Using a Hydrophone – Context

Having just received the microphone, I am still yet to venture out into a deep water environment where I can test the microphone to its full capacity. However, I was keen to immediately test the microphone to get an idea of how sensitive it was going to be, how accurate it was going to potentially be in capturing the original sound source, and how much noise it may or may not inherently have. Using my Zoom H6 with this hydrophone to gather a number of preliminary samples, I considered the options I had immediately around me. I chose the 60,000 litre salt water fibreglass swimming pool found in our front garden as my first test environment. A place where my partner and I have spent considerable hours over the past two decades, it is surely a significant part of our lives, and therefore somewhere I am going to need to gather sample events for my composition. In saying that, embarking on this test I acknowledged there would be some limitations of using this test environment to trial the functionality of this condenser microphone. Namely, the structure of the pool – the pool is 4 metres wide, 9.5 metres long  and 1.9 metre deep (reducing to about 1.4 metres in the shallow end) and made of a fibreglass shell with the sides and bottom curved into one continuous surface. Due to this particular environment, the hydrophone microphone would likely display a narrower bandwidth than what it would optimally have in deeper waters; and the captured sound source was likely to include the original sound source and a number of reflections off the hard surfaces of this domestic swimming pool.  Irrespective, as I was going to need samples of this environment eventually, I considered it a useful initial test environment.

Using a Hydrophone – Part 1

The first 5 sample events I believe demonstrate the sensitivity this condenser microphone has in underwater situations. I was surprised how sensitive the microphone was, despite the large amount of water residing between/separating the subject and the microphone capsule during these recordings. As indicated above regarding the reflections, the captured sample events demonstrates a cacophony of sonic textures resulting from a fusion of both the intended sound source and its’ multiple reflections.

Note also the frequency range of each sample event relative to the micopphones’ depth and proximity to either the surface, the bottom, or the sides of the swimming pool. I have been reminded that in a shallower water environment: there is likely to be less fully developed low frequencies due to the shorter distance between any surfaces. Additionally, in calm water conditions the sound waves under the surface are likely to rebound back off a flat water surface, phase cancelling the original signal below it. This phenomena of a varying frequency range is particularly noticeable in Using a Hydrophone – Part 2 sample events 7 and 8 when the condenser microphone capsule is being bounced up and down at variable depths under the surface, and then breaches the surface of the water. Listen and compare the frequency range and the sonic texture of each sample event as the condenser capsule moves through the water.

Sample Event 1  (click to access audio)

In the first sample, the hydrophone was submerged in the swimming pool to a depth of about 1.5 metres. The Zoom H6 track 3 gain level was set to 6 (of 10). My friend (the subject) was in the pool and approximately 2 metres away from the hydrophone, facing it and blowing bubbles under water in the direction of the microphone. The reverberations off the nearby pool surfaces are quite noticeable from about 1/3 third into the sample event, providing a minor delay of the original signal until the end of the sample event.

Sample Event 2 (click to access audio)

In the second sample, the hydrophone was maintained in the swimming pool at a depth of about 1.5 metres. The Zoom H6 track 3 gain level was set to 7 (of 10). The subject was in the pool and approximately 3 metres away from the hydrophone, facing it and blowing bubbles. The overall levels are softer in this second sample event while she was mimicking what she had done previously – with the exception of when the hydrophone capsule got knocked by something (tall volume spike midway) – despite the gain level being increased marginally. See image i below. The reverberations off the nearby pool surfaces are quite noticeable from about one third into the sample event, providing a minor delay of the original signal until the end of the sample event for the second third, but then decays and releases back to mainly the original signal in the final third of the sample event.  As a result of the decaying signal, the amplitude reduces. With the return to the original signal in the final third, there is greater clarity of the signal.

