miniDSP EARS User manual
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Revision history
Revision
Description
Date
0.1
First draft
12 Sept 2017
0.2
Second draft
15 Sept 2017
0.5
Fifth draft
15 Dec 2017
0.7
Preliminary version, OK for public release
17 Dec 2017
1.0
First official version
20 Dec 2017
1.1
Added RAW and HEQ compensation, simplified SPL calibration
16 April 2018
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TABLE OF CONTENTS
Important Information.................................................................................................................................. 4
1Product Overview ................................................................................................................................... 6
2Getting Started with EARS ....................................................................................................................... 7
2.1 A note on measurement conditions...........................................................................................................................7
2.2 Check the EARS gain setting .......................................................................................................................................7
2.3 Download the calibration files ...................................................................................................................................8
2.4 Connect for measurement .........................................................................................................................................8
2.5 Check computer settings............................................................................................................................................9
2.6 Step-by-step: Room EQ Wizard ..................................................................................................................................9
2.6.1 Calculate SPL calibration target.........................................................................................................................9
2.6.2 Configure REW.................................................................................................................................................10
2.6.3 Measure the first channel................................................................................................................................12
2.6.4 Measure the other channel .............................................................................................................................13
2.6.5 Changing the calibration file............................................................................................................................14
2.6.6 Measuring at higher SPLs ................................................................................................................................14
3More about measurement .....................................................................................................................15
3.1 The frequency response graph.................................................................................................................................15
3.2 Other types of measurement ...................................................................................................................................16
3.3 More on headphone measurements .......................................................................................................................17
3.4 More on IEM measurements ...................................................................................................................................19
3.5 Effect of the amplifier (and DAC) .............................................................................................................................20
3.5.1 Amplifier frequency response .........................................................................................................................20
3.5.2 Output impedance...........................................................................................................................................20
3.5.3 What to do.......................................................................................................................................................20
4Calibration/compensation curves ...........................................................................................................21
4.1 Raw vs compensated response ................................................................................................................................21
4.2 Headphone EQ compensation (“HEQ”)....................................................................................................................22
4.3 IEM diffuse-field compensation (“IDF”) ...................................................................................................................22
4.4 Headphone measurement compensation (“HPN”)..................................................................................................23
4.5 Raw calibration with no compensation (“RAW”) .....................................................................................................23
5Further information ...............................................................................................................................24
5.1 Specifications............................................................................................................................................................24
5.2 Obtaining support ....................................................................................................................................................24
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IMPORTANT INFORMATION
Please read the following information before use. In case of any questions, please contact miniDSP via the
support portal at minidsp.desk.com.
DISCLAIMER/WARNING
miniDSP cannot be held responsible for any damage that may result from the improper use or incorrect
configuration of this product. Please read this manual carefully to ensure that you fully understand how to
operate and use this product, as incorrect use or use beyond the parameters and ways recommended in this
manual have the potential to cause damage to your audio system.
Please also note that many of the questions we receive at the technical support department are already
answered in this User Manual and in the online application notes on the miniDSP.com website. So please take
the time to carefully read this user manual and the online technical documentation. Thank you for your
understanding!
WARRANTY TERMS
miniDSP Ltd warrants this product to be free from defects in materials and workmanship for a period of one year
from the invoice date. Our warranty does not cover failure of the product due to incorrect connection or
installation, improper or undocumented use, unauthorized servicing, modification or alteration of the unit in any
way, or any usage outside of that recommended in this manual. If in doubt, contact miniDSP prior to use.
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FCC CLASS BSTATEMENT
This device complies with Part 15 of the FCC Rules. Operation is subject to the following two conditions:
•This device may not cause harmful interference.
•This device must accept any interference received, including interference that may cause undesired
operation.
Warning: This equipment has been tested and found to comply with the limits for a Class B digital device,
pursuant to Part 15 of the FCC Rules. These limits are designed to provide reasonable protection. This
equipment generates, uses and can radiate radio frequency energy and, if not installed and used in accordance
with the instructions, may cause interference to radio communications. However, there is no guarantee that
interference will not occur in a particular installation. If this equipment does cause harmful interference to radio
or television reception, which can be determined by turning the equipment off and on, the user is encouraged to
try to correct the interference by one or more of the following measures:
•Reorient or relocate the receiving antenna.
•Increase the separation between the equipment and receiver.
•Connect the equipment into an outlet on a circuit different from that to which the receiver is connected.
