G.r.a.s. 42ac User manual

Instruction Manual

www.gras.dk

LI0126 – 4 July 2013

G.R.A.S. 42AC/42AC-S2 Pistonphone

G.R.A.S. Sound & Vibration

Pistonphone Type 42AC - Page 2

Note: unless individually specied, the term “Pistonphone” through-

out this document refers to both types, i.e. Type 42AC / 42AC-S2.

Revision History

Any feedback or questions about this document are welcome at [email protected].

Revision Date Description

1June 2012 Version June 2012

2 4 July 2013 Section 3.1 “Shipment and transport” added

http://www.gras.dk

Any technical documentation that is made available by G.R.A.S. is the copyrighted work of G.R.A.S.

and is owned by G.R.A.S.

The content in this document is subject to change without notice. G.R.A.S. Sound & Vibration A/S is

not liable or responsible for any errors or inaccuracies that may appear in this document.

Trademarks

Product names mentioned in this document may be trademarks or registered trademarks of their

respective companies and are hereby acknowledged.

Before shipment, remove the batteries and wrap separately. Assure cautious handling

during transport.

Remove the battery as soon as it is discharged or if the 42AC is stored for a prolonged period

of time. Leakage from the batteries may otherwise destroy the electronic components.

G.R.A.S. Sound & Vibration

CONTENTS

1. General Description ..............................................4

1.1 Operating Frequency ...............................................4

1.2 Calibration Level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2. Theory .........................................................6

2.1 Principle .........................................................6

2.2 Volume Corrections. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.3 Static-pressure Corrections ..........................................7

2.3.1 Barometer ..........................................................8

3. Handling and Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.1 Shipment and Transport .............................................9

3.2 Batteries .........................................................9

3.3 Calibrating Microphones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.3.1 Microphone Size .....................................................9

3.3.2 ½" Microphones ......................................................9

3.3.3 ¼" Microphones .....................................................10

3.3.4 ⅛" Microphones .....................................................11

3.3.5 1" Microphones .....................................................12

3.4 Calculation of Microphone Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.5 Calibrating Hydrophones ...........................................13

3.5.1 Calibrating Hydrophone Type 10CT. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4. Specications ..................................................15

A Appendix ......................................................17

A.1 Corrections for Static Ambient Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

A.2 Corrections for Humidity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

G.R.A.S. Sound & Vibration

Pistonphone Type 42AC - Page 4

1. General Description

The G.R.A.S. Pistonphone Type 42AC (Fig. 1.1) is a battery-operated, precision sound source

for accurate and reliable calibration of measurement microphones, sound level meters and other

sound measuring equipment.

• Type 42AC includes ½" microphone coupler.

• Type 42AC-S2 includes 1" microphone coupler.

Otherwise, there is no difference between the two types.

1.1 Operating Frequency

The Pistonphone type 42AC and type 42AC-S2 operate at a frequency of 250Hz.

1.2 Calibration Level

With a microphone inserted in the Pistonhone’s coupler and the Pistonphone and switched on,

the nominal calibration level is:

• 134 dB

1 re. 20 µPa

• 125.4 dBA re. 20µPa (with A-weighting applied)

At a static ambient pressure of 1013 hPa, no further correction factors need be applied. An indi-

vidual calibration chart is delivered with each Pistonphone.

The Pistonphone is an extremely stable laboratory-standard sound source which can also be

used for eld calibrations - it retains its high accuracy even under hostile environmental conditions.

1 134 dB is equivalent to 100 Pa

The Pistonphone complies with all the requirements of IEC Standard 942 (1988) Sound Calibra-

tors Class 1 when corrected with barometer ZC0003K.

The Pistonphone works on the principle of two reciprocating pistons actuated by a precision-

machined cam disc with a sinusoidal prole. The rotation speed of the cam disc is controlled to

within 0.5% via a tachometer signal in a feed-back loop.

Fig. 1.1 Pistonphone Type 42AC

G.R.A.S. Sound & Vibration

Pistonphone Type 42AC - Page 5

The operating procedure is straight forward, simply t the microphone into the coupler of the

Pistonphone and switch on. The Pistonphone will now produce a constant sound pressure level

on the diaphragm of the microphone.

The Type 42AC is delivered with a 1/2" coupler1 (RA0048) for calibrating 1/2" microphones

directly. The Type 42AC-S2 is delivered with a 1" coupler1 (RA0023) for calibrating 1" micro-

phones directly. Adaptors for calibrating ¼" and ½" microphones are included.

1 The coupler is individually calibrated with the pistonphone, and these must be considered as a pair

G.R.A.S. Sound & Vibration

Pistonphone Type 42AC - Page 6

2. Theory

2.1 Principle

The Pistonphone works on the principle of two pairs of similar opposing, reciprocating pistons

(Fig. 2.1) actuated by a precision-machined cam disc with a sinusoidal (SHM) prole. The cam

disc is mounted on the shaft of a small electric motor. The prole of the cam disc is such that the

pistons follow a sinusoidal movement at a frequency equal to four times the speed of rotation.

