Home Theater Forum and Systems banner

Status
Not open for further replies.
1 - 20 of 45 Posts

·
Elite Shackster
Joined
·
1,468 Posts
Discussion Starter #1


Welcome to the Subwoofer Tests forum!

This is the place to get accurate and comparable info how different subwoofers measure against each other. Many magazines (both paper and online) also measure subwoofers, but their time is limited and not too many subwoofers gets measured per year. It is also much harder to get one’s favourite subs measured in a magazine, not to mention DIY subs. Also different testing methodologies used by reviewers/magazines often makes comparing results almost impossible. But people are always interested to find out how different subwoofers measure against each other, and also because most specifications released by subwoofer manufacturers themselves are somewhat “unreliable” and hard to compare, I decided to start helping people (and of course to satisfy my own enquiring mind) to find out how they compare using accurate gear and more advanced and versatile tests than many magazines take (sad but true fact).

Since I don’t have access to an anechoic chamber, I'm performing my tests outdoors on a big field with no nearby large reflective surfaces (usually ~10 m distance is considered adequate for testing frequencies down to 10 Hz). Naturally testing ourdoors has its cons like random background noise, wind, ambient temperature, air humidity and atmospheric pressure. All these are unfortunately uncontrollable variables and therefore can and will affect on measurements. The key thing is try to keep their influence as small as possible.

Another great source for accurate subwoofer measurements can be found at AV Talk forum. It should be noticed that our testing methods and rigs are not identical, meaning one shouldn’t compare our results directly.

Naturally measurements are only one part of the subwoofer evaluation. If the particular subwoofer doesn’t sound good in your own room and to your ears, it doesn’t matter how good it looks on paper or sounds to some random reviewer. Measured performance and perceived performance/sound are of course tightly connected, which means that one can have a pretty good picture how a specific subwoofer will perform/sound by just looking at the measurements. Of course the final judgement should be done in your own room with your own ears.

One should not forget the importance of the final in-room frequency response and the integration between the sub and the mains, which IMO are the most important factors (if the max output level is high enough for your needs) when pursuing good bass reproduction. Especially the importance of good/strong mid-bass output from the main speakers is often underestimated. Pair of 5.25” or 6.5” woofers won’t be able to keep up with a strong subwoofer even at 15 dB below reference or higher (in a 15-25 m^2 room), when a typical 80 Hz crossover frequency is used. 80-300 Hz range must be balanced with the range below at all levels in order to have a solid and strong sounding bass. Subwoofer should be just the dot on the i on a system, not the dominating component which around the system is built to.
 

·
Elite Shackster
Joined
·
1,468 Posts
Discussion Starter #2
Testing methodology

Test Gear:

Laptop: Acer Aspire 5024WLMi (AMD Turion 64-bit 3000+, 512MB DDR, 80GB HD)
Soundcard: Creative Labs SB Live! 24-bit External
Microphone: IBF-Akustik EMM-8, serial number 31070504
SPL-calibrator: IBF-Akustik SC-1, serial number 0503003
Programs used: TrueRTA, SpectraPro, ETF5



I measured the frequency response of the soundcard using RMAA 5.5 program and it was +/- 0.3 dB 10 Hz - 15 kHz. Without a correction file, the frequency response of the microphone is +/- 1 dB 10 Hz - 20 kHz or +/- 0.3 dB 10 Hz - 5 kHz. With a correction file both soundcard and microphone frequency responses are corrected to flat. Microphone was calibrated on 31.7.2005 by IBF-Akustik, the reference microphone used was B&K 4133. The measured frequency response of the pre amp is +/- 0.1 dB 10 Hz - 20 kHz. The frequency response of the whole measuring chain is therefor better than +/- 1 dB 10 Hz - 20 kHz.



The SPL-calibrator gives a 94 dB @ 1 kHz signal with a +/- 0.5 dB accuracy. The calibrator was calibrated on 17.8.2005, the reference calibrator used was Quest CA-22. I did the calibration for the absolute SPL indoors at the ambient temperature of around 24 degrees Celsius. The SPL-calibrator was originally calibrated at this temperature. The atmospheric pressure was around 1035 hPa (Finnish weather service), which is very close to the 1007 hPa where the calibrator was originally calibrated. The atmospheric correction for the calibrator is around 1 dB / 200 hPa. The microphone is practically independent for variations in temperature and atmospheric pressure. A hairy windshield was used around the microphone, which helped to eliminate most wind noise (wind noise mainly occurs at very low frequencies).

