God of Bass postings (Tom Danley)
From: Thomas Danley
Subject: Re: We need more power, Capn!
> From: Mike Ford
> At 7:16 PM -0800 2/6/98, Cathleen & John Halliburton wrote:
> > Ten BassTech 7 cabinets running in the warehouse, or twelve Contrabasses,
> > or a sonic boom
> Makes me ask a couple of questions,
> What is the largest array of basstech 7 cabinets ever fired up and tested?
> Running the two sets of 6 BT7s as described in the U2 concert at full
> power, how far away would I have to be to get a good blend with my Quad ESL
> 63s (average max spl around 98 dB)?
I don't recall the largest number we tested, probably about 12 or 16,
there begins to be a point that doing this indoors even in a wharehouse
becomes dangerous, remember that 132 dB is about 2 pounds of pressure
per square foot and hearing decent bass on the out side of a cinder
block and brick wall means its moving and there not supposed to.
Inside, you can actually get to the point where you have "enough bass".
12 BT-7's can produce 2400 acoustic watts from 28 to 125 HZ, steady
state which is pretty loud. 12 units, close coupled also is large enough
to have some directivity but disregarding that increase in on axis SPL,
2400 acoustic watts from a point source would be about 126.8 dB at 10
meters, at 80 meters about 108 dB and at 160 meters, 102 dB and at 320
meters would have fallen to 96.8 dB. By 5620 meters (3 1/2 miles) the
sound would have fallen to a modest 72.8 dB. I guess thats why the World
music theater got "Bass" complaints from more than 5 miles away when U-2
A fellow named Gene Patronis, an Acoustic consultant for the gov't
spec'd in 12 contras (with a bunch of other high range stuff) in a
"battle field simulator", a special room at Aberdeen proving grounds
that had 36" thick concrete walls and could reproduce the sound of a 105
mm Howitzer at some distance. The sound system in this room could induce
"shell shock". Gene (a fellow who also like loud noises) said it was
very realistic, although I have no idea what actual levels it got to.
I think the loudest audible range sound we ever made was a compressed
air siren in the 300-600 HZ range (John do you remember that beast ugh).
It measured 155 dB at 6 feet and drained John's Big air compressor tank
in a few seconds, it also broke the welded seams in the steel horn it
was attached to. That was so loud at 20 feet that even with ear plugs
AND muffs with your hands clamped on them, it was still too loud, it
even made the bones in your face hurt. It was designed to shake the dust
out of cloth bag air filters at power plants and fortunatly we only had
to build and test one. Nasty Nasty Nasty.
Ey, the dylithium crystals canna take any more Cap'n
From: Thomas Danley
Cc: Erik Olson
Subject: Re: BassTech 7 and SPL ratings
> Subject: BassTech 7 and SPL ratings
> Date: Fri, 20 Feb 1998 13:20:16 -0800 (PST)
> From: Erik Olson
> Check out this brouchure on the ContraBass Page:
> It is for the folded horn BassTech 7.
> They claim a sensitivity of:
> 1 unit (1W, 1m) 107.5 dB
> 4 units (1W, 1m) 110.0 dB
> SPL at full rated power: (28Hz - 125Hz)
> 1 unit 134 dB
> 4 units 141.5 dB
> Low Frequency Corner of: (-3dB)
> 1 unit 30 Hz
> 4 units 28 Hz
One of the problems you run into measuring large speakers is that the
size of the boxes and the area of the source effects the measurments.
The spec sheet above was a preliminary one and had aparently type'o in
it although the response / phase / distortion measurments are ok.
To resolve the size issue we hired an independant consultant called
"Summit Labs" to set the units up (out in the desert in California) and
measure the units at 20 meters, a distance which reflects the true
acoustic power of the units and eliminates the size effect.
For the sensitivity measurments, the units were driven at 1/4 rated
power and then levels calculated back to 1 watt, 1 meter.
The sensitivity and efficiencys they reported are as follows.
1 BT-7 = 98 dB 1w/1m 3.8% efficiency
2 BT-7 = 104 dB 1w/1m 15.1% efficiency
4 BT-7 = 107 dB 1w/1m 30.2% efficiency
6 BT-7 = 109 dB 1w/1m 47.9% efficiency
I think ServoDrive is in the process of getting a web site
at www.servodrive.com, they can be reached at 847-724-5500.