Image I – Pro Tools 12 Sample event 1 (top) and Sample Event 2 (bottom)

Sample Event 3  (click to access audio)

In the third sample event, the hydrophone was submerged in the swimming pool to a depth of about 1.5 metres. The Zoom H6 track 3 gain level was maintained at 7 (of 10). The subject was in the pool and approximately 3 metres away from the hydrophone, facing it and trying to talk underwater.  I note that despite her being farther away from the hydrophone capsule than she was in the first sample event, as she was trying to talk loudly under water toward the microphone capsule, the audio is louder than both sample events 1 and 2. As you can see in image ii below, the overall mass of the wav file is exponentially greater in this third event than both the previous two sample events, with the subject’s speaking voice producing far greater mass and density than she did when blowing bubbles underwater. This mass and density represents increases in sound pressure levels, and reverberant signals, resulting in a cacophony of sonic textures.  Had I included a longer sample, you would observe, as per the sample event 2, at a certain point the signal decays and releases back to mainly the original signal, with reduce amplitude, but greater clarity.

Image II – Pro Tools 12 Sample event 1 (top), Sample Event 2 (middle, Sample Event 3 (bottom)

Sample Event 4  (click to access audio)

In the fourth sample event, the hydrophone was submerged in the swimming pool to a depth of about 1.5 metres. The Zoom H6 track 3 gain level was maintained at 5 (of 10). The subject is in the pool and approximately 0.5 metres away from the hydrophone, facing it and blowing bubbles. Sonically, this fourth sample event demonstrates a cacophony of sonic textures, resulting from excessive sound pressure levels due to the close proximity of the transducer relative to the sound source, and the accompanying  reverberant signals from the multiple surfaces of the pool.  The inherent distortion results from excessive sound pressure levels, with an over-gained signal. For non-audiophiles: note the clean flat line along the top of the wav form indicating a form of dynamic limiting. Given that no dynamic processing was used to achieve this limiting of the audio signal, the limiting effect indicates acceptable gain levels for the equipment were exceeded, resulting in what is referred to as digital (signal) clipping. See image iii below (top wav form).

Image III – Pro Tools 12 Sample event 4 (top) and Sample Event 5 (bottom)

Sample Event 5  (click to access audio)

In the fifth sample event, the hydrophone was maintained at a depth of about 1.5 metres. The Zoom H6 track 3 gain level is reduced to 5 (of 10). The subject was in the pool and approximately 0.5 metres away from the hydrophone, facing it and trying to talk underwater. As you can see in image iii above (bottom wav form), the overall mass of the wav file is exponentially greater in this fifth event than the previous sample events, with the subject’s speaking voice producing far greater sound pressure levels than she did when blowing bubbles underwater. Sonically, this fifth sample event is heavily distorted due to the excessive sound pressure levels due to the close proximity of the transducer relative to the sound source. The digital recording is therefore clipped given the amplitude far exceeded the specified gain levels of the equipment. For non-audiophiles: in this example the cleaner flatter line along the top of the wav form – relative to the previous example – indicating extreme limiting of the audio signal. Again, as no dynamic processing was used – it similarly indicates excessive sound pressure levels at unacceptable gain levels for the equipment, resulting in severe digital (signal) clipping across almost the entire length of the audio wav file.  It is also worth noting the very thin sound of this sample event as a result of the absence of low frequencies in the shallow depths; and yet as per sample event 4, there is a cacophony of sonic textures given the multiple reverberant signals arriving from the numerous surfaces of the pool.

Using a Hydrophone Part 2

In the following examples, I gathered a number of sample events using the hydrophone closer to the surface of the water line.  I hope the sample events further show how sensitive the hydrophone microphone is, effectively capturing sonic qualities of very subtle movements.

Sample Event 6  (click to access audio)

In the sixth sample event, the Zoom H6 track 3 gain level was set at 6 (of 10). The hydrophone was being dragged along the surface of the swimming pool at a relatively slow walking pace. The sound of rushing of water is the wake of water that the small condenser capsule (25mm wide, but 46 mm long, weighing 105 grams.) is creating and capturing as it breaches the surface of the water. I think you will agree that this confirms both the sensitivity and low noise levels of this particular microphone. The deeper frequency you hear (boomy quality) in the audio file is when the transduction surface of the microphone capsule is re-submersed  under the surface of the water.

Sample Event 6wp (click to access audio)

Sample event 6wp indicates that it is the same sample as sample event 6, but with post-production audio processing added. In the studio – following recording the sample – I chose to add two (2) reverb processing devices – a Eventide and a Lexicon reverb – to the initial audio file. While doing this, and listening to the altered sonic textures of the audio, I am imagining the many applications that I could use such an effect in my sonic compositions and sound design.

Sample Event 7 (click to access audio)

The seventh sample event is a similar execution as sample event 6, with the hydrophone being dragged along the swimming pool at a relatively slow walking pace, but being bounced in and out of the water in an approximately 30 centimetre arc.  The popping and gurgling sounds are occurring as the capsule breaches the surface of the water (popping), then followed by the re-submersion (gurgling). It is a similar but more exaggerated version of sample event 6, with the sample event’s frequency varying dependent on where the condenser microphone capsule is relative to the water: being just under the surface, at depth (only about 30 cms in this example), breaching the surface, or above the surface of the water.

Sample Event 8 (click to access audio)

The eighth sample event is a similar execution as sample event 7, with the Zoom H6 track 3 gain level remaining at 6 (of 10). The hydrophone was being dragged along the surface of the swimming pool at a relatively slow walking pace, but being bounced in and out of the water over a much larger arc – approximately 1.5 metres.   This is a more exaggerated version of sample event 7, with the popping and gurgling sounds associated with the breaching and re-submersion are relatively deeper in tone due to the greater depth, speed and height the capsule was dropped from, back into and under the water.  Sonically, you may hear what sounds like wind noise in this audio sample event. I noted at the time that this was due in combination to both the faster movement of the capsule above the surface of the water after breaching; but also partially due to the wind in our local area picking up nearing the end of the test. You will also note that near the end of the sample event you can hear a voice – talking, describing my actions. This voice was captured by the microphone capsule after it had breached the surface of the water, with the speaker’s mouth about 2 metres away.

Sample Event 9 (click to access audio)

The ninth and last sample event had the hydrophone submerged in the swimming pool to a depth of about 1.5 metres held stationary. The Zoom H6 track 3 gain level remained at 6 (of 10). The subject was approximately 2 metres away from the hydrophone drop point, swimming up and down the pool in freestyle form. The low frequency plop occurred every time the subject kicked her feet, with training flippers on.  The bass frequency was pronounced, reverberating off the surfaces of the  pool, producing a sound somewhat similar to a deep tom sonic boom after the skin had been struck. And yet, the hydrophone microphone still clearly captured what sounds to be running water – the sound of the subject’s hand and arms entering and breaching the surface of the water with each and every stroke. Again, I am imagining the many applications that I could apply some processing to this sample event, and use such an effect in my sonic compositions and sound design.

Summary

The Aquarian Audio Products Hydrophone H2a-XLR microphone is an extremely sensitive fully submersible condenser microphone, with minimal extraneous noise. It is well designed and constructed to be impact resistant, using sturdy materials. Whilst it is designed to be submersed in a far greater depth than I have tested to date, I believe I have made a good purchase with this hydrophone, something that will complement my current stock of studio and live microphones. I believe this microphone will allow me even greater versatility in a range of recording scenarios that I can foresee me being presented. I daresay I will probably now go searching further afield, exploring less predictable outdoor terrain, and feeling the need to be less mindful than I usually would taking my more expensive studio microphones. I am looking forward to progressing my sonic compositions and sound designs using water samples across the range of contexts which I have experienced in my life – the ocean – including boating, body surfing, snorkelling and scuba diving – rivers,  waterfalls, natural pools, and domestic water use – in order to capture specific sample events that represent significant events and memories. I look forward to this next chapter in my creative practice.

It is intended for this series of microphone-related blogs to continue.

References

AE Project Studio Microphone Case image courtesy of: DLP Pinterest site  Accessed 28th August, 2015

Aquarian Audio Products. 2016a.  H2a-XLR Hyrophone Users Guide http://www.aquarianaudio.com   Accessed 17th November 2016

Aquarian Audio Products. 2016b. http://www.aquarianaudio.com  Accessed 15th November 2016

Hydrophone images courtesy of: Aquarian Audio Products  Accessed 16th November 2016

Page, David. L 2016. Soundcloud.  DLP Soundcloud  Accessed 17th November 2016

Pro Tools 12 Sample Event Images courtesy of: David L Page  Accessed 16th November 2016

Zoom H6 image courtesy of: Sound on Sound  Accessed 16th November 2016

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