 

·
Elite Shackster
Joined
·
1,468 Posts
Discussion Starter #3
Frequency response

Round 3 ->
Frequency response was measured crossover at its max and min settings and also bypassed if available. 10 second sine sweep from 200 Hz to 10 Hz was used. Level was matched at 90 dB at 50 Hz at 2 m distance. Mic was on the ground and the distance was measured from the acoustic center of the subwoofer. For sealed subs acoustic center lies right in front of the voice coil (front baffle was used for consistency). For vented and subs with passive radiator(s) acoustic center lies somewhere in between the driver(s) and the port(s)/PR(s). But because most of the output (vent/PR affects only near tuning frequency) comes from the driver, I measured the 2 m distance from the (active) driver also with vented/PR subs (using a directional line from the mic). If the sub had a port or PR on the back, the sub was rotated so both were equidistant to the mic. In some cases I also measured all combinations and choosed the one which gave better FR. Being consistant and fair to all subs regarding this matter is extremely difficult because of the various driver/port/PR combinations subwoofers have. If all subs were single driver, sealed, front-firing subs, there wouldn’t be any problems. The key thing was to measure the 2 m distance from the point where the most of the output comes from, but also keep the other sources (if there was one or more) as close as possible to the 2 m distance. Subs were kept in their normal operating positions, for example base plates weren’t removed or subs turned on their sides etc. Same orientation/position was used during all measurements. All graphs are using a full 1/24 octave resolution. +/- 3 dB points were calculated using maximum or bypass (if available) crossover setting.

Round 1 and Round 2
Subwoofer’s frequency response was measured with TrueRTA’s Quick Sweep signal. The signal is a short-duration, digitally synthesized logarithmic sine sweep from 0 Hz to 24 kHz (when using a 48 kHz sampling frequency). It takes only a few hundred ms’s to sweep the normal subwoofer operating range. Unfortunately this method isn’t as accurate as the normal sine sweep (which was the reason why I switched to it) under relatively loud background noise. Therefore small variations can be expected between these methods. +/- 3 dB points were calculated using maximum or bypass (if available) crossover setting.




+/- 3 dB points: 18.5 Hz - >200 Hz

The best possible anechoic FR depends of the room/space where the subwoofer will be put in. Usually gradually sloping (6-12 dB/oct.) response gives the flattest in-room response due to room gain. The bigger the room the lower the response can be flat without having problems with elevated low end response. Below can be seen two frequency responses. The first one is more suited for small/mid-sized rooms while the second one is more suited for larger spaces with less low frequency room gain. If the second sub will be put in a small (rigid walled) room, frequency response will be strongly elevated below 30-40 Hz, which will sound too "bottom heavy”.



 

·
Elite Shackster
Joined
·
1,468 Posts
Discussion Starter #4
Max output level and power compression

Maximum long term output level was measured using a 30 second linear sine sweep from 100 Hz to 10 Hz. First sweep was level matched at 90 dB at 50 Hz. Drive level was raised by 5 dB after each sweep. Sweeps were taken up to the point where the output level exhibited clear compression. This is a very demanding test and the results should not be compared to other tests with faster sweeps etc. Power compression graph is showing the relative compression to the 90 dB sweep.

Round 3 ->
Sweeps were taken up to the point where the output level stopped rising (5 dB or more of compression). I also tried 2 dB as the last step, but usually it didn’t help.

Round 1 and Round 2
Maximum long term output level was evaluated with a 45 second linear sine wave sweep from 100 Hz to 10 Hz. Sweeps were conducted at progressively louder (3 dB increments) levels. If the sound of the subwoofer became too ”stressed”, the sweep was stopped. The levels shown are the very maximum levels, meaning the next sweep was totally compressed.


Max output should be as high as possible over as wide frequency range as possible. Power compression should be as low as possible.



 

·
Elite Shackster
Joined
·
1,468 Posts
Discussion Starter #5
THD sweeps

Round 3 ->
The THD was measured at the same time with max level sweeps. The frequency resolution is around 1.4 Hz. Note that the SPL isn’t absolute but follows the frequency response and possible power compression instead! One should also notice that if the amplitude of the fundamental goes very low, the accuracy of the THD measurement is reduced. For example if the fundamental is at 70 dB, the random background noise at around 30 dB causes 1% THD. Measuring THD on the fly is more demanding for a subwoofer than measuring with single 2-3 second sine waves. Therefore the results are not perfectly comparable with each other.

