Bass Response- proper measuring technique

[DaveD]

I have been reading a thread on AudioKarma that was linked in a thread here titled Stacking 17s. The member in both cases goes by Zilch.

Near the center of this page is a photo from member Paratima of his measurement technique. Zilch used the same close miking with an

expensive measurement setup. Both of them measured strong bass rolloff below 80Hz.

http://www.audiokarma.org/forums/showthrea...8877&page=6

Not being technically inclined and steeped in solder, the thought that came into my head was of all of those speaker reviews that I read

by Julian Hirsch years ago. He always used a combination of close miking and room response for the lower frequencies. I believe that the

reason was that the close miking can't pick up the lower frequencies, whose complete wavelengths are longer than the distance between the

close microphone and the bass driver. They are picked up by the in-room microphone (carefully placed, I guess, to account for room node

effects) and then spliced together by some formula with the close miked measurement from the woofer.

I don't want to start an argument about surround doping-- the subject matter that is being covered in conjunction with these measurements by

Zilch and Paratima-- but would like to know if their close miking technique is flawed.

I have a pair of 17s whose surrounds were sealed by a restoration technician, using the latex based material from Orange County Speaker. I have

learned on this forum that this product may do some harm to the bass as it hardens. The bass is deeper after the doping. It has been almost

three years since this was done. Subjectively, using the warble tones from Stereophile, I have similar 50Hz and 40Hz response now as I had after the

work was done. I would have to play the low E bass note on the intro to Golden Slumbers on Abbey Road to tell if it sounds like the bass I had when the speakers were new. My 17s have subjectively louder response at 40Hz than at 50Hz, and just a hint at 31.5Hz. This is much more than the authors of those close miked measurements report.

[Zilch]

Not being technically inclined and steeped in solder, the thought that came into my head was of all of those speaker reviews that I read by Julian Hirsch years ago. He always used a combination of close miking and room response for the lower frequencies. I believe that the

reason was that the close miking can't pick up the lower frequencies, whose complete wavelengths are longer than the distance between the close microphone and the bass driver. They are picked up by the in-room microphone (carefully placed, I guess, to account for room node effects) and then spliced together by some formula with the close miked measurement from the woofer.

I don't want to start an argument about surround doping-- the subject matter that is being covered in conjunction with these measurements by

Zilch and Paratima-- but would like to know if their close miking technique is flawed.

You remember it backwards. It's the techniques used for higher-frequency measurements that include the room influence and thus do not accurately reflect the low-frequency response. The 1/4" close-mic technique we employ was developed by Small, and is industry standard for measuring response below 200 Hz in small spaces indoors; the nearfield LF and MLS or sinusoidal HF are then "stitched" together to show the complete spectrum.

Nearfield bass response measurements are inherently 2-Pi, the cone itself comprising a boundary. Thus, they illustrate how the speaker will respond with one boundary, such as a floor or wall. The curve must be corrected for comparison to anechoic or freespace response by reducing the response accordingly.

It's quite straightforward with closed-box designs like these KLHs and other acoustic suspension speakers. When measuring vented alignments, the technique becomes a bit more complex; the nearfield response of the port and driver must be measured separately and summed.

Read up in Dickason and/or D'Appolito. There's also an excellent series of articles on loudspeaker measurement procedures by John Atkinson in Stereophile, available online:

http://www.stereophile.com/features/99/index.html

http://www.stereophile.com/features/100/index.html

http://www.stereophile.com/reference/103/index.html

[Carlspeak]

I don't think Dave D. has the tools to do the nearfield testing. I suspect his post related simply to what he heard when playing some test tones. What we hear and what we measure can be two different things sometimes. Correctly done measurements take out most of the subjectivity of listening tests by applying numerical values to levels of sound. Subjective measurements can be fraught with unintended variables which get overlooked and ultimately, anecdotal comments surface which may or may not have validity.

[Zilch]

Hi, Carl!

On the page Dave D linked above, Paratima did it with a $50 R/S SPL meter and obtained results comparable to mine using $2K of measurement gear.

