Structure-born noise

[archive]

From: "Richard Plourde" <richardplourde@m...>
Date: Mon Jan 8, 2001 4:14 pm
Subject: Structure-born noise

I was puzzling over something very different from air-
born 'noise' -- the difficulties of eliminating
structure-born noise. It occurs to me that it might
be possible to sample structure-born noise (for
example, the vibration from a passing train), "tweak"
the sample for frequency-dependent phase and gain,
and use it to electronically cancel in-room microphone
pickup of that noise. Has anyone ever done this for
frequencies below, say, 70 Hz (5 meter wavelength in
air)? Back in the straight analog days, this would
have been just about impossible, but a DSP using
finite impulse response filters, might be able to do
it by self-adaption.

Initial research and firmware development would, of
course, be quite expensive, but the DSP's are
themselves dirt cheap. And building a "room in a
room, suspended on springs" is damned expensive.
Assuming that such an approach were possible, does
a sufficient market exist to justify development
costs?

-R

[archive]

From: SRF7@a...
Date: Mon Jan 8, 2001 5:33 pm
Subject: Re: [acoustics] Structure-born noise

In a message dated 1/8/01 11:17:59 AM Eastern Standard Time,
richardplourde@m... writes:

>I was puzzling over something very different from air-
born 'noise' -- the difficulties of eliminating
structure-born noise. It occurs to me that it might
be possible to sample structure-born noise (for
example, the vibration from a passing train), "tweak"
the sample for frequency-dependent phase and gain,
and use it to electronically cancel in-room microphone
pickup of that noise.

Train noise would be a difficult first candidate for the system you propose.
I get daily experience with this particular problem (my home studio is
adjacent to a RR track. First, no two trains are alike ... there is infinite
variety in the noise they put off, as the sources are quite numerous (out of
round wheel assmblies, sticky brakes, stuttering car connections, thrum from
the diesel electric motor in the locomotive, the air horn ... etcetera). The
low stuff is obviously the most troublesome as it is ground bourne (the earth
literally shakes the foundation). When the train finds a mode resonate in
the slab you can actually feel a pronounced bounce in your chair, though
fortuneately this is fairly rare.

If the project focused on a consistent noise source the job would be worlds
simpler ... as the system could be "tuned" to cancel the noise ... with
trains you'll have latency in both locking onto the freq. and in tracking its
dynamics .... the cure might be worse than the disease, what with the system
having to detect the noise, lock onto its freq, then calculate the proper
amplitude, then continuously predict the non-linear changes necessary to keep
the canceling signal locked onto the noise's modulations (rising then falling
amplitude, build up then decay of harmonic structure, doppler shift ????) all
this for treating one point in the room.

>Has anyone ever done this for
frequencies below, say, 70 Hz (5 meter wavelength in
air)? Back in the straight analog days, this would
have been just about impossible, but a DSP using
finite impulse response filters, might be able to do
it by self-adaption.

> Initial research and firmware development would, of
course, be quite expensive, but the DSP's are
themselves dirt cheap. And building a "room in a
room, suspended on springs" is damned expensive.
Assuming that such an approach were possible, does
a sufficient market exist to justify development
costs?

-R

For trains, given that my mid and hi-band isolation is quite good (brick
veneer over a grout-filled concrete block building shell) my only real
disturbance is the ery low freq. rumble you describe... for this I find dual
60 to 80 Hz hi-pass filtering knocks the problem out (I use one on the mic
pre when tracking, and one in the DAW if necessary when mixing, this gives a
double dose that usually defeats any gentle rumble entirely) ... on bass
instruments where this approach is not feasible, I just make sure the mic is
in a heavy stand (I have a one which weighs about 60 lbs. and rides on three
rubber casters), and/or has a spider suspension .... this has proven
sufficient (haven't lost any great takes to a train yet ... knock on wood).

Given that I would concievably be a prime customer for the system, but have
found a cheap and not too annoying way around the problem, I question the
size of the potential market for anything that wasn't fairly cheap and easy
to use. That said if you want a site to field test the project I'm game.

Scott R. Foster

[archive]

From: steve <whitaker@p...>
Date: Mon Jan 8, 2001 6:33 pm
Subject: Re: Structure-born noise

Hello,

That is a very good idea, and products doing similar functions already
exist. (sorry, i don't remember the specific product, you might try a web
search). The structure borne noice from large HVAC systems being an example
of a structural noise which would be fairly consistant.