•Consult the dealer or an experienced radio/TV technician for help.
Notice: Shielded interface cable must be used in order to comply with emission limits.
Notice: Changes or modification not expressly approved by the party responsible for compliance could void the
user’s authority to operate the equipment.
CE MARK STATEMENT
The MINIDSP EARS has passed the test performed according to European Standard EN 55022 Class B.
ANOTE ON THIS MANUAL
This User Manual is designed for reading in both print and on the computer. If printing the manual, please print
double-sided. The embedded page size is 8 ½” x 11”. Printing on A4 paper will result in a slightly reduced size.
For reading on the computer, we have included hyperlinked cross-references throughout the manual. In
addition, a table of contents is embedded in the PDF file. Displaying this table of contents will make navigation
easier.
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1PRODUCT OVERVIEW
Thank you for purchasing a miniDSP EARS headphone measurement system. miniDSP EARS is a measurement jig
for headphones and IEMs (in-ear monitors) that we've built using the same core technology as our extremely
popular UMIK-1 calibrated speaker and room measurement microphone.
EARS designed to be a useful and practical tool for headphone enthusiasts. It is ideal to:
1. Check that your headphones are operating correctly e.g. balance of left/right frequency response.
2. Take measurements that you can use as a basis for equalizing your headphones.
3. Observe the effect on frequency response of different earpads and eartips.
4. Measure the effect of modifications to the headphones.
5. Compare your headphone measurements with other EARS users.
Please note that EARS is not an industry-standard measurement head. Even expensive (i.e. tens of thousands of
dollars) heads can produce measurements that are different to each other. Our goal with the EARS is to produce
something that is affordable and fun to use!
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2GETTING STARTED WITH EARS
In a nutshell, you mount your headphones on the EARS jig (or put your IEMs in the “ear canal”) and run a
measurement sweep using your headphone amp and favorite measurement program. To get a useful
measurement, you will need to first load a suitable calibration/compensation file (which we provide) into the
measurement program.
2.1 ANOTE ON MEASUREMENT CONDITIONS
Measurements should be performed under good conditions. While the sweep technique used by modern
acoustic measurement programs is quite robust, external noise can still corrupt a measurement. Be aware of
external noise sources such as computer fans, air conditioning, traffic noise, aircraft and so on. If necessary,
choose a location or time of day for measurement that minimizes noise.
Low frequencies in particular are susceptible to external noise. Even if you cannot hear (or are not aware of)
external noise sources, they can still show up in your measurements. If you are not getting consistent
measurements at low frequency, external noise is the most likely reason.
Try and keep the space around the EARS clear of other objects, and keep both sides similar. For example, don’t
position the EARS so one channel is next to a wall and the other is facing into the room.
2.2 CHECK THE EARS GAIN SETTING
There is a bank of DIP switches on the front of the EARS.
Switches 1 to 3 control the internal gain of the microphone amplifier, as shown in Table 1. (Switch 4 is not used.)
The default gain as shipped and as indicated in the diagram above is 18 dB. The remainder of this section
assumes that the default gain is used. If you change the analog gain setting and want to display correct SPL
values, you will also need to read Measuring at higher SPLs on page 14.
Table 1. Gain switch settings
Gain (dB)
SW1
SW2
SW3
0
Down
Down
Up
6
Down
Up
Down
12
Down
Up
Up
18
Up
Down
Down
24
Up
Down
Up
30
Up
Up
Down
36
Up
Up
Up
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2.3 DOWNLOAD THE CALIBRATION FILES
You will need two calibration files, one for the left channel and one for the right. You can download these files
from the EARS page on miniDSP.com by entering your EARS serial number.
You will also need to select a compensation (see Section 4 for more information). Table 2 lists the recommended
compensation curves for headphones and IEMs with the corresponding downloaded file names. (xxxyyyy in the
file name is the serial number of your EARS unit.)
Table 2. Naming of calibration files for EARS
Code
Type
Channel
File name
HEQ
Over-ear headphone
Left
L_HEQ_xxxyyyy.txt
HEQ
Over-ear headphone
Right
R_HEQ_xxxyyyy.txt
IDF
IEM diffuse field
Left
L_IDF_xxxyyyy.txt
IDF
IEM diffuse field
Right
R_IDF_xxxyyyy.txt
2.4 CONNECT FOR MEASUREMENT
Figure 1 shows a typical connection scheme for headphone measurement. The diagram shows a miniDSP HA-
DSP but you can use any other headphone amplifier. If your headphone amplifier doesn’t have a USB input, then
you will need to adjust accordingly i.e. by using a separate DAC and headphone amplifier. If your headphone
amp has DSP or EQ in it, make sure that it is completely disabled when running measurements.
