Plexiglass as a sound barrier

What does plexiglass do to the sound behind it or in front of it?

Plexiglass shields are used to manage live drums, stage monitors, guitar amps & to protect musicians ears from other instruments on stage. What frequencies are actually affected by the plexiglass and are there downsides to using it?

Let’s find out…
The measurement setup:
DUT – Plexiglass shield
RESPONSE – Earthworks TC30K.
APP – Spectra Foo Complete
PLATFORM – Mac Book Pro 2010
AUDIO I/O – Metric Halo 2882.

Here we go:
UPJ plexi @ midpointThis is the trace with the plexiglass mid way between the mic & the speaker
UPJ plexi mid
UPJ plexi @ speakerThis is the trace with the plexiglass a few inches away from the speaker
UPJ plexi @ UPJ
UPJ plexi @ mic 1
UPJ plexi @ mic 2This is the trace with the plexiglass a few inches away from the mic
UPJ plexi @ micThis is the trace with all the captures visible.
UPJ Plexi all measurementsWhat is happening to the sound behind the plexi once the plexi is blocking it’s path? For example, how does wrapping a drummer in plexiglass affect the drum sound? If the drummer has an open vocal mic inside the plexiglass cage, how is that affected?
UPJ mic behind plexi
UPJ mic behind plexiThere are obviously two sides to every plexiglass panel & while it may solve issues on one side, it may cause others. After seeing the FR of the various traces & how plexiglass affects the sound, I will go forward with a better understanding of what I am losing, what I’m gaining & how to approach correcting those inherent aspects of plexiglass use.

Classical Mystery Tour – Beaumont Texas

Screen Shot 2014-02-02 at 12.44.50 PM

The trace above is from a venue in Beaumont Texas where I worked with Classical Mystery Tour & the SOST. The pink trace represents the house left side of the PA. The yellow trace represents house right side. Obviously there is something wrong with the house left side of the PA. I was sent a text the night before the gig revealing there was a problem so I knew I had to deal with it before I arrived. Due to the tight orchestra rehearsal schedule & arriving from Lubbock that morning on a plane, I had to get thru a band sound check and then an orchestra rehearsal before I was able to measure the PA. The band typically does a last minute warm up on stage before doors open so I knew I could hear the band again but didn’t want to change the PA balance too much. I also didn’t have time to try to address the cause of the low end drop off with the house left side of the PA. I ended up using the console (Yamaha M7CL-48) parametric EQ to split the difference between the two sides.


Mansfield ISD PAC 021214

Working for Texas Ballet Theater on “Peter and the Wolf – the children’s ballet”, I spent the last two days at the Mansfield ISD PAC. Huge place that barely gets used. The PA is made up of a stereo L/R JBL Vertec rig with subs at the top of the arrays. The balcony has it’s own (3) shorter JBL Vertec columns but that part of the venue wasn’t being used so that part of the PA was off. The venue also has curtains that block the balcony when not used so that helps the sound on the main floor. All in all, a fairly nice rig considering the venue gets used mainly for high school graduations. This gig was another reminder that being able to measure a PA is vital in this day & age. I could of EQed the PA based on listening to music & my own voice but measuring is simply a superior way of doing the work.

This capture shows that the house PA has excessive low / low mid content. Not a good thing. Especially when dealing with narration. Excessive low / low mid content for human voice makes it hard to understand the words.

MISD PA stock

If you set up your rig correctly, you can compare the input signal going to the console, the output signal coming from the console & the room response at the reference mic. After selecting the “pair” of signals you want to compare in the right order, you can see how well the compensative eq follows the room response at the microphone.

MISD corrective EQ

This is a capture of the original trace, the corrective eq trace & a measurement I made of the audio I/O. I have included the audio i/o measurement (green) because it helps explain the frequency & phase response of the console output. The low & high end phase drift & roll offs are an artifact of the audio i/o. Not the EQ itself.
MISD overlay

With the compensative eq in place via the FOH consoles parametric eq, the end result is this which worked much better.
MISD PA corrected

common FFT mistakes

Here are a few common mistakes users make. Maybe by reading over them, you can avoid making the same mistakes:

1. forgetting to configure the rig to measure the desired speaker with the desired mic
2. forgetting to configure the PA so that only one speaker is producing sound for the measurement
3. forgetting to compute delay time prior to beginning the measurement process
4. forgetting to move the mic when taking the next measurement
5. forgetting to capture a measurement before moving onto the next measurement position