Tower speakers and floor-bounce frequency response notch?

Archived from groups: rec.audio.tech (More info?)

I am trying to better understand the cause of the "300 Hz floor bounce
notch" effect that has been reported in frequency response measurements
of tower-style speakers. I've read about this effect in test reports
from several audio magazines, especially Tom Nousaine's tests for Sound
and Vision Magazine. (I see that Mr. Nousaine sometimes posts to the
rec.audio groups - I wonder if this message will catch his attention.)

I've seen an explanation (please correct if I'm not repeating it
correctly) that the "floor bounce notch" is a wave interference effect,
caused by destructive interference between the sound wave that travels
directly from the bass/mid-range driver to the listener's ear, and the
wave that undergoes a reflection ("bounce" ) from the floor on its way to
the listener's ear. If that were the full explanation, then I would
think the effect should occur equally strongly in tower speakers and
stand-mounted "bookshelf"-style speakers when the bass/mid-range driver
is at a similar height above the floor. However, the Sound and Vision
tests suggest that the floor bounce notch really is stronger in tower
speakers than in stand-mount speakers under "normal listening
conditions". So maybe the effect isn't purely "floor bounce" but has
some correlation with the tower speaker geometry (tall and narrow
enclosure).

Here are some example S&V reports where the effect is discussed:

(A) Paradigm Monitor 11 tower speakers, Monitor CC-370 center:

http://www.soundandvisionmag.com/a [...] e_number=1

http://www.soundandvisionmag.com/p [...] gm_lab.pdf

The Monitor 11 is a large tower with 1 bass-midrange and no less than 3
bass drivers. It can be seen that the "floor bounce notch" is not
eliminated by having an array of drivers at different distances from the
floor, although it may be weakened. Notice also that the floor bounce
notch does not occur for the CC-370 center.

(B) Wharfedale EVO-30 tower, Wharfedale EVO-Centre, speaker systems
from Atlantic Technology and Celestion:

http://www.soundandvisionmag.com/a [...] e_number=1

http://www.soundandvisionmag.com/a [...] 132655.pdf

The Wharfedale EVO-30 tower shows a huge (-9 dB) floor bounce notch at
about 400 Hz, while none of the other, non-tower speakers in this report
show a similar effect.

(C) Phase Technology Velocity V10 tower, Sapphire Audio ST2 tower, other
Phase Technology, Sapphire Audio, and JBL Northridge series speakers:

http://www.soundandvisionmag.com/a [...] e_number=1

http://www.soundandvisionmag.com/a [...] 145133.pdf

The Phase Technology and Sapphire Audio towers both show a moderate
"floor bounce notch". The non-tower speakers do not show the effect.

Thanks for any help with this question!

Archived from groups: rec.audio.tech (More info?)

Ethan Winer wrote:
> Lawrence,
>
>
>>I've seen an explanation (please correct if I'm not repeating it
>
> correctly) that the "floor bounce notch" is a wave interference effect <
>
> I'm not a speaker designer, but I have a few comments anyway. :-> )
>
> Simple acoustic boundary interference is indeed a major cause of nulls. It's
> also the cause of peaks. At frequencies and distances where the direct and
> reflected waves are some amount "out of phase" you get a null. And when they
> combine more or less in phase you get a peak. Further, this occurs not at
> one frequency, but at a series of related frequencies - comb filtering.
>
> A single bounce off the floor is hardly the biggest problem, even if it's
> severe. All rooms create a series of many peaks and deep nulls throughout
> the entire low end (and beyond). Some of these peaks and nulls are modal,
> and others are positional like the floor bounce you mentioned. My point is
> that it's pointless to worry about a single null caused by a single factor
> when there are numerous other nulls and peaks to contend with in all rooms.
>
> --Ethan
>
>

I realize that the speaker-room interaction is complex (and that
my respondent has a strong interest in room acoustics), but I am
still puzzled by the Sound and Vision Magazine frequency response
charts. Maybe what puzzles me is not so much that the tower
(tall, narrow, floor-standing) speakers do show a "floor-bounce"
dip at close to 300 Hz, but that the other speaker styles DO NOT
show such a dip.

As I wrote previously, I started to wonder whether the 300 Hz
notch might be an "internal" effect, related to some type of
resonance or standing wave in the tower speaker cabinet, rather
than any kind of room interaction. However, if that were
correct, then the effect should be seen even when the speaker is
tested in an anechoic chamber, and results from other measurement
labs do not show this. In particular, the 300 Hz notch is not
seen in tests from Soundstage Magazine / Network, which are
conducted in an anechoic chamber (National Research Council of
Canada).

http://www.soundstageav.com/avreviews_speakers.html

(example of SoundStage test results for a tower speaker)
http://www.soundstagemagazine.com/ [...] isseur_c9/

It would be useful to have a description of the Sound and Vision
Magazine speaker test setup, which I couldn't find on their web
site, but may exist in the print edition.