Advanced Animatronics Fursuit Muzzle Loudspeaker Design ...

Advanced AnimatronicsFursuit Muzzle Loudspeaker Design Part II v0.1

The Electric Fursuits Show & Tell – 12/12/2020

Floere T. Pillowcase, Devourer of Automobiles

[email protected]

12/12/2020The Electric Fursuits Show & Tell2

Danger, Will Robinson !

I just started learning about speaker design …… which means I’m probably right at Mount Stupid

Thus, don’t bet the farm on what’s in these slides and feel free to correct me !

Saturday Morning Breakfast Cereal


So, we Built a (Tuned) Speaker Enclosure… Now What ?

● So, does it actually work as expected ?● Is the enclosure actually tuned where we think it is tuned ?● Are there any “surprising features” ?

● Resonances, air leaks, speaker driver problems, rattling, etc...● Also: don’t forget to use your ears: does it actually sound any good ?

● “Meten is weten, maar er is ook al veel tijd mee verscheten.” – Roland Reekmans (former technician-in-chief, KULeuven, dep. ESAT-Electa)● “To measure is to know, but a lot of time has been wasted (lit: turned to shit) with it as well.”● You MUST know: what to measure, how to measure, the limitations of the measurement, what

can go wrong, and how to identify how it (always to some degree) went wrong !


Device Under Test: Speaker with Passive Radiator

● Quick & dirty design using:● Qspeakers (Free)

● Ignores everything beyond basic T/S● WinSpeakerz ($40)

● Includes cone excursion, baffle diffraction loss● Compatible with DATS V3 measured data● Because I’m suspicious, because of test results ...

● System: ● Enclosure: ~0.2 L (4th order vented)● Driver: Tectonic TEBM28C10-4/A● PR: Dayton Audio ND65-PR● Fb : 100 Hz target tuning


Hoffman’s Iron LawAs Expressed by Richard H. Small

● Half-space efficiency η0 cfr. Small:● η0 = Kηf3³Vb (η0 * 100) for %, f3 in Hz, Vb in m³

● Close box: Kη = 1.5 x 10^-6● Vented box: Kη = 2.5 x 10^-6

● Wowzer !● f3 (-3db frequency) is qubed !

● Half f3 need→ 8 x power or volume !● And that’s why small speakers that have

to play low are absolute power hogs● Know as: “having poor sensitivity”

● Another fun one: dB SPL vs η0

● dB SPL = 112.1 dB + 10 Log η0

(dB SPL for 1 Watt / 1 meter)

→ η0 = 10^[(dB SPL – 112.1 dB) / 10]● We’ll come back to this one !

● Warning: there will be a quiz


Simulation: Baffle Diffraction Loss

● Small enclosure = spherical radiation pattern happens for higher frequencies● Baffle diffraction “loss” is - 6 dB due to effectively

going from half-space full-space radiation→● The speaker baffle becomes small vs the wave length

● Human voice also spreads-out (head & torso are finite baffle)

● It likely won’t be as bad as - 6 dB, as we still have torso & head (or resin base) near by

● A more spherical radiation pattern is not necessarily a bad thing … ● Helps people standing not directly in front of you

hear what you’re saying !

-6 dB


Simulation: Deer Lord ! Xmax is Doing WHAT !!!

● Big issue : Xmax is quickly exceeded < 500 Hz !!!● Curve is with 10 W drive power (max for driver)● Can only use 0.5 W max in lower-end of voice range !

● With baffle diffraction: SPL 66 dB @ 100 Hz● FFS ! This is not nearly the targetted 80 – 90 dB. ● What gives ? The prototype seems to do better ...

● Ah, yes. Plot is for one frequency at a time !● Speech is harmonics : each harmonic has some

contribution, and 2nd is already in a lower cone excursion range (100 Hz F0).

● Also: 10 W vs 1 W : only twice as loud (hearing is logarithmic !)

● But, what do your Spider senses say ?

XmaxSPL

ZExcursion

Hic Sunt Dracones !(aka: Exit Conus)


Simulation: Deer Lord ! Xmax is Doing WHAT !!!

● Oops … ● When I used measured T/S data, things

suddenly made sense● I had Sd off by a factor of ten

(messed-up the unit conversion)● And this is why cross-checking and

measuring is important● Because you too can be a doofus !

● Now we get the expected power handling, and the cone excitation is congruent with the prototype. Whew !

XmaxSPL

ZExcursion

Hic Sunt Dracones !(aka: Exit Conus)


Measuring: Speaker T/S Parameters

● T/S Parameters: ● Do we have the actual values, or those from the

marketing department ?● Did we enter them right into the software ?

