoned in 1938 with the invention of Radar!

WW1 30ft Sound Mirror

The listening ‘trumpet’ seen on this dish could be rotated to find the strongest signal. This would enable the listener in the control room beneath the dish to calculate the direction of the incoming aircraft. The information could then be relayed to anti-aircraft batteries.

Parabolic Ears

A ‘stereo’ device like this could be used to pick up activity in enemy positions and provide early warning of attack. (That is if you could hear anything above the sound of your mates laughing their heads off!)

The Parabolic Microphone

In the post war years with the rise of television and the increasing popularity natural history broadcasts on both wireless and TV, sound recordists were quick to adopt the parabolic microphone i.e. an omni or a cardioid microphone, mounted facing inwards, at the central focal point of a portable parabolic dish.


Parabolic MIcrophone

The author recording birds in the trees down at the bottom of the garden.  CLICK HERE to share in the joy!

A parabolic reflector has one significant advantage over other sound pick-up devices: it is a noiseless acoustic amplifier. The frequency response and polar pattern are a function of the size of the dish used. The enormous concrete military dishes of the 1930’s were often 30ft in diameter or more, enabling them to detect the lowest frequencies of an aircraft engine over huge distances. For the modern wildlife recordist a somewhat smaller portable dish is something of a compromise! For a narrow forward beam of 10 degrees a 60cm diameter dish gives around 14db (x5) gain to frequencies above 500Hz. A 1m dish will give 20db (x10) gain to frequencies above 300Hz. The forward gain of a reflector is defined as the difference in output level between a microphone which is reflector-mounted compared to the same microphone unmounted. Even with a modest sized reflector, the on-axis sound of the chosen subject is greatly magnified without adding any of the hiss and hum associated with electronic amplification. However, given the narrow beam characteristics of the dish, one thing that the recordist should bear in mind is that if the subject moves off axis the tonal quality of the sound will change. This can be difficult if for instance the subject is moving around in a group of birds. For that reason I would suggest that the parabolic dish is at its best when recording an isolated single subject.

Frequency/ Directional Response Plot for a typical parabolic reflector for wildlife recordingParabolic Dish Frequency plot

Using an unmounted microphone there are many wildlife recording situations in which it is simply not possible for the sound recordist to get close enough to achieve a good signal to noise ratio. As you creep up with your microphone the subject simply flies off or runs away! ……..Or in the case of dangerous wild animals it may be the sound recordist who does the running! The parabolic dish enables the sound recordist to record from a safe distance without risk of disturbing the subject or getting eaten!

One of the most common problems with wildlife recording is that quite often the perfect recording opportunity comes up without warning. Animals and birds are not predictable. Hence, my minimalist, rapid response kit, illustrated below is light, portable and very fast to set up. I can be in record in a matter of seconds!

The Parabolic Microphone. Minimalist Wildlife Sound Recording Kit

  • Extremely light weight, flexible, 50cm plastic parabolic dish available from new UK company innercore
  • Aluminium handle on the back which can also be screwed on to a tripod
  • Simple microphone mount marked at the focal point.
  • HMN Sound MicroLav. N.B. to prevent wind noise a Rycote Furry windshield is recommended. (Not pictured.)
  • M-Audio MicroTrack II set to record in mono at 96kHz 24bit.
  • M-Audio in-ear headphones with industrial ear defenders over the top for isolation.

Walking by a local lake the other day I came across a small flock of Canada geese about 40ft away quietly pottering about on the grassy bank. Suddenly one of them started squawking/honking. I started recording and within a few seconds it flew high into the air and passed straight over my head. On the recording the strange creaking sound of the bird’s wings and the movement of air can be clearly heard. It sounds close up, even though the bird is at least 30ft above me. It then flies round the lake and lands back with the others and carries on honking very loudly. Definitely not a bird you would want to get close to! Would probably make a good guard dog!                                                                           CLICK HERE to take a listen.

Other Uses for the Parabolic Microphone.

When listening to sporting coverage on radio or TV you may have enjoyed the additional excitement of hearing the ball striking the bat or the racket. You may hear the close-up thunder of the horse’s hooves in a race or the grunts and shouts of a rugby scrum and…… Not a microphone in sight!

In Conclusion.

My purpose in writing this post has not been to advertise the merits of a particular set of equipment but merely to suggest some of the possibilities and fun to be had recording sound using a parabolic dish.

P.S. (Nov 2019)  I have just come across this fascinating patent by British Acoustic Films Ltd from 1931 ( !! ) for a Parabolic Microphone for use in film production, recording and broadcasting.  Parabolic Microphone B.A.F Ltd 1931       It even includes a setup whereby sound can be recorded (and mixed) from 3 independent reflectors aimed at 3 different sound sources simultaneously using a single microphone!  Wow!

P.P.S. 2020 update during COVID 19 lockdown with Sennheiser MKE2 microphone.