AudioFrog GB60 6-Inch Midwoofer Review (Car Audio product)

[hardisj]

As per usual, please read the review on my site for the tables since they don't translate here.
https://www.erinsaudiocorner.com/driveunits/audiofroggb60/


AudioFrog GB60 6 Inch Midwoofer Review

• Monday, Aug 17, 2020

Information
Note: These speakers are designed specifically for car audio use and come with a kit of mounting gear such as adapter rings, grilles and additional installation hardware. This review is focused only on the performance aspect and does not consider the “install” aspect as it relates to the provided hardware.
Per the manufacturer:

Designed to be used in two- or three-way systems, the GB60 can be used in small enclosures or in infinite baffle applications. High power handling, low distortion and 9mm of linear one-way excursion make the GB60 a great foundation of any main speaker system. Mounting depth of under 2-3/4” makes installation in many factory locations possible. Like all the GB Series midrange speakers, the GB60 includes a grille and installation kit that can be customized to blend with or highlight nearly any installation.​

Retail price is about $900/pair.

Manufacturer product specs can be found here.

Test Results
Foreword: Subjective Analysis vs Objective Data of Drive Units (click here)

Small Signal Testing (Thiele-Small Results)
Using Klippel’s Distortion Analyzer 2, Linear Lumped Parameter Measurement Module, Pro Driver Stand and provided Panasonic ANR12821 Laser along with Klippel’s Training 1 - Linear Lumped Parameter Measurement tutorial, I measured this drive unit’s impedance and small-signal parameters. Below are the results.
Note: The impedance graph can be found in the Frequency Response Linearity graphic later in this review.
Electrical Parameters Re3.64Ohmelectrical voice coil resistance at DCLe0.202mHfrequency independent part of voice coil inductanceL20.430mHpara-inductance of voice coilR21.51Ohmelectrical resistance due to eddy current lossesCmes379.64µFelectrical capacitance representing moving massLces22.23mHelectrical inductance representing driver complianceRes67.18Ohmresistance due to mechanical lossesfs54.8Hzdriver resonance frequencyMechanical Parameters(using laser)Mms20.594gmechanical mass of driver diaphragm assembly including air load and voice coilMmd (Sd)19.058gmechanical mass of voice coil and diaphragm without air loadRms0.807kg/smechanical resistance of total-driver lossesCms0.410mm/Nmechanical compliance of driver suspensionKms2.44N/mmmechanical stiffness of driver suspensionBl7.365N/Aforce factor (Bl product)Lambda s0.095suspension creep factorLoss factorsQtp0.453total Q-factor considering all lossesQms8.780mechanical Q-factor of driver in free air considering Rms onlyQes0.476electrical Q-factor of driver in free air considering Re onlyQts0.452total Q-factor considering Re and Rms onlyOther ParametersVas8.7295lequivalent air volume of suspensionn00.290%reference efficiency (2 pi-radiation using Re)Lm86.82dBcharacteristic sound pressure level (SPL at 1m for 1W @ Re)Lnom87.23dBnominal sensitivity (SPL at 1m for 1W @ Zn)Sd122.70cm²diaphragm area


Large Signal Modeling (Linear Xmax Results)
Using Klippel’s Distortion Analyzer 2, Large Signal Identification Module, Pro Driver Stand and provided Panasonic ANR12821 Laser along with Klippel’s Training 3 - Loudspeaker Nonlinearities tutorial, I measured the linear, nonlinear and thermal parameters of this drive unit.
Nonlinearities
Traditionally, Xmax has been defined in one of the following ways:

• the physical overhang of the voice coil (height of the voice coil relative to height of the gap)
• 115% times the physical overhang above
• the point where displacement limit(s) is/are exceeded

The third option is where the Klippel LSI module comes in to play. It permits a more “apples to apples” approach of defining the displacement (Xmax) limits based on the XBL, XC, XL and Xd. The displacement limits XBL, XC, XL and Xd describe the limiting effect for the force factor Bl(x), compliance Cms(x), inductance Le(x) and Doppler effect, respectively, according to the threshold values Blmin, Cmin, Zmax and d2 used by the operator.
There are one of two sets of thresholds which can be used to define linear excursion:

1. Non-Subwoofer Drivers: The thresholds Blmin= 82 %, Cmin=75 %, Zmax=10 % and d2=10% generate for a two-tone-signal (f1=fs, f2=8.5fs) 10 % total harmonic distortion and 10 % intermodulation distortion.
2. Subwoofer Drivers: The thresholds Blmin= 70 %, Cmin=50 %, Zmax=17 % create 20 % total harmonic distortion which is becoming the standard for acceptable subwoofer distortion thresholds.