● Affordable ways to measure:● DATS software + DATS V3 device (€150)● REW + little circuit (free + few €)● Manually (needs various, common test equipment)

● Big fat note: ● Loudspeakers have rather large tolerances● Properties change appreciably during burn-in (when new)● All in all, if it’s within 20 % after burn-in, it’s good enough

(Some parameters can even be 50 % off and the result is still reasonably comparable. Others, not so much ...)


Measuring: System Fb (and Unexpected Resonances)

● Using DATS or REW● Look for valley between both peaks● Easy, fast, accurate

● Manually● Using techniques from references

● Visually: sweep high to low● Look for pattern in cone motion: max,

min, max as you go lower in frequency● The minimum motion is at Fb● Not easy to see with small speakers

Fb


Fixing: System Fb is Wildly Out

● Are you sure the components match the T/S specifications used in the simulation ?

● Is the passive radiator or vent reasonably tuned ?● Are you sure the box is air-tight ?

● Seams, speaker gasket seal, wire seal, missing screw, etc...● MDF is surprisingly and ridiculously porous !

Seal it with paint, varnish, or water-diluted PVA wood glue● Mind the pressure-equalization vent !

● This is functionally a HP filter “tuned” at a very low frequency to pass ~DC (ambient air pressure changes)

● Test: push PR cone in fast but gently and hold while observing the driver cone. Driver cone should should take at least a second or so to return to the neutral position.

● Note: not so for a ported enclosure (big hole in case)● Option A: seal the port & try again● Option B: seal the vent and re-measure Fb. If no change, it wasn’t the vent

Fb shift due to one M3 screw removed !


Fixing: System Fb (When not Wildly Out)

● This is the actual Fb tuning !● Tweak port length● Tweak passive-radiator mass

● Iterative process● Plot: nothing, washer, wingnut, 2 X M4 nut● Keep the set-up otherwise identical

● Easy to chase your own tail due to:● Holding the case tight. Clamping force varies !● Non-static objects close to driver, port, passive radiator

● Make sure that the seals are in right and tight for every try ! Screw holes matter !

● May require some “getting creative”Fb = 99.9 Hz = Target


Fixing: Unexpected Resonances

● Cabinet resonances can be damped using damping materials, stuffing● Note: stuffing will change tuning somewhat !

● Panel resonances need bracing and /or mass loading● Sweep frequency and feel the sides of the box

gently with your finger tips● If some sides vibrating a lot at a frequency that

matched the bumps in the plot: bingo ! ● Air leaks can also do bad things …

● Listen for breating / shuffing noises at seams, gaskets, wire holes, …

Resonances


Measuring: Speaker Response

● Using REW + UMIK 1 + DAC + AMP● Measured at 1 m, in half-space

Using floor as image mirror boundary● Microphone calibration state unsure


Measuring: Speaker ResponseExpected Measured

Due to PR

● Simulation used a tuned port● Build used a passive radiator

● Passive radiator has a steeper slope + resonance wobble at the bottom● Agreement is not too bad actually

● Tuning seems about 5 Hz too low ? Depends on how I smooth ...

Due to driver


Fixing: Ugly Response Looks Like the Alps

● This is normal● Apply smoothing, like the manufacturers do ;-)

● Measuring indoors results in lots of room modes, reflections, etc …● Each of these contributes peaks & valleys● Very location-dependent. Move 10 cm and be amazed …

● Baffle diffraction pattern also contributes● High-frequency response of larger drivers is bumpy

● Compare to data sheet to get an idea of what’s what● Don’t worry too much, as long as there are no huge

errors in the smoothed response (few dB wobble is ok)● Our brain is good at “connecting the dots”● EQ can help with some of these disturbances● This ain’t a multi k€, high-end speaker in a carefully treated room

It will never be perfect ! Also, re-measure when installed in suit→

EQ EQ


Fixing: Low-Frequency Response is Completely Wrong

● Are you sure your test set-up is not HP filtering ?● Be very suspicious! Many sound cards & microphones have a

steep HP filter there (to reject sub-sonics, rumbles, etc … )● DAC + amplifier path: measure the output to the

speaker using a good multi-meter (that has the bandwidth !) or an oscilloscope and sweep frequency

● ADC path : use a signal source (validated using above meters) to map magnitude response using eg, REW or audacity and sweep the frequency

● Microphone: tough one… “Easiest” is with a known acoustic source (which is €€€)● Really, just use a calibrated USB measurement mic + REW !● Or use a cheapy electret condensor capsule and do what ESP tells

you to do to it and get something quite useable, for cheap● Or use a very flat studio mic and trust the vendor’s specs …● Or spend €2K- €4K for a real, free-field, lab-grade mic + ADC

Blue Yeti ($130)

AKG C451 B ($500)

Nope !


