Constructive Interference — Any Scientists Here?!

I have developed tinnitus as a result of a “training incident” that took place at a new place place of employment on 13th May. I've posted full details in my 'Introduce Yourself' thread, so I won't give full details here. In short, I was being shown an alarm lock up/unlock procedure by a senior member of staff at my work and they did it in an unnecessarily stupid way, and I've had tinnitus since (13 days now).

I reported the incident to the organisation they brought in an engineer, and they say he recorded the db level of the alarm at 98 (and therefore safe in their eyes), though they don’t say at what distance (I was a foot from the alarm).

Anyway, since the incident I’ve been feeling very anxious about the long-term implications and have been reading a lot about tinnitus and exposure to loud sounds. I’ve read about "constructive interference" which describes how sound waves interact when reverberating around a space. It relates to when two sound waves of the same amplitude meet, meaning they combine and their sums are added together (e.g. the sound will get louder). The training took place in a small enclosed space with flat walls and windows, and I can't help but think this phenomenon could very easily have occurred over the course of 1 minute (the duration of the incident).

However I don't really know what this ACTUALLY means in terms of dB level. Is it simply that they combine and it becomes 2 x 98 dB, or is there some kind of magical logarithmic relationship that comes into play? I've tried Googling, but that mainly gives me equations and graphs, whereas I want SOUND WAVES FOR DUMMIES type stuff.

Does anyone know?

 

Yes, but it would be too complex to explain it here (you can already tell this is true since you've seen some info online).

Instead of thinking about wave interference (constructive or destructive) and what happens at the micro-level, just look at it from a very high level.

You're in a room, whenever sound is emitted, it reflects on walls. That's called echo (I'm sure you know what it is), and it's also the same system that bats use to figure out distance to objects. So the sound comes back and essentially the power adds up. But the sound that comes back from reflection has had to travel before coming back (it has had to reach the wall, and come back like bouncing off a mirror, sometimes hitting multiple walls before coming back), so it is much more attenuated that the original sound (sound power drops fast as sound travels through air), unless your walls are boxing you into a very small enclosure.

The net net is that, unless you are in a very small enclosed space and the sound is loud to begin with, the reflections are probably not something to worry about. Night clubs typically fit the criteria above, so they would be environments to worry about.

 

Thanks for replying, but I'm still confused. I WAS in a really small enclosed room - like 3 metres x 3 metres sorta thing, with a piercing digital intruder alarm sounding constantly for 1 minute. Basically if there's a chance that constructive interference could have come into play in that situation then it could help any case I may have against my employer for negligence (and I'll push to get extra sound level tests done accordingly).

 

@JaneTC - sorry to hear that. Someone else was in the same situation as he was installing an alarm. From his latest posts I understand that he is back to living a normal life although it was bad in the beginning. Was actually thinking about him lately as his started at a similar time as mine.

It could be that your exposure to music predisposed you to it. However your work is still responsible, there’s something called “acoustic shock” for example. You’ve done the right thing by reporting it, keep all the emails in case you need to take action at some point.

Do not be angry with yourself, you didn’t know - we’re all the victims of a lack of education - never understood why Public Health aren’t doing much about it.

I got prednisone, didn’t really help, but took it soon after it started.

This is a great community and you’ll find some good info. When you have the info you need to try and move on if your tinnitus is mild, it will help you not focus on it.

 

You may have suffered an acoustic trauma due to the sound source and the small enclosed room, but using the terminology of "constructive interference" is not helpful, in my opinion.

 

Ok, thank you. I guess what I need to understand is what I SHOULD be saying. Basically employer is saying the alarm was 98db at moment A, but if there's a chance that at moment B, C, D, E, F, G.... >Z it may have been GREATER than 98, then I need to know how to put that case forward. I still don't understand if you're categorically of the feeling the sound level couldn't have changed over time or not...? Sorry for pushing.

 

It's likely to have been greater than 98 dB, simply because of laws of reflection and sound attenuation. But how much greater is the question. Perhaps 99 dB? Perhaps 110 dB? That's the part that gets quite complicated to compute: the easiest is to measure if the conditions can be reproduced, rather than try to compute with laws of physics.

Look at this to figure out how complicated it gets once you start modeling a few reflection iterations: http://arqen.com/acoustics-101/reflection-free-zone/

Compare the left (1 reflection) to the right (3 of them). Then understand that in real life, there's an infinite amount of them, but it doesn't matter so much because the sound also attenuates quickly, so at some point (say after 3 reflections), the sound power has dropped so much that it doesn't really contribute much to what you are hearing.

 

Ok, thank you! That makes me more confident to insist on some additional sound level tests (or, given that I have keys to the space, to get someone to do it for me discreetly on a weekend!). I still work there, so feel very awkward making a fuss, but also don't want to be completely shafted by them because I'm too polite. Thanks again.

 

The alarm I was exposed to was 85 db at ten feet. Maybe it's possible that yours is the same. They told me - from when I called - that alarms have only been getting louder. To hear yours was 90 dbl plus and you don't know from how many feet is scary. Alarms also come with level registers. You need to ask your employers a lot of questions.

 

How is your T now? Have you seen much improvement since it happened? I feel like such a fool for just following orders, and I'm hoping mine will start to fade soon.

 

I was also a foot away from the alarm which would make it at a 115-120 db possibly to my ears if I did the math right. My T has faded, yes, a good amount. But I've been vigilant in protecting against loud noises and avoiding noisy events. I have taken the Bill Bauer route.

 

Glad to hear yours is improving. Gives me hope!

 

The alarm was 98 dB. This is really, seriously loud. I think it is an academic question if it was louder or softer after that. The point is that 98 dB was more than enough to produce the acoustic trauma with noise related tinnitus you are suffering.