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RE: Measuring Decibel Separation

From: Jonathan Avila <jon.avila@levelaccess.com>
Date: Tue, 18 Sep 2018 18:54:56 +0000
To: Greg Gay <greggay@rogers.com>, "w3c-wai-ig@w3.org" <w3c-wai-ig@w3.org>
Message-ID: <CY1PR03MB22045776E2C0F91134BDAADEF11D0@CY1PR03MB2204.namprd03.prod.outlook.com>
I haven’t tried it – but I wonder if you could use Audacity to remove the vocals and have the background leftover?  You would of course have to get the audio recorded from the video – but that should be straightforward with an audio driver or patch cable from the sound card out jack to the mic jack.

Random Youtube video on using Audacity to remove vocals.


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From: Greg Gay <greggay@rogers.com>
Sent: Tuesday, September 18, 2018 2:42 PM
To: w3c-wai-ig@w3.org
Subject: Re: Measuring Decibel Separation

Thx David,

After posting the original question, I did come across an old article by David MacDonald that essentially suggests the same strategy you've outlined. Take a sample of the background, take a sample of the background with speech in it, then compare the decibels of each sample.


I happen to have ProTools available, which apparently will do the job.


Does anyone know of an open source or free tool that can be recommended for client who want to do their own audio sampling, who might not otherwise have a use for a tool like ProTools?


On 2018-09-18 1:50 PM, David Woolley wrote:
On 18/09/18 18:23, Greg Gay wrote:

How does one measure whether speech in multimedia is 20db louder than background sound, from an auditing perspective.

During production its easy enough to control speech and background levels, but what I'm looking for is a way to show clients post production, that the speech volumn level in a video is less than 20db greater than the volume of the background audio in that video. (re WCAG 2.1 SC 1.4.7)

This really needs to be done white box to properly demonstrate compliance, but 20dB is 10 times more power, so, I'd suggest determining sections of the audio with no speech and measuring peak amplitude (probably fast A weighted**), then measuring the peak amplitude in a section with speech.  If the ratio is more than 11 times power (about 20.8 dB, say 21 dB)you are getting close to complying, although one needs some safety margin to account for poor selection of the quiet and speech sections.

You'll need to exclude the periods with the allowable occasional sounds, first.

** The criteria should specify this, although they appear not to do so!

Received on Tuesday, 18 September 2018 18:55:21 UTC

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