Round 1 and Round 2
Subwoofer’s THD levels measured with single sine waves. We adjusted the gains on subwoofers so that we got 90 dB (+/- 0,5 dB) at 50 Hz - this was our starting point. Then we run the “sweeps” using 5 dB increments up to 105 dB. The program measures the THD level at 10 frequencies between 100 Hz - 16 Hz. The program interpolates the values in between. Each tone plays around 2.5 seconds, so the whole ”sweep” takes around 25 s. Notice that the absolute SPL is correct only at 50 Hz. For other frequencies one must look at the power compression sweeps and find the correct SPL.


THD levels should be as low as possible over the whole frequency range. THD at low frequencies (below 40-50 Hz) isn’t as harmful or audible as THD at high frequencies (above 40-50 Hz).

 

·
Elite Shackster
Joined
·
1,468 Posts
Discussion Starter #6
Harmonic distortion by component

The individual levels for harmonics from second (H2) to sixth (H6) were manually plotted at 15 Hz, 20 Hz, 25 Hz, 32 Hz, 40 Hz, 50 Hz, 63 Hz, 80 Hz and 100 Hz. Some subwoofers didn't have enough output at the lowest frequencies, so those frequencies couldn't be plotted. The Y-axis scale is logarithmic instead of linear. The chosen sweep level wasn’t kept the same for all subs because of different max output levels. Usually the second highest sweep was chosen. That’s why you shouldn’t compare the absolute levels between different subwoofers, only relative levels between harmonics.

All harmonic components should be as low level as possible (at all levels). Due to masking effect, high-order harmonics are more audible than low-order harmonics, meaning same THD level with different kind of component distribution will sound different. It is also said that even-order harmonics (H2, H4, H6 etc.) are less likely to be perceived than odd-order harmonics (H3, H5, H7 etc.).

 

·
Elite Shackster
Joined
·
1,468 Posts
Discussion Starter #7
Group delay

Mathematically group delay is the negative derivative of the phase response. Phase response was measured using the ETF-5 program which calculates it from the measured impulse response. Since frequency response and phase response correlate with each other, also FR and GD correlate with each other. Meaning flat or gently sloping FR results in low GD. That's why the final in-room frequency response should be as flat as possible. There are no studies defining the audibility of GD at low frequencies, but the threshold is suggested to be in the range of 1-1.5 cycles. Therefore I have included the 1 cycle curve on the graphs.

Ideally GD should be as low as possible, or at least gradually/gently rising.

 

·
Elite Shackster
Joined
·
1,468 Posts
Discussion Starter #8
Spectral decay

Any sudden change (especially if high-Q) in frequency response always causes ringing (sometimes also referred as ‘stored energy’), and that’s why a smoothly sloping FR down to as close to direct current (0 Hz) as possible causes the least ringing and results in fast decay at all frequencies. Also same applies to the upper end of the frequency response. A steep low pass filter causes a rise in group delay and ringing near cut-off/crossover frequency. Spectral decay graph shows the frequency response after the input signal has stopped. I used a 200 ms gate time and a 40 ms slice spacing up to 200 ms. The spectral decay is evaluated down to -24 dB level from the highest amplitude point. The measurement itself was taken at around 90 dB level, which allowed a good S/N ratio but didn’t push subwoofers into compression. The same info is often presented with a ‘waterfall graph’.

Spectral decay should be as fast as possible over as wide frequency range as possible. Ideally the graph should only have one frequency response slice. Ringing at low frequencies isn’t as harmful or audible as ringing at higher frequencies.

 

·
Elite Shackster
Joined
·
1,468 Posts
Discussion Starter #9
Spectral contamination

”Spectral contamination is a graphic analysis of cross-modulation products (’self-noise’) produced by a system excited by a multi-frequency signal. Multi-tone tests are far more representative of the rigors of music or speech reproduction than traditional stimuli. The resulting non-linear distortion products (the noise between the tones, which are distortion products generated at frequencies where no excitation energy is present) correlates with subjective perceptions of quality, such as ’clarity’ and ’coloration’.” (Spectral Contamination Measurement, by Deane Jensen and Gary Sokolich, AES preprint 2725, 85th AES Convention, 1988).

This is a new test which hasn’t been used by subwoofer reviewers before. It has been used with full range speakers, but not with subwoofers. Since there wasn’t any info how this test should be performed on subwoofers, I and my good peer Ed Mullen had to start things from the scratch. Based on the SC tests performed on speakers, 10 stimulus frequencies ranging from 20 Hz to 77 Hz were chosen. A multiplier of 1.1618 was used between stimulus frequencies. 8 kHz sampling frequency and a 64k FFT size resulted in 0.122 Hz frequency resolution. I measured SC starting at 90 dB and then raised the level by 5 dB as long as the sub could produce a full 5 dB step. Three highest SPL screens are showed. Frequency range goes from 15 Hz to 200 Hz.

Spectral contamination test will show all non-linear distortions and mechanical ‘self-noises’ (panel vibrations, driver motor noise, port noise etc.). Also the limiting circuits often became very audible during this test. Many subwoofers started sounding like rumbling and popping V2 engines instead of steadily humming subwoofers. Some subs were already struggling at the lowest test level of 90 dB (combined power of all 10 tones, each individual tone at 80 dB). Interpretting the results isn’t very easy because as said earlier, these are the first tests ever published for large number of subwoofers. It is pretty clear that subwoofers with high output capabilities have also low overall spectral contamination at both low and high test levels. Whether this test will show anything usefull beoynd the ‘old tests’ is yet to be examined.

Ideally spectral contamination graph should only have the ten stimulus frequencies and nothing else between or around them (some amount of background noise and system noise is of course always present). The lower level the 'self-noise' is, the better.

 

·
Elite Shackster
Joined
·
1,468 Posts
Discussion Starter #10
CEA-2010 standard

The CEA-2010 (Consumer Electronics Association) is a standard which defines a method for measuring the performance of powered subwoofers. It present tone bursts centered at 1/3 octave frequencies in 20 - 63 Hz range. I also measured some additional frequencies below and above that range. The test is performed by increasing the input level until the SPL is limited by prescribed frequency-dependent distortion threshold (staircase function) or compression (won’t go any louder). The test signal is 6.5 cycle long, shaped (Hann window) sine wave burst. So the length of the tone burst stimuli goes longer as the frequency decreases. The staircase function defines the allowed distortion level for each harmonic. The staircase function allows higher SPL for harmonics closer to the fundamental, and lower for harmonics further to the fundamental. This is because human hearing has a decreased tolerance for distortion components at higher harmonics (based in part on studies of distortion audibility and masking [Shorter (BBC 1950’s), Harman]). Allowed SPL by harmonic (compared to the fundamental): 2nd -10dB; 3rd -15dB; 4th and 5th – 20dB; 6th – 8th -30dB; 9th and above -40dB. CEA-2010 rating is defined by calculating an average distortion or compression-limited SPL (GP @2m, dB RMS values) for each of two one-octave low-bass performance ranges - Ultra Low-Bass: 20, 25, 31.5Hz and Low-bass: 40, 50, 63Hz. If max SPL is not measurable or the S/N ratio becomes too low through some portion of ultra-low band, then the measurer may choose to state “NA” for that band.




Intermodulation distortion

IMD is also one of the variables which hasn’t been used in subwoofer reviews before. By definition the intermodulation distortion occurs when the non-linearity of a device or system with multiple input frequencies causes undesired outputs at other frequencies. IMD is measured by inputting two stimulus frequencies using certain spacing between them, a few common stardards are 250 Hz / 8020 Hz (with a 4:1 amplitude ratio), 60 Hz / 7000 Hz (with a 4:1 amplitude ratio) and 19 kHz / 20 kHz. These stardards were designed for measuring electrical devices such as amplifiers. Naturally they are not suitable for subwoofers, so again I and my peer Ed Mullen had to start from the scratch with this test too.

