This._howl.loop is not a function

I am using PsychoPy V2021.1.4 (because I have found with previous experiments that this works most reliably) and am now trying to get an online version of my experiment to work.

The experiment comprises of a mental comparisons task in which participants have to rate which of two audio stimuli refer to objects that are louder, sharper, lighter etc. The stimuli appear in pairs (e.g., nail or needle) along with a target sensory modality (e.g., sharper), so for each trial three audiofiles are called up and played sequentially.

When playing audio files in a loop, they mostly work, but not all the way through so I keep getting this error:

Unfortunately we encountered the following error:

  • TypeError: this._howl.loop is not a function

Try to run the experiment again. If the error persists, contact the experiment designer.

I have tried different formats of files (.wav, .mp3, .ogg), checked that these are working properly, looked through the forums etc. played around with the loops, limiting these etc. to try and isolate the stimulus that is causing the problem. Nothing has worked, the error always appears sooner or later.

Any suggestions would be appreciated!

URL of experiment:

Cheers, Sebastian

As is usually the case when one finally asks for help, one (nearly) finds the problem…

The programmers of Psychpy have remarkably incorporated an aesthetic sense for language… the java version just does not like the German words “Huf” and “Kralle”, possibly also “Spritze” and “Lineal”. So it crashes, take them out, and it is fine.

I have no idea why, doesn’t seem to be due to an adblocker. But anyway, these stimuli were somehow problematic!

Thanks, Sebastian


in what context do you “Hut”, “Kralle”, “Spritze” or “Lineal”? As stimuli, as filenames? I’m running experiments in German and never had problems similar to the one you report.

Best wishes Jens

Hi Jens,
They are audio files, specifically *.wav oder *.mp3 and the name of the files was Huf.wav etc. I don’t know if that was the problem, in fact I did try changing it by putting an “f” in front (e.g., “fHuf.wav”) but that didn’t help. I reconverted the files, checked that they played etc. I just couldn’t find any reason for them not working, but when I removed them, it was fine (just for the record, I was kidding about Psychpy having an innate language device).

So, all I really know is that the experiment with around 200 audiofiles of a variety of German nouns crashed before, but now it works…

Best wishes,


it could simply be that the four files in question did not have the proper sampling frequency.

Best wishes Jens

Thanks for the suggestion - however, I had standardised them all to 44100 Hz, they were recorded in my lab and then I standardised them along with the film clips. So I think that this wasn’t the issue (although that one has tripped me up in the past!).