GRAS 42AP User manual

Instruction Manual
Pistonphone Type 42AP
Skovlytoften 33, 2840 Holte, Denmark
www.gras.dk [email protected]
SOUND & VIBRATION
G R A S
...
.
LI0053

G.R.A.S. Sound & Vibration
Revision History
Any feedback or questions about this document are welcome at [email protected].
Revision Date Description
1 1 October 2005 First publication
2 4 July 2013 Section 3.1 “Shipment and transport” added
Copyright Notice
© 2013 G.R.A.S. Sound & Vibration A/S
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 han-
dling during transport. Remove the battery as soon as it is discharged or if the 42AP
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 ..............................................5
1.1 Operation ........................................................5
1.1.1 Coupler Size. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2 Operating Frequencies ..............................................6
1.3 Calibration Level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2. Theory .........................................................7
2.1 Principle .........................................................7
2.2 Volume Corrections. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3 Static-pressure Corrections ..........................................8
2.3.1 Built-in Barometer ....................................................9
2.3.2 Thermometer ........................................................9
3. Handling and Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1 Shipment and Transport ............................................10
3.2 Batteries ........................................................10
3.3 External Connector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.3.1 External Power Supply. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.3.2 RS-232 Connection ..................................................10
3.4 Calibrating Microphones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.4.1 Microphone Size ....................................................11
3.4.2 ½"Microphones .....................................................11
3.4.3 ¼"Microphones .....................................................12
3.4.4 ⅛"Microphones .....................................................12
3.4.5 1"Microphones .....................................................13
3.5 Calculation of Microphone Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4. Remote Control via RS-232 Interface ...............................15
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.2 Interface ........................................................15
4.3 Behaviour .......................................................17
4.4 Commands and Responses .........................................17
4.4.1 Interrogational Commands ............................................17
4.4.2 Setup Commands ...................................................18
4.4.3 Special Responses ..................................................18
5. Specications ..................................................19
A Appendix ......................................................21
A.1 Corrections for Static Ambient Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
A.2 Corrections for Humidity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

G.R.A.S. Sound & Vibration

Button beside the LED
G.R.A.S. Sound & Vibration
Pistonphone Type 42AP - Page 5
1. General Description
The G.R.A.S. Pistonphone Type 42AP (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.
1.1 Operation
When power is turned on, the coupler size will be displayed and the LED will light red.
After 1-2 sec. the frequency will be displayed, the Pistonphone will start and the LED will turn
green indicating that the Pistonphone is now locked to the operating frequency.
After another second or two the Pistonphone will be in normal operation mode, pressing the
button beside the LED will step the display to show the following (see Fig. 1.1):
• actual corrected sound pressure level in decibels re. 20 µPa
• actual corrected sound pressure level if measured with A-weighting in decibels re. 20 µPa
• static air pressure in h Pa
• calibration temperature in °C
• calibration temperature in °F
All information, including calibration data, can be read remotely via the RS-232 interface.
Fig. 1.1 Pistonphone Type 42AP shown with display examples
1.1.1 Coupler Size
It is important that the size of the coupler agrees with what is registered in the Pistonphone’s
memory. The Pistonphone can be tted with either a ½"coupler (RA0048) or a 1"coupler
(RA0023). Note: the coupler is calibrated together with the Pistonphone.
If the coupler size shown on the display (0.5 in or 1in) when rst switched on does not agree

G.R.A.S. Sound & Vibration
Pistonphone Type 42AP - Page 6
with the size of the coupler currently tted, do one of the following:
• Change the coupler to agree with what is displayed when the Pistonphone is rst switched
on.
OR
• Switch the Pistonphone off then while holding down the button beside the LED switch the
Pistonphone on again. The alternative coupler size will now be displayed.
1.2 Operating Frequencies
The operating frequency of the Pistonphone is programmable (see chapters 4 and 5) and is
selected via the RS-232 interface to be either 250 Hz
1or 251.2 Hz 2.
The current operating frequency is memorised and will be displayed for a short time on the
screen whenever the Pistonphone is switched on.
1.3 Calibration Level
With a microphone inserted in the Pistonhone’s coupler and the Pistonphone switched on, the
nominal calibration level is:
• 114 dB 3re. 20 µPa
The actual sound pressure level is shown on the display of Pistonphone, corrected for the static
ambient pressure. The display can be selected to correct for using an A-weighting lter.
An individual calibration chart is delivered with each Pistonphone.
The Pistonphone complies with all the requirements of IEC Standard 60942 (2003) LS and 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.
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 0.1% via a tachometer signal in a feed-back loop.
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 Pistonphone is delivered tted with the coupler (RA0048) for calibrating ½"microphones.
Adapters for calibrating ¼" and ⅛"microphones are included. A coupler (RA0023) for calibrating
1"microphones is also included.
Note: When the Pistonphone is calibrated, it must be calibrated with its ½”coupler (RA0048), its
1"coupler (RA0023), and, if purchased, the two-port calibration coupler (RA0024).
1Factory setting
2 True centre frequency of a 250 Hz ⅓-octave band lter
3114 dB is the equivalent of 10Pa

