peculiar 168 hz in tap response

You can ask questions here about Trevor and Gerard's exciting new book on Luthiery.

Moderators: kiwigeo, Jeremy D

User avatar
kiwigeo
Admin
Posts: 10580
Joined: Sat Sep 29, 2007 5:57 pm
Location: Adelaide, Sth Australia

Re: peculiar 168 hz in tap response

Post by kiwigeo » Wed Feb 19, 2014 2:17 pm

Trevor,

Here's an excel plot of data from spectrum text file off VA.

The setup is a Shure condensor Mic running through a preamp (supplying phantom power) and then to input jack on laptop. Preamp gain is wound back to lowest setting. As previously stated the only change from previous setup was the Mac computer.
Workbook3.jpg
Martin

User avatar
kiwigeo
Admin
Posts: 10580
Joined: Sat Sep 29, 2007 5:57 pm
Location: Adelaide, Sth Australia

Re: peculiar 168 hz in tap response

Post by kiwigeo » Wed Feb 19, 2014 4:35 pm

I don't know what I did but the problem has suddenly disappeared!! I went out for a coffee..came back and the bldi thing is working like it used to.

Spectrum for top below. Note I did a control run without tapping the top and the regular peaks were absent. Back gives a strong sharp peak at around 275Hz.
013 Raw Box Top.jpg
Martin

User avatar
Trevor Gore
Blackwood
Posts: 1605
Joined: Mon Jun 20, 2011 8:11 pm

Re: peculiar 168 hz in tap response

Post by Trevor Gore » Wed Feb 19, 2014 4:46 pm

:lol:

Here's the post I was about to post! Sounds more like a bad connection (no signal), but here's the post anyway.


I'd be guessing that that's either no signal at all or an earth loop (or ground loop, depending on where you come from), of the digital variety.

I can only tell you about XP rather than later versions, but they seem similar when I've tried later versions (just more annoying)

First, make sure you have the mic input switched on on the lap top:

If your system has the equivalent of the XP speaker icon, click on that, open the volume control panel (or equivalent), select options, properties, recording, OK, select the mic as the input. Also click advanced (same window) and select mic boost if you have the option.

Also try running the lap top off its battery pack with the power supply completely disconnected and see what you get. That step my break the earth/ground loop

Other things to think about:

1) Are you sure the Shure is working (and its pre-amp)?

2) Do you know how the jack socket is wired on the laptop? Generally, they expect an electret mic with a stereo jack and often have a bias voltage on the ring (around 2v, I think). If you plug in a mono plug you short that 2v to ground, which probably does no damage, but screws things around

3) Check the spec on the jack socket on the lap top (should be in the handbook) and see what signal level it takes. If it will take line level, plug in a MP3 player or something and see if you get a signal/output.


There still seems to be a lot of "grass" on your plot.

User avatar
kiwigeo
Admin
Posts: 10580
Joined: Sat Sep 29, 2007 5:57 pm
Location: Adelaide, Sth Australia

Re: peculiar 168 hz in tap response

Post by kiwigeo » Wed Feb 19, 2014 4:59 pm

Thanks alot Trevor. Yes the plot is a bit furry. Ill check everything you've suggested. I have to get my head into Windows mode first......
Martin

User avatar
kiwigeo
Admin
Posts: 10580
Joined: Sat Sep 29, 2007 5:57 pm
Location: Adelaide, Sth Australia

Re: peculiar 168 hz in tap response

Post by kiwigeo » Fri Feb 21, 2014 3:44 pm

Ive redone tap testing of the top (raw boxed up guitar) using the Shure SM57 mic running via an XLR/USB adapter straight into a USB port on the HP laptop. Results of both the original setup (mic into PC mic input via preamp) and current setup plotted up on Excel. Not a huge amount of difference with the amount of "grass" on the plot but main top peak is much clearer.
013 Raw Box.jpg
Martin

Post Reply

Who is online

Users browsing this forum: No registered users and 18 guests