Eventually we ended up using fundamentals (or carrier signals) at 30 Hz and at 72 Hz. 30 Hz isn’t too low for most subwoofers to produce and 72 Hz is still below the commonly used 80 Hz crossover frequency. This combination also doesn’t share any THD/IMD harmonics. The Excel spreadsheet we made calculates all IMD harmonics up to 6th order, which was more than enough because most subs didn’t have any 5th or 6th order IMD harmonics (already below background noise floor at ~30-40 dB, system's noise floor is at much lower level). There isn’t a very large database yet, but these first results suggest that subwoofers with high output capabilities and low THD levels also have low IMD levels. One should carefully examine if IMD is even worth measuring with subwoofers due this relationship.

Ideally IMD should of course be as low as possible at all levels.

IMD tests haven't been performed since Round 3.
 

·
Registered
Joined
·
27 Posts
Re: Finnish Subwoofer Tests Explained

Hello Ilkka,

Thanks very much for the excellent explanation of your measuring techniques.

I have been using ETF5 for several years and recently added TrueRTA. I use stereo ACI Maestros to help out the L/R mains and a Servodrive ContraBass for the dedicated LFE channel. I have a few questions. All my measurements are done indoors.

10 second sine sweep from 200 Hz to 10 Hz was used.
How do you do a reverse sweep with TrueRTA? For a 10 second sweep do you just set the sampling frequency to 24k and then use QuickSweep? Did you ever compare the ETF low frquency sweep (short or long) against TrueRTA? Curious as to why you picked TrueRTA over ETF to do the frequency measurements.

Mathematically group delay is the negative derivative of the phase response. Phase response was measured using the ETF-5 program, which calculates it from the measured impulse response. Since frequency response and phase response correlate with each other, also FR and GD correlate with each other. Meaning flat FR results in low GD.
My experience with ETF5 and its Minimum Phase graph has been very inconsistent. I can do five measurements in row and each one will have a different phase graph but the frequency responses will be identical. I asked Doug Plumb (ETF designer) about how the phase graph is calculated and he told me not to bother with it (his new program does not have a phase graph). I had hoped to use the phase graph to help phase align the subs and mains but it has not proved very useful.

I wonder if it has to do with measuring indoors with its complex reflections. Have you every compared the phase graphs outdoors vs. indoors? Do you have the same problem indoors that I have?

Thanks for your help.
George
 

·
Elite Shackster
Joined
·
1,468 Posts
Discussion Starter #12
Re: Finnish Subwoofer Tests Explained

Hello Ilkka,

Thanks very much for the excellent explanation of your measuring techniques.

I have been using ETF5 for several years and recently added TrueRTA. I use stereo ACI Maestros to help out the L/R mains and a Servodrive ContraBass for the dedicated LFE channel. I have a few questions. All my measurements are done indoors.
Hi George,

I think I recall you from AVS. You know there aren't that many people who own a dual Maestro & ContraBass system. :hail:

How do you do a reverse sweep with TrueRTA? For a 10 second sweep do you just set the sampling frequency to 24k and then use QuickSweep?
I use an external signal generator to produce both 10 second and 30 second reversed sweeps. So TrueRTA acts only as a "receiver". TrueRTA can not do anything else than the (forward) quick sweep.

Did you ever compare the ETF low frquency sweep (short or long) against TrueRTA? Curious as to why you picked TrueRTA over ETF to do the frequency measurements.
I have compared them and they produce pretty much identical results. I choose the TrueRTA because it's much better with this kind of measurements. The ETF can only show one measurement at a time, it doesn't show the absolute SPL, it has a linear frequency range instead of log...etc. ETF is good for some things but not for all.

My experience with ETF5 and its Minimum Phase graph has been very inconsistent. I can do five measurements in row and each one will have a different phase graph but the frequency responses will be identical. I asked Doug Plumb (ETF designer) about how the phase graph is calculated and he told me not to bother with it (his new program does not have a phase graph). I had hoped to use the phase graph to help phase align the subs and mains but it has not proved very useful.

I wonder if it has to do with measuring indoors with its complex reflections. Have you every compared the phase graphs outdoors vs. indoors? Do you have the same problem indoors that I have?
The phase measurement is pretty accurate and consistent/repeatable, but you need to know a few things. Since the ETF uses the impulse response as a "base" for its every graph, you need to make sure the impulse itself is aligned correctly. Copy/paste from the help file:

The impulse response from the system must be aligned properly in time for the loudspeaker/mic distance and the phase response. Move the Impulse Time Alignment slider one step back or forward and press "Update Graph". Repeated clicking on "Update Graph" will cause the impulse response to move in animation.