From: Thomas Danley
Subject: Re: Intersonic BT-7 - Indestructible?
> From: Douglas Purl
> On Tue, 24 Feb 1998, Nicholas McKinney wrote:
> > >Evidently, the poster's suggesting that the Intersonic BT-7 is easy to
> > >destroy. Wasn't this one of those systems based around the almost
> > >indestructable Contrabass-type system?
> > Hello Brian,
> > The people owning the Basstechs over here just had me put in regular sub
> > drivers because they kept blowing the motors. They could not get used to
> > the low distortion and just kept adding power until they exploded the
> > brushes and center shafts. Supposedly they have replaced these motors befor
> > I think that it is operator ignorance that kills these things. Turn it up
> > until it distorts, then back down a little kinda mentality.
> I had a long discussion a year or two ago with a corporation exec who had
> dealt with the problems of warranting destroyed company products. (Voice
> coils not uncommonly return to the factory carbonized.) There is much
> experience in the company in engineering, operating, etc., professional
> sound systems. Their conclusion is that many fellows operating gear have
> no more engineering understanding of the equipment they are operating than
> the average driver has of the operation of the engine and running gear of
> his automobile. Such operators burn up scads of equipment, even nominally
> bullet-proof gear.
> No doubt Tom will respond on this, but surely Nicholas' caution is a good
> one. Many operators judge speaker overload by the predominance of
> harmonic distortion. The bass power of the CB systems misleads them into
> overdriving them.
> Doug Purl
A person that says they have made something idiot proof or bullit proof
has not met some of the idiots I have run across in the pro sound biz.
Some are very sharp but some are well lets just say, not.
When we got in the pro speaker biz, we were using a overcomming the
skepticism of a new aproach which many didn't think could work and
overcomming the lack of a track record. The BT-7 was the last horn
system we designed and it was also the best. As John H. mentioned at
Brians web site, the system far surpassed the AES measurment criteria,
infact would technically pass even at 2X rated power but to be
conservative we kept the rating at 400 Watts. At that level, there "life
span" at full rated power from 28 to 90 HZ was about 96 - 108 hours, a
number of times the life of big name drivers in the same frequency
On the other hand there are a small number of people who judge how hard
there driving speakers by the level of distortion and /or consider the
duty cycle of the red lights on there power amp as indicators of getting
there monies worth. The bass tech 7 can be damaged by either type of
person, frequent clipping and / or substancial over powering will damage
the driver, rather than instant failure, this damage accumulates until
parts fail (in this order acording to repair record) 1st drive belts,
cone assemblies and then motor repair.
In some cases, the large touring sound companies can aparently afford a
substancial driver mortality. On Micheal Jacksons thriller tour, 16
ServoDrives were added to improve the bass in a Giant sound system where
24 dual 18" vented boxes were the subwoofer system. The cone system was
loosing 10 to 12 drivers PER SHOW. Switching to ServoDrives gave them
more and deeper bass in less truck space and they went 51 shows before
service was needed. The sound company was pleased with both performance
and relaibility and has since used more of them for other artists. In
there case getting the most sound in the smallest space was there main
concern and reduced service was a plus.
Many servodrives (about 500) are hidden in various places at Disney
theme parks & Epcott, in there case, the reliability in difficult to
service locations was there concern, getting better bass was a plus.
In short, the Bass Tech 7 got the reputation for producing the loudest,
cleanest bass you could get in a given space and was proved reliable if
driven within reason.
From: Thomas Danley
Subject: Re: driver measurements and jerks
> Oliver Friedman wrote:
> > Limit the input bandwidth!
> Since our directional hearing sense indicates a differential time
> perception of 10 us or better, bandwidth less than 100KHz results in
> time distortion.
Differential is as in a 10us difference BETWEEN your ears not that you
can hear a delay of 10us presented to both ears.
