[pianotech] What is bloom,

[Dean May]

The following are based on my limited rembrance of engineering studies on
vibrating systems with a single event forcing function (the hammer blow). I
wonder if what this curve is showing is the initial sound contains a great
deal of inharmonic noise. Without continued external excitement the system
wants to settle into vibrating at its natural frequency with its attendent
natural frequency partials. What I wonder is if the steep curve represents
the brief period of extra chaos where the un-natural partials will rob more
energy out of the vibrating string. As they fade  away and the natural
partials remain, those natural partials will rob less energy which then
slows the rate of decay.
 
Belly systems that favor extra extraneous noise will have reduced bloom.
Belly systems that enhance the natural frequency through good terminations
and matched impedence will have the knee in the curve appearing sooner. 
 
Dean
  _____  

From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf
Of Delwin D Fandrich
Sent: Friday, March 18, 2011 3:15 AM
To: pianotech at ptg.org
Subject: Re: [pianotech] What is bloom,



Yes, well, I continue to wonder just what it is that we're actually hearing.
Below is an idealized illustration of what is happening at, and following,
hammer impact. (It's a little more idealized than I would like but I don't
have any of my own on this computer. This one is borrowed from the Five
Lectures website.)The hammer strikes the strings at about 3 sec. There is a
chaotic spike immediately following (the period of chaos is typically a bit
wider than shown here). The sound immediately begins to decay at some fairly
rapid rate but, for this note, at around 5 sec. the rate of decay changes. 

Description: Description: Fig 1. Typical decay of a piano tone as
illustrated by the sound pressure level versus time (Eb3 = 311 Hz). The
decay process is divided into two parts; an initial attack part with a fast
decay (prompt sound) followed by a sustained part with slow decay
(aftersound).

>From what I've been able to figure out, the knee (at around 5 or 6 sec.) is
where the strings vibration pattern changes from a predominately transverse
motion (perpendicular to the bridge) to a more random, or rotational
pattern. The note is still dying out but at a slower rate. It continues thus
until the sound dies out or, as in this illustration, the damper drops.

In all the samples I've recorded and studied over the years I've never seen
the sound level increase after hammer impact and that first chaotic wave
pattern. They all end up looking like some variation of this. More ragged
and uneven sometimes but they follow this generally pattern. 

It leaves me wondering if what we think we hear as "bloom" isn't at least
partially-perhaps predominately-psychoacoustic. Our ears-or our brain's
interpretation of what our ears detect-quickly become accustomed to that
rapid drop-off following the chaotic hammer impact and, when the waveform
gets to the knee and the decay rate slows (sometimes dramatically) we
interpret the change as "bloom."

 

ddf

Delwin D Fandrich

Piano Design & Fabrication

6939 Foothill Court SW, Olympia, Washington 98512 USA

Phone  360.736.7563 - Cell  360.388.6525

 <mailto:del at fandrichpiano.com%20> del at fandrichpiano.com -
<mailto:ddfandrich at gmail.com> ddfandrich at gmail.com

 

From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf
Of Dale Erwin
Sent: Thursday, March 17, 2011 5:20 PM
To: pianotech at ptg.org
Subject: Re: [pianotech] What is bloom,

 

Del
  Understood. I can't measure it empirically either. Fortunately we can hear
it.

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