Basketball Floor Isolation Design Using CDM-ISO … Floor Isolation Design Using CDM-ISO-DPM by ......

Basketball Floor Isolation DesignUsing CDM-ISO-DPM

by RPG Diffusor Diffusor Systems, Inc.

Isolation of sports floors used for fit-

ness, basketball, etc. is a common

problem and CDM have investigated

the relevant issues in their research lab-

oratory. Before describing the solution,

it is important to consider what the audi-

ble disturbance really is. It is also very

typical that when these facilities are

above grade the structural floor will

have a low resonance, typically in the

vicinity of 5 Hz. When studying noise

and vibration problems it is also impor-

tant to understand if we are trying to

eliminate vibration or audible noise,

whether direct or indirect. When vibra-

tions are concerned, a typical rule of

thumb is that the resonant frequency

of the isolator is equal to the resonant

frequency of the floor divided by the

square root of 10. With a 5 Hz floor

resonance, this leads to an unreasonable iso-

lator resonance frequency of 1.6 Hz. But it is

important to realize that we are not dealing

with vibrations, because a dribbling basketball

does not have sufficient mass to excite vibra-

tions in the structure and the disturbance is

actually the audible sound of the impact.

There is a benefit here in that while the

threshold of hearing is typically 20 Hz, few

people can hear frequencies this low and our

sensitivity to these low frequencies is also sig-

nificantly lower than higher frequencies.

To determine the frequency of this audible

sound, a series of tests have been conducted

in the CDM research facility, using an ac-

celerometer and a sonometer, to measure the

effect of a basketball impact from various

heights and at various dribbling frequencies.

This report is presented at the end of this

white paper.

Let’s consider the important frequencies to be

considered.

- The dribbling frequency of a basket ball

typically ranges from 1 to 10Hz. High frequen-

cies here require low bouncing heights of

roughly 3” (0.08m)

- The impact noise frequency range is

between 50 and 150Hz with peaks at roughly

100 Hz. Low bouncing heights lower the peak

energy.

- The resonance frequency of the struc-

tural floor is typically between 3 and 10Hz, de-

pending on the span and floor buildup. The

mid-span deflection, d, can be calculated by

dividing the total span distance in inches by

360. The resonance frequency is then given

by 3.13*sqrt(1/d).

- The isolator frequency is typically be-

tween 5 and 15Hz for natural rubber pads

(and between 3 and 5Hz for steel springs)

- The floor system resonance frequency,

including the air stiffness and the mass of the

structural floor is normally > 10Hz

The Isolation Efficiency, I in % and dBv, is ex-

pressed below in terms of the transmissability,

T, in terms of the ratio, beta, of the disturbing,

fd, to the resonant frequency, fr, and xi, the

damping factor. Therefore, when the ratio of

the disturbing frequency and isolator reso-

nance frequency is greater than a factor of 4,

one can achieve greater than 90% isolation.

But one should also stay away from the struc-

Figure 1. Isolation efficiency in % (blue) and dB (red)

as a function of the ratio of the disturbing frequency,

fd, and the isolator resonant frequency, fr. Solid lines

are without damping and dashed lines include 10%

damping typically associated with elastomers.

tural resonance frequency of the base floor

with at least a factor 1.41 (square root of 2) to

avoid amplification. So, if the structural fre-

quency is 5Hz, it is best to use an isolator with

a minimum resonance frequency of 7Hz.

However, we also need to consider the stiff-

ness of the entrapped air below the floor. The

stiffness of the air and isolator combine to

raise the resonant frequency of the system

significantly. To minimize the entrained air

layer stiffness, one can increase the air space

by installing the isolators on sleepers. In-

creasing the air cavity will then lower the stiff-

ness and lower the combined system

resonance. If the walls are resting on the

floating floor, then there is also the opportunity

to vent the air in floor cavity into the space be-

tween the floating wall and structural wall to

further lower the influence of the air. Venting

the air is most effective for smaller floors, be-

cause on large floors the air along the perime-

ter will be more easily vented than the air in

the center.

In normal basketball games the dribbling fre-

quency is < 3Hz for 99% of the time, so there

is no amplification effect.

Flanking noise

caused by the

common walls will

also be an issue,

but this may not be

able to be con-

trolled, without a

box-in-box con-

struction.

The goal then is to

provide a system

resonant frequency

as far removed

from 50 Hz as pos-

sible, with pads

that are roughly 1.5

times the structural

floor resonance.

These goals can

be met with the

CDM-ISO-DPM

system as shown

in Figure 2. The

floating floor

should be made as

heavy as practical.

Figure 2. Drawing of a floating basketball floor containing 3/4” plywood/2

layers 5/8” Durock/3/4” plywood/3/4” typical typical basketball floor. The

floor is supported by 2” cube isolators on sleepers to increas the air cavity.

The walls are resting on the floating floor and the air cavity is vented into

the floating wall-structural wall air cavity to lower the system resonance.

