{"id":2378,"date":"2010-06-12T07:19:45","date_gmt":"2010-06-12T11:19:45","guid":{"rendered":"http:\/\/languagelog.ldc.upenn.edu\/nll\/?p=2378"},"modified":"2010-06-12T13:50:39","modified_gmt":"2010-06-12T17:50:39","slug":"lowpass-filtering-to-remove-speech-content","status":"publish","type":"post","link":"https:\/\/languagelog.ldc.upenn.edu\/nll\/?p=2378","title":{"rendered":"Lowpass filtering to remove speech content?"},"content":{"rendered":"

In the \"halfalogues\" research that I've discussed in a couple of posts recently (\"Halfalogues<\/a>\", 6\/9\/2010; \"Halfalogues onward<\/a>\", 6\/11\/2010), one of the experimental manipulations was intended to establish that \"it is the unpredictable informational content of halfalogues that result [sic] in distraction\", and not (for example) that the distraction is simply caused by an acoustic background that comes and goes at irregular intervals like those of conversational turns.<\/p>\n

As I suggested in passing, the method that was used for this experimental control has some intrinsic problems, and the paper doesn't give enough information for us to judge how problematic it was. Today I'm going to explain those remarks in a bit more detail. (Nerd Alert: if you don't care about the methodology of psychophysical experimentation, you may want to turn your attention to some of our other fine posts.)<\/p>\n

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From the paper's abstract:<\/p>\n

We show that merely overhearing a halfalogue results in decreased performance on cognitive tasks designed to reflect the attentional demands of daily activities. By contrast, overhearing both sides of a cell-phone conversation or a monologue does not result in decreased performance. This may be because the content of a halfalogue is less predictable than both sides of a conversation. In a second experiment, we control for differences in acoustic factors between these types of overheard speech, establishing that it is the unpredictable informational content of halfalogues that result in distraction.<\/span><\/p>\n

Here's a bit more of their reasoning, from the body of the paper:<\/p>\n

Relative to dialogues, halfalogues are less predictable in both their informational content and their acoustic properties. Whereas dialogues involve the nearly continuous switching of speech between speakers, in halfalogues, speech onsets and offsets are more unpredictable. If these unpredictable acoustic changes were solely responsible for the detrimental effects of overhearing halfalogues, then filtering out the informational content while leaving the low-level acoustic content the same, would still result in significant distraction. On the other hand, if the increased distraction observed while overhearing halfalogues is due to the relative unpredictability of the content of half of a conversation, then filtering the speech should nullify the distraction resulting from overhearing halfalogues. Thus, Experiment 2 controlled for the unpredictable acoustics of the halfalogue speech by filtering out the information content (i.e. rendering the speech incomprehensible) while maintaining the essential acoustic characteristics of speech onsets and offsets.<\/span><\/p>\n

How did they \"control for the unpredictable acoustics of the halfalogue speech by filtering out the information content … while maintaining the essential acoustic characteristics of speech onsets and offsets\"?<\/p>\n

[A]ll sound files were low-pass filtered such that only the fundamental frequency of the speech was audible. This manipulation rendered the speech incomprehensible while maintaining the essential acoustic characteristics of speech; similar to hearing someone speak underwater.<\/span><\/p>\n

But the problem with lowpass filtering is that it removes a very large portion of \"the essential acoustic characteristics of speech\". Specifically and obviously, it removes the higher frequencies. And since the typical range of fundamental frequencies for female conversational speech is something like 100-300 Hz, while the range of speech-relevant frequencies goes up to 8,000 Hz or so, we're talking about removing a very large fraction of the \"essential acoustic characteristics of speech\".<\/p>\n

This has many psychological consequences, but I'll focus on one simple aspect of the outcome: it's likely to make the overall psychologically-relevant sound level of the audio signal a great deal lower.<\/p>\n

Here's the peak-normalized original\u00a0 for a short piece of female speech (specifically, TEST\/DR2\/FCMR0\/SI1105.wav from the TIMIT<\/a> collection):<\/p>\n

Audio clip: Adobe Flash Player (version 9 or above) is required to play this audio clip. Download the latest version here<\/a>. You also need to have JavaScript enabled in your browser.<\/span><\/p>\n