TMP: True Piston Motion

Eighteen Sound Patented TPM
True Piston Motion (TPM) technology has been developed by Eighteen Sound in order to improve the high frequency behavior of top of the range compression drivers.

The further big advancement achieved with TPM technology, when compared to already optimum performances of pure titanium diaphragms, has been obtained by extensive research made on the new material treatment.

The special Titanium Nitride Coating dramatically improves stiffness with obvious benefits in transient and intermodulation distortion response. With its very high value of elasticity modulus (five times more than titanium), nitride thin film is capable of increasing regular titanium stiffness by 45%.

The piston frequency range motion extends 25% higher in frequency, showing a predictable, ideal frequency response decay and avoiding top-end spurious resonances. The nitride-free ellipsoidal suspension shape has been designed to maintain inherent titanium constant stiffness, assuring a 3rd harmonic distortion figure lower than 0.05% over the whole working frequency range.


It is known that a diaphragm for an electro-acoustic transducer (for example, a compression driver) must be light-weight and have a high value of flexural rigidity. This is because of its superior sound reproduction characteristics (higher efficiency, lower distortion, faster transient response).

It is also known that the flexural rigidity (EI) is given by the product between Young Modulus E and the second moment of Inertia I of the cross sectional area around a neutral axis.

Hence, the most important physical properties for diaphragm materials are:

  • density ρ (Kg/m³)
  • Young Modulus E (GN/m²)

By the higher value of the E/ ρ ratio, it is possible to widen the range of piston motion and consequently to improve the high frequency reproduction. On the table below, the physical properties of the main metal foils used for compression driver diaphragms are summarized

  E (GN/m²) ρ (Kg/m³) E/ρ (m/s)²
Titanium 119 4.54 x 10³ 26 x 106
TiN 600 5.22 x 10³ 115 x 106
Titanium - TiN composite layer 173 4.60 x 10³ 37 x 106

State of the art

The most popular metal film used for compression driver’s diaphragms is titanium. This because it has high E modulus, it is easily formed, it is reliable and its sound characteristics have become a reference on the pro audio market. Other interesting metal foils for compression driver diaphragms are aluminum and magnesium as well as beryllium.

TPM technology

Eighteen Sound has developed TPM (True Piston Motion) technology, consisting of a composite board diaphragm formed with two skin layers of TiN (Titanium Nitride) ceramics material deposited on both sides of a light weight titanium core layer.

Moreover, in order to maintain the original suspension stiffness of the titanium diaphragm, a special PVD (Physical Vapor Deposition) suspension masking has been developed and adopted during the  deposition process.

Looking at the picture below, it is easy to  understand how the two TiN skin layers operate on the composite material during diaphragm vibration. It is clearly evident that higher composite diaphragm compression and tension resistance can be reached by the.

By the picture below it is easily understood how the two TiN skin layers operate on the composite material during diaphragm vibration. It looks clearly evident that higher composite diaphragm compression and tension resistance can reached by the:

  1. bigger distance between two skin
  2. higher value of E young modulus
  3. higher thickness of the skin

The most important properties of the skin layers is the flexural (or stiffness) rigidity that is a function of the intrinsic material properties E (Young Modulus) and by the geometry of the composite diaphragm (i.e. distance between two skins layer).

From a physical point of view, we can calculate the flexural rigidity of the composed diaphragm by the formula:

EI = ETiNI1 + ETiI2 + ETiNI3


  • E - composite young modulus
  • ETiN - TiN young modulus
  • ETi - titanium young modulus
  • I - second moment of inertia of the composite
  • I1 - second moment of inertia of the upper TiN layer
  • I2 - second moment of inertia of the titanium
  • I3 - second moment of inertia of the lower TiN layer

Because the composed diaphragm is symmetric around a neutral axis it follows that I1 = I3 = ITiN

Therefore: EI = 2ETiNITiN + ETiI2

It is possible to show that with only 8% of increment of diaphragm mass the composite Young Modulus increases respectively by the 45% while the composite E/ρ ratio raises by the 42%. (as shown on the table above).

Measurements and Considerations

The TPM technology, based on its composite nitride-titanium diaphragm, thanks to its better piston motion behaviour, is capable of  improving sound quality and transient response.

In order to demonstrate this, we have compared two different diaphragms; one made with standard titanium foil and a similar one treated with TPM technology. Using Laser Doppler Vibrometry Analysis (LDV) we can see different diaphragm behaviors at various frequencies. The pictures below show the diaphragm velocity map at 13kHz of both TPM and conventional titanium diaphragm.




It is now evident how the composite diaphragm break-up modes are moved up on frequency domain.

From an acoustical point of view, TPM composite diaphragms offer linear piston decay at high frequencies (see Fig.1), shorter and smoother impulse response (see Fig.2) and lower Intermodulation – IMD - distortion (see Fig.3) when compared to pure titanium diaphragms. In fact, the dome break-up modes have very high influence on the IMD and impulse response with clearly audible effects.

Fig.1 – SPL Response comparison: 3" TPM diaphragm (white) – 3" Titanium diaphragm (cyan)

Fig.2 – Impulse Response comparison: 3" TPM diaphragm (white) – 3" Titanium diaphragm (cyan)

Fig.3 – IMD Response comparison: 3" TPM diaphragm (white) – 3" Titanium diaphragm (cyan)

  Patented TPM (True Piston Motion) PDF (300 Kb)