These parameters are defined in more detail in the (Klippel) papers:

• “AN04 – Measurement of Peak Displacement Xmax”
• “AN05 - Displacement Limits due to Driver Nonlinearities”
• “AN17 - Credibility of Nonlinear Parameters”
• “Prediction of Speaker Performance at High Amplitudes”
• “Assessment of Voice Coil Peak Displacement Xmax”
• “Assessing Large Signal Performance of Loudspeakers”

Below are the displacement limits’ results for this drive unit obtained from Klippel’s LSI module:
Displacement Limits X Bl @ Bl min=82%4.7mmDisplacement limit due to force factor variationX C @ C min=75%5.9mmDisplacement limit due to compliance variationX L @ Z max=10 %>8.2mmDisplacement limit due to inductance variationX d @ d2=10%23.3mmDisplacement limit due to IM distortion (Doppler)Asymmetry (IEC 62458)Ak-13.13%Stiffness asymmetry Ak(Xpeak)Xsym1.15mmSymmetry point of Bl(x) at maximal excursion
Per the above table, this drive unit’s linear excursion is limited to 4.7mm due to exceeding the Bl displacement limit of 82% for the distortion limit of 10%.
We can break the above information down further. The below text is written by Patrick Turnmire of Red Rock Acoustics and used with his permission, substituting data from this drive unit’s test results where applicable.
Large Signal Modeling
At higher amplitudes, loudspeakers produce substantial distortion in the output signal, generated by nonlinear ties inherent in the transducer. The dominant nonlinear distortions are predictable and are closely related with the general principle, particular design, material properties and assembling techniques of the loudspeaker. The Klippel Distortion Analyzer combines nonlinear measurement techniques with computer simulation to explain the generation of the nonlinear distortions, to identify their physical causes and to give suggestion for constructional improvements. Better insight into the nonlinear mechanisms makes it possible to further optimize the transducer in respect with sound quality, weight, size and cost.
Nonlinear Characteristics
The dominant nonlinearities are modelled by variable parameters such as
Bl(x)instantaneous electro-dynamic coupling factor (force factor of the motor) defined by the integral of the magnetic flux density B over voice coil length l as a function of displacementKMS(x)mechanical stiffness of driver suspension a function of displacementLE(i)voice coil inductance as a function of input current (describes nonlinear permeability of the iron path)LE(x)voice coil inductance as a function of displacement

Nonlinear Parameters
Bl change with excursion

The electrodynamic coupling factor, also called Bl-product or force factor Bl(x), is defined by the integral of the magnetic flux density B over voice coil length l and translates current into force. In traditional modeling this parameter is assumed to be constant. The force factor Bl(0) at the rest position corresponds with the Bl-product used in linear modeling. The red curve displays Bl over the entire displacement range covered during the measurement. You see the typical decay of Bl when the voice coil moves out of the gap. At the end of the measurement, the black curve shows the confidential range (interval where the voice coil displacement in this range occurred 99% of the measurement time). During the measurement, the black curve shows the current working range. The dashed curve displays Bl(x) mirrored at the rest position of the voice coil – this way, asymmetries can be quickly identified. Since a laser was connected during the measurement, a coil in / coil out marker is displayed on the bottom left / bottom right.

Suspension Stiffness change with excursion

The stiffness KMS(x) describes the mechanical properties of the suspension. Its inverse is the compliance CMS(x).

Inductance change with excursion

The inductance components Le (x) and Bl(i) of most drivers have a strong asymmetric characteristic. If the voice coil moves towards the back plate the inductance usually increases since the magnetic field generated by the current in the voice coil has a lower magnetic resistance due to the shorter air path. The nonlinear inductance Le(x) has two nonlinear effects. First the variation of the electrical impedance with voice coil displacement x affects the input current of the driver. Here the nonlinear source of distortion is the multiplication of displacement and current. The second effect is the generation of a reluctance force which may be interpreted as an electromagnetic motor force proportional to the squared input current.
The flux modulation Bl(i) has two effects too. On the electrical side the back EMF Bl(i)*v produces nonlinear distortion due to the multiplication of current i and velocity v. On the mechanical side the driving force F = Bl(i)*i comprises a nonlinear term due to the squared current i. This force produces similar effects as the variable term Le(x).