Figure 1. Typical connection for headphone measurement
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2.5 CHECK COMPUTER SETTINGS
Some simple settings will need to be checked in your computer:
•The sample rate of your DAC is set to 48 kHz
•The input gain of the EARS is set to 0 dB. On the Mac, use Audio MIDI Setup. On Windows, use the
Sound control panel and open its Properties dialog. On the Levels tab, right-click to convert the display
to dB and adjust the slider so it reads 0.0 dB.
2.6 STEP-BY-STEP:ROOM EQ WIZARD
Room EQ Wizard (REW) is a free acoustic measurement program that runs on Windows, Mac and Linux. This
section provides step-by-step instructions with screenshots. Here is the documentation.
2.6.1 Calculate SPL calibration target
1) Work out your SPL calibration target. By default, this is −40 for headphones and −30 for IEMs.
a) Open one of your calibration files (e.g. by double-clicking on it). At the top, you will see two lines like
this:
"Sens Factor =-1.7dB, EARS Serial 8601001, compensation HEQ V1"
"Use this file on the LEFT channel. Your sensitive side is LEFT."
Write down the sensitivity factor (“Sens Factor”), and which channel is the “sensitive side.” In the
example above, the sensitivity factor is −1.7 and the sensitive side is the left.
b) Add the sensitivity factor to the SPL calibration target. For example, with sensitivity factor of −1.7, the
SPL calibration target is −41.7 for headphones and −31.7 for IEMs.
Note: the calculated SPL calibration target is valid only for REW, and only if the option “Full scale sine rms is 0
dBFS” is unchecked i.e. off:
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2.6.2 Configure REW
2) Start REW and open the Preferences window.
3) On the Soundcard tab, set the sample rate to 48 kHz. Select the headphone amplifier (or DAC) as the output
and the EARS as the input source. Choose the “sensitive” channel (Left in this example) for both output and
input, and set the Sweep Level to −20 dBFS.
4) On the Mic/Meter tab, click on Browse. Select the downloaded calibration file for the “sensitive” channel.
Make sure that the “Mic or Z weighted SPL meter” option is set.
5) On the View tab, ensure that the option “Full scale sine rms is 0 dBFS” is unchecked i.e. off.
6) Turn the volume on the headphone amplifier all the way down.
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7) Open the REW Generator window, select Sine Wave output and set the frequency to 300 Hz. Set the RMS
Level at −20 dBFS and press the green Play button.
8) Open the REW SPL Meter window and turn it on (the red button at lower right). Increase the volume on
your headphone amplifier until the dBFS meter (lower half of window) reads the SPL calibration target. (In
this example we are using –40.)
The very first time you do this, we suggest that you increase the volume with the headphones
on your head instead of EARS. This will help ensure that you don’t blast your headphones at
full power if something is not set up correctly.
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9) Press the Calibrate button. In the dialog that comes up, enter 84 for headphones or 94 for IEMs. Then click
Finished.
10) Click on OK on the confirmation dialog that comes up. Close both the Generator and SPL Meter windows.
2.6.3 Measure the first channel
11) Press the Measure button (top left of the REW main window). Set Start Freq to 20 and End Freq to 20,000.
Check that Level is set to –20 dBFS and that the correct channel is selected for output. Then press the Start
Measuring button.
12) When the measurement process finishes, you will get a new measurement in the REW main window.
Rename the measurement so that you don’t lose track of which is which.
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2.6.4 Measure the other channel
13) Open the Preferences window. On the Soundcard tab, select the other channel for output and input. (In
other words, if you selected Left before, select Right now. If you selected Right before, select Left now.)
14) On the Mic/Meter tab, click on Browse. Select the other calibration file for the selected channel. Make sure
that the “Mic or Z weighted SPL meter” option is set.
15) Press the Measure button and ensure that the correct channel is selected for output. Do not change the
Level setting or the volume on your headphone amplifier. Press the Start Measuring button.
16) When the measurement finishes, you will get a new headphone measurement in the REW main window.
Rename the measurement so that you don’t lose track of which is which.
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2.6.5 Changing the calibration file
If you forgot to load a calibration file or had the wrong one loaded (i.e. left instead of right or vice versa), REW
allows you to set or change the calibration file after taking a measurement. Just click on the “Change Cal…”
button underneath the measurement thumbnail on the left-hand side. Then select the correct calibration file.