> Further, most of the time distortion occurs in
> speakers, where it grows to 90-180 degrees at resonance. At 40 Hz, this
> is 12-25 milliseconds. Since harmonics typically shift phase with
> distance from the source, this doesn't sound at all bad; but moving cone
> drivers have a universal delay distortion mechanism as follows:
In the 15 1/2 years I have been measuring drivers with a TEF machine I
have never seen a driver alone that had more than -80 degrees of phase
shift, most are more like + - 30 to 60 deg. Point sources (if they have
a flat response) are constant acceleration devices.
The acceleration force is NOT the voltage input to the system but rather
the current and so the acoustic phase mirrors the phase of the current.
The moving mass of the system appears as capacitive reactance and hence
the 1 pole filter (of the velocity) is realised that is needed to offset
the changing radiation resistance vs frequency. In theory this would
yield a 90 deg phase shift but it is never that much in practice.
What makes real world things more complicated is the effect of
additional reactances in the system, for example in a sealed box the
driver has a leading phase below resonance, at resonance it is at zero
degrees and above resonance it is lagging (capacitive reactance). As the
frequency is increased, one reaches a point where the series inductance
and the motional reactance form a conjugate and the phase is again zero
degrees (this is the R-min point in the impedance curve). Above this
frequency the phase leads (inductive reactance). The actual acoustic
phase is typically no more than 40-60 deg either way due to the additional
effects and as you can see is at times leading, lagging and resistive
depending where one is in frequency.
> You also say "Since harmonics typically shift phase with
> distance from the source" What mechanism acounts for this? Sound does not
> chan ge velocity with frequency, why doesn't this show up when you measure
> (1) The ratio of sound pressure level to cone displacement rises with
> the ratio of cone area to wavelength at +6db/octave.
What are you getting at here?
> (2) The moving mass of the cone system and the combined compliance of
> the speaker suspension and air spring formed by the box volume comprise
> a first-order resonator. The amplitude response of this resonator to
> applied force is -6db/octave above and below the resonance. This
> combines with the "air transduction curve" in (1) to yield flat response
> above the resonance and -12db/octave below the resonance.
The slope on the radiation resistance DOES not impose a phase shift, it
is simply a changing resistance. The 6dB per octave slope on the
driver's velocity is as explained above, at most a -90 degree shift.
> (3) At resonance, steady state response approaches 180 degrees out of
> phase with the driving force.
No, at resonance the reactive components are equal but opposite, both
the electrical load and acoustic output are at zero degrees as explained
> This means that during amplitude buildup
> at the start of a note, the output begins with zero phase shift and
> gradually lags behind, making the speaker output substantially off-key
> until equilibrium is reached. At the end of a staccato note, the
> speaker will continue to output at the resonant frequency for some > time,
> which is also off-key.
Any input signal you can think of can be broken it its equivolent
frequency components, when fed into a driver those frequency components
are reproduced at an amplitude and phase acording to the drivers
amplitude and phase response at those frequencies.
In other words, for example a simple step function, the instantanious
change in input signal when reproduced is limited in rise time by the
drivers high frequency response and the low frequency response governs
how long (along with x-max) it can remain a straight line.
How well it can reproduce the wave shape is both a function of its
amplitude and phase response.
> Because the drive is 180 degrees out of phase with the response,
> feedback systems (such as the Velodyne) will become chaotic at
Instability results when the feed back signal is significantly past 90
degrees before the gain is zero.
> The only musically acceptable commercial solutions to this problem are
> to either attenuate bass transient information by bass rolloff and/or
> compression or move the speaker resonant frequency below recorded bass
> transient information.
> During recording, the microphone bass rolloff switches are mostly on,
> and direct bass guitar is always compressed. Both of these techniques
> result in recorded music with limited bass transient information, which
> doesn't sound too objectionable on commercial speakers.
> Greiner and Eggers (J. Audio Eng. Soc. 37(4)) determined that commercial
> CD recordings all have substantial bass rolloff with the exception of
> classical organ music. The organ is a high-Q instrument that produces
> no bass transient information. The worst rolloffs were on jazz
> recordings, presumably because full frequency pizzicato contrabass
> sounds awful on dynamic monitors.
Perhaps your not familliar with some of the other research on CD's with
significant low frequency components. Dolby labs has looked at this and
also one of the car stereo labs found large components on cds.
I have played the Ripington's Killamonjaro into my Hyperception system
and found several songs have significant bass at 25 HZ. Do an FFT on a
bass drum and you will see the peak at 10HZ or below.
