Psychoacoustics

[jkeny]

prompted by this post from Raffles, I thought it might be useful to start a thread on Psychoacoustics & hopefully we can all learn something from it. Note it isn't framed as a challenge to any group (like similar thread in the past) so there's no need for argumentation. Let's try to make this a learning resource like some of the informative threads of the past!

I think that our brains are good at compensating for 'natural' 'distortions' such as listening to a recording in a non-anechoic room. The brain is not so happy with a fixed non-flat 'formant' over a moving audio spectrum. A guitar or violin or human voice, however, is often modelled as an excitation waveform filtered by a fixed formant, so perhaps judging frequency response accuracy is not easy on a recording of a single instrument compared to, say, an orchestra where the combination of multiple instruments is constantly changing.

I am told that we are remarkably tolerant towards 'harmonic' distortion, but an amplifier with harmonic distortion generates inharmonic intermodulation distortion when fed with multiple input waveforms. Again, another reason why perhaps a complex signal e.g. a choir rather than a solo voice is a better test of an audio system..?

So let me add this resource - a presentation from JJ Johnston which is his summary of the state of affairs in psychoacoustics circa 2011 http://www.aes.org/sections/pnw/ppt/jj/hearingtutorialv1.ppt

If I could start by asking if Raffles could explain what he meant in his post & maybe tease it out some more?

I hope Amir can also give some input into how psychoacoustics are relevant to our playback systems?

 

[DonH50]

There is more emphasis on IMD than THD in almost every system I have worked on, from audio to W-band radars...

 

[jkeny]

There is more emphasis on IMD than THD in almost every system I have worked on, from audio to W-band radars...

Don, do you mean more attention is paid to IMD than THD in measurements, etc.? Or do you mean that IMD is more important/crucial than THD?

 

[opus111]

Even the arch audio objectivist himself Doug Self admits IMD products outweigh THD in an amplifier. Doesn't stop him relying almost exclusively on THD though.

 

[DonH50]

Don, do you mean more attention is paid to IMD than THD in measurements, etc.? Or do you mean that IMD is more important/crucial than THD?

IMO/IME both, but in general THD is seen in the audio world and IMD rarely.

For others (I am sure you know this!), IMD creates tones unrelated (harmonically) to the fundamentals so in audio they tend to "stick out" more. In the RF world, IMD causes mixing products that fall back in-band and so cannot be filtered out. IMD also creates DC/LF spurs that corrupt dc balance and can cause LF "pumping" in high-gain circuits such as log amps. IMD and THD are fundamentally related, however, so given one you can calculate the other. For example, the 3rd-order IMD tones are 9.54 dB above the 3rd-order HD terms, and IMD2 is about 6 dB above HD2 (IIRC, don't have my reference at hand; the calculation is tedious but straight-forward). THD can be measured with just a single tone so is easier to do than IMD, which requires at least two tones.

FWIWFM - Don

 

[jkeny]

One of the most intriguing & fascinating areas of psychoacoustics is that the mechanism of action of hearing is not yet fully determined & this leaves open the way for new discoveries & fresh insights into how best to construct & organise our audio playback systems! It also may give us insights into the boundary conditions of our hearing - an area that Amir has recently written about i.e what really is our audibility threshold at different frequencies & how does this effect our audio playback systems particularly our digital audio systems & bit depth!

Another example from the JJ Johnston slide is this which is probably old information for most but it is cogent to some of the recent discussions about sound stage & depth!

• The center speaker is essential for getting depth cues
right.

– Exactly how you handle the center channel isn’t clear, but
there is no doubt that you need it.
– Fortunately, this also increases the sweet spot for the
stereo soundstage.

Another fascinating area is the cochlear amplifier which is the actual active, local feedback mechanism that is central to the function of our hearing:

The cochlear amplifier is a positive feedback mechanism within the cochlea that provides acute sensitivity in the mammalian auditory system.[1] The main component of the cochlear amplifier is the Outer Hair Cell (OHC) which increases the amplitude and frequency selectivity of sound vibrations using electromechanical feedback.[2]

http://en.wikipedia.org/wiki/Cochlear_amplifier

Lest anyone think that all this has been settled many years ago & we know all that we need to know about how the ear works (never mind the processes after the ear does it's translation into electrochemical signals), here is another summary paper, this time from 2010, which outlines the different views that experts in the field have with regard to the cochlear amplifier http://en.wikipedia.org/wiki/Cochlear_amplifier

So, let's hope that we can continue this thread as a reading & discussion resource about psychoacoustics!