But, as Scott stated, a train is a moving target (literally) and so is its
sound content. This applies to traffic noise and airplane noise too (yes,
i'm still talking about the structure borne content of these noise sources).

There is a processing delay for the moving target, so you'd need a delay on
all the mic inputs to compensate for that delay.

Hope this helps.

Steve

>I was puzzling over something very different from air-
>born 'noise' -- the difficulties of eliminating
>structure-born noise. It occurs to me that it might
>be possible to sample structure-born noise (for
>example, the vibration from a passing train), "tweak"
>the sample for frequency-dependent phase and gain,
>and use it to electronically cancel in-room microphone
>pickup of that noise. Has anyone ever done this for
>frequencies below, say, 70 Hz (5 meter wavelength in
>air)? Back in the straight analog days, this would
>have been just about impossible, but a DSP using
>finite impulse response filters, might be able to do
>it by self-adaption.
>
>Initial research and firmware development would, of
>course, be quite expensive, but the DSP's are
>themselves dirt cheap. And building a "room in a
>room, suspended on springs" is damned expensive.
>Assuming that such an approach were possible, does
>a sufficient market exist to justify development
>costs?
>
>-R
>
>
>
>For more info, unsubscribe, large file uploads, ect:
http://www.studiotips.com
>Send small drawing files to dan@s...
>To Unsubscribe: Send email to
>acoustics-unsubscribe@e????ups.com
>
>
>

[archive]

From: "Richard Plourde" <richardplourde@m...>
Date: Tue Jan 9, 2001 5:52 pm
Subject: Re: Structure-born noise

--- In acoustics@e????ups.com, SRF7@a... wrote:
> In a message dated 1/8/01 11:17:59 AM Eastern Standard Time,
> richardplourde@m... writes:
>
> >I was puzzling over something very different from air-
> born 'noise' -- the difficulties of eliminating
> structure-born noise. It occurs to me that it might
> be possible to sample structure-born noise (for
> example, the vibration from a passing train), "tweak"
> the sample for frequency-dependent phase and gain,
> and use it to electronically cancel in-room microphone
> pickup of that noise.
>
> Train noise would be a difficult first candidate

Train, subway, truck, etc. noise would be the primary
candidate -- and while this would present some
difficulties, I doubt that it would be insurmountable.
With wavelengths (in air) on the order of 5 meters
(and, in the structure, probably on the order of 10
meters) a single structure-noise pickup would not
suffice. But many pickups are practical. Just to
put some numbers on it, let's assume each structure
pickup consists of a 3-axis accelerometer. It would
be easy enough to package such a widget, complete
with A:D conversion (ten or twelve bits should be
plenty over the frequency range of interest) for a
manufacturing cost of about $10US (let's say a
street price of $50). Such a widget could be
easily daisy-chainable. In a small to moderate
sized recording studio, three such widgets per
surface would probably suffice to provide a mapping
that would allow 20dB post-processing attenuation of
structure-born vibration -- even given processing
electronics, we're probably talking about a street-
price on the order of $5000. It doesn't take too
many 3 A.M. recording sessions at union overtime
rates to pay off $5000.

But, do enough studios exist that treat recording
as a business to support the several million
dollar development cost of such a system? (That
development cost, by the way, is part of the
difference between the $10 manufacturing cost and
the $50 street price.) Putting numbers to it
again, let's assume a "per-installation" street
price of $5000, with $1000 going to pay for the
development. With a development cost of $2e6,
that would (using back of the envelope estimates)
make the break-even point somewhere around 2000
installations.

Of course, there are plenty of ways of playing
with the numbers, and of playing with the cost
of the electronics. (Given a big enough
quantity, the distributed "widget" could probably
get down to $1 manufacturing cost.) Let's say
that the system could sell for any number you
pick -- just so long as the total sales (street
price) amount to $1e7. Is there a ten million
dollar world market for a machine that does not
reduce room vibration, but that does remove it
from recordings, or sound reinforcement systems,
without removing any of the 'desired' material
from the recording at the same time?