P = γ⋅P 0⋅2 A p⋅S
V ⋅√ 2(1)
G.R.A.S. Sound & Vibration
Pistonphone Type 42AP - Page 7
2. Theory
2.1 Principle
The Pistonphone works on the principle of a pair 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.)
P
0is the ambient pressure in pascals (Pa)
A
pis the cross-sectional area of one piston in square metres (m
2)
Sis the stroke length of the pistons in metres (m)
Vis the coupler volume in cubic metres (m
3)
A
pand Sare 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 = 20 ⋅Log ⋅( )
V
V + ∆ V
(2)
G.R.A.S. Sound & Vibration
Pistonphone Type 42AP - Page 8
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 ∆ Vis given by:
Where:
V= 15540mm
3, including the effective load volume of 40AG
∆ Vis 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
4.
Table 2.1 shows the equivalent volumes of G.R.A.S. ½"microphones and the corresponding
Pistonphone corrections.
4The 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 0.077 dB.
The equivalent volume uncertainty is ±7mm
3.
The correction uncertainty is ±0.004 dB.
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
3and requires a correction -0.011 dB.
2.3 Static-pressure Corrections
The Pistonphone is factory adjusted to give a nominal sound pressure level of 114 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 %
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 0.011
40AD 20 – 0.011
40AE 25 – 0.014
40AF 30 – 0.017
40AN 25 – 0.014
40AP 25 – 0.014
40AQ 15 – 0.008
40AR 25 – 0.014
40AU – 137 0.077

SPL = 20 ⋅Log ⋅( ) + SPL
ref
P
a
P
r
(4)
∆ P = 20 ⋅Log ⋅( )
P
a
P
r
(3)
G.R.A.S. Sound & Vibration
Pistonphone Type 42AP - Page 9
The displayed pressure level is corrected (by an amount ∆ P) for static pressure variations given
by the following equation:
Fig. 2.2 Corrections for static ambient pressure
2.3.1 Built-in Barometer
The Pistonphone has a built-in precision barometer which measures and displays the static
ambient pressure. This value can also be used by the user for correcting sound pressure levels
for static ambient pressure. The Pistonphone already displays the corrected sound pressure
level, with and without A-weighting. The corrected sound pressure level (SPL) is calculated as
follows:
Where:
P
a = the displayed static ambient pressure (hPa)
P
r= reference static pressure (1013 hPa)
SPL = the actual sound pressure level in the coupler (decibels re. 20µPa)
SPL
ref = the sound pressure level at reference condition (decibels re. 20 µPa)
Note: SPL
ref is both stored in the memory of the Pistonphone and given on the calibration chart.
2.3.2 Thermometer
The Pistonphone has a thermometer that measures the temperature of the Pistonphone. The
thermometer tracks the actual calibration temperature in the measuring chamber to within a
degree. The measured temperature can be displayed both as °C and °F.
Where:
P
a= measured static ambient pressure (hPa)
P
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).

G.R.A.S. Sound & Vibration
Pistonphone Type 42AP - Page 10
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 10 hours on a new set of batteries.
Remove the battery as soon as it is discharged or if the 42AP is stored for a
prolonged period of time. Leakage from the batteries may otherwise destroy
the electronic components.
Fig. 3.1 Access to batteries
3.3 External Connector
The Pistonphone has a 4-pin LEMO socket for both an external power supply and an RS-232 con-
nection (Fig. 3.1). Use the cable (AA0050) supplied for this purpose. The other end of this cable has
a 9-pin D-sub connector (for RS-232) with a 2mm DC socket for connecting to an external 6V DC
power supply (e.g. G.R.A.S. mains/line power supply AB0005). See also Fig. 4.1
3.3.1 External Power Supply
When an external 6 V DC (e.g. from a G.R.A.S. mains/line power supply AB0005) is applied to
the Pistonphone via the input socket, the Pistonphone is automatically switched on independ-
ent of the on/off (I/0) button (unless programmed otherwise, see section 4.4.2) and the internal
batteries are disabled.
3.3.2 RS-232 Connection
The RS-232 meets the electrical specications dened by EIA/TIA-232, the 9-pin D-sub connec-
tor is dened by EIA-574 Specications. Where:
Pin 2 for receive data (RX)
Pin 3 for transmit data (TX)
Pin 5 is ground which is connected to screen.
No other pins are connected.