2.6.6 Measuring at higher SPLs
If you measure headphones at a reference SPL higher than 84 dB, or IEMs at a reference SPL higher than 94 dB,
you may find that the measurement signal clips. In that case:
1. Reduce the analog gain of the EARS (see page 7).
2. Reduce the SPL calibration target by the amount that you decreased the analog gain. For example, if you
reduced the gain from 18 to 12 dB, change the SPL calibration target from e.g. –40 to –46.
3. Recalibrate REW using a 300 Hz sine wave and its SPL meter.
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3MORE ABOUT MEASUREMENT
This section explains various aspects of headphone measurement in more detail.
1
3.1 THE FREQUENCY RESPONSE GRAPH
Figure 2 shows an example frequency response graph. This is the most common and useful graph correlating to
headphone sound. Some of the important controls are highlighted in red. In REW, this is the default display, and
is selected with the highlighted button “SPL & Phase.”(The specifics here relate to REW, but any measurement
program will have similar functions.)
The frequency response graph (in red) shows how the measured SPL of the headphone varies with frequency.
For example, at 4 kHz the SPL is 75 dB, as shown by the blue markers overlaid on the graph. Underneath the
graph is a set of checkboxes; usually, you will need to display only the amplitude response (named “Left” in this
example), and leave Phase and Mic/Meter Cal unchecked.
The frequency range is 20 Hz to 20 kHz, as indicated by the scale along the bottom. The SPL range is 50 to 100
dB, as indicated by the scale on the left. You can set the scales by clicking on the Limits button at the top right.
While it is certainly possible to measure over a larger frequency range e.g. 10 Hz to 24 kHz, note that:
•Low frequency measurements are easily corrupted by noise, and
•The calibration/compensation files don’t go higher than 20 kHz.
1
The measurements in this section are intended to illustrate certain points. They are not intended as reference
measurements of the headphones and IEMs mentioned.
Figure 2. An example frequency response measurement
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Measurements are easier to interpret if some smoothing is applied to the graph. This can be done from the main
Graph menu. For most purposes, we suggest using 1/12th-octave smoothing, as illustrated in Figure 3.
3.2 OTHER TYPES OF MEASUREMENT
The bar along the top of Figure 2 can be used to select other types of measurement. Some of these will be the
subject of future app notes on our web site, but here is a brief overview of the most relevant and useful for
headphones (and IEMs):
All SPL Shows multiple SPL measurements. To take an average of several measurements, click All SPL
and select the ones that you want to average using the checkboxes at the bottom. Then click on
the Average the Responses button.
Distortion Displays measured harmonic distortion versus frequency. The graphs are displayed in dB, but
you can read off the distortion in percent at any frequency by placing the cursor on the graph,
then reading off the distortion % next to the graph names underneath.
Impulse EARS is not suited for impulse response measurements unless the measurement program uses
the phase information in the EARS calibration file when it displays the impulse response. REW
doesn’t currently do this.
Waterfall The “waterfall” or CSD (cumulative spectral decay) plot is a hybrid time-and-frequency display
that is useful to show resonances in the response. Note: while it seems likely that a headphone
with a “clean” CSD will sound better than a headphone with a “messy” CSD, we are not currently
aware of formally-published research in this area.
Another type of measurement you will see in some online measurement sets is the headphone impedance vs
frequency. EARS can not be used to make impedance measurements.
EARS is also not well suited for sound isolation measurements, as the omnidirectional microphone capsules pick
up sound from the back of the “ear,” limiting the maximum attenuation that can be measured.
Figure 3. The effect of smoothing on a graph
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3.3 MORE ON HEADPHONE MEASUREMENTS
The measurement obtained from a headphone will vary with headphone position and seal. In Headphone
Measurement Procedures - Frequency Response, Tyll Hertsens explains that he takes measurements at five
positions of the headphones on the measurement jig (that is, five on each side).
Figure 5 shows measurements taken at five headphone positions (one channel only) for an open headphone
(AKG K702), while Figure 4 shows measurements at five positions for a closed headphone (Audeze LCD-XC). We
recommend that you measure your own headphones at several different positions to establish how much
variation there is with position.
If you find that you get significant variation, you may wish to use your measurement program to generate an
average of several measurements. You may also find that, with practice at headphone positioning, you can
consistently get a “center” position measurement that is very close to this average.