Perhaps such signals do sound bad on monitors, does that means the music
sounds bad or the speakers sound bad? Have you ever heard this kind of
music played through a really good, powerfull subwoofer that has a very
From: Thomas Danley
Subject: Re: Reply to Summary of bass transients
>Larry de Martin wrote
> > David E. Hyre, PhD wrote:
>> One has only to look at the equations for inductors and capacitors to see
>> what Noah means: V = L dI/dT for inductors, and I = C dV/dT for caps, where
>> dI/dT is the rate of change in the current and dV/dT is the rate of change in
> > the voltage.
> If speakers behaved as differentiators, the distortion I describe would
> not exist. The problem is that speakers are double integrators -
> displacement is the second integral of current.
> Because NO RESPONSE CAN OCCUR PRIOR TO ITS STIMULUS, speaker cone
> displacement cannot have a leading phase with respect to drive in the
> time domain.
Recall that for a point source, it is acceleration which correlates with
acoustic pressure not displacement.
What happens when you present a woofer with a step function?
The input voltage instantly changes and at first nothing happens because
the voice coil inductance looks like a very high resistance. After a short
time, current starts to flow and the coil produces a force and hence
acceleration proportional to the current flowing. In the time region
(frequency) where the inductance is effecting the results, a crossover
normally shifts these time/frequency components to a higher frequency
driver with less inductance.
When measuring the "Energy vs time" (ETC) of a speaker, the inductance
shows up as part of the acoustical delay between the mic and radiator.
Once the inductance is "out of the way", the Rdc of the coil and the
moving mass (which looks like a capacitor) form an RC filter which sets
the 1st order slope needed for an acceleration response.
As the coil starts to move, it produces a back emf voltage in phase with
the acceleration, which shows up as the voltage across the capacitor.
This voltage is at the other end of the Rdc and reduces the voltage
across the Rdc and hence reduces the current and acceleration .
> A simplification made so early in the application of this
> math that it has been lost, is that 90 degrees of lead is actually 270,
> 630 or 990 degrees of lag, and this is the physical meaning of "leading
If you eliminate the time delays, one is limited to the phase response
predicted by the equivolent electrical circuit, IE: <180 deg.
> The back EMF on a particular cycle of speaker displacement is the result
> of energy stored during previous cycles (>180 degrees in the past).
NO, back emf is proportional to coil velocity at that instant, allways.
> On the first cycle, there is no energy stored and time domain measurement
> will clearly show lagging phase to all stimulus, whether it is in the
> "leading phase" compliance limited region or mass-limited "lagging"
> region of the response curve.
Remember that a rapid change in level is the same as a much higher
frequency and unless the driver has sufficient high frequency response,
cannot reproduce all of it.
> After the last cycle, the speaker should exhibit no response, but it
> always does.
> If Qms > 1, there is a measurable and audible portion of loudspeaker
> response which is left over from more than 360 degrees at resonance
> frequencies, even if Qtc < .5. This creates bad-sounding distortion
> which has been co-opted by removing the sharp edges from envelopes in
> virtually all recordings.
Again, as with starting up, the drivers ability to "stop on a dime" is
governed by its high frequency response, dictated by its coil inductance
In addition, the acoustic waveshape produced is defined by the drivers
amplitude and phase response to all of the significant harmonic
components of the complex signal.
> Any measurement system which results in leading phase is therefore
> making the assumption that the "note" is continuous, that is, that it
> has no start and stop. If your instruments counted cycles from the
> beginning (or end, since everybody quotes negative time mathematics) of
> stimulus and response, it would be clear that this is the case.
> In fact, the distortion of which I write is only present at the
> beginning and end of a bass note near a speaker system pole/zero pair.
> The information regarding the beginning and end of sine wave stimuli is
> typically thrown out to arrive at a phase measurement.
> > I think the term "minimum" comes from the fact that values derived this way
> > are limited to a range of 360 degrees due to the cyclical (i.e. 0-360 degree)
> > nature of the sine wave components.