This is used to align the measured impulse so that the first part of it aligns with the sound card impulse response. In most cases ETF will have these aligned perfectly, but is some rare cases an adjustment is necessary.
Although I would say that the last sentence doesn't hold water. In some rare cases an adjustment isn't necessary. ;)

When measuring outdoors and only subwoofers, getting the peaks aligned is pretty simple. But when measuring indoors with lots of reflections and output coming from both speakers and subwoofer(s) (usually sub's peak lags a bit), it isn't that easy. So it doesn't surprise me at all that you got different results each time.

I haven't used ETF that much indoors. I usually tweak the phase by basic FR measurements and by ear. I can try some indoor phase measurements and post the results here if you like.
 

·
Registered
Joined
·
75 Posts
"Maximum long term output level was measured using a 30 second linear sine sweep from 100 Hz to 10 Hz. First sweep was level matched at 90 dB at 50 Hz. Drive level was raised by 5 dB after each sweep."

How long was the time between sweeps, and was it consistent? It would affect voice coil temp and thus the amount of compression.

"But we do sence the second derivative, acceleration, without any help."

Actually, it takes one more derivative, called "jerk".

The best example is the constant acceleration of gravity.

Also notice that when taking off on a plane, the noticeable part is when the thrust is being increased, but when it reaches its maximum, it's less perceptible because it's constant.
 

·
Elite Shackster
Joined
·
1,468 Posts
Discussion Starter #14
How long was the time between sweeps, and was it consistent? It would affect voice coil temp and thus the amount of compression.
The time between the sweeps was around 60 seconds. I didn't use a stopwatch to measure it, but it was fairly consistent. And actually it doesn't matter that much because at high output levels voice coil heats up really fast. I remember Dan Wiggins quoting that the VC in AA Tumult takes around 2 seconds to go above 200 deg. C with 1000W input power. So it doesn't matter that much whether the starting temperature is 30 C or 60 C.

Actually the heated up air inside the enclosure is a bigger problem. It would be interesting to measure Bossobass' "cooling system" to find out how much it actually helps.

Actually, it takes one more derivative, called "jerk".

The best example is the constant acceleration of gravity.

Also notice that when taking off on a plane, the noticeable part is when the thrust is being increased, but when it reaches its maximum, it's less perceptible because it's constant.
Yes, we do sense a changing acceleration better than a constant one. What I wrote is based on Mr. Geddes' research. IMO group delay is one of the least important variables in subwoofer's performance.
 

·
Registered
Joined
·
75 Posts
"And actually it doesn't matter that much because at high output levels voice coil heats up really fast. I remember Dan Wiggins quoting that the VC in AA Tumult takes around 2 seconds to go above 200 deg. C with 1000W input power. So it doesn't matter that much whether the starting temperature is 30 C or 60 C."

Good point.

I've quoted that same thing from Dan several times when people start talking about the continuous power capacity of some subwoofer.

"Actually the heated up air inside the enclosure is a bigger problem. ♠"

Yes. I wonder why more car audio subs don't burst into flames with a kW or two being fed into a small, sealed, well-insulated box.

Maybe the crest factor is high enough that the average power isn't nearly so high.

Or maybe they do; I don't follow car audio, I've just read references to what they're up to.
 

·
Registered
Joined
·
109 Posts
"And actually it doesn't matter that much because at high output levels voice coil heats up really fast. I remember Dan Wiggins quoting that the VC in AA Tumult takes around 2 seconds to go above 200 deg. C with 1000W input power. So it doesn't matter that much whether the starting temperature is 30 C or 60 C."

Good point.

I've quoted that same thing from Dan several times when people start talking about the continuous power capacity of some subwoofer.

"Actually the heated up air inside the enclosure is a bigger problem. ♠"

Yes. I wonder why more car audio subs don't burst into flames with a kW or two being fed into a small, sealed, well-insulated box.

Maybe the crest factor is high enough that the average power isn't nearly so high.

Or maybe they do; I don't follow car audio, I've just read references to what they're up to.
Hi Noah,

The high power boxes don't burst into flames for the same reason the drivers heat up so fast. Most drivers are very in-efficient at radiating heat away from the voice coil. You can't heat up the enclosure unless there is a good means of transferring the heat out of the VC. Over time you can heat things up in pro use where averages are high, but with typical crest factors, a dark colored subwoofer sitting in the shade vs sun for extended time probably has a greater impact.