Figure 4. Effect of headphone positioning 2 (LCD-XC)
Figure 5. Effect of headphone positioning 1 (K702)
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Earpads will also make a difference to a headphone measurement. While the EARS can be used to identify the
differences in frequency response that you hear with different earpads, this is also information to provide if
sharing your measurements with others. For example, the top two traces in Figure 6 show the
Massdrop/HIFIMAN HE4XX with FocusPad in green and the FocusPad-A in purple. The lower two traces show the
same FocusPad in green compared to an old worn-out FocusPad in red. (The difference in the bass area is most
likely due to the old pads being torn and therefore unable to seal properly.)
Some headphones are very sensitive to the amount of seal and/or any pressure applied. Figure 7 shows a small
over/on-ear headphone (Bose QuietComfort 35 II in passive mode). The curves in purple and blue are with the
headphone positioned so that the headphone doesn’t seal to the jig. The curve in red would be considered the
normal measurement; the curve in green is with additional pressure applied to the earcup. As you can see, some
care will be needed to get consistent and repeatable measurements when measuring a headphone such as this.
Figure 7. Effect of seal/pressure on small sealed headphone (QC35II)
Figure 6. Effect of pad differences (HE4XX)
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3.4 MORE ON IEM MEASUREMENTS
Figure 8 shows the effect of different eartips (each graph is the average of left and right on an Etymotic ER4SR).
While EARS can be used to measure the frequency response changes that you hear with different eartips, this
also means that the eartips you used should be mentioned if sharing or publishing your measurements.
Insertion depth also makes a big difference. Figure 9 shows measurements at three insertion depths (ER4SR with
Comply foam isolation tip). It is important to note that not only does the level change with insertion depth, but
so does the frequency response.
Because of this, when measuring IEMs that don’t have a fixed insertion depth, we recommend that you repeat
the reference SPL level check on each insertion and for both sides –that is, don’t change the headphone amp
volume, just check the level and move the IEM out or in a little until the level is close to your reference level.
Additional notes and things to be aware of when measuring IEMs:
•IEM measurements vary quite a lot at high frequencies, and generally drop off very quickly at 15 kHz or so.
Be careful when doing high frequency EQ and don’t try and EQ up the frequency response above 15 kHz.
•It’s possible with some IEMs to inadvertently obstruct the end of the IEM tube. This will show up as greatly
reduced bass response.
•Some IEMs will show strong resonant peaks in the response. Sometimes these are the IEM itself, but other
times it’s a resonance in the “ear canal” that moves around with insertion depth. Don’t EQ the latter.
•Avoid inserting a “deep insertion” IEM in as far as it will go, as the tip will collide with the microphone
capsule. This will obstruct the tube and produce a poor measurement. It could also (if done forcibly)
potentially damage the capsule.
Figure 8. Effect of different IEM eartips
Figure 9. Effect of IEM insertion depth
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3.5 EFFECT OF THE AMPLIFIER (AND DAC)
Everywhere else in this manual, it is assumed that the amplifier (and the DAC if you have one in the
measurement chain) are perfect. In practice, this may not be the case.
3.5.1 Amplifier frequency response
Figure 10 shows a typical amplifier (and/or DAC) frequency response. If your amplifier (and DAC) “droops” in
frequency above 20 Hz or below 20 kHz, this will affect your headphone measurement.
3.5.2 Output impedance
If your headphones have a non-constant impedance and your amplifier has a high output impedance (say 10% or
more of the headphone’s nominal impedance), this may affect your measurements. The worst case variation is
given in this table, where Zamp is the amplifier’s output impedance and Zphones is the headphone’s nominal
impedance:
Zamp/Zphones (%)
10
20
50
100
Worst case variation (dB)
1
2
3.6
6
In practice, real headphones will have less variation than this. For example, planars have an almost constant
impedance and so will not show variation due to the amplifier output impedance.
3.5.3 What to do
To make headphone measurements that are not affected by the amplifier, a solid-state headphone amplifier
with a bipolar power supply (and no coupling capacitor on the output) will usually be a good choice. The criteria
are flat frequency response to the measurement limits and an output impedance less than 10% of the
headphone’s impedance.
In some cases, you may wish to do a measurement and then EQ that includes the effect of the amplifier. That
way, the EQ compensates for the amplifier as well as the headphones. In that case, measure using the same
amplifier that you plan to listen with.
Figure 10. Typical DAC+amplifier frequency response (exaggerated)
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