> "Miniumum phase" means that the phase response is the minimum to account
> for the frequency response - which means, as you stated, that if the
> Hilbert transform matches the frequency response, then the system is
> minimum phase. This can be over 360. Note that the reverse transform
> is in a mathematical sense. Accuracy of measurement is an issue in
> applying this math, there is no physical reality to "perfect
One the other hand, one can use a TDS system, like the TEF machine which
actually measures energy-vs-time with no extra asumptions and use that
figure to set the TDS delay and one gets real phase information.
I think I'm seeing what you mean but minimum phase or no, a driver
CANNOT reproduce an acoustic replica of a complex input signal if it
does not have flat response and zero degrees phase (or 180 deg if you
flip the polarity) over a wide enough bw to include all of the
significant harmonics of the signal, which none have (driven the
I took a sine burst (4 cycles on, 4 off) and fed it inot a hilbert transform
(in Hyperception) and looked at what comes out, it is NOT the same.
If your looking for a replica of your input signal, it won't happen
unless the criteria is met.
> I am not listening to or measuring square waves. I am describing a sine
> wave burst as a square wave modulation at 1/4 or 1/8 fundamental to
> indicate that the transition from no signal to sine wave and from sine
> wave to no signal is nearly intantaneous (less than 1 microsecond in my
> apparatus), the corner is extremely sharp.
Less than 1 ms read as frequency components over 1 kHz.
> Since you claim competancy with Fourier analysis, could you calculate
> the spectrum of my test signal? I think you will find frequencies at
> other than integral multiples of the sine wave fundamental, yet it is
> heard on-key.
I measured it, it is far more than a sine wave in harmonic content.
> > Anyone can test this with a digital synthesizer,
> > CoolEdit, etc.
> Than why hasn't anyone on the list repeated my experiment? Everyone I
> have tried this on agreed to the gross audibility of "group delay
> distortion" or whatever is the most appropriate term.
> Here is the procedure:
> 1. Find the impedance peak(s) of the driver/enclosure system
> 2. Generate a sine wave of this frequency.
> 3. Gate this sine wave so that the output of the gate is zero for four
> cycles and 2.83V*sin(wt) for four cycles alternately. Frequencies above
> 80HZ are clearer with eight cycles.
I did it.
> I used an LM393 to detect zero crossing, a CD4024 as a cycle counter and
> the other half of the LM393 as an open collector AC mute. I can supply
> schematics in hardcopy or print files to those interested. (My CAD
> software won't run under Windows)
> 4. Listen to speakers and compare to headphones.
> > a delay in acquisition, due to distance between the microphone and
> > speaker, will appear as a time-independent constant phase shift plus an
> > additional frequency-dependent, linear phase shift.
Remember the coil inductance also acts as a delay (and rolloff and phase
slopes) added to the acoustical distance.
> My microphone is less than 2.5cm from the cone and I am measuring 500Hz
> maximum. The "C" weighting has a low zero which shifts phase, but I
> alway measuring relative phase shifts at one frequency and looking at
> waveform distortions. Further, my measurement system is just an attempt
> to account for what I hear and don't hear coming from speaker
> reproductions of my string motion. If the measurement agrees with my
> ears, then it is a valid test tool. If it agrees with the physics, then
> it is a valid design tool.
While "C" weighting is the closest to your ears response, It would be a
mistake to use anything other than a "flat" microphone weighting as the
phase shifts in the filter will alter the waveshape you see on the scope.
Also, without a crossover, you are presenting an unrealistic test signal
(so far as a hifi speaker test goes) as your rapid changes in amplitude
represent frequencies well above where the woofer is normally used.
> This has all been an attempt to communicate an audible distortion that
> is not measured or discussed accurately by any literature of which I am
> aware. I have played my test for others who unanimously support my
> subjective observations and I have found other speaker builders who have
> observed and dealt with this distortion.
> Before you argue that it can't exist, please try the experiment. It
> takes less time than composing another post re-iterating the perfect
> applicability of Fourier amd Thiele-Small analysis to musical
Regardless of the point of view, Larry's testing and listening does
bring up that old question about waveshape, can you hear it?.
John H. was over this afternoon and I showed him what a square, triangle
and sine burst look like when passed through a hilbert transform (broad
band -90 deg phase shift), none have much resemblence to the input
signal but all have the original number and level of harmonics. The
question remains " can you hear the difference?".