-R

[archive]

From: steve <whitaker@p...>
Date: Tue Jan 9, 2001 6:42 pm
Subject: Re: Structure-born noise

Interesting analysis.
problem is, structure born noise travels *faster* than air born, so the
wavelength will be dramatically less than the 5m you refer to.

As for use/applications, broaden your vision a bit. In the 5000 price
range, large 'sound reinforcement' situations would use the device too.
there are plenty of those.
I remember working on a classical FM radio station where that would have
been useful. There was a parking lot nearby and the cars would cause a
structure born noise which would be picked up by the record stylus.
not so many record players in use today, but once a device is developed,
applications come out of the woodwork ;-)

I wouldn't be surprised to find the device being used to cancel the noise
inside jet airplanes. I haven't been around Boeing's acoustical lab since
around 1980, so they might have one, but if they develop it, it'll likely
remain in-house. (they'll probably publish a paper on how they did it tho).

Technologically, the 3-axis method wouldn't work as each material used has
an inherently different acoustic velocity, meaning that the device would
have to add the materials and distance of each material into the
calculation... ugly :-( not to mention the variability of each material.

Finally, are you suggesting this be used with a loudspeaker which would
cancel the sounds once they again become airborn, being radiated from the
various surfaces, or as a replacement for a good decoupling/isolation of
the microphone from its mounting?

Interesting idea :-)

Steve

>Train, subway, truck, etc. noise would be the primary
>candidate -- and while this would present some
>difficulties, I doubt that it would be insurmountable.
>With wavelengths (in air) on the order of 5 meters
>(and, in the structure, probably on the order of 10
>meters) a single structure-noise pickup would not
>suffice. But many pickups are practical. Just to
>put some numbers on it, let's assume each structure
>pickup consists of a 3-axis accelerometer. It would
>be easy enough to package such a widget, complete
>with A:D conversion (ten or twelve bits should be
>plenty over the frequency range of interest) for a
>manufacturing cost of about $10US (let's say a
>street price of $50). Such a widget could be
>easily daisy-chainable. In a small to moderate
>sized recording studio, three such widgets per
>surface would probably suffice to provide a mapping
>that would allow 20dB post-processing attenuation of
>structure-born vibration -- even given processing
>electronics, we're probably talking about a street-
>price on the order of $5000. It doesn't take too
>many 3 A.M. recording sessions at union overtime
>rates to pay off $5000.
>
>But, do enough studios exist that treat recording
>as a business to support the several million
>dollar development cost of such a system? (That
>development cost, by the way, is part of the
>difference between the $10 manufacturing cost and
>the $50 street price.) Putting numbers to it
>again, let's assume a "per-installation" street
>price of $5000, with $1000 going to pay for the
>development. With a development cost of $2e6,
>that would (using back of the envelope estimates)
>make the break-even point somewhere around 2000
>installations.
>
>Of course, there are plenty of ways of playing
>with the numbers, and of playing with the cost
>of the electronics. (Given a big enough
>quantity, the distributed "widget" could probably
>get down to $1 manufacturing cost.) Let's say
>that the system could sell for any number you
>pick -- just so long as the total sales (street
>price) amount to $1e7. Is there a ten million
>dollar world market for a machine that does not
>reduce room vibration, but that does remove it
>from recordings, or sound reinforcement systems,
>without removing any of the 'desired' material
>from the recording at the same time?
>
>-R
>
>
>For more info, unsubscribe, large file uploads, ect:
http://www.studiotips.com
>Send small drawing files to dan@s...
>To Unsubscribe: Send email to
>acoustics-unsubscribe@e????ups.com
>
>
>

[archive]

From: "Richard Plourde" <richardplourde@m...>
Date: Wed Jan 10, 2001 6:11 pm
Subject: Re: Structure-born noise

--- In acoustics@e????ups.com, steve <whitaker@p...> wrote:
>
> Interesting analysis.
> problem is, structure born noise travels *faster* than
> air born, so the wavelength will be dramatically less
> than the 5m you refer to.

It's so nice to see that I'm not the only person who has
terms slip from the numerator to the denominator.
wavelength = wave-velocity / frequency
A higher wave velocity -> longer wavelength.

We can generally expect wavelengths in structures to
exceed those in air by up to an order of magnitude.

> As for use/applications, broaden your vision a bit.
> In the 5000 price range, large 'sound reinforcement'
> situations would use the device too.

