Audio Kit Help Page

Audio Kit is a collection of four applications which are essential for those working in audio and sound. The individual applications are a real time Spectrum Analyser, a Scope to display waveforms, a SPL (sound pressure level) Meter, and a Signal Generator producing sine waves, white noise and pink noise.

Spectrum Help
Scope Help
SPL Meter Help
Signal Generator Help
Startup Settings Help

The App will automatically detect when a headset or headphone is plugged in. If an iPhone or iPad headset with microphone (4-conductor jack plug) is plugged in, both the microphone and the earphones on that headset will be used for input and output.

If an iPhone or iPad headphone without a microphone is plugged in, the device's built-in microphone will continue to be used, but the output will be from the earpieces.

If an external microphone is used, the output will be from the external 4 pin jack also. In this case if output from the Signal Generator is also needed at the same time, an adapter is required to split the headphone and microphone signals. An example of this is the SP-IPHONE-CABLE from The Sound Professionals.

Always start Audio Kit before plugging in external devices for the first time.

FFT Spectrum

Spectrum is a FFT (Fast Fourier Transform) based real time spectrum analyser. It is simple to operate and displays a spectrum of around the entire normal human hearing range (20-20,000Hz).

The FFT method calculates the frequencies in groups (or bins), rather than discrete individual frequencies.

In Audio Kit there are 3 selectable FFT data sizes - 2048, 4096 and 8192. These correspond to bin sizes of 21.5332Hz, 10.7666Hz and 5.383308Hz respectively. Data from these bins is then used to calculate the highest peak values exactly by interpolation, typically giving an accuracy to within 1Hz.

The initial display is with the frequency (x) scale in logarithmic format. A linear frequency scale can also selected in the options. In both the log and linear scales the vertical green lines enable you to estimate the frequency of various other peaks besides the strongest, which is indicated with moving text, and with large number in the center.

In the upper right hand side of the screen there are labels displaying the number of FFT windows, the FFT size, scale type of either "Log" or "Lin" to reflect the scale currently being used, and just underneath these the FFT Window Type is displayed.

By pressing the Info button you will be presented with a transparent view. This allows you to:

The strongest frequency is displayed in two places:

  1. Above the peak of the strongest frequency in a small font.
  2. In the upper center of the screen in a large font. This large number can be quickly toggled on or off by double tapping in the main area of the Spectrum screen.

All of these quick change settings can be made the default startup settings in the Settings view.

FFT Parameters

Audio Kit allows for the selection of three types of parameters for FFT calculations used in calculating the Spectrum display:

Changes to all 3 of these FFT parameters can be made from the Settings -> Spectrum Settings screen, and are effective immediately as well as being retained as the startup settings.

The number of FFT windows used to calculate and display the graph is selectable from 1 to 4 windows. Selecting one window displays readings from a single sample. As more windows are selected samples are averaged and a more stable display is achieved with less noise. More windows also has the effect of a slight inertia to the display, giving a memory effect to the peaks.

Three different FFT data sizes or lengths can be selected - 2048, 4096 and 8192. The current running FFT size is displayed next to the Log/Lin label in the upper right hand corner of the Spectrum display.

There are 6 different FFT window function types available: Hamming, Hann, Rectangular, Bartlett, Triangular and Blackman. The current running FFT window type is displayed in the upper right hand side of the Spectrum screen under the Log/Lin display.

Other Spectrum screen controls

By giving the Spectrum screen a single tap, the screen will freeze. Another single tap will unfreeze it. When the display is frozen other functions such as screen capture, toggle big number display and the info button can still be accessed. The option to freeze the screen in this way can be switched off in the Settings, Spectrum Settings screen. Switching this option off will require the App to be stopped completely by removing it from the background tray and restarting it. This feature works with a short tap, a touch and hold gesture will have no effect.

