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Intermodulation tests

svart-hvitt, modified 5 Years ago.

Intermodulation tests

Padawan Posts: 35 Join Date: 5/6/16 Recent Posts
Do Genelec active speakers pass intermodulation tests?

Take a look at the web page below:

http://xiph.org/~xiphmont/demo/neil-young.html

Scroll down to the section "Intermod tests".

Here, you have the following options:
************************
Intermod Tests:

30kHz tone + 33kHz tone (24 bit / 96kHz) [5 second WAV] [30 second FLAC]
26kHz - 48kHz warbling tones (24 bit / 96kHz) [10 second WAV]
26kHz - 96kHz warbling tones (24 bit / 192kHz) [10 second WAV]
Song clip shifted up by 24kHz (24 bit / 96kHz WAV) [10 second WAV]
(original version of above clip) (16 bit / 44.1kHz WAV)
************************

The web page states:

"Assuming your system is actually capable of full 96kHz playback [6], the above files should be completely silent with no audible noises, tones, whistles, clicks, or other sounds. If you hear anything, your system has a nonlinearity causing audible intermodulation of the ultrasonics. Be careful when increasing volume; running into digital or analog clipping, even soft clipping, will suddenly cause loud intermodulation tones".

So what are Genelec active monitors supposed to show?

Some noise (on higher volumes) or "completely silent"?

(I have run the test on my 8351a, with USB out to AES-EBU input, using Mac OS Sierra, played back from Safari browser. Volume controller is 9310a via GLM 2).
ilkka-rissanen, modified 5 Years ago.

Re: Intermodulation tests

Yoda Posts: 2564 Join Date: 3/23/09 Recent Posts
Hi,

We have performed intermodulation tests but not with these tests signals and not at those frequencies. Typically intermodulation that can be witnessed by the listener is caused by signals within the audible bandwidth i.e. below 20 kHz. Of course higher frequencies can and will cause intermodulation effects at lower frequencies but those can be typically triggered only by special test signals, such as those linked.

What were your observations/results?
svart-hvitt, modified 5 Years ago.

Re: Intermodulation tests

Padawan Posts: 35 Join Date: 5/6/16 Recent Posts
Ilkka,

my tests gave audible effects running all the tone tests (I am uncertain about the song clip, though).

In the book "Audio in Media" Stanley R. Alten writes:

"Intermodulation distortion usually occurs in the high frequencies because they are weaker and more delicate than the low frequencies. Widenes and flatness of frequency response are affected when IM distrotion is present. In addition to its obvious effects on perception, even subtle distortion can cause listening fatigue".

In "The Loudness War: Background, Speculation and Recommendation", an AES paper from the 129th convention in November 2010, Earl Vickers (of STMicroelectronics) writes:

"Marketing claims of “reduced listener fatigue” frequently appear in sales pitches for various audio products. In the audio engineering literature, listening fatigue has been attributed to a wide variety of causes including fast-acting compression and limiting [35], a lack of variation in loudness over the duration of a recording [35], contradictory location cues [44][45] [46], stereo image processing and active matrix upmix steering fluctuations [35][47], phantom image instability [45], phasiness [48], poor equalization [35], low frequency rumble [49], too much boost between 1-4 kHz [50], clipping and intermodulation distortion [35] [51], Doppler speaker distortion [52], low bit rate lossy encoding [34][42], stacked codecs [42], excessive hearing aid amplification of soft background sounds [53], listening to monophonic instead of stereo recordings [54], listening to stereo instead of 3-channel recordings [46], and listening with headphones to audio mixed for speakers [55]".

My point is:

1) IM distortion is usually a high frequency issue.
2) IM distortion may cause listening fatigue.

From a Genelec perspective this may have two implications:

1) If the monitors themselves introduce IM distortion, the monitors can be the source of listening fatigue.
2) If the monitors themselves produce IM distortion, the engineer may not detect IM distortion that originates from other parts of his audio production chain.

In other words:

Listening fatigue may origin both from the hardware playback side as well as from the source (recorded digital file) side.

Both points would be interesting to hear your opinions on :-)

And as we know, listening fatigue is one of the themes on the next AES convention in Los Angeles, September 29th to October 2nd.

Just one last note: A recent AES paper, "A Meta-Analysis of High Resolution Audio Perceptual Evaluation" from June 2016, concludes that "the perceived fidelity of an audio recording and playback chain can be affected by operating beyond conventional resolution".

It casts new light on the debate on our ability to hear and detect higher frequencies. And I thought it may be relevant for discussing the overall playback qualities of monitors as well :-)
svart-hvitt, modified 5 Years ago.

Re: Intermodulation tests

Padawan Posts: 35 Join Date: 5/6/16 Recent Posts
Ilkka, will you come back on this thread?

Is high-frequency IM distortion specific to my 8351as, or is it a general Genelec or 8351a issue?

:-)
ilkka-rissanen, modified 5 Years ago.

Re: Intermodulation tests

Yoda Posts: 2564 Join Date: 3/23/09 Recent Posts
Hi,

Sorry for the delay, it took some time to run the tests. I used an 8351A monitor connected to an RME UCX sound card using 96 kHz sampling frequency. Playback software was foobar2000. On the first three samples I could detect some very low level noise (barely above the background noise floor) at close distance while the fourth sample (music) was completely silent. Whether that noise is caused by the sample clips themselves, sound card or the monitor, it would not cause any issues because the level was too low for any sort of detection at longer distance and over the program material. It's a clean pass for the intermodulation tests. :)
svart-hvitt, modified 5 Years ago.

Re: Intermodulation tests

Padawan Posts: 35 Join Date: 5/6/16 Recent Posts
Ilkka,

thanks for coming back to the IM distortion question!

:D

Interestingly, I hear the same noise on my Sennheiser HD800-HDVD800 combo. Played from another Mac computer.

:?

I always played these tests in Mac OS, via the web page and Mac internal sound app. And I change the settings to 96kHz.

However, I am not sure if this is a real-scenario problem at all.

:geek:

But it's related to my perception of sound at really high volumes (uncomfortably high); at these volumes it's hard to tell if the ability to hear nuances falls apart, or if it's the speaker that starts to distort.

At normal listening volumes, I guess that means always below 90dB (I don't have a precision sound-level meter), the 8351a is really good. My acoustics are a greater problem than my speakers' ability to reproduce sound...
ilkka-rissanen, modified 5 Years ago.

Re: Intermodulation tests

Yoda Posts: 2564 Join Date: 3/23/09 Recent Posts
It is a combination of many things. Our ears will start to distort at medium to high levels as well as of course the monitor itself (not because of ultra high frequency intermodulation distortion though). And as you wrote, the acoustics of your listening space will have a huge effect as well. If you have lots of reflective surfaces and little absorption, sound energy will decay slower which will deteriorate sound quality and accuracy. I would say the latter is the biggest factor is many cases I have seen and heard.

How does your GLM setup file look like? Can I see the curves?
svart-hvitt, modified 5 Years ago.

Re: Intermodulation tests

Padawan Posts: 35 Join Date: 5/6/16 Recent Posts
Ilkka,

thanks for your reply!

In the link below you will be able to see the curves of the left and right 8351a monitors.

https://abload.de/img/screenlfu7s.png

This is a single mic position setup.

The setup is made for two individual speakers.

Ca. 180 cm between the speakers. Speakers are near the back wall. Mic is about 180 cm from the upper line in the listening position triangle.

:)


PS: Please indicate if you need more data, for example the setup file.