Yes, it could reduce the amplification of stage
vibration. But so much of sound reinforcement does
not require the microphone pickup of low frequencies.
For human voice, for example, there's no reason that
I can think of for not using a sharp high-pass filter
at 100Hz or even a bit higher.

Also, I doubt that this would be useful for mobile
installations -- both the necessity for installation
and the time required for the system to develop its
own heuristics would probably rule it out.

> . . . but once a device is developed,
> applications come out of the woodwork ;-)

That's true. From a business point of view, it's
probably good to have a clear map to profitability
with a fairly predictable market. (The dot-com's
seem to have forgotten that.) The unanticipated
markets then become "gravy." Also, it does seem
that every widget that does something triggers
somebody's skull into operation -- a new widget!
>
> I wouldn't be surprised to find the device being
> used to cancel the noise inside jet airplanes.

I would. Physical noise cancellation is very
very different from feed-forward cancellation.
I did some work on noise-cancelling headphones
where wavelength just wasn't much of a factor.
The problem was, when you took the headphones
off, the acoustic conditions changed so radically
that the headphones would oscillate -- in practice,
the cure was much worse than the disease. The
problem with active noise cancellation in a
structure is that if you have the ability to feed
energy into the structure at a higher rate than
that energy gets converted into heat by the
structure, uncontrolled oscillation can build
until some other function reestablishes a balance
between input power and power dissipation. That
other phenomenon might be structural failure.

Anecdote: About fifteen years ago, I worked on
a servo-project that involved a 200kg gantry
on a billiard-table sized table. The require-
ments included both precision and high slewing:
"move towards the specific location at 30cm/sec,
then position to within 1 micrometer." The
system did, in fact, slew at that rate, with
deceleration starting 1mm from the targetted
location -- it then slowed from 30cm/sec to the
1 micrometer requirement in that last mm without
overshoot (critically damped). That was a tough
job -- but we did it. At installation, the
system was turned on and, about five seconds
later, turned off. Damage was limited to some
cracked walls and a few broken windows (plus
the random coffee-cup that shook off a desk.)
The development took place on a concrete slab
basement floor, the installation itself was on
the third floor of a steel/concrete office
building. (I hate to think of what would have
happened if it'd been a brick building.)
Eigenmodes are not just mathematical concepts.

Oops.

I'd think long and hard before trying to do
active noise cancelling in the physical world.
And I'd get a *lot* of liability insurance.

I feel comfortable introducing a signal into
a recording chain, and maybe doing sound
reinforcement cancellation of stage vibrations
(where the intent is not so much to keep
people in the audience from hearing those
vibrations, but rather to keep them from
getting amplified by the sound reinforcement
system.)

I'd also be comfortable (although less so)
trying to make a table that self-cancels
structural vibrations, perhaps for optical
measurements or for semiconductor fabrication.
But I'd draw the line at trying to cancel
vibrations of a large structure -- cancelling
the pickup of the resulting sound (by a
microphone) would be about as far as I'd
want to go.

> Finally, are you suggesting this be used with a
> loudspeaker which would cancel the sounds once
> they again become airborn, being radiated from the
> various surfaces, or as a replacement for a good
> decoupling/isolation of the microphone from its
> mounting?

No. I doubt that getting rid of airborn sound
would even be possible except at discrete locations
following some learning process of how structural
vibrations map into sound pickup at those discrete
locations. This may be a fatal flaw in the whole
plan. At low frequencies, a change in microphone
location of a few cm would probably not totally
destroy the heuristics -- however, a change in
orientation of a directional microphone (one that
uses air velocity as a part of its metric) might
invalidate the heuristics.

In general, I question the use of directional
microphones at low frequencies. (The behavior
changes too radically with frequency for wave
propagation that can't be treated as planar.) But,
the fact remains -- directional microphones *do*
get used at low frequencies.

The more I think of it, the more it seems that this
is an idea better set aside.

Thanks for those who participated in this discussion.
A conversation is a *lot* cheaper than finding
investors and setting up a company.

