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

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.

• 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

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.)

0is the ambient pressure in pascals (Pa)

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

Sis the stroke length of the pistons in metres (m)

Vis the coupler volume in cubic metres (m

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

(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

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

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

(4)

∆ P = 20 ⋅Log ⋅( )

(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:

a = the displayed static ambient pressure (hPa)

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:

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).

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.

C= L

N+ L

B+ L

V(5)

G.R.A.S. Sound & Vibration

Pistonphone Type 42AP - Page 11

3.4 Calibrating Microphones

3.4.1 Microphone Size

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

special adapters are included. For 1"microphones, the 1"coupler

1 RA0023 has to be tted (see

Fig. 3.6). Each case is described in the following four sections.

3.4.2 ½"Microphones

Note: make sure the Pistonphone is set for the ½"coupler, see section 1.1.1.

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

1as 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.

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

Band the volume correction L

V, i.e.:

Fig. 3.2 Loosening the microphone retention collar

Fig. 3.3 ½"microphone inserted in the coupler

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

C= L

N+ L

B+ L

V(6)

G.R.A.S. Sound & Vibration

Pistonphone Type 42AP - Page 12

3.4.3 ¼"Microphones

Note: make sure the Pistonphone is set for the ½"coupler, see section 1.1.1.

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.