This results in a corresponding sinusodial variation in the effective volume of the closed coupler

and, consequently, an acoustic signal within it. The RMS pressure, P (in pascals), of this acous-

tic signal is given by:

Where :

γ is the ratio of specic heats for the gas in the coupler (for air this is 1.402 at 20 °C and

1 atm.)

0 is the ambient pressure in pascals (Pa)

p is the cross-sectional area of one piston in square metres (m

S is the stroke length of the pistons in metres (m)

V is the coupler volume in cubic metres (m 3)

p and S are determined by the physical dimensions of the pistons and the cam disc; the ambi-

ent pressure refers, in most cases, to the barometric pressure of the atmosphere.

2.2 Volume Corrections

The volume of the coupler is dened partly by the dimensions of the coupler itself and partly the

effective load volume of the microphone. The effective load volume of the microphone is sum of

the equivalent volume and the front cavity volume. The front cavity volume of the microphone

is the volume between the microphone’s diaphragm and the protection grid plus the volume

contained in the slits of the protection grid.

The equivalent volume of the microphone is the apparent volume of the microphone behind the

diaphragm. Since the diaphragm is not perfectly stiff, it will be deected slightly by a sound pres-

sure. This slight deection of the diaphragm can be considered as equivalent to a small volume

and, as such, should be added to the sum of the grid and coupler volumes. Since the grid and

equivalent volumes can vary slightly from one type of microphone to another, the total volume of

the coupler will, accordingly, also vary slightly.

Fig. 2.1 Principle of the Pistonphone

P = γ⋅P 0⋅4 A p⋅S

V ⋅√ 2(1)

G.R.A.S. Sound & Vibration

Pistonphone Type 42AC - Page 7

A change in the total volume of the coupler will cause a change in the sound pressure level

generated within the coupler. A change in the sound pressure level ∆P, in decibels, for a change

in coupler volume ∆V is given by:

Where:

V = 12530mm

3, including the effective load volume of 40AG

∆ V is the equivalent volume correction

The Pistonphone is calibrated using a G.R.A.S. ½" Microphone Type 40AG. Since all G.R.A.S.

½" microphones have the same grid volume, the only correction necessary when calibrating the

various types is the correction for the various equivalent volumes*.

Table 2.1 shows the equivalent volumes of G.R.A.S. ½" microphones and the corresponding

Pistonphone corrections.

* The G.R.A.S. Microphone Type 40AG is equivalent to the Brüel & Kjær Type 4134 and the volume correction for these are

0 dB. As a matter of interest, the volume correction for a G.R.A.S. Type 40AU and a Brüel & Kjær Type 4180 is 0.095 dB.

The equivalent volume uncertainty is ±7mm

The correction uncertainty is ±0.004dB

No equivalent volume correction is needed for calibration of ¼” microphones (G.R.A.S. type

40BD, 40BE, 40BF, 40BP) using the adaptor RA0049, nor 1” microphone (G.R.A.S. type 40EN)

using the adaptor RA0023.

When calibrating 1/8” microphones (G.R.A.S. type 40DD, 40DP) using the adaptor RA0069 the

equivalent volume correction is +19mm

3 and requires a correction +0.013 dB

2.3 Static-pressure Corrections

The Pistonphone is factory adjusted to give a nominal sound pressure level of 134 dB re.

20 µPa. This nominal value is valid for the following ambient reference conditions:

• Temperature 23º C

• Static pressure 1013 hPa

• Relative humidity 50 %

For other static pressures, the nominal Pistonphone level will have to be corrected. The correc-

tion ∆P, in decibels, is given by the following equation:

∆ P = 20 ⋅Log ⋅( )

V + ∆ V

(2)

∆ P = 20 ⋅Log ⋅( )

(3)

Table 2.1 Corrections for G.R.A.S. ½"microphones re.

the G.R.A.S. Microphone Type 40AG

G.R.A.S. Equiv-volume Correction

Mic. Type correction (mm

3) (decibels)

40AC – 20 0.014

40AD 20 – 0.014

40AE 25 – 0.017

40AF 30 – 0.021

40AN 25 – 0.017

40AP 25 – 0.017

40AQ 15 – 0.010

40AR 25 – 0.017

40AU – 137 0.095

G.R.A.S. Sound & Vibration

Pistonphone Type 42AC - Page 8

Where:

a = measured static ambient pressure (hPa)

r = reference static pressure (1013 hPa)

The corrections for ambient static pressure are shown in Fig. 2.2 (see also Appendix A.1 for an

extended pressure range). These values should be applied to the nominal Pistonphone level.

Fig. 2.2 Corrections for static ambient pressure

2.3.1 Barometer

The Pistonphone is delivered with a small barometer (ZC0003K) as a standard accessory for

use when applying static-pressure corrections

Analogue Barometer ZC0003K (Fig. 2.3)

This barometer has the static-pressure corrections printed directly on its scale.

