woofer speed

this is a good paper for speaker designWoofer speedSo, back to that loudspeaker model. a loudspeaker is a coil of wire wound on a former that attaches to thcone. The current flows through the coil, creating an alternating magnetic field that interacts with the staticmagnetic field of the permanent magnet. So, what could limit current flow? Well, what does a voice coillook likeHow about an inductor? You know, those coils of wire (hey isn t that what a voice coil is)that you use incrossovers? Guess what-a voice coil IS an inductor! In fact, an inductor stores its energy in the magneticfield(as opposed to a capacitor which stores its energy in the electric field). It is this magnetic field of thevoice coil"inductor"that interacts with the permanent magnet field we talked about above. Hey,aloudspeaker is an inductor hung on the end of a cone in a static magnetic fieldSo, what about an inductor will alter the way current flows? Well, inductors don 't like to have the currentflowing through them change. They like to hold the current constant. They will allow you to change thecurrent flowing in them, but the bigger the inductor(or, the higher the measured inductance) the longer itwill hold the current before it starts to change(I'll leave it to the reader to go research inductance on theirown, to learn why this happens)So, the voice coil is an inductor. And we see that inductors don ' t like to change current. but we also seefrom equation 4 above that we need to change the current if we want to change the acceleration So, thevoice coil doesn ' t want us to change the current. How good is it at holding the current? Depends upon theinductance! The higher the inductance of the driver, the longer it can hold the current flowing through itWhich means the more time elapses before it starts to respond to the amplifier's applied voltage. Whichmeans we have slower transient responseGuess what-we just answered the original question! It turns out that transient response of a woofer is nota function of the moving mass, as is commonly espoused (one of the most infamous audio myths). Inactuality, it is based upon the inductance of the driver. And the greater the inductance, the slower thedriver- the lower the transient responseSo let's put this to the test- let's add some mass to a driver, and also increase the inductance to see whathappens. Out to the lab! This is what we didUsed a prototype Extremis 6-8(8 Ohm 6.5 woofer). Note that we had Klippel data measured by the goodDr Wolfgang Klippel himself at CES 2004Added mass was 28.5 grams(Mms of the Extremis 6-8 is 24.39 grams per Klippel). Mass added todustcap only; none was placed on the cone itself (all within the diameter of the former)Added inductance was a 10 AWG, 0.47 mH inductor (inductance of the Extremis 6-8 is 0.6 mH perKlippel)We left the driver mounted in our test baffle, which is 48"square. Driver centered on baffle (not flushmounted), anechoic (no box ). Nearest reflective surface is 6 feet from the speaker, for good results down to90Hz.Test gear used wasPraxis measurement systemM Audio Audiophile usb sound cardAudio Control MP200 test mic and preamp (last cal at factory November 2002 -time to getanother cal!)QSC PLX3402 amplifierCopyrightcAdireAudioAllRightsReservedVisitAdireaudioathttp://www.adireaudio.comWoofer speedWe measured the impulse response of the driver raw, with 28. 5 grams added(more than doubled themoving mass ), and with 0.47 mH in series (78% increase in inductance ) In other words, we more thandoubled the mass, and didn't double the inductance. So the mass- if it really limits transient response(andhence frequency response) should really stand out compared to the inductive changesHere's what the impulse tests showComparison of impulse response Extremis 6-8 raw [red). 28.5g added (blue), 0. 47 mH in line (greenLiberty =pfdxii=Extremis 6-8 anechoic raw Impulse px2ITime Domain Dataaeg2/20200491109AM4m5.5mMkr 1Mkr 21.854msec2. 062msecExtremis 6-8 anechoic 470 uh inductor Impulse. p:2: Loudspeaker Impulse ResponseExtremis 6-8 anechoic 28 gram loaded Impulse px2: Loudspeaker Impulse ResponseRaw image dump from Praxis. Red is the raw driver-nothing added blue is the driver with 28.5 gramsmass added. Green is the driver(no added mass )with the inductor in seriesNow, look at the time of the peaks. I've set the markers where the raw driver has its turn-on transient peaksThe mass(blue line)corresponds nicely. The inductor (green line) has a delay in the negative ANd positiveCopyrightcAdireAudioAllRightsReservedVisitAdireaudioathttp://www.adireaudio.comWoofer speedHere's a zoomed in view of the start(note the sampling time is 2l microseconds, a 48 kHz sampling rateComparison of impulse response Extremis 6-8 raw [red], 28. 5g added (blue), 0.47 mH in line green)yax/s=Extremis 6-8 anechoic raw Impulse px2Time Domain dataaq2/20/200491109AM2.2m2.3m2.4m25m261020Mkr 1Mkr 21463m14.63mExtremis 6-8 anechoic 470 uh inductor Impulse pN2: Loudspeaker Impulse ResponseExtremis 6-8 anechoic 28 gram loaded Impulse px2: Loudspeaker Impulse ResponseYou can clearly see that the raw and mass loaded transient peaks- first negative and first positive peaksoccur at the same time. The inductively loaded(green) peaks are definitely delayed in time. As one wouldexpectCopyrightcAdireAudioAllRightsReservedVisitAdireaudioathttp://www.adireaudio.comWoofer speedNow, how would this translate to frequency response? Here's a graph of the raw(unsmoothed, wide gate)frequency response for the same 3 casesLoudspeaker Frequency ResponseLiberty =paNEExtremis 6-8 anechoic raw FR Dx2Frequency Responseacq2/20/20049:0942AM[DComp =1.77imsec)5400Mkr t500Hz1kHz38243623910013Extremis 6-8 anechoic 470 uh inductor FR px2: Loudspeaker Frequency ResponseExtremis 6-8 anechoic 28 gram loaded FR px2: Loudspeaker Frequency ResponseNote the red and blue- raw and mass loaded, respectively. They are pretty much equal from 700 Hz and up(I assume we all accept the Fourier transforms, and that the Fs/efficiency of the driver is affected by theadded mass, accounting for the differences down lowNow look at the green. It is 5+ db down at 4+ kHz. The high frequency -the transient part of transientresponse -is reduced! The inductance cut the high frequency extension -mass did not.Mass isn't the problem-inductance is. So if you want faster transient response, ignore that moving massparameter that some manufacturers push-look at the inductance! And if they don' t list the inductance, askyourself why -is there something they don't want to show? Inductance is the key to driver transientresponse-ask for it when transient response comes upCopyrightcAdireAudioAllRightsReservedVisitAdireaudioathttp://www.adireaudio.com