VG I'll try & answer some of your questions. On 30 November 2010 17:28, V G wrote: > Hi all, > > I've been recording lectures for my classes using the Zoom H2 ( > http://www.zoom.co.jp/english/products/h2/) portable audio recorder. I > normally sit near the front (no more than 5 or 6 m away from the professo= r) > and therefore tend to catch a lot of background noise, as well as noise f= rom > other students. > > 1. At this distance, the high gain mode is more than enough to capture wh= at > the professor is saying, however if someone sitting near the recorder cou= ghs > or sneezes or flips a page or smashes their face against their desk, I ge= t > high amplitude clicks in the recording. Such noises are far louder than t= he > sound from the professor. Because of this, I find it hard to "normalize" = the > recording using Audacity since the maximum amplitude wont be adjusted to > more than 0 db. > > How would i clip off these loud spikes? In Audacity, they're clearly visi= ble > as tall spikes, compared to the rest of the recording. ..Some sort of directional mic (preferably with built-in noise cancelling) will help getting the desired audio while minimising the background. It may not be practical in your situation but would give a massive improvement. > > 2. Audacity also has a really nice noise removal function. You select a > section of audio (as a sample) that is "noise", and it filters that noise > out from the entire recording. For lectures, the beginning and end of the > lectures are noisy, as everyone settles down. The quietest sections can b= e > found in the middle of the lectures. However, during that time, I find it > hard to find a gap where it's *only* noise and the professor isn't talkin= g > (since he is almost always speaking). Any ideas as to how to remove this > noise? > Audacity lets you amplify beyond clipping. There is a tick box and you can amplify as required. You can bank out the noise peaks before amplifying or you can reduce the harshness by low pass filtering the clipped result after amplifiction. Experiment to find the best cutoff frequency & rolloff rate. > 3. I have a question about decibels. I understand that when measuring vol= ume > or sound intensity, 0 dB is the level at which sound is barely audible fo= r > the average human. So, any sound that is louder will use that barely audi= bly > sound as a reference on the decibel scale. > > 3.1. However, the normalizing function says it will stretch the maximum > amplitude to 0 dB. What does this mean? What is the reference? > > 3.2. My recorder has a sound intensity scale (I think) that is showing > values from -48 dB all the way up to 0 dB on the high end. You can see th= e > bar moving higher if you speak louder. I don't understand this scale. > -- 0dB is simply a reference point. There is a tendency to use it as a sound pressure level of 1pascal/m^2 or something close to the audible threshold, but unless the reference is quoted it has no absolute value. Audacity uses the clipping level as the 0dB reference point and levels measure downward from here (negative values). So Audacity is showing sound levels from 0 to -48dB or so and others are using the threshold of hearing as 0dB and it goes up from there - e.g to a jet engine of +140dB or whatever. To emphasize, dBs are a measure of power ratio - not absolute levels. In electronics power may be expressed as dBm or dBV etc. Here the reference level IS included as the (m)illiwatt or (V)olt etc. Audio levels can also be expressed as dBA or dBC etc. In this case the reference level IS standardised, along with the frequency response of the listening device. I'll leave it up to you to google the details. Hope this assists. RP --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .