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JohnA

Auto Dash Speakers

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What does placing speakers in the corner of a car dash and bouncing the sound off the windshield do to frequency response?  How can I calculate/estimate/guestimate the effect? 

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Use REW and a mic to see what is going on


Sent from my iPhone using Tapatalk

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16 hours ago, JohnA said:

What does placing speakers in the corner of a car dash and bouncing the sound off the windshield do to frequency response?  How can I calculate/estimate/guestimate the effect? 

The issue that you're looking for, I believe, are the 1/4 wavelength boundary reflections.  These reflections will probably occur in the 700-2000 Hz band, and will be cancellations (nulls).  Frequencies below that band will be supported like you have a horn there, so you're going to get a lot of gain (as much as 20 dB vs. the null frequencies). 

 

Additionally, this means that the polar lobing issues will be pronounced in this frequency band, so as you move the microphone around (up/down, side to side) from the seated position head heights, you're going to see pretty wide swings in output. 

 

I'd recommend drawing a side view of the loudspeaker/windshield and a top (plan) view of the same to understand where these 1/4 wavelength cancellations will occur.  These are fairly straightforward to draw if the curvature of the windshield is known, but more difficult if the windshield shape or the side wall reflectors aren't straight.  But I think you'll see how to estimate the problem areas once you lay it out and calculate the path lengths from the driver to the reflector to the listener's ears.

 

Chris

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On 6/28/2020 at 5:49 PM, carlthess40 said:

Use REW and a mic to see what is going on


Sent from my iPhone using Tapatalk

 

Because it is part of a Blose audio system.  The unknown, built-in EQ interferes with the test and dragging the gear out there to bypass the Blose, is not yet worth the effort. 

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12 hours ago, Chris A said:

The issue that you're looking for, I believe, are the 1/4 wavelength boundary reflections.  These reflections will probably occur in the 700-2000 Hz band, and will be cancellations (nulls).  Frequencies below that band will be supported like you have a horn there, so you're going to get a lot of gain (as much as 20 dB vs. the null frequencies). 

 

............

 

Chris

 

 

Hi Chris,

 

Why 1/4 wave and not 1/2 wave? 

 

The OEM drivers are rather flat from 200 to 2000 and then have a spikey plateau 6 dB above 200 to 2k band, from 3500 to 12k.  I curved them out of the car.  My first attempt with replacements has been good, but there is some oddness in tenor and Alto voices.  I dont think they are fed any thing below ~300 Hz. 

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See http://arqen.com/acoustics-101/speaker-placement-boundary-interference/

 

The characteristic boundary reflection distances for dash mounting drivers (i.e., those without "waveguides" or horns to control their polar responses) is about 4 inches or less. 

 

Chris

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