This will depend on allowed floor heights and

whether ramps or steps will be used. A typical

floating floor will consist of 3/4" plywood / 2

layers 5/8” Durock / 3/4” plywood / 3/4” bas-

ketball floor for a weight of 68.7 kg/m2 (14.1

lbs/ft2). With an estimated live load of 500

kg/m2 (102.5 lbs/ft2), this results in an

acoustical design load of 22.1 psi (0.15 MPa).

With this loading the CDM 79 natural rubber

pad offers a resonant frequency of 10 Hz and

depending on the density and thickness of the

concrete structural floor of roughly a 27 Hz

system resonance, if the air cavity is not

vented. Venting the air cavity lowers the sys-

tem resonance to 14 Hz.

Under these conditions, the Transmission

Loss R (dB) can be predicted by the Progno-

sis Method and shown in Figure 3.

Figure 3. Left: Transmission Loss, R(dB), and Insertion Loss, Ln(dB), for the floor without a vented air

cavity below the floating floor; Right: Transmission Loss, R(dB), and Insertion Loss, Ln(dB), for the floor

with a vented air cavity. Notice the isolator resonance is 10 Hz in both cases, but the vented floor low-

ers the system resonant frequency from 27 Hz to 14 Hz. If you consider the Isolation efficiency for 50

Hz, for example, venting the cavity raises the efficiency from 55%, for the 27 Hz system resonance, to

85%, for the 14 Hz system resonance.

Reutenbeek 9-11, 3090 Overijse, Belgium

Tel : +32-2-686.15.60 Fax : +32-2-687.35.52

[email protected]

EXTERNAL TEST REPORT

CDM Lab report n°: 20091113-01 form: QS-lab01ap05

Issued on: 13-11-2009 FOR DISTRIBUTION

Page n°: 1 Total pages: 4

CDM nv/sa

Reutenbeek 9-11

BE-3090 OVERIJSE BELGIUM

BTW/TVA/VAT: 428.322.207

HRB/RCB/TRB: 477.294

Tel. +32-2-6877907 fax +32-2-6873552

www.cdm.be

E-mail: [email protected]

Handled by: Approved by:

Veelhaver Bram Vanstraelen Michael

TEST SUBJECT: Determination excitation frequency spectrum of a bouncing basket ball

CONTENTS: 4 pages

TEST PERIOD: November 2009

1. INTRODUCTION ____________________________ page 2 2. DESCRIPTION OF TEST PROCEDURE ____________ page 2

3. TEST DEVICES & SOFTWARE __________________ page 2 4. TEST RESULTS – VIBRATION MEASUREMENTS ___ page 3

5. TEST RESULTS – ACOUSTIC MEASUREMENTS______page 4


Tel : +32-2-686.15.60 Fax : +32-2-687.35.52

[email protected]





CDM nv/sa

Reutenbeek 9-11


BTW/TVA/VAT: 428.322.207


Tel. +32-2-6877907 fax +32-2-6873552

www.cdm.be


1. Introduction

The aim consists in determining the impact frequency spectrum generated by bouncing a basketball on a concrete slab (groundfloor / upper floor).

2. Description of test procedure

- Vibration measurements: FFT spectral analysis (from 3Hz to 350Hz) measured at a point located half a meter away from the impact, first on a ground floor (lab) and secondly on an upper floor (hall).

- Noise measurements: in one third octaves and dB(A) by means of a sonometer, only in the room located under the floor of impact (stockroom)

All possible external noise & vibration sources were kept minimal.

3. Test devices & software

- Accelerometer = dytran 3055A2, S/N 179 - Analyzer / Sonometer SVAN 947


Tel : +32-2-686.15.60 Fax : +32-2-687.35.52

[email protected]





CDM nv/sa

Reutenbeek 9-11


BTW/TVA/VAT: 428.322.207


Tel. +32-2-6877907 fax +32-2-6873552

www.cdm.be


4. Test results – vibration measurements

In both cases it seems that most of the measured energy is situated between 50 and 150Hz. The peaks in the second graph are due to structural resonance effects since that floor is more flexible. The exact location of these peaks is depending on the floor parameters (stiffness, span, etc).

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FFT spectrum [3-350Hz]

measuring point - labo

background level

with ball bouncing at 1Hz



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FFT spectrum [3-350Hz]

measuring point - hall

background level





Tel : +32-2-686.15.60 Fax : +32-2-687.35.52

[email protected]





CDM nv/sa

Reutenbeek 9-11


BTW/TVA/VAT: 428.322.207


Tel. +32-2-6877907 fax +32-2-6873552

www.cdm.be


5. Test results – acoustic measurements

Again, most of the disturbance energy seems to be situated in the zone 50 to 150Hz (peaking at 100Hz). This is important when designing acoustic isolation for the rooms situated under the impact floor.

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stockroom - individual bounces [dB(A)]

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stockroom - dribbling [dB(A)]

Basketball Floor Isolation Design Using CDM-ISO … Floor Isolation Design Using CDM-ISO-DPM by ......

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