Digital Barometer RA0168 (Fig. 2.4)

This has an LCD which can display (via a toggle button) either the static-pressure correction

(e.g. – 0.13 dB) or the ambient static pressure (e.g. 997 hPa). It runs on standard 9 V battery

which can be accessed by removing the back plate which is held in place by four screws.

Fig. 2.3 Barometer ZC0003K Fig. 2.4 Barometer RA0168

On/Off switch

Toggle button

G.R.A.S. Sound & Vibration

Pistonphone Type 42AC - Page 9

3. Handling and Operation

3.1 Shipment and Transport

Before any major shipment where rough handling is to be expected, remove the batteries to pro-

tect connectors and other internal parts from accidental damage.

Before shipment, remove the batteries and wrap separately.

Assure cautious handling during transport.

3.2 Batteries

The Pistonphone runs on four 1.5 V batteries (type AA [LR6]). To install or replace the batter-

ies, remove rst the lid of the battery compartment which is held in place by the screw shown in

Fig. 3.1. Slide the lid in the direction shown after removing the screw. Insert four new batteries

observing the correct polarity as indicated on the gure inside the battery compartment.

The Pistonphone can operate continuously for about 20 hours on a new set of batteries.

Remove the battery as soon as it is discharged or if the 42AC is stored for a

prolonged period of time. Leakage from the batteries may otherwise destroy

the electronic components.

3.3 Calibrating Microphones

3.3.1 Microphone Size

The Pistonphone is normally delivered ready to calibrate ½" microphones. For ¼" and ⅛"

microphones, special adapters are included and for 1" microphones, a 1" coupler (available from

G.R.A.S.) has to be tted. Each case is described in the following four sections.

3.3.2 ½"Microphones

To calibrate a ½" measurement microphone, rst loosen the microphone retention collar as

shown in Fig. 3.2. Then insert the microphone into the ½" coupler

1, as shown in Fig. 3.3. Make

sure that the microphone is all the way in, then tighten the microphone retention ring so that the

microphone is held rmly in place.

Fig. 3.1 Access to batteries

1 The ½" coupler is individual calibrated with the pistonphone, and these must be considered as a pair

Fig. 3.2 Loosening the microphone retention collar

G.R.A.S. Sound & Vibration

Pistonphone Type 42AC - Page 10

Switch the Pistonphone on via the on/off (I/0) button. The LED above the on/off button is a

dual-colour LED for showing red or green. The LED shows green if the Pistonphone is operating

properly at the specied frequency. If the LED shows red or ashing red, the Pistonphone is not

operating at the specied frequency and the batteries should be changed (see section 3.2).

Wait approximately for 15 seconds for the static pressure in both the Pistonphone and the

microphone to stabilise, and for the microphone itself to stabilise within the coupler.

The static pressure within the coupler volume is equalised via an air-equalisation tube located

under the cap which protects the pistons and retention spring shown in Fig. 2.1.

With the Pistonphone switched on, the microphone is subjected to a sound pressure level L

given as the sum of the Pistonphone’s nominal sound pressure level L

N, the static pressure cor-

rection L B and the volume correction L

V, i.e.:

Fig. 3.3 ½"microphone inserted in the coupler

3.3.3 ¼"Microphones

To calibrate a ¼" measurement microphone, rst loosen the microphone retention collar as

shown in Fig. 3.2. Then insert the ¼" microphone adapter (RA0049) into the ½" coupler as

shown on Fig. 3.4. Make sure that the adapter is all the way in, then tighten the microphone

retention collar so that the adapter is held rmly in place. Insert the ¼" microphone into the ¼"

adapter.

Switch the Pistonphone on via the on/off (I/0) button. The LED above the on/off button is a

dual-colour LED for showing red or green. The LED shows green if the Pistonphone is operating

properly at the specied frequency. If the LED shows red or ashing red, the Pistonphone is not

operating at the specied frequency and the batteries should be changed (see section 3.2).

Fig. 3.4 ¼"microphone adapter inserted

in the coupler

C= L

N+ L

B+ L

V(4)

G.R.A.S. Sound & Vibration

Pistonphone Type 42AC - Page 11

Wait approximately for 15 seconds for the static pressure in both the Pistonphone and the

microphone to stabilise, and for the microphone itself to stabilise within the coupler.

The static pressure within the coupler volume is equalised via an air-equalisation tube located

under the cap which protects the pistons and retention spring shown in Fig. 2.1.

With the Pistonphone switched on, the microphone is subjected to a sound pressure level L

given as the sum of the Pistonphone’s nominal sound pressure level L

N, the static pressure cor-

rection L

B and the volume correction L

V, i.e.:

3.3.4 ⅛"Microphones

To calibrate a ⅛" measurement microphone, rst loosen the microphone retention collar as

shown in Fig. 3.2. Then insert the ⅛" microphone adapter (RA0069) into the ½" coupler as

shown on Fig. 3.5. Make sure that the adapter is all the way in, then tighten the microphone

retention collar so that the adapter is held rmly in place. Insert the ⅛" microphone into the ⅛"