I suspect some people can, consider that horns and electrostats both
have a nominally zero degree acoustic phase and both have small legions
of devoted fans, on the other hand the majority of people live with the phase
shift. I think at the BBQ we will do a listening test, the Hyperception system
and dsp board has 4 in 4 out at 16 bits up to 100KHZ sampling rate and a
hilbert transform is one of the blocks available. I'll drag the setup
outside and we'll run music through it and see what it sounds like.
Also, if I have time, I have an old switchmode amplifier prototype which
ought to still work (if I can find the controler chip around here), it
had "extra" circuitry which forced a woofer to have a zero degree
acoustic phase. It sounded great driving the sub under electrostatic
speakers and the full range rotary line sources are also about zero
degrees above about 250 HZ.
From: Thomas Danley
Subject: Re: Human Perception of Sound & BBQ notice
> TAN BAC NGUYEN wrote:
> > In response to your question about people having similar experiences, I
> > occasionally get dizzy and severe headaches while listening to my system
> > for a while at high volumes, but it's a combination of the volume and the
> > frequency that cause the illness. I've got two Bass Shakers and I don't
> > get sick from those, but my SPL levels are in the 140-142 range, and it
> > almost seems dangerously unhealthy sometimes.
First I would ask how did you measure 142 dB, this is significant sound
level. Given a very efficient speaker with no power compression that was
96 dB 1/w it, would take 40,000 watts to get 142 dB (if your willing to
sit a meter away).
There certinly are limits to what is safe to endure but also Low
frequency sound has its own significant amount of "lore".
To save time and finger tips I am re-posting a post from 9/12/97 on the
effects of low frequency sound.
Ever since the first Trade Show we went to showing the Servo-Drive
woofers I have been told many storys about what low frequencies do to
you. Stories of uncontroled bowel movements (at 7 HZ), genital
stimulation, instant nausea and other similar stories come to mind, some
aparently spread by the compatition of the day.
In my work developing transducers at Intersonics Inc, I have built
devices and systems to produce high sound levels at both ends of the
spectrum. One sonic boom simulator supplied to BBN could produce 163 dB
at 5 HZ (in a concrete bunker) and could remove windows and doors from
there frames and fixtures.
Another sonic boom simulator (3 way system called "speakers from hell"
on "beyond 2000" TV show) had the displacement of an 8 foot by 12 foot piston
moving 18" peak to peak, the system could produce >132dB on the outside wall
of an old house from 3HZ to 5KHZ.
This is about 2 lb presure acoustic presure per sq ft and with a sine
wave at 3 HZ, made the middle of the wall move 8-10 inches in and out
and "house parts" frequently would fall off (I thought we broke the
house at one point when there was a loud crash from the crawl space so
we stopped "teasing" the house with the oscilator).
In testing, I intentionally experienced many slow TEF sweeps from 28
down to 3 HZ at full intensity (cause I'm a bass sicko) with never the
need to change underware. Certinly there was a strong feeling of relife
when it stopped but it had no other Ill effect. BTW at 3 HZ the
threshold of aubibility is about 125 dB so at 132 dB 3 HZ IS audible! I
felt fairly safe exposing myself to this as I had found a very good book
on the subject called "Infrasound and Low Frequency Vibration" edited by
W. Tempest, published by Academic press.
It is a summary of the research done largely to find out what the limits
of "the acoustic right stuff" was. It turns out that the ears
sensitivity curve roughly tracked its ease of hearing damage and that
for low frequencies one could safely experience very loud low frequency
sound. For example, The 8 minute recomended maximum exposure limit is 140 dB
from 12-20 HZ, 145 dB 11-8 HZ, 150 dB 7-1 HZ.
As your body is acoustically an elastic bag filled mostly with water, very
little airborn sound energy is transmitted to your body (*as is the case for
electronic gear also) but directly coupled mechanical vibration is another
matter. Page 212 has a nice list of Biodynamic resonances and to answer the
question everyone is asking, Bowel/Bladder presure is listed from 10-27
HZ in a standing subject with up/down vibration applied to its feet.
In short there are many many resonances for different functions/parts of
the body depending on where the vibration is applied and its axis of
Anyway for those interested in scientific look at "Bass", vibration and
its actual effect on the Body check out this book.