Fs shift with excursion

Qts change with excursion

Asymmetrical Nonlinearities
Asymmetrical nonlinearities produce not only second- and higher-order distortions but also a dc-part in the displacement by rectifying low frequency components. For an asymmetric stiffness characteristic the dc-components moves the voice coil for any excitation signal in the direction of the stiffness minimum. For an asymmetric force factor characteristic the dc-component depends on the frequency of the excitation signal. A sinusoidal tone below resonance (f<fS) would generate or force moving the voice coil always in the force factor maximum. This effect is most welcome for stabilizing voice coil position. However, above the resonance frequency (f>fS) would generate a dc-component moving the voice coil in the force factor minimum and may cause severe stability problems. For an asymmetric inductance characteristic the dc-component moves the voice coil for any excitation signal in the direction of the inductance maximum. Please note that the dynamically generated DC-components cause interactions between the driver nonlinearities. An optimal rest position of the coil in the gap may be destroyed by an asymmetric compliance or inductance characteristic at higher amplitudes. The module Large Signal Simulation (SIM) allows systematic investigation of the complicated behavior.
Bl symmetry xb(x)
This curve shows the symmetry point in the nonlinear Bl-curve where a negative and positive displacement x=xpeak will produce the same force factor Bl(xb(x) + x) = Bl(xb(x) – x).
If the shift xb(x) is independent on the displacement amplitude x then the force factor asymmetry is caused by an offset of the voice coil position and can be simply compensated.
If the optimal shift xb(x) varies with the displacement amplitude x then the force factor asymmetry is caused by an asymmetrical geometry of the magnetic field and cannot completely be compensated by coil shifting.

Kms Symmetry xc(x)
This curve shows the symmetry point in the nonlinear compliance curve where a negative and positive displacement x=xpeak will produce the same compliance value kms(xc(x) + x) = kms(xc(x) – x).
A high value of the symmetry point xc(x) at small displacement amplitudes x » 0 indicates that the rest position does not agree with the minimum of the stiffness characteristic. This may be caused by an asymmetry in the geometry of the spider (cup form) or surround (half wave). A high value of the symmetry point xc(x) at maximal displacement x» xmax may be caused by asymmetric limiting of the surround.

Parameters at the Rest Position
For accurate system modelling “Large + Cold” parameters are preferable to “Small Signal” parameters because they more closely reflect the parameters in their typical operating range.
Symbol Large + Warm Large + Cold Small Signal Unit Comment Note:for accurate small signal parameters, use LPM module———————–———————————————————————————————————————————————–Delta Tv [referenced]58.60Kincrease of voice coil temperature during the measurement referenced to imported Re(Delta Tv=0K)Delta Tv = Tv-Ta38.000Kincrease of voice coil temperature during the measurementXprot9.39.31.2mmmaximal voice coil excursion (limited by protection system)Re (Tv)4.293.753.75Ohmvoice coil resistance considering increase of voice coil temperature TvLe (X=0)0.220.220.21mHvoice coil inductance at the rest position of the voice coilL2 (X=0)0.530.530.37mHpara-inductance at the rest position due to the effect of eddy currentR2 (X=0)0.840.840.92Ohmresistance at the rest position due to eddy currentsCmes (X=0)426426394µFelectrical capacitance representing moving massLces (X=0)39.2139.2130.10mHelectrical inductance at the rest position representing driver complianceRes (X=0)54.7254.7222.71Ohmresistance at the rest position due to mechanical lossesQms (X=0, Tv)5.705.702.60mechanical Q-factor considering the mechanical system onlyQes (Tv)0.390.340.43electrical Q-factor considering Re (Tv) onlyQts (X=0, Tv)0.370.320.37total Q-factor considering Re (Tv) and Rms onlyfs39.039.046.2Hzdriver resonance frequencyMms19.94419.944g(calculated from imported Bl) mechanical mass of driver diaphragm assembly including voice-coil and air loadRms (X=0)0.8560.8562.361kg/smechanical resistance of total-driver lossesCms (X=0)0.840.840.56mm/Nmechanical compliance of driver suspension at the rest positionKms (X=0)1.201.201.78N/mmmechanical stiffness of driver suspension at the rest positionBl (X=0)7.327.327.32N/A(imported) force factor at the rest position (Bl product)Vas17.761917.761911.9136lequivalent air volume of suspensionN00.2570.2950.262%reference efficiency (2Pi-sr radiation using Re)Lm86.386.886.3dBcharacteristic sound pressure levelSd122.70122.70122.70cm²diaphragm area