Nearfield LF measurements of AS speakers is a slam dunk, and I encourage anyone who's interested in knowing the facts regarding LF response to ascertain these themselves for their own satisfaction.

Measure before and after redoping surrounds, for example. Did you fix them or screw them up?

As RayW suggested in that thread, "Come over to the Dark Side. We have cookies...."

[Pete B]

You remember it backwards. It's the techniques used for higher-frequency measurements that include the room influence and thus do not accurately reflect the low-frequency response. The 1/4" close-mic technique we employ was developed by Small, and is industry standard for measuring response below 200 Hz in small spaces indoors; the nearfield LF and MLS or sinusoidal HF are then "stitched" together to show the complete spectrum.

Nearfield bass response measurements are inherently 2-Pi, the cone itself comprising a boundary. Thus, they illustrate how the speaker will respond with one boundary, such as a floor or wall. The curve must be corrected for comparison to anechoic or freespace response by reducing the response accordingly.

It's quite straightforward with closed-box designs like these KLHs and other acoustic suspension speakers. When measuring vented alignments, the technique becomes a bit more complex; the nearfield response of the port and driver must be measured separately and summed.

Read up in Dickason and/or D'Appolito. There's also an excellent series of articles on loudspeaker measurement procedures by John Atkinson in Stereophile, available online:

http://www.stereophile.com/features/99/index.html

http://www.stereophile.com/features/100/index.html

http://www.stereophile.com/reference/103/index.html

The nearfield technique was first reported by Keele in an AES paper (refer to #6 in the link below)

where he gives credit to Raymond Newman at Electrovoice as the first to point it out:

http://www.xlrtechs.com/dbkeele.com/papers.htm

[Carlspeak]

I believe the best assessment for experimenting with various types of doped textile fabric surrounds is bare woofer resonant frequency. Much of the discussion seems to be on white (pva) types vs the recently discovered Permatex High Tack gasket sealant. Some folks have claimed Aileen's adhesives or the stuff OC sells stiffen over time. If they stiffen the surround's compliance characteristics, it will show up as measurably higher Fs. I suggest someone dope with Aileen's and test Fs when dry. Then put the driver in an oven at low temp for an extended period to accelerate the ageing process. Continue testing Fs periodically to see if it rises significantly over time.

OTOH, doping and putting the driver in a cabinet and measuring frequency response adds an undesirable variable to the assessment (degree of sealing of the surround).

So, I guess to assess type of doping sealant, use bare woofer Fs testing. To assess degree of success in sealing the woofer, conduct before and after FR testing of the woofer in a cabinet using close mic techniques brought to life by D. B. Keele, Jr. in his 5/15/73 AES paper, "Low-Frequency Loudspeaker Assessment by Nearfield Sound-Pressure Measurement".

[Zilch]

O.K., O.K., Small et al., then.

How 'bout more of us do it, and then we can debate the data....

[Pete B]

Forgot to mention that Keele won an AES award for that paper IIRC.

That paper was required reading when I studied audio engineering

and did some loudspeaker analysis under Professor Wadesworth at WPI.

[DaveD]

Hi, Carl!

On the page Dave D linked above, Paratima did it with a $50 R/S SPL meter and obtained results comparable to mine using $2K of measurement gear.

Nearfield LF measurements of AS speakers is a slam dunk, and I encourage anyone who's interested in knowing the facts regarding LF response to ascertain these themselves for their own satisfaction.

Measure before and after redoping surrounds, for example. Did you fix them or screw them up?

As RayW suggested in that thread, "Come over to the Dark Side. We have cookies...."

I still have that low note in Golden Slumbers. It is in the second verse. The cartridge is the same as 30 years ago. The turntable is better quality and the amplifier is much better quality. The room is very much different, much larger and harder. Subjectively, I think I hear and feel about as much of that low

bass as I did when the speakers were new. Too many variables to be sure. I am blessed with not having perfect pitch, and no insturments around anymore to pick out notes, but I think it is low E, 41.2Hz. I don't think Macca detuned to low D (36.7Hz), but he might have. I have to walk around the room to find the bass, but the same was true in the smaller room years ago, as it still is in that room today.