-R

[archive]

From: steve <whitaker@p...>
Date: Wed Jan 10, 2001 7:11 pm
Subject: Re: Structure-born noise

Thanks for the conversation. I like the challenge of thinking in new
directions

>> Interesting analysis.
>> problem is, structure born noise travels *faster* than
>> air born, so the wavelength will be dramatically less
>> than the 5m you refer to.
>
>It's so nice to see that I'm not the only person who has
>terms slip from the numerator to the denominator.
> wavelength = wave-velocity / frequency
>A higher wave velocity -> longer wavelength.

ooops <blush>

oh well, it happens.
better in a mail list than in the lab :)

steve

[archive]

From: Stephen Foster <oncourse@i...>
Date: Wed Jan 10, 2001 8:25 pm
Subject: Re: [acoustics] Re: Structure-born noise

>Yes, it could reduce the amplification of stage
>vibration. But so much of sound reinforcement does
>not require the microphone pickup of low frequencies.
>For human voice, for example, there's no reason that
>I can think of for not using a sharp high-pass filter
>at 100Hz or even a bit higher.

Gosh... hate to disagree, but as a vocalist and engineer, I know that a
great deal of my vocal sound is generated below 100 cycles. Ever been to a
Pink Floyd, or Zappa, or U2, or Eagles concert? These groups are famous for
their great sound systems and live mixes. They're running masses of 18s to
develop and separate the sound at low freqs. I watched a Pink Floyd
engineer send hundreds of people to the public restrooms running 130 DB of
12 cycles during sound check.

I can understand not using very low freqs, like maybe 50 cycles and under,
for live sound, but that's only if you have either an inadequate sound
system or a scared engineer (or both). I can guarantee you that if you do
not get the bass drum and bass separated in the 30-60 cycle ranges, you can
only produce a limited mix, and the more instruments you have going the
worse the problems will be. This comes from 30 years of dealing with
exactly this same problem.

Harmonic resonance is not just a textbook problem. It is the single most
important criteria in layered mixing. Confusion at 30 cycles produces
confusion at 120 cycles, 480 cycles, etc, right on up the chain. You can't
fix a midrange problem with midrange EQ if the problem is being caused in
the low end. If I caught a live engineer high-passing my vocals (or the
bass, kick, or guitars) like that I'd replace him if he insisted on
continuing the practice.

As a matter of fact, the best engineers actually cut a hole in the drum EQ
at exactly 100 cycles to fit the vocal into. They say that's where the
meat of the vocal is, as does the chart in my Columbia School Of
Broadcasting textbook.

That's my story.... ($.02, as Scott puts it)

sf

Stephen Foster
MillKids/Howler Studios/MFoV Info
http://www.idnmusic.com/howler
WhiteHorse Records
IDNMusic.com
http://www.idnmusic.com
all music. all indie.

[archive]

From: Lou D <MR_LOU_D@y...>
Date: Thu Jan 11, 2001 4:26 am
Subject: Re: Structure-born noise

Might be a good candidate for a U.S. DARPA or EPA contract.
At least, for seed funding.
Just a thought...

-Lou

Subject: Re: Structure-born noise---
In acoustics@e????ups.com, SRF7@a... wrote:
> In a message dated 1/8/01 11:17:59 AM Eastern Standard Time,
> richardplourde@m... writes:>
> >I was puzzling over something very different from air-
> born 'noise' -- the difficulties of eliminating
> structure-born noise. It occurs to me that it might
> be possible to sample structure-born noise (for
> example, the vibration from a passing train), "tweak"
> the sample for frequency-dependent phase and gain,
> and use it to electronically cancel in-room microphone
> pickup of that noise. > > Train noise would be a difficult first candidate
Train, subway, truck, etc. noise would be the primary
candidate -- and while this would present some
difficulties, I doubt that it would be insurmountable.
With wavelengths (in air) on the order of 5 meters
(and, in the structure, probably on the order of 10
meters) a single structure-noise pickup would not
suffice. But many pickups are practical. Just to
put some numbers on it, let's assume each structure
pickup consists of a 3-axis accelerometer. It would
be easy enough to package such a widget, complete
with A:D conversion (ten or twelve bits should be
plenty over the frequency range of interest) for a
manufacturing cost of about $10US (let's say a
street price of $50). Such a widget could be
easily daisy-chainable. In a small to moderate
sized recording studio, three such widgets per
surface would probably suffice to provide a mapping
that would allow 20dB post-processing attenuation of
structure-born vibration -- even given processing
electronics, we're probably talking about a street-
price on the order of $5000. It doesn't take too
many 3 A.M. recording sessions at union overtime rates to pay off $5000.
But, do enough studios exist that treat recording
as a business to support the several million
dollar development cost of such a system? (That
development cost, by the way, is part of the
difference between the $10 manufacturing cost and
the $50 street price.) Putting numbers to it
again, let's assume a "per-installation" street
price of $5000, with $1000 going to pay for the
development. With a development cost of $2e6,
that would (using back of the envelope estimates)
make the break-even point somewhere around 2000installations.
Of course, there are plenty of ways of playing
with the numbers, and of playing with the cost
of the electronics. (Given a big enough
quantity, the distributed "widget" could probably
get down to $1 manufacturing cost.) Let's say
that the system could sell for any number you
pick -- just so long as the total sales (street
price) amount to $1e7. Is there a ten million
dollar world market for a machine that does not
reduce room vibration, but that does remove it
from recordings, or sound reinforcement systems,
without removing any of the 'desired' material
from the recording at the same time?-R

[archive]

From: frobass@a...
Date: Thu Jan 11, 2001 9:59 am
Subject: Re: [acoustics] Re: Structure-born noise

If I might jump in here. Your idea is, obviously a good one, as, the military
and several high-end sports car manufacturers are already using this
technique. It is also used in the cockpits of many fighter jets, as a noise
abatement solution. Further, I just recently spoke with an optical cable (for
high speed data transmission) developer, and they are using similar
techniques in order to increase light wave length speeds. Granted, light is a
different animal, however, the wave property concepts are somewhat similar.
Peace.

[archive]

From: frobass@a...
Date: Thu Jan 11, 2001 10:18 am
Subject: Re: [acoustics] Re: Structure-born noise

My 2 cents. Most 'studio' engineers I know (and I dare not mention them
before asking their permission, five of these fellows currently account for
about 20 of the Billboard all time Top 100 greatest hits), consistently use
such a rolloff. Live sound reinforcement, however, is a different animal,
which I really can't comment on. I am not that experienced in live sound.
As a reminder for those acousticians out there, and for the purposes of
discussion, the order of harmonics goes something like this: fundamental
frequency, octave harmonic (aka 'first order harmonic', also present at
one-half the volume of the fundamental), a harmonic of a fifth (one-half of
the previous volume), and then the rest is left to the texts, however the
next most present (non fundamental multiple octave harmonics), are the major
third and the flat seven (b7). So, although you are correct in discussing the
first order multiples, my question, especially for live sound, is: given the
amount of power used in live applications, what effect do the lower
frequencies upper harmonics have on the over all sound and distortion? Am I
wrong in assuming that in a live situation, these would be more troublesome
than in a studio situation? Just a question. Peace.

[archive]

From: frobass@a...
Date: Thu Jan 11, 2001 10:31 am
Subject: Re: [acoustics] Re: Structure-born noise

Density, interesting component. I forget about it all the time (maybe because
I am so........dense! LOL). I just wanted to concur and then add something. I
have been following the thread on overhead sound transmission (through a
ceiling which is a floor). I am not that experienced as a studio builder
(hence, the reason I have joined the list, as I am about to do it again). I
have only built four rooms and have never been totally happy, even though
asthetically, they were all beautiful. However, in one of the quietest rooms,
(in fact, the only problem with that room was it was too dead) a local fellow
who had built some thirty rooms in Nashville, recommended we try .060
rubberoid on a floor which was built upon a cement block structure, and
hollow underneath. We had A LOT of low frequency resonance from the cavity
below, and when we added the rubberoid, it was amazing!!! I believe we used a
meter and calculated that the frequencies below 250hz were attenuated some
12db or so. Could this rubberoid material be placed on the floor above? It
definately cuts down on the vibrations in a more dense environ! Just a
thought. Peace.

[archive]

From: SRF7@a...
Date: Thu Jan 11, 2001 4:31 pm
Subject: Re: [acoustics] Re: Structure-born noise

In a message dated 1/11/01 2:07:36 AM Pacific Standard Time, frobass@a...
writes:

> If I might jump in here. Your idea is, obviously a good one, as, the
> military
> and several high-end sports car manufacturers are already using this
> technique. It is also used in the cockpits of many fighter jets, as a noise
> abatement solution.

It is a good idea ... and it is being used in other circumstances as a means
of noise reduction. My point however is that it is, as they say, a
nontrivial feat.

When I examine the typical uses of the tech, I notice that it is generally
used as a means of last resort to damp a specific known source (the engine
the designers put in the jet) at and for a specific site (the work position
in front of a machine where the operator must stand for 8 hours during his
shift). In these circumstances the device can be "tuned" (both in freq. and
in time alignment) and "aimed" (create a zone of cancelation around the
pilot's head for example).

For general studio use against a random strucutre transmitted noise source
you'd have a number of moving targets and you'd need to hit them all dead on
at the same time ... and then do it again ... say 100 times a second. Noise
source attributes change radically in the example cited (different trains,
loaded differently, with different locomotives, different freight cars (the
one with the bad wheel), different horns, different drivers (the guy who
likes to do two short horn blasts then one long one), passing at different
speeds, in different stages of acceleration (brakes on, or
pedal-to-the-metal) and in the midst of all this the device must sense the
first increasing, then diminishing excitation of different modes in the
different structure components (is that the drywall ceiling rumbling, or the
concrete floor) and the varying extent to which these structures will convey
this noise to the studio air volume, and the different rates such transmitted
rumble will decay in the air volume (the studio's itself has modes) and the
different degree to which a certain mic at a certain position pointed in a
certain direction would pick up this noise and thereby pollute the pollute
the source signal.

Yep ... non-trivial ... and given that good old isolation (suspension mounted
microphones) and hi-pass filtering (not to mention floating floors on
neoprene and extra wall layers on resilient channel) are pretty effective and
reasonably priced, the project has some steep challenges as a money making
proposition. That said, I have trains and a studio .... bring your project
over and I'll buy the beer (Hell, I'll even help you write the grant
proposal).

Scott R. Foster

[archive]

From: SRF7@a...
Date: Thu Jan 11, 2001 5:46 pm
Subject: Re: [acoustics] Re: Structure-born noise

In a message dated 1/11/01 2:19:18 AM Pacific Standard Time, frobass@a...
writes:

> my question, especially for live sound, is: given the
> amount of power used in live applications, what effect do the lower
> frequencies upper harmonics have on the over all sound and distortion? Am I
> wrong in assuming that in a live situation, these would be more troublesome
> than in a studio situation?

Well, sort of ... you are correct that low freq. harmonics are a problem in
SR. But same's true in a studio.

The way I see it, you just have two different animals as you say. One is real
time the other is anything but. In the studio I use eq first (band-pass
filtering where I can, parametric shaping / notch filtering as needed), then
gating. If I were good enough ... all this would then be fixed by the
world's greatest mastering engineers ... who'd do even more parametric
notching, compression, etc. SR teams have only one shot so they do a "run
what you brung", "tweak as you go" sort of thing, sound checks are great, but
rooms change when you fill them with people (it takes a cool head to stay on
top of it all).

My view is that a great SR man (as to setup and component quality anyway) can
be quickly noticed by listening to the low end of his system (no ... not that
system ... get your mind out of the gutter Bevis!). There is nothing quite
so exhilerating to the masses (sound wise anyway) than the raw energy of a
solid low end that thumps the chest. Making this happen without getting
things muddy is an art. As Stephen points out notch filtering is a big part
of the answer, but 8 jillion watts pushing a convoy of chain drive elephant
horn LF drivers helps also ... and as to your question, that's where studio
work and SR start to vary.

When you have that much power in the LF you eq differently than one does when
mixing ... you are tuning the system to a room to shake a crowd in real time
using lots of power .. very different than doing a mix down for playback in
any system. The issue of controlled power doesn't come up in the studio in
the same way. Power gives one the means to avoid/cure (and even create) some
of the problems of physics (and to some extent ... in the hands of a master
... even some of the shortfalls coming from a lack of talent ... erase the
jazz noise that the rhythm guitar is making) but not the way a studio does.
Power equals head room, and head room means low distortion. It takes a ton
of power to make a clean low end ... the idea is to get the best effect given
the room (or lack thereof), the system, the crowd ... all in one shot, very
specific. Same thing as studio mixing ... 'cept different.

Scott R. Foster