Frequency Response
Frequency Response data is generated using Klippel’s Transfer Function Mueasurement module in conjunction with Klippel’s In-Situ Compensation module. This pairing enables me to generate anechoic-quality measurements in the farfield with a resolution of response down to 20Hz increments; far better than splicing nearfield with gated farfield data (the latter of which is often limited to 200Hz or 300Hz resolution and will not show high-Q resonance in the midrange and bass frequencies). Data is represented at 2.83v/1m.

On-Axis Linearity and Impedance

On and Off-Axis Frequency Response

Total Harmonic Distortion (THD) and Compression:
Klippel’s 3D-DISTORTION MEASUREMENT (DIS) module is used to calculate the Total Harmonic Distortion and Compression for this drive unit.
Distortion and Compression measurements were completed in the nearfield (approximately 0.3 meters). However, SPL provided is relative to 1 meter distance.
Harmonic Distortion and Compression: What does this data mean? (click me for info)

Maximum Long Term SPL (Multitone Distortion Testing)
Klippel’s Multi-Tone Measurement (MTON) module is used to calculate the maximum SPL for this drive unit.
The below data provides the metrics for how Maximum Long Term SPL is determined. This measurement follows the IEC 60268-21 Long Term SPL protocol, per Klippel’s template, as such:

• Rated maximum sound pressure according IEC 60268-21 §18.4
• Using broadband multi-tone stimulus according §8.4
• Stimulus time = 60 s Excitation time + Preloops according §18.4.1

Each voltage test is 1 minute long (hence, the “Long Term” nomenclature).
The thresholds to determine the maximum SPL are:

• -20dB Distortion relative to the fundamental
• -3dB compression relative to the reference (1V) measurement

When the speaker has reached either or both above thresholds, the test is terminated and the SPL of the last test is the maximum SPL. In the below results I provide the summarized table as well as the data showing how/why this SPL was deemed to be the maximum.
This measurement is conducted twice:

• First with a 20Hz to 20kHz multitone signal
• Second with a limited bandwidth multitone signal

The reason for the two measurements is because it is unfair to expect a small bookshelf speaker to extend low in frequency. Applying both will provide a good idea of the limitations if you were to want to run a speaker full range vs using one with a typical 80Hz HPF. And you will have a way to compare various speakers’ SPL limitations with each other. However, note: the 80Hz signal is a “brick wall” and does not emulate a typical 80Hz HPF slope of 24dB/octave. But… it’s close enough.
You can watch a demonstration of this testing via my YouTube channel:

Test 1: 20Hz to 20kHz
Multitone compression testing. The red line shows the final measurement where either distortion and/or compression failed. The voltage just before this is used to help determine the maximum SPL.

Multitone distortion testing. The dashed blue line represents the -20dB (10% distortion) threshold for failure. The dashed red line is for reference and shows the 1% distortion mark (but has no bearing on pass/fail). The green line shows the final measurement where either distortion and/or compression failed. The voltage just before this is used to help determine the maximum SPL.

Test 2: 80Hz to 5kHz
Multitone compression testing. The red line shows the final measurement where either distortion and/or compression failed. The voltage just before this is used to help determine the maximum SPL.

Multitone distortion testing. The dashed blue line represents the -20dB (10% distortion) threshold for failure. The dashed red line is for reference and shows the 1% distortion mark (but has no bearing on pass/fail). The green line shows the final measurement where either distortion and/or compression failed. The voltage just before this is used to help determine the maximum SPL.