I do have a R/S meter and the warble tones CD. It isn't likely that I will feel like trying to measure, even though I have some curiosity about this. It appears the three year old latex based sealant didn't hurt the speakers very much, but I do wonder what the Permatex would have done instead. It's much less stressful to just put on another record.

Thanks to all of you for helping me understand.

[soundminded]

In Keele's paper #6, the fatal flaw in the ported design is easily seen in figure #2 where the normalized pressure is compareed to the inverse of the frequency (lambda = velocity/frequency) times the area. The alternating peaks and troughs, the peaks being spaced one octave apart and the troughs also being spaced an octave apart but midway between the peaks is exactly what you'd expect from Newton's second law of motion as applied to forced oscillation for a tuned port. The resistance pressure in the port will be at a minimum and therfore the efficiency of the driver's ability to move and to push air through it at a maximum at the wavelengths to which the port is tuned (and to multiples of it) while the troughs will occur where the resistance pressure is at a relative maximum (maximally de-tuned.) In Newton's second law this parameter is "K" or the spring constant which is multipled by displacement. The equation can be written as F(t)= ma + bv +kx where F(t) is the motion as a function of time, a, v, and x are acceleration, velocity, and displacement, and m,b, and k are the mass, viscousity (velocity related frictional loss) and k is the spring constant proportional to displacement. Because of the port, K is a function of frequency with oscillating amplitude in an exponentially increasing envelope as frequency is lower. Since P=F/A and the energizing electromechanical force applied for measurement is a constant independent of frequency while the resistive force K oscillates, so would the pressure. You'd think someone would have come up with the bright idea of improving ported designs by using two woofers with ports tuned half an octave apart or three woofers with ports tuned a third of an octave apart but nobody has been even this clever that I am aware of. Maybe there's one out there.

The plot gets more interesting when the port is at the back of the enclosure and the speaker is installed about 4 feet from the wall behind it. This puts the sound from the port considerably out of phase with the sound from the front of the woofer where the listener sits but not necessarily at 180 degrees. For instance, at 40 hz, at sea level the wavelength is 27 feet 3 inches putting the output of the port reflected by the back wall about 120 degrees out of phase with the sound coming directly from the woofer itself. Further collisions with the side walls create additional cancellations and reinforcements. Therefore, in a steady state FR measurement in a real room, what you get will depend on where you are and it should vary substantially from point to point and invariably does. This does not even take into consideration standing waves in the room. Therefore published theoretical data, or measured average data is a pure fudge, a fiction compared to what you will hear but one thing you won't hear is anything close to flat FR.

The acoustic suspension design does not suffer this flaw of alternating peaks and valleys because the spring constant of the air trapped in a sealed box is independent of frequency. This is according to the ideal gas law where PV=nRT, and P1*V1=P2*V2. Not only is the restoring force independend of frequency but is applied to the cone uniformly over its entire surface while in the ported design, the mechanical force is applied at the circumference in the center of the cone and at the edges. Any differences either radially or tangentially from point to point will tend to twist and flex the cone in a ported speaker. Therefore, all other things being equal, the driver cone in a ported design is far more prone to breaking up into Bessel function modes the way a drum head vibrates and therefore has inherently higher harmonic distortion.

I am puzzled by Keele's subsequent statement "this surprising result shows that for low frequencies the nearfield sound pressure is directly proportional to the far soundfield pressure." Why should that be a surprise since it is propagation of the same acoustic energy which creates both the nearfield and farfield pressue waves. I'll have to read this more carefully but so far I don't buy it.

[Zilch]

I have to walk around the room to find the bass, but the same was true in the smaller room years ago, as it still is in that room today.

Thanks to all of you for helping me understand.

Seems we're more in agreement on this than originally supposed....