A screen capture facility is included to allow spectrum displays to be saved. The images from the screen capture are saved to the Photos App and can be later accessed and sent via email or other method. To access screen capture tap the Capture Screen button on the upper left of the screen. The first capture may take a few seconds, but subsequent captures are quicker. The timestamp is in UTC time.


This is a simple oscilloscope type display of the raw sounds received by the microphone.

Using a pinch gesture with 2 fingers will allow you to expand and contract the display in both directions so as to best present the waveform for viewing.

Zero crossing of the waveform is detected so that it appears to be starting from zero at the left axis. The default is positive zero crossing. Negative zero crossing can be selected in the options using the Info button in this screen.

The App help page is also accessed from the Info button.

Note that at very high frequencies the waveform appears less smooth, and more jagged. This is due to less samples being taken at those frequencies. Using the included Signal Generator at frequencies in the mid range will display very smooth sine waves, even given the limitations of the device speaker and microphone. The Signal Generator itself is producing very uniform waveforms.

A screen capture facility is included to allow waveform displays to be saved. The images from the screen capture are saved to the Photos App and can be later accessed and sent via email or other method. To access screen capture tap the "Capture Screen" button on the upper left of the screen. The first capture may take a few seconds, but subsequent captures are quicker.

If you get an error about not being able to save the image, check you device Settings. Go to Settings -> Privacy -> Photos on the iPhone, you should see an option to allow Audio Kit access to Photos. Make sure this is in the ON position.

SPL Meter

A Sound Pressure Level Meter presents sound levels showing the current level as the main large display, and smaller displays showing minimum and maximum over the long term sampling period. Pushing the reset button resets all the digital displays and puts a mark on the ticker type plot.

There is also a colored bar type indication of the measured SPL at the left hand side of the screen.

A sliding ticker type plot of the SPL readings over time is displayed across the bottom of the SPL screen. If the reset button is pressed the peak registers a mark of full scale then zero.

The SPL meter has been calibrated for the built-in iPhone and iPad microphones. Other external microphones may read slightly differently.

The output is adjusted for the A-Weighting of frequencies.

The SPL Meter starts running once its screen has been viewed for the first time. After this it is always running in the background until the Audio Kit App terminates.

The Info button on the SPL Meter view brings up these help pages.

While this meter gives a reasonably accurate measurement, it should not be used for any critical measurements or for any legal purposes. Sinusoid Pty Ltd bears no responsibility or liability for any use of this SPL Meter or any other parts of this iPhone App in such a way. Please see the web site for more details.

SPL Calibration

If a reference SPL meter is available or if you would like to calibrate two or more Audio Kit SPL Meters to read similar values, follow this calibration procedure.

While in the SPL view, press the Calibrate button. This will bring up a small window with a slider control, an End Cal. button, and a Cancel button. During calibration the display will be colored red. If the slider is not moved and the End Cal. button is tapped, or if the Cancel button is tapped at any time, the calibration window will disappear and no change will be made. If the slider has been moved and the End Cal. button is tapped, a dialog will appear asking you to confirm or cancel the calibration.

To make the actual calibration, choose a stable sound source that is not changing, and make sure the microphones of both devices are near each other, pointing in the same direction, and the same distance from the calibration source. Move the slider control towards either end to make the display of the Audio Kit SPL meter read the same as the reference device. When finished, tap the End Cal. button and confirm the calibration.

For best results, calibrate at a sound level near to what you intend to be measuring. For example if you are measuring loud sounds, calibrate at a high dBA level.

Signal Generator

This is a Signal Generator with 3 different output types:

The sine wave has variable frequency output from 0-20,000Hz. Due to the characteristics of the built-in speaker the effective useful range is about 200-20,000Hz, but output from the 3.5mm audio jack will give a greater usable range at the low end.

The controls on the Signal Generator are a frequency control picker, and to its right, a volume control slider and signal type selector panel. Use this signal type selector to select signal Off, Sine Wave, White or Pink Noise, or Sweep Generator.

The volume slider controls the volume for all signal types.