The above data can be summed up by looking at the tables above but is provided here again:

• Max SPL for 20Hz to 20kHz is approximately 101dB @ 1 meter. The compression threshold was exceeded above this SPL.
• Max SPL for 80Hz to 5kHz is approximately 105dB @ 1 meter. The compression threshold was exceeded above this SPL.

Bottom Line
T/S Parameters and Linear Excursion:

• My measured linear excursion is limited to about 4.7mm one-way. This is based on 10% distortion limits.
• For those who do not know, the GB60 was tested a couple years ago by Justin Zazzi (link here) and found to have linear xmax limits of 6.3mm in the lowest of (3) units tested. However, that test used the more relaxed displacement limits for 20% distortion, whereas I use 10% (20% is typically relegated to subwoofer use only). Changing my limits to match that of the 20% THD limit yields 5.9mm one-way linear excursion which is more in-line with what Justin measured but still not as high as his highest of the (3) samples of 7.3mm one-way linear excursion.
• I spoke with Andy Wehmeyer of AudioFrog noting the difference in my results vs the specification. He clarified the GB60 specification for 9mm linear xmax is based on the Voice Coil - Gap Height + 15% method (described earlier). This was Andy’s reply: “Coil height is 20.45. Gap height is 5. Overhang is 7.75. Overhang + 15% is 8.9125."
• As I mentioned earlier, manfucturers specify these limits differently. This is exactly why I like using Klippel’s product because it provides apples-to-apples comparisons per IEC 62458 standard. You don’t have to assume or guess as to how the spec is derived. You know. And it is an IEC standard.

Frequency Response:

• The average sensitivity is measured at about 88.6dB from 300Hz to 1000Hz.
• On-axis response linearity is ±1.5dB within 90-3.5kHz (excluding the ~0.5dB bump around 2.7kHz) with some mild bumps here and there.
• On-axis response linearity is ±3.0dB within 78-4kHz.
• Breakup is limited to approximately +12dB above the mean SPL at 5-7kHz. A proper LPF will help mitigate this effect; a steeper order filter will make more of a difference.
• Off-axis response shows nice linearity until about 4kHz but would be better served crossing below 3kHz to avoid beaming as well as the breakup mode mentioned above.

Distortion and Compression:

• Distortion within the nominal passband of 80-2kHz is reasonably low. Above 200Hz the distortion is mostly below 1%. Below 200Hz the distortion increases to about 5% THD. A proper HPF will remedy most of this (AudioFrog recommends a bandpass range of 70Hz to 3.5kHz with at least 12dB/octave slopes)
• Compression is mostly under 0.25dB at 12vRMS input, but at approximately 60Hz and 1.1kHz the compression increases to about 0.75dB.
• Full bandwidth (20Hz - 20kHz) multitone distortion gets above about 1% distortion thresholds below 200Hz, and above 1kHz at 101dB @ 1 meter equivalent. That’s low.
• When supplied an 80Hz to 5kHz signal the multitone distortion is well below 1% THD with a max SPL of 105dB.

I use this speaker in my own car. Its compact size combined with high power handling, “made for car audio” design and relatively good linear excursion make it a nice choice if you are trying to eek out the last n-th percentage of performance in your car audio system. Used with a proper high-pass filter (mine are crossed at 80Hz/48dB L-R slope), they have plenty of low distortion output.
AudioFrog recommends a bandpass range of 70Hz to 3.5kHz with ≥12dB/octave slopes. Based on this data, that’s reasonable. Output desires and implementation in to your own system will yield where to fall within those bounds.


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[Lonestar1027]

Thanks for the data! I installed these and a set of GB15s in my truck last year and couldn't be happier. Glad to see the data supports a solid speaker.

 

[Hemi-Demon]

Great work! I have always been curious about this brand, considering the rather high price per driver. Rhe recommendation for a 3.5K cut, with that off axis response is being real generous, considering these would be placed in the car door. I bet these would be far better in a 3 way, cut at 2K.

 

[Lonestar1027]

I spoke with Andy for his recommendations and with the larger GB15, he recommended a crossover around 1800. I'm using a 4th order LR and crossing somewhere around there in a 2 way.

 

[elberoth]

Anyone using those as 2-way ?

I'm complememting using those in a hi-end 2-way system (no place to put a sub in a car, unfortunately).

 

[Vini darko]

I have much to learn about speakers barely understood any of this review.