PS - It looks like I might make it out your way again late September.
 

·
Premium Member
Joined
·
74 Posts
Multi-Frequency Testing

Hi Ilkka,

Don't all of your tests including the CEA test use a spectrally pure, single frequency input at a time?

The reason I ask, it seems it would be useful to also have a different test that measures the ability of a sub (or any speaker for that matter) to accurately reproduce multiple frequencies at the same time. One of the things that floored me when I got my DD-15 several years ago was that I could hear musical (intended) harmonics and textures that I could not hear with my previous subs.

This relates to something I've always wondered about ported subs. At port tuning, the acoustic load on the driver increases dramatically thereby reducing cone excursion. This is great if the input only has this one frequency while the port creates most of its output, but what happens if there are other frequencies present in the input signal? Won't they be affected (attenuated?) by the non-linear acoustic loading over frequency? Is the lack of any acoustic loading in IB/OB subs the reason why many claim they have superior SQ?

As a proposed test method, it will be necessary to create a test signal that contains a constant frequency tone in the range of 50-80 Hz (referred to as the "dut" tone) combined with an equal amplitude swept tone that varies from 1/2 octave below the dut tone to lower than box tuning. The spectral content/amplitude of the dut tone can then be compared against several different frequency points of the swept tone. As a reference data point, the spectral plot of the dut tone by itself should also be included. In order to minimize other distortion sources that might obscure the spectral plots, the test should be restricted to a moderate starting SPL of say 90 db with the input amplitude of both tones held constant during the sweep.

Any feedback is appreciated as I am half trying to propose something, and half trying to learn.

Thanks,
Darrell
 

·
Registered
Joined
·
2,398 Posts
Re: Multi-Frequency Testing

darrell, he used to include just such a measurements in his tests. The results can be found here. Ultimately I believe he stopped because the results showed a very strong correlation to regular THD and spectral contamination measurements and they didn't offer enough unique information on their own. You will notice that ported subs have nothing inherently holding them back from having very low IMD.

As for your theory about behavior around tuning, why wouldn't you assume the same type of thing would happen in a small sealed where pressures can get very high at low frequencies?
 

·
Premium Member
Joined
·
74 Posts
Hi Steve,

Thanks for the response and link. I missed the IMD measurement sticky before, but after reading it, it still doesn't exactly address my concerns. Instead of measuring IM content for 30 Hz and 72 Hz fundamentals, I wanted to measure the affects on the upper fundamental as the lower fundamental is varied down to box (port) resonance and lower. When I say the affects on the upper fundamental, I mean how is its magnitude and any spectral contamination affected as the lower fundamental is varied.

The idea for this multi-frequency test actually came about as I was reading recent posts on the AVS "Lord of the Bass" thread. Instead of questioning whether or not a linear air spring minimizes THD in smaller sealed subs, I was wondering about the opposite affect in ported subs. Since a ported sub presents a non-linear and much higher magnitude acoustic load to its driver (at port resonance) compared to a sealed sub, I wanted to see if there was a test that could isolate and measure this affect (if any). Logically since a ported sub's driver is contributing virtually no output at port resonance (if that's the only frequency present), a second "victim" frequency must be introduced to determine if this peak acoustic load is having any detrimental affects on the driver. I proposed one method, but there may be others that are better.

It may be that a well implemented ported sub closely approximates a linear system in spite of its non-linear acoustic load. If true, the principle of frequency superposition says that individual frequencies will not be affected by one another, and everything is good. I just wanted to see if there is a direct way to test this.

Thanks,
Darrell
 

·
Registered
Joined
·
75 Posts
"At port tuning, the acoustic load on the driver increases dramatically thereby reducing cone excursion. "

An interesting question.

The loading at Fb ought not restrict excursion at higher freq, and would have certainly been noticed in ported systems.

The loading at Fb isn't acoustic, it's mechanical pressure loading (hydraulic, if you will) from the air in the port compressing the air in the box.

If it were increased acoustic loading, the efficiency would be higher. Peaked response from underdamping doesn't count, the same can happen in a poorly designed sealed box.

Mark,

Thanks for the response, I missed it til just now!
 
1 - 20 of 45 Posts
Status
Not open for further replies.
Top