From: Erik Olson
Subject: Re: Human Perception of Sound
> From: Thomas Danley
> Another sonic boom simulator (3 way system called "speakers from hell"
> on "beyond 2000" TV show) had the displacement of an 8 foot by 12 foot
> piston moving 18" peak to peak, the system could produce >132dB on
> the outside wall of an old house from 3HZ to 5KHZ.
8' x 12' = the same surface area at a 132" cone.
18" peak to peak is an Xmax of 229mm.
Goto my piston excursion calculator at:
and plug in drivers=1 diameter=132" Hz=3 and SPL=132dB or Xmax=229mm
> This is about 2 lb presure acoustic presure per sq ft and with a sine
> wave at 3 HZ, made the middle of the wall move 8-10 inches in and out
> and "house parts" frequently would fall off (I thought we broke the
> house at one point when there was a loud crash from the crawl space so
> we stopped "teasing" the house with the oscilator).
Now this is bass! I've been wanting to do an experiment with some tactile
transducers and a suspended floor on the second level. Energize the floor
with a sine wave at the resonant frequency of the floor and see if I can
"Tacoma Narrows" it? Any guesses at what frequency a 12'x12' suspended
concrete floor resonants at? It shakes a lot when you walk on it.
> felt fairly safe exposing myself to this as I had found a very good book
> on the subject called "Infrasound and Low Frequency Vibration" edited by
> W. Tempest, published by Academic press.
Unfortunatly this book is out of print, anyone have any luck finding a
source for it?
From: Thomas Danley
Subject: How low to go & 1 way speaker &BBQ
How low do you go? Well one way to look at it is "what can you hear?"
There was a paper done by (Benjiman and Fielder I think) Dolby labs on
subwoofer performance that was excelent, it had the frequency content of
some music and had the hearing sensitivity chart extended down to a few
HZ. The threshold of hearing at 3 HZ is about 120 dB for example.
Some one ought to dig it up and post it, I have not found my copy yet.
Interestingly low frequencies (below 20 HZ) are common in everyday life,
not to mention explosions etc but also there was little hope (due to the
displacement and harmonic distortion requirements) in reproducing them at
realistic levels. Anyway, there is no sense extending the response of the
system below that where the woofer displacement is not enough to produce
For example looking at the equal loudness curves in digest 1038, one can
see that at 20 HZ the threshold of audibility is about 75 dB. Sitting 2
meters from a 12" woofer in a sealed box, one requires about 1/4 inch
peak to peak at 20 HZ reaches the threshold of hearing in a quiet room
but if one wanted a 20 HZ tone to have an aparent loudness of 90 dB this
requires about 30" peak to peak or 30, 12" drivers at 1" peak to peak.
Even at 30 HZ, an aparent level of 90 dB requires a displacement of
4 1/2 inches peak to peak. As you can see, even 2 meters from the woofer,
low bass at "normal" musical levels requires lots of cubic inches of
displacement. Low frequency room gain, to what ever extent is present is
not included in the above.
What does a system sound like with "enough" displacement?
As you might guess, the displacements we are talking about do not escape
unnoticed. Personally I find a graphic eq on the bass usefull and for my
taste often I add a rising slope below 35-40 HZ with perhaps 3-6 dB
boost at 16 HZ. This (once you get the subwoofer level set right) adds a
real sense of weight or size to the music. Those that bought Contra kits
and have them running should try this. The new low sounds you will hear
are often a percussive spectra, much of music is transient and that
often means low frequency content also.
At last years DIY BBQ we had a pair of Contra's radiating in a figure 8
pattern. We were a little short on Bass watts (I think it was 75w or
150W ea). This year Brad has agreed to supply 4 and there is a 2kW Delta
Omega in the other room that is happy driving them, even out doors the
2400 cubic inch displacement at 16HZ should be "enough" even out side.
Also speaking of one way speakers and bbq's at last years BBQ I had
demonstrated a wide band rotary radiator line source
loudspeaker which was flat more or less (+ -2) from 75 HZ to 25 KHZ and
the phase response was nominally zero degrees from 300 HZ to 20 KHZ.
It is hard, not impossible.
Back to the quest for more bass page.