Fixing: Random Rattles

● Wires are often the culprit● Work wires into Damping material● Wrap them with foam tape (common in automotive)● Glue them down over the whole length● Strain relief at speaker ! (Eg: using heat shrink)

Vibration = highway to metal fatigue and broken wires● Anything else that can move?

● You’d be surprised at what can move with little energy when driven at its natural resonance frequency

● See: Tesla Earthquake machine● Locktite your fasteners, silastic / hot snot any

connector that does not really lock in place ! ● Vibration has patience: screws, nuts, plugs will loosen !

https://www.fordxr6turbo.com/forum/topic/26338-tutorial-fitting-an-aftermarket-deck-to-a-ba/


Fixing: Audible Distortion due to Amplifier Clipping

● Identification:● Increasing, harsh distortion as sound gets louder● Cone excursion definitely within Xmax limits● Pure sine wave has lots of harmonics (using microphone + FFT)● Oscilloscope voltage over speaker terminals reveals clipping

● Solution : don’t drive the amplifier that hard !● But it may not be loud enough …● Use a line-level HP filter (remove energy below Fb, which is

wasted energy anyhow, and dangerous for the driver)● Solution : P = U²/R

● Amplifier with higher supply voltage● Bridge-tied amplifier (doubles effective supply voltage)● Lower speaker impedance = more power with same supply

voltage (if the amp can handle it)(4 Ohms is usually better for battery-operated systems)

https://factualaudio.com/post/distortion/


Fixing: Audible Distortion due to Exceeding Xmax

● DON’T DRIVE IT THAT HARD !YOU WILL BREAK THE DRIVER !

● Can be due to cone unloading● Applies to vented enclosures (also PR)● Driving speaker below Fb will quickly

reduce air spring effect● Causes wild, unrestrained cone movement that

quickly exceeds the driver’s mechanical limits Xmax, Xmaxmech

● Use a HP filter tuned at or around Fb

● Really, always use a high-pass filter with a tuned enclosure. It’s much safer that way https://ledgernote.com/columns/studio-recording/blown-speaker/


Fixing: Remaining Speaker Response Issues

● Use an equalizer (EQ)● Can deal with resonance peaks● Tune to smooth-out response, but

ignore small (few dB) wobbles● Caution: don’t fill valleys (dips)

too much !● Dynamic range is severely limited in

our case, especially at low frequencies● And the rest is up to our brain ...

https://sourceforge.net/projects/peace-equalizer-apo-extension/


Wrapping-Up

● Good-enough-for-voice muzzle speakers can be done !

● Design using simplistic tools is quite OK, but surprises may surprise ...

● Validating and tuning a basic speaker is not that hard● Requirements for voice are relaxed● Good tools are accessible● Measuring at home is good enough

● Don’t call it a flaw, call it a feature… It adds character !→


Quiz !

● Q: What is the electric acoustic →energy conversion efficiency η0 of this speaker ?● Sensitivity: 80 dB @ 1 W 1 m (generous !)● Ignoring: LF drop-off over most of the voice F0

range, baffle diffraction losses, etc ...

● A: η0 = 10^[(80 dB - 112.1 dB) / 10]

= 0.062 %

With a big enough rocket, even pigs fly ...John Steinbeck’s Motto – image unknown source


Resources

● Loudspeaker Database (also useful to get vendor T/S parameters in coherent format)http://www.loudspeakerdatabase.com/

● Books (used them all – Note: all but the first require some engineering background !)● Introductory : “Introduction to Loudspeaker Design,” John L. Murphy● More depth : “Loudspeaker Design Cookbook, 7th edition,” Vance Dickason● How to Measure : “Testing Loudspeakers,” Joseph D’Appolito

● Sites● Elliot Sound Products

https://www.sound-au.com/● Wikipedia (eg: Thiele-Small)● Many, many more … But: beware the opinions untroubled by facts ! This is audio, after all ...

http://www.loudspeakerdatabase.com/

https://www.sound-au.com/


Resources

● Design Software (free and inexpensive):● Qspeakers (used, basic, simple to use, no documentation)

http://brouits.free.fr/qspeakers/● WinSpeakerz (used, $40, more advanced, quite simple to use and decent manual)

https://www.trueaudio.com/win_abt1.htm● Basta!

http://www.tolvan.com/index.php?page=/basta/basta.php● WinISD

http://www.linearteam.org/● VituixCAD

https://kimmosaunisto.net/Software/Software.html● Visaton BoxSim

https://www.visaton.de/en/literatur/software● And many, many more !

http://brouits.free.fr/qspeakers/

https://www.trueaudio.com/win_abt1.htm

http://www.tolvan.com/index.php?page=/basta/basta.php

http://www.linearteam.org/

https://kimmosaunisto.net/Software/Software.html

https://www.visaton.de/en/literatur/software

Advanced Animatronics Fursuit Muzzle Loudspeaker Design ...

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