Doppler anyone?

[fishinbob]

I would like to know what is the cause for a change (rise) in pitch at the end of a piece of music. :banghead:

It's very annoying and only is audible on the decay of a signal.

JBL 001 w/original everything except my newly re-coned 130As.

Rats. :(

I erroneously posted this in another part of the forum and have found out what I believe to be a Doppler effect is part of my listening enviornment.

It is noticible on every speaker I've tried so far. Some more so than others. The prime example would be the end of the THX intro. At the end the pitch rises most notcibly on the JBLs and Klipch(center) and to some degree on the others I've tried including a new set of MTM towers.

I'm going to be doing some WAF acoustic stuff (family room) and was wondering if someone might have any tips? My treatments will be mostly absorbtive in nature so hopefully no baffles.

Thanks
Bob

[pos]

I have noticed this effect also.
I think this is pure psychoacoustic, due to the brain being acoustumed to the doppler effect, and trying to correct it: in the real world when a sound is fading out it generally means that it is going away, and so its pitch is lower than normal due to the doppler effect. So the brain might tend to "correct" that shifted pitch back to the real one, by artificially rising it. That would be why you hear a pitch rising when fading out: the sound is fading out without modification of the pitch (no doppler effect, just a variation of level), but the brain tries to correct the doppler effect anyway!
If that is true then the rising effect will depend on the acceleration of the fade. With a linear fade you should not have this effect.

[grumpy]

Sounds plausible. Vision/mind plays analogous tricks... watch scrolling movie credits
in the dark for awhile, then look at something fixed... often, it will distort or 'move'.

This effect is particularly evident after a long session of Guitar Hero :D, whether
playing or watching.

[fishinbob]

I wouldn't quote myself on this but it is my understanding that it is real, and is caused by a loudspeaker trying to produce a low and a higher frequency at the same time and the higher one is affected by the lower.
This is said to be most common in a full range application and to lesser degrees in two way and then three way designs and so on. This of course is also affected by the individual speakers themselves (response, crossover etc.) I know this because of my side by side listening experience.

Bob

[pos]

The doppler effect in loudspeakers is a kind of intermodulation distortion. It is real but I don't think this is what you are experiencing here.
It only appears at high levels, when the large excursion implied by the LF causes fast moves of the cone that modulate the other parts of the frequency band reproduced by the driver. What you would then notice is some sort of distortion, not a change in pitch.

[grumpy]

Note that this is true because of physical limitations of the transducer, and why going
multi-way can reduce this effect (limiting each transducer to an optimum range of
response).

If it were able to follow the waveform, no matter if multiple tones were produced
with larger or small separation in frequency, there would be no IM contribution from
the speaker... that it can't is a non-linear effect and therefore a distortion.

Combine and a 100Hz tone and a 1000Hz tone... there will be 900Hz and 1100Hz
(at least) distortion products. If bad enough, I suppose one could perceive this as a
frequency shift.

It's certainly a measurable quantity, if difficult to portray other than specific
snapshots/examples. Check Wikipedia for a graphic RF example.

My personal opinion is that calling this a "Doppler" effect would mislead most readers.

[fishinbob]

The audibility of Doppler distortion depends upon three criteria:

the FM threshold of hearing,
masking by the low frequency tone,
FM to AM conversion due to room resonance
This is from http://www.linkwitzlab.com/frontiers.htm

It is my hope that number three could be changed favorably.:)

[grumpy]

I read the analysis (thanks). I'll change my tone slightly :)

If a sound reproducing device were able to follow the waveform,
by creating an analogous sound pressure, without moving...

Any moving plane device (even ideal) will experience the effect
described. It gets worse with real devices.

Do you have known room resonance issues?

[fishinbob]

My personal opinion is that calling this a "Doppler" effect would mislead most readers.

would you buy phase modulation distortion?

http://sound.westhost.com/doppler.htm

[fishinbob]

Do you have known room resonance issues?

I think we all do. I feel my equipment is up to par so I'm looking that way.

[grumpy]

funny, I remember (now) reading that quite awhile back, but had since forgotten.

works for me :)

re room resonances... I suppose I meant identified/quantified, as well as known.

(why do I ask? curious about what was noticed and/or measured, what would be
used to remedy the cause, and if post audition or measurements gave you joy ...
I did read the initial post, but hopefully your findings will make it back here).

[fishinbob]

re room resonances... I suppose I meant identified/quantified, as well as known.

No I haven't made any attempt to measure room response or anything like that. I was going to make a few bass traps and place a few panels for relections. Sort of by gosh and by golly.:)

[grumpy]

gotcha. would still be interested to hear how you fare. :cheers:

[fishinbob]

gotcha. would still be interested to hear how you fare. :cheers:
Biggest hurdle is the WAF:argue: :D

[pos]

I am quite sure the effect you describe has nothing to do with this doppler distortion. If it where then it would be much more noticeable a high levels, and not at fade outs. Plus, this would not result in an audible change of pitch: the sound would be modulated by the lower frequencies. If a 100Hz tone is responsible for this modulation then the pitch of the modulated higher tones will alternatively go up and down 200 times a second. I don't think you could interpret that as any audible change in pitch. This is more like a very fast vibrato.

As stated in my other messages, I think what you are hearing is a psychoacoustical effect, caused by a brain correction of the doppler effect (not the one you hear in a fullrange driver, but the more common one with slow variations, such as a car passing by, etc.). But to be sure you would have to mesure it. Maybe you can try this with a simple tone, and a guitar tuner?

[fishinbob]

I am quite sure the effect you describe has nothing to do with this doppler distortion. If it where then it would be much more noticeable a high levels, and not at fade outs. Plus, this would not result in an audible change of pitch: the sound would be modulated by the lower frequencies. If a 100Hz tone is responsible for this modulation then the pitch of the modulated higher tones will alternatively go up and down 200 times a second. I don't think you could interpret that as any audible change in pitch. This is more like a very fast vibrato.

As stated in my other messages, I think what you are hearing is a psychoacoustical effect, caused by a brain correction of the doppler effect (not the one you hear in a fullrange driver, but the more common one with slow variations, such as a car passing by, etc.). But to be sure you would have to mesure it. Maybe you can try this with a simple tone, and a guitar tuner?

So it's my brain playing tricks? I assume that I am among the select few who experience this and that there is a good chance that others will not? If this is the case then I would hear this regardless of enviornment. Assuming the source material/amplification were to remain constant,then I would I hear this no matter where or what full range loudspeaker I listen to?:(
I have downloaded a guitar tuner and will burn a disc to see what happens.

One thing I've found since joing these forums that it is awful easy to jump into deep water. My understanding of physics and things electronic is very limited so bear with me. :)

Bob