Using the frequency control picker wheels, frequency of the sine wave can be selected accurately to within 1Hz.

To set the Sweep Generator, use the frequency control picker to select start and stop frequencies, then tap the number under the "Start f", "Stop f" labels to set the value. Also use this picker to select the duration of the sweep, in milliseconds and tap the number under the "Sweep time" label to set it. So that for example 15,000 milliseconds will be 15 seconds sweep duration.

Sweep can go in both increasing and decreasing directions, as long as different frequencies are set, and the Sweep time value is not zero.

The sweep can either be linear or logarithmic, and this is set in the Signal Generator Startup settings screen.

If the Off button is selected, no signal is generated. If the Sine wave is selected there will be a red badge over the Sig Gen icon in the lower right of the tab bar. This will remain regardless of which view the App is displaying, and will show the set frequency of the Signal Generator. Likewise, in the pink and white noise positions the badge will show which noise setting is in effect.This red badge does not appear when the Signal Type selector is in the Off.

If an external output plug is to be used with the Signal Generator, make sure that Audio Kit is started before plugging it in or the output signal will be very low. If this has already occurred it can be fixed by stopping Audio Kit and removing it from the app tray, then restarting it.

Changes to the signal type settings also remain in startup settings. For volume and sine wave frequency startup settings, make changes permanent in the Startup Settings -> Signal Generator View.

The Info button on the Signal Generator view brings up these help pages.

Startup Settings

The Settings view allows for user preferences in each of the above applications to be made persistent.

In this way preferred initial settings are retained between restarts of the App.

In each case the quick changes to settings can still be made from within the individual applications. For example, in the Spectrum application the frequency scale, line style and whether the large number display is on or off can still be quickly changed, but the startup settings remain as set.

Note that these persistent Startup Settings apply to a complete restart of the App. If the App has been run recently and is still in the recent Apps tray, the last running settings still apply.

Spectrum Startup Settings

The following four are startup settings, and changes won't have effect until the App is stopped completely, including from the background. The following three settings all take effect immediately and do not need a restart. They also are saved as startup settings.

Scope Startup Settings

Zero crossing detection direction - Set to either Positive or Negative direction.

SPL Meter Settings

There are currently no persistent settings available for the SPL Meter. This screen is a placeholder for now.

Signal Generator Startup Settings

Startup frequency. This displays the saved startup frequency, and if the signal generator is running, the current frequency. To set a new startup frequency, set the desired frequency from within the Signal Generator and then tap the Yes button in this screen. If you don't want any changes, just leave the setting as it is and tap the All Settings button.

Startup Volume. This displays the saved startup volume, and if the signal generator is running, the current volume. To set a new startup volume, set the desired volume from within the Signal Generator and then tap the Yes button in this screen. If you don't want any changes, just leave the setting as it is and tap the All Settings button.

Note that with the Signal Generator Signal Type switch in the Off position, the volume in the Settings Screen will show zero. Switch to sine wave or one of the noise types to set a volume level first. This is to prevent an unintentional loud signal on startup.

Permanent changes to the signal type are made in the Signal Generator screen as the type is selected.

The Help button on the Settings view brings up these help pages.

All Screens Settings

Disable Idle Timeout

Set this switch to the on position to prevent from going into auto-lock if you have this set on your device. This allows for long term monitoring and will only take effect when the app is in the foreground.

Screen Orientation Preference

This setting allows you to control the orientation of the screens in view. There is a control with 3 choices:

The orientation lock take effect the next time the device is rotated.

These orientation settings remain between restarts of Audio Kit until changed by the user.

Note that there is no portrait orientation for Audio Kit.

Quick Fixes

These are some simple fixes for a couple of issues running Audio Kit which we have seen recently. They relate to privacy settings on the OS.


Constructive feedback is welcome as are suggestions for features in future updates. See the web site for contact details.

Audio Kit version 2.0
Sinusoid Pty Ltd
March 2016

A copy of this help file can be found on the website under Attachments for Audio Kit.