wolfkristiansen
389 posts
Aug 07, 2016
1:33 AM
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Don't put the back of your open backed amp close to the wall.
Most of us play through open backed amps, am I right? As opposed to closed back.
I much prefer the sound of my Fender Concert and Fender Blues Deluxe amps (both open backed) when they are well away from a back wall. I don't like small stages where you are forced to put your amp right against the wall. The sound suffers.
It's hard to describe the sound difference between "free" and "imprisoned" (squashed against the wall) amps, but there is a difference.
Anybody else notice this?
wolf kristiansen
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hot4blues
11 posts
Aug 07, 2016
5:05 AM
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Why would anyone place an open back amp directly against a wall? That's no different than doing the same thing to a refrigerator. The unit needs circulation of air, otherwise, overheating can occur. Besides, an amp with an open back was made to be open completely. If the manufacturer wanted to enclose the back, they would have done it already.
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shakeylee
568 posts
Aug 07, 2016
7:01 AM
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Sometimes,if the stage is small and tight,I face the amp against the wall with the back facing out .
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www.shakeylee.com
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hvyj
3087 posts
Aug 07, 2016
8:46 AM
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Yes, there's defmitely a difference. I happen to like the sound I get from my Super Reverb Reissue when its back is against or close to a wall. But regardless of personal preference, there is a very perceptible difference in sound.
Last Edited by hvyj on Aug 07, 2016 8:46 AM
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Owen Evans
182 posts
Aug 07, 2016
8:51 AM
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The problem with placing the back of the amp near the wall is that it acoustically couples with it and this can increase bass so as to muddy up the sound.instead of allowing the "back-wave" of the speaker to bounce off the wall and then "harmonize" with the front wave; placing the speaker near the wall makes the back wave either become absorbed by the wall or reflects it too fast & you get cancellation both the front & back wave.
Physics is physics. Audiophiles have known these principles for years because speaker placement is so important when recreating a soundstage. An example is electrostatic speakers with a sub-woofer (sound below 100Hz).The electrostatic sends sound waves in both directions & placing these in a room is a challenge. If you then place a bass speaker near a corner, it can double it's output due to wave generation from 4 surfaces.
(both walls in a corner and the ceiling & floor) That's why a lot of bands place carpets in front of the drum kit as well as plexiglass walls surrounding the kit so that feedback is reduced to the microphones. Sorry to be the messenger here, but it is complex.
If you play an amp with a closed back & put it near a wall (6-12 inches from it) it's bass only will be reinforced and your sound will be muddy as heck. Here's some overkill but info if you want to understand more about this.
http://www.audiophysic.com/aufstellung/regeln_e.html
I'll end with a personal experience of a couple of weeks ago. My friend's band was playing in a fair sized room. He had his Bassman set up & sound checked before the gig. We sat in a table with 6-8 other friends. I was at the furthest spot from the stage and I was having problem hearing his playing? I mentioned this to him at the break. He asked the sound guy to check it out. No problem. They started up again & I couldn't hear his playing very much. I got up to go to the can & the sound was outstanding only 3 feet from where I sat. I came back from the can & moved my chair. No more muddy sound! Room acoustics & speaker placement are serious concerns when trying to deliver your sound to an audience. Hope this is helpful @ wolfkristiansen
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STME58
1800 posts
Aug 08, 2016
8:09 AM
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I can remember my physics instructor stating that the distance from a speaker to the back of the cabinet is important as the sound wave that bounces off the wall can be in phase or out of phase with the wave coming straight froward from the speaker. I don't know why I remember this little piece of info as I never did much with it ore really studied the phenomenon. In thinking about it now, sound in the audible range has wavelengths from 17 millimeters to 17 meters with the low notes being the longest. Placing the wall at 1/2 wavelength from the source would cause the reflected wave to be in phase with the direct wave, but only at one frequency. Then again, you will get similar reinforcement at 1.5 wavelengths 2 wavelengths etc. It gets complicated real fast. One thing becomes clear just from the comments here and the link Owen provided, the interaction between a speaker and its surroundings is critical to what you hear in a given position.
I can also rember my instructor talking about ripple tank simulation of room acoustics. I just found this cool ripple tank simulator that lets you place speakers and draw in walls to see how the waves react to different placements.
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Greg Heumann
3262 posts
Aug 08, 2016
11:20 AM
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There is no question the amp will interact with its surroundings. Stick it in a corner and you'll get the most bass boost. But don't get too concerned with "...sound wave that bounces off the wall can be in phase or out of phase with the wave coming straight froward from the speaker. "
That is a true statement, but the actual wavelength depends on the frequency. The wavelength at 40Hz is a whopping 28.2 feet. The wavelength at 2000 hz is only 6 3/4 inches. So in the real world of music where we make lots of noise across a wide range of frequencies - just accept the fact that some reflected frequencies are going to be "in phase" and some are not. This is true ALSO for mic'ing an amp, front side or back side, as well as the phase relationship between a mic element and the amp's speakers. Interesting philosophical discussion but in practice - not an adequate description of all that's going on.
Here's a useful link if you want to see the wavelength for any frequency: http://www.mcsquared.com/wavelength.htm
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/Greg
BlowsMeAway Productions
See my Customer Mics album on Facebook
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Last Edited by Greg Heumann on Aug 08, 2016 11:21 AM
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garry
658 posts
Aug 08, 2016
3:33 PM
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It is common practice among players using Vox amps (closed backs) to pack the inside with insulation or sound absorbing foam. The claim is that it helps the sound, avoiding cancellation effects. What do y'all think of that?
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MP
3389 posts
Aug 10, 2016
2:03 PM
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As a habit, I try to have my amp mid stage if possible. If I'm forced to be a few feet from the back wall, I like to mic the amp from the rear open backed area w/ an EV 609/906 mounted on a small desk mic stand. All amps are different but I tend to get all kinds of overtones from mine when I do that. Maybe it's all in my head. :-)
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Have good day. M.A.P.
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Goldbrick
1569 posts
Aug 10, 2016
3:54 PM
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I talked to a friend who is an amp tech and a vox player for 40 years. He said he has not seen or heard of people putting insulation or foam in the cabinet
Maybe thats a local practice where you are ?
Tube Vox's have a reputation for running hot so it doesnt seem like a great idea .
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garry
659 posts
Aug 10, 2016
5:02 PM
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The Vox thing is commonly recommended on valvetronix.net forums. They're hybrid modelling amps, so the heat thing not likely an issue.
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CarlosJones
1 post
Aug 15, 2016
9:43 AM
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Hi..as per my knowledge there's defmitely a difference. I happen to like the sound I get from my Super Reverb Reissue when its back is against or close to a wall. But regardless of personal preference, there is a very perceptible difference in sound.
pcb assembly prototype
Last Edited by CarlosJones on Sep 16, 2016 7:36 AM
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