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How to calculate sound pressure level at the front wall?
Old 13th December 2018
  #1
Here for the gear
 

How to calculate sound pressure level at the front wall?

Hi,

I am contemplating changing my current speakers (PMC) to a pair of Dutch and Dutch 8C and I have a question about how to calculate the sound pressure level at the front wall in two different scenarios. Below are a couple of links for those of you who are not familiar with the 8C monitors.

8c - Dutch & Dutch

The Dutch & Dutch 8Cs are remarkable speakers..

The DSP-based 8Cs are different from most monitors in that the woofers handling frequencies below 100 Hz are on the backside of the monitors and the rest of the frequency spectrum is handled by a forward firing midrange and tweeter. The 8Cs are cardioid above 100 Hz with 20 dB backward attenuation starting at 100 Hz. The backside of the monitors should be placed close to the front wall, less than 40 cm, to obtain boundary coupling. The 8Cs are full range and extends flat to 20 Hz.

Since I am not a professional user and intend to have the 8Cs in our living room, I am a bit worried that two powerful subwoofers shooting bass below 100 Hz straight into the (concrete) front wall from a very short distance will disturb our neighbours on the other side of the wall. With my current speakers, which extends flat to about 25 Hz, they are not disturbed.

My current speakers are, however, front firing. Both the woofers and the transmission line ports are on the front of the speakers with the ports just above the floor. That is, the backward radiation of the omni-directional bass will travel a longer distance before hitting the front wall compared to the 8Cs in the same position in the room.

Assume the following two scenarios:

(1) I reproduce bass frequencies below 100 HZ at 75 dB in the listening position with my current speakers, the listening position being 2.5 meters (equidistant) from the front of the speakers. The fronts of the speakers are in turn about 80 cm from the front wall.

(2) I reproduce bass frequencies below 100 HZ at 75 dB in the listening position with the 8Cs, with the back of the monitors (where the woofers are mounted) 35 cm from the front wall.

My question is the following: will scenario (2) produce a higher or lower sound pressure level at the front wall in the bass region? That is, will the 8Cs increase the risk of disturbing my neighbours compared to my current setup for a given sound level in the listening position?

Since I am an amateur in the area of acoustics, I cannot figure out exactly how the boundary reinforcement (about 5-6 dB I guess) from having the woofers of the 8Cs close to the front wall will balance the fact that the woofers and transmission line ports of my current speakers in scenario (1) are a bit more than twice the distance to the front wall compared to the 8Cs in scenario (2).

I am grateful for any input...
Old 13th December 2018
  #2
Gear Nut
 

Under 100 Hz soundwaves are longer than 3,4 meter.
So it doesn't matter at all how the speaker is located as at frequencies with wavelength larger than the speaker enclosure they are all omni directional.
Old 13th December 2018
  #3
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Quote:
Originally Posted by bert stoltenborg View Post
Under 100 Hz soundwaves are longer than 3,4 meter.
So it doesn't matter at all how the speaker is located as at frequencies with wavelength larger than the speaker enclosure they are all omni directional.
Thanks for the input. So, if I interpret you correctly, the sound preassure at the wall from a woofer (in a "small" monitor) located 1 meter from the wall will be the same as if I move the woofer to a distance 50 cm from the wall?
Old 13th December 2018
  #4
Lives for gear
Quote:
Originally Posted by Peter E View Post
Thanks for the input. So, if I interpret you correctly, the sound preassure at the wall from a woofer (in a "small" monitor) located 1 meter from the wall will be the same as if I move the woofer to a distance 50 cm from the wall?
The difference would be negligible, if there is a measurable/noticeable difference in practice it would be because of different room mode excitation rather than distance to the wall.
Old 13th December 2018
  #5
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mensink's Avatar
 

Transmission loss of sound through a wall gets higher with frequency. The higher the frequency, the better the wall's isolating properties. At low frequencies the isolation is not very high, but this limitation is worsened by two factors: wall-resonances and standing waves in the room.

At a wall-resonance, a lot of sound is transferred to the next room. The extent to which a wall-resonance is excited is determined by the sound pressure at the wall surface at that frequency. When a room-mode is excited, the sound pressure at the room boundaries is very high. If a room-mode frequency and a wall resonance frequency coincide, you're in trouble.

For a given sound pressure level at the listening position, I don't know if with the 8c's the average sound pressure at the walls is higher or lower than with a conventional speaker placed some distance from the wall. What I do know is that it is very beneficial to notch room-modes. It improves the sound and significantly reduces the noise to neighbors. You can do this with the on-board DSP in the 8c's.
Old 13th December 2018
  #6
Here for the gear
 

Quote:
Originally Posted by mensink View Post
Transmission loss of sound through a wall gets higher with frequency. The higher the frequency, the better the wall's isolating properties. At low frequencies the isolation is not very high, but this limitation is worsened by two factors: wall-resonances and standing waves in the room.

At a wall-resonance, a lot of sound is transferred to the next room. The extent to which a wall-resonance is excited is determined by the sound pressure at the wall surface at that frequency. When a room-mode is excited, the sound pressure at the room boundaries is very high. If a room-mode frequency and a wall resonance frequency coincide, you're in trouble.

For a given sound pressure level at the listening position, I don't know if with the 8c's the average sound pressure at the walls is higher or lower than with a conventional speaker placed some distance from the wall. What I do know is that it is very beneficial to notch room-modes. It improves the sound and significantly reduces the noise to neighbors. You can do this with the on-board DSP in the 8c's.
Thanks all of you for the valuable input. Notching room-modes with the on-board DSP seems like a very good starting point.
Old 13th December 2018
  #7
Gear Nut
 

Quote:
Originally Posted by Peter E View Post
Thanks for the input. So, if I interpret you correctly, the sound preassure at the wall from a woofer (in a "small" monitor) located 1 meter from the wall will be the same as if I move the woofer to a distance 50 cm from the wall?
No. In the free field there is a 6 dB difference as the distance differs 100 %. But as Ryan and Martijn tell you the conditions in a room are different and the neighbours will probably not living with their ears on the wall.
If the distance increases the 100% difference becomes smaller.
Old 13th December 2018
  #8
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Quote:
Originally Posted by bert stoltenborg View Post
No. In the free field there is a 6 dB difference as the distance differs 100 %. But as Ryan and Martijn tell you the conditions in a room are different and the neighbours will probably not living with their ears on the wall.
If the distance increases the 100% difference becomes smaller.
Ok, thanks.
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