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# Noise Level

## Noise Level

(OP)
Hello !! I am currently facing a small problem. I am installing an avionic equipment in an helicopter. And I wonder if it is possible with a simple analysis to determine if the alarm produced by my equipment will be correctly audible by my pilot. Knowing that my equipment will be installed on an unswitch input of my audio panel not allowing the pilot to turn down the volume of this alarm for safety reasons. I would like to make a small calculation only for my personal knowledge. Even though I know that the equipment is standard and that my pilot will hear this alarm. I am just curious and would like to know how with a calculation we can say that we are confident that the alarm will be audible.

The ambient noise level of my rotorcraft is 115dB

My equipment has the following characteristics:
- 600 ohm -> 0.625 to 80 mW

My audio panel has the following characteristics:
- Input impedance: 500Ohm
- Input Isolation: 60dB minimum

The speaker has the following characteristics:
- Output power : 10 Watts into 4 or 8 ohm

I have been asking myself this question for a very long time, whether it is possible or not, but unfortunately I don't know the audio world very well. If you have an answer to my question + explication I'll be able to finally sleep well ahah

Thank you very much for your help

### RE: Noise Level

You need to know about the speaker efficiency in order to turn you power into volume.
And there are rules about how loud a sound needs to be to be heard against the background but I don't recall them.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

### RE: Noise Level

"not allowing the pilot to turn down the volume of this alarm for safety reasons" ... that sounds very unsafe to me, and probably not acceptable to many pilots.

### RE: Noise Level

(OP)
Hello @rb1957 I'm not saying that the alarm cannot be inhibited but only that volume of this alarm cannot be turned down.

### RE: Noise Level

A 115dB ambient noise level is definite concern for crew/Pax hearing protection.

For this reason most helo pilots, crew and Pax will be using/wearing high quality heads-sets which block-out/muffle/cancel ambient noise yet receive all aural comm/nav/system/warning tones/notifications/messages with clarity. IF this is the case, then human factors with this 'primary' aural channel... hearing quality... is always a wild-card.

IF my presumption is real, then multiple/arranged/discrete/directional cabin speaker(s) or piezoelectric devices will be required to flood the cockpit/cabin with their warning tones/notifications/messages directly... along with these same messages/tones in the headsets.

A series of flight tests of various speakers/piezoelectric devices-configurations, with people having various hearing issues/degradation in the nominally loud environment... having high quality headset with volumes 'starting at full-down/off'/rising to max volume for emergency notifications... may be the simplest to way to validate Your design for the real world.

NOTE. People, seat arrangements, insulation, window panes, mechanical components [engines, transmissions] and wind-noise, at various flight conditions will all play various damping/muffling roles.

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

### RE: Noise Level

(OP)
Hi @WKTaylor, Thank you for your very interesting answer, in fact when I say 115dB it is to take the worst case, in reality the ambient sound level of an helicopter is between 85dB and 100dB- 115dB worst case. Many pilots have actually ANR headset. Noise attenuating headset have 15dB to 28dB of ambient noise reduction. I think the question I am asking to myself is maybe not as simple as that, but I am asking how an equipment manufacturer designs its equipment so that the alarm it will provide is adaptable to "all" environments (airplane or helicopter) that's why I am asking if with a "simple" calculation as well as with a rough knowledge of the ambient level one is able to determine if the crew will hear this alert or not. Of course I specify again that it is simply a question that I ask myself because I do not know very well this field which in spite of everything is I find very interesting. It is also that if I connect I take an example at random but an EGPWS with an audio panel properly qualified we know very well that the alarms will be heard by the crew without doing great calculation. As well as if I'm connecting a Garmin GTS to a GMA340 i know pretty well that everything will be heard. However it is a personal desire to try to inform me and perhaps to learn full of thing on this domain which is very obscure to me.

### RE: Noise Level

how do noise cancelling headphones work ? don't they detect the external noise ? Could these work in a similar way ?

### RE: Noise Level

In the case of the AF447 (Air France crash of a perfectly fine Airbus A330):

#### Quote (Craig E. Geis, M.B.A., M.A.Psychology)

The stall warning sounded continuously for 54seconds.
The pilots did not comment on the stall warnings and apparently did not realize that the plane was stalled.

Yet for the duration of the flight, none of the pilots will mention it, or acknowledge the possibility that the plane has indeed stalled — even though the word "Stall!" will blare through the cockpit 75 times.

A tone by itself seems to be a popular, but not always effective, way to alert a crew to problems in a machine with so many variations of trouble. It may make things worse if pilots decide that they don't need to frequently scan the instruments, confident that an alarm will draw their attention should a problem arise. In one case to solve this problem the Mooney Mite, famed for being the smallest production plane with retractable landing gear, they added a device that flicked back and forth in the sightline of the pilot to remind pilots to put the gear down when the flaps and throttle were set suitable for landing. This was due to the large number of wheels-up accidents.

### RE: Noise Level

1) none so deaf as those that will not hear ? The pilots (of AF447) had completely lost situational awareness, something I think is easy to do late at night, high over an ocean. Still their training should have prepared them for this and given them a roadmap back to safety. As I heard it, the two side-sticks were giving opposite inputs.

2) they needed a dog in the cockpit ... a big, nasty, angry dog (to keep the pilots from touching things).

### RE: Noise Level

They were well rested. As indicated in that report the co-pilot was apparently sidetracked by what he thought the problem was. The additional alarm did not fit with his thinking and the more senior pilot could not figure out why the plane was acting that way because it could only do so if it was being forced into that attitude and why would the co-pilot do that? So neither knew what was going on and both ignored the plane.

### RE: Noise Level

rb1957, Witn NC headphones they do take the outside sound and create an inverse signal. The real trick is how much do they block (60%, 80%?) and over what frequency range. With my Bose phones on it is actually easier to hear people speak than without them in a plane, they are designed to not block as much in the vocal range.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

### RE: Noise Level

But people talk with modulated tones, which NC headsets don't cancel.
They are designed to generate a "white noise" to cancel the environmental noise.
The alarm is a "pure tone" and might not be modulated, so there is a chance that the headset could cancel it, more or less.
You want to patch it into the audio system AND put it on speaker.
This is a solved problem. I helped solve it, on a project years ago.
You seem to be past the point of already selecting the equipment, but just in case...

My project was done for these folks (see attached):

### RE: Noise Level

Oh. I forgot.
That operator isn't in business any more.

he's gone deaf ?

### RE: Noise Level

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