[LAU] Amp / speaker combo for Piano

Gerhard Zintel gerhard.zintel at web.de
Fri Mar 8 15:26:25 CET 2019

(sorry Chris to have sent it to your private mail address accidentally)

On Thursday 07 March 2019, Chris Caudle wrote:
> > For sure you can tune such a system to be efficient in
> > a small frequency range.
> The article mentioned "a" high pitched tone, singular article in English
> implying single frequency or small spread of frequency.  Does not seem
> likely to be useful for music use,  might help for machinery with a
> specific annoying high pitch frequency.

Now that I looked closer to the picture of the device I assume I understand the 
principle. Nothing new, used in e.g. thousands of exhaust systems for 50 years 
and more.

Split a pipe into two pipes with equivalent sections (same area in both 
downstream pipes). Make pipe 2 longer than pipe 1. Let both pipes join together 
at an outlet orifice and radiate into the open.

The difference in length between the pipes is L. At the frequencies where L is 
half a wavelength the both waves at the opening are out of phase. This gives 
(ideally) total reflection at the opening.

The picture gives the impression of about a  few centimetres difference between 
outer path (the spiral inside the plug) and the direct path (length of the 
plug). The first efficient reduction is to be expected at 

f = C / (2*L)

thus about 5 kHz for L = 3.4 cm. It is efficient furthermore at multiples of 
that frequency.

From the article

"If, however, you peered into the PVC pipe, you would see the loudspeaker's 
subwoofers thrumming away."  

At 5 kHz you will never see the speaker thrumming! It is written as if they 
could cancel not only the "high-pitched note" but also the low end. That is 
obviously due to the writers lack of knowledge or pure advertisement bullshit.

just my 2 ¢

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