Active noise-cancelling silencers.

I remember when they were going to be the next big thing - silencers
with speakers & sound generators that sent out an opposite sound wave
to cancel out the exhaust noise. They disappeared without a trace for
whatever reason.

But stumbling across an article about them t'other day got me thinking.
The ins & outs are way beyond my brain, but is it as simple as
recording the exhaust noise, loading that into something like Audacity
& using some functions in there to generate an 'opposite' sound file?
And if so, would that only work for the RPM the recording was done at,
or does the exhaust sound wave stay roughly the same frequency at all
revs (or a band of n RPM)?

 

LOL, you're bored then ...

Thinking back to when I had a Suzuki X7 with expansion chambers I doubt the
exhaust note stays the same frequency, the X7 certainly high-pitch screamed
at high revs and quietly-ish burbled (fsvo burbled) at low revs.

 

I wish... I've been battling with an extremely complex problem for the
last couple of weeks, & have to keep kicking it out of my brain in the
hope I have a eureka moment when it pops back in.

Yeah I thought as soon as I posted that it was a stupid question. The
first part still stands though.

 

That wouldn't work - it's not just the frequency which must be exactly
right, but the phase that must be exactly right also. Even if you
had a continuous frequency sound you wanted to cancel out, a recording
wouldn't be sufficient because it's doubtful you'd be in phase (well,
precisely 180 degrees out of phase). At best you'd end up with a
WwwwwwooOOONNGGgggwwwwoooONNNNGGggggwwwwwwoooONNNNGGGgggg sound (if you
see what I mean) because the two sound waves wouldn't be exactly at the
same frequency and so you'd end up with a "phasing" sound louder than
what you're trying to cancel for some of the time. (Think of the sound
you hear on a turboprop airliner when they don't bother to keep the
props precisely in sync, that really annoying LOUD noise).

Even when canceling a fixed frequency sound, you still need to actively
keep the sound wave you're using for cancelation precisely 180 deg.
out of phase with the sound you're trying to cancel, and this simply
cannot be done passively.

All bets are off trying to do it with a simple recording for a simple
fixed frequency sound, all bets are definitely off for a bike exhaust
which will run a large range of frequencies (and different wave forms).

 

Not really, even in so far as there is such a thing as `phase' on
these signals. You can't cancel the noise everywhere so you aim to
reduce it at selected points without increasing somewhere else that
people might hear it.

I suspect that is more likely to be due to a gust, than loss of sync.

You don't: you take a feed off the reference sigal (on aircraft, you
use the prop shaft rotation pulse). You don't have a feedback signal
in this system anyway, so you do some cancellation at source, just
past the end of the can.

The way I would do it is put a ring speaker on the end of the can (not
inside it) and drive it with a feed from the rpm signal through a
smart bit of electronics.

 

(M J Carley) gurgled happily, sounding much like they
were saying:

I'd just lob some money at Lotus. They've been doing it in production for
20yrs.

 

We are actually in the building next to where Southampton University
used to have their automotive research labs: there's a big department
called ISVR (Institute of Sound and Vibration Research) who partnered
with Lotus for some of the work that you mention.

Unfortunately they got shut down because the science park people felt
that running JCBs etc all day whilst pointing microphones at them didn't
fit in with the general ambience they were aiming for. Which I reckon is
a bit sad.

 

Which could set up backpressures, equivalent to blocking the pipe.
If it were that simple it would have been done already.

Active noise reduction only works for a small point in space no larger
than wavelenth/10 of the sound to be reduced. Noise cancelling
headphones only need to create a null point at the ear entrance so
they work (give or take some leakage) so they are the simple solution.

 

Using the patented Mavis Beacon "Hunt&Peck" Technique,
typed

<pulls up chair>

crn teaches TBC about acoustics.

This should be fun.

 

Which is why I said to put on the end, not inside it. As for
backpressure, you are cancelling the radiating mode on the pipe
termination, not the full modal system inside the pipe, which requires
much less energy with no need to stop the flow (the mean flow doesn't
make noise).

Best tell NASA:

http://www.nasa.gov/centers/glenn/about/fs03grc.html

Best tell Saab their Saab 2000 system doesn't work, then.

 

Different case. This is similar to the method Bombadier use in the Q400
turboprop. If you know that noise is being transmitted by vibration
through a panel or bulkhead you would normally add a layer of absorbant
padding between the panel and the interior lining and/or stiffen the panel.
It is possible to add sensors and actuators to the interior lining to cancel
the transmitted vibration, thus reducing both noise and padding weight.
This is not the same as cancelling airborne noise, stopping the lining
vibrating prevents the airborne noise being _created_ rather than trying
to fix it later for a listener which is much more difficult.

 

Saab's noise team say they used speakers to cancel airborne noise
inside the cabin, which is a lot bigger than one tenth of a
wavelength:

www.a2acoustics.com/publikationer/AIAA_98_2231.pdf


As for exhaust noise, this is a very sweet idea:

http://www.mech.kuleuven.be/mod/other/topic_03_01

 

I also seem to recall some success in industrial applications with
fixed frequency sources such as fans.

http://www.hse.gov.uk/noise/casestudies/soundsolutions/pneumatictransport.htm

 

There has indeed. These guys have done some nice work lately:

http://www.usherbrooke.ca/gmecanique/recherche/domaines/vibro-acous/themes/controle/

This was really cute (using the fan as the loudspeaker):

www.rthcorp.com/publications/ANC_Fan.pdf

 

No, if it's continuous in still air, it's because the props aren't
synced. (Most large turboprops should have some kind of automatic
sync but it may not be perfect. Small twin engine aircraft have
to be synced by the pilot manually (example, the elderly Piper Apache
I did my multiengine rating in).

You've just defined an active system Using a feed off a prop shaft
to keep in sync or using a sound wave to keep in sync, effectively
you're having to do something actively to keep the cancellation signal
180 degrees out of phase with the sound you're trying to cancel.

 

Or propeller asymmetry.

You can't keep your signal 180 degrees out of phase. ANC does not give
you full cancellation. In aircraft active systems, the ANC uses
feedback microphones to adjust phase and